US20040068996A1

US20040068996A1 - [automatic control system of liquid chillers]

Info

Publication number: US20040068996A1
Application number: US10/249,868
Authority: US
Inventors: Lyman Tseng
Original assignee: Macronix International Co Ltd
Current assignee: Macronix International Co Ltd
Priority date: 2002-10-14
Filing date: 2003-05-14
Publication date: 2004-04-15
Also published as: JP4439842B2; TW567299B; JP2004132686A

Abstract

An automatic control system of the invention comprises a plurality of liquid chillers, a principal controller, and a plurality of secondary controllers respectively connected to the liquid chillers and the principal controller. The automatic control system stores a British Thermal Unit (BTU) table that includes a predetermined high load limit parameter and a predetermined low load limit parameter varying according to the number of liquid chillers in operation. The principal controller compares an actual load of the currently operating liquid chillers with the high and/or low load limit parameter to determine an adequate number of liquid chillers that have to be operated. By evaluating the number of liquid chillers currently operating, the principal controller then determines whether liquid chillers have to be switched on or switched off through the secondary controllers.

Description

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the priority benefit of Taiwan application serial no. 91123555, filed on Oct. 14, 2002.

BACKGROUND OF INVENTION

1. Field of the Invention

The invention relates generally to an automatic control system of liquid chillers and, more particularly, to an automatic control system that controls the liquid chillers based upon a BTU table.

2. Description of the Related Art

Liquid chilling machines (also called “liquid chillers”) are conventionally used to process in low temperature conditions. More particularly, when the low temperature conditions have to be maintained with a relatively high precision, an automatic control system of the liquid chillers is needed to prevent undesirable temperature fluctuations due to malfunctions of some of the liquid chillers.

Referring to FIG. 1, a simplified schematic view illustrates an automatic control system of liquid chillers known in the prior art. The known

automatic control system

10 comprises a primary loop 12 and a secondary loop 14 that are connected to each other via a de-coupling pipe 18. Chilled liquid outputted by the liquid chillers 102 travels through a chilled liquid pipe 106 to a plurality of control valves 112 and secondary pumps 114, and is delivered through a chilled liquid outlet. Through a liquid inlet, the chilled liquid after having been used returns through the chilled liquid pipe 106 to the primary pumps 104. However, a part of this used liquid also flows through the pipe 108 to the control valves 112, which causes an increase in temperature of the chilled liquid at the chilled liquid outlet.

Along the chilled

liquid pipe

106 are mounted a plurality of temperature sensors 110 to sense the temperature of the liquid flowing through along the pipe 106. Any abnormal temperature variation thereby is detected, which enables to take adequate decisions such as increasing the load of currently operating liquid chillers or, if necessary, activating additional liquid chillers.

The conventional operation of the control system as described above is based upon the detection of the temperature of the circulating liquid. The mount of several temperature sensors is therefore needed, which increases the equipment cost.

SUMMARY OF INVENTION

An aspect of the invention is therefore to provide an automatic control system of liquid chillers that determines the on-switch or off-switch of liquid chillers according to the actual load of currently operating liquid chillers compared against an acceptable load limit of the liquid chillers.

To accomplish the above and other objectives, an automatic control system of the invention comprises a plurality of liquid chillers, a principal controller, and a plurality of secondary controllers respectively connected to the liquid chillers and the principal controller. The automatic control system stores a British Thermal Unit (BTU) table that includes a predetermined high load limit parameter and a predetermined low load limit parameter varying according to the number of liquid chillers in operation. The principal controller compares an actual load of the currently operating liquid chillers with the high or low load limit parameter to determine an adequate number of liquid chillers that have to be operated. By evaluating the number of liquid chillers currently operating, the principal controller then determines whether liquid chillers may have to be switched on or switched off. To achieve the above purpose, a plurality of secondary controllers are respectively connected between the liquid chillers and the principal controller. To switch on or switch off liquid chillers, the principal controller sends a corresponding command signal to the secondary controllers that accordingly switch on or switch off the proper liquid chillers and equipment associated with the liquid chillers.

According to an embodiment of the invention, the principal controller includes a plurality of control units among which is alternately switched a control right. Thereby, an adequate control of the liquid chillers is maintained even if one control unit has a malfunction.

According to the invention, an on-switch delay time and an off-switch delay time are further set in the BTU table. The on-switch delay time indicates a delay time after which the liquid chiller to be switched on is effectively switched on. This delay time allows a confirmation that the outputted on-switch command signal corresponds to a real need of capacity increase and not due to erroneous noise signals, which therefore prevents undesirable on-switch operations. The off-switch delay time indicates a delay time after which the liquid chiller to be switched off is effectively switched off, which prevents erroneous off-switch operations.

The equipment associated with the liquid chillers comprises chilled liquid pumps, liquid recycle pumps, and control valves.

It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed.

BRIEF DESCRIPTION OF DRAWINGS

The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. [0015]
FIG. 1 is a schematic view illustrating an automatic control system of liquid chillers known in the art. [0016]
FIG. 2 is a schematic view illustrating the general structure of an automatic control system of liquid chillers according to an embodiment of the invention. [0017]
FIG. 3 is a chart schematically illustrating a BTU table used by the automatic control system according to an embodiment of the invention. [0018]
FIG. 4 is a flow chart illustrating the control operation of an automatic control system according to an embodiment of the invention. [0019]
FIG. 5 is state diagram of an automatic control system according to an embodiment of the invention. [0020]
FIG. 6 is a flow chart illustrating the control operation of a secondary chilled liquid pump system according to an embodiment of the invention.[0021]

DETAILED DESCRIPTION

The following detailed description of the embodiments and examples of the present invention with reference to the accompanying drawings is only illustrative and not limiting. Furthermore, wherever possible in the description, the same reference symbols will refer to similar elements and parts unless otherwise illustrated in the drawings. [0022]
Referring to FIG. 2, a block diagram schematically illustrates an automatic control system of liquid chillers according to an embodiment of the invention. A major aspect of the invention is that a [0023] principal controller 20 operates according to the data provided by a BTU table (as illustrated in FIG. 3) to control the operations of a plurality of liquid chilling machines 26 (also called “liquid chillers” hereafter). The principal controller 20 compares the actual load of chilled liquid production with a high (and/or low) load limit in the BTU table to determine an adequate number of liquid chillers 26 to be operated. Consequently, the principal controller 20 outputs a command signal for either switching on or switching off liquid chillers 26 in order to optimally satisfy the actual load of chilled liquid production. If the number of currently operating liquid chillers is equal to the adequate number of liquid chillers to be operated, no liquid chillers are switched on or switched off.
The [0024] principal controller 20 comprises a plurality of control units 202 that alternately possess a control right. Therefore, if a malfunction occurs to one control unit 202 currently possessing the control right, the automatic control system transfers this control right to another control unit 202. The control units 202 are, for example, programmable logic circuits. The command signal travels through a modular bus 22 to secondary controllers 24 that, accordingly, effectively switch on or switch off liquid chillers 26 and associated equipment 28. The equipment 28 includes at least a supply pump 282, a liquid recycle pump 284, and a control valve 286 that are respectively connected to the liquid chillers 26.
FIG. 3 describes an example of BTU table that is used in an automatic control system according to an embodiment of the invention. In the BTU table, the first row indicates the number of liquid chillers. The second row indicates the accumulated capacity corresponding to the number of liquid chillers. “High load limit” indicates a load limit of chilled liquid production over which at least one additional liquid chiller has to be switched on to allow an optimal operation of the liquid chillers. “On-switch delay time” indicates a delay time after which the liquid chiller to be switched on is effectively switched on. This delay time allows the performance of a confirmation operation ensuring the outputted on-switch command signal corresponds to a steady need of capacity increase and not due to erroneous noise signals, which therefore prevents undesirable on-switch operations. “Low load limit” indicates a load limit of chilled liquid production below which at least one liquid chiller has to be switched off. “Off-switch delay time” indicates a delay time after which the liquid chiller to be switched off is effectively switched off. Similar to the function of the on-switch delay time, this off-switch delay time allows the performance of a confirmation operation to ensure there is a steady capacity decrease, which prevents undesirable and erroneous off-switch operations. [0025]
Reference now is made to FIG. 2 through FIG. 4 to describe the operation of the automatic control system of liquid chillers according to an embodiment of the invention. First, the actual load of chilled liquid production is compared with the high (and/or low) load limit of the BTU table to determine the adequate number of [0026] liquid chillers 26 that have to be operated. This adequate number of liquid chillers 26 is compared with the number of currently operating liquid chillers 26 to determine whether liquid chillers may have to be switched on or switched off. Being activated, the principal controller 20 checks the state of the automatic control system (step S102), and establishes an on-switch sequence of the liquid chillers 26 according to their service time (step S104). Subsequently, power is supplied to the liquid chillers 26 and equipment 28 (step S106). If power is normally supplied to each of the selected liquid chillers 26, the on-switch sequence previously established is confirmed and the liquid chillers 26 are effectively activated (step S108). Otherwise, step S104 is re-executed to establish a new on-switch sequence of liquid chillers.
Meanwhile, the [0027] principal controller 20 determines the adequate number of liquid chillers 26 to be operated according to the BTU table (step S110), and through the modular bus 22, outputs a command signal of the liquid chillers 26 and equipment 28 (step S112). Subsequently, the automatic control system supplies power to the pump 282 (step 114). After having received the command signal (step S116), the secondary controllers 24 activate the liquid chillers 26 and the equipment 28 (step S118). While the liquid chillers 26 are operating, the automatic control system continuously records the operating state and the service time of each of the operating liquid chillers 26 (step S120). The states of the liquid chillers 26 and equipment 28 are feedback through the modular bus 22 to the principal controller 20.
When a [0028] liquid chiller 26 malfunctions, the principal controller 20 switches to a manual control mode (step S122). From the principal controller 20, the secondary controllers 24 are therefore manually controlled (step S124) to activate one or more liquid chiller 26 and equipment 28 (step S126). As described above, the states of the liquid chillers 26 and equipment 28 are similarly sent back through the modular bus 22 to the principal controller 20 (step S128).
Referring to FIG. 5, a state diagram illustrates an input state of the principal controller and a system control state according to an embodiment of the invention. In FIG. 5, [0029] reference numeral 30 refers to the principal controller, reference numeral 32 designates an input state, and reference numeral 36 designates a system control. The states and parameters from the input state 32, including the states of the liquid chillers and equipment, are delivered to the principal controller 30 that accordingly performs various control operations as indicated in the system control 36. According to the actual demand in chiller liquid production and the parameters of the BTU table, the principal controller 30 controls the operation of the automatic control system.
In the [0030] input state 32, a power state 320 indicates the state of power supply in the system. A liquid chiller state 322 continuously informs the principal controller 30 of the operating state of the liquid chillers. If a chilled liquid temperature 324 exceeds a predetermined limit more than three minutes, the automatic control system activates a first set of chilled liquid towers and the liquid chillers. The input state 32 also includes a pump state 326, a cooling tower status 328, a BTU and power meter 330, a liquid chiller equipment status 332, an outdoor temperature 334, a PDT 336, a control network system state 338, and a sensor 340.
The [0031] system control 36 comprises: a liquid treatment system control 360, a 5 Â° C. on-switch/off-switch system control 362, a cooling tower filtration system control 364, a cooling tower control 366, a secondary chilled liquid pump control 368, a chilled liquid load management system control 370, a warm/hot liquid system control 372, a 9 Â° C. on-switch/off-switch system control 374, and a plumbing system control 376.
Referring to FIG. 6, a flow chart schematically illustrates a control process of the secondary chilled liquid pumps according to an embodiment of the invention. According to the secondary chilled liquid pump/liquid chiller unit ratio, the principal controller determines the number of secondary chilled liquid pumps that may have to be activated or stopped, and accordingly delivers a command signal (step S[0032] 202). Subsequently, the automatic control system supplies power to the secondary chilled liquid pumps (step S204), and activates the secondary chilled liquid pumps. If secondary chilled liquid pumps have to be stopped, pumps that have been operating longer are first stopped (step S206).
After having received the command signal from the principal controller, the control valves respectively control the activation and the speed of the secondary chilled pumps (step S[0033] 208). The operating states of the secondary chilled liquid pumps are feedback to the principal controller (step S210). Subsequently, the on-switch/off-switch control system of the secondary chilled liquid pump system is activated, and the state of the on-switch/off-switch control system is automatically checked, the result of this check being sent to the principal controller (step S212).
As described above, the invention therefore includes the following advantages. [0034]
(1) By comparing the actual load in chilled liquid production and the predetermined high or low load limit of the BTU table, the automatic control system determines the adequate number of liquid chillers that should be operating. According to the actual number of liquid chillers that are operating, the automatic control system consequently determines whether additional liquid chillers have to be switched on or currently operating liquid chillers have to be switched off. [0035]
(2) Via a load control of the chilled liquid production from the BTU table, the invention allows an optimal operation-of the liquid chillers. [0036]
(3) The principal controller of the invention includes a plurality of control units, between which a control right can be alternately switched. The control of the entire system can be therefore ensured even if some control units have malfunctions. [0037]
It should be apparent to those skilled in the art that other structures that are obtained from various modifications and variations of different parts of the above-described structures of the invention would be possible without departing from the scope and spirit of the invention as illustrated herein. Therefore, the above description of embodiments and examples only illustrates specific ways of making and performing the invention that, consequently, should cover variations and modifications thereof, provided they fall within the inventive concepts as defined in the following claims. [0038]

Claims

1. An automatic control system of liquid chillers, comprising:

a plurality of liquid chillers for producing chilled liquid; and

a principal controller, operating from data of a British Thermal Unit (BTU) table, wherein the principal controller compares an actual load of chilled liquid production with a load limit of the BTU table to determine an adequate number of liquid chillers to be operated, and after having compared a number of currently operating liquid chillers with the adequate number of liquid chillers to be operated, the principal controller accordingly outputs a command signal to command an on-switch, off-switch, or no-operation of the liquid chillers.

2. The system of claim 1, further including a secondary controller that receives the command signal to either switch on or switch off at least one of the liquid chillers.

3. The system of claim 1, wherein the principal controller comprises a plurality of control units that are alternately given a control right.

4. The system of claim 3, wherein the control units are programmable logic circuits.

5. The system of claim 1, wherein the BTU table includes a high load limit; if the actual load in chilled liquid production is higher than the high load limit, additional liquid chillers are switched on.

6. The system of claim 1, wherein the BTU table includes a low load limit; if the actual load in chilled liquid production is lower than the low load limit, currently operating liquid chillers are switched off.

7. The system of claim 1, wherein the BTU table further includes an on-switch delay time that allows a confirmation that the outputted command signal commanding an on-switch corresponds to a steady need of capacity increase and not due to erroneous noise signals.

8. The system of claim 1, wherein the BTU table further includes an off-switch delay time that allows a confirmation that the outputted command signal commanding an off-switch corresponds to a steady capacity decrease and not due to erroneous noise signals.

9. The system of claim 1, wherein an on-switch/off-switch time interval is automatically set.

10. A control method of liquid chillers, comprising:

comparing an actual load of chilled liquid production with a predetermined high and low load limit of the liquid chillers to determine an adequate number of liquid chillers to be operated;

comparing the adequate number of liquid chillers to be operated with a number of currently operating liquid chillers;

if the adequate number of liquid chillers to be operated is greater than the number of currently operating liquid chillers, at least one additional liquid chiller is switched on;

if the adequate number of liquid chillers to be operated is less than the number of currently operating liquid chillers, at least one currently operating liquid chiller is switched off; and

if the adequate number of liquid chillers to be operated is equal to the number of currently operating liquid chillers, the currently operating liquid chillers are maintained.

11. The method of claim 10, wherein if the adequate number of liquid chillers to be operated is greater than the number of currently operating liquid chillers, an additional liquid chiller is switched on after an on-switch delay time has elapsed, the on-switch delay time being used to confirm whether there is a steady need of capacity increase.

12. The method of claim 10, wherein if the adequate number of liquid chillers to be operated is less than the number of currently operating liquid chillers, a currently operating liquid chiller is switched off after an off-switch delay time has elapsed, the off-switch delay time being used to confirm whether there is a steady capacity decrease.