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CHW Variable Primary

CHW Variable Primary

CHW Variable Primary

(OP)
I have been asked to look into converting a chilled water primary/secondary system to variable primary.  Unfortunately I have little information to start with, so I am looking more or less for the "do's" and "dont's".

I have 2 centrifugal chillers, one 1,500 tons and one 1,800.  They currently do not have VFD's and part of this project is to retrofit them.  Likewise, the primary pumps will now have VFD's.  Each chiller has a pump discharging into an 18" header which circles the chiller room in to an 18" suction header.  

Teed off the 18" loop for hydraulic separation is another set of 18" pipes that go out of the room.  I have no additional information on the secondary side.

So if I put the primary pumps in VFD's what can I expect?  I forsee the secondary flow overcoming the primary and reversing flow in the common pipe, thereby degrading the secondary supply temperature.  There is no check valve in the common piping.    

What should I look out for?

RE: CHW Variable Primary

What is the pressure at the take-off into the secondary loop?  Do you have any idea of what's out there?

What do the pump curves look like?

If you only know what you've told us so far, don't do it.

**********************
"Pumping accounts for 20% of the world's energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies) http://virtualpipeline.spaces.live.com/

RE: CHW Variable Primary

When you go for variable primary, take out the decoupler from the system. The speed control of the primary pump is better done with a dp setpoint at the end of the headers. If the secondary flow exceeds the primary flow, the dp across the primary supply and return headers decrease and the primary pump speeds up to maintain the dp setpoint and vice versa.

The links given below deal with all variable speed chillers in detail and are very informative

http://www.automatedbuildings.com/news/mar02/art/hrtmn/hrtmn.htm

http://www.automatedbuildings.com/news/apr02/art/htmn/htmn.htm

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