heat exchanger for compressed air

[PaulLag]

Hello
Hope everybody is fine.
Please I am contacting you for following topic.
I’d need to calculate an heat exchanger that cools compressed air.
Normally these calculations are realized considering dry air.
In this case I need to consider relative humidity of compressed air.
Therefore, if the air touches a surface under dew point temperature, would generate condensate.

Please, does anybody knows any correlation I could consider for this specific case ?
Can anybody recommend me any publication I could refer in order to study the physical model ?

I thank you in advance for any help

 

[1503-44]

Assuming your air is saturated at its intake temperature is the worse case. It won't condense until it gets below that temperature again.

A black swan to a turkey is a white swan to the butcher.

 

[PaulLag]

@1503-44
Many thanks for your answer.
I apologize for not having presented all the data.
I fear that the case I am involved is more complex.
Just to present you some numbers.
Compressed air is entering the heat exchanger at following conditions
Entering @194F, 135 psig 100% RH
Outlet @86F
Therefore the outlet temperature is lower than inlet temperature and therefore saturation.
I deem that we are moving form 2 to 3 and then down to saturation line.

Please, is my interpretation correct ?
Do you know how to realize a calcualtion at these conditions ?

Thanks in advance

 

[1503-44]

You have to include the cooling of the water content in the air.

https://www.engineeringtoolbox.com/maximum-moisture-content-air-d_1403.html

This chart for air at various temperatures and pressures, with example,

https://www.engineeringtoolbox.com/water-content-compressed-air-d_1275.html

Be sure to include the latent heat of condensation of the amount of water that condenses, as it must be carried off by the coolant stream.

 

[MJCronin]

Wouldn't a more prudent heat exchanger calculation include an assumption of completely dry air ?

Why take credit for the cooling of the liquid for a 100% RH input stream when it may not be there ?

(... and this is probably only the mistake of a "dull normal" specification writer .....)

MJCronin
Sr. Process Engineer

 

[georgeverghese]

a)What is the cooling duty of the dry air alone?
b)What is the cooling duty of the water vapor component, including the condensing duty and subcooling duty?

Since water vapor in air is only a small mole fraction of total air, suspect (b) may be a small fraction of (a) even though latent heat of steam is high. If this is so, it may be conservative to include the duty of (b) into (a), then increasing the total surface area required for (a) alone by a factor equal to

A2 = A1 x (Qa + Qb)/Qa,

Where A1 is the surface area required for sensible cooling of dry air alone.

We can do this on the assumption that (a) sensible cooling htc is much lower than that for condensing water vapor and (b) assume this is shellside cooling of wet air stream, therefore all water condensate drops out onto shell bottom without occupying tube OD surface.

You can specify this increase in surface area from A1 to A2 in the excess surface area entry on your TEMA datasheet.

 

[PaulLag]

Hello all
I would like to thank you for your kind answers.
I will try to answer to these questions, defining also my issues.
Unfortunately, I am still stuck in defining the mathematical model.

Staring with answering or commenting your posts.

@1503-44
Do I simply consider the max water content at inlet conditions and consider that all the content condenses?

@MJCronin
I need to consider wet air and not dry in order to calculate the condnesate part and how much condensate to drain

@georgeverghese
My problem is indeed to determine the b) part, so that I can utilize your equation.

My problem is as follows.
How can I determine if and when the condesation will occour ?
I would expect when the temperature will be lower of the dew point temperature.
But how to determine this point ?
Could you maybe recommend me some literature concernin this topic, maybe there are already some standardized methods ?

I thank you in advance for any help

 

[1503-44]

The water content at discharge conditions will remain in the cooling air stream carried off.. The amount that condenses is the difference between the content at inlet conditions minus the content at outlet conditions. The 1st link I posted shows water content kg/m3 at various temperatures.

If inlet temperature is 30°C, water content is 0.0304 kg/m3 , see 1st link
C kg/m3
20 0.017
30 0.030

At 20C it is 0.017 kg/m3
So 0.030 - 0.017 = 0.013 kg/m3 condenses

A black swan to a turkey is a white swan to the butcher ... and to Boeing.