Just move up to grand Junction and am working for a outfit that does alot of commrercial refrigeration, Anyway I have run across at least 3 systems walk in freezers that are running 404a. Problem I'm seeing is high head pressures and or compressors out on internal over load. Usually when I check them they are running low superheat 3-9 and high subcoolng 40+. When I start to adjust charge it willl not keep temp inside box. Only happens when ambients reach over 105. I suspect the systems are undersized and the tech before me is overcharging the system. Also does 404 usaully run at such hi pressures, 350 to 370 with 105 to 111 outside.one more question where is the best place to check sub cooling? at evap, liquid line or at condensor liquid line? And I have always checked superheat at evap, and used pressure at compressor suction port, is this correct. These systems are measured in ounces and I'm use to working on large ac systems, It's been a good learning experience and i've really had to study up on all the new freons and such.
Not sure but how is air flow or when was coils cleaned last. also 404a @105 amb. head press should be aproximately 377.8 but that is a ballpark idea
is this a txv system might want to check bulb insulation
and or adjustment as alast resort
Well, 3-9 degrees of superheat at the evap is probably fine.
40 degrees of subcooling is hard to believe in that ambient, but also, is OK.
Check subcooling right before the TXV.\
Do these systems have crankcase pressure regulators? If not, they might need them to preven the compressor from over-amping after defrost, which would cause the overloads to trip out.
You say that these systems are "measured in ounces", are these tiny little close-coupled walkin freezers? Usually, a walkin freezer with a reciever won't be "measured in ounces", charge-wise.
And, 404a is NOT new....
I also, almost ALWAYS check superheat at the compressor suction service valve. Remember, one of the two main purposes of checking superheat is to prevent floodback to the compressor. If you have more than 20 degrees, but less than 40 degrees at the SSV, you won't be flooding, or overheating a (refrigerant cooled) compressor.
3-9 degrees measured at the evap, again, will probably be OK.
Saw that "measured in ounces" and really cracked up. At those ambients, think I would be more concerned about superheat at compressor. Keep them thar cond. coils clean
If you really know how it works, you have an execellent chance of fixin' er up!
Tomorrow is promised to no one...
anyone do any research or know if superheat at the compressor and at the evaporator, which one is more..better.
i know compressor sh is good for the compressor, but what about why its there in the 1st place..to make a box cold..
what if they dont...jive?
MOP TXV's would also be advisable
I find the 40ºF subcooling really hard to accept on a typical commercial low-temp system unless the system is waaaaayyyy overcharged......like as in enough to fill the receiver and flood half the condenser coil.
Originally posted by azhogdog
........Problem I'm seeing is high head pressures and or compressors out on internal over load. Usually when I check them they are running low superheat 3-9 and high subcoolng 40+. When I start to adjust charge it will not keep temp inside box. Only happens when ambients reach over 105. I suspect the systems are undersized and the tech before me is overcharging the system.....
A gross overcharge would help explain the high discharge pressures and cut-out on overload. If it's not overcharged, I'd be looking to find where that "subcooling" is coming from.
[Edited by icemeister on 07-17-2005 at 07:25 PM]
you are correct I overlooked subcooling and it is sign of over charge
Get a print out of the compressor curve which shows what amount of work that unit will accomplish given the temps and pressures you have read.
This will tell you if the unit is working within ballpark guidelines.
The answer to your question about which superheat is more important ....
The overall system superheat is most important.
If the superheat at the evap is not right ... but it is right at the compressor ... then your compressor is gonna live.
However, if the superheat at the coil is fine but your unit takes a dump ...well .... you can answer that question yourself.
If the condensing unit is sized correctly for the load and the evaporator is correct for the load ... then you have a decent chance of being within balance.
With adjustments and high ambient conditions ... you can exceed those factory designed parameters and begin to lose effeciency/ performance.
But as long as you maintain that superhat at the unit ... at least you wont be droping compressors like flies of a hot day.