Effectiveness of Heat Exchanger

[Eelcos]

At our company building we have a heat recovery ventilation unit with cross flow heat exchanger. We have calculated the Effectiveness of the heat exchanger by measuring the outdoor inlet air temperature, the indoor supply air temperature and the indoor return air temperature. I measured an effectiveness of about 70%. To my feeling this is quite low, because the brochure of the unit specifies up to 90%. But probably that is under optimal conditions.
Someone told to me that the Effectiveness of the unit (in %) strongly depends on the temperature difference between indoor and outdoor. But I have the feeling that the Effectiveness is quite constant (because it is related to the physical heat transfer properties of the heat exchanger), and that the relationship with indoor/outdoor temperature only applies to the absolute value of recovered heat.
I have looked around at the internet, but I could not find any chart of heat exchanger Effectiveness as a function of the indoor/outdoor temperature delta.
Does anyone know how this works? And which role plays the air humidity in the Effectiveness of the unit?

 

[Latexman]

I have looked around at the internet, but I could not find any chart of heat exchanger Effectiveness as a function of the indoor/outdoor temperature delta.

That's because the effectiveness method [aka the Number of Transfer Units (NTU) method) was derived to deal with situations when there is insufficient temperature information to use the log mean temperature difference (LMTD) method. There are equations that can be used though. See NTU method and heat exchanger effectiveness for a start.

Air humidity and condensate formation adds another degree of complexity to the picture. It is not impossible to deal with simultaneous sensible and latent heat transfer, but it is much more difficult.

Good luck,
Latexman

To a ChE, the glass is always full - 1/2 air and 1/2 water.

 

[EdStainless]

Temperatures are only part of the picture. You need to know flow rates on both sides and RH of both.
Because you really need to figure out how much energy is being recovered.

As an extreme example if you have 5 times as much cold air flowing in as warm air leaving you might only heat the incoming air a few degrees even if you recover all of the energy.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[Eelcos]

Ok yes, I forgot to tell part of the story. The ventilation unit applies 'balanced ventilation', so that forward and return air have equal quantity. The air flow has been measured recently, and both flows were equal in air quantity. I have calculated the Effectiveness as: (T_supply_indoor - T_inlet_outdoor)/(T_return_indoor - T_inlet_outdoor) * 100%.
The calculated value is 70% (at 0 degrees Celsius outdoor and 20 degrees Celsius indoor).
The brochure of the unit specifies 90% at -10 degrees Celsius outdoor and 20 degrees Celsius indoor. The manufacturer of the unit says to me that the difference in Effectiveness is caused by the difference in outdoor temperature (0 vs. -10). But I cannot imagine that the effectiveness decreases with 20% if the temperature delta decreases from 30 to 20 degrees. So that's the background of my question.

 

[Latexman]

It does not seem unreasonable to me that the effectiveness increases 29% (90/70) if the temperature delta increases 50% (30/20).

Good luck,
Latexman

To a ChE, the glass is always full - 1/2 air and 1/2 water.