Pressure setpoint for VFD control on refrigeration compressor

[BigBill53]

Hi,
I have been asked to control some refrigeration compressors with VFDs to save power. I've done a lot with VFDs over the years, but this has never come up yet.

I've done a fair bit of research and am pretty confident I have everything figured out - I was going to control the compressor motor's speed via a PID loop in the VFD. The feedback would be the pressure on the suction side of the compressor. There is just one thing I can't find any information about - how do I determine the pressure setpoint?

I managed to take a look at another existing setup at another factory, and this is how it is controlled. In this case it is trying to keep to a suction pressure of 3 BAR.

Does anybody know how to determine this? I have looked at every one of the manufacturer's datasheets and whitepapers, and scoured the internet and come up with nothing. Everything else is explained except for that one little bit of critical information, and yet from what I can tell this is a very commonly done thing.

 

[BrianPetersen]

The pressure on the suction side will be related to the desired evaporator temperature and what refrigerant you are using. Do you know those?

 

[BigBill53]

Sorry about the late reply to this thread - it took me a while to get the information I was after.

The desired evaporator temperature is -17 degrees Celsius. The refrigerant is R404A.

While I was snooping around the machine I found an existing pressure switch that may have the answers I am looking for. The cut-in pressure was 2.7 BAR and the cutout pressure was 2 BAR.

Am I right in thinking that if I try to control the speed of the compressor so that the low side pressure sits between this range (e.g. 2.4 BAR) then the VFD can slow the compressor down as needed. If you can't slow the compressor down any further, then eventually the pressure switch will cut out. The basic operation of the system will still be the same, just the VFD slows the compressor down if needed.

Thanks

 

[itsmoked]

That sounds about right.

You need to be aware of the oil situation. If your compressor speed drops too low you will likely starve the bearings and.. well that would be bad.

Hopefully you could install an oil pressure gauge in the oil circuit somewhere, especially for this experiment. Limit the speed to whatever value drops the oil pressure, something reasonable, and then as you mentioned let the suction pressure drop until 2BAR and stop the compressor altogether. Bring it back on at some reasonable hysteresis point 2.3BAR?

A lot of energy can also be saved via the evaporator fans, possibly with the condenser fan too. Monitor the exit air temp from the condenser and run the fan only fast enough to keep the air temp below X degrees, to prevent 'over blowing' the air.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

That oil pressure gauge will not be simple Keith. Most gauges measure pressure relative to atmospheric pressure. The oil pressure in a refrigeration compressor must be measured relative to crankcase pressure. Refrigeration compressor oil pressure safety switches are differential switches, with a connection to both the oil delivery port and to the crankcase.
The test gauge manifold may be used by connecting one gauge to the oil delivery port and the other gauge to the suction port and comparing the readings.
Most larger compressors use a gear pump for lubrication oil. A larger issue is oil starvation. Large refrigeration systems are designed for optimum flow velocities on both the liquid and the vapor lines. The flow velocity is a compromise between excess pressure drop and oil transport. If the velocity is too low to transport the carry over oil, then the crankcase oil will eventually end up distributed throughout the piping. In a properly sized system, the refrigerant velocity in all parts of the system will be sufficient to sweep the oil back to the crankcase.
You may be better to use itsmoked's suggestion to control the fans.

Bill
--------------------
"Why not the best?"
Jimmy Carter