Appendix 2A: Chiller Control Principles... 104 2A.1 Features of Chiller Control Components... 104 2A.2 Start-up and Shutdown... 105 2A.2.1 Start-up...105 2A.2.2 Shutdown...106 2A.3 Chilled-Water Supply Temperature Control... 107 2A.3.1 Refrigerant Pump and Level Control...107 2A.3.2 Steam Valve Control...109 2A.3.3 Solution Pump and HTRG Level Control... 111 2A.4 Cooling-Water Supply Control...112 2A.4.1 Cooling-Water Pump Control... 112 2A.4.2 Cooling Water Fan Control... 113 2A.5 Vacuum Maintenance...114 2A.6 Crystallization Detection and Decrystallization...115 2A.6.1 Crystal Detection... 116 2A.6.2 Crystal Removal...116 2A.7 Safety Control and Diagnostics...117 102
Figure 2A - 1: Control logic of startup procedure...106 Figure 2A - 2: Control logic of shutdown procedure...107 Figure 2A - 3: Configuration of the refrigerant level meter...108 Figure 2A - 4: Principle of the chilled-water supply temperature control...108 Figure 2A - 5: Configuration of HTRG solution level probe...109 Figure 2A - 6: Control principle of opening the steam valve... 110 Figure 2A - 7: Control principle of opening the steam valve... 110 Figure 2A - 8: Control principle of closing the steam valve... 111 Figure 2A - 9: General rule for the variation of solution pump frequency... 112 Figure 2A - 10: Control principle of the cooling fan (increasing cooling-water temperature)... 113 Figure 2A - 11: Control principle of the cooling fan (decreasing cooling-water temperature)... 113 Figure 2A - 12: Principle diagram of automatic gas purge device, AGPD... 114 103
Appendix 2A: Chiller Control Principles The chiller operation program is a sequence of processes to start up, follow load, and shut down the chiller by adjusting the corresponding chiller control components listed in Table 2-4 on the basis of the measurements in Table 2-3. The chiller instrumentation and control system the manufacturer has provided was described in chapter 2. The chiller control principles will be discussed in detail in this appendix. The chiller control program can be divided into start-up, shutdown, and load-follow. Chiller operation involves chilled-water supply temperature control, cooling-water supply temperature control, noncondensable gas detection, crystallization detection and control, and chiller diagnostics. Chiller start-up starts with the start-up command and ends with the steam valve fully open. Chiller shutdown starts with the shutdown command to the chilled-water pump stop. While operating, the chiller continuously provides chilled water within a preset temperature range at the imposed chilled-water flow and return temperature. Load changes complicate the chiller control program. Higher cooling loads require higher heat input and sorbent solution flow. Additional factors need to be considered, such as the detection of noncondensable gas, crystallization formation, and abnormal conditions. 2A.1 Features of Chiller Control Components The chiller components to be adjusted during the chiller start-up, load-flow, and shutdown procedures are listed in Table 2-4; they have the following features: The chilled-water pump (CHWP) is a constant speed pump. It starts or stops immediately once it received an on or off command. The CHWP is usually the first component to start in the chiller and the last component to stop. Similar to the CHWP, the cooling-water pump (CWP) is a constant speed pump. It starts or stops immediately once it has received an on or off command. It operates only when the CHWP is running. The solution pump has a variable-frequency drive that adjusts the pump speed to maintain the sorbent solution level in the HTRG constant. It starts or stops immediately once an on or off command has been received. The refrigerant pump is a constant-speed pump. It starts or stops immediately once an on or off command is received. The cooling-tower fan (CTF) has low, medium, and high speeds to maintain the desired coolingwater supply temperature. 104
The steam valve has four staged openings. The steam valve opens or closes in steps between the first and fourth stages. The opening or closing of the steam valve is not measured directly but is estimated by the corresponding operating time of the drive motor. The city-water switch (CTS) is a magnetic valve. It opens or closes immediately once an on or off signal has been received. The CTS remains open to supply city water to the cooling tower because the cooling tower loses water to the atmosphere while the chiller operates. The cooling-water drain device (CWDD) is motor driven. The motor runs for a certain time to open or close the valve completely once an on or off command has been received. The cooling water begins to drain the cooling-water tank until it is empty, once the CWDD has opened for a certain length of time. When the cooling tower begins to fill with city water, the CWDD must be closed. The cooling-water by-pass valve (CWBPV) is motor driven. The motor runs until the valve is fully open or closed, as soon as it has received an on or off command. The cooling-water supply temperature is kept sufficiently high by the by-pass of circulating cooling water around the cooling tower. The electric refrigerant pump heater (RPH) prevents refrigerant from freezing in the evaporator. 2A.2 Start-up and Shutdown 2A.2.1 Start-up The chiller start-up command can be given through the push button on the chiller control panel or through the control software in the computer. The chiller starts the CHWP, the CWP, and the solution pump, and opens the steam valve in sequence. To operate these four major components, four conditions must be met: ambient temperature must be higher than a preset value cooling-water tank must be filled with water chilled-water flow must be sufficient chilled-water supply temperature must be higher than a preset value To meet these conditions, the chiller control system needs to open or close the corresponding valves (the CWDD and the CTS). The start-up procedure is illustrated in Figure 2A-1. 105
Figure 2A - 1: Control logic of startup procedure When the chiller control system receives a start-up command, the chiller checks the ambient (surrounding) temperature. If this temperature is as high as the preset ambient temperature for cooling, the chiller will start; otherwise, the start-up will halt. If cooling is required, the chiller will sequentially close the CWDD and open the CTS to provide city water to the cooling tower. But if the cooling-water level probe fails to detect water for 3 minutes, the start-up program will terminate. When water is detected in the cooling-tower tank, the CHWP will start; and the status of the chilled-water flow will be checked by the chilled-water flow detector. If an adequate chilled-water flow is detected CWP will be started to circulate cooling water, and the solution pump will be started to circulate dilute solution from the absorber to the two regenerators. Once the solution pump is operating, and the chilled-water supply temperature is higher than its preset value, the steam valve will open in stages to supply thermal energy into the HTRG. After the start-up procedure has been completed, the control system modulates the refrigerant pump and the steam valve to meet the preset value of the chilled-water supply temperature. 2A.2.2 Shutdown The chiller starts to shut down when it receives the command through the push button or monitoring software, or when the chiller detects an operational malfunction or component failure. The shutdown process is simpler than the start-up process, but it requires roughly 45 minutes to cool the solution temperature with the circulating cooling water. 106
Figure 2A - 2: Control logic of shutdown procedure Shutdown Close SV Stop SP Stop CTF No Dilute process Yes Stop RP Stop CWP HTRG solution temperature<preset? Close CWV Stop CHW End The shutdown process is illustrated in Figure 2A-2. First the chiller closes the steam valve and then starts the sorbent cooling process by mixing the cold and dilute sorbent solution in the absorber with the hot and concentrated sorbent solutions in the two regenerators until the temperature of the sorbent solution in the HTRG drops from 155 o C to 85 o C. The chiller components stop one by one in sequence, as shown in Figure 2A-2. 2A.3 Chilled-Water Supply Temperature Control The chilled-water flow rate and the chilled-water supply and return temperature determine the cooling load. The absorption chiller responds to cooling-load changes by adjusting the amount of refrigerant supplied to and vaporized in the evaporator. The chiller operates the steam valve to supply steam to the HTRG that provides refrigerant. In response, the refrigerant generation rate and the corresponding vaporization pressure variations in the HTRG, the dilute sorbent solution flow rate must be adjusted to maintain a constant solution level in the HTRG. In general, the chiller control system responds to the cooling load changes and maintains the chilled-water supply temperature by modulating the refrigerant pump, the steam valve, and the solution pump. 2A.3.1 Refrigerant Pump and Level Control Figure 2A-3 illustrates the arrangement of the refrigerant pump and the refrigerant level probe. The level probe has three pins in a cylinder connected with the water tray and the top of the evaporator. The probe pins detect four levels of the refrigerant as illustrated in Figure 2A-3. The refrigerant pump control is detailed in Figure 2A-4. 107
The refrigerant pump starts when two conditions are met: first, the refrigerant level submerges refrigerant Pin 1; second, the chilled-water supply temperature is the same as the above preset temperature. Once the refrigerant pump operates, it does not stop until the chilled-water supply temperature drops below the preset value or until the water leveling the refrigerant tray drops below Pin 1. Two additional conditions can also stop the refrigerant pump: first, the chiller executes a shutdown; second, the chiller executes the de-crystallization program. Figure 2A - 3: Configuration of the refrigerant level meter Pin 3 Pin 2 Pin 1 A B C D RBPSV L2 RP Chilled water Refrigerant tray The steam valve also plays a critical role in controlling the chilled-water supply temperature; its control principles will be discussed in the following section. Figure 2A - 4: Principle of the chilled-water supply temperature control 108
2A.3.2 Steam Valve Control For an absorption chiller, the cooling load is determined by the amount of refrigerant provided to the evaporator from the regenerators. The steam valve regulates the steam supply to the HTRG and the quantity of refrigerant produced in the HTRG. As a result, the steam valve must be controlled to respond to the cooling load variations. The control of the steam flow, the adjustment of the steam valve must In general respond to: the level of the sorbent in the HTRG. Maintaining this level assumes adequate surface for heat transfer from the steam to the sorbent. the temperature of the sorbent solution in the HTRG. Nominally 155 o C, maintaining this temperature assumes that the sorbent concentration and absorber pressure are appropriate the chilled-water supply temperature is, nominally, 7 o C. The structure of the HTRG solution level probe (L1) is illustrated in Figure 2A-5. Similar to the refrigerant level probe (L2), L1 has 4 pins and accordingly 5 areas along the height of the probe cylinder. Figure 2A - 5: Configuration of HTRG solution level probe Sorbent solution inlet L3 LTRG Condenser L1 Cooling-water outlet Cooling-water inlet Steam SV HTRG A B C D E Pin 4 Pin 3 Pin 2 Pin 1 Condensate Sorbent solution inlet and outlet Refrigerant To open the steam valve, the initial requirements must be met: the sorbent solution pump must be operating; the solution level must be above Pin 1 of the solution level probe in the HTRG 109
Figure 2A - 6: Control principle of opening the steam valve Figure 2A-6 presents the overall logic for initial opening of the steam valve. Under continuing operating conditions, the operation of the steam valve, opening and closing, is dependent on the chilled-water supply temperature from the evaporator in Figure 2A-7 and 2A-8. Figure 2A - 7: Control principle of opening the steam valve 4th stage Steam valve position 3rd stage 2nd stage 1st stage Close 5 o C 6 o C 7 o C 8 o C 9 o C CHW supply temperature 110
Figure 2A - 8: Control principle of closing the steam valve 4th stage Steam valve position 3rd stage 2nd stage 1st stage Close 5 o C 6 o C 7 o C 8 o C 9 o C CHW supply temperature When the steam valve receives a command from the control system, the valve will be adjusted in stage from closed to fully open by an electric motor. The first-stage opening ranges from 5% to 30%; the second, from 30% to 60%; the third, from 60% to 80%; and the fourth, from 80% to 100% open. There is one special case for the steam valve closing procedure. When the refrigerant level in the water tray rises into area D, (this happens when the cooling load drops dramatically), the steam valve closes one stage from its current opening. If the refrigerant level remains in D for more than 3 minutes, the steam valve will close one additional stage. If the level remains in area D for more than 10 minutes, the steam valve will close completely. When the level drops into area B, the steam valve will open again. 2A.3.3 Solution Pump and HTRG Level Control The solution pump control is set to minimize the solution level variations through a variable frequency pump. The configuration of the sorbent 4-pin-level probe is illustrated in Figure 2A-5. The target of HTRG level control is to hold the solution level constant in area C. The level control is implemented by modulating the solution pump frequency. The frequency band of the pump is between 15 and 42 Hz; the change rate is 2 Hz/s. The control formulation s variable frequency pump is illustrated in Figure 2A-9. 111
Figure 2A - 9: General rule for the variation of solution pump frequency Frequency (Hz) Maximium 42 40 Targeting frequency 35-2 Hz -1 Hz -1 Hz/5s -3 Hz/5s 30 25 +2 Hz/5s +1 Hz/5s 20 Minimium 15 A B C D E 1 2 3 4 Solution level probe When the solution pump is activated during the start-up procedure, with the solution level within area A, the pump motor will operate at 25 Hz when the solution temperature is below 80 o C and at 30 Hz when the solution temperature is between 80 and 120 o C. The frequency will be 38 Hz when the solution temperature is higher than 120 o C. Once the solution hits area C, the solution pump will keep its current frequency unchanged until the level drops below or rises over area C. The pump frequency subsequently will decrease by 2 Hz when the solution level rises up one. Figure 2A-9 gives the general rules for the frequency variation corresponding to the solution level changes, increasing or decreasing. For both directions, the control system is aimed to hold the solution level in area C. 2A.4 Cooling-Water Supply Control The cooling tower has six control points, more than half the total control points for the chiller. Appropriate control for the cooling tower is required for effective and efficient chiller operation. Three control points deal with cooling-water quality: the CTS, the CWDD, and the CWDV. Three control points pertain to the control of cooling-water flow and supply temperature: the CWBPV, the CWP, and CTF. 2A.4.1 Cooling-Water Pump Control The CWP is a single-speed pump. The cooling-water pump can be started when the cooling-water level probe detects the required cooling-water level, and when the CHWP is operating. 112
2A.4.2 Cooling Water Fan Control The fan can be started only when the CWP in operating. The cooling-water fan has three speeds controlled by the measured cooling-water supply temperature. In the chiller, the cooling-water supply temperature is usually preset at 30 o C; at this temperature, the fan runs at medium speed. An acceptable temperature range is preset at 2 o C. When the water temperature hits at 32 o C, the fan speed will operate at high speed until the temperature drops back to 30 o C. If the cooling-water supply temperature drops to 28 o C, the fan will operate at low speed. The preset temperature and the temperature range for the fan speeds can be adjusted through the control system. The relation of the cooling-water supply temperature to the fan speed is illustrated in Figures 2A-10 and 2A-11. Figure 2A - 10: Control principle of the cooling fan (increasing cooling-water temperature) Fan status High Speed Midium Speed Low Speed Off 26 o C 28 o C 30 o C 32 o C Figure 2A - 11: Control principle of the cooling fan (decreasing cooling-water temperature) Cooling water supply temperature Fan status High Speed Midium Speed Low Speed Off 26 o C 28 o C 30 o C 32 o C Cooling water supply temperature 113
2A.5 Vacuum Maintenance Because of the high vacuum in the evaporator and the absorber vessel, air can leak into the water-libr absorption refrigeration system. Corrosion also occurs in the chiller, although corrosion inhibitors are added to the water-libr sorbent solution. Non-condensable gases, such as H 2, can be generated. An automatic gas purge device, AGPD, is installed to continuously remove these noncondensable gases to maintain the required low pressures in the evaporator and absorber. The vacuum can be maintained through the AGPD and periodic manual purge services. Figure 2A - 12: Principle diagram of automatic gas purge device, AGPD Figure 2A-12 illustrates the AGPD system of the chiller. The noncondensable gas in the absorber are separated and stored automatically by the AGPD until the storage chamber is full. The noncondensable gas generated in the upper tank (the HTRG and the LTRG), however, is difficult to remove through sorbent solution migrations. Even if the AGPD is installed, therefore, manual vacuum services are still required to purge noncondensable gas from the storage chamber and the upper vessel. In the chiller, the AGPD comprises purge chambers, solution supply and return pipes, pressure balancing pipes, gas pickup pipe, and a gas storage chamber. When noncondensable gases are generated in the system during operation, they tend to migrate to the absorber where the pressure is lowest. The solution 114
pump supplies a small portion of dilute solution to chamber A on the top of the automatic purge unit. Chamber A also connects the open space of the absorber. A second pressure-balancing pipe connects the absorber solution reservoir and Chamber A open space. The absorber and Chamber A have the same total pressure. The sorbent solution pumped to Chamber A flows back to the absorber reservoir through a drain pipe. A short pipe termed gas pickup pipe is inserted into the drain pipe at the upper end; the other end is opened to the open space of Chamber A. Gas from Chamber A flows into the sorbent solution and down the drain pipe because of the reduced pressure at the lower end of the gas pick up pipe. Water vapor in the gas will be reabsorbed into solution while the noncondensable gas will remain bubbles entering in Chamber B. Chamber B has one end connected to the absorber reservoir, the other end is connected to gas storage Chamber C. This configuration will cause level 3 to move down because of the increased quantity and pressure of the noncondensable gas in Chambers B and C. When the level 3 probe indicates that too much gas has accumulated in Chambers B and C and a manual vacuum process is required. To remove the noncondensable gas generated in the upper vessel (regenerators), a vacuum pump can be connected to Vacuum valve 2, at the same time; Vacuum valve 3 must be opened. Then gases in two regenerators are removed through the vacuum pump. This operation requires operating at ambient temperature. In this process, a small amount of water vapor is removed from Chamber B and C. As a result, the sorbent solution composition is increased. New machines tend to generate more noncondensable gases than older machines. 2A.6 Crystallization Detection and Decrystallization Crystals of LiBr solid can deposit from the sorbent solution at high concentrations and low temperatures 63% to 68% LiBr and 398 to 70 o F. Such deposits in the Broad chiller are most likely to form on the surfaces where concentrated sorbent solution leaves the HTHX and the LTHX interchangers and enters the absorber. Solid deposits of LiBr on such surfaces can reduce or block sorbent solution flows from the HTRG and/or the LTRG. Crystal formation is significantly affected by operating conditions in the chiller: The cooling-water supply temperature to the absorber. The resulting low temperature in the outlet sorbent stream from the absorber results in low temperatures where the concentrated sorbent leaves the HTHX and LTHX and increases the probability of deposition there. 115
Noncondensable gas accumulation in the evaporator and chiller. This gas inhibits water evaporation from the refrigerant and absorption in the sorbent. The temperature rises in the evaporator and falls in the absorber. Steam flow is increased by the chiller control. Consequently, the concentration of LiBr in the circulating sorbent solution increases and the temperature decreases, resulting in an increased probability of LiBr deposits. Abrupt shutdown. A shutdown results in decreasing sorbent solution temperature prior to reducing the sorbent solution concentrations by mixing the various sorbent and refrigerant reservoirs in the chiller. This can cause crystals to form. 2A.6.1 Crystal Detection The chiller detects the crystallization indirectly by monitoring the temperature and solution level in the HTRG and the LTRG. When the solution temperature is above 140 o C in the HTRG and the solution level remains at area E for more than 30 seconds, the chiller control concludes that crystals have formed in the HTHX, and the sorbent solution is blocked. Crystallization detection in the LTHX is simpler than in the HTHX. If the solution level in the LTRG is higher than its upper limit for more than 120 seconds, the chiller control concludes that crystal has formed in the LTHX and sorbent solution flow is blocked. 2A.6.2 Crystal Removal Once crystallization is detected, the decrystallization process is conducted immediately to maintain chiller operation. Usually when crystals start to form on the inner surface of tubes, they do not immediately block flows. The sorbent solution stream can still pass through in the pipes. If proper actions are taken, the crystals can be dissolved back into the flow of sorbent solution. If no action is taken, the crystals accumulate in the tubes and eventually completely block the solution flow. If this occurs, professional services are required to clear the blockage. Crystal removal involves stopping the CWP, the cooling-water fan, and the refrigerant pump sequentially. If crystals are detected in the HTHX, the solution pump will stop until the level drops to C. After the level has reached to C for 30 seconds, the chiller starts the solution pump at maximum frequency to circulate the dilute solution and dissolve the crystals back to solution. This process continues for 10 minutes. If crystals are detected in the LTHX, the chiller control stops the solution pump until the solution level in the LTRG drops to B. Once the level reaches B, the solution pump starts again at maximum frequency. 116
When the process has continued for 5 minutes, decrystallization will be concluded. If the power is interrupted, the decrystallization process is similar to the HTHX decrystallization process. 2A.7 Safety Control and Diagnostics Chiller control diagnostics and protection procedures are followed when malfunction or abnormality occurs during chiller operation. The chiller safety control and diagnostics can be categorized into the following three levels: malfunction shutdown, malfunction alarming, and abnormal warnings. In malfunction shutdown, the shutdown program is executed if the cooling-water supply, the city-water supply, the cooling water pump, the solution pump, and the steam valve fail, and if the chilled-water supply temperature and the HTRG solution temperature sensors fail. Any of these malfunctions trigger the shutdown program or ends the start-up program. In malfunction alarming, messages appear on the monitor. The alarm messages include: any one of the two chilled-water temperatures, and the cooling-water supply temperature sensor failures and the coolingwater pump, the refrigerant pump, and the HTRG solution level control failures. Some of the alarm messages, such as any one of the two chilled-water temperature sensor failure, will not shut down the chiller operation, but the cooling-water supply temperature sensor and the cooling-water pump problem do trigger the shutdown program if the problems persist. In abnormal warning, a message appears. A warning message is a notice such as the noncondensable is full. Warnings do not affect the chiller operation but are reminders that service or regular checks are required. 117