heat pump loop water flow

[tbedford]

I have just reviewed a TAB report and 35 McQuay heat pumps indicated supply air temperatures ranging from 6 to 8 degrees C in the air conditioning mode. This cannot be correct and is too cold. I am wondering if excessive water flow through the heat pumps would lower head pressure sufficiently to reduce suction pressure and ultimately supply air temperature.
I haven't yet ruled out mis-calibration of the DDC sensors, although re-caibration of sensors was a part of the project scope.
Water flow in the loop was not measured... although EWT=27.2C and LWT=32C so delta T seems in the correct range.
Airflows across the coil were 500 to 600 L/s after air balance
Suggestions for further troubleshooting??

Tom

 

[Yorkman]

Tom, You'll have to excuse me my metric conversions are a bit rusty, but the condenser water temps looked to be normal for standard a/c 80F in 89.6F out. If the water flows were excessive your delta T would be much smaller. Your suction pressures would be more affected by low condenser water temps rather than too much flow.
You did not mention the tonnage of these heat pumps. But I would typically look for 350-500 CFM/ton on the airside at sealevel. In Denver Colo. 5,280ft I'd shoot for 450-600 CFM/ton. I'm sorry I don't know what these values would be in liters/min. Other items to look for all relate back to insuffecient air flow: fire dampers closed, dirty filters or evap coils, were these units selected for your area air density? I only mention it because it's something that some engineers overlook when designing systems operating above 2000ft. If these all check out, go after the sensors. Better yet verify your sensors first.

I'm not a real engineer, but I play one on T.V., A.J. Gest York Int.

 

[tbedford]

Thanks for the info.

The heat pumps are all 3 ton and at sea level...Victoria BC.
airflows are approx.1250 to 1400 cfm, very little static pressure (.1"wc)...and I am satisfied with the delta t's approx 10F for my water side. I am most suspicious about the temperature sensors.

Tom