Heat Exchanger Efficiency

[miclwl]

Hi all,

I am working on a combined heat and power project, which will use a gas-water heat exchanger to capture the waste heat from exhaust gas of turbine and utilize this heat to heat up water. I need to have a general figure about the efficiency of gas-water heat exchanger.

I would like to ask what is the general range of efficiency of gas-water heat exchanger?

Regards,
Michael

 

[BB78]

It depends from the arrangement of the exchanger and flow rates. There are some experimental relations between efficiency and NTU (Number of Transfer Units), for several geometries.
Roughly, for a single pass cross-flow heat exchanger you can assume 0.75/0.85.

 

[miclwl]

For my case, I have hot exhaust gas with flow rate of about 1000 kg/hr into the heat exchanger, and water flow rate of about 6000 kg/hr. I assume the overall heat transfer coefficient to be 130 W/m2.K, and surface area of about 23m2.

I have found that the efficiency of the heat exchanger calculated from the above data is above 95%. Is my calculation correct?

 

[TD2K]

How are you defining 100% efficient and thereby, calculating your efficiency as 95%?

 

[miclwl]

I define the heat energy in the exhaust gas flowing into the heat exchanger as energy input. The heat energy transferred to the water as energy output. The efficiency is defined as:

Efficiency = energy output/energy input

 

[TBP]

Efficiency isn't really a good term for heat exchangers. If they're insulated properly, heat out will be almost equal to heat in. I think "effective" is a better term. You can have a poorly designed unit transfer the same heat as a well designed one, just by making it have a larger surface area. The other consideration is pressure drop across the heat exchanger. A small unit with an ugly pressure drop won't usually be that much good.

Normally, the best thing to do is provide some HX manufacturers with your application data, and let them work out the unit best suited. Unless this type of calculation is something you're going to do regularly (and for most people, it's not) let the guys who do it all the time handle it. They'll be able to zip through it pretty fast, and may even have some standard designs that they can offer, which will cut the price and delivery time down considerably.

 

[CountZero]

Efficieny in this instance would be the 2nd law efficiency: that is the total amount of energy transfered divided by the total possible heat that could be transfered. From there you could determine how effective your h.e. design is and how much more this design could be improved.

I hope that helps.