CN118149514A - Capacity adjusting method and system of screw unit - Google Patents

Abstract

The invention provides a capacity adjusting method and a system of a screw unit, wherein the method comprises the following steps: setting four capacity control points of a screw unit; obtaining refrigerating capacity corresponding to the four capacity control points; detecting an end load; the end load is compared with the four refrigeration capacities and then the maximum allowable refrigeration capacity of the screw unit is defined, after which the capacity of the screw unit is adjusted to match the end load by loading and unloading. According to the capacity adjusting method, the capacity of the screw unit is divided into three capacity sections through four capacity control points, so that the total refrigerating capacity of the screw unit is correspondingly divided into three refrigerating capacity sections, when the tail end load falls into a certain refrigerating capacity section, the allowable maximum refrigerating capacity of the screw unit is limited to be the maximum refrigerating capacity in the refrigerating capacity section, the refrigerating capacity of the screw unit can be effectively prevented from exceeding the tail end load far, the change speed of the output temperature of the screw unit is slowed down, and the output temperature of the unit is stable.

Description

Capacity adjusting method and system of screw unit

The application relates to a capacity adjusting method of a screw unit and a system for realizing the method, which are divisional patent application with the application date 2020.08.15 and the application number CN 202010821848.6.

Technical Field

The invention relates to the technical field of refrigeration devices, in particular to a capacity adjusting method and system of a screw unit.

Background

At present, the screw unit changes the capacity of the compressor through the loading or unloading of the screw compressor so as to adjust the refrigerating capacity of the unit to match the load change of the tail end, thereby meeting different requirements of users. Namely, when the end load is full load, the screw compressor is loaded continuously until the capacity is full capacity, so that the screw compressor is easy to realize; when the end load is a partial load, the screw compressor needs to continuously adjust the capacity of the compressor through loading or unloading until the refrigerating capacity corresponding to the capacity is matched with the partial load, so that the output temperature (such as the air outlet temperature of the evaporator or the water outlet temperature of chilled water) of the unit can be stabilized, the process is not easy to realize, and the adjustment precision is not high.

Taking a screw chiller as an example, the end part load only requires the compressor to run at 60% capacity to meet the user's demand, the compressor should stop loading from start-up to approximately 60% capacity, but because the adjustable capacity range of the compressor in the prior art is between maximum capacity and minimum capacity (e.g., from 100% capacity to 25% capacity) and the change in chilled water temperature is not synchronous with the change in compressor capacity, the compressor is caused to load continuously until the current chilled water outlet temperature is equal to the target temperature value, or the compressor is loaded to 100% capacity (maximum capacity) to stop loading and unloading. Although the compressor has stopped the loading action, the capacity of the compressor is greatly overshot at this time, i.e. the refrigerating capacity of the compressor is already much greater than the partial load demand at the end, resulting in a continuous drop in chilled water temperature, which necessarily results in a chilled water temperature below the target temperature value, thus causing the compressor to unload. The refrigerating capacity of the compressor is far larger than the partial load of the tail end, so that the temperature of the chilled water is fast to drop, at the moment, if the unloading speed is low, the chilled water outlet temperature is too low to cause frost cracking of an evaporating pipe or trigger the anti-freezing protection of a unit, in order to ensure the safety of the unit, only fast unloading can be selected, the quick unloading can cause the temperature of the chilled water outlet temperature to rise too fast, and therefore, the chilled water temperature can finally oscillate, and the requirement of stably outputting the chilled water temperature cannot be met.

Disclosure of Invention

In order to overcome the defects of capacity adjustment of the screw unit, the invention aims to provide a capacity adjustment method of the screw unit, which can realize stable output temperature under the load of a tail end part.

Accordingly, another object of the present invention is to provide a capacity control system for a screw machine set, which can stabilize the output temperature under the load of the end portion.

Regarding the capacity adjustment method of the screw unit, the capacity adjustment method for solving the technical problems comprises the following steps: setting four capacity control points of the screw unit, wherein the four capacity control points are sequentially a minimum capacity, a first capacity, a second capacity and a maximum capacity from small to large, and the capacity difference between the latter capacity and the former capacity is 20-35; the refrigerating capacity respectively corresponding to the minimum capacity, the first capacity, the second capacity and the maximum capacity is obtained, and the minimum refrigerating capacity, the first refrigerating capacity, the second refrigerating capacity and the maximum refrigerating capacity are sequentially obtained from small to large; detecting an end load; setting a target load, a loading threshold and an unloading threshold, wherein the loading threshold is greater than the unloading threshold; when the minimum refrigerating capacity is less than the end load and is less than or equal to the first refrigerating capacity, the screw machine set can be started, and when the screw machine set operates, the allowable maximum refrigerating capacity of the screw machine set is limited to be equal to the first refrigerating capacity; when the first refrigerating capacity is less than the end load and less than or equal to the second refrigerating capacity, the screw machine set can be started, and when the screw machine set operates, the allowable maximum refrigerating capacity of the screw machine set is limited to be equal to the second refrigerating capacity; when the end load is greater than the second cooling capacity, the screw unit may be started, and when the screw unit is operated, the allowable maximum cooling capacity of the screw unit is defined to be equal to the maximum cooling capacity; when the screw machine set is operated, after limiting the allowable maximum refrigerating capacity of the screw machine set, when the difference value between the end load and the target load is larger than or equal to the loading threshold value, the capacity of the screw machine set is gradually increased, and when the difference value between the end load and the target load is smaller than or equal to the unloading threshold value, the capacity of the screw machine set is gradually reduced.

According to the capacity adjusting method, the capacity of the screw unit is divided into three capacity intervals with small difference (for example, the capacity interval is 20-35), so that the total refrigerating capacity of the screw unit is correspondingly divided into three refrigerating capacity intervals, when the tail end load falls into a certain refrigerating capacity interval, the allowable maximum refrigerating capacity of the screw unit is limited to be the maximum refrigerating capacity in the refrigerating capacity interval, which is equivalent to providing a proper adjusting range for capacity adjustment, thus the refrigerating capacity of the screw unit can be effectively prevented from far exceeding the tail end load, the capacity overshoot amplitude is greatly reduced, the change speed of the output temperature (for example, the evaporator air outlet temperature or the chilled water outlet temperature) of the screw unit is slowed down, the allowable maximum refrigerating capacity of the screw unit is limited by the tail end load, the degree of asynchronization of the compressor capacity change and the output temperature change can be reduced, then the capacity of the compressor is adjusted to be matched with the tail end load by the loading threshold or the unloading threshold, and the output temperature of the screw unit is finally stable.

Preferably, the capacity difference is 25.

Further, the capacity adjustment method further includes: setting a minimum starting load, wherein the minimum starting load is smaller than the minimum refrigerating capacity; when the minimum starting load is less than or equal to the end load less than the minimum refrigerating capacity, the screw unit can be started, and when the screw unit operates, the allowable maximum refrigerating capacity of the screw unit is limited to be equal to the minimum refrigerating capacity; and when the end load is less than the minimum starting load, stopping the screw unit.

In one embodiment, the screw machine set is a screw water chiller, and the detecting step of the end load includes: acquiring the chilled water inlet temperature of the screw unit; obtaining the chilled water outlet temperature of the screw unit; acquiring the frozen water mass flow of the screw unit; the end load is calculated according to the formula: q=c×m×Δt; wherein Q is the end load, C is the chilled water specific heat capacity of the screw unit, M is the chilled water mass flow of the screw unit, T _{Feeding in} is the chilled water inlet temperature, T _{Out of} is the chilled water outlet temperature, Δt is the chilled water temperature difference of the screw unit, i.e. Δt=t _{Feeding in} －T _{Out of} .

In one embodiment, the screw machine set is a screw water chiller, and the capacity adjustment method further comprises: setting a target outlet water temperature; setting a minimum water inlet temperature, a first water inlet temperature and a second water inlet temperature which are arranged from small to large, wherein the relation between the first water inlet temperature and the first refrigerating capacity is as follows: q ₁＝C×M×(T _{Is provided into 1}－T _{Set out} ), the relationship between the second inlet water temperature and the second refrigerating capacity is: q ₂＝C×M×(T _{Is provided into 2}－T _{Set out} ); wherein, Q ₁ and Q ₂ are the first refrigerating capacity and the second refrigerating capacity respectively, T _{Is provided into 1} and T _{Is provided into 2} are the first water inlet temperature and the second water inlet temperature respectively, T _{Set out} is the target water outlet temperature, C is the chilled water specific heat capacity of the screw unit, and M is the chilled water mass flow of the screw unit; when the minimum water inlet temperature is less than the chilled water inlet temperature and less than or equal to the first water inlet temperature, the screw unit can be started, and when the screw unit operates, the allowable maximum refrigerating capacity of the screw unit is limited to be equal to the first refrigerating capacity; when the first water inlet temperature is less than the chilled water inlet temperature and less than or equal to the second water inlet temperature, the screw unit can be started, and when the screw unit operates, the allowable maximum refrigerating capacity of the screw unit is limited to be equal to the second refrigerating capacity; the screw unit may be started when the second inlet water temperature < the chilled water inlet temperature, and the allowable maximum cooling capacity of the screw unit is defined to be equal to the maximum cooling capacity when the screw unit is operated.

In another embodiment, the screw machine set is a screw water chiller, and the capacity adjustment method further includes: setting a target water inlet temperature; setting the maximum water outlet temperature, the first water outlet temperature and the second water outlet temperature which are arranged from large to small, wherein the relation between the first water outlet temperature and the first refrigerating capacity is as follows: q ₁＝C×M×(T _{Is provided into} －T _{Set out 1}), the relationship between the second outlet water temperature and the second refrigerating capacity is: q ₂＝C×M×(T _{Is provided into} －T _{Set out 2}); wherein, Q ₁ and Q ₂ are the first refrigerating capacity and the second refrigerating capacity respectively, T _{Set out 1} and T _{Set out 2} are the first water outlet temperature and the second water outlet temperature respectively, T _{Is provided into} is the target water inlet temperature, C is the chilled water specific heat capacity of the screw unit, and M is the chilled water mass flow of the screw unit; when the first water outlet temperature is less than or equal to the chilled water outlet temperature and less than the maximum water outlet temperature, the screw unit can be started, and when the screw unit operates, the allowable maximum refrigerating capacity of the screw unit is limited to be equal to the first refrigerating capacity; when the second water outlet temperature is less than or equal to the chilled water outlet temperature and less than the first water outlet temperature, the screw unit can be started, and when the screw unit operates, the allowable maximum refrigerating capacity of the screw unit is limited to be equal to the second refrigerating capacity; the screw unit may be started when the chilled water outlet temperature < the second outlet temperature, and the allowable maximum refrigeration capacity of the screw unit is defined to be equal to the maximum refrigeration capacity when the screw unit is operated.

In another embodiment, the screw machine set is a screw air cooler, and the detecting step of the end load includes: acquiring the air inlet temperature of an evaporator of the screw unit; acquiring the air outlet temperature of an evaporator of the screw unit; acquiring the heat transfer area of an evaporator of the screw unit; acquiring the heat transfer coefficient of an evaporator of the screw unit; the end load is calculated according to the formula: q ₃ = a x K x Δt; wherein Q ₃ is the end load, a is the evaporator heat transfer area of the screw unit, K is the evaporator heat transfer coefficient of the screw unit, t _{Air inlet} is the evaporator air inlet temperature, t _{Air outlet} is the evaporator air outlet temperature, Δt is the air inlet and outlet temperature difference of the evaporator of the screw unit, i.e. Δt=t _{Air inlet} －t _{Air outlet} .

Further, the capacity adjustment method further includes: setting an anti-freezing temperature, and stopping the screw unit when the output temperature of the screw unit is less than or equal to the anti-freezing temperature.

In terms of a capacity adjusting system of a screw unit, the capacity adjusting system for solving the technical problems comprises a cold quantity measuring device, a loading oil circuit, an unloading oil circuit, a first capacity oil circuit, a second capacity oil circuit, a loading electromagnetic valve, an unloading electromagnetic valve, a first capacity electromagnetic valve and a second capacity electromagnetic valve; the cold quantity measuring device is used for detecting the end load; the loading electromagnetic valve is connected in series in a loading oil way of the screw unit and used for controlling the on-off of the loading oil way; the unloading solenoid valve is connected in series in an unloading oil way of the screw unit and is used for controlling the on-off of the unloading oil way; the first capacity electromagnetic valve and the second capacity electromagnetic valve are respectively connected in series in a first capacity oil way and a second capacity oil way of the screw unit and are respectively used for controlling the on-off of the first capacity oil way and the second capacity oil way; when the screw unit operates, and the minimum refrigerating capacity is smaller than the end load and smaller than or equal to the first refrigerating capacity, the first capacity electromagnetic valve is normally open so as to limit the allowable maximum refrigerating capacity of the screw unit to be the first refrigerating capacity; when the first refrigerating capacity is smaller than the end load and smaller than or equal to the second refrigerating capacity, the first capacity electromagnetic valve is normally closed and the second capacity electromagnetic valve is normally open so as to limit the allowable maximum refrigerating capacity of the screw unit to be the second refrigerating capacity; when the end load is greater than the second refrigerating capacity, a first capacity electromagnetic valve (17) and a second capacity electromagnetic valve (18) are normally closed to limit the allowable maximum refrigerating capacity of the screw unit to be the maximum refrigerating capacity;

After limiting the allowable maximum refrigerating capacity of the screw unit, when the difference value between the end load and the target load is more than or equal to the loading threshold value, the capacity of the screw unit is gradually increased, and when the difference value between the end load and the target load is less than or equal to the unloading threshold value, the capacity of the screw unit is gradually reduced.

The capacity adjustment system detects the end load by a cold measuring device so as to provide control parameters for the capacity adjustment method; on the other hand, the unloading solenoid valve, the first capacity solenoid valve and the second capacity solenoid valve are used for respectively controlling the on-off of the unloading oil way, the first capacity oil way and the second capacity oil way to control the allowable maximum refrigerating capacity of the capacity regulating system so that the refrigerating capacity provided by the unit is matched with the load required by the tail end, the refrigerating capacity of the screw unit can be effectively prevented from far exceeding the tail end load, the capacity overshoot amplitude is greatly reduced, the change speed of the output temperature (such as the evaporator air outlet temperature or the chilled water outlet temperature) of the screw unit is slowed down, the asynchronism degree of the compressor capacity change and the output temperature change can be reduced, then the compressor capacity is regulated to be matched with the tail end load through the loading threshold value or the unloading threshold value, and finally the output temperature of the unit is stable.

In one embodiment, the capacity adjustment system further comprises a controller electrically connected with the cold quantity metering device, the loading electromagnetic valve, the unloading electromagnetic valve, the first capacity electromagnetic valve and the second capacity electromagnetic valve, wherein the controller collects the end load detected by the cold quantity metering device and automatically controls the on-off of the loading electromagnetic valve, the unloading electromagnetic valve, the first capacity electromagnetic valve and the second capacity electromagnetic valve.

Drawings

In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following brief description of the drawings of the embodiments will be made, it being apparent that the drawings in the following description relate only to some embodiments of the present invention and are not limiting of the present invention.

FIG. 1 is a flow chart of a capacity modulation method of an embodiment of the present invention;

FIG. 2 is a flow chart of controlling refrigeration capacity by chilled water intake temperature in accordance with an embodiment of the invention;

FIG. 3 is a flow chart of controlling refrigeration capacity by chilled water outlet temperature in accordance with an embodiment of the invention;

FIG. 4 is a schematic diagram of on-off of an oil circuit when the refrigerating capacity is a first refrigerating capacity according to an embodiment of the present invention;

FIG. 5 is a schematic diagram of on-off of an oil circuit when the refrigerating capacity is the second refrigerating capacity according to the embodiment of the invention;

FIG. 6 is a schematic diagram of on-off of an oil circuit when the refrigerating capacity is the maximum refrigerating capacity according to the embodiment of the invention;

fig. 7 is a schematic diagram of on-off of an oil path when the refrigerating capacity is the minimum refrigerating capacity according to an embodiment of the present invention.

Detailed Description

It should be noted that the terms "first," "second," and the like as used herein do not denote any order, quantity, or importance, but rather are used to distinguish one element from another. The following description of the embodiments of the present invention will be made clearly and completely with reference to the accompanying drawings, in which it is apparent that the embodiments described are only some embodiments of the present invention, but not all embodiments. All other embodiments, which can be made by those skilled in the art based on the embodiments of the invention without making any inventive effort, are intended to be within the scope of the invention.

As described in the background art, at present, under the partial load of the tail end, the capacity adjustment range of the screw unit is between the maximum capacity and the minimum capacity, so that the capacity is easily over-adjusted to ensure that the refrigerating capacity of the compressor is far greater than the partial load demand of the tail end, and finally the output temperature of the unit is oscillated, and the requirement of stable output temperature cannot be met.

Referring to fig. 1, in order to solve the above technical problem, an embodiment of the present invention provides a capacity adjustment method of a screw unit capable of achieving stable output temperature under an end part load, comprising the steps of:

S01: setting four capacity control points of the screw unit, wherein the four capacity control points are sequentially a minimum capacity, a first capacity, a second capacity and a maximum capacity from small to large, and the capacity difference between the latter capacity and the former capacity is 20-35 so as to divide the capacity of the screw unit into three capacity sections with small differences;

it will be appreciated that the refrigeration capacity of the screw unit is provided by the screw compressor, and therefore the capacity of the screw unit corresponds to the capacity of the screw compressor.

It should be noted that, in the existing screw compressor, the minimum capacity and the maximum capacity are 25% and 100%, respectively, in order to facilitate positioning and manufacturing of the capacity control points, the capacity intervals formed by two adjacent capacity control points may be equal, that is, the capacity difference between the next capacity and the previous capacity is 25, so that the first capacity and the second capacity may be set to be 50% and 75%, respectively, so that the capacity of the screw unit is divided into three equal capacity intervals.

In addition, the screw compressor allows a certain deviation of capacity control points other than the minimum capacity and the maximum capacity, for example, a permissible deviation of ±5, due to the limitation of the manufacturing process, and thus four capacity control points of the screw set may be set to values as shown in table 1:

TABLE 1

Minimum capacity	First capacity of	Second capacity of	Maximum capacity of
				25％	50％	75％	100％
25％	50％	70％	100％
				25％	50％	80％	100％
25％	45％	70％	100％
				25％	45％	75％	100％
25％	45％	80％	100％
				25％	55％	70％	100％
25％	55％	75％	100％
				25％	55％	80％	100％

S02: the refrigerating capacities corresponding to the minimum capacity, the first capacity, the second capacity and the maximum capacity are obtained, namely the capacities of the compressors are approximately stabilized at the four capacity control points respectively so as to obtain four corresponding refrigerating capacities, and the refrigerating capacities are sequentially the minimum refrigerating capacity, the first refrigerating capacity, the second refrigerating capacity and the maximum refrigerating capacity from small to large;

It can be understood that the minimum refrigerating capacity, the first refrigerating capacity, the second refrigerating capacity and the maximum refrigerating capacity can be obtained through the model selection software of the screw compressor according to the working condition of the screw unit, and can also be obtained through testing the corresponding screw compressor under the working condition of the screw unit.

S03: detecting an end load;

The screw machine set can be a screw water chiller or a screw air cooler. When the screw unit is a screw water chiller, the output temperature of the screw unit is the chilled water outlet temperature, and at this time, the end load can be obtained by a cold energy measuring device, and can be detected by the following steps (not shown in fig. 1):

s0311: acquiring the chilled water inlet temperature of the screw unit;

s0312: obtaining the chilled water outlet temperature of the screw unit;

s0313: acquiring the frozen water mass flow of the screw unit;

S0314: the end load is calculated according to the formula: q=c×m×Δt, wherein Q is the end load in kW; c is the specific heat capacity of the chilled water of the screw unit, and the unit is KJ/(kg. DEG C); m is the mass flow of chilled water of the screw unit, and the unit is M3/h; t _{Feeding in} is the inlet temperature of the chilled water, and the unit is the temperature; t _{Out of} is the outlet water temperature of the chilled water, and the unit is the temperature; Δt is the chilled water temperature difference of the screw unit, i.e., Δt=t _{Feeding in} －T _{Out of} .

It will be appreciated that the chilled water inlet and outlet temperatures described above may be detected by temperature sensors and the chilled water mass flow may be detected by flow meters.

When the screw unit is a screw air cooler, the output temperature of the screw unit is the air outlet temperature of the evaporator, and the end load can be obtained through a cold energy measuring device at the moment, and can be detected through the following steps (not shown in fig. 1):

s0321: acquiring the air inlet temperature of an evaporator of the screw unit;

s0322: acquiring the air outlet temperature of an evaporator of the screw unit;

s0323: acquiring the temperature of an evaporator refrigerant of the screw unit;

s0324: acquiring the heat transfer area of an evaporator of the screw unit;

s0325: acquiring the heat transfer coefficient of an evaporator of the screw unit;

S0326: the end load is calculated according to the formula: q=a×k×Δt, wherein Q is the end load in kW; a is the heat transfer area of an evaporator of the screw unit, and the unit is square meter; k is the heat transfer coefficient of the evaporator of the screw unit, and the unit is W/(squaremeter) x ℃; t _{Air inlet} is the air inlet temperature of the evaporator, and the unit is the temperature; t _{Air outlet} is the outlet air temperature of the evaporator, and the unit is the temperature; t _Refrigerant is the temperature of the refrigerant of the evaporator; Δt is the logarithmic average temperature difference of the evaporator, i.e., Δt= (t _{Air inlet} －t _{Air outlet} )/ln((t _{Air inlet} －t _Refrigerant )/(t _{Air outlet} －t _Refrigerant )), where ln is the natural logarithm.

It can be understood that the inlet air temperature and the outlet air temperature of the evaporator can be detected by a temperature sensor, the heat transfer area of the evaporator can be calculated according to structural parameters (such as the diameter and the length of an evaporation tube, the form of fins, the number of fins and the like) of the evaporator, the refrigerant temperature of the evaporator can be obtained by inquiring a refrigerant pressure-enthalpy diagram according to the refrigerant pressure and the refrigerant type of the evaporator, and the heat transfer coefficient of the evaporator can be determined by practical tests.

S04: comparing the end load with the minimum refrigeration capacity, the first refrigeration capacity, the second refrigeration capacity and the maximum refrigeration capacity respectively, and then limiting the allowable maximum refrigeration capacity of the screw unit according to the following refrigeration capacity control logic:

First case: when the minimum refrigerating capacity is less than the end load and is less than or equal to the first refrigerating capacity, the screw machine set can be started, and when the screw machine set operates, the allowable maximum refrigerating capacity of the screw machine set is limited to be equal to the first refrigerating capacity;

Second case: when the first refrigerating capacity is less than the end load and less than or equal to the second refrigerating capacity, the screw machine set can be started, and when the screw machine set operates, the allowable maximum refrigerating capacity of the screw machine set is limited to be equal to the second refrigerating capacity;

Third case: the screw unit may be started when the end load > the second cooling capacity, and the allowable maximum cooling capacity of the screw unit is defined to be equal to the maximum cooling capacity when the screw unit is operated.

Fourth case: the screw unit may be started when the end load is equal to the minimum cooling capacity and an allowable maximum cooling capacity of the screw unit is defined to be equal to the minimum cooling capacity when the screw unit is operated.

The maximum allowable cooling capacity of the screw unit refers to the maximum value of the maximum allowable cooling capacities of the screw unit, and for example, under the condition that the maximum allowable cooling capacity of the screw unit is limited to be equal to the second cooling capacity, the cooling capacity output by the screw unit does not increase after reaching the second cooling capacity, and even when the compressor is continuously loaded, the cooling capacity output by the screw unit does not increase.

In the screw unit capacity adjustment method according to the present embodiment, the order of steps S01, S02 and S03 is not fixed, but may be exchanged, for example, S03 may be used as the first step and S01 may be used as the third step, and such exchange does not substantially change the screw unit capacity adjustment method according to the present embodiment.

Therefore, according to the capacity adjusting method, the capacity of the screw unit is divided into three capacity intervals with small difference (for example, the capacity interval is 20-35), so that the total refrigerating capacity of the screw unit is correspondingly divided into three refrigerating capacity intervals, when the end load falls into a certain refrigerating capacity interval, the allowable maximum refrigerating capacity of the screw unit is limited to be the maximum refrigerating capacity in the refrigerating capacity interval, which is equivalent to providing a proper adjusting range for capacity adjustment, thus the refrigerating capacity of the screw unit can be effectively prevented from far exceeding the end load, the capacity overshoot amplitude is greatly reduced, the change speed of the output temperature (for example, the evaporator air outlet temperature or the chilled water outlet temperature) of the screw unit is slowed down, the degree of asynchronism between the capacity change and the output temperature change of a compressor can be reduced by limiting the allowable maximum refrigerating capacity of the screw unit through the end load, and finally the output temperature of the screw unit is stable.

As shown in fig. 1, in this embodiment, the step S04 further includes: setting a minimum starting load, wherein the minimum starting load is smaller than the minimum refrigerating capacity; when the minimum starting load is less than or equal to the end load less than the minimum refrigerating capacity, the screw unit can be started, and when the screw unit operates, the allowable maximum refrigerating capacity of the screw unit is limited to be equal to the minimum refrigerating capacity; and when the end load is less than the minimum starting load, stopping the screw unit. It should be noted that the above-mentioned screw machine set stop includes two cases, one is to prohibit the screw machine set from starting; the other is to stop the running screw unit. In general, the load required by the tail end can allow a certain deviation (for example, the chilled water inlet temperature or the outlet water temperature is allowed to fluctuate within a deviation of +/-0.5 ℃), although the tail end load is smaller than the minimum refrigerating capacity, and the minimum refrigerating capacity is the lowest limit of the operation of the screw unit, the unloading cannot be continued any more, but when the tail end load is not greatly different from the minimum refrigerating capacity, the refrigerating capacity output by the screw unit still can meet the lower deviation value of the load required by the tail end, namely, the chilled water inlet temperature or the outlet water temperature still falls within the allowable deviation, and when the tail end load is smaller than the minimum starting load, the chilled water temperature is inevitably caused to fall below the lower deviation of the set target value, so that the screw unit cannot meet the tail end requirement. On one hand, the anti-freezing protection of the unit can be triggered to cause shutdown; on the other hand, each time the compressor is started, the minimum operation time exists, and in the period of time, if the temperature of the chilled water is reduced too fast, hidden danger of freezing and cracking of the evaporating pipe exists. Therefore, in order to protect the safety of the screw unit, the frequent start and stop of the compressor are prevented, the service life of the compressor is prolonged, and when the end load is smaller than the minimum start load, the screw unit is prohibited from being started or the running screw unit is stopped.

After the allowable maximum cooling capacity of the screw unit is defined, the capacity of the compressor needs to be finely adjusted so that the output temperature of the screw unit meets the terminal requirements. As shown in fig. 1, in this embodiment, the method for adjusting the capacity of the screw unit further includes step S05:

Setting a target load, a loading threshold and an unloading threshold, wherein the loading threshold is greater than the unloading threshold; when the screw machine set is operated, if the difference value between the end load and the target load is more than or equal to the loading threshold value, the capacity of the screw machine set is gradually increased, and if the difference value between the end load and the target load is less than or equal to the unloading threshold value, the capacity of the screw machine set is gradually reduced. The capacity of the screw unit, namely the capacity of the compressor is increased, so that the refrigerating capacity is increased; and when the capacity is reduced, the refrigerating capacity is reduced, and the terminal load value is gradually close to the target load value by continuously adjusting the refrigerating capacity output by the compressor until the terminal load value meets the requirement, so that the output temperature of the screw unit meets the requirement of the terminal.

As shown in fig. 2, in this embodiment, the screw unit is a screw chiller, and in order to facilitate implementation of control logic, the method for adjusting capacity of the screw unit controls the allowable maximum refrigerating capacity of the screw unit by using the chilled water inlet temperature, and specifically includes the following steps:

s11: setting a target outlet water temperature;

S12: setting a minimum water inlet temperature, a first water inlet temperature and a second water inlet temperature which are arranged from small to large, wherein the relation between the first water inlet temperature and the first refrigerating capacity is as follows: q ₁＝C×M×(T _{Is provided into 1}－T _{Set out} ), the relationship between the second inlet water temperature and the second refrigerating capacity is: q ₂＝C×M×(T _{Is provided into 2}－T _{Set out} );

Wherein, Q ₁ and Q ₂ are the first refrigerating capacity and the second refrigerating capacity respectively, and the unit is kW; t _{Is provided into 1} and T _{Is provided into 2} are the first water inlet temperature and the second water inlet temperature respectively, and the unit is the temperature; t _{Set out} is the target outlet water temperature, and the unit is DEG C; c is the specific heat capacity of the chilled water of the screw unit, and the unit is KJ/(kg. DEG C); m is the mass flow of chilled water of the screw unit, and the unit is M3/h;

S13: the chilled water inlet temperature is respectively compared with the minimum inlet temperature, the first inlet temperature and the second inlet temperature, and then the allowable maximum refrigerating capacity of the screw unit is limited according to the following refrigerating capacity control logic:

When the minimum water inlet temperature is less than the chilled water inlet temperature and less than or equal to the first water inlet temperature, the screw unit can be started, and when the screw unit operates, the allowable maximum refrigerating capacity of the screw unit is limited to be equal to the first refrigerating capacity;

When the first water inlet temperature is less than the chilled water inlet temperature and less than or equal to the second water inlet temperature, the screw unit can be started, and when the screw unit operates, the allowable maximum refrigerating capacity of the screw unit is limited to be equal to the second refrigerating capacity;

the screw unit may be started when the second inlet water temperature < the chilled water inlet temperature, and the allowable maximum cooling capacity of the screw unit is defined to be equal to the maximum cooling capacity when the screw unit is operated.

In another embodiment, as shown in fig. 3, to facilitate implementation of control logic, the method for adjusting capacity of a screw unit controls the allowable maximum refrigerating capacity of the screw unit by chilled water outlet temperature, and specifically includes the following steps:

S21: setting a target water inlet temperature;

S22: setting the maximum water outlet temperature, the first water outlet temperature and the second water outlet temperature which are arranged from large to small, wherein the relation between the first water outlet temperature and the first refrigerating capacity is as follows: q ₁＝C×M×(T _{Is provided into} －T _{Set out 1}), the relationship between the second outlet water temperature and the second refrigerating capacity is: q ₂＝C×M×(T _{Is provided into} －T _{Set out 2});

Wherein, Q ₁ and Q ₂ are the first refrigerating capacity and the second refrigerating capacity respectively, and the unit is DEG C; t _{Set out 1} and T _{Set out 2} are the first water outlet temperature and the second water outlet temperature respectively, and the unit is the temperature; t _{Is provided into} is the target water inlet temperature, and the unit is DEG C; c is the specific heat capacity of the chilled water of the screw unit, and the unit is KJ/(kg. DEG C); m is the mass flow of chilled water of the screw unit, and the unit is M3/h;

S23: the chilled water outlet temperature is respectively compared with the first outlet temperature and the second outlet temperature, and then the allowable maximum refrigerating capacity of the screw unit is limited according to the following refrigerating capacity control logic:

When the first water outlet temperature is less than or equal to the chilled water outlet temperature and less than the maximum water outlet temperature, the screw unit can be started, and when the screw unit operates, the allowable maximum refrigerating capacity of the screw unit is limited to be equal to the first refrigerating capacity;

When the second water outlet temperature is less than or equal to the chilled water outlet temperature and less than the first water outlet temperature, the screw unit can be started, and when the screw unit operates, the allowable maximum refrigerating capacity of the screw unit is limited to be equal to the second refrigerating capacity;

The screw unit may be started when the chilled water outlet temperature < the second outlet temperature, and the allowable maximum refrigeration capacity of the screw unit is defined to be equal to the maximum refrigeration capacity when the screw unit is operated.

As shown in fig. 2 and 3, in order to prevent frost cracking of the evaporating pipe of the water chiller, the foregoing embodiments each include an anti-freezing protection step S06:

Setting an anti-freezing temperature, and stopping the screw unit when the chilled water outlet temperature of the screw unit is less than or equal to the anti-freezing temperature. It will be appreciated that the chilled water outlet temperature described above may be detected by a temperature sensor.

As shown in fig. 4 to 7, an embodiment of the present invention also provides a capacity adjustment system of a screw unit capable of stabilizing an output temperature under an end partial load, including a cold amount metering device (not shown), a loading oil passage 11, an unloading oil passage 12, a first capacity oil passage 13, a second capacity oil passage 14, a loading solenoid valve 15, an unloading solenoid valve 16, a first capacity solenoid valve 17, and a second capacity solenoid valve 18; the loading solenoid valve 15 is connected in series in the loading oil way 11 of the screw unit and is used for controlling the on-off of the loading oil way 11; the unloading solenoid valve 16 is connected in series in the unloading oil way 12 of the screw unit and is used for controlling the on-off of the unloading oil way 12; the first capacity electromagnetic valve 17 and the second capacity electromagnetic valve 18 are respectively connected in series in the first capacity oil path 13 and the second capacity oil path 14 of the screw unit, and are respectively used for controlling the on-off of the first capacity oil path 13 and the second capacity oil path 14.

It will be appreciated that the volumetric flow rate (i.e. capacity) of the screw compressor is regulated by the slide valve 21, the slide valve 21 being provided with radial discharge ports, the length of the rotor 22 sealed being correspondingly different when the slide valve 21 is in different positions in the compressor, and thus the amount of gas fed into the compression chamber during a revolution. The sucked air outside the sealing section flows back to the suction end through a bypass port below the machine body. The method can achieve the purpose of adjusting and changing the air delivery of the compressor, thereby realizing capacity adjustment. When the slide valve 21 moves, high-pressure oil from the compressor oil passage system is communicated with the oil chamber on one side of the oil cylinder, and the other side of the piston 23 is led to the low-pressure side of the compressor. The resultant force generated by the pressure difference on both sides of the piston 23 pushes the piston 23 to move in one direction, so that the slide valve 21 connected with the piston is driven to move in the same direction correspondingly, thus changing the relative position of the slide valve 21 in the compressor, and changing the refrigerating capacity output by the compressor. The compressor oil circuit system comprises a loading oil circuit 11, an unloading oil circuit 12, a first capacity oil circuit 13 and a second capacity oil circuit 14, and the refrigerating capacity output by the compressor can be changed by controlling the on-off of each electromagnetic valve on the compressor oil circuit system. For example:

as shown in fig. 4, the allowable maximum cooling capacity of the screw unit is the first cooling capacity when the unloading solenoid valve 16 is normally closed and the first capacity solenoid valve 17 is normally open, so as to implement the first case of step S04.

As shown in fig. 5, when the unloading solenoid valve 16 and the first capacity solenoid valve 17 are both normally closed and the second capacity solenoid valve 18 is normally open, the allowable maximum cooling capacity of the screw unit is the second cooling capacity, so as to implement the second case of step S04.

As shown in fig. 6, when the unloading solenoid valve 16, the first capacity solenoid valve 17, and the second capacity solenoid valve 18 are all normally closed and the loading oil passage 11 can be communicated, the allowable maximum cooling capacity of the screw unit is the maximum cooling capacity, so as to implement the third case of step S04.

As shown in fig. 7, when the unloading solenoid valve 16 is normally open, the allowable maximum cooling capacity of the screw unit is the minimum cooling capacity, so as to implement the fourth case of step S04.

The cold quantity measuring device for detecting the end load comprises a thermometer or a temperature sensor for detecting the inlet water temperature and the outlet water temperature of chilled water or the inlet air temperature and the outlet air temperature of an evaporator and the refrigerant temperature of the evaporator, and a flowmeter for detecting the flow quantity of chilled water, so as to realize the step S03.

The capacity adjustment system detects the end load by a cold measuring device so as to provide control parameters for the capacity adjustment method; on the other hand, the unloading solenoid valve 16, the first capacity solenoid valve 17 and the second capacity solenoid valve 18 are used for respectively controlling the on-off of the unloading oil way 12, the first capacity oil way 13 and the second capacity oil way 14 to control the allowable maximum refrigerating capacity of the capacity regulating system, so that the refrigerating capacity of the screw unit can be effectively prevented from far exceeding the end load, the capacity overshoot amplitude is greatly reduced, the change speed of the output temperature (such as the evaporator air outlet temperature or the chilled water outlet temperature) of the screw unit is slowed down, the unsynchronized degree of the capacity change and the output temperature change of the compressor can be reduced, and finally the output temperature of the unit is stable.

For automatic control, in this embodiment, the screw unit capacity adjustment system further includes a controller electrically connected to the first temperature sensor, the second temperature sensor, the loading solenoid valve 15, the unloading solenoid valve 16, the first capacity solenoid valve 17 and the second capacity solenoid valve 18, where the controller collects the chilled water inlet temperature and the chilled water outlet temperature detected by the first temperature sensor and the second temperature sensor respectively, and controls on-off of the loading solenoid valve 15, the unloading solenoid valve 16, the first capacity solenoid valve 17 and the second capacity solenoid valve 18.

It is understood that the controller may employ a PLC controller to collect control parameters (e.g., temperature, flow, etc.) and write the above-described method for adjusting the capacity of the screw unit into the controller so as to automatically control the on/off of each actuator (e.g., solenoid valve).

The technical scheme of the invention is further described by two specific embodiments, in which the screw machine set is a screw water chiller.

In the first embodiment, the end full load is 370.48kW, and the specific requirement is that the chilled water outlet temperature is stabilized at 4.8+/-0.5 ℃ when the chilled water inlet temperature is any one of the temperatures in the range of 7.4-12+/-0.5 ℃, the water flow rate is 44+/-2.7 m < 3 >/h, and the specific heat capacity of the chilled water is 4.21 KJ/(kg DEG C).

To meet the above requirements, the screw unit includes a cold amount metering device including a temperature sensor for detecting the temperature of the chilled water inlet and outlet water and a flow meter for detecting the flow rate of the chilled water, and the screw compressor includes a loading oil passage 11, an unloading oil passage 12, a 50% capacity oil passage (i.e., a first capacity oil passage 13), a 75% capacity oil passage (i.e., a second capacity oil passage 14), a loading solenoid valve 15, an unloading solenoid valve 16, a 50% capacity solenoid valve (i.e., a first capacity solenoid valve 17) and a 75% capacity solenoid valve (i.e., a second capacity solenoid valve 18), each of which controls the on-off of the corresponding oil passage.

It will be appreciated that the unloading oil circuit 12 and the unloading solenoid valve 16 of the existing screw compressor correspond to 25% capacity of the screw compressor, i.e. the minimum refrigeration capacity that the screw compressor can deliver. It can be seen that the screw machine set has four capacity control points, 25% capacity, 50% capacity, 75% capacity and 100% capacity in this order from small to large. The refrigerating capacity corresponding to the four capacity control points is obtained through compressor type selection software, and specific type selection parameters are shown in table 2.

TABLE 2

Project	Results
		Application conditions of	Water cooling condition
Using a cooling medium	R134a
		Compressor suction saturation temperature DEG C	2
Degree of superheat K of inspiration	5
		Compressor discharge saturation temperature DEG C	40
Condensation supercooling degree	5
		100% Capacity refrigeration capacity (maximum refrigeration capacity) kW	378.93
75% Capacity refrigerating capacity (second refrigerating capacity) kW	288.48
		Refrigeration capacity (first refrigeration capacity) kW of 50% capacity	192.62
Refrigerating capacity (minimum refrigerating capacity) kW of 25% capacity	134.83

The inlet temperature of the chilled water, the outlet temperature of the chilled water and the mass flow of the chilled water can be detected by the corresponding temperature sensor and flowmeter of the screw unit, and then the value of the end load can be calculated according to the formula q=c×m×Δt in the foregoing embodiment, and the meaning of each code in the formula can refer to the relevant content in the foregoing embodiment, which is not repeated here.

As shown in table 3, the end load is compared with the minimum cooling capacity (25% capacity), the first cooling capacity (50% capacity), the second cooling capacity (75% capacity) and the maximum cooling capacity (100% capacity), respectively, and then the allowable maximum cooling capacity of the screw unit is defined according to the following cooling capacity control logics L11 to L15:

L11. when the end load is equal to the minimum cooling capacity (134.83 kW), the screw unit may be started and when the screw unit is running, the 25% capacity solenoid valve is normally open to define the allowable maximum cooling capacity of the screw unit to be equal to the minimum cooling capacity.

L12. when the minimum refrigerating capacity (134.83 kW) < the end load is less than or equal to the first refrigerating capacity (192.62 kW), the screw unit can be started, and when the screw unit is operated, the 50% capacity electromagnetic valve is normally open so as to limit the allowable maximum refrigerating capacity of the screw unit to be equal to the first refrigerating capacity;

L13. when the first refrigerating capacity (192.62 kW) < the end load is less than or equal to the second refrigerating capacity (288.48 kW), the screw unit can be started, and when the screw unit is operated, the 50% capacity solenoid valve is normally closed and the 75% capacity solenoid valve is normally open, so as to limit the allowable maximum refrigerating capacity of the screw unit to be equal to the second refrigerating capacity;

L14. when the end load is > the second capacity (288.48 kW), the screw unit may be started and when the screw unit is running, both the 50% capacity solenoid valve and the 75% capacity solenoid valve are normally closed to define that the maximum allowable capacity of the screw unit is equal to the maximum capacity.

L15. setting the minimum startup load as 108.06kW, when the minimum startup load (108.06 kW) is less than or equal to the end load and less than the minimum refrigerating capacity (134.83 kW), starting the screw unit, and when the screw unit operates, the 25% capacity electromagnetic valve is normally open so as to limit the allowable maximum refrigerating capacity of the screw unit to be equal to the minimum refrigerating capacity; when the end load is less than the minimum start-up load (108.06 kW), the screw unit is shut down.

Table 3 (inlet temperature in degrees Celsius and end load in kW)

It should be noted that, with respect to table 3, "inlet water temperature" in the table refers to the chilled water inlet water temperature; the end load value is calculated according to the condition that the outlet water temperature of chilled water is 4.8 ℃ and the water flow rate is 44m 3/h.

As can be seen from table 3, in this embodiment, the allowable maximum cooling capacity of the screw unit can also be controlled by the chilled water inlet temperature, and the specific steps are as follows:

setting the target outlet water temperature to be 4.8 ℃;

setting the limited startup water inlet temperature to be 6.9 ℃, setting the minimum water inlet temperature to be 7.4 ℃, setting the first water inlet temperature to be 8.5 ℃ and setting the second water inlet temperature to be 10.4 ℃;

The chilled water inlet temperature (hereinafter referred to as inlet temperature) is respectively compared with 6.9 ℃, 7.4 ℃, 8.5 ℃ and 10.4 ℃, and then the allowable maximum refrigerating capacity of the screw unit is defined according to the following refrigerating capacity control logics L21-L25:

l21. when the temperature of the incoming water is less than or equal to 6.9 ℃ and less than or equal to 7.4 ℃, the screw unit can be started, and when the screw unit is operated, the 25% capacity electromagnetic valve is normally open to limit the allowable maximum refrigerating capacity of the screw unit to be equal to the minimum refrigerating capacity.

L22. when the water inlet temperature is more than 7.4 ℃ and less than or equal to 8.5 ℃, the screw unit can be started, and when the screw unit operates, the electromagnetic valve with 50% capacity is normally open so as to limit the allowable maximum refrigerating capacity of the screw unit to be equal to the first refrigerating capacity;

L23 when the water inlet temperature is less than or equal to 8.5 ℃ and less than or equal to 10.4 ℃, the screw unit can be started, and when the screw unit operates, the 50% capacity electromagnetic valve is normally closed and the 75% capacity electromagnetic valve is normally open, so as to limit the allowable maximum refrigerating capacity of the screw unit to be equal to the second refrigerating capacity;

L24. when the inlet water temperature is > 10.4deg.C, the screw unit can be started, and when the screw unit is operated, the 50% capacity solenoid valve and the 75% capacity solenoid valve are normally closed to define the allowable maximum refrigeration capacity of the screw unit to be equal to the maximum refrigeration capacity.

And L25, stopping the screw machine set when the temperature of the inlet water is less than 6.9 ℃.

In the second embodiment, the end full load is 370.48kW, and the specific requirement is that the chilled water inlet temperature is stabilized at 11.5+/-0.5 ℃ when the chilled water outlet temperature is any one of the temperatures in the range of 4.3-8.9+/-0.5 ℃, the water flow rate is 44+/-2.7 m < 3 >/h, and the specific heat capacity of the chilled water is 4.21 KJ/(kg DEG C). At this time, the allowable maximum cooling capacity of the screw unit can be controlled by the chilled water outlet temperature, as shown in table 4. The compressor option of this embodiment is the same as that of the first embodiment, and will not be described here again.

Table 4 (outlet temperature in degrees Celsius and end load in kW)

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It should be noted that, in table 4, "water outlet temperature" refers to chilled water outlet temperature; the end load value is calculated according to the condition that the inlet water temperature of chilled water is 11.5 ℃ and the water flow rate is 44m 3/h.

The method comprises the following specific steps:

setting the target water inlet temperature to be 11.5 ℃;

Setting the limited startup water outlet temperature to 9.4 ℃, setting the maximum water outlet temperature to 8.9 ℃, setting the first water inlet temperature to 7.8 ℃ and setting the second water inlet temperature to 5.9 ℃;

The chilled water outlet temperature (hereinafter referred to as outlet temperature) is compared with 9.4 ℃, 8.9 ℃, 7.8 ℃ and 5.9 ℃ respectively, and then the allowable maximum refrigerating capacity of the screw unit is limited according to the following refrigerating capacity control logics L31-L35:

L31. when the temperature of the discharged water is more than or equal to 8.9 ℃ and less than or equal to 9.4 ℃, the screw machine set can be started, and when the screw machine set operates, the 25% capacity electromagnetic valve is normally open so as to limit the allowable maximum refrigerating capacity of the screw machine set to be equal to the minimum refrigerating capacity.

L32. when the water outlet temperature is more than or equal to 7.8 ℃ and less than 8.9 ℃, the screw unit can be started, and when the screw unit operates, the 50% capacity electromagnetic valve is normally open so as to limit the allowable maximum refrigerating capacity of the screw unit to be equal to the first refrigerating capacity;

l33, when the water outlet temperature is less than or equal to 5.9 ℃ and less than 7.8 ℃, the screw unit can be started, and when the screw unit operates, the 50% capacity electromagnetic valve is normally closed and the 75% capacity electromagnetic valve is normally open, so as to limit the allowable maximum refrigerating capacity of the screw unit to be equal to the second refrigerating capacity;

L34. when the water outlet temperature is < 5.9deg.C, the screw unit can be started, and when the screw unit is operated, the 50% capacity solenoid valve and the 75% capacity solenoid valve are normally closed to limit the allowable maximum refrigerating capacity of the screw unit to be equal to the maximum refrigerating capacity.

L35. when the temperature of the water discharged is less than 9.4 ℃, the screw machine unit is stopped.

In order to ensure the safety of the unit and prevent the evaporator of the unit from frost cracking, the control steps of the first embodiment and the second embodiment are further provided with a water outlet antifreezing step, which is as follows: the freezing prevention temperature is set to be 2.8 ℃, and when the chilled water outlet temperature of the screw unit is less than or equal to the freezing prevention temperature (2.8 ℃), the screw unit is stopped.

The present invention is not limited to the above embodiments, but is intended to be within the scope of the present invention as long as the technical effects of the present invention can be achieved by any same or similar means.

Claims (9)

1. A capacity adjusting method of a screw unit is characterized by comprising the following steps: the capacity adjustment method includes:

setting four capacity control points of the screw unit, wherein the four capacity control points are sequentially a minimum capacity, a first capacity, a second capacity and a maximum capacity from small to large, and the capacity difference between the latter capacity and the former capacity is 20-35;

The refrigerating capacity respectively corresponding to the minimum capacity, the first capacity, the second capacity and the maximum capacity is obtained, and the minimum refrigerating capacity, the first refrigerating capacity, the second refrigerating capacity and the maximum refrigerating capacity are sequentially obtained from small to large;

setting a target load, a loading threshold and an unloading threshold, wherein the loading threshold is greater than the unloading threshold; detecting an end load;

when the minimum refrigerating capacity is less than the end load and is less than or equal to the first refrigerating capacity, the screw machine set can be started, and when the screw machine set operates, the allowable maximum refrigerating capacity of the screw machine set is limited to be equal to the first refrigerating capacity;

when the first refrigerating capacity is less than the end load and less than or equal to the second refrigerating capacity, the screw machine set can be started, and when the screw machine set operates, the allowable maximum refrigerating capacity of the screw machine set is limited to be equal to the second refrigerating capacity;

When the end load is greater than the second cooling capacity, the screw unit may be started, and when the screw unit is operated, the allowable maximum cooling capacity of the screw unit is defined to be equal to the maximum cooling capacity;

When the screw machine set operates, after limiting the allowable maximum refrigerating capacity of the screw machine set, when the difference value between the end load and the target load is larger than or equal to the loading threshold value, the capacity of the screw machine set is gradually increased, and when the difference value between the end load and the target load is smaller than or equal to the unloading threshold value, the capacity of the screw machine set is gradually reduced.

2. The method for capacity adjustment of a screw machine set according to claim 1, wherein: the capacity difference was 25.

3. The method for capacity adjustment of a screw machine set according to claim 1, wherein: the capacity adjustment method further includes:

Setting a minimum starting load, wherein the minimum starting load is smaller than the minimum refrigerating capacity;

When the minimum starting load is less than or equal to the end load less than the minimum refrigerating capacity, the screw unit can be started, and when the screw unit operates, the allowable maximum refrigerating capacity of the screw unit is limited to be equal to the minimum refrigerating capacity;

and when the end load is less than the minimum starting load, stopping the screw unit.

4. The method for capacity adjustment of a screw machine set according to claim 1, wherein: the screw machine set is a screw water chiller, and the detection step of the end load comprises the following steps:

Acquiring the chilled water inlet temperature of the screw unit;

Obtaining the chilled water outlet temperature of the screw unit;

acquiring the frozen water mass flow of the screw unit;

The end load is calculated according to the formula: q=c×m×Δt;

Wherein Q is the end load, C is the chilled water specific heat capacity of the screw unit, M is the chilled water mass flow of the screw unit, T _{Feeding in} is the chilled water inlet temperature, T _{Out of} is the chilled water outlet temperature, Δt is the chilled water temperature difference of the screw unit, i.e. Δt=t _{Feeding in} －T _{Out of} .

5. The method for capacity adjustment of a screw machine set according to claim 4, wherein: the capacity adjustment method further includes: setting a target outlet water temperature;

setting a minimum water inlet temperature, a first water inlet temperature and a second water inlet temperature which are arranged from small to large, wherein the relation between the first water inlet temperature and the first refrigerating capacity is as follows: q ₁＝C×M×(T _{Is provided into 1}－T _{Set out} ), the relationship between the second inlet water temperature and the second refrigerating capacity is: q ₂＝C×M×(T _{Is provided into 2}－T _{Set out} );

Wherein, Q ₁ and Q ₂ are the first refrigerating capacity and the second refrigerating capacity respectively, T _{Is provided into 1} and T _{Is provided into 2} are the first water inlet temperature and the second water inlet temperature respectively, T _{Set out} is the target water outlet temperature, C is the chilled water specific heat capacity of the screw unit, and M is the chilled water mass flow of the screw unit;

When the minimum water inlet temperature is less than the chilled water inlet temperature and less than or equal to the first water inlet temperature, the screw unit can be started, and when the screw unit operates, the allowable maximum refrigerating capacity of the screw unit is limited to be equal to the first refrigerating capacity; when the first water inlet temperature is less than the chilled water inlet temperature and less than or equal to the second water inlet temperature, the screw unit can be started, and when the screw unit operates, the allowable maximum refrigerating capacity of the screw unit is limited to be equal to the second refrigerating capacity; the screw unit may be started when the second inlet water temperature < the chilled water inlet temperature, and the allowable maximum cooling capacity of the screw unit is defined to be equal to the maximum cooling capacity when the screw unit is operated.

6. The method for capacity adjustment of a screw machine set according to claim 4, wherein: the capacity adjustment method further includes:

setting a target water inlet temperature;

Setting the maximum water outlet temperature, the first water outlet temperature and the second water outlet temperature which are arranged from large to small, wherein the relation between the first water outlet temperature and the first refrigerating capacity is as follows: q ₁＝C×M×(T _{Is provided into} －T _{Set out 1}), the relationship between the second outlet water temperature and the second refrigerating capacity is: q ₂＝C×M×(T _{Is provided into} －T _{Set out 2});

wherein, Q ₁ and Q ₂ are the first refrigerating capacity and the second refrigerating capacity respectively, T _{Set out 1} and T _{Set out 2} are the first water outlet temperature and the second water outlet temperature respectively, T _{Is provided into} is the target water inlet temperature, C is the chilled water specific heat capacity of the screw unit, and M is the chilled water mass flow of the screw unit;

When the first water outlet temperature is less than or equal to the chilled water outlet temperature and less than the maximum water outlet temperature, the screw unit can be started, and when the screw unit operates, the allowable maximum refrigerating capacity of the screw unit is limited to be equal to the first refrigerating capacity; when the second water outlet temperature is less than or equal to the chilled water outlet temperature and less than the first water outlet temperature, the screw unit can be started, and when the screw unit operates, the allowable maximum refrigerating capacity of the screw unit is limited to be equal to the second refrigerating capacity; the screw unit may be started when the chilled water outlet temperature < the second outlet temperature, and the allowable maximum refrigeration capacity of the screw unit is defined to be equal to the maximum refrigeration capacity when the screw unit is operated.

7. A method of capacity adjustment of a screw assembly according to any one of claims 4 to 6, characterized in that: the capacity adjustment method further includes:

Setting an anti-freezing temperature, and stopping the screw unit when the chilled water outlet temperature of the screw unit is less than or equal to the anti-freezing temperature.

8. The method for capacity adjustment of a screw machine set according to claim 1, wherein: the screw machine set is a screw air cooler, and the detection step of the end load comprises the following steps:

acquiring the air inlet temperature of an evaporator of the screw unit;

acquiring the air outlet temperature of an evaporator of the screw unit;

acquiring the temperature of an evaporator refrigerant of the screw unit;

Acquiring the heat transfer area of an evaporator of the screw unit;

acquiring the heat transfer coefficient of an evaporator of the screw unit;

the end load is calculated according to the formula: q=a×k×Δt;

Wherein Q is the end load, a is the evaporator heat transfer area of the screw unit, K is the evaporator heat transfer coefficient of the screw unit, t _{Air inlet} is the evaporator air inlet temperature, t _{Air outlet} is the evaporator air outlet temperature, t _Refrigerant is the evaporator refrigerant temperature, Δt is the air inlet and outlet temperature difference of the evaporator of the screw unit, i.e., Δt= (t _{Air inlet} －t _{Air outlet} )/ln((t _{Air inlet} －t _Refrigerant )/(t _{Air outlet} －t _Refrigerant )), and ln is the natural logarithm.

9. A capacity modulation system for implementing the capacity modulation method according to any one of claims 1 to 8, characterized in that: the capacity adjusting system comprises a cold quantity measuring device, a loading oil circuit (11), an unloading oil circuit (12), a first capacity oil circuit (13), a second capacity oil circuit (14), a loading electromagnetic valve (15), an unloading electromagnetic valve (16), a first capacity electromagnetic valve (17) and a second capacity electromagnetic valve (18);

the cold quantity measuring device is used for detecting the end load;

The loading electromagnetic valve (15) is connected in series in a loading oil way (11) of the screw unit and is used for controlling the on-off of the loading oil way (11);

the unloading solenoid valve (16) is connected in series in an unloading oil way (12) of the screw unit and is used for controlling the on-off of the unloading oil way (12);

The first capacity electromagnetic valve (17) and the second capacity electromagnetic valve (18) are respectively connected in series in a first capacity oil way (13) and a second capacity oil way (14) of the screw unit, and are respectively used for controlling the on-off of the first capacity oil way (13) and the second capacity oil way (14); when the screw machine set is in operation,

When the minimum refrigerating capacity is less than the end load and less than or equal to the first refrigerating capacity, the first capacity electromagnetic valve (17) is normally opened to limit the allowable maximum refrigerating capacity of the screw unit to be the first refrigerating capacity;

when the first refrigerating capacity is less than the end load and less than or equal to the second refrigerating capacity, the first capacity electromagnetic valve (17) is normally closed and the second capacity electromagnetic valve (18) is normally open so as to limit the allowable maximum refrigerating capacity of the screw unit to be the second refrigerating capacity;

when the end load is greater than the second refrigerating capacity, a first capacity electromagnetic valve (17) and a second capacity electromagnetic valve (18) are normally closed to limit the allowable maximum refrigerating capacity of the screw unit to be the maximum refrigerating capacity;

After limiting the allowable maximum refrigerating capacity of the screw unit, when the difference value between the end load and the target load is more than or equal to the loading threshold value, the capacity of the screw unit is gradually increased, and when the difference value between the end load and the target load is less than or equal to the unloading threshold value, the capacity of the screw unit is gradually reduced.