Hi,
I took some measurements on our dryer doing a fairly small load:
Maytag Neptune, 2002, electric.
Wet load weight 7.9 lbs, Dry weight 4.2 lbs, Water weight 3.7 lbs
Outlet duct 4 inch dia, 0.0872 sf
Outlet velocity (very constant through load) = 1850 fpm
Flow rate (0.087sf)(1850 ft/min) = 161 cfm
Time to dry 48 minutes including 6 minute "cool down" -- cool down is fan running, but no heat.
Temperature -- I've got a plot of the logged temps, but don't know how to include it here. Anyway, it built up from 70F to about 140F over first 7 minutes (I believe heat element was on all of this time), then temp cylced up and down about 140 F (turning element off and on) until the start of the "cool down".
Also (sort of) measured relative humidity, but did not want to leave the logger in the high temperature stream to long as its not designed for those conditions. I believe the RH was about 42% near start and about 24% about half way through.
Sensible Energy Vented Out of House By Dryer Duct:
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Flow rate = (0.0872 ft^2)(1850 ft/min) = 161 cfm
Say that average temp was 140F for about 42 minutes (leave out cool down).
Heat Rate = ((161 ft^3/min)(0.062 lb/ft^3)(60 min/hr)(140F – 65F)(0.24 BTU/lb-F) = 10,800 BTU/hr
Or, for 42 mins, 7550 BTU (2.2 KWH)
This agrees almost exactly with the Canada study that says 2.23 KWH per load for 416 loads a year and 930 KWH total for year.
Energy to heat air pulled in from outside:
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Infiltration Energy = (161 ft^3/min)(48 min)(0.062 lb/ft^3)(70F – 30F)(0.24 BTU/lb-F) = 4600 BTU or 1.3 KWH -- for this small load.
Total air pulled into house = (161 cfm)(48 min) = 7700 cf (about 1/3 rd of a house full).
Latent Energy in Water Vapor Vented Out Dryer Duct:
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Heat that could be recovered if the water vapor in the outlet stream were converted to water inside the house:
Heat = (3.7 lb)(970 BTU/lb) = 3590 BTU or about 1 KWH.
So, for this not full sized load, about 2.2 + 1.3 + 1 = 4.5 KWH of energy per load that could be recovered if you got all of it.
For us, this is our largest single electrical load and is a significant fraction of our monthly bill.
Still thinking about whether something could be done with a heat exchanger made for Coroplast. The Coroplast is polypropylene, which is a good high temp and chemical resistant plastic. I think its about $20 a 4 by 8 sheet. Comes in thicknesses up to 10 mm.
Two sheet of Coroplast would offer 128 sf of heat exchange area -- about the same as 120 ft of 4 inch duct.
The flow area could be quite large, so, I don't think that back pressure would be an issue?
Some sort of condensation drain would be needed.
The dryer already provides a blower for one side of the HX. Other side could be natural convection or a fan of some sort?
Any thoughts on a design?
Gary
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