ON/OFF control 1

[remp]

How does a BMS or PLC ontroller control the sequencing ON and OFF of say 4 nr. pumps that are trying to maintain a set point pressure in a pipe of say 2 bar. The usage of water and a subsequent drop in pressure will call for more pumps to maintain the set point. How does it do that in control logic? I know how it would work if it was a single pump on a variable speed drive but how does PI control work when you have say 4 stages of ON/OFF. It would also expalin how say 4 nr cooling towers in parallel with fans on them stage ON/OFF to maintain a set pont return temp??

 

[imok2]

With a four stage controller

The Model UCS-421 is a solid-state multi-stage device
used for floating/tri-state control of VAV boxes, or multistage
control in HVAC systems from a single analog signal.
The UCS-421 can be used to obtain a digital output
from a voltage or current producing sensor. In its base
configuration, the UCS-421 has four relay outputs. Units
may be “daisy chained” to provide additional stages of
control. The UCS-421 is supplied with mounting track
for easy installation.

go here:

http://www.hvacc.com/pdf/kele/UCS-421.pdf

 

[ChasBean1]

Remp,

First note: these pumps should have a DP control bypass valve (sort of a poor man’s VFD) if you want some kind of automated pump staging.

With this type of setup and this type of variable flow control: at a standard operating point with 3 pumps maintaining a 20 psid set point with the bypass valve fully closed; the shift to the 4th pump should occur after the DP set point diminishes to say, 18 psid with the bypass fully closed after X amount of time.

The stage-back to 3 pumps should be the point that with 4 pumps running and the bypass open X percent (found out by testing) produces a DP of, for example, 16 psid.

With what you have:

Test with a fixed load to see that (for example) 3 pumps produce 20 psid, 2 pumps produce 15 psid, and 4 pumps produce 24 psid: Allow more slop. If the loop DP set point is 20 psid and you are at 13 psid for X amount of time, add a pump. If you are at 25 psid for X amount of time, drop a pump.

The cooling tower scenario is easier as fan operation doesn't have an immediate impact on CWS temp.

CB

 

[marcoh]

Assuming all 4 pumps are VSD controlled, all pumps would be controlled in parallel as per single pump control. If the VSD speed drops to minimum speed, turn one of the pumps off, if the VSD speed increased to maximum speed, start another pump.

Not that operating 4 pumps in parallel would result in a very flat pumping curve making pressure control more difficult.