heat pump problems

[ian-edwards]

Hi all

Need help with heat pump chiller whilst still got hair left. 2 identical machines, 2 circuits each. 1 circuit works perfect, other three all suffer high superheat and low suction pressure in cooling mode, work fine in heating mode - short of gas I hear you say? - no, weighed it in to manufacturers instructions, charge spot on. Duff txv perhaps? - no - new ones fitted to cooling circuit - no difference. Also checked: reversing valve - no leaks between high/low, equalisation line on txv is correctly fitted. Each circuit has 2 compressors running simultaneously, a 4 port reversing valve, 2 txv's, 4 check valves, brazed plate evaporator and air cooled condenser. System will regularly trip out on LP switch, but whilst running we are getting 1.9bar suction, 18bar discharge, 30k superheat and 110c discharge temp (R407c) Water on off is about 21/9.
Each circuit holds 40kg gas so taking ages to re-claim and re-charge every time we change anything (no valves) - driving me nuts so any suggestions gratefully received.

[install monkey]

make/model number might help, tev orifice clogged?

[The Viking]

Well, the obvious answer would be to check the subcooling and if >8ºK then adjust TXVs if <8ºK add charge.

Also as it is R407c, have you recharged with virgin refrigerant? Any leaks in the past that only been topped up might have changed the mixture...

What machine are you looking at?

[ian-edwards]

TEV's are new so not clogged, adjustment makes no difference, they are correct valves supplied by manufacturer, machine is Airedale KKHS50D, two of the faulty circuits have new refrigerant and are still not working, adding more refrigerant above factory charge improves back pressure slightly but it then starts flooding back in heating mode and the compressor starts to freeze up - with correct charge, heating cycle works fine.

[al]

Any chance the water connections are mixed up on the plate heat exchanger, inlet/outlet wrong way around?

al

[goshen]

Hi
as AL suggested what is your water pressure drop?
temp in - out cooling and heating ?
try ballancing the system on the water side this will have a significant affect on refregirent side !!
good luck

[stufus]

Check the flow rate and pressure drop on waterside and pull the strainers.
Cheers
Stu

[frank]

The delta t on the water side is a little high - is that with both circuits running in cooling?

Increase the flow to achieve around 6K on full load

[ian-edwards]

water flow temps vary considerably as i am running it in heating to 40c outlet temp to create load for testing on cooling cycle, so system is seeing water outlet temp vary from 40 down to 6c - at all values we are getting same symptoms of high superheat and low suction pressure. If water flow was too low we would get low suction pressure but low superheat because the txv would be throttling the valve, conversely, water flow too high would give high superheat but high suction pressure as well - neither scenario fits the symptoms i am getting so i am not looking too closely at water side. I am coming to the conclusion that the tev may be undersized - it is the correct one according to the manufacturer but the chillers are about 8 years old and may have been modified - next step is to check all the components and see if they balance correctly.

[frank]

What is the rated duty of the chiller and what flow rate have you got?

[marc5180]

TEV's are new so not clogged, adjustment makes no difference.

Did you get the the bottom of this after all?

I know it's 4 months late but......

Did you measure the sub cooling and did you leave it long enough for the superheat changes to take effect?

What happened if you held the TEV bulb, did it flood the coil, if not maybe your right and the TEV isn't sized right.