Piping System Retrofit 1

[ct4242]

Hello all,

To preface this question, please note I don't deal with fluids engineering very often.

I have a client that asked a question I am not normally used to dealing with. Albeit it doesn't seem terribly difficult, I want to make sure it wont prove much more difficult than expected before I accept.

The Problem:
Customer has a CWS for cooling a retirement facility. The system was very old and outdated and many components were removed. He feels that now operating the pump (which has a VFD) at 100% is no longer needed as the pressure drop across the components should be much less than before.

The Details: (so far)
I have the information on the pump, the air cooled chiller they are using, and the AHU. I believe I have the correct recommended minimum flow rate for the components. I also have the type of piping (3" PVC pipe, a little strange but nonetheless) and the length/elevation change for the piping.

So far I have compiled all that information but am a little rusty on the process of getting to the solution. I need to get the head loss through the pipe and the components (the HL through the components is given and varies at different flow rates) and I'm assuming I use an equation from my old fluids book I have somewhere to get the friction head loss. Not sure where to go from there.

Any and all help is appreciated!

 

[LittleInch]

A diagram would help to understand what you're looking at, but from the description I assume you are talking about a Chilled Water system operating on a closed loop between a chiller unit(s) and some AHU(s).

As such you can ignore the elevation change as in a circuit the effect cancels out.

all you really need is to calculate the head loss at various flow rates and plot the curve in head (m or feet). This comes from the losses in the pipe and fitting and across the particular elements ( chiller, AHU etc).

You then superimpose the differential head applied by the pump at its various speeds and where the two line intersect, that is more or less what your flow will be.

The question then is what is the min / max flow required under various cooling conditions and how does the pump meet it. Running a chilled water system at a flow greater than needed tends to waste money pumping water round faster than needed and also doesn't do the chiller any good as it doesn't have the differential temperature to work with that it should.

Your return temp to the chiller should be calculated or be the key control to control water flow. Too high and the chiller won't be able to lower the temperature enough and too low and you're wasting energy.

Is that what you were looking for?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[ct4242]

LittleInch,

That was exactly what I was looking for. I just was a bit lost on what to do with varying head loss based on the different flow rates.

The air cooled chiller's manual has specs on the minimum flow rates to achieve meet the cooling requirements of the building. I will finish this up and compare the two numbers at work tomorrow.

Thanks for your help!

 

[LittleInch]

Ok, let us know the outcome.

For a closed loop system all you need to worry about is differential pressure across the pump. Static pressure is irrelevant so long as the static pressure at the top / highest point is greater than atmospheric and the pressure at the lowest point is less than the design pressure.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.