Heat Exchanger EES Code

[mfdoom]

I am writing a code on EES that simulates the exchangers in a boiler with hot air being flue gas, interfacing with water in the tubes.

My aim is to see how raising the flow rate of water would reduce the air's temp. at the exit.

my equation for this is m_dot_water*cp*ΔT=UA*((Tf_h-Ts_h-Tf_c+Ts_c)/ln(Tf_h-Ts_h/Tf_c-Ts_c)) LMTD is counter flow oreintation

Tf_h=Temp hot air in
Ts_h=Temp hot water out
Tf_c=Temp cold air out
Ts_c=Temp cold water in

Now if I leave my Tf_c as unknown, EES will solve it.

The problem I am facing is that when I increase the m_dot_water in the equation, the value of the Tf_c starts increasing, but it should decrease as more energy is absorbed from the hot air.

Any input would be greatly appreciated.

 

[hendersdc]

Is the equation you have in your post exactly how you wrote it in EES? I think you are probably missing a few parentheses to get the order of operations right. Admittedly I don't know what the correct equation should be but I would bet that you should at least change it to this so that the math is performed correctly. All the parentheses in bold are new:

m_dot_water*cp*ΔT=UA*(((Tf_h-Ts_h)-(Tf_c+Ts_c))/ln((Tf_h-Ts_h)/(Tf_c-Ts_c))