Sizing a coil for face and bypass. 1

[rhwiet]

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!

 

[Drazen]

pressure drop over coil has to be the same as via bypass.

 

[ChasBean1]

Sorry, too many unknowns to answer your question with accuracy.

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).

 

[MTES93]

Appears you are doing a built up system and not a factory furnished device? If that's the case size the coil for the load. If heating coil size at a reasonable face velocity/pressure drop w/out making the coil too costly (economics). If a cooling coil size for load as noted and a max face velocity that will not cause moisture carry over. In either heating or cooling applications with variable volume systems watch the minimum face velocity of the coil at min air flow to prevent laminar heat transfer characteristics. Typically at less than 200 FPM. So you may make the coil smaller to avoid this. Different views on that subject however...

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.

 

[rhwiet]

Thanks for the help/tips. I should have what I need!