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HHC
14-07-2006, 08:33 PM
Head pressure should correspond to the pressure of ambient plus 30-35F. My question is, how high above this psig is cause for alarm? If the compressor isn't overheating and the system is cooling, but the head pressure is 20-25 psig higher than it's supposed to be, is that worth worrying about?

US Iceman
14-07-2006, 09:05 PM
Hi HHC,

Welcome to the RE forums.



Head pressure should correspond to the pressure of ambient plus 30-35F.


That in itself doesn't mean too much. It depends on the type of condenser. The numbers you are using would be based on much older systems for air-cooled condensers. For water cooled and evaporative condensers the numbers would be different.

I think for the most part you can "assume on newer air-cooled systems" the difference would be closer to 15-20 degrees above the highest ambient. (I'm using the Fahrenheit scale here). This depends on how the system was designed, so these are really guesses without that detail.

As a "general rule" this might be a good way to initially evaluate a system. If the discharge pressure is higher than the refrigerant saturation temperature of ambient temp. + 20 degrees, this would mean something is not quite right.

Dirty condenser, worn fan belts, etc.

Anytime the discharge pressure is higher than it needs to be you are using excess energy and increasing the stresses placed on the compressor. You are also loosing some cooling capacity doing this.

Is it worth worrying about?... I would look into it if it was me.

HHC
14-07-2006, 10:27 PM
Thanks for the quick reply. The unit is a 20+ year old Beverage-Air 3-keg direct draw beer cooler running R12, fan cooled, with a new hermetic compressor. Condenser is clean, but does have some rust on it.

With regard to increasing stress on the compressor, would this be reflected in the amperage? My amp readings show it to be about 6.3 amps; the compressor is rated at 6.7.

US Iceman
14-07-2006, 11:11 PM
If the condenser was able to have cooler air flowing through it, the discharge pressure should decrease. That would reduce the amp draw.

If the compressor was recently changed, was the system evacuated with a vacuum pump? If the system has air in it, the discharge pressure may be higher than normal also.

The discharge pressure may be a bit higher also due to pull down load with a warm box.

It's hard to troubleshoot something you can't see or hear.:confused:

HHC
14-07-2006, 11:39 PM
Yes, a vacuum pump was used, but I don't think it was adequate. It was an older single-stage pump. Do you have any estimation of how much pressure could be attributable to a small amount of air in the system?

By the way, the cabinet is cooling adequately (cuts in at 40F and cuts out at 32F); all readings are being taken at the end of the cooling cycle, just before the compressor cuts out.

A point worth noting is that even with the head pressures described, the suction is reading between 10-14 psig.

US Iceman
15-07-2006, 04:19 AM
Do you have any estimation of how much pressure could be attributable to a small amount of air in the system?


Here is something to check:
1) Record the discharge pressure
2) Measure the liquid refrigerant temperature leaving the condenser.
3) Find the pressure for the saturation temperature for R-12 liquid.
4) Subtract the pressure for the liquid temperature from the discharge pressure.

The difference in pressure may be attributed to non-condensables (air, etc.) or subcooling.

Since it is hard to tell the difference between subcooling or non-condensables at this point you have two options.

1) recover the refrigerant from the system, and completely evacuate the system.

or,

2) let it run as is. If the system is cooling OK you might be spending more money fixing something than it may be worth.

Andy
15-07-2006, 10:19 AM
Thanks for the quick reply. The unit is a 20+ year old Beverage-Air 3-keg direct draw beer cooler running R12, fan cooled, with a new hermetic compressor. Condenser is clean, but does have some rust on it.

With regard to increasing stress on the compressor, would this be reflected in the amperage? My amp readings show it to be about 6.3 amps; the compressor is rated at 6.7.

R12:eek: probably one of the drop in replacements, they do run 10 to 15 psig higher;)

Kind Regards Andy:)

HHC
15-07-2006, 05:23 PM
2) let it run as is. If the system is cooling OK you might be spending more money fixing something than it may be worth.

That's at the heart of what I'm trying to assess. I just want to make sure I'm not setting myself up for problems later on.

By the way, condenser subcooling is running between 21-24F.

Thanks again for the great input!