Face and Bypass Dampers
Face and Bypass Dampers
(OP)
I am new to HVAC and am in the process of retrofitting a control system for several industrial air handlers. The air handler equipment includes face and bypass dampers on the steam coils. I am looking for some good reference material on face and bypass damper control or the answers to these questions: Why are face-and-bypass dampers typically used? (To prevent steam coil freeze-up by allowing a constant flow of steam through the coil?) How are they typically controlled? Any information would be greatly appreciated.





RE: Face and Bypass Dampers
The steam coil should have a low limit protection device (freeze stat) to protect from freezing in colder climates.
RE: Face and Bypass Dampers
RE: Face and Bypass Dampers
Provide manual shut-off valve so steam to coil can be valved off during summer
Make preheat control on face & bypass coil active enen when unit is off during winter. Make sure face dampers fail open to protect the cooling coil from freezing, in case of loss of power during winter.
Put freezestat at face of cooling coil, evenly distributed to automatically shut down unit & close OA damper when freezestat trips.
RE: Face and Bypass Dampers
Below 35-40°F, the steam control valve(s) fully open and the F&B dampers modulate to maintain discharge air temperature setpoint.
RE: Face and Bypass Dampers
"Why are face-and-bypass dampers typically used"?
This stragity is used many times to control supply air temperature. we normally use a controler with the sensor in supply air to maintain steam valve and dampers
RE: Face and Bypass Dampers
RE: Face and Bypass Dampers
What about face & bypass damper applications for DX cooling? I have seen some designs involving heat pumps calling for this feature in order to control 100% outdoor air applications. Dampers above the evaporator allow 40% of the outdoor air to bypass the coil. The evaporator coil itself handles 60% of the outdoor air.
Looking at a psych chart, it appears that the mixing of the two resultant air streams brings you to a point on the chart that appears to be "unreachable" through conventional dx cooling. can someone verify this for me or discuss?
thanks
RE: Face and Bypass Dampers
You can do trial & error solution for the recirculated air CFM. The recirculation air should tempers the OA condition to a point that would be within the tabulated performance of the unit. Interpolate for the capacity. Total CFM should be in the range of about 250 to 300 CFM/ton. Verify with manufacturer that the total CFM would be above the minimum required (which would be the minimum CFM tabulated in the performance tables).
RE: Face and Bypass Dampers
Actually, I'm not designing this system...it's a design that is being used by some consulting engineers in my area on large heat pump applications. I am interested in knowing why they would opt for this configuration (face & bypass cooling). What would be the intent of the design?
Like I mentioned...my only lead is based on the psychrometrics.
ex: 10,000 CFM
90/75 entering air
308 MBH cooling load (on evaporator)
compare:
A) 100% passing over DX coil vs...
B) 40% bypass mixing with...
60% through coil
the final temperatures are on different points on the psychrometric chart.
Thanks
RE: Face and Bypass Dampers
Recirculating a portion of the discharge air around the cooling coil is a trick to make the DX unit perform within the tabulated performance range with regards to rated cooling coil inlet air and discharge. Also it makes sure you have more than the minimum CFM across the DX coil. If not you can get frosting/icing at the coil.
RE: Face and Bypass Dampers
If you have a modern coil with a low amount of bypass (say 10% bypass and 90% of the air contacts the coil), you're dehumidification is not optimized. There is a dehumidification 'sweet spot' at a point where some percentage (maybe 60-70%) of the air contacts the coil and the remainder bypasses the coil that optimizes dehumidification. It is the point at which you can bypass the most possible air while still maintaining the discharge air temperature set point. It defies common sense, and you have to go through a pyschrometric mixing exercise of a given inlet air at some enthalpy, x amount contacting the coil and y amount bypassing the coil, while maintaining a steady discharge temperature. At the point where your discharge temperature starts to rise because you've reached the cold limit off the coil, you've maximized dehumidification.
Dehumidification may be a reason why this control is in place, or it may be to improve cooling staging of a DX system.