[Greenbuilding] dehumidification

ARCH59 at aol.com ARCH59 at aol.com
Mon Jun 11 15:23:34 CDT 2012


Bj, Tom,
 
Can you speak about your unit we designed into the Bright'n Green project.  
This thread needs your input please.
 
Robert
 
 
In a message dated 6/11/2012 4:20:26 P.M. Eastern Daylight Time,  
nick at early.com writes:

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/B00002N5
Z9/ref=sr_1_1?ie=UTF8&qid=1339445503&sr=8-1

Nick  


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