Variable primary chilled water loop 2

[rskrumha]

I am adding chillers and pumps to an existing variable primary chiller plant. We will have different size chillers and pumps in the common loop. Does anyone have experience with this configuration?

 

[DrRTU]

Trane has a couple of engineering news letters on this topic.

http://www.trane.com/commercial/library/vol31_4/adm_apn005_en.pdf

http://www.trane.com/commercial/library/vol284/enews_28_4_093099.pdf

 

[koubi]

From hydronic point of view,you should consider following points: How are the pumps configured; are they dedicated (1 chiller 1 pump) or headered? Are all chillers equaly sized? How is bypass line and bypass control valve designed? Is flow meter in use and where it is located?

 

[rskrumha]

the pumps are headered and the chillers are different sizes. Bypass line design is by me, so what I find out to be the best scenario will be the answer.

 

[koubi]

Sorry I missed the point that all the chillers have different sizes. With this fact and a fact that the pumps are headed you have very demanding installation. Flows through the chillers should be dynamically balanced with automatic balancing valves equipped with modulating actuator. Valves should have an ability to close against high pressure and they should have also shut off function to isolate non operating chiller from back flow.
The bypass line should be sized for the minimum flow rate of the largest chiller. According to chiller producers recommendations, minimum chiller flow should be 50% of the nominal flow. Selecting appropriate bypass control valve deserves special concern. It should have high range ability, stable control of the flow, linear control valves characteristic. Consider using pressure independent control valves.

 

[pretendfarmer]

rskrumha,

Accurate system flow control will be the key to your success. A little more information is required:
1.) Are the chillers variable speed?
2.) What is the total tonnage of the chiller plant?
3.) What will be the total number of chillers installed?
4.) What will be the total number of pumps installed?
5.)Are you adding more chillers because the you have added load to your
system or because the chillers were not large enough to begin with?
Controlling dissimilar pumps can be pretty tricky. Parallel pump flow analysis is not a linear relationship and will be hard to accurately predict prior to installation. For instance, if three dissimilar pumps are operating in parallel, three different speed signals may be sent to the three dissimilar pumps for optimum flow control. Some trial and error as far as programming the pump control will probably be required.
My recommendation would be to use a pressure independent 2 way modulating control valve as the bypass control valve. It should be sized for the minimum flow of the largest chiller and should be installed out in the loop such that a chilled water buffer tank is not necessary when a single chiller at low load is operating. This valve should be controlled by differential pressure transducers installed across the evaporators to insure that minimum flow requirements are met.
Lastly, your job will be much easier if pressure independent 2 way modulating control valves are installed at the individual chilled water coils. These valves are like using pressure independent VAV boxes except they are used on the water side. These valves use much less water to cool the space (some even guarantee large chilled water coil Delta T performance). And they can be used as individual flow meters (eliminating the requirement for the large flow meter in the main). Large flow meters in the main do not work very well when the total load is small.
You might try the attached website for some additional help. Good luck.

 

[rskrumha]

the great pretender thanks for your reply;

1) 1 of the 5 chillers will be variable speed
2) 5500 tons
3) 4 chillers
4) 5 pumps
5) we have added load onto the chiller plant

 

[DRWeig]

RSKrumha,

Here are some articles with some other considerations.

http://www.kele.com/archive/solutions/SelValvesHydroSys.pdf

http://www.kele.com/archive/solutions/ControllingChillers98-1.pdf

http://www.kele.com/archive/solutions/ChillersMax.pdf

There are a few more at

http://www.kele.com/archive/solutions/

at the bottom of that page, from Gil Avery. He's kind of a guru of variable flow hydronics...

Let us know how it works out for you!

Good on ya,

Goober Dave

 

[pretendfarmer]

rskrumha

FYI

We have just recently worked on a chilled water system that is approximately half the size of the one you are working on and the addition of pressure independent control valves saves approximately $140,000/year (we used $.07/KwH for electricity). This equals about a 2 year payback period. The improvement in control keeps additional chillers from running. The trick is in the programming. Contact me for additional info (jdschmidt@mac.com).

 

[cry22]

The whole VPF system rests on a VERY GOOD flow control system and Very Good control valves on the by-pass.
What I noticed is that people put X stand-by chillers, Y stand-by pumps and forget to add a STAND-BY VALVE, which is more likely to fail than a pump. The whole sequence goes out of wack when the valve fails, yet no one ever considers a stand-by valve in teh by-pass.

 

[rskrumha]

awesome observation! i can go to sleep today! I've attended 100's of hazzard/operational analysis meetings and that is a Least Common Denominator no one had pointed out to me before!