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kizza231188
10-05-2009, 03:47 PM
Went to a med sized system at a local fast food restaurant. Its 2 stage, with a shared evaporator, in which each stage has half of the evaporator (upper/lower). TXv system, r22. System has suction accumulator before compressor. Outdoor unit is 13 meters or so from evap. Outdoor unit is at ground level.

Stage one was out on hp, condenser fan was dead. Stage one has a Head pressure controller (fan speed), which has a variable set point. Fitted the new fan, and ran the system. Noted that the compressor ran for abut 6 seconds before head pressure rose high enough to start the fan. Sight glass full.

System readings (stage one) after being run for a few hours:

High side was 1600kpa (230 psi) converts to 43C sat temp. Liquid Line temp after condenser = 27C therefore Subcooling = 16 K

Low side was fluctuating from 345-365 kpa (50-52 psi) fluctuations occured over around one minute, gives sat temps -3 / -2 C.
Suction Line temp near compressor 7.1 C = minimum 9 K Superheat at compressor

Suction line temp measured at evap was fluctuating 4.3 - 8.3 C, over a period of one minute. Gives superheat of 6 K at evap.
Seems like tx valve hunting to me, although the valve is externally equalized.


Evap air on / off was 18C / 11C. --> dT = 7K

Condenser air on /off was 17C / 34C. --> dT = 17K

Apart from the valve hunting, does the system seem to be working properly? The high subcooling seems to be pointing to overcharge... also, the head pressure of 1600kpa (230 psi) seems a little high, perhaps the head pressure controller should be set a bit lower.
I understand that the evap temperatures are both low for aircondtioning but for comfort of staff, they have the set point at 17.5 C, as the radiant heat from all the kitchen equipment is quite high.

Will be returning there shortly to get discharge temp.

Gary
10-05-2009, 05:07 PM
Remove refrigerant until the subcooling is 8.5K/15F.

Gary
10-05-2009, 06:01 PM
As a rule, when there are over and under coils the lower coil must be the first stage. Depending on the particular design, if the upper coil is active and the lower coil is not, then condensate from the upper coil can drip down onto the lower coil and re-humidify the air as it flows through the lower coil.

You are showing a very low delta-T. I'm wondering if you are measuring mixed supply air, rather than measuring the air as it leaves the coil? You need to measure as close to the coil as possible.

Does the air handler have bypass dampers?

I'm wondering also if the first stage has unloaders?