[RomanKatz]

Hi all,
 
I would really appreciate your assistance in a problem I have:
 
A flue gas (10% humidity, 140 DegC, 80m3/hr) is to be cooled down by a tap water stream to 70DegC. Water stream temp is about 30DegC.
 
This flue gas is discharged by an ID FAN, 525 mmWater gage, 3 HP.(1.05 atm???)
 
What is the necessary water stream and its final temp and what is the methodics for solving this problem?
 
If my calculation is correct too much water is needed and that cant be done.

I did the mass balance Q=mCpdT and found the Cp online but I am not sure that this is the right way to solve this problem.

Thanks very much

Roman Katz

 

[awolokay]

I haven't actually run the numbers you gave, but my expectation is that it should be possible to cool the stream as you describe - what you are asking for is quite common as a first step in flue gas treatment which is what I suspect you are doing.

Using specific heats should get you quite close to the answer, but you do have to remember to account for the latent heat of vaporization of the water, which in fact probably contributes the majority of the cooling. Because this is in part an evaporation problem, you have to check that you are not exceeding 100% humidity in the cooled gas (or practically, some value less than this) to have a driving force for evaporation.

The more accurate way to do the problem is to work with enthalpies and to do an enthalpy balance. When doing the enthalpy balance you have to pay close attention to the standard state of the water - i.e. whether it is liquid or gaseous water and make adjustments if necessary to account for the latent heat of evaporation.

[curiouscat]

Try looking up a Psychometric Table. It'll have a lot of data that you'll need.

As stated, your problem seemed underdetermined: I'm a little unsure if you can find both the  "necessary water stream and its final temp"; one of those will determine the other, won't it?