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re: heat exchanger
8 aug 2000
cary wrote:
>i'm going to use a 55 gallon barrel as a storage reservoir for the hot
>water from my solar panels.
a steel drum would last longer with a plastic film drum liner.
>i'll have a coil of refrigerator copper tube in the barrel running
>from my water supply to the water heater. is there any way to calculate
>what the temperature of the water exiting the tube would be?
the ntu technique, as described in the ashrae handbook of fundamentals.
>my bathtub fills at 4 gpm,
so heat capacity rate cmin = 4gpmx8lb/gx1btu/lb-fx60m/h = 1920 btu/h-f.
>our water supply is now 62 f. i'm hoping the panels can get the water
>in the barrel up to 130 f.
and tci = 62 and thi = 130 f...
>i'm wondering if i should buy the 50' coil of 1/2" or two parallel 50'
>of 3/8" or ???.
the 50' 1/2" coil holds about 1/2 gallon of water, which will emerge
from the coil at close to 130 f right after you turn on the hot water.
it has a = 50pi(1/2/12) = 6.54 ft^2 of surface, and a water-water thermal
conductance u = 30 btu/h-f-ft^2, approximately, so the number of heat
transfer units ntu = au/cmin = 6.54x30/1920 = 0.102. the capacity rate
ratio z = cmin/cmax = 0, since there's lots of hot water in the drum,
compared to the cold water heated, so the heat exchanger effectiveness
e = 1-exp(-0.102) = 0.0972. after the stored hot water runs out, the
water will exit the coil at tco = tci+e(thi-tci) = 62+0.0972(130-62)
= 68.6 f.
two 50' 3/8 coils store 0.6 gallons of water, with 9.8 ft^2 of surface,
which makes e 0.153 and tco = 62+0.153(130-62) = 72.4. better...
my 55 gallon drum on top of a woodstove has 300' of 5/8" 160 f reinforced
garden hose inside, 2 75' pieces (swan's model pm5875, grainger #2p098,
$34.95 each, with a lifetime guaraowntee) in series, paralleled with 2
more in series. it stores about 1.6 gallons of water, with a = 49.1 ft^2
and u = 10, approximately, so ntu = 0.26 and e = 1-exp(-0.026) = 0.23,
so it would deliver 77.3 f water.
this would work a lot better with a convection loop below the existing
water heater or a tiny recirculation pump (grainger's 1/150 hp $82 4rg98?)
through the existing water heater. we might use a differential thermostat
to pump water from the hot end of the hose to a t at the upper cold water
input to the water heater whenever it's warmer than the lower water heater
drain, which would be connected to the cold end of the hose...
nick
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