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SteG
17-02-2016, 08:08 AM
Hi, looking for help.

For setting up a cold room to to run between 3oC-5oC on R134a gas with a 35oC average ambient temperature, what should I set my pressure switch to cut in and out at?
It's a thermostat controlled pump down system, Thanks.

mikeref
17-02-2016, 08:29 AM
3 psi and 35 to 40 Psi if it is a TX system that hasn't had it's valve fiddled with.
Might as well grab a PT chart from your local distributor.

RANGER1
17-02-2016, 09:06 AM
Hi, looking for help.

For setting up a cold room to to run between 3oC-5oC on R134a gas with a 35oC average ambient temperature, what should I set my pressure switch to cut in and out at?
It's a thermostat controlled pump down system, Thanks.


Cut out pressure should be above 0 kpa so system never goes in vacuum

3 deg C = 226 kpa
5 deg C = 250 kpa
suction pressure would probably operate at about -6 deg C or 130 kpa

suggest cut out pressure between 20 to 50 kpa, depending on size of evaporator.
You don't want really long pump down time & it could pump down a few times during off cycle.

suggest cut in pressure 226 kpa or 3 deg C = lower limit of room temp.
If it takes to long to start lower it a bit, as you want to avoid any floodback on start up &/or short cycling when pumping down.

remember pressure means nothing, it's the temperatures that mean everything, then convert it to pressure to achieve required result.
Setting up switch with dry nitrogen can save a lot of time & more accurate.

http://www.achrnews.com/articles/87415-automatic-pumpdown-systems

SteG
17-02-2016, 09:48 AM
Thank you, is the cut in temperature measured in relation to the saturation temperature as room reaches temperature? Or because my desired room temperature is 3oC, just so I can apply this for using different refrigerants e.g r404a.

sedgy
17-02-2016, 03:07 PM
hi steg. no you cannot use this on any type of refrigerant the idea of using r134a as you quoted, you where given the temp/ pressure for that refrigerant you will have to get a p/t chart ( you may require a few of them before you retire) because every gas has its own footprint ( as mikeref) suggested. enjoy your new trade. you will never stop learning( I havent and I have been retired 14 years) all the best = sedgy

monkey spanners
17-02-2016, 05:43 PM
I'd say about 5psi cut out and about 25psi cut in. If you go too high with the cut in the unit may not start in cold weather.

Magoo
18-02-2016, 02:16 AM
Interesting reading,
a pump down cycle with a thermostat and a solenoid in the perfect situation is great, straight forward and simple, but walk in chillers are notorious for someone either loading room and /or leaving door open for extended periods. So evaporator turns to ball of snow, so setting on LP control is critical to protect compressor and get an air defrost for a possible snow up.
Generally I wire control circuit through stat and HP/LP in series, so either or will cycle compressor, then cycle solenoid with compressor start /stop, to avoid liquid migration in off cycle. Some time spent on setting LP setting coresponding to room temp and set LP cut out 3>5 psig below that suction pressure. Compressor stops, solenoid closes and evaporators defrosts and restarts on cut in setting at P/T +5 'C, other wise system cycles on stat if evaporator is operating normally at normal conditions
Saves a lot of call backs and nuisance frost ups

RANGER1
18-02-2016, 06:48 AM
Interesting reading,
a pump down cycle with a thermostat and a solenoid in the perfect situation is great, straight forward and simple, but walk in chillers are notorious for someone either loading room and /or leaving door open for extended periods. So evaporator turns to ball of snow, so setting on LP control is critical to protect compressor and get an air defrost for a possible snow up.
Generally I wire control circuit through stat and HP/LP in series, so either or will cycle compressor, then cycle solenoid with compressor start /stop, to avoid liquid migration in off cycle. Some time spent on setting LP setting coresponding to room temp and set LP cut out 3>5 psig below that suction pressure. Compressor stops, solenoid closes and evaporators defrosts and restarts on cut in setting at P/T +5 'C, other wise system cycles on stat if evaporator is operating normally at normal conditions
Saves a lot of call backs and nuisance frost ups

Magoo, so no real pump down with this scenario!
Would it have to have a suction slop pot for this setup?
Otherwise possible slug back or carryover of liquid!

mikeref
18-02-2016, 09:07 AM
I'd say about 5psi cut out and about 25psi cut in. If you go too high with the cut in the unit may not start in cold weather.

That is true Jon. In this case, i chose an LP reset pressure higher than 30 Psig because somewhere down the track,an iced evaporator has a hard time raising suction pressure beyond 30-32 Psi when the solenoid kicks in.

This setting gives the Evaporator some extra time to clear itself of Ice.
Liquid flooding after LLSV has opened? A suction accumulator. Heatcraft have moved away from the Conventional pump down. Both Compressor and LLSV shut down on the off cycle.
NO Pump Down.

chemi-cool
18-02-2016, 03:03 PM
You don't really need pumpdown, as mikeref said, let compressor and SV shot at the same time, make sure it installed close to the TXV to avoid excess liquid in the evaporator.