Sizing a coil for face and bypass.
Sizing a coil for face and bypass.
(OP)
I need to select a coil for a face and bypass system, but am new to F&B. This is more of a general question than for a specific job, so I was wondering if anyone can advise on an effective approach to sizing coils for face and bypass systems? Thanks!





RE: Sizing a coil for face and bypass.
RE: Sizing a coil for face and bypass.
If it is a heating F&B coil, the valves (steam or HW) modulate to maintain discharge air temperature (DAT) whenever there is no risk of the freeze-stat tripping, usually over 40°F outdoors.
When <40°F, the heating control valves might be fully open while the F&B dampers modulate to maintain discharge temp. Typically you’d size the coil for a design winter condition (e.g. 5°F outside) if you’re in the northern part of the world.
A cooling bypass damper might have another purpose altogether. If you need to dehumidify more with a given cooling coil, a lower apparatus dew point and a higher coil bypass factor gives you more dehumidification for the same leaving air temp. You’d run cooler chilled water to accomplish this, and start to open the bypass around your cooling coil to maintain the same LAT (e.g., typical 55°F set point).
RE: Sizing a coil for face and bypass.
Once you have your coil select a damper (and weep damper (downstream one)) if you so choose, depending on your applications. I think they call this an integral coil.? Can't remember. . Then as Drazen says, size your bypass path/damper to match the pressure drop of your combined face coil/damper assembly. Chasebeam has additional helpful info on operation/control and sizing.
The integral face and bypass (if I have the term right) has a damper downstream of the coil in addition to the upstream one. The downstream damper is used to prevent unwanted heat transfer from the bypass air swirling out of the restricted opening and pulling heat (or cool I suppose) off of the hot/cold coil. Its use is depending on your application and how critical your control of discharge air temperature is.
As far as becoming familiar with them, google face and bypass and you will get everything from mfgr's lit to HPAC/ASHRAE articles on the subject.
Hope that helps.
RE: Sizing a coil for face and bypass.