Heat Exchange using cooling water

[asifraza0]

Hi All,

I've got a process stream at 390 deg C and wants to cool up to 38 deg C using cooling water at 29 deg C. I understand to limit the pipe wall temperature at a high process inlet temp. I have to cool/temper the process sream with air up to a certain temp and then intoduce into the H-E using cooling water.

Can somebody guide me as how to estimate the process inlet temperature which would not result in a high tube wall temperature.

Thanks in advance,

Asif Raza

 

[TD2K]

You need to estimate the heat transfer coefficients for both sides of the system and calculate the tube wall temperature from that. The methodology is in your heat transfer textbooks.

 

[epoisses]

First of all, what is a "high" tube wall temperature? Are you planning to use an existing exchanger? What is its DT?
IMO you will need a train of exchangers in series to create good countercurrent, otherwise you will never get down to 38 deg.
Other than that, there is room for improvement in the sense that you would be extremely energy-inefficient by doing nothing useful with the 390 deg stream except heating up cooling water (or cooling air) to an unpractically high temperature. Isn't there any other 250-300 deg stream that can be used for initial cooling, then a ~150 deg stream, then finally cooling water or air to reach the target of 38 deg?

 

[25362]

Is asifraza0's intention to limit temperatures to avoid hard water scale formation ?

 

[asifraza0]

Yes, I want to avoid the hard scale formation due to high skin temperatures.
I'm planning to cool the process up to 200 deg C using air (blowing air in a double pipe H-E) and then use cooling warter to further cool it to 38 deg C.

Asif Raza

 

[DickRussell]

From the last post, mentioning cooling to 200 C by blowing air through a double-pipe HX makes me wonder about the scale of this operation. What is the flow of the hot stream? Is this a very small-scale system or something on a commercial scale?

Normally an air-fin array would be used, with a fan below each cell, and the process side outlet temperature might be around 60 C. Then cooling water can be used to finish the job. I wouldn't put 200 C process fluid against cooling water, because of the possibility of excessive fouling and film boiling.

 

[AzChaudhary]

Mr. Asif if you can give details of the process stream, like type, properties, enthalpy, flow rate. Also provide available margin in your cooling water system. What are your ambient air temp, humidity?

Also if you can give brief description of your process streams, so that some other option may be considered.

Azhar Chaudhary
Asstt Operations Manager
Bosicor Pakistan Ltd
E-add: azharchaudhary.tec@bosicor.com.pk
Cell: +92-300-2943166