CN114527159B - Anti-condensation method for environmental test box - Google Patents
Anti-condensation method for environmental test box Download PDFInfo
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Abstract
The invention provides an environment test box dew prevention method, which comprises the following steps: s1: measuring the temperature and relative humidity outside the environmental test chamber; s2: calculating dew point temperature according to the temperature and relative humidity outside the environmental test chamber; s3: calculating the critical temperature in the environmental test chamber under the critical condition of dew condensation by adopting a thermodynamic formula according to the dew point temperature and the heat conduction characteristic of the heat insulation material of the environmental test chamber; s4: determining a sampling period and a sampling frequency, and measuring and recording the temperature in the environmental test box according to the sampling frequency in one sampling period; s5: calculating a power coefficient according to the change condition of the temperature in the environmental test chamber in a sampling period and the critical temperature in the environmental test chamber; s6: the heating power of the resistive heating wire is set to be graded and adjustable, and the heating power grade of the resistive heating wire is controlled according to the power coefficient. The invention provides an anti-condensation method for an environmental test box, which solves the problem that the performance of the environmental test box is affected after a resistive heating wire is electrified.
Description
Technical Field
The invention relates to the field of environmental tests of products, in particular to an anti-condensation method of an environmental test box.
Background
The environment test box is a necessary test device for testing various environmental stresses (such as temperature, humidity, air pressure, solar radiation, wind, rain, snow, mould, salt fog and the like) of various products (such as various materials, electromechanical parts and components and complete machine products), detecting and verifying the environment adaptability and evaluating the quality and reliability of the products. Common environmental test chambers are: constant temperature and humidity test box, rapid temperature change test box, alternating damp and heat test box, cold and hot impact test box, photo aging test box, mold test box, salt spray test box, etc.
Generally, the temperature in the temperature-humidity environment test box has a large range, the high temperature limit is close to 200 ℃, and the low temperature limit reaches-80 ℃. A typical low temperature environment test procedure may undergo three phases: a cooling stage, a constant low temperature stage and a heating stage. Cooling from a certain high temperature or normal temperature state (for example, 25 ℃ normal temperature) at a certain cooling rate, maintaining the target low temperature value for a period of time after reaching the target low temperature value, generally from several hours to several days, and returning to normal temperature or another temperature state at a certain heating rate, wherein common temperature changing rates are as follows: 1 ℃/min,3 ℃/min, 5 ℃/min, 10 ℃/min, 15 ℃/min, 20 ℃/min, etc.
For the environment test box with the low-temperature test function, if the internal temperature of the environment test box is low enough and heat transfer is carried out for a period of time enough, when the local temperature of the outer wall surface of the environment test box is reduced to be lower than the dew point temperature of the external environment humid air of the environment test box, the dew formation occurs on the outer wall surface of the environment test box. At present, a heating device (such as a resistive heating wire) is generally arranged on the outer wall of an environmental test box, a box door and a glass observation window, and when the temperature in the environmental test box is reduced to a set temperature value, the resistive heating wire is electrified to heat so as to achieve the purpose of preventing condensation. The temperature value in the box for controlling the electric heating wire to be electrified and heated is about-10 ℃ to 10 ℃, and is generally set by the manufacturer of the environmental test box in a factory, or can be set by the user.
However, if the temperature value for controlling the energization of the resistive heating wire is low, dew condensation may already occur on the outer wall surface of the environmental test chamber, but the resistive heating wire does not start to energize and generate heat yet. In order to ensure that the phenomenon of dew condensation does not occur, technicians usually set the temperature value in the box for controlling the electric heating of the resistive heating wire to be higher, and the resistive heating wire works with the maximum electric power after being electrified. The method is simple and practical, can prevent dew condensation to a certain extent, is unfavorable for energy conservation, and can influence the performance of the environmental test chamber. As long as the temperature in the environmental test chamber is lower than the set value, the resistive heating wire is electrified, and the resistive heating wire works with the maximum electric power, which is equivalent to artificially introducing a heating source around the working area of the environmental test chamber, which inevitably has a certain negative influence on the technical indexes such as the temperature fluctuation degree, the temperature uniformity, the temperature change rate and the like of the environmental test chamber.
Disclosure of Invention
The invention provides a dew prevention method of an environment test box, which aims to overcome the technical defect that the performance of the environment test box is affected by the maximum electric power operation of a resistive heating wire after the resistance heating wire is electrified.
In order to solve the technical problems, the technical scheme of the invention is as follows:
an environment test box dew prevention method, wherein a resistive heating wire is arranged in the environment test box, and the method comprises the following steps of:
s1: measuring the temperature and relative humidity outside the environmental test chamber;
s2: calculating dew point temperature according to the temperature and relative humidity outside the environmental test chamber;
s3: calculating the critical temperature in the environmental test chamber under the critical condition of dew condensation by adopting a thermodynamic formula according to the dew point temperature and the heat conduction characteristic of the heat insulation material of the environmental test chamber;
s4: determining a sampling period and a sampling frequency, and measuring and recording the temperature in the environmental test box according to the sampling frequency in one sampling period;
s5: calculating a power coefficient according to the change condition of the temperature in the environmental test chamber in a sampling period and the critical temperature in the environmental test chamber;
s6: the heating power of the resistive heating wire is set to be graded and adjustable, and the heating power grade of the resistive heating wire is controlled according to the power coefficient.
According to the scheme, the critical temperature in the environment test box under the critical condition of dew condensation is calculated, the power coefficient is calculated by combining the change condition of the temperature in the environment test box in a sampling period, the heating power of the resistive heating wire is set to be graded and adjustable, and the heating power grade of the resistive heating wire is controlled according to the power coefficient, so that the purpose of dew condensation prevention is achieved, and meanwhile, the energy conservation is achieved, and the influence on the performance of the environment test box after the resistive heating wire is electrified is avoided.
Preferably, the dew point temperature θ in step S2 dew The calculation formula of (2) is as follows:
θ dew ＝100(a+b·θ out )·U out +c·θ out -19.2
wherein coefficient a= 0.1980, coefficient b=0.0017, coefficient c= 0.8400, θ out For the temperature outside the environment test chamber, U out Is the relative humidity outside the environmental test chamber.
Preferably, in step S4, the sampling period is n minutes, and the value range of n is: n is more than or equal to 45 and less than or equal to 75.
Preferably, n is an integer.
Preferably, the sampling frequency in step S4 is once per minute.
Preferably, in step S5, the calculation formula of the power coefficient K is:
K＝A/B＝A/[n·(θ out -θ cv )]
wherein A is the temperature difference D i Discrete integral value over time, D i ＝θ out -θ i ，θ out Is the temperature outside the environment test chamber, theta i For the temperature in the environmental test chamber, the reference value b=n· (θ out -θ cv ) Sampling period is n minutes, theta cv Is the critical temperature in the environmental test chamber.
In the scheme, the temperature in the environment test chamber is constant at theta cv And the duration of n minutes is a critical condition for dew formation, and b=n· (θ out -θ cv ) B may be a reference value. Comparing A with B: if A is significantly less than B, this indicates less likelihood of condensation; if A is approximately equal to B, indicating that the condensation is in a critical state; if A is significantly greater than B, this indicates that condensation is more likely to occur and that the amount of condensation may be greater. Therefore, the ratio of A to B can be used as a control quantity to control the heating power of the heating wire, thereby obtaining excellent anti-condensation effect.
Preferably, the temperature difference D is represented by the following cumulative formula i Discrete integral value a over time:
taking the starting time of the cooling stage in the low-temperature test process as the starting point of a time axis,
when the temperature change time t is smaller than n, the number of sampling points is smaller than n, and when no sampling point value is found in the accumulation and summation operation of the discrete integral value A, (theta out -θ i )＝0；
When t is more than or equal to n, n sampling points are respectively: t-n+1, t-n+2, … …, t-1, t;
where i represents the ith sample and Δt represents the time between two adjacent samples.
Preferably, in step S6, the heating power of the resistive heating wire is divided into five levels, namely P 0 ＝0、P 1 ＝0.25P max 、P 2 ＝0.5P max 、P 3 ＝0.75P max And P 4 ＝P max Wherein P is max Is the maximum heating power of the resistive heating wire.
Preferably, the method comprises the steps of,
when K is less than 0.5, controlling the heating power of the resistive heating wire to be P 0 A stage of the process, the stage,
when K is more than or equal to 0.5 and less than 0.8, the heating power of the resistive heating wire is controlled to be P 1 A stage of the process, the stage,
when K is more than or equal to 0.8 and less than 1.2, the heating power of the resistive heating wire is controlled to be P 2 A stage of the process, the stage,
when K is more than or equal to 1.2 and less than 1.5, the heating power of the resistive heating wire is controlled to be P 3 A stage of the process, the stage,
when K is more than or equal to 1.5, controlling the heating power of the resistive heating wire to be P 4 A stage.
Preferably, during the warm-up phase of the low temperature test process,
if the temperature theta in the current environmental test chamber is detected i Above dew point temperature theta dew The heating power level of the resistive heating wire controlled according to the power coefficient is reduced by one level, and the lowest level is P 0 A stage;
if the temperature theta in the current environmental test chamber is detected i Higher than the temperature theta outside the environment test chamber out Directly adjusting the heating power level of the resistive heating wire to P 0 A stage.
Compared with the prior art, the technical scheme of the invention has the beneficial effects that:
the invention provides an environment test box anti-condensation method, which is characterized in that the critical temperature in an environment test box under the critical condition of condensation is calculated, the power coefficient is calculated by combining the change condition of the temperature in the environment test box in a sampling period, the heating power of a resistance heating wire is set to be graded and adjustable, and the heating power level of the resistance heating wire is controlled according to the power coefficient, so that the anti-condensation purpose is achieved, and meanwhile, the energy conservation is realized, and the influence on the performance of the environment test box after the resistance heating wire is electrified is avoided.
Drawings
Fig. 1 is a diagram of the implementation steps of the technical scheme of the invention.
Fig. 2 is a schematic module connection diagram of an environment test chamber anti-condensation system according to the present invention.
Wherein: 1. an environmental test chamber; 2. a temperature sensor; 3. a dry-wet ball temperature and humidity sensor; 4. a power supply; 5. a microcontroller; 6. resistive heating wire heating device.
Detailed Description
The drawings are for illustrative purposes only and are not to be construed as limiting the present patent;
for the purpose of better illustrating the embodiments, certain elements of the drawings may be omitted, enlarged or reduced and do not represent the actual product dimensions;
it will be appreciated by those skilled in the art that certain well-known structures in the drawings and descriptions thereof may be omitted.
The technical scheme of the invention is further described below with reference to the accompanying drawings and examples.
Example 1
As shown in fig. 1, an environment test box anti-condensation method, wherein a resistive heating wire is arranged in the environment test box, and the method comprises the following steps:
s1: measuring the temperature and relative humidity outside the environmental test chamber;
s2: calculating dew point temperature according to the temperature and relative humidity outside the environmental test chamber;
s3: calculating the critical temperature in the environmental test chamber under the critical condition of dew condensation by adopting a thermodynamic formula according to the dew point temperature and the heat conduction characteristic of the heat insulation material of the environmental test chamber;
s4: determining a sampling period and a sampling frequency, and measuring and recording the temperature in the environmental test box according to the sampling frequency in one sampling period;
s5: calculating a power coefficient according to the change condition of the temperature in the environmental test chamber in a sampling period and the critical temperature in the environmental test chamber;
s6: the heating power of the resistive heating wire is set to be graded and adjustable, and the heating power grade of the resistive heating wire is controlled according to the power coefficient.
In a specific implementation process, the critical temperature in the environment test box under the critical condition of dew condensation is calculated, and then the power coefficient is calculated by combining the change condition of the temperature in the environment test box in a sampling period, the heating power of the resistive heating wire is set to be graded and adjustable, and the heating power grade of the resistive heating wire is controlled according to the power coefficient, so that the purpose of preventing dew condensation is achieved, and meanwhile, the energy conservation is realized, and the influence on the performance of the environment test box after the resistive heating wire is electrified is avoided.
Example 2
An environment test box dew prevention method, wherein a resistive heating wire is arranged in the environment test box, and the method comprises the following steps of:
s1: a pair of dry-wet ball temperature and humidity sensors are arranged outside the environment test box, and the dry ball measures the temperature theta outside the environment test box out According to the dry bulb temperature and the wet bulb temperature, the relative humidity U outside the environment test box is converted by looking up a table out ；
In the specific implementation process, the temperature theta outside the environment test box is measured and recorded only 1 time in one sampling period due to slower temperature and humidity change outside the environment test box out And relative humidity U out 。
S2: according to the temperature theta outside the environment test chamber out And relative humidity U out Calculating dew point temperature theta dew ；
More specifically, the dew point temperature θ in step S2 dew The calculation formula of (2) is as follows:
θ dew ＝100(a+b·θ out )·U out +c·θ out -19.2
wherein coefficient a= 0.1980, coefficient b=0.0017, coefficient c= 0.8400, θ out For the temperature outside the environment test chamber, U out Is the relative humidity outside the environmental test chamber.
In a specific implementation process, since the dew point temperature does not need to be precisely measured and calculated in this embodiment, the dew point temperature can be estimated by adopting the above formula, and in addition, the dew point temperature can be precisely calculated by referring to the methods recommended by the following three standard specifications: GB/T35226-2017 climate observation guide for ground Meteorological observation Specification air temperature and humidity, ASHRAE handbook (American Society of Heating, refrigerating and AirConditioning Engineers) and World Meteorological Organization (WMO).
S3: according to dew point temperature theta dew And the heat conduction characteristic of the heat insulation material of the environment test chamber, and calculating the critical temperature theta in the environment test chamber under the critical condition of dew condensation by adopting a thermodynamic formula cv ；
In the specific implementation process, the temperature in the environment test chamber is higher than the dew point temperature theta of the environment humid air outside the environment test chamber due to the heat insulation material of the wall of the environment test chamber dew To a certain extent and provided that the temperature in the environment test chamber is maintained for a sufficient time to reduce the local temperature of the outer wall surface of the environment test chamber to the dew point temperature theta dew Reaching the critical condition of dew condensation. Let the temperature lower than the dew point temperature be theta cv (θ will be described below cv Referred to as critical temperature) for a sufficient period of time, n minutes. θ cv And n is mainly related to the heat conduction characteristics of the insulation material of the environmental test chamber and also related to the roughness of the outer wall surface of the environmental test chamber. Given the boundary conditions, if the thickness and thermal conductivity of the insulation material, and the convective heat transfer coefficient of the environment outside the environmental test chamber are known, then θ cv Can be calculated by using the thermodynamic formula of the steady-state heat transfer process.
S4: determining a sampling period and a sampling frequency, and measuring and recording the temperature in the environmental test box according to the sampling frequency in one sampling period;
more specifically, in step S4, the sampling period is n minutes, and the range of n is: n is more than or equal to 45 and less than or equal to 75.
More specifically, n is an integer.
More specifically, the sampling frequency in step S4 is once per minute.
In the specific implementation process, the temperature theta in the environment test box is measured by installing a temperature sensor in the environment test box i The temperature θ in the environmental test chamber was measured and recorded every one minute i A total of n theta are recorded once in one sampling period time i 。
S5: based on the change of temperature in the environmental test chamber in one sampling period and critical temperature theta in the environmental test chamber cv Calculating a power coefficient K;
more specifically, the calculation formula of the power coefficient K in step S5 is:
K＝A/B＝A/[n·(θ out -θ cv )]
wherein A is the temperature difference D i Discrete integral value over time, D i ＝θ out -θ i ，θ out Is the temperature outside the environment test chamber, theta i For the temperature in the environmental test chamber, the reference value b=n· (θ out -θ cv ) Sampling period is n minutes, theta cv Is the critical temperature in the environmental test chamber.
In the specific implementation process, the temperature in the environment test chamber is constant at theta cv And the duration of n minutes is a critical condition for dew formation, and b=n· (θ out -θ cv ) B may be a reference value. Comparing A with B: if A is significantly less than B, this indicates less likelihood of condensation; if A is approximately equal to B, indicating that the condensation is in a critical state; if A is significantly greater than B, this indicates that condensation is more likely to occur and that the amount of condensation may be greater. Therefore, the ratio of A to B can be used as a control quantity to control the heating power of the heating wire, thereby obtaining excellent anti-condensation effect.
More specifically, the temperature difference D is represented by the following cumulative formula i Discrete integral value a over time:
taking the starting time of the cooling stage in the low-temperature test process as the starting point of a time axis,
when the temperature change time t is smaller than n, the number of sampling points is smaller than n, and when no sampling point value is found in the accumulation and summation operation of the discrete integral value A, (theta out -θ i )＝0；
When t is more than or equal to n, n sampling points are respectively: t-n+1, t-n+2, … …, t-1, t;
where i represents the ith sample, Δt represents the time between two adjacent samples, and Δt=1 when the sampling frequency is once per minute.
In the implementation process, the temperature theta in the environment test box is measured and recorded every one minute in one sampling period i Calculate and record the temperature difference D i ＝θ out -θ i The following table (θ in one sampling period time) out Is unchanged):
| time t (min) | 1 | 2 | …… | n-1 | n |
| Temperature theta i (℃) | θ 1 | θ 2 | …… | θ n-1 | θ n |
| Temperature difference D i ＝θ out -θ i (℃) | θ out -θ 1 | θ out -θ 2 | …… | θ out -θ n-1 | θ out -θ n |
S6: the heating power of the resistive heating wire is set to be graded and adjustable, and the heating power grade of the resistive heating wire is controlled according to the power coefficient.
More specifically, in step S6, the heating power of the resistive heating wire is divided into five levels, P 0 ＝0、P 1 ＝0.25P max 、P 2 ＝0.5P max 、P 3 ＝0.75P max And P 4 ＝P max Wherein P is max Is the maximum heating power of the resistive heating wire.
More specifically, the process is carried out,
when K is less than 0.5, controlling the heating power of the resistive heating wire to be P 0 A stage of the process, the stage,
when K is more than or equal to 0.5 and less than 0.8, the heating power of the resistive heating wire is controlled to be P 1 A stage of the process, the stage,
when K is more than or equal to 0.8 and less than 1.2, the heating power of the resistive heating wire is controlled to be P 2 A stage of the process, the stage,
when K is more than or equal to 1.2 and less than 1.5, the heating power of the resistive heating wire is controlled to be P 3 A stage of the process, the stage,
when K is more than or equal to 1.5, controlling the heating power of the resistive heating wire to be P 4 A stage.
In the specific implementation process, the heating power level of the resistive heating wire is controlled according to the power coefficient K as follows:
| value range of K | K＜0.5 | 0.5≤K＜0.8 | 0.8≤K＜1.2 | 1.2≤K＜1.5 | K≥1.5 |
| Heat generation power level | P 0 | P 1 | P 2 | P 3 | P 4 |
More specifically, during the warm-up phase of the low temperature test process,
if the temperature theta in the current environmental test chamber is detected i Above dew point temperature theta dew The heating power level of the resistive heating wire controlled according to the power coefficient K is reduced by one level, and the lowest level is P 0 A stage;
if the temperature theta in the current environmental test chamber is detected i Higher than the temperature theta outside the environment test chamber out Directly adjusting the heating power level of the resistive heating wire to P 0 A stage.
In the concrete implementation processIf the temperature theta in the current environmental test chamber i Above dew point temperature theta dew Indicating that the condensation phenomenon is about to end (there may be a small time delay due to heat transfer, and the heating wire needs to generate heat to remove the condensation), the heating power level determined according to K should be lowered by one step, for example, the heating power level is determined to be P2 according to K, and then the heating power level is adjusted to be P1); if the temperature theta in the current environmental test chamber is detected i Higher than the temperature theta outside the environment test chamber out The heating power level should be directly adjusted to P0 level to stop the heating of the resistive heating wire. Therefore, the heating power just meets the anti-condensation requirement, the influence of the heating of the resistive heating wire on the performance of the environmental test box is reduced to the maximum extent, the good anti-condensation effect is realized, and the energy is saved.
Example 3
As shown in fig. 2, an environment test box anti-condensation system is used for implementing the environment test box anti-condensation method, and comprises an environment test box 1, a temperature sensor 2, a dry and wet ball temperature and humidity sensor 3, a power supply 4, a microcontroller 5, a memory and a resistive heating wire heating device 6;
the temperature sensor 2 is arranged inside the environment test box 1, and the dry and wet ball temperature and humidity sensor 3 is arranged outside the environment test box 1;
the temperature sensor 2, the dry and wet ball temperature and humidity sensor 3, the microcontroller 5 and the resistive heating wire heating device 6 are respectively connected with the power supply 4;
the temperature sensor 2 and the dry and wet ball temperature and humidity sensor 3 are respectively connected with the input end of the microcontroller 5, the memory is connected with the microcontroller 5, and the output end of the microcontroller 5 is connected with the resistive heating wire heating device 6;
the temperature sensor 2 is used for measuring the temperature theta in the environmental test chamber 1 i ；
The dry-wet ball temperature and humidity sensor 3 is used for measuring the temperature theta outside the environmental test chamber 1 through the dry ball out And the relative humidity U outside the environment test chamber 1 is converted according to the dry bulb temperature and the wet bulb temperature by looking up a table out ；
The power supply 4 is used for supplying power to the temperature sensor 2, the dry and wet ball temperature and humidity sensor 3, the microcontroller 5 and the resistive heating wire heating device 6;
the microcontroller 5 is used for receiving temperature data and humidity data acquired by the temperature sensor 2 and the dry and wet bulb temperature and humidity sensor 3, and realizing the control of the heating power level of the resistive heating wire heating device 6 through programming;
the memory is used for storing data acquired by the temperature sensor 2 and the dry and wet ball temperature and humidity sensor 3 and can be called by the microcontroller 5;
the resistive heating wire heating device 6 is used for heating and removing dew under the control of the microcontroller 5.
In the specific implementation process, a plurality of groups of resistive heating wire heating devices 6 are installed at the places where dew condensation is easy to occur, such as the outer wall of the environmental test chamber 1, the chamber door, the glass observation window and the like, the number of groups of the resistive heating wire heating devices 6 to be installed is determined according to the size and the shape of the environmental test chamber 1, generally 3 to 4 groups are installed, and the larger the size of the environmental test chamber 1 is, the more the groups of the resistive heating wire heating devices 6 to be installed are, but generally not more than 8 groups are needed.
Example 4
The embodiment simulates a low-temperature test process in an environmental test chamber and adopts the anti-condensation method of the environmental test chamber to carry out a first test, and specifically comprises the following steps:
cooling from normal temperature of 25 ℃ in an environment test box at 2 ℃/min, after 30min, the temperature reaches the target low temperature of-35 ℃, maintaining the low temperature for 12h, heating at 5 ℃/min, and returning to the normal temperature of 25 ℃ after 12 min.
Whole course time is t=30+12×60+12=762 (min).
Let the laboratory environment be: the temperature is 23 ℃ and the relative humidity is 90%, and the humidity of the experimental room temperature is unchanged and theta is supposed to be the same in the whole experimental process out ＝23℃，U out =90%, and estimating the dew point temperature corresponding to this temperature and humidity:
θ dew ＝100(a+b·θ out )·U out +c·θ out －19.2
＝100×(0.1980+0.0017×23)×90％+0.8400×23－19.2
＝21.46(℃)
in general, the wall of the environmental test chamber is generally made of a metal sheet and internally covered with two layers of heat insulation materials, wherein the inner layer is made of heat insulation materials resistant to high temperature, such as foam glass, the outer layer is made of heat insulation materials with low heat conductivity coefficient, such as polyurethane, the thickness of the two layers of heat insulation materials is 0.05m, the heat conductivity coefficient of the foam glass is 0.06W/m.K, and the heat conductivity coefficient of the polyurethane is 0.03W/m.K. Since metal is a good conductor of heat, the effect of the sheet metal can be ignored in the calculations below.
Calculating critical temperature value θ using steady state heat transfer calculation cv . The environmental chamber wall may be considered an infinite plate and the temperature field distribution is one-dimensional, i.e. the temperature varies along a direction perpendicular to the environmental chamber wall. Neglecting local temperature inside the wall of the environmental test chamber and temperature theta in the environmental test chamber i Is equal to theta in the local temperature inside the wall of the environmental test chamber i 。
The heat per unit time passing through the flat plate with area S is:
Q＝λSΔθ/l
where Q is the heat transfer rate, in units of: w is a metal; λ is the thermal conductivity, in units: W/m.K; s is the area, unit: m is m 2 The method comprises the steps of carrying out a first treatment on the surface of the Δθ is the temperature difference across the plate in units of: k, performing K; l is the plate thickness, unit: m is m
The local temperature at the contact point of the inner layer heat insulation material (foam glass) and the outer layer heat insulation material (polyurethane) is theta m The heat quantity of the inner-layer heat insulating material having a passing area S per unit time is:
Q 1 ＝λ 1 SΔθ 1 /l 1 ＝λ 1 S(θ m -θ cv )/l 1
the heat quantity of the outer layer heat insulation material with the passing area S in unit time is as follows:
Q 2 ＝λ 2 SΔθ 2 /l 2 ＝λ 2 S(θ dew -θ m )/l 2
according to the law of conservation of energy, there is Q 1 ＝Q 2
Obtaining the formula (1): lambda (lambda) 1 (θ m -θ cv )/l 1 ＝λ 2 (θ dew -θ m )/l 2
The heat exchange between the outer layer heat insulating material and the environment outside the environment test box belongs to natural convection heat transfer, and the natural convection heat transfer coefficient of air is 5.6W/m 2 K, the heat exchange amount per unit time having a surface area S is:
Q＝hSΔθ
where Q is the heat transfer rate, in units of: w is a metal; h is the heat transfer coefficient, unit: w/m 2 K; s is the area, unit: m is m 2 The method comprises the steps of carrying out a first treatment on the surface of the Δθ is the temperature difference, unit: k (K)
The heat exchange amount between the outer heat insulating material with the surface area S in unit time and the environment outside the environment test chamber is as follows:
Q 3 ＝h 3 SΔθ 3 ＝h 3 S(θ out -θ dew )
according to the law of conservation of energy, there is Q 2 ＝Q 3
Obtaining the formula (2): lambda (lambda) 2 (θ dew -θ m )/l 2 ＝h 3 (θ out -θ dew )
The formula (1) and the formula (2) are combined, and the known number is substituted, so that the heat conductivity coefficient lambda of the foam glass 1 Foam glass plate thickness l =0.06W/m·k 1 =0.05m, polyurethane coefficient of thermal conductivity λ 2 =0.03W/m·k, polyurethane sheet thickness l 2 External ambient air natural convection heat transfer coefficient h of environmental test chamber =0.05m 3 ＝5.6W/m 2 K, environmental test chamber external temperature θ out =23 ℃, dew point temperature θ dew The critical temperature θ can be obtained by using =21.46℃ cv ≈0(℃)。
For a typical environmental test chamber, the empirical value for one sampling period is about 45min to 75min. In this embodiment, the sampling period is selected to be 60min, n=60.
Starting the low temperature test process, measuring and recording theta i Calculating discrete integral value A and power coefficient K, determining heating power level, and controlling resistive heating wireIs provided.
In general, there is a slight deviation between the actual measured value of the temperature in the environmental test chamber and the set point of the low temperature test process, and this deviation is ignored below for the purpose of illustrating the algorithm only, depending on the temperature control accuracy of the environmental test chamber.
(1) When t=30 min, the recorded data are as follows:
in the above formula, i>At 30, θ i No measurement was taken (θ out -θ i )＝0
The power coefficient k=a/b=a/[ n ] (θ out -θ cv )]＝870/[60×(23-0)]＝0.63
K is more than or equal to 0.5 and less than or equal to 0.8, the heating power level is judged to be P1, and the heating and dew removal are carried out by 25% of the maximum heating power. Although the temperature in the environment test chamber has been reduced to-35 ℃, due to the effect of the heat insulation material of the wall of the environment test chamber, a certain time is required for heat conduction, at this time, the possibility of dew condensation on the outer wall surface of the environment test chamber is also relatively small, and the heat generating power is not required to be too great.
(2) When t=45 min, the recorded data are as follows:
in the above formula, i>45 times, θ i No measurement was taken (θ out -θ i )＝0
The power coefficient k=a/b=a/[ n ] (θ out -θ cv )]＝1740/[60×(23-0)]＝1.26
K is more than or equal to 1.2 and less than or equal to 1.5, the heating power level is judged to be P3, and the dew is removed by heating with 75% of the maximum heating power.
(3) When t=100 min, the sampling points were 41, 42, 43, … …, 100 (min), 60 sampling points in total, and the recorded data were as follows:
the power coefficient k=a/b=a/[ n ] (θ out -θ cv )]＝3480/[60×(23-0)]＝2.52
K is more than or equal to 1.5, the heating power grade is judged to be P4, and dew is removed by the maximum heating power.
(4) The cooling stage takes 30min, the temperature is maintained at minus 35 ℃ for 12h, the temperature is raised from the time t=30+12×60=750 (min), and when t=762 min, the recorded data are as follows:
when t=762 min, the temperature in the environmental test chamber is raised to 25 ℃, the temperature is higher than the external environmental temperature of the environmental test chamber by 23 ℃, the A value and the K value are not needed to be calculated, the heating power level is directly judged to be P0, and the heating is stopped.
Example 5
The embodiment simulates a low-temperature test process in an environmental test chamber and adopts the anti-condensation method of the environmental test chamber to carry out a second test, and specifically comprises the following steps:
let the laboratory environment be: the temperature 23℃and the relative humidity 95% are the same as in example 4. Theta is then out ＝23℃，U out =95%, dew point temperature:
θ dew ＝100(a+b·θ out )·U out +c·θ out －19.2
＝100×(0.1980+0.0017×23)×95％+0.8400×23－19.2
＝22.64(℃)
the critical temperature value θ was obtained by calculation in the same manner as in example 4 cv Sample period was also chosen to be 60min, n=60, =17.60 (°c).
When t=30 min, the recorded data are as follows:
the power coefficient k=a/b=a/[ n ] (θ out -θ cv )]＝870/[60×(23-17.60)]＝2.68
K is more than or equal to 1.5, and the heating power grade is judged to be P4. As the relative humidity of the environment outside the environmental test chamber increases, the dew condensation probability increases, and a larger heating power is required for dew removal, as compared with example 4.
It is to be understood that the above examples of the present invention are provided by way of illustration only and not by way of limitation of the embodiments of the present invention. Other variations or modifications of the above teachings will be apparent to those of ordinary skill in the art. It is not necessary here nor is it exhaustive of all embodiments. Any modification, equivalent replacement, improvement, etc. which come within the spirit and principles of the invention are desired to be protected by the following claims.
Claims (9)
1. The environment test box dew prevention method is characterized in that the environment test box is internally provided with a resistive heating wire, and the method comprises the following steps:
s1: measuring the temperature and relative humidity outside the environmental test chamber;
s2: calculating dew point temperature according to the temperature and relative humidity outside the environmental test chamber;
s3: calculating the critical temperature in the environmental test chamber under the critical condition of dew condensation by adopting a thermodynamic formula according to the dew point temperature and the heat conduction characteristic of the heat insulation material of the environmental test chamber;
s4: determining a sampling period and a sampling frequency, and measuring and recording the temperature in the environmental test box according to the sampling frequency in one sampling period;
s5: calculating a power coefficient according to the change condition of the temperature in the environmental test chamber in a sampling period and the critical temperature in the environmental test chamber;
s6: setting the heating power of the resistive heating wire to be graded and adjustable, and controlling the heating power grade of the resistive heating wire according to the power coefficient;
the calculation formula of the power coefficient K in step S5 is:
K＝A/B＝A/[n·(θ out -θ cv )]
wherein A is the temperature difference D i Discrete integral value over time, D i ＝θ out -θ i ，θ out Is the temperature outside the environment test chamber, theta i For the temperature in the environmental test chamber, the reference value b=n· (θ out -θ cv ) Sampling period is n minutes, theta cv Is the critical temperature in the environmental test chamber.
2. The method for preventing dew condensation of an environmental test chamber according to claim 1, wherein the dew point temperature θ in step S2 dew The calculation formula of (2) is as follows:
θ dew ＝100(a+b·θ out )·U out +c·θ out -19.2
wherein coefficient a= 0.1980, coefficient b=0.0017, coefficient c= 0.8400, θ out For the temperature outside the environment test chamber, U out Is the relative humidity outside the environmental test chamber.
3. The method for preventing dewing in an environmental test chamber according to claim 1, wherein the sampling period in step S4 is n minutes, and the value range of n is: n is more than or equal to 45 and less than or equal to 75.
4. The environmental test chamber anti-condensation method of claim 3, wherein n is an integer.
5. The method for preventing dewing in an environmental test chamber according to claim 1, wherein the sampling frequency in step S4 is once per minute.
6. The method for preventing dew condensation of an environmental test chamber according to claim 1, wherein the temperature difference D is represented by the following cumulative formula i Discrete integral value a over time:
taking the starting time of the cooling stage in the low-temperature test process as the starting point of a time axis,
when the temperature change time t is smaller than n, the number of sampling points is smaller than n, and when no sampling point value is found in the accumulation and summation operation of the discrete integral value A, (theta out -θ i )＝0；
When t is more than or equal to n, n sampling points are respectively: t-n+1, t-n+2, … …, t-1, t;
where i represents the ith sample and Δt represents the time between two adjacent samples.
7. The method for preventing dewing in an environmental test chamber as claimed in claim 1, wherein in step S6, the heating power of the resistive heating wire is divided into five levels, P respectively 0 ＝0、P 1 ＝0.25P max 、P 2 ＝0.5P max 、P 3 ＝0.75P max And P 4 ＝P max Wherein P is max Is the maximum heating power of the resistive heating wire.
8. The method for preventing dewing in an environmental test chamber according to claim 7, wherein,
when K is less than 0.5, controlling the heating power of the resistive heating wire to be P 0 A stage of the process, the stage,
when K is more than or equal to 0.5 and less than 0.8, the heating power of the resistive heating wire is controlled to be P 1 A stage of the process, the stage,
when K is more than or equal to 0.8 and less than 1.2, the heating power of the resistive heating wire is controlled to be P 2 A stage of the process, the stage,
when K is more than or equal to 1.2 and less than 1.5, the heating power of the resistive heating wire is controlled to be P 3 A stage of the process, the stage,
when K is more than or equal to 1.5, controlling the heating power of the resistive heating wire to be P 4 A stage.
9. An environmental test chamber anti-condensation method according to claim 7 or 8, wherein, during the temperature rising phase of the low temperature test process,
if the temperature theta in the current environmental test chamber is detected i Above dew point temperature theta dew The heating power level of the resistive heating wire controlled according to the power coefficient is reduced by one level, and the lowest level is P 0 A stage;
if the temperature theta in the current environmental test chamber is detected i Higher than the temperature theta outside the environment test chamber out Directly adjusting the heating power level of the resistive heating wire to P 0 A stage.
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