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re: using car radiators as heat exchangers
2 jun 2000
news wrote:
>there are many of these air-to-air heat exchangers on the market. the most
>efficient claims 90% heat reclaim... an led gives the percentage of heat
>reclaimed by sensing the difference.
interesting thought, but one would have to measure two differences to find
e = (thi-tho)/(thi-tci), eg (65-35)/(65-30) = 0.86(%) in a 65 f house with
outgoing 35 f air and incoming 30 f air, ignoring latent heat.
with equal flow rates, the efficiency of a counterflow heat exchanger is
ntu/(1+ntu), where ntu = au/c. a is the heat transfer area in square feet,
u is the thermal conductance through the area in btu/h-f-ft^2, and c is
the flow rate in btu/h-f, approximately the same as the airflow in cfm.
>these units only just about pay for the running costs of the motors with
>what they reclaim in heat... they are not cheap and you have to install
>ductwork about the house.
seems to me the fresh air would diffuse around the house. one might make
a passive version with no motors, eg a double-wall woodstove chimney with
warmer indoor air rising up the inner pipe and cooler outdoor air falling
down between the outer and inner pipes, even without a fire in the stove.
in a less-airtight house, warm air might rise up in both pipes.
a 16' chimney with an 6" inner and 8" outer pipe has a = 25 ft^2 and
u = 0.75 btu/h-ft^2 with slowly-moving air on both sides of the inner
pipe, so e = 18.8/(18.8+c), eg 24% at 60 cfm.
a blower (outdoors, in series with a cooling thermostat on the inner pipe)
could improve thermal efficiency by increasing u and allow control with
a humidistat, as you suggest.
nick
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