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re: dryer air heat exchanger
30 dec 2001
tony  wrote:

>how hard would it be to make a heat exchanger for the discharge air of an
>electric clothes dryer?  i would imagine the moisture would be a problem,
>but why wouldn't this be practical?

you can filter the lint, but moisture can be a problem. 

i measured an empty speed queen 5400 watt electric dryer cycling from 140
to 167 f, averaging 153 f, with a duty cycle close to 50%. that's 2700 w
or 9200 btu/h in about 9200/(153-70) = 110 cfm of dry airflow. running it
loaded for 20 minutes on a 30 f day might remove 0.33h(70f-30f)110btu/h-f
= 1466 btu of heat from the house as well as consuming 0.33h(5400w) = 1.8
kwh (6140 btu) of electricity.

you might make a 12'x2'x2' plywood shoebox with a furnace filter at one
end and a drain at the other and a poly film liner and 25 10'x4" pvc pipes
inside. with 250 ft^2 of surface and slow-moving air and no condensation,
ntu = 1.5x250/110 = 3.41. the ashrae hof says a heat exchanger like this
with a zero capacity rate ratio (the dryer air has a much smaller heat
capacity than the pipe water) has an effectiveness e = 1-exp(-ntu) = 0.967,
so the initial dryer air might emerge from the box at 153-0.967(153-70)
= 72.7 f.

the pipes would warm at most 0.33x5400x3.412/1400 = 4.4 f in 20 minutes.

as a slower but less labor-intensive alternative, you might hang
the clothes in a 110 f "warm closet" made from 4 4'x8' $11 sheets of
1" atlas energy shield 350 f foamboard and a $9.97 500 w home depot 
work lamp and a $9.95 cooling thermostat (tm93hvc2416) and a $2.50
heating thermostat (tm02hvc2293) and a $4.95 humidistat (tm89hvc5203)
and a $4.95 100 cfm fan (tm89fan4893) from herbach and rademan
(800) 848-8001, 

we might arrange them like this (in courier font):

         < 50%      < 200 f                < 100 f
       humidistat    heating       |       cooling    switch    |
                     thermostat    <       thermostat         -----
120 v                              < lamp                    | fan |
                                   <                          -----
                                   |                            |

when you put wet clothes in, the humidity rises, which turns on the lamp. 
if the temperature reaches 100 f and the humidity is still more than 50%,
the fan turns on. when the clothes are dry, the box stays warm enough to
keep the rh less than 50%, with very little power, so there's no chance
of mildew, with the lamp switch on. no need for a timer :-)

the fan blows room air into the top of the box, with one-way plastic
film dampers over the vent holes at the top and the bottom of the box
to prevent natural airflow in "idle" mode.

with an air gap on each side, the foil-faced foamboard becomes us r15, 
so a 2'x4'x8' tall box has a thermal conductance of 112ft^2/r15 = 7.5.
with the humidistat in the "on" position and the fan switched off, the
closet would become 70+500x3.41/7.5 = 297 f, but for the 200 f heating/
safety thermostat in "sauna mode," with no wet clothes inside. a 4'x8'
closet or sauna might have 2 layers of foamboard with an air gap (r26.) 


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