Q. I am working on the design of a Dedicated Outdoor Air System ( DOAS ) unit. I am leaning towards using double wheel units (latent and sensible wheels). What is your opinion on this approach vs. using a wraparound heat pipe.
Rick… From my viewpoint the dual wheel design is a much less efficient approach than using a wraparound heat pipe. There are a few simple reasons:
- the sensible reheat wheel gets its energy from the exhaust air stream and does not have any pre-cooling effect, the wrap around heat pipe gets its energy from the leaving air off the cooling coil and pre-cools the air coming into the cooling coil by 8-9 deg F
- the sensible reheat wheel is inherently designed to be 70+% efficient due to its size and velocity, which is much more than you need, you could get a 25 deg rise out of that which you could never use, the ideal reheat amount is 8-10 deg
- the wrap around heat pipe is 2 rows and 30% efficient, which gives you exactly the 8-10 deg you want, and by precooling the air it saves almost 30% more capacity than the sensible wheel
- the heat pipe is over 30% lower static pressure drop
- the heat pipe takes up less space and is less costly
- both devices can modulate the discharge temperature, the heat pipe has solenoid valves and the heat wheel has a VFD, so both are equally controllable
- the heat pipe is passive and the wheel is not. There will be more maintenance associated with the wheel
Note that we are still using the latent wheel in the wrap around heat pipe choice. We are just eliminating the sensible wheel in favor of a wrap around heat pipe.