AC Compressor damage in VAV system

[t1111]

I have now gotten liquid into the compressor two springs in a row. Not Good! After several sleepless nights I think I see a problem. Our building works well in normal operation, We have 16 VAV zones wih a total air flow capacity of 18,000 cfm and a 50 ton AC Condenser unit. The problem occurs when we rent the dining room to a 300 person dinner/dance party on Saturday night. The 3 vav boxes serving the dining room will be supplying 4500 cfm and demanding maximum cooling while the rest of the building is shut down and pulling maybe 3000 cfm and returning it at 65F. The mechanical contractor's only advice is to run the boilers and heat up the return air. we would prefer to have some criterion like RAT - DAT times estimated CFM of flow to shut off the compressors before damage occurs and subject our guests to warmer temperatures. Anyone know how to do that?

 

[MintJulep]

How about pressure and temperature sensors in the compressor's suction line. When super-heat gets too low do something. Maybe hot-gas bypass.

 

[imok2]

This control may solve your problem.

http://www.emersonclimate.com/Documents/FlowControls/Final FC-TD EC3-X32.pdf

 

[MintJulep]

Well that takes all the fun out of it.

 

[KiwiMace]

That nice controller might prevent you from damaging your compressor, but you describe a physical problem that a controller will not fix by itself. Mint's suggestion of hot gas bypass is valid and much more efficient than running the boiler. If you don't have HGB then thats not much help. You need to look at how your system is supposed to operate at low loads and assess what you need to do.

What kind of compressors/staging do you have? Your numbers above indicate around 25% loaded which should be within reach of a multi-scroll or recip in a system this size, I would have thought.

 

[imok2]

I have had similer situations as this where
we installed a seperate cooling system for a srecific area and not use the main system to cool the area and when not in use the secondary system would be locked out
your only talking about 12 tons

 

[BronYrAur]

Do you have a suction accumulator on the system? I think it is always good practice on a split system with VAV.

The other thing you could do is charge your dinner parties more money to cover the compressors - ha ha.

Seriously, I like the hot gas idea and i believe a suction accumulator is necessary. If you contractor can only tell you to run the boiler, then you need to find a new one!!!

 

[t1111]

Many thanks for all of your helpful suggestions.
I think I'm about to be eased out of this particular mess and won't miss it at all!

 

[AnotherEllis]

Might be a nice application for a RAWL device.

http://www.ellisconsultingengineers.com

 

[cturg]

Ok, so the compressors are taking a drink and wreaking havoc inside. More than likely you are having a low load situation and the condensing unit cannot shed enough capacity.

How many stages is the condensing unit/evaporator set up for???
How many circuits, TXV's per circuit, etc. What is controlling the staging? What are the set-points? Start simple.

If low load is your culprit then capacity reduction on the refrigeration side would be best. Let us know what you have for staging and other details and we can get the proper solution.

Cheers!