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re: venting clothes dryer inside for heat?
10 nov 2000
maxine in ri  wrote:

>a better way i've heard of is to add fins to the vent pipe, to retain
>some of the heat in the house, while still venting to the outdoors. 

venting indoors in wintertime wastes less energy, but you may need to
remove most of the humidity by condensation if you live in an airtight
house or a damp climate. 

i measured an empty speed queen 5400 watt electric dryer that cycled
from 140 to 167 f, an 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.

a thick fin with a good thermal connection might have 3 btu/h-ft^2
(counting both sides) of thermal conductance from the duct surface to
the slow-moving room air. the 4" duct has 0.087ft^2 of cross-section,
so the duct velocity is 110ft^3/min/0.87ft^2 = 1261 ft/m or 14 mph.

the internal warm-air-to-duct-surface conductance might be 2+14/2 = 9
btu/h-f-ft^2, and a linear foot of 4" duct has about 1 ft^2 of surface.
attaching 3 ft^2 of fins to each foot of duct (say 3 circular 1 ft^2
disks with 7" outside diameters and 4" holes in the middle) would make
the heat transfer conductance u = 9 btu/h-f per foot.

the ashrae hof says a heat exchanger like this with a zero capacity
rate ratio (the room air has a much larger heat capacity than the air
moving through the duct) has an effectiveness e = 1-exp(-ntu), so we
might remove 90% of the sensible heat from the air by making e = 0.9
or ntu = 2.3 = du/cmin = 9d/110 = 0.082d using d = 28' of finned duct.

that seems large and expensive. how about putting 120 2 liter plastic
bottles in 2 stacked plastic 55 gallon drums instead? at 2.5 ft^2 of
surface and 4.2 pounds of water per bottle, the total is 147 ft^2 with
220 btu/h-f for slow-moving air and 504 btu/h-f, for a charging time
constant rc = 504btu/f/220btu/h-f = 2.29 hours, so the bottles would
warm by (153-70)(1-exp(-0.33/2.29)) = 11.2 f in 20 minutes, capturing
11.2x504 = 5666 btu of the 6140 btu from the dryer, at 92% efficiency. 


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