medium temp pumpdown vs no pump down

[Harrison]

hi all,
i was wondering about the purpose of pumpdown for small medium (4 deg) temp coolroom, 1.25hp condensing unit. why do pumpdown? is it to prevent refrigerant migration on the off cycle and slugging at startup? i mostly see non-pumpdown on smaller units so is there any reason for doing it? like if the condensing unit is likely to get colder than the room?
thoughts, experiences.....
cheers

[chillerman2006]

hi all,
i was wondering about the purpose of pumpdown for small medium (4 deg) temp coolroom, 1.25hp condensing unit. why do pumpdown? is it to prevent refrigerant migration on the off cycle and slugging at startup? i mostly see non-pumpdown on smaller units so is there any reason for doing it? like if the condensing unit is likely to get colder than the room?
thoughts, experiences.....
cheers

with your ambient temp above your "coolroom" (4deg) refrigerant will migrate to the evaporator, then bring liquid back to the compressor on start up, or with your ambient temp below your "coolroom" refrigerant will migrate to the compressor unless crankcase heater is in use

[Harrison]

yeah as i thought, but non pump down is a very common arrangement on small systems that dont have sump heaters, so it would seem that practically they work. is there a rule of thumb when to use which, under what circumstances?
cheers

[chillerman2006]

small splits years ago i used to look at had no expansion valve which when used on larger units would open fully upon evaporator temp increase allowing full flow, capillary distributor still restricts flow. By the time it takes to equalise pressure the condensor and receiver are still below compressor temp and majority of refrigerant will remain there. But I suppose in theory migration to compressor as well is still possible under right conditions

[Slatts]

Harrison, one reason that LP pump down is used on remote condensed coolrooms is to make it easier to set up. no wiring between cold room and condensing unit. It also helps with cyclic defrost. System pumps down when down to temp, set the LP cutin to say 4 or 5C equivalent and to cut out 3 or 4 PSI below normal operating pressure. Then if you get an ice up, your suction pressure will drop and cut out the LP control and it should stay off till the evap is defrosted.
If you're just worried about floodback on start up, you could wire your LLSV to cycle with the compressor and put it at the condensing unit.