[Greenbuilding] dehumidification

[nick pine]

Gennaro Brooks-Church - Eco Brooklyn <info at ecobrooklyn.com> writes:

> Has anyone experimented with the powder desiccants on rotation? Say have 
> two trays and when one is in the house getting wet the other is outside on 
> a south wall protected from the rain drying out.

I've thought about using bentonite clumping cat litter in a raised glazed 
tray in the sun with a concentric pipe air-air heat exchanger that makes a 
wet airstream rising up from a house less dense than a dry airstream falling 
down into a house, at the same temperature.

Jason Holstine <jason at amicusgreen.com> wrote:

> The univ of maryland solar decathlon house has done dessicants with water 
> wall features. The maybe kind of might have info to share.

They are patent-crazy and don't share information well, but they used 
calcium chloride, which is not a very effective desiccant.

John Straube <jfstraube at uwaterloo.ca> wrote:

> The reasons for the ratio of sensible to latent heat are in the physics of 
> air, not government regs.

I've heard DOE rating systems encourage this ratio.

> Get a psychrometric chart and draw on the process in terms of enthalpy 
> (total energy).

Why bother with drawings? Air at 40 F and 100% RH contains about 0.0053 
pounds of water per pound of dry air... 70 F air at 50% RH it contains about 
0.0079.  Cooling 1 cfm of 70 F air to 40 F takes about 30 Btu/h.  Condensing 
the vapor takes about 1000x0.075(0.0079-0.0053) = 0.19 Btu/h. Rewarming the 
air to 70 F takes about 30 Btu/h. The latent energy change is 0.19 Btu/h, 
and the net sensible energy used is 0 Btu/h, for a 100% latent/sensible 
ratio.

> Drying to 40%RH requires me to cool to just above 40F.
> Now I need to reheat that air to get it to 70F, or I will overcool the 
> space.  This is a normal one-speed fixed output air conditioner.

Normal air conditioners cool air, n'est-ce pas?

> You cant buy a small AC unit that puts arbitrary amounts of heat back into 
> the air and the remainder to the outside.

Why not? Basic physics or DOE ratings?

> If you reduce airflow through the AC, the air gets colder, and this drags 
> more moisture out the limit is 32F, at which point frost forms and quickly 
> blocks it off.

So stop before the output air temp drops to 32 F...

> The easiest current answer is to get a really good efficient variable 
> output AC unit, like a ductless split, run them slow (low airflow, low 
> capacity) and then add back heat for the hours needed.

Duct tape over some air outlets seems easier :-)

> Enthalpy controlled ventilation is a common technique which reduces 
> cooling needs exactly in the cool dry hours that are not a big problem. 
> Often they only work when the outdoor temperature is below 60F...

Bad designs? Why don't they work at higher temps and lower humidities?

> Surprisingly, the efficiencies of compression based AC is now so high, 
> that economizers often use more energy to run the fan through filters 
> ducts and dampers, than the AC uses when it is 60F outside.

This efficient 2-watt filter with a low pressure loss: 
http://www.nseproducts.com/electronicaircleaner.htm is for houses, not big 
buildings with badly-designed HVAC systems. A 20"x20" version works well 
with a $69 90 watt 2470 cfm Lasko 2155A window fan: 
http://www.amazon.com/Lasko-Electrically-Reversible-Window-2155A/dp/B00002N5Z9/ref=sr_1_1?ie=UTF8&qid=1339445503&sr=8-1

Nick