Increase heat transfer capabilities from Heat Exchanger

[Rpsfinest]

Curently fuel gas entering our gas preheater is at 170 deg F. The gas is raised to that temperature from a gas compressor. The gas leaving the preheater (shell and tube heat exchanger) is at around 370 deg F. We are trying to find a way to get away from the compressors because of the maitenaince issues surrounding it. Is there any way we can possibly increase the surface area of the heat exchanger to still control our gas temperature at 370 deg F with only 40 deg gas entering it? I understand we would have to do a heat mass balance to assure the feedwater entering the heater is ample enough to provide the heat source. Just curious if there is any way we can modify the existing preheater?

thanks guys

 

[25362]

In the absence of details, one can only speculate on an increase of ~65% on heat load. The main HT resistance being on the gas side, have you considered adding another (probably finned hairpin-type) exchanger ?

 

[insult2injury]

If you're pulling a large amount of heat out of your feedwater, what does that do to the boiler efficiency and the amount of fuel required?

I2I

 

[magicme]

there are lots of unknown variables at the moment, but i will throw my pennies on the table....

assumptions:
internal fluid is cool and is heating up.
internal fluid goes in at Tinlet and exits at Texit.
mass flow rate and Cp are known.
*hotside fluid temperature is constant at Tamb*
mass flow rate and Cp are known.
U = overall heat transfer coefficient
A = effective surface area
(needless to say, watch the units)

the exit temperature of a fluid from such a device is...

Texit = Tamb - (Tamb-Tinlet)*exp(-U A / mdot / Cp)

to start with, if you know everything but U and A, you can back-calculate U*A from this equation and the known data.

then, re-using the equation, you can re-calculate the required U * A for the new condidtions you desire.

usually U will stay the same and you are trying to simply increase Area, so the increase in required U * A really is an increase in the required Area.

this may not be the final answer but it should be a good "sanity check" on your results.

regards

magicme

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"not all that glitters is gold"

 

[magicme]

on second thought, a more useable form of that equation is....

( U * A ) = - (mdot * Cp) * ln { (Tamb - Texit) / (Tamb - Tinlet) }

again, Tamb is the external fluid temperature and i assumed it is constant. (and don't miss the leading negative sign.)

so to generally re-size a design:

1. use this equation with the original variables (Tinlet, Texit, etc etce) to calculate U * A;
2. plug in the new variables and re-calculate U * A;
3. assuming you don't change the design approach, U is constant , so the ratio of these numbers gives the required change in effective area for the new design.

regards

magicme

------------------------------------
"not all that glitters is gold"

 

[pmover]

magicme,

U is not a constant. think about the equation for U, which is a function of fluid flow rates, fluid properties, and exchanger physical geometry. it would be best to have the enchanger thermally rated at new design conditions by a heat transfer specialist or OEM to obtain reasonable results.

good luck!
-pmover

 

[magicme]

pmover

technically you are correct for the final re-design of the heat exchanger; however, the original question implied the mass flow rates would not change and simply asked how much would the heat exchanger area need to increase to eliminate the compressor.

however, it could be that i got the question wrong.

magicme

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"not all that glitters is gold"