# Heat exchanger effectiveness

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?

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russ_watters
Mentor
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.

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