Face and Bypass Dampers 2

[PEMSENGR]

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.

 

[rkostynu]

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) to protect from freezing in colder climates.

 

[Yeldud]

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...

 

[lilliput1]

Make sure you have enough height at the bottom of the coil to install trap per coil manufacturer's requirement.

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.

 

[ChasBean1]

Good points above. Typical control: monitor outdoor temperature. Above some OA temp. range (usually 35-40°F), the F&B dampers should be aligned for full coil face (no bypass). Modulate steam valves as necessary to maintain discharge temperature.

Below 35-40°F, the steam control valve(s) fully open and the F&B dampers modulate to maintain discharge air temperature setpoint.

 

[imok2]

PEMSENGR

"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

 

[lilliput1]

Face & bypass damper control of steam coils is recommended for 100% Outdoor Air applications. You can also use steam distributing tube type coil.

 

[koolair]

I see a lot of discussion online about face & bypass dampers for steam heating coils.

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

 

[lilliput1]

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).

 

[koolair]

Good point lilliput1...re-circ would probably be a good way of controlling humidity.

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

 

[lilliput1]

Not having the OA bypass the cooling coil would prevent humid air from getting int the space.

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.

 

[ChasBean1]

koolair, this is a late response so hopefully you're still around. Cooling coil bypass factor has been around since HVAC school days, but we (me included) might have glazed over a bit during that session. Bypassing the coil while maintaining average discharge temperature helps to dehumidify and in a nutshell is this:

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.