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re: heat exchanger efficiency
23 mar 2001
m russon  wrote:

>hiya group,

hiya m,

>i am reading conflicting reports claiming that liquid to air heat
>exchangers work quite well. other reports tell me the transfer rate
>stinks and to not waste my time.

you might look at numbers vs words. a liquid-liquid heat exchanger
might transfer 20x more heat with the same surface, but so what?

>...sure hate to build it in a liquid type if the transfer from liquid
>to air is going to be worthless...

another word. we might use a room full of small containers of water with
about 1.5 btu/h-f-ft^2 of still air film to liquid thermal conductance
to store heat from solar-warmed air during the day and warm house air
at night. this is simple, with no plumbing, and it has the advantage of
combining heat storage with heat exchange, but we need lots of container
surface to be thermally efficient with a small air-liquid temp difference
during "charging" and after a few cloudy days in a row. 

a radiant floor surface is also part of a liquid to air heat exchanger,
with something like 1.5 btu/h-f-ft^2 of air film conductance. 

a radiant wall could absorb solar heat in the winter or cool a room in
the summer, with condensation with a very high surface conductance, if
the room air reaches the dew point. we might use a "waterwall fountain"
or a licl fountain...

baseboard radiators with fin tubes have about 5 btu/h-f-ft of conductance.

a 212 f auto radiator which removes the heat from burning a gallon of gas
per hour at idle (about 100k btu) on a 100 f day has an effective thermal
conductance (with its fan) of about 100k/(212-100) = 900 btu/h-f. 

magic aire's $150 all-copper 2'x2' shw 2347 duct heat exchanger transfers
45k btu/hour between 125 f water and 68 f air at 1400 cfm with a 0.1" h20
pressure drop, so its conductance is 45k/(125-68) = 800 btu/h-f. 

how well would it work in still air? i don't have the curves in front of
me, but the ashrae hof says a 1'x1' rough surface has 2+v/2 btu/h-f of
thermal conductance, where v is in mph. we might figure it has 50% open
area, with 1400cfm/2ft^2 = 700 lfm or 8 mph of air flowing through it,
so 45k = (125-68)(2+8/2)a, with an equivalent rough surface a = 132 ft^2.
we might get something like 2x132 = 264 btu/h-f or 15k btu/h in still air. 

we might put a vertical version near the base of a closet with a 2 ft^2
hole at the top and 16.6(2ft^2)sqrt(6'dt) = 81sqrt(dt) cfm of air flowing
through it at 0.46sqrt(dt) mph. it might supply 132(2+0.23sqrt(dt))(125-68)
= 15k+1956sqrt(dt) of heat flow, while 81dt^1.5 flows out of the hole at
the top, so dt = 7.07^2 = 50 f, with a 120 f closet temp and 28.8k btu/h
from the heat exchanger.

for zoning in multiple rooms, the holes at the tops of the closets might
have automatic foundation vents or motorized dampers with thermostats...

nick




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