Heat recovery

[mechbuild]

Hi every one,
Can anybody advice me about reclaimimg the ventilation. At present, I am designing an HVAC system in which 25000 cfm of cool air to be axhausted outside. I want to reclaim the cooling of this exhaust air.
Any advice would be highly appreciated.

Regards,

 

[IRstuff]

Generally, the quality of the heating, or cooling, content of the final product is too low to be of much use. If there's little increase in heat content from the interior of the building, then there's too much cooling going on in the first place, and the thermostats should be adjusted. Your A/C should be taking, say, 90°F outside air, cooling to 55°F, resulting in 78°F exhaust air. So, a 35°F delta has been reduced by 23°F, which cannot be recovered.

In a normal situation, the only practical thing the exhaust air could be used for is to pre-cool the make-up outside air, which would reduce the overall cooling that the A/C needs to generate That is, if you could knock the inlet air down a few degrees, your A/C will have less load, but, the savings need to be balanced against the cost of the pre-cooling and its maintenance.

TTFN

FAQ731-376

 

[HVAC68]

25,000 cfm is a lot of air being exhausted ! This means, you will be taking at least 25,000 cfm of outside air, if not more for maintaining positive pressure.

You can use a heat recovery wheel or a plate type heat exchanger for energy savings. Considerable savings is possible, especially, if the outside air temperature is high.



HVAC68

 

[davidcamell]

What isn't mentioned is the possibility to take LATENT load out of the incoming OA (in other words, moisture). 90 deg humid air takes a lot more cooling to get down to 55 than 90 deg dry air.

An energy recovery unit with a desiccant energy recovery wheel can help pre-"dry" the OA with the exhaust stream. A wheel rotates through each airstream, and it is covered with a material engineered to carry over molecules the size of water vapor or smaller (in other words, it doesn't carry over the larger particles you probably want exhausted out in the first place).

Take a look at your sensible cooling ratio on your calculated loads. This can give you an indication of how much moisture you can pre-treat before the OA hits your cooling coil.

You're best bet is to contact a few manufacturer's reps for different brands of units and tell them what you need. A good rep will ask for a lor of information and help you with selections.

 

[daly1]

can you add some info on where the 25,000 cfm of exhaust comes from? If this is bathrooms, hallways, and similar areas I have used total enthalpy wheels with success (being sure to apply good filtration on both sides).

For labs or areas with potential contaminants I would never use any wheel and prefer heat pipe or run around coils.

 

[mechbuild]

Hi,
This 25000 cfm is coming from a body washing center where dead bodies are being washed and finally sent to their relatives for a last visit.

I beleive this air is highly contaminated. so what type of heat recovery would you recommend.

Regards,

 

[ChrisConley]

You could have just said contaiminated air and left it at that.... *shudder*

For a contaminated air stream you'd best look at a 0 transfer air system. I'd recommend a heat pipe system if the air streams are close together. Glycol run around systems aren't as efficient, but the fresh and exhaust air streams can be anywhere in the building.

 

[mauricestoker]

I think you are stuck with indirect heat recovery only. That means using only sensible heat recovery unless you have a waste heat stream that you can regenerate an indirect desiccant or salt, like a Kathabar system.

From an energy standpoint I think you might have better luck trying to reduce air flow or temperature setpoint when not occupied.