Portable chiller shutting down on High Head

[Arkma]

My customer has a air cooled 10 ton portable chiller. r-22, copeland scroll compressor, sh-12, sc-17,. ls psi 90, hs psi 340, ambient is 95. Unit is consistently shutting down on high head. We have manufactured this chiller the same way for many years. in fact the customer has two other chillers of ours, both 10 ton and operating properly with normal pressures. One thing that is odd, we tripped the head pressure switch by blocking off the condenser the one day. On the next restart all our pressures were fine and the system ran well all day. the next day it was right back to it's tricks again, shutting down on high head. Anyone have any ideas?

[rbartlett]

is there any form of head pressure control occasionally they can be a tad slow to take off and the head soars before the fans catch up.?

dodgy hp trip
dodgy fan
high load surge
very warm water on start up

first thoughts only-gotta go out ;-))

cheers

richard

[frank]

ls psi 90,

What are the water entering and leaving temps?

[Arkma]

65-70 entering

Asking for 50 leaving, obtaining 55.

[frank]

I would assume that 10F differential across the machine is acceptable and so the flow rate seems OK. The entering temp is not excessive which should not cause the high LS pressures. Is the air flow across the condenser within spec? Is the gas charge correct? It is difficult to fault find from a distance so numbers are all important.

A partially blocked condenser will give the symptoms you describe but then again so will other factors.

[Arkma]

10F is acceptable, Air flow good.

I am going to bank on th echarge is high.
What about non condensables?

[binman54]

Your head pressure of 340psig seems high. Typically we design for 260psig with a high pressure cutout set for 350psig. Could you be operating too close to the high pressure cutout? Could the cutout be set too low?

We've had the same thing when the water is too hot on startup. The compressor can run at high suction (and possibly low discharge) and, often, more subcooling than we expect. There is a higher THR than allowed and the result is a condenser that is too small. The head pressure rises faster than the fans/head pressure controls can react and the units trips on high head. This is the same as blocking the condenser air flow. A CPR valve or a TX valve with a pressure limiting head can solve the problem.

Monday morning when the customer forgets to turn on the chiller until the water gets really hot is the usual time for an irate phone call. Often we find the equipment works best when a mechanic is standing by and fails just after he leaves.

Brian

[Andy W]

Reminds me of a problem some years ago when I was a contracts manager, one of our guys installed some Airedales on the Isle of Wight, on commissioning he could not get them to run, just tripping out constantly on HP which he could not understand, our engineering manager asked me to pop down to take a look, the cause of the problem...........he forget to let the nitrogen out of the units, for some reason thought that it was refrigerant, the result, we had to reclaim all of the gas from 3 systems, arrange for more gas to be delivered on the overnight ferry and recommission them again, I was asked to stay until hand over this time.

Now back to your problems, I would test for non condensables first, check the calibration of the HP switch ( particular if it is a Danfoss ) I presume fan motor and blade are the right size, no fan speed control fitted, expansion valve not oversized so requiring more refrigerant to shut it down, is the compressor sweating back excessively, is the TEV bulb fitted correctly and tightly and I would fill the water tank with warmer water to try and create the fault, I once had a system that supplied chilled water to a machine via a motorised 3 way actuator valve, depended on the stage of the process either cold or hot water was required, these valves sometimes leaked past the seal and bled hot water into the cold tank causing increased load.

[benijoseph]

INTERESTING:

After compresing gas into condensor and next day if unit behaved well it means flow rate of the gas is not proper or gas is mixed with some other gas or gas might be poorly recycled gas, Try pouring some cold water on condensor once! If works well, change the orifice size of the expansion valve or reset the setting of the valve or change the positon of sensor bulb try!

beni joseph,

[rbartlett]

check the charge

cheers

richard

[Arkma]

Turns out I had them pull the charge. It weighed out almost 2# heavy. Thus an overcharge was evident. Thanks for all the input guys!