[fb1234]

Hey!

I'm working on a project that requires the use of a bubble column. I'm currently trying to set up a very simple model that does the following:   

                air out (T)
                   ^
             ____|_____
 hot  -->|                |
 water   |                 |
 in  (T1) |_________|----> water out (T)
                   ^
                   |
                cold air in (T2)


1) Inputs: Hot water inlet temperature (T1 <100C), cold air inlet temperature (T2 < T1)
2) Outputs: Tcolumn (Tcolumn = Twater,out = Tair,out = T)
3) Solve an enthalpy balance by assuming that an equal amount of heat Q is transferred from the hot water to the cold air in the column

The balance would be something as follows:

Water: -Q [kJ/s] = (Fwater [kg/s]*Cpwater [kJ/kg/K]*T1 [K])in - (Fwater [kg/s]*Cpwater [kJ/kg/K]*T [K])out
Air : Q [kJ/s] = (Fwater [kg/s]*Cpwater [kJ/kg/K]*T2 [K])in - (Fwater [kg/s]*Cpwater [kJ/kg/K]*T [K])out

4) Set Q = -(-Q)
5) Solve for T

However, when I try to do this the temperature always ends up being something ridiculous, and I don't know why.

Any ideas on what's going wrong? Would appreciate any advice! 

[mjc123]

Why do you have Fwater*Cpwater in the air equation?
Is this just a typo? Do you still get a wrong answer when you use Fair*Cpair?

[fb1234]

Oh wow my bad, thought I changed it. It's a typo! Meant to be Fair*Cpair.

[Enthalpy]

I don't quite understand your convention for signs.

If you want to use signs, the usual convention is that an energy change is positive when the body receives some. So it should be out - in for water as well as air.

Or you could write the conservation of energy : air in + water in = air out + water out.