Effectivenss Calculations 1

[jrrocks]

Effectiveness of heat exchanger can be calculated as (Tin - Tout)/Max Differential Temperature. I have Energy wheel to extract heat from outside air. OAT=46°C, after energy recovery it drop down to 30°C. Exhaust air Temperature EAT=24°C. so, Effectivess = (46-30)/(46-23), it gives 70% effectiveness which is matching with the manufacturer data. Flow rate is same. OA supply airflow rate is equal to exhaust flow rate.

But when I put Heat Pipe in series with energy wheel, I am getting relatively lower temperature of outside temperature after energy wheel i.e. 26°C. So overall heat recovery system effectiveness is (46-26)/(46-23) = 87%.
In some AHUs the OA temperature drop is even low, i.e. after Heat pipe - Energy wheel OAT is less than 26°C. But i think, in any case my OA temperature will not go below 24°C (EAT temp). So my effectivess of Heat recovery system may come to Unity.

Someone please confirm, if I am on the right track?

 

[GT-EGR]

The equation you are not applying isn't exactly correct, although your calculations seem fine.

The equation for heat recovery effectiveness is:

Effectiveness = [OACFM • (OAT - SAT)] / [MinCFM • (OAT - EAT)]

MinCFM is the lower of the two flowrates, either OA or EA.

That being said, you aren't going to reach unity, because after each heat recovery section, your exhaust air temperature is going to have less and less heat to recover, so at some point you'll reach a point with minimal heat to take, and you'll just be wasting pressure drop. You'd need an infinitely long heat exchanger to reach unity.

Once you consider the physical requirements of doing anything beyond an energy recovery wheel with a heat pipe, you'll see why you've probably gone far enough.

In order to reach an effectiveness of 1, your supply air temperature would have to equal your exhaust air temperature, which isn't going to happen. The other way would be an infinitely lower outside air temperature, which isn't practical in real world situations.

 

[jrrocks]

Thank you, GT-EGR

We have reviewed manufacturer documents. We have found that at several AHUs, the OA temperature drops from 46°C to 21°C after passing through Energy wheel and then Heat Pipe, respectively. However, exhaust flow temperature remain same 23°C.

Yes, individually, we may not receive Unity but when two heat exchanger (Energy wheel and Heat pipe) in series, SYSTEM/COMBINED EFFECTIVENESS is relatively high, as depicted from manufacturer data.