Heat Recovery Wheel in Lab??

[ChasBean1]

Comrades,

I'm reviewing a design that incorporates a heat recovery wheel in a chemical lab school building (post-grad studies)...

I love the idea of heat recovery. I cringe at the thought of using a wheel that spins through a contaminated exhaust air stream into the make-up air stream. The exhaust stream includes hood exhaust.

My thought is that there's been a good salesman for this application and that the designer should re-think this.

I could be off-base and designs may exist that eliminate odor and contaminant reentrainment.

Your thoughts are welcomed and appreciated. -CB

 

[quark]

CB!

You are right. Generally heat recovery wheels are not at all recommended for containment applications. It is a 'must not to do' rule as far as pharmaceutical companies are concerned. Even after using some filters (I doubt, what are they?) the entire system is to be validated for traces of active items which is very difficult as the concentration of containment may not be homogenous.(Even when HEPA filters are used)

Transfer efficiency of Heat recovery wheels is less when compared to other means. (High density may be the reason)

Moreover, If one AHU covers multiple areas, cross contamination is another problem.

Regards,

 

[Yeldud]

CB - I'm in complete agreement with you on this one. All heat recovery wheels i've seen have at least some cross leakage, and this is a real concern if hood exhaust is going through the wheel. I would not want to be the Engineer trying to defend this design during litigation.

I was surprised by Quarks reference to lesser transfer efficiency with Heat Recovery Wheels. Not sure what they are being compared to... heat pipes, run around glycol recovery loop, air to air... Perhaps Quark will clarify for us.
Regards

 

[stevenw]

Heat wheels will always have some cross leakage.
We use flat plate HX for heat recovery in this application.
Venmar, Trane, Engineered Air, etc.. offer this so you can get an off the shelf size/item.

 

[Whoops]

I'll second that. A thermal wheel is a no-no. A flat plate heat exchanger shouldn't cause any contamination of the supply air. The extract fan should be downstream of the heat exchanger & the supply fan upstream, so that any leaks are from the supply and into the extract and not vice versa.

If by 'hood extract' you're referring to what I'd call a fume cupboard, then I wouldn't attempt to recover heat from these air streams, but just dump the air. They will contaminate the ducts, extract fans and heat exchanger, causing maintenance problems. They're probably only a small proportion of the total air extract.

 

[ChasBean1]

Thanks for all your replies. Common sense rules on this one. The closest I got to an answer was in the 1999 ASHRAE Applications Handbook, Ch. 13.17. They reference NFPA-45 indicating that the NFPA strictly forbids it. Reading NFPA-45, they actually don't strictly forbid it but vaguely advise against it (U.S. stuff)... Nonetheless I'm going with common sense here. -CB

 

[blutfort]

ASHRAE 90.1 (1999) advises against it.
Can you get the efficiencies you need from a heat loop?

 

[ChasBean1]

blutfort, that's another topic and a good question. I don't know, but I'd like to know. I know latent wheels perform very well when new - as far as glycol heat recovery closed water loops, I've wondered about the exchange efficiency, at what differential temperatures does it become cost effective to run the HR pump, how do added maintenance costs such as exhaust filtration changing, special handling of filters as hazmat in some applications, coil cleaning, etc., add in when figuring the true benefits of heat recovery...

 

[Whoops]

I refurbished an existing heat recovery run-around system some years ago. When we ran it, we got about 50% heat recovery without any difficulty. The system had 6 row exhaust coils and 4 row supply coils.

I think that the present costs of such coils could make such a system uneconomic for a new installation. A cross-flow heat exchanger, if applicable, could achieve similar efficiencies with a much lower initial cost.

 

[chicopee]

Hi, The Heat recovery wheel concept is not a new idea. Power boilers used by utility companies have such devices to preheat combustion air. Such device is known as the Lungstrum air preaters. Such device is heated by the products of combustion and as the device rotates the much cooler make up air is heated. Obviously health and life safety is not an issue for this application but in you case it would be. It is better for you to check at parallel plate type of heat exchangers keeping in mind though that leakage of contaminated air could still present a hazard.