Centrifugal Compressor part-load efficiency

[new2hvac]

I'm trying to identify the part load efficiency on a chiller with a centrifugal compressor. I have the installation specs and it appears to be a little over 0.5kW/ton. I don't know if this was measured at full load or some portion of full load.

How does decreasing the load affect the efficiency? I suspect it decreases by some amount as the vanes close for lower loads.

Are there typical numbers for centrifugal compressors or does it vary by the manufacturer (it's a Trane)? Is it a linear relationship?

Thanks!
Brian

 

[quark]

Generally manufacturers give full load efficiency. You can get the part load data on request. I suggest you to recheck the figure 0.5kW/Ton even for full load condition (for ex. for what cooling water temperature and what chilled water temperture)This is the lowest figure I came across with so far. Even VFD controlled centrifugal chillers consume around 0.7 to 0.8 kw/ton at full load conditions. Chiller efficiency will be low at part load condition and is not linear.

Regards,


Eng-Tips.com : Solving your problems before you get them.

 

[KenRad]

new2hvac,

Your kW/ton will be lowest at or near full load, and will rise non-linearly as the load is decreased.

0.5 - 0.6 kw/ton is a typical full load value for a newer Trane centrifugal using an 85F condenser inlet temperature, 44F chilled water outlet. This can be improved considerably if condenser water temperature is lowered. For example, I have two 700 ton Tranes that will get as low as 0.43 kW/ton when I lower the entering condenser water temperature, as can often be done when low load coincides with low outside wet bulb. Of course, this will result in some increase in power consumption at the cooling tower, but if you work it right, the net effect will be less power consumed for the entire system.

The local Trane rep should be able to give you part-load efficiencies for whatever conditions you request.

---KenRad

 

[imok2]

carrier claims the following:
Carrier has significantly reduced the power consumption of HFC-134a positive-pressure chillers. The result is ultrahigh energy efficiencies, giving the Evergreen chillers the highest efficiency of any chlorine-free chiller in the world.

With the use of unit mounted refrigerant cooled (19XRV) variable frequency drives, IPLV values in the range of 0.35 kW/ton can be achieved.

Roger

 

[Cooky]

I work for Carrier, and have selected many centrifugal and screw chillers.

Normally, centrifugal chillers are selected at 100% duty. If the customer requests part-load performance, there are two options to do this. The first (and most common) method is with constant condenser water temperature. The second method is to lower the condenser water temperature as the % chiller load is reduced. I'd normally offer a part-load selection in 10% increments, from 100% down to, say, 30%.

Lowering the condenser water temperature for a part-load selection, will give a much higher COP at part-load, so you need to be aware which method the manufacturer is using.

The relationship between % part-load and COP (efficiency) is generally non-linear.

Brian: I'd advise you to speak directly to Trane, they are the only people who can answer your specific question.

I hope this helps.

Cooky

 

[new2hvac]

The local Trane rep has been not been very responsive to my inquiries. The 0.5 kW/ton comes from the original installation specs. I base this on a nominal 700-ton chiller with a kW draw of 382. This, of course, does not include the kW of the cooling tower, fans and pumps in the system so the system's kW/ton would increase slightly.

The specs also include CHW and CW temperatures, but not flow rates so I cannot calculate the tonnage on which the kW draw was based.

Are there typical efficieny decreases associated with a Trane centrifugal that was mfg in the early 80s?

Thanks for all the replies.

Brian

 

[Centrif]

Be careful in trying to lower the condenser temp on a compressor with a hermetic motor. The manufacturers usually cautions us against this and suggests a min of 85 degrees or so. The delta P between the evap and condenser is what drives the hermetic motor cooling. Drop the condenser to improve efficiency and the motor may run hotter.

 

[jherbert]

hi,

Trane can provide a graph for your machine.
They are probably being lazy, write to Trane
HQ and let them know HVAC chiller experts
(eng-tips) from all over the planet are noting
Trane's lack of tech. support.

Why would one spec a Trane unit if they don't
provide the necessary tech. support?

 

[Centrif]

Your local Trane rep should be able to provide you with all of the information you need. Was your unit performance tested at their factory? If so you can get a copy of the factory test results. Usually these types of tests are performed at 100%, 75%, 50% and 25% load. Their test blocks are ARI certified so you should get accurate information.

Cooky -

Can the Carrier machines utilize lower condensing temperatures but at full load? Seems like 100% motor current but with reduced condensing temp would not provide enough delta P to adequately cool the motors. I am sensitive to motor cooling issues after having had a motor burn...(insulation cracked and shorted to the stator)...yuck what a mess. However if I can get 100% capacity/load at reduced tower temps that would save me money.

 

[Cooky]

Centrif,

I would not select a Carrier 19XR centrif with a condenser water temperature below 18 deg.C, normally, the lowest condenser water temperature is about 20 deg.C. The chilled water temperature will also impact on the selection.