line or coil and is drawn back into the compressorintermittently in slugs, there is danger of injury to themoving parts of the compressor.I n g e n e r a l ,t h e e x p a n s i o n v a l v e s f o rair-conditioninga n d w a t e r - c o o l i n g p l a n t s(high-temperature installations) normally are adjustedfor higher superheat than the expansion valves for coldstorage refrigeration and ship’s service store equipment(low-temperature installations).You may not be able to adjust expansion valves tothe desired settings, or you may suspect that theexpansion valve assembly is defective and requiresreplacement. In either case, you should makeappropriate tests. First you should be sure that the liquidstrainers are clean, that the solenoid valves areoperative, and that the system is sufficiently chargedwith refrigerant.The major pieces of equipment required forexpansion valve tests is as follows:l A service drum of R-12 or a supply of clean, dryair at 70 to 100 psig. The service drum is used tosupply gas under pressure. The gas does not haveto be the same as that used in the thermal elementof the valve being tested.l A high-pressure and a low-pressure gauge. Thelow-pressure gauge should be accurate and ingood condition so that the pointer does not haveany appreciable lost motion. The high-pressuregauge, while not absolutely necessary, will beuseful in showing the pressure on the inlet sideof the valve. Refrigeration plants are providedwith suitable replacement and test pressuregauges.The procedure for testing is as follows:1. Connect the valve inlet to the gas supply withthe high-pressure gauge attached to indicate the gaspressure to the valve. Connect the low-pressure gaugeloosely to the expansion valve outlet. The reason thelow-pressure gauge is connected loosely is to allow asmall amount of leakage through the connection.2. Insert the expansion valve thermal element in abath of crushed ice. Do NOT attempt to perform this testwith a container full of water in which a small amountof crushed ice is floating.3. Open the valve on either the service drum or inthe air supply line. Make certain that the gas supply issufficient to build up the pressure to at least 70 psi onthe high-pressure gauge connected in the line to thevalve inlet.4. The expansion valve can now be adjusted. If youwant to adjust for 10°F superheat, the pressure on theoutlet gauge should be 22.5 psig. This is equivalent toan R-12 evaporating temperature of 22°F. Since the icemaintains the bulb at 32°F. the valve adjustment is for10°F superheat (difference between 32 and 22). For a5°F superheat adjustment, the valve should be adjustedto give a pressure of approximately 26.1 psig. Theremust be a small amount of leakage through thelow-pressure gauge connection while this adjustment isbeing made.5. To determine if the valve operates smoothly, tapthe valve body lightly with a small weight. Thelow-pressure gauge needle should not jump more than1 psi.6. Now tighten the low-pressure gauge connectionto stop the leakage at the joint and determine if theexpansion valve seats tightly. If the valve is in goodcondition, the pressure will increase a few pounds andthen either stop or build up very slowly. But with aleaking valve, the pressure will build up rapidly until itequals the inlet pressure. With externally equalizedvalves, the equalizer line must be connected to thepiping from the valve outlet to the test gauge to obtainan accurate superheat setting.7. Again loosen the gauge to permit leakage at thegauge connection. Remove the thermal element, orcontrol bulb, from the crushed ice. Warm it with yourhands or place it in water that is at room temperature.When this is done, the pressure should increase rapidly,showing that the power element has not lost its charge.If there is no increase in pressure, the power element isdead.8. With high pressure readings showing on bothgauges, the valve can be tested to determine if the bodyjoints or the bellows leak This can be done by using ahalide leak detector. When you perform this test, it isimportant that the body of the valve have a fairly highpressure applied to it. In addition, the gauges and otherfittings should be made up tightly at the joints toeliminate leakage at these points.Replacement of ValvesIf the expansion valve is defective, it must bereplaced. Most valves used on naval ships havereplaceable assemblies. Sometimes it is possible toreplace a faulty power element or some other part of the5-8
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