Re:LP pressure switch

[peter h]

hi all
Just recently joined a refrigeration company after many years of air conditioning installs.
Thing are going well and rely enjoying the change.
But having a few issues setting lp switch cut in and cut out .Can some one please explain in plain English how to do this? and what methodology they used
I want to use the pressure as the thermostat ..

chillier freezer
set point 2c -20
134a 404a

regards peter

[marc5180]

Hi Peter, try the link below. When you've found what your looking for click on literature and that should explain it for you.
http://www.danfoss.com/Products/Categories/Group/IA/Pressure-switches/789d59ba-1f8c-43b1-9170-88ad1e0d4f82.html

[passandscore]

Check this out

LOW PRESSURE CONTROL APPLIED AS A THERMOSTAT (http://books.google.ca/books?id=eS_2UGJVA2EC&pg=PA483&lpg=PA483&dq&source=bl&ots=bgW5A_KeHb&sig=gWIsUJtVCYJjePtdzbUb-Lk-qO8&hl=en&sa=X&ei=4BE8U_3BMcGe2AWm3oDwDw&ved=0CEUQ6AEwAQ#v=onepage&q&f=false)

[NewmanRef]

Do mean you want to use the LP switch to control your compressor? I am assuming you will have thermostat of some sort controlling a liquid line solenoid valve and you want your compressor to control on a pump down?

If so your cut in should be set no higher than your running suction pressure so that when your thermostat energises valve, suction pressure rises, switch makes switch and starts compressor. If the cut in is set too high the compressor will not start promptly and your room/ cabinet temperature will rise too high as a result. If the cut in is set too low your compressor may short cycle due to vapour boiling off from oil in low side and raising suction pressure with valve shut. Find a happy medium. Use a PT chart to get a start point.

The cut out is determined by the diff scale on your switch. The cut in minus the diff equals the cut out. On a pump down system you are aiming to pump the low side down to 0 but not into a vacuum. So when the thermostat drops feed to the valve the compressor continues to pump against the closed valve whilst pulling the low side pressure down. Therefore, on a single compressor condensing unit the diff should be set exactly the same as the cut in or very slightly less. So cut in 30psi, diff 29psi equals cut out 1psi.

The scales on the pressure switch are a guide and are generally a good starting point but will not be completely accurate. You need to test with your gauges on the system and fine tune.

You can set up before charging the system by connecting your gauges directly to the switch and use OFN to apply pressure and set cut in then release the OFN to set the diff.

You can set the switch whilst the system is running, either by watching the system through a cycle several times or by closing your suction service valve and opening again to replicate the valve closing.

Hope this helps. Had a couple of beers and babbled on a bit...

[frank]

Do mean you want to use the LP switch to control your compressor? I am assuming you will have thermostat of some sort controlling a liquid line solenoid valve and you want your compressor to control on a pump down?

I don't think this is what he wants. You can use the pressure switch to control the compressor cycle based on the SST/pressure of the refrigerant.


Hope this helps. Had a couple of beers and babbled on a bit...

Been there..done that :D

[peter h]

yes well worded... did an small food chillier on a fishing boat and the boss said to set up the lp switch to cut out at room temp of 0c just in case the mechanical thermostat fails(set point 2c)!

so using pt. chart 134a how do I work this out 0degc =27.8psig but the evaporating temp is going to be lower than this so I can not have it cutting out at 27.8psi as it would never get close to temp.

cheers peter

[hyperion]

You need to know/guess the the difference between the evaporating temperature and the air off temperature in comparison to the achieved room temperature. As a rough guide the evaporating temperature will be 8degK lower than room temp. So your suction pressure could be about 18 psig at 2degC, so set your pressure switch to cut out at 15psig. You may need to do a little fine tuning to make sure that it is working correctly and allowing the system to get down to temperature and operate on the room thermostat.

[peter h]

so what your saying is if I got rid of the thermostat and relied on the lp switch as temp control I would set cut in at 32psig(3.5degc) and cut out at 17.5psig(-7degc) which gives me a diff of 14.5 psig...which should leave me with a room temp of around 2degc and a diff of 1.5degc?
cheers peter



You need to know/guess the the difference between the evaporating temperature and the air off temperature in comparison to the achieved room temperature. As a rough guide the evaporating temperature will be 8degK lower than room temp. So your suction pressure could be about 18 psig at 2degC, so set your pressure switch to cut out at 15psig. You may need to do a little fine tuning to make sure that it is working correctly and allowing the system to get down to temperature and operate on the room thermostat.