Close-off pressure for low flow bypass valve
Close-off pressure for low flow bypass valve
(OP)
Hi,
While selecting the low flow bypass valve across the chiller to bypass the chilled water at low load conditions, which of the following should be considered as the close-off pressure for the valve selection?
1) The pressure developed by the chilled water pump at its minimum speed? or
2) The pressure developed by the chilled water pump when it is running at its design full load? or
3) The shut-off pressure of the chilled water pump? or
4) The differential pressure across the bypass valve when it is fully open and when the pump is running at its minimum speed? Or
5) The pressure required at the bypass valve to allow it to close, when the chilled water pump starts to ramp up as the building load increases?
Please help, as I haven't arrived at a firm answer even after exploring about this for a full day. Thanks.
While selecting the low flow bypass valve across the chiller to bypass the chilled water at low load conditions, which of the following should be considered as the close-off pressure for the valve selection?
1) The pressure developed by the chilled water pump at its minimum speed? or
2) The pressure developed by the chilled water pump when it is running at its design full load? or
3) The shut-off pressure of the chilled water pump? or
4) The differential pressure across the bypass valve when it is fully open and when the pump is running at its minimum speed? Or
5) The pressure required at the bypass valve to allow it to close, when the chilled water pump starts to ramp up as the building load increases?
Please help, as I haven't arrived at a firm answer even after exploring about this for a full day. Thanks.





RE: Close-off pressure for low flow bypass valve
For minimum chiller flow, your pump needs to be sized that min pump flow is higher than min bypass, or the bypass won't work (not enough flow to keep bundle from freezing). The dP across the bypass should be field set to min bypass flow setting, measured from something similar to an Onicon.
I think you will need a separate dP across discharge to correspond to max flow provided by chiller; something needs to control the VFD/IGV.
RE: Close-off pressure for low flow bypass valve
RE: Close-off pressure for low flow bypass valve
if it's pump with fixed three speeds, you can just set design rpm, controller cannot change it, so once set up at commissioning the speed is not relevant for further discussion.
RE: Close-off pressure for low flow bypass valve
So a 50 psi shutoff would be a minimum criteria for this pump, however I would shoot for a higher rating.
RE: Close-off pressure for low flow bypass valve