adjustment), maintain a superheat rangingapproximately from 4° to 12 °F at the cooling coiloutlet. The proper superheat adjustment varies,of course, with the design and the serviceoperating conditions of the valve, and the designof a particular plant. Increased spring pressureincreases the degree of superheat at the coil outlet;decreased pressure on the other hand, has theopposite effect. Many thermostatic expansionvalves are initially adjusted by the manufacturerto maintain a predetermined degree of superheat,and no provisions are made for further ad-justments in service.When the expansion valves are adjusted to givea high degree of superheat at the coil outlet, orwhen a valve is stuck shut, the amount ofrefrigerant admitted to the cooling coil is reduced.With an insufficient amount of refrigerant, thecoil is “starved” and operates at a reduced capac-ity. Compressor lubricating oil carried with therefrigerant tends to collect at the bottom of thecooling coils, thus robbing the compressorcrankcase, and providing a condition wherebyslugs of lubricating oil are drawn back to the com-pressor. If an expansion valve is adjusted for toolow a degree of superheat, or if the valve is stuckopen, the liquid refrigerant may flood from thecooling coils back to the compressor. Should theliquid refrigerant collect at a low point in the suc-tion line or coil, and be drawn back to the com-pressor intermittently in slugs, there will be dangerof injury to the moving parts of the compressor.In general, the expansion valves for air con-ditioning and water cooling plants (hightemperature installations) are adjusted for highersuperheat than the expansion valves for coldstorage refrigeration and ship’s service storeequipment (low temperature installations).If it is impossible to adjust expansion valvesto the desired settings, or if it is suspected thatthe expansion valve assembly is defective and re-quires replacement, make appropriate tests. (Firstmake sure that the liquid strainers are clean, thatthe solenoid valves are operative, and that thesystem is sufficiently charged with refrigerant.)The major equipment required for expansionvalve tests is as follows:1. A service drum of R-12, or a supply ofclean dry air at 70 to 100 psig. The service drumis used to supply gas under pressure. The gasused does not have to be the same as thatemployed in the thermal element of the valve be-ing tested.2. A high pressure and a low pressure gage.The low pressure gage should be accurate and ingood condition so that the pointer does not haveany appreciable lost motion. The high pressuregage, while not absolutely necessary, is useful inshowing the pressure on the inlet side of the valve.Normally, refrigeration plants are provided withsuitable replacement and test pressure gages.The procedure for testing is as follows:1. Connect the valve inlet to the gas supplywith the high pressure gage attached to indicatethe gas pressure to the valve, and the low pressuregage loosely connected to the expansion valveoutlet. The low pressure gage must be connectedup loosely so as to provide a small amount ofleakage through the connection.2. Insert the expansion valve thermal elementin a bath of crushed ice. Do not perform this testwith a container full of water in which only a smallamount of crushed ice is floating.3. Open the valve on the service drum or inthe air supply line. Make certain that the gassupply is sufficient to build up the pressure to atleast 70 psi on the high pressure gage.4. Adjust the expansion valve, if it is desiredto adjust for 10°F superheat, the pressure on theoutlet gage should be 22.5 psig. This pressure isequivalent to the pressure of an R-12 evaporatingtemperature of 22°F. Since the ice maintains thebulb at 32°F, the valve adjustment is for 10°Fsuperheat (difference between 32 and 22). For a5 °F superheat adjustment, the valve should beadjusted to give a pressure of approximately26.1 psig. Allow for a small amount of leakagethrough the low pressure gage connection whilethis adjustment is being made.5. To determine if the valve operatessmoothly, tap the valve body lightly with a smallweight. The low pressure gage needle should notjump more than 1 psi.6. Tighten the low pressure gage connectionand stop the leakage at the joints. Determine ifthe expansion valve seats tightly. If the valve isin good condition, the pressure will increase a fewpounds and then either stop or build up veryENGINEMAN 1 & C6-10
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