CPR question

[cretan]

I worked on a coffin freezer with a 3/4 horse semi.Capillary system.changed from 502 over to 408.I noticed when i turned the system on the high side took a long time to get to operating pressure.would a wrong adjustment on CPR valve cause this?What kind of pressure should I see on the gauges?The freezer will not go below -16F.

[US Iceman]

If you have a CPR valve in the suction line it could very well limit how fast the system pulls down to the expected evaporating temperature.

The pressure setting for the CVPR is dependent on the type of refrigerant used and the desired temperature in the evaporator (after pull down has been completed).

[Toolman]

If you have a CPR valve in the suction line it could very well limit how fast the system pulls down to the expected evaporating temperature.

The pressure setting for the CVPR is dependent on the type of refrigerant used and the desired temperature in the evaporator (after pull down has been completed).

Iceman your correct on the pull down time being slower , but dont you mean use an EPR for desired temp in evap ? I would only expect a CPR to control pressure at the compressor .

CRETAN - you say -16F in your posting - thats pretty cold in celsius its -26C , what temp are you after ?
I would be happy with -26C in a freezer .

[Samarjit Sen]

Sorry Toolman,

16oF is -8.88 oC

[US Iceman]

...but dont you mean use an Epr for desired temp in evap ? I would only expect a Cpr to Control pressure at the Compressor .

You are right, an EPR directly controls the evaporating pressure and hence it's operating saturation temeprature.

I was thinking in terms of the CPR valve being set for the original refrigerant as opposed to the requirements of the the new refrigerant.

Of course, there may be additional problems that have not even been discussed yet either.

[Toolman]

Mmm Ok your right Samarjit Sen

I have this little convert program it has failed me or I punched in the amount in the wrong column ,
yes minus 8 C isnt too good for Mr Cretan .

[Toolman]

I worked on a coffin freezer with a 3/4 horse semi.Capillary system.changed from 502 over to 408.I noticed when i turned the system on the high side took a long time to get to operating pressure.would a wrong adjustment on CPR valve cause this?What kind of pressure should I see on the gauges?The freezer will not go below -16F.

I wouldnt worry about the CPR ( as previously discussed with your other system and its CPR problem ) the CPR is just to stop high pressure at the valve plate ( suction side only )
LOW Head pressure is possibly low gas charge and this also relates to not being able to get system down to temp.

[Javabones]

What about setting the CPR during highest suction pressure so that motor amps remain at or under the nameplate rating? I have found that CPRs will definitely cause some capacity loss on pulldown if not set properly - i.e. set too low.

[Dan]

I worked on a coffin freezer with a 3/4 horse semi.Capillary system.changed from 502 over to 408.I noticed when i turned the system on the high side took a long time to get to operating pressure.would a wrong adjustment on Cpr valve cause this?What kind of pressure should I see on the gauges?The freezer will not go below -16F

As discussed, a CPR will give you a slow start, thus you will not see a familiar pull-down load on the high side because the compressor will not be operating at a high suction pressure. But sooner or later, you should see the temperature that is designed for the application. If you are not seeing the proper temperature you may not have an issue with the CPR. You could have other issues. Ideally, you should measure no pressure drop across the CPR once the evaporator has achieved its normal temperatures. Regrettably, you often cannot see the pressures on both sides of the CPR valve.

Does this case have fans? You could be moving too much air (somebody changed motors or blades) 200 fpm is about what you need.. anything more will cause high temperatures)

If you could name the manufacturer and model of the case it would help. When you say -16 Deg F is as cold as it will get, is this the evaporating temperature?

If it is a cold wall case, Toolman's advice is spot on.

Regardless, 10 psig suction and 210 psig discharge might be what I would expect. What do you have?

[cretan]

Its a gravity type evarorator(no fans)with box temp at-12f or so,low side is around 12 and high is 300.The condensor has pipes no cooling fins.Got it going and has been running about a week.you figure 300 is too high?It is a critical charge unit.I put in the required amount.

[paul_h]

Did you weigh it in? Do you know what 408 charge it should have, or did you just weigh in to the amount of r502 it should take?
Anyway, since it's a static coil, is it icing up? What's the suction temp out of the evap?

[cretan]

I put in the 502 weight.coil is not frozen condensor is clean.So I should of went 80% or of 502?