View Full Version : High Pressure problem HELP!

16-07-2014, 11:39 AM
Hi guys,

Hopefully someone can help here:

We've been having problems with a York YCUA150 condenser feeding a cooling coil in an AHU.

I was originally called to site due to having a leak on the system at the outlet of the compressor discharge pipe. Once the leak was repaired, we charged the circuit with 29kgs of R407C (as found in the original commissioning data).

When I run the system up, within minutes the discharge pressure rises and keeps rising until it cuts out on 28bar. However, the condenser fans run ok, the coil appears clean and free of obstruction, and all solenoid valves are open. The system has 2 circuits that are operated through the same condenser (seperate coils either side) and 1 circuit runs fine all the time.

My operating readings are:

Ambient temp - 23 deg C
Standing pressure - 9 BarG
Suction pressure - 4.5 BarG constant
Suction saturated temp - 0 deg C
Evap outlet temp - 7 deg C
Sucion s/heat - 7K
Suction pipe temp at compressor inlet - 4 deg C (I find this odd)
S/heat at compressor - 4K (see above)

Discharge pressure - 25 bar (I manually cut compressor at this pressure but all following readings are relative)
Discharge temp - 61 deg C
Discharge pipe temp - 61 deg C
Discharge superheat (desuperheating) - 0K (very odd)
Liquid line temp outlet of condenser - 32 deg C
Subcooling - 29K
Liquid line temp before TEV - 32 deg C
Suncooling - 29K

As you can see from my readings, there is no discharge superheat at all it seems. I find it odd that the discharge pipe at the compressor would be the same temp as the saturation temp in the condenser.

Another reading that concerns me, is that there is no temperature difference across the evaporator....after the expansion valve I measured 7 deg C, and at the evaporator outlet I measured 7 deg C, yet the evap coil is in the same AHU as th circuit that runs continuously and is fine.

I also find it odd that the evaporator outlet is approx 7degC but my compressor inlet is 4degC...

Unfortunately I didn't take many readings from the circuit running fine, only that the Suction pressure was also about 4.5 bar and the discharge pressure was approx 18.5 bar.

If I remember any other info I will add.

Any feedback would be very helpful, cheers guys.

16-07-2014, 12:42 PM
Sounds like could be non condensables in the system.

Is it possible air or nitrogen could of been trapped or left in the ref pipe circuit?

had a similar situation a few years back on a Daikin r22 split ducted system my apprentice attended, he said the hp switch was faulty as the unit would run for a few minutes then knock out on the hp. I told him to keep looking but anyhow.. Part arrives he reclaims gas removes and welds in new hp sensor, vacs unit recharges machine with the recovered gas - at this point I turn up to check on him just as he turns unit back on... unit runs for 2 minutes and cuts out again.

Turns out another service co replaced a compressor 12 months earlier and unit hadn't really worked since - we reclaimed the gas out of the machine and did a pt to bottle temp and found it out to buggary.

Gave the system another good vac and weighed in a fresh charge, now up and going pressures fine.. I could of strangled him for not checking unit hp pressure when a system is tripping on hp lol.

16-07-2014, 02:14 PM
If discharge temperature is cooler than the condensing temp, liquid is being vapourized by the compression stroke, wet compression. Liquid flood back somehow but your superheat is 7 degrees so one of your readings must be wrong.

The problem may well be non condensables but looking at your standing pressure and your ambient temperature these aren't too far away so I'd suggest the problem to be due to the system to be overcharged.

Silly question but the condenser could have been cleaned and the fans are running in the correct direction?

16-07-2014, 03:22 PM
Did you charged R407C in liquid phase?
Check condenser pressure while fans are working and compressor is stopped for about 10 min. Second circuit also turned off. Measure temperature of air entering in condenser. Post your measuring results here.

The MG Pony
25-07-2014, 04:32 PM
I'd be on the side of non condenables too, as if the coils are clean with good air flow that is all that remains. The non condencibles would allow liquid refrigerant to return to compressor causing the 0 super heat.

25-07-2014, 07:17 PM
Non-condensables make good sense for me too. however dont ignore the silly question asked by marc5180 regarding the direction of the fans. ask yourself what has changed since the unit was last working fine?