Desicant Wheel Dehumidifier

[remp]

I am upgrading an existing air handler and its BMS controls that is about 15-18 years old.
The air handler looks pretty typical for Pharma with full fresh air, filters, heating coil, cooling coil and fan with zone reheats. Because the %RH requirements for the space are low, humidity 30%, they added a desicant dehumidifier to the air handler.
The dehumidifier is basically an air handler in itself with a desicant wheel. Its about 1/5 the size of the main air handler and sits on top of the main air handler. This is where it get interesting.... The dehumidifier unit sucks in a portion of air from the main AHU (1/5th say) after the filters and before the cooling coil. It dries it and delivers it out through its own dedicated cooling coil and down stream of the main air handler cooling coil. Why would they do this? Whey not take it in and deliver it out again before the main cooling coil and have one less to coil to deal with ( i.e. the one downstream of the de humidifier)
Has anyone seen this set up?

 

[Drazen]

i did not, but it sounds like someone made detailed psychro calculation so mixing very dry air with main stream gives needed relative humidity.

that way you save on dehumidifier size, don't you? five times smaller wheel saved much on investment and maintenance.

cooling energy will not be changed, and there is indeed cost of extra cooling coil, but it sounds negligible compared to other equipment.

i would firstly guess that they found to need just small additional dehumidification to reach specs, that's why they used such bypass setup.

 

[remp]

Drazen
I understand the bit about taking in a small portion because that's all that is need to get to the desired RH, but why take in before the main cooling coil and deliver out after the main cooling coil.
I have seen another installation on the same site and the dehumidifier takes in a portion of air off the main supply duct down stream of the AHU and conditions it and discharges back on again 2 meters downstream of the inlet. Seems it doesn't really matter...whereever fits.
Regards
R

 

[Drazen]

Desiccant wheel process is heating air, so one way or another you have to cool it downstream in summer conditions.

 

[d5708c]

If the discharge air from the desiccant wheel was added back in before the main cooling coil, than the amount of moisture that could be removed by that coil would be reduced. Using a separate coil for sensible post-cooling of the active desiccant wheel air stream will result in dryer leaving air for the overall process.

 

[EnergyProfessional]

is it somethingn with the dual ERV and cooling coil in between, like the Semco Pinacle system?

 

[quark]

The dehumidifier might have been added at a later stage. The original coil may not be sufficient to take care of the additional load. Dehumidification be dessicants is isenthalpic.

 

[317069]

Can you find the product data of the original air handler

 

[Leon75]

This to me looks like a retrofit (maybe design) because the latent cooling in the existing coil was insufficient for humidity control.

By doing it this way the latent cooling / dehumidification is controlled rather than before the coil where the 'cooling coil' will also have a dehumidification effect. This could really dry the air out.

I have seen this done because under certain conditions (normally a very partly loaded coil & high amounts of moist outside air) there is insufficient latent cooling in the main cooling coil. The bypass system is set up to dry the air out separately.

From a BMS perspective the de-hum coils then be only activate when the 'cooling coil' was not able to handle the latent cooling, then add the latent coiling coil as required.