Optimizing Heat Exchanger Efficiency: Understanding Effectiveness and Capacity

[cabellos2]

If I calculate a heat exchanger effectiveness of 0.5 does this mean the heat exchanger is only 50% efficient in its transfer of thermal energy? Therefore if the capacity of the heat exchanger chosen is based on Q, then would you need to select a unit double this to account for only 0.5 effectiveness?

 

[russ_watters]

No. By conservation of energy, any energy that is lost on one side must be gained on the other so in that way you would have to call them 100% efficient. But the effectiveness is actually just a measure of the ability of a heat exchanger to exchange temperatures. Ie, a perfect counterflow heat exchanger should be able to get the two fluids to swap temperatures (assuming the same fluid and mass flow rate). If you have a=50F air and b=90F air going through a perfect heat exchanger, you should get a=90F air and b=50F air out of it. 50% effective would give you 70F air out from both streams.

 

[cabellos2]

Ok I understand the effectiveness.

But suppose I have calculated a value of Q and this determines my selection of a heating capacity of a heat exchanger. Is it advisable to select an exchanger with a capacity slightly higher than this in the same way a 'safety factor' might be incorporated into a design problem?

 

[russ_watters]

It may depend on the application, but yes, you probably want a safety factor.