Heat transfer flow relationship

[zenapp]

Hi, Im hoping someone can help me out,

Id like to understand the rules around flow in a heat exchanger and how they link to heat transfer. I have a CHP engine generating electricity and heat is recovered from a water cooling system on board through plate heat exchanger where it heat process water for a plant.

The equation im using to calculate heat transfer is -
Dh = C_p *M * dT
where
Dh = Change in heat (energy)
C_p = heat capacity
M = Mass (volume in heat ex)
dT = Change in temperature

On the engine side water is in the exchanger, inlet temperature is 90C, outlet temperature is 80C flow rate is 40m3/h
On the process side of the exchanger it is also water, inlet temperature is 61C, outlet temp is 68C flow rate is also 40m3/hr

Energy Removed from engine side circuit - 4.18kj/kg * 40,000 * 10 = 1672000kJ (464kW equiv)
Energy taken on into process side circuit - 4.18kj/kg * 40,000 * 7 = 1117200kJ (310kW equiv)

My calculations show in an excel model that increasing the flow rate increases heat recovered, although I can manipulate the flow to achieve impossible results, such as increasing the engine flow to 60m3/hr would mean (in the model) 696kW heat taken from the engine, I know this isnt possible from the engine data sheet, in the same way the process side recovers more heat than removed from the engine circuit if I double the flow so that not possible either.

I have attached a model that shows what im working with.

When it comes to optimising or designing flow rate is there a set of rules to use to help. Any help with this appreciated.

Thanks Zen

 

[IRstuff]

This is not a simple equation(s) based problem. You need to hire a thermal engineer.

TTFN
faq731-376

 

[Terminus0]

Heat Exchangers are tricky and require a whole bunch of different equations for different types of conditions and types of heat exchangers.

 

[zenapp]

Thanks for your help.

Please can you point me in the right direction, Terminus, you mention multiple equations, can you tell me what they are.. IRStuff, you say this is for a thermal engineer, at the moment Im the best we have got ! Where do i start looking that (s)he would look? Rheology etc? must have the equations...

I hope you can help me out, this would really help me in my career.

Regards

Zen

 

[Terminus0]

I'm going to assume this heat exchanger is some type of Shell and Tube heat exchanger for the purpose of giving you a series of example equations.

Here's an example of what a Shell and Tube looks like

http://www.spiraxsarco.com/images/resources/steam-engineering-tutorials/2/13/Fig_2_13_4.gif

You then need to know if the exchanger is a parallel flow or counter flow heat exchanger.
Parallel Flow

http://marineinsight.com/wp-content/uploads/2010/12/parallel-flow-heat-exchanger.gif

Counter Flow

http://farm3.static.flickr.com/2303/2090037059_c5c57422a7.jpg

You need to figure out the overall heat transfer coefficient through doing probably Thermal resistance calculation

http://en.wikipedia.org/wiki/Thermal_resistance

You then use the log mean temperature difference equation

http://en.wikipedia.org/wiki/Log_mean_temperature_difference

So everything then plugs into this final equation
q=U*A*deltaT(log mean)
With U being the overall heat transfer coefficient, A the heat transfer surface area, and deltaT(log mean) being the log mean temperature difference.

Now you wanted to relate the flow rate to the total heat transfer?

I believe the flow rate would factor in somewhere when you would be calculating the h(avg) the avg heat transfer coefficient inside the pipes and the shell maybe through getting using a Heat Transfer correlation?

Needless to say this would all be very messy and make you very unhappy being that you don't seem to have any background in heat exchangers.

 

[Terminus0]

If you have to be the one that does this get a good Heat Transfer Book if you don't already have one.

I'd suggest 'Fundamentals of Heat and Mass Transfer' by Incropera and DeWitt

 

[IRstuff]

This one

http://web.mit.edu/lienhard/

http://www/ahtt.html

is free to download. It's a bit too theoretical for my tastes.


TTFN
faq731-376

 

[zenapp]

Thanks guys, appreciate your answers, ill let you know how I get on
Z