[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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