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.

Good face and bypass damper control requires the individual discharge air streams (coil discharge/bypass discharge) be properly mixed for proper temperature sensing. If mixing of the air streams is not through vanes or louvres the ideal spot to sense air temperature is after the discharge of the fan ( a fan is a great air mixing device). Sometimes the use of averaging temperature sensors are used but placement of the sensors are critical. Sometimes space limitations restrict the ability of the averaging sensor to correctly sense the mixed temperature. I have found better success using discharge air temperture after the fan as ideal.

The steam coil should have a low limit protection device (freeze stat-Motorized Valve Actuators) to protect from freezing in colder climates.

You are correct, and rkostynu's advice is also on target. Keeping full steam pressure on the coil helps to keep the coil free of liquid (which could freeze) by keeping the maximum pressure available for operation of the trap. If you were to modulate steam flow, and by inference coil pressure due to the condensing steam accompanying volume change, there will be times when the coil pressure would drop below atmospheric (vacuum), effectively causing condensate to hang up in the coil. It is also recommended the trap be at least 18-36 inches below the coil outlet to provide sufficient head on the trap to allow complete coil draining in the event of loss of steam pressure. Armstrong should have a good web site and reference documents for trap installation. There is much to know about steam, so be careful...

No you don't bypass outdoor air. If you do you lose control of space humidity. What you can do with DX is to recirculate a portion of the dehumidified air from the supply fan discharge and introduce it back upstream of the DX coil to mix with the outdoor air. Thus the coil sees more CFM through the DX coil than actual OA CFM. The OA CFM would equal the net supply to the room. The supply fan CFM & DX coil CFM = OA CFM + recirculated CFM.

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).

MORE NEWS