Need some advise on a PAC500DZ-T4-S.
There are three of them, an outside contractor called us in.
Of the three machines, (under warranty) two have tossed compressors, one has had a TXV replaced in the past few weeks.
I need to evaluate all three machines, but for today I need to focus on the one on which the TXV was recently replaced.
-complaint is that this unit goes off in high head while under low ambient conditions. The fans are cycled by ambient temp. settings are on, 55, 62.
This unit floods its condenser for low ambient operation. It floods the evap and has unloaders for capacity control.
The max setting for the low ambient control is 225 psi. today I ran all three fans an 65 ambient and had 300 psi head. I turned in the flooding valve and pressure dropped to 220. this is an issue because the valve shouldn't hold back above 225, but it's out of my way for the time being. Im not sure this justifies condemning the headmaster or not, to be honest.
-Next issue. Contractor says the data center has an 18,000 BTU load, and I have 50 tons of cooling per machine. I did not have the unit short cycling, but it did satisfy fully loaded (there are two compressors, one unloader each). there are controls for the unloaders, set to 60 PSI. the unit satisfies at about 61 psi. Contractor says it short cycles, this this is the cause for the failures.
The building engineer said he DID change the settings on the unloaders of all machines. He also does not remember the default setpoint, but claims the unit stopped short cycling after he made the changes. (He does not realize that he set them in the opposite direction, IE now they can't unload, therefore they would satisfy more.)
-next issue. I turned the unloader to 80 psi (unit was ten deg above setpoint). When the switch made to unload, the compressor unloaded for about a second then shut off as if it had satisfied. After the time delay it came back on. I jumped the unloader switch and the compressor shut off again. no alarms. Put a tattletale on the motor protector and it did not trip. Tomorrow I will cut the wire on the unloader and jump it again to determine if it is a bad unloader, or factory miswired.
This unit is pretty new and has a colorfull history, a bad unloader would be odd. I suspect that the engineers observed it shutting down at unload and thought that was a short cycle.....thats why turning the unloader down gave them resullts.
-big issue. My company may take this job over on a contract (we already have the big chillers). I need to have these units set up properly and the manufacturer can't really do much without an autopsy of the failed compressors. I could use some advise for setup.
I need to balance condenser fans, head pressure control, the hot gas, and the unoaders along with the low load.
I'm pretty sure I just wrote all this for my own opportunity to think about it, (theres no way anyones reading all this) but I'll throw in some values.
discharge psi- 200
liquid temp- 92
discharge S.H.- 50
suction PSI- 61 (70 entering accumulator, 61 at compressor)
suction S.H.- 13
cond approach- 39 (air cooled)
evap approach- 14
setpoint (entering)- 50
barrell TD- 6
no drop at drier
I'm going back tomorrow. I'm not exactly sure where I'm going to go with the unloaders and hot gas. If anyone actually took the time to read all this, thanks! Any advise is appreciated. I really want to get this right.