Differential pressure switch

[HASSAN NAIM]

Hi ,

We are installing a primary/secondary chiller plant . As known , the secondary pumps must be variable speed type and its VFD is controlled by a differential pressure switch .

My problem is where i should install this DPS and what is the optimum pressure setpoint so teh system will work properly and efficiently .

Thank you .

Hassan Naim .

 

[DRWeig]

Hi Hassan,

I think you mean "differential pressure transmitter" instead of "differential pressure switch." You'll also need a controller (your VFD on the secondary pump(s) will probably have one built in).

Best place to put the dp transmitter is across the valve/coil combo that is physically the farthest from the pumps - or if you study the piping system, you may come up with a different worst-case coil. This way, you are controlling the secondary pumps to produce adequate pressure for the worst-case coil. Note that your transmitter goes across both the control valve and the coil, not just the coil.

Good luck with your project!

Best to you,

Goober Dave

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[GT-EGR]

You'll find there are multiple answers that will end up giving you a system that works. But I would you should put the differential pressure (DP) sensor/transmitter (not switch) where your system has the most consumers that are representative of your typical system loads. Then go 2/3 down that supply/return pipe and put it there.

As for your pressure setpoint, you can't really know that for sure until the balancing contractor gets there. In theory it would match the pressure drop you calculated from your supply branch takeoff, through all the coil valving, through the coil itself, and back to your return branch takeoff. But calculation and actual don't always line up. As far what is optimal, without a complicated optimization strategy where you track control valve positions - you are looking for a target differential pressure that leaves your worst case control valve satisfied at 95-100% open.