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# Chiller VFD installation2

## Chiller VFD installation

(OP)
All you HVAC engineers! Question? Can a VFD or soft start controller be installed on a Rotary Liquid Chiller? I have received conflicting info regarding this action. We are trying to reduce the initial elec. demand on startup. Any ideas?

Run1on

### RE: Chiller VFD installation

Run1on,

First of all, if your system is not a primary/secondary system, I am not 100% convinced variable flow through a chiller is all that great.  I know that some manufacturers say that it can work with their chillers, but some manufacturers still don't believe in it.  The only place I have seen VFD's on pumps work is on a primary/secondary system (or as ASHRAE calls it now, a "decoupled system").

My personal opinion is that water flow should be constant through the chillers for stable control.  A chiller typically controls discharge water temperature, and the heat exchange rate in the evaporator is directly related to flow.  Therefore, your system is trying to control too many variables.  Control becomes unstable, and if your system is even semi-critical, unreliable.

With a decoupled system, your chiller circulating pumps are lower head pumps and therefore have smaller horsepower requirements.  The main system pumps are then VFD.  This reduces starting hp for the chiller pumps and gives you lower operating hp's on the delivery side.  This way the chillers only have to control leaving water temperature with a constant flow.

Not sure what your system is or how it operates, but hopefully this gives you some insight as to some of your choices.  Maybe a little more detail on the system could help to get more detailed answers.

### RE: Chiller VFD installation

We have scheduled VFD on chillers and pumps for soft start.
It does help. The other \$ saver is not to start the pumps and chiller all at the same time

### RE: Chiller VFD installation

We are about to upgrade our Mcquay Screw Drive Chillers with "soft Start" kits from the manufacturer as an upgrade due to previous compressor failure after starting up with liquid at the intakes. THEY claim this will curb the problem, it has only been out for about a year from Mcquay but with positive results, howeve it is noted certain internal compressor modifications have to done as well.

### RE: Chiller VFD installation

(OP)
The chiller in question is a Trane model RHTB 300 ton unit 274 KW, 480 v

Run1on

### RE: Chiller VFD installation

I have a site w/ 4 rthb 250  that have soft sart and have had no problems with them, they are cutler hammer and were factory installed.I believe they are availible as after maket products through your local trane rep.As far as vfds, I dont think they are availible for screw chillers at this time just centrifugals.

### RE: Chiller VFD installation

(OP)
Thanks, David.

As far as I have been able to determine York is the only manufacturer that has taken on solid state controls for screw chillers. I have been told that due to the pressure imbalances that can exist in a screw chiller (front to rear) it is a difficult design challenge to achieve equal pressure before starting. I was just looking for a way to reduce the amount of inrush current needed as we are on a demand meter system. I have been told a "hot gas bypass" can run the chiller in a constant state at a limited load. It is more expensive for us to start the unit than it is to run it.

Run1on
Run1on

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