4 replies to this topic

[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.

What is the necessary water stream and what is the methodics for solving this problem?

If my calculation is correct too much water is needed and that cant be done.


Thanks very much.
Roman Katz

[Flame]

Underlying principle

Heat given out by flue gas = Heat taken in by water

Calculate the average cp (specifc heat) & M.W (for evaluating density using ideal gas law) of flue gas by identifying individual flue gas components & their respective cps (@ average temp. of 105°C). Caclulate flue gas Mass flow (kg/h) x cp (kcal/kg°C) x Delta T (°C) to find out the heat released (kcal/hr). Divide the same by specific enthalpy of water @ atmospheric pressure, this is roughly equal to terminal temperature difference of water (70°C-30°C) in kcal/kg. You will obatin the water flow rate.

My very rough estimate is around 30 kg/h (in the absence of much data), please verify exactly.

Edited by Flame, 29 August 2012 - 11:53 AM.

[Art Montemayor]

Roman:

Flame is not correct in his estimates. Why don’t you simply furnish YOUR calculations? The Forum response would be much more detailed and positive. We don’t have the slightest idea of what you call “too much water”.

The algorithm you have to follow is:

• Calculate the amount of water vapor in the flue gas;
• Calculate the mass flow rate of the flue gas;
• Calculate the dew point of the water vapor in the flue gas; this will tell you at what temperature you will start to condense some of the water vapor (this is latent heat of vaporization);
• Calculate the amount of water vapor left in the flue gas after it is cooled to 70 ^oC;
• Now calculate the three (3) heat transfers taking place:

• The sensible heat from the flue gas (140 ^oC down to the water dew point);
• The water latent heat of condensation at the dew point;
• The sensible heat cooling the flue gas and the condensed water (dew point to 70 ^oC)

Add up the three heat transfers; that is the total cooler duty.
Calculate the cooling water required using the total cooler duty.

[Flame]

Thank you Art for the methodological explanation.

I had completely missed out the heat given up by condensation of water vapor in the flue gas.

[Santoshp9]

Dear Art Montemayor Sir,

As you described to calculate the three (3) heat transfers taking place:
1.The sensible heat from the flue gas (140 oC down to the water dew point);

2.The water latent heat of condensation at the dew point;

3.The sensible heat cooling the flue gas and the condensed water (dew point to 70 oC)

for calculating sensible heat transfer for flue gas ,I think no need to calculate it from 140°C to dewpoint of water vapor & then dew point to 70°c,one can calculate it oneshort from 140°c to 70°c.