R717 compressor discharge control

Hi Everyone,

This new chiller-duty refrigeration plant under my supervision is currently set on the HMI a discharge pressure of 950 kpa. Heat rejection is via PHE condenser with cooling tower water flow. The cooling water from the CT is set to 24 Deg C and never changes so the discharge pressure/temp keeps changing due to compressor load changes.

In this setup, I am unable to figure out how the DP of 950 kpa is maintained when the process variables are not in any algorithm. On cold ambient the DP lowers to anywhere around 650 kpa and at warm ambient will rise to almost 1200 kpa? Is this wasting energy and/or should the compressor discharge be wet bulb algorithm controlled? I'm very keen to know your thoughts here. Thanks

I should say, try to go maintain as low as possible the HP, this will give you the biggest savings...waste heat is something for free but the main goals should be saving electricity in the first place.

Mataura,
I think should be controlling fans on cooling tower on WB algorithm to gain maximum efficiency with fan control.
Do you have VSD fans on cooling tower?
3 way valve on water circuit?
I can’t understand sump temperature control & set discharge pressure.

Cooling tower sump control can sometimes be used for water cooled oil cooler control temperature control.

Quote:

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Originally Posted by RANGER1

Mataura,
I think should be controlling fans on cooling tower on WB algorithm to gain maximum efficiency with fan control.
Do you have VSD fans on cooling tower?
3 way valve on water circuit?
I can’t understand sump temperature control & set discharge pressure.

Cooling tower sump control can sometimes be used for water cooled oil cooler control temperature control.

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The cooling tower fan is on VSD control. When compressor is running on light load the CT fan will slow or stop and yet the DP can get to as low as 600 kpa. Being a PHE condenser, could the gas velocity damage the stainless steel plates by pitting them? This PHE has been problematic and many platenhave been changed out due to pitting and leakage.

Mataura,
Common things that can damage plates are poor water treatment, excessive chlorine causing stress corrosion cracking.
High discharge temperatures especially if reciprocating compressor can cause gasket failures.

Attachment 16208

Here are the common values for max chlorine concentration in heat pump applications.
Stress corrosion can occur under a déposit....
If water start boiling you also get cavitation at low water flow.