Question:
I have a couple of questions on a 20 ton, 2 stage commercial air
conditioner and I would appreciate any help.
First, would a high suction pressure (80 psi. with return air at 75 F
and discharge pressure of 180 psi.) and a high superheat at the
compressor while the compressor is running at about 2/3 of rated
amperage indicate an inefficient compressor?
Second, if the compressor is a scroll, what method is used to determine
if it is inefficient?
Answer:
More information is needed to pin down your problem but you could
just simply have a restriction problem or low refrigerant charge. Your
symtoms represent an ineffeiciant compressor except the high super
heat. If your compressor was failing your super heat would not increse
persay, (unless of course you where sensing directly at the compressor
and you where getting back feed from the high side dischage valve, and
in that case would be obvious because of the extream temperatures
measured) Remember your metering device doesnt know if your compressor
is pumping or not all it knows is that, if properly charged,and if a TXV
is used, that a specific superheat is to be maintained regardless the
pressure. As a matter of fact you may even have a lower superheat
because of the lack of heat transfer tacking place. And with a captube
feed, well thats constant also regardless of efficiantcy of compressor.
Your symtoms could represent low charge or restriction: Theres a point
at which a system can be undercharged and produce high super heat, high
suction and low head presure. The best way I can discribe it is that at
some point when a system gos from being properly charged to being
undercharged there is enough refrigerant (%of vapor and liquid) to
maintain a higher presures within in the evaporator. Kind of like a
refrigerant tank. If you where to have your gauges connected to a full
refrigerant and begin to release gas, you could watch your gauge
pressure begin to drop until you shut off the tank. You would continue
to watch your presure raise to meet atmospheric temperature until the
percentage of refrigerant could no longer meat the expanstion of the
tank.
