ON/OFF control
ON/OFF control
(OP)
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??





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