Reducing Air Flow through Dehumidifier

[arcticguy00]

We have an existing dehumidifier that we would like to re-purpose at a lower air volume. Has anyone had any experience with blocking off portions of an existing DX coil to reduce dehumidifier capacity?

Here's what we have: The AHU was originally specified to dehumidify 3500 CFM of air from 72*F/50% RH to about 42*F Saturated. The evaporator and condenser are in series in the dehumidified air stream and both are in the same air handler plenum as the unit's 15 ton scroll compressor. Refrigerant is 410A. Both coils were selected such that the face area is 3'x3', and the piping arrangement is full face with one distributor into the evaporator. There is hot gas-by pass, and with the heat of compression added into the dehumidified air, the air is supplied to our process at about 85*F/25%RH.

Due to process changes we are now limited to supplying about 1000 CFM of dehumidified air with the same target of 85*F/25%. We will do this for about a year, then the process will be permanently shut down. Instead of doing a complete reselection of the unit, would it be possible (and feasible) to replace the 15 ton compressor with (say) a 5 ton compressor, block +/- 2/3rds of the coil face area, re-balance the air and just start running at 1000CFM?

thanks

 

[EnergyProfessional]

the problem will be refrigerant still will expand int he part of the coil that doesn't get airflow if you block half the coil.

but couldn't you just reduce the airflow (and let the compressor cycle more often)? You probably want to talk to the compressor manufacturer how it will be have with the large coil.

 

[317069]

"The evaporator and condenser are in series in the dehumidified air stream and both are in the same air handler plenum"
I didn't understand it, do you mean 3500 cfm at(72F) leaves the evaporator at(42F) then the condenser at(85F) then to the space and back to evaporator at(72F).

 

[arcticguy00]

Thanks. With respect to refrigerant continuing to expand in the blocked section of coil: I was thinking that as long as the amount of coil that was blocked to get performance reasonably close to optimal, then the expansion valve and hot gas bypass would work their magic together to control the suction pressure back to the compressor. I will talk to the compressor manufacturer about cycling, but the dehumidified air is supplied directly into a process (paint spray booth) that needs a pretty steady temperature and humidity.

 

[arcticguy00]

With Respect to 317069's question, i hope this clarifies: The air enters the dehumidifier at 72*F/50%, goes through the evaporator coil to reach 42*F saturated, then is sensibly reheated ad it passes through the condensing coil to regain the heat lost during dehumidification and gain the added heat of the compressor. After the air passes through the condenser coil it is physically in inlet plenum of the supply fan, which also happens to house the compressor. So the air ends up leaving the dehumidifcation unit at about 85*F/25%. For further clarification, I will attach a photo or a sketch or plot on a psych chart in the next post if requested.

 

[MintJulep]

Not likely to work well.

You might be able to recircuit the coils, taking the no longer needed tubing out of the picture.

This would also involve changing the distributor and the expansion valve.

And probably the suction line - to get the velocity back up high enough to have oil return.

And you'll need to tinker with the controls, and the compressor motor protection, and the fan motor protection, and there is probably a sail switch, differential pressure switch or current sensor that interlocks compressor operation with proof of airflow.

And you are likely to need to change the fan as well, since making such a big reduction in airflow will probably move the current one into the unstable portion of its performance curve.

And.....


How badly to you want to become a dehumidifier manufacturer?

 

[317069]

Thank you, now let say you have reduced the air flow from 3500cfm to 1000cfm, if there is a duct system, will you have an issue with it since the duct would be oversized and air velocity will go way down?

 

[317069]

Does this unit serve one paint booth or number of paint booths and you want to shutdown part of them?