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Exhaust gas heat exchanger calculations

by dubber91
Tags: calculations, exchanger, exhaust, heat
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May6-09, 10:47 AM
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Hi Guys newbie here.

Im trying to design a CHP system in which an engine with steady exhaust gas flow uses a heat exchanger to heat water for use around the home (central heating, washing water etc). The engine specifics are to be developed but i need to be able to size a heat exchanger so i can develop the system. The exhaust composition is largely air but obviously steam is present. The water is standard, chalky mains water.
Now the hot tank of water can be sized accordingly, and the flow rate specified by using a pump or pumps. How would you go about setting up a spreadsheet using standard heat transfer equations to model the system? Ive got pretty basic heat transfer knowledge (i.e. newtons law of cooling and heat transfer coefficients).

Any help would be great appreciated :)

Kindest regards

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May6-09, 11:40 AM
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brewnog's Avatar
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Conservation of energy, i.e. the energy taken out of the exhaust gas equals the energy recovered for your hot water system.

Heat available in the exhaust is mass flow, multiplied by the specific heat capacity (is this a Diesel or petrol engine?), multiplied by the temperature difference over the heat exchanger (so exhaust outlet temperature minus whatever you can cool down to). Then you need to look at what return temperature and flow rate you want for your hot water system, and size a cooler for that capacity and required effectiveness.
May6-09, 11:50 AM
P: 3
OK. But how would i then go about sizing a heat exchanger? Assume an exchanger efficiency? How much energy is recovered over what size of heat exchanger? thanks in advance.....

May6-09, 12:02 PM
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Exhaust gas heat exchanger calculations

That can be pretty tricky; there are people who do just that as a profession. If you have a Heat Transfer book, there should be some empirical data in there. My particular book has a section in there. Work through some stuff like that, and you should be in the ballpark.
May6-09, 12:13 PM
P: 3
any way i can avoid the cost of a book!???

any pointers for appoximate calcs? im not after any particular accuracy....
May6-09, 05:17 PM
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I'd suggest purchasing one unless you want to go into the heat exchanger business. There are hundreds of manufacturers of all kinds of heat exchangers. A simple plate type might work well and be economincal for this application.

The first thing to do is write down what you know about your process, such as:
Fluid being cooled:
Allowable pressure drop though HX:
Inlet temp:

Fluid being heated:
Allowable pressure drop though HX:
Inlet temp:

Most manufacturers can then size a heat exchanger for you and make a recommendation. Manufacturers also sometimes have software on the net you can use to size one yourself.

Take a look for manufacturers here:

Here's a typical manufacturer's web page that includes selection software:

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