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Old 12-23-12, 07:46 PM   #68
theworldtrekker
Lurking Renovator
 
Join Date: Oct 2012
Location: Corvallis, Oregon
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Overview: a 12 foot co-axial heat exchanger does not have the necessary surface area with PVC pipe and the spare small blower I had laying around to extract a reasonable amount of energy from the dryer exhaust...~325watts

So today I acquired some 6" insulated duct and put it over the existing 4" thin walled (~1/8" wall thickness, drainage) PVC pipe. I re-routed my dryer vent line so it is now only about 14 feet long, and managed to get the 6" insulated duct over about 12 feet of that. I ran a load of laundry through the dryer, put a blower on the 6" pipe in the crawl space and got the following numbers:

Dryer exhaust air temp 130°F (plastic flex surface temp about 2 feet from dryer), at end of 14' of pipe 112°F.
No RH measurements
Dryer estimated 81cfm using handheld anemometer (4" pipe, 4.7m/s measured)
Pipe surface temp where duct began 118°F, ended 90.5°F
Ambient temp in crawlspace today ~60°F
Blower exit temp 85°F, estimated 50cfm using handheld anemometer (2.5" pipe, 7.4m/s measured)

The heat exchanger looks like it was recovering about 1108BTU/hr (325wh) using Gary's earlier formula (50cfm)(0.062lb/ft3)(60min/hr)(85°F-60°F)(0.24BTU/lb-F).

Notes: Judging by the 4" pipe external temps, the blower should have been moving more air, which should improve the above performance numbers some.

Changing the heat conducting material (PVC) to something with a higher thermal conduction coefficient (ie AL) should see a small improvement.

Increasing air turbidity might help ensure that the hottest exhaust air is next to the PVC.

Ensuring the 6" duct isn't resting on the 4" pipe would help, increasing heat exchanging surface area, unlike what occurred in the above test where no spacer was used.
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