Centrifugal Lift as Pressure VS Temperature
Holden Shamburger
Chiller Training | Centrifugal Chillers
3 minute read
Centrifugal Lift as Pressure VS Temperature:
I am going to share a thought process I’ve been working through on how Chiller technicians think of centrifugal compressor lift.
This was shared with me by someone else and after some time/thought I feel it could help.
There is still some nuance to this to be aware of.
One of the main concerns we have with centrifugal compressors in the lift across the impeller.
If we push the lift too far the compressor will surge.
Bad for impeller, bearings, efficiency, etc.
I was taught, Lift = Discharge pressure - Suction pressure
Most of the time condenser and evaporator pressure is what we have available.
These are close enough.
To be the most accurate here we would need to reference the compressors surge graph to track where we are on the surge curve.
Unless you work for the factory this graph is not typically available to you.
So we are left using our experience and understanding of the technology to determine our own patterns.
Another factor, using pressure as the measurement then with each refrigerant we could have drastically different numbers. Low vs high pressure chillers as examples.
Another factor, lol, the lift we can sustain changes based on load. (flow for the technical out there but flow is varied based on load…)
This means to say one set of numbers is the maximum safe lift without surging per compressor won’t work. You’ll see what I mean in a moment.
Another factor, is the water flow, heat exchangers, refrigerant charge, refrigerant quality, etc…..
It’s ok if this feels complicated. It is. It gets much easier to keep track of with study and experience as all these factors start to become more apparent.
Now to my point. Try this.
Lift = Condenser Saturation - Evaporator Saturation
This is not a perfect solution but I feel it has merit.
So let me give some examples of how I have seen these compressors operate in the field.
These examples are the amounts of lift I expect the chillers can do before they push into surge conditions based on load with a 45F setpoint.
YK:
Low load, 25-30F
High load, 40-50F
CVH:
Low load, 30-35F
High Load, 45-55F
These are Degrees of Lift.
Something to ponder on for yourself.
MTT
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