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Hi
just wondering what “rule of thumb temperatures” converted to pressure you lads are using when setting up your pressure controls.
based in England so ambients of 30’c plus aren’t to common.
id like to see what other people use when setting there controllers up.
head pressure settings?
Low pressure Cut in? Diff?
Thanks
Kind of open ended question, what service are you concerned with?
We work to the standards.Quote:
Originally Posted by Airconking
Standards state 55degc for condensing temp (high side) and 32degc for low pressure side.
If setting up safeties then you set HP to x 0.9 or 90% of that value for the high side. If you are setting PRV then that is x 1.1 or 110% of maximum working pressure.
I see you are mainly AC? R410A at 55degc is about 33barg so safeties set to 30ish and blow offs to 36ish
Hp switch auto reset at 32degc so it would need to cool down below 20barg before restarting.
R404A on the other hand is a lot lower pressure so R404A @ 55degc = 25barg. Safeties set to 22barg - 23barg and blow offs to 28ish barg. auto rest on HP at 14barg.
So no guess work, it should all be done to EN378 and obviously manufactures specifications but manufacturers always work to EN378 anyway.
Rob
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Low side is temperature related or safety for gas loss.
LP for low pressure due to a leak could be set to 0.5barg all the way to 3barg or even 4barg system dependant. For frost protection on evap see what your refrigerant is a 0degc and set pressure accordingly. R410A at 0degc is 7barg but R404A at 0degc is 5barg so frost protection is temperature dependant.
Freezer with R404A type refrigerants would be set to as low as 0.5barg ish because the evap would be down to -30degc at a pressure of 1barg.
Rob
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Thanks for the reply rob...
I’m using 50’c as a ball park figure for my high side safety’s this obviously takes into account the 90% rule you mentioned using EN378.
If I use 55’c - 10% or just a 50’c with no deduction there pretty much the same figure anyway using r404a.
ill stick to this then because like you said it’s working to EN378
The low pressure controlls I’ve come across are for systems working on pump down, mainly walk-in chillers and freezers with electric defrosts.
I’m finding these set at a range pressures depending on the installer.
my question is is there a “rule of thumb” when setting pressure controlles used for pump down.
i know I want to be cutting out just above 0bar but at what point do I want to be cutting in at to avoid problems such as short cycling or even low ambient temperatures affecting cut in.
Hi brianQuote:
Originally Posted by Brian_UK
mainly AC but venturing into the fridge world recently hence the pressure controls question.
Just wanting to see how you guys are setting up your pressure switches obviously high side for safety reasons and low side for pump down aswel as safety.
thanks
A decent LP control would include anti-recycle pumpdown, this way it won't keep restarting the compressor until the temperature control calls for a go condition.
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The LP with Pumpdown needs to cut in before the product temp.
So R404A on a fridge could pumpdown to about 3 barg and cut in at 4.5bargish this would give protection for gas loss and also allow the compressor to start up again at -5degc ish.
Freezer on R404A would cut out at 0.8barg ish and cut in at 1.8barg - 2barg ish this gives the comp time to start well before the product gets to -20degc.
There is not an exact rule of thumb as such that I know of other than the fact that the compressor needs to be running before the pressure increases to the product temperature, fridge 0degc ish and freezer -20degc ish.
The differential should be wide enough to prevent compressor short cycling in the off cycle but not too wide that it won't allow the compressor to restart.
so a freezer running at -20degc need to start the compressor at -22degc which is 1.8barg on R404a, if it warms up to 5barg then the product would warm up to 0degc before the compressor fire up.
Does that make sense??? As I'm writing I'm trying not to spout rubbish or at least no more rubbish than normal :)
Rob
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Thanks again Rob and yes I understand what your saying regarding the product temp Dictating the cut in temp etc.Quote:
Originally Posted by Rob White
its all system dependant I suppose and obviously the location of the equipment might call for minor adjustments.
I am with Rob this time, only want to add that in a system with pump down pressure control, I use for the low pressure a manual reset one to avoid the in, out, in, out of the compressor in event of gas loosing, blocked drier, faulty TXV etc.
The pump down control should be automatic one.