retrofitting r422a

Hi

I am overseeing someone trying to commission a system which has just been converted from R22 to R422a or isceon 79 if you like, I have read previous threads and followed the links like duponts website and from what I read something has gone astray somewhere?

system is domestic fridges consisting of 2x walk in deep freeze and 1 veg room. bitzer v/w compressor (original), original condensor, new evaps and TEVs

As per the info taken from the net the new system is running at a higher Discharge pressure 220-230psi but am running into a vacuum on the LP side of compressor, Have expressed dislike about vacuum and the noise the compressor makes when at vacuum only to be told that this vacuum is normal running parameters. which I can clearly see is not from the charts supplied from duponts. I have since adjusted the LP cut out to be above zero but should my LP pressure not be in the region of 30 40 psi to achieve a temp of -4 deg F at chilled room?

any help and any other references would be welcomed.

is there a problem with my compressor and compatibility to gas or am overlooking the system requiring more gas( will my HP press go up more by adding more) one piece of lit says the pressure diffrences between 422a and r22 should be the same or similar.

please help getting desperate:confused:
cheers
Joapers

R422a is a replacement for R22.

Operating in vacuum is NOT normal using R422a at -4F.

Having 220 PSI condensing is normal for 37 centigrade condensation.

You are right your pressure should be around 30 PSI to get -4F.

I would not blame this on the compressor but there's not enough information on where could the problem be.

Low suction in general may be caused by:
Low thermal load, insufficient refrigerant fed to the evaporator, large compressor and in this case may I add not having the refrigerant you think you have.

So check catalog data on system componets with R422a, check temperature and pressure and see if it consistent with refrigerant data in places you know there is mixture of liquid and vapour phase (in condenser,in evaporator, liquid receiver).

Check that your expansion valve is open and the evaporator well fed.

thanks for prompt reply

I am en route to see if we can rectify this problem finally.

just a small query with your advice, mentioned "large compressor" is there a notable size difference between a 422a and R22 compressors.

as before much thanks for reply and as so many before me have said this site is solid gold. not the first time you guys have saved my bacon.

cheers
Joapers

If you use the same compressor the capacity is less with R422a than R22 (specific volume is about the same at -4F but enthalpy difference about 19% better with R22) so you should expect HIGHER sucction pressures with R422a IF the thermal load is constant.

thermal load varies as I have 3 spaces to cool at different temps, 2 x deep freezes at -17 deg C and -20 deg C and 1 x veg store at 3-5 deg C all three evaps are different capacities, and compressor has no unloading capabilities.

have come to the conclusion that compressor is trying to move a fixed volume of gas, and the volume of gas being passed through the system varies depending which rooms are demanding chilling hence the fluctuating LP pressure.

My system no longer goes into vacuum since adjusting LP cut out but does still appear to short cycle, do you have any suggested pressures for LP cut in/out? just incase I have to increase the band between the two.

thanks for all your help and assistance without it would have surely lost the will to carry on.

cheers
Joapers

Quote:

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Originally Posted by joapysoe

thanks for all your help and assistance without it would have surely lost the will to carry on.

cheers
Joapers

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Sorry It took me too long to come back!

I would always set LP catout within the compressor operating envelope!

This may cause short cycling and if it can be solved with a timer I'd use it if it can't you need to install a capacity control.

Sometimes you can prevent this controling the cut out temperature of the different evaps.

But you must avoid operating the compressor outside its desigh range and from short-cycling.