[danish]

In a typical Heat exchanger (chilled water) system, in secondary side I have VFD connected pumps controlled by Differential Pressure Sensor.

How could I improve delta T?
How to make sue that cold water do not go back to Hex?
Where should be bypass line before secondary pumps or after the secondary pumps?
When it should open or close? What should be complete process?

 

[jemawalton]

A lot of open ended questions. I'll give it a shot.


1. To improve delta T on the secondary side, slow the water down. Thats the easy answer but you have a million other things that have to be considered. Why are you worried about your delta T? Check you approach temps.

2. In order not to get cold water back to the Hex, make sure it gets rejected somewhere out in the loop. Again, not sure what you are getting after with the question.

3. If you have a bypass in a VFD system I would want it at the end of the loop. That way it can circulate through the farthest point from the pumps. I am assuming it will be set up to maintain a minimum amount of circulation when all other chilled water valves are closed.


Give us some more details on the setup and we can probably give you some better info to help you out.

 

[db7803]

Check your Delta P across the chiller barrel before you slow down your VFD

 

[Randy S.]

Start by identifying any coils that have a lower return temp than others, or lower than design. The coils with the lower return temps are most likely getting too much flow.

 

[PSI]

Check your Delta P across the chiller barrel before you slow down your VFD

I agree, this is a great starting point. I've had many issue like this one and it usually started with incorrect delta P across chiller, in some cases it didn't help that the chilled water had a VFD that increased & decreased flow.

 

[echo20]

Lower the DPs if you can. If you can increase the return temps on the fans. Return 72-to 74 74 should be good. See if any chill water valves are passing by. That will lower the loads on the loop.

 

[Randy S.]

If it's a VFD primary, then figure out the minimum flow in terms if Hz, and set up the VFD accordingly. Also try to have those same limits in the DDC, whether OEM or third party.

 

[danish]

I tried a lot but I never succeed, I can control deta P by DPS and secondary VFD pumps but to stop LOW return temperature back to chillers/Hex never achieved.

I believe the only thing we can do is to select high return temperature coil with modulating valve actuators. Has any one tried with PICV (pressure independent control valve) in FCUs?

 

[Randy S.]

The low delta T is either from too much water through the coil or an unrestricted path from supply to return like back flow through a decoupling bridge, or, as exists in some systems a bypass open too much, or a PRV stuck open.

 

[Pneuma]

You need to get the original plans, blue prints, and see how it is supposed to work.

The low return temperature may not be a problem at all. Under low loads you will get a low return water temperature. You only get the design split across the chiller when it is running at 100% of design.

Why are you worried about the low return temperature?