Face and By-Pass The DX Evaporator Coil For Dehumidifcation

[ChaChaMan]

I have been told if you By-Pass some air around the avaporator coil, you will dehumidify the air while not needing to reheat it.

What is more, the fact that you reduce the air flow through the evaporator coil, you reduce the suction pressure of the cold gas, thereby reducing the evaporating temperature of the coil. Therefor the dew point is lowered and you wring out that much more moisture out of the air. It is claimed you can get the evap temp down to 35*F by starving some some air flow by By-Passing it.


Finally when the By-Pass air is mixed with the Dehimidified air, the result is much drier air that is nonetheless warm, and usually does not need to be reheated before supplying to the space.

Does anybody have any experience with this they would like to share ?

Thank you in advance.

 

[imok2]

If the entering water temperature and flow rate are unchanged, the velocity of the air passing through the coil will drop at part load, allowing the air to get colder and more moisture to condense. A space thermostat will reset the space temperature until the appropriate supply-air temperature is achieved. Face and bypass dampers on DX systems are sometimes encountered for air-side capacity control, but they must be used with extreme caution since the low air flow rates that occur across the coil when the dampers are in bypass can cause severe problems in the refrigeration circuit.

 

[ChaChaMan]

There is a DDC controller modulating the dampers, thus ensuring that there is enough air passing through the DX evap coil so that it does not freeze up.

Am I too understand that :
- the suction pressure will drop as there is less cold gas flowing ?
-The temp of the refrigerant evaporation (saturation temp) and thus superheat will drop ?

What can go wrong with the equipment ?

Thx

 

[berkshire]

"""What can go wrong with the equipment ?"""

If you over do it the coil will freeze up.
B.E.

The good engineer does not need to memorize every formula; he just needs to know where he can find them when he needs them. Old professor

 

[imok2]

Set the DDC controller modulating the dampers to make sure that the load set point does not fall below 50*F or it will module the bypass damper to close

 

[ChasBean1]

You’re right. A higher coil bypass factor (e.g., via coil bypass dampers or less fins per inch) and lower apparatus dew point will have better dehumidification. If you can cool half an airstream to 45°F and the other half to 65°F versus a combined 55°F 14 fins-per-inch coil leaving temperature, your leaving airstream with the same net temperature has a lower dew point and is drier.

There is no need to ask us for experience. You can work it out psychrometrically.