adjustment),   maintain   a   superheat   ranging approximately from 4° to 12 °F at the cooling coil outlet. The proper superheat adjustment varies, of  course,  with  the  design  and  the  service operating conditions of the valve, and the design of a particular plant. Increased spring pressure increases the degree of superheat at the coil outlet; decreased  pressure  on  the  other  hand,  has  the opposite  effect.  Many  thermostatic  expansion valves  are  initially  adjusted  by  the  manufacturer to maintain a predetermined degree of superheat, and  no  provisions  are  made  for  further  ad- justments in service. When the expansion valves are adjusted to give a high degree of superheat at the coil outlet, or when   a   valve   is   stuck   shut,   the   amount   of refrigerant admitted to the cooling coil is reduced. With an insufficient amount of refrigerant, the coil is “starved” and operates at a reduced capac- ity. Compressor lubricating oil carried with the refrigerant tends to collect at the bottom of the cooling  coils,  thus  robbing  the  compressor crankcase, and providing a condition whereby slugs of lubricating oil are drawn back to the com- pressor. If an expansion valve is adjusted for too low a degree of superheat, or if the valve is stuck open, the liquid refrigerant may flood from the cooling coils back to the compressor. Should the liquid 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 danger of injury to the moving parts of the compressor. In general, the expansion valves for air con- ditioning   and   water   cooling   plants   (high temperature  installations)  are  adjusted  for  higher superheat  than  the  expansion  valves  for  cold storage  refrigeration  and  ship’s  service  store equipment (low temperature installations). If  it  is  impossible  to  adjust  expansion  valves to the desired settings, or if it is suspected that the expansion valve assembly is defective and re- quires replacement, make appropriate tests. (First make sure that the liquid strainers are clean, that the solenoid valves are operative, and that the system is sufficiently charged with refrigerant.) The  major  equipment  required  for  expansion valve tests is as follows: 1. A service drum of R-12, or a supply of clean dry air at 70 to 100 psig. The service drum is used to supply gas under pressure. The gas used  does  not  have  to  be  the  same  as  that employed 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 in good  condition  so  that  the  pointer  does  not  have any appreciable lost motion. The high pressure gage, while not absolutely necessary, is useful in showing the pressure on the inlet side of the valve. Normally,  refrigeration  plants  are  provided  with suitable replacement and test pressure gages. The procedure for testing is as follows: 1. Connect the valve inlet to the gas supply with the high pressure gage attached to indicate the gas pressure to the valve, and the low pressure gage loosely connected to the expansion valve outlet. The low pressure gage must be connected up loosely so as to provide a small amount of leakage through the connection. 2. Insert the expansion valve thermal element in a bath of crushed ice. Do not perform this test with a container full of water in which only a small amount of crushed ice is floating. 3. Open the valve on the service drum or in the  air  supply  line.  Make  certain  that  the  gas supply is sufficient to build up the pressure to at least 70 psi on the high pressure gage. 4. Adjust the expansion valve, if it is desired to  adjust  for  10°F  superheat,  the  pressure  on  the outlet gage should be 22.5 psig. This pressure is equivalent to the pressure of an R-12 evaporating temperature of 22°F. Since the ice maintains the bulb at 32°F, the valve adjustment is for 10°F superheat  (difference  between  32  and  22).  For  a 5 °F superheat adjustment, the valve should be adjusted  to  give  a  pressure  of  approximately 26.1 psig. Allow for a small amount of leakage through  the  low  pressure  gage  connection  while this adjustment is being made. 5.  To  determine  if  the  valve  operates smoothly, tap the valve body lightly with a small weight. The low pressure gage needle should not jump more than 1 psi. 6. Tighten the low pressure gage connection and stop the leakage at the joints. Determine if the expansion valve seats tightly. If the valve is in good condition, the pressure will increase a few pounds  and  then  either  stop  or  build  up  very ENGINEMAN  1  &  C 6-10