Help w/ conficting info from vendors on dehumidifiers

[tikitime]

I have a building in central florida that is being used to dry parachutes. The existing ahu to dry the parachutes is an off the shelf dx split unit. Consequently it takes a week to dry the parachutes. One of the mods I am working on is to replace the ahu so that drying time is reduced to a day. The user wants the new unit to use a dehumidification coil - not a desicant system.

I have talked to several vendors and they have been giving me conflicting info. One says to use 100% outside air, another says to modulate the Oa and Ra values bbased on the dewpoint of the OA, and another says to use 100% RA.

100% OA vendor says that the dewpoint of the OA will be less than the air in the building. I don't think that is true for florida where the ASHRAE design values 93 db / 80 wb have a 76 dewpoint.

The OA/RA vendor says that the amount of OA should be based on the OA dewpoint. This way when the OA dp is low enough, less dehumidification will be required. But this option will require more complicated controls. I don't think this is justified considering the number of times we will be able to use OA. Also the unit will be run only a few times a year.

The 100% RA vendor says that the time when the Oa dp is low enough to justify using OA is so limited, it's not worth using. Also he says that the moisture from the parachutes is a known quatitiy that can be designed for whereas OA is unknown and requires a larger unit.

Does anyone have a take on all this?

Thanks!

 

[MintJulep]

Ask each vendor to provide an engineering justification for their position, and a life-cycle cost analysis.

Given only these three choices, my gut tells me that the 100% RA is probably correct.

You can only drive the DP so far with a dx coil alone. I would be tempted to investigate a combination dx/desiccant system.

 

[hvaclane]

You must have a RH level you want. ie.60% 50% 20% ??????
Have your your temp @75'f
cfm of unit 6000+
50% inside air
50% ouside OR humidiy controled unit and or temp when one turns off unit air switches to fan on recirulating in side air 100%
hvac lane

 

[imok2]

you may want to takea look at this:

http://www.drirotors.com/hrw.htm

 

[Whoops]

An interesting problem. The US parachute association must publish some guidance, or have names of consulting engineers with experience in this area. There must be some guidance on fabric drying in your wonderful ASHRAE guide; are we trying to re-invent something here?

Could the parachutes be dried overnight? My thinking is that the outside air temperature must drop, so some of the water vapour would condense from the OA. The production of dried air would then use less energy and the energy would be cheaper, if you can get cheaper off-peak electricity. The ASHRAE figures quoted refer to peak day-time conditions. A chiller/dehumidification plant could be used for AC during the day and for drying at night.

You’d have to do some calculations to determine whether you’re better off dumping the moisture-laden RA and bring in 100% OA, or cooling the RA to condense the moisture and then reheating it. If the answers depend on the outside conditions, a DDC/BMS system could modulate between the two conditions, saving energy. The initial cost may be outside the budget. Such control systems usually operates on the air enthalpy, a function of air temperature & Rh. You’d also have to calculate whether you’re better off lowering the Rh of the supply air by dehumidification (cool & reheat, so probably electric energy) or by simple heating (heat only so oil or gas, if available).

If the existing DX system is not performing, it’s probably undersized for this application. If it's an off-the-shelf unit, is there an external compressor/ condensor unit? If so, the rejected heat could be used in the drying process. A custom built AHU with the evaporator and condensor coils in the supply airstream would seem to be the way to go.

Are there any lower limits on the moisture content of the fabric? I’d imagine you might get static electric problems if the fabric was too dry. I believe this caused ‘roman candles’ with silk parachutes of the WW2 era. I'm interested, being ex-X Para.

 

[Whoops]

Then again, how about a south-facing drying shed, with glazed or polycarbonate roof and walls, and an extract fan activated by Rh? The solar gains should dry out most things on most days.