Refrigeration and Air Conditioning Equipment Cooling Episode 3

26 Figure28. Absorptionrefrigerationcycle. circulated in a closed cycle to the cooling coils. This refrigeration effect is known as flash cooling. 5. In reference to item 3 of figure 28, note the addition of the generator and accessory equipment. These components are necessary for continuous and efficient operation. The salt solution would become diluted and the action stopped if it were not for the regeneration of the salt solution. To keep the salt solution in the absorber at its proper strength so that it will have the ability to absorb water, the salt solution is pumped to a generator where heat is used to raise its temperature and boil off the excess water. The salt concentrate is then returned to the absorber to continue its cycle. The water that is boiled off from the salt solution in the generator is condensed in the condenser and returned to the evaporator as shown in item 4 of figure 28. The heat exchanger uses a hot solution from the generator to preheat the diluted solution. This raises the overall efficiency because less heat will be required to bring the diluted solution to a boil. Condensing water, which is circulated through the coils of the absorber and the condenser, removes waste heat from the unit. By comparing figure 29 with figure 27, you will get a better understanding of the relation between basic operating principles and an actual installation. 6. Controls. Figure 30 illustrates a typical control panel for an absorption refrigeration unit. The purpose of each control listed in this figure is described in the following paragraphs. Turning the off-run-start switch (1) the START position energizes the electric pneumatic switch (2), which activates the control system of the absorption machine. Supply air pressure of 15 p.s.i.g. (3) passes to the chilled water thermostat (4), then to the concentration limit thermostat (5), and finally to the capacity control valve (7). 7. The chilled water thermostat (4) is a direct acting control with a 7° F. differential. For every degree change in the chilled water temperature, there is approximately a 2-pound change in its branch line air pressure. Its thermal element is located in the leaving chilled water line. As the leaving chilled water temperature drops below the control setting of the thermostat, the supply air pressure (3) is throttled, causing the capacity control valve (7) to throttle the condenser water quantity. With a constant load on the machine, the capacity control valve throttles just enough condensing water to balance the load. 8. The concentration limit thermostat (5) is a direct acting bleed type control, with the thermal element located in the vapor condensate well. Its purpose is to prevent the solution from concentrating beyond the point where solidification results. At startup, the capacity control valve (7) is closed and remains closed until the vapor condensate well temperature rises above the control point of the concentration limit thermostat. As it does, the thermostat begins to throttle the air bleeding to the atmosphere, thus raising the branch line pressure (6) and opening the capacity control valve. This control valve on some absorption models may be controlled electrically instead of pneumatically. 9. Safety controls. Two safety controls are usually used in the control systems. They are the chilled water safety thermostat and the solution pressurestat. In moist instances, any malfunction occurring during operation is immediately reflected by a rise in the chilled water temperature. The thermal element of the chilled water safety 27 Figure29. Absorptionrefrigerationcycle. thermostat is located in the chilled water line leaving the machine. The control point is set approximately 10° F. above the design leaving chilled water temperature. A temperature rise above the control point shuts off the air supply. All control lines are then bled and the system is shut down. When the off-run-start switch is in the START position, this control is bypassed. The switch should not be placed in the RUN position until after you obtain a chilled water temperature below the control setting. 10. The solution pressurestat located in the 28 Figure30. Controlpanel. discharge line of the solution pump is set to cut in on a rising pressure at 40 p.s.i.g. and cut out on a falling pressure at 30 p.s.i.g. If for any reason the discharge pressure falls below the control point, the system will be shut down in the same manner as described above. 11. Special control. Special chilled water controllers may be installed in the field for special applications. These controls are used to maintain the chilled water temperatures within a plus or minus 2° F. Explosion-proof controls and motor are installed for special applications. Refer to the manufacturer`s manual on the operation and maintenance of these controls and motors. 12. Thermometers. Thermometers are installed in several locations in the system. Below is a general listing of thermometer locations and their purposes: (1) Chilled water piping to indicate the entering chilled water temperature. (2) Chilled water pump suction piping to indicate leaving chilled water temperature. (3) Condensing water piping entering the absorber section. (4) Condensing water piping leaving the absorber section. For proper temperature measurements, the thermometer is located in the generator bypass line. (5) Condensing water piping leaving the condenser section. (6) Condensing water piping to indicate the total condensing water temperature to the cooling tower or drain. 13. Pressure Gauges. Pressure gauges are installed in several locations in the system. The following is a general listing of gauge locations: (1) Purge water line after the strainer and before the purge water jet. (2) Purge water line after the jet. (3) Steam line before the generator section. (4) Discharge line from the chilled water pump. 29 (5) Discharge line from the condenser water pump. 14. Water Seals. Older models of absorption machines require mechanical seals on the solution and evaporator pumps. However, the newer machines have hermetically sealed pumps that eliminate the need for mechanical seals. The older models require external water seals; therefore, it is necessary to supply a water seal tank to maintain water on the seals for lubrication purposes and so that water rather than air leaks into the machine in case the seals break or leak. 15. The water seal tank has a float control to limit the quantity of water to the seals when the machine is in operation. The operator must open the manual valve supplying the seal water tank before startup and must close the manual valve on shutdown. This is the standard method of control. The alternate method is one where a check valve is installed in the supply line to the tank, as well as an antisyphon vacuum breaker. When the machine is shut down a visual check can be made to determine the condition of the seal and to prevent a large quantity of water from leaking into the machine if the seal is worn or cracked. If mechanical seals have to be replaced, the manufacturer`s instructions must be carefully followed in order to do the job correctly and prevent the new seals from leaking. During operation, the evaporator pump makes up for the water lost by a seal; but during shutdown, it is possible to lose a large amount of water from the tank if a large leak exists. Therefore, leaky seals must be replaced immediately. Having learned the importance of water seals in the absorption system, we can now discuss the starting procedures. 6. Starting Procedures 1. Some absorption systems are completely automatic and can be started by simply pushing a start button, while in other systems the machine is automatic but the auxiliary equipment is manually operated. The type of startup determines the starting procedure. Therefore, each starting procedure is outlined separately, and the machine operator can perform the starting operations applicable to the type of startup required. Even though some systems are automatic, it would be advisable to check the system as described below before starting the unit. 2. Daily Startup. Use the following steps in performing a normal startup. (1) Check vacuum in machine (see Maintenance, Section 8). (2) Check mechanical seals for leakage (see Maintenance, Section 8). (3) Check water level in evaporator sight glass. (4) Check absorber section for presence of water. (5) Start condensing water pump. (6) Check temperature of condensing water going to machine. Do not start cooling tower fan until the condenser water it has warmed up to the recommended setting. (7) Start the purge unit. • Push start button on the purge control panel. • Open purge steam supply valve. • Check the standpipe for water seal circulation before starting the pumps. (8) Start the chilled water pump and open the valves to insure circulation through the evaporator tubes and air-conditioning equipment. (9) Start the refrigerant pump and open the valve in the refrigerant pump discharge line. (10)Start the purging machine. Open the absorber purge valve located in the purge line to the absorber. The generator purge valve located in the purge line between the absorber and generator must be open. (11)Wait until the machine is completely purged. There will be a substantial drop in the leaving chilled water temperature when the machine is completely purged. If the leaving chilled water temperature does not drop and there are no leaks in the machine, then the steam jets should be cleaned. (12)Open the main steam valve to the machine. (13)Check steam pressure supply to see that it is within the proper range. (14)Place the control panel switch in the START position. (15)Check the main air supply pressure gauge to insure that 15 p.s.i.g. is supplied to the control panel. (16)Start solution pump. Be sure the strong solution return valve is open at all times. (17)When the leaving chilled water temperature has dropped below the safety thermostat setting, move the control panel switch from START to RUN. 3. Startup After Standby Shutdown. This procedure is basically the same as for daily startup. There are, however, additional preparation steps that must first be performed in order to put the machine in operational condition for startup. In order to prepare the machine for startup, the nitrogen with which the machine has been charged must be removed and a vacuum pulled on the machine. This is done by operating the purge unit until the machine has been purged of nitrogen and a satisfactory vacuum reading attained. 30 ... - tailieumienphi.vn