Document 4bxOj30N8JXL2wXDKnk0MNde

762 CHAPTER 70 1962 Guide And Data Bo^ switch and motor overloads) so that the refrigerant solenoid valve will close when the compressor stops through the action of any one of these safety devices. 5. Low pressurestat settings such that the cut-in point will correspond to a saturated refrigerant temperature lower than any compressor ambient air temperature which may be ex pected. For example, if the ambient temperature can go as low as 50 F, the cut-in point of the low pressure switch must be lower than 46.7 prig (when using Refrigerant 12). If the cut-in setting of the low pressure switch is higher than this, then liquid refrigerant can accumulate and condense in the crank case at a pressure corresponding to the ambient temperature. In this case, the crankcase pressure would not rise high enough to reach the cut-in point and effective automatic pump-down would not be obtained. Crankcase Oil Heater with Single (Nonrecycling) Pump-out at the End of Each Operating Cycle (DX Systems) This arrangement is not as positive as automatic pump- down control, in keeping liquid refrigerant out of the crank case, but is a satisfactory substitute where pump-down con trol meets with customer objections, or results in excessive compressor short cycling. Crankcase oil heaters cannot prevent liquid entering the crankcase during off cycles as the result of poor piping ar rangements or leaking liquid feed devices. They will, however, maintain the crankcase oil at a temperature higher than other parts of the system, thus minimising the absorption of the re frigerant by the oil and also the Histillatinn process. Operation with this arrangement is as follows. Whenever the temperature control device opens - the circuit, or the manual control switch is opened for shut-down purposes, the crankcase heater is energized and the compressor keeps run ning until it cuts off on the low pressure switch. The crank case heater remains energized during the complete off cycle and it is thus important that a continuous live circuit be made available to the heater during the off time. The compressor cannot start again until the temperature control device or manual control switch closes, regardless of the position of the low pressure switch. The use of a crankcase heater with singly pump-out at the end of each operating cycle requires: 1. A liquid line solenoid valve in the main liquid line or in the branch to each evaporator. 2. Use of a relay, or the compressor motor starter auxiliary maintaining contact, for obtaining a single pump-out operation before stopping the compressor. * 3. A relay or auxiliary starter contact for energizing the crankcase heater during the compressor off cycle ana de energising it during the compressor on cycle. 4. Electrical interlock of the refrigerant solenoid valve, with the evaporator fan or chilled water pump, so that the refrigerant flow will be stopped when either the fan or pump are out of operation.' 5. Electrical interlock of the refrigerant solenoid valve with the safety devices, such as the high pressure cut-out, oil safety switch and motor overloads, so that the refrigerant flow valve will close when the compressor stops through the action of one of these safety devices. Compressor Control with Flooded Type Evaporators It is recognized that neither automatic pump-down control, nor a single pump-out operation, are practical in systems em ploying flooded evaporators, unless suction line solenoid valves are added to the system. Therefore, with flooded type evapo rators, the following control arrangements are considered satisfactory: 1. Manual operation as described in following text. No crank case heaters required. 2. Automatic control from temperature controllers or <wl devices, provided crankcase beaters are used and enerrixaH* the off cycles aud the liquid solenoid valve closes whenever,?1 compressor is stopped. Where water cooling of the comprea^* employed, a solenoid valve in the water supply Une shout?)?.8 whenever the compressor is stopped. Ctos* 3. Same as item 2 except with the added precaution of a pump-down of the compressor for night or weekend shutdo*?8 This can be accomplished by manually closing the eompr^J^ suction stop valve. If the single pump-out procedure is useit? compressor will automatically pump down once, then shutoff* low pressure cut-out, and stay on until manually re-staroS1 (This feature cannot be used if the low pressure cut-outsS acts as the last capacity control step.) * Manual .Compressor Operation Compressors may be controlled manually without the useof automatic pump-down control, or single pump-out and crank case heater, provided the system is at all times under the co> trol of a qualified operator. The operator will pumpout the compressor by use of the manual valves and will keep liquid suction and discharge valves closed when the machine ^ ^ operating. This method of manual operation may be n^d fOT any system, but is particularly applicable to flooded systems. Effect of Short Operating Cyde It is characteristic of the reciprocating compressor operation that oil will leave the crankcase at an accelerated rate im mediately after starting. It is necessary, therefore, that each start be followed by a sufficiently long operating period to per mit the regain of the oil level. Operation under normal control demand in response to room air temperature will, in nv^t cases, provide sufficient operating time. However, if the com pressor is controlled in response to supply air temperature, or to water temperature leaving a water chiller, a rapid cycle may result. Accordingly, thermostats used for compressor control from these indices should have differentials wide pTwngh so that the running cycles will not be less than seven or eight minutes. Short cycles caused by action of safety devices, such as low pressure cut-out or high pressure cut-outs, will, if continued, also cause the compressor to lose oil. This kind of service trouble should always receive prompt attention. HOT GAS BYPASS ARRANGEMENTS Most reciprocating compressors are equipped with capacity control facilities which allow the compressor to start with most of its cylinders unloaded and modulate compressor capacity down to within 25 to 50 percent of full load. In spite of this feature, however, it is sometimes necessary to accomplish further unloading of the compressor for the fol lowing reasons: 1. To reduce storting torque requirements, so that the compres sor can be started up with low starting torque prime movers, and also to allow starting of the compressor on low current Ups of reduced voltage starters. 2. To permit capacity control down to 0 percent load conditions without stopping the compressor. Full (100 percent) Unloading for Starting Purposes When the requirement is for 100 percent unloading during the starting period only, this can be done with a manual or automatic valve in a bypass line between the hot gas and suc tion lines at the compressor. This valve should only be open during the starting period, and then closed after the compressor is.up to full speed and full voltage is applied to the motor terminals. Use of this by pass for longer periods will result in overheating. i? Refrigerant Piping Systems for Refrigerants 12, 22 and 500 763 A. Simple Hot Gas Bypass 8. Hot Gas Bypass to fJtif of fraponrior C Hot Gas Bypass to Entrance of Evaporator D. Hot Gas Bypass with Liquid Injection for Dotupetbeuting fig. 49 .... Hot Gas Bypass Arrangements1 The sequence of control b as follows: Hie unloading bypass valve should be energized immediately on demand of the cool ing thermostat, or other control calling for compressor opera tion. This permits equalization of pressures across the com pressor. After an adequate time delay (time necessary for complete valve opening), a timing relay should close a pair of normally open contacts to start the compressor. After a farther time delay (adequate to allow the compressor motor to come up to running voltage and speed), a pair of normally closed timing relay contacts should open, de-energizing the unloading bypass valve and closing the bypass. Pwfl (100 percent) Unloading for Capacity Control Where up to 100 percent compressor unloading b required for capacity control purposes, it b posable to use hot gas bypass arrangements in ways that will not tend to overheat the compressor., h> using these arrangements, it b highly desirable that bot Su is not bypassed until after the last unloading step on the oppressor has taken place. Some positive arrangement should be made to Insure thin. Various methods of bypassing hot gas have been used. They do not all have equal merit. In selecting a hot gas by pass, careful consideration should be.given to the following three requirements: 1. It should give acceptable regulation throughout the range of loads. 2. It should not cause excessive superheating of the suction gas. 3. It should not cause any slop-over to the compressor. A hot gas bypass for capacity control purposes b an arti ficial loading device to maintain a minimum evaporating pres sure, during continuous compressor operation, regardless of the evaporator load. This b usually accomplished by means of an automatic or manual pressure reducing valve which acts to maintain a con stant pressure on the downstream side. Four of the more common methods of using hot gas bypass are shown in Diagrams A, B, C, and D of Fig. 49. Diagram A illustrates the simplest type of hot gas bypass. It will dangerously overheat the compressor if used for pro tracted periods of time. Diagram B shows the use of hot gas bypass to the exit of the evaporator. Here dependence b made on the evaporator