MIL-HDBK-1003/3
APPENDIX A (Continued)
A-3.01
Rotary Air Wheel. With the rotary air wheel, heat
transfer takes place as the finned wheel rotates between the
exhaust and supply duct. See Figure A-3. There are two types of
rotary air wheels - one transfers only sensible heat, the other
transfers both sensible and latent heat. The wheel is 70 percent
effective for an equal supply and exhaust mass flow rates, but a
certain amount of unavoidable leakage will reduce this
effectiveness. Closely investigate cross contamination effects
on the application, especially when the exhaust air is from a
process source. Give this system full consideration in air
conditioning and ventilating systems where exhaust air is 4,000
cfm or greater.
A-3.02
Plate Heat Exchanger. With the plate heat exchanger
method heat transfers across alternate passages carrying exhaust
and supply air in a counterflow or crossflow pattern. See Figure
A-4 and Figure A-5.
Plate heat exchangers are 40 to 80 percent efficient in
recovering heat, depending on the specific system design,
temperature differences, and flow rates. Crossflow methods are
usually more convenient, but counterflow methods are more
efficient. With the plate exchanger method, only sensible heat
is transferred. Plate heat exchanger is a static device having
no moving parts, allowing for only a minimal chance of cross
contamination. It is a relatively simple method of heat
recovery.
A-3.03
Heat Pipe Method. The heat pipe method involves a
self-contained, closed system which transfers sensible heat.
This method consists of bundles of finned copper tubes, similar
to cooling coils, sealed at each and filled with a wick and
working fluid. The working fluid may be water, refrigerant, or
methanol.
For the most efficient system, the exhaust and supply
air shall be counterflow. Performance also is improved by
sloping the heat pipe so the warm side is lower than the cool
side. See Figure A-6. For more information refer to ASHRAE
Equipment Handbook, the chapter entitled "Air-to-Air Energy
Recovery Equipment."
A-3.04
Runaround System (Closed Loop) Method. With the closed
loop systems method, a hydronic system transfers sensible heat
from the exhaust air to the outdoor air using water, glycol, or
some other sensible heat fluid. See Figure A-7.
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