Design of a Double Pipe Heat Exchanger

[AAMAIK]

Homework Statement

It is desired to heat 5130 kg/hr of cold benzene from 80 to 120℉ using hot toluene using double pipe heat exchanger. Toluene will be cooled from 160 to 100℉. The specific gravities at 68℉ are 0.88 and 0.87 respectively. A fouling factor of 0.001 should be provided for each stream, and the allowable pressure drop on each stream is 10 psi. A number of 9-ft hairpins of 3-by 2-in. IPS pipe are available. How many hairpins are required?

Homework Equations

The Attempt at a Solution

The calculated pressure drop is smaller than the allowable, so what changes to the original design can I implement. Should I consider reducing the flow area?
Attached Files contains my attempt at solving the problem whereas in the attached thumbnails contains the relevant figures and tables necessary to calculate the physical properties and heat transfer coefficients.[/B]

 

[Chestermiller]

I had trouble reading your handwriting. Were you told the pipe diameters?

 

[AAMAIK]

I had trouble reading your handwriting. Were you told the pipe diameters?

My bad, can you point out to me parts which are unclear.
Yes
Outer Pipe
Outer Diameter, (in)=3.50
Inner Diameter, (in)=3.068

Inner Pipe
Outer Diameter, (in)=2.38
Inner Diameter, (in)=2.067

 

[Chestermiller]

If you had all the properties in Imperial units, is there a reason you didn't solve the problem using Imperial units? Are you familiar with the PCU, the pound-centigrade-unit, the amount of heat to raise the temperature of water 1 degree centigrade (1.8 BTU)?

 

[AAMAIK]

If you had all the properties in Imperial units, is there a reason you didn't solve the problem using Imperial units? Are you familiar with the PCU, the pound-centigrade-unit, the amount of heat to raise the temperature of water 1 degree centigrade (1.8 BTU)?

My professor insisted we solve the problem in SI units. Yes, I am.

 

[Chestermiller]

You indicated that the pressure drops are both <10 psi. What values did you get? You're trying to decide whether you should go to longer lower-diameter tubes. This is a judgment call. Is it really worth changing?

 

[AAMAIK]

I miscalculated the pressure drop in psi but after going through the calculations again, the pressure drops across the annulus and inner pipes are approximately 14 psi and 6 psi respectively.

 

[AAMAIK]

You indicated that the pressure drops are both <10 psi. What values did you get? You're trying to decide whether you should go to longer lower-diameter tubes. This is a judgment call. Is it really worth changing?

Should I consider an outer pipe with higher nominal pipe size instead to reduce the pressure drop across the annulus?

 

[Chestermiller]

I miscalculated the pressure drop in psi but after going through the calculations again, the pressure drops across the annulus and inner pipes are approximately 14 psi and 6 psi respectively.

Well, since the 14 psi is above specifications, I guess you could go to a little larger pipe diameters.

 

[Chestermiller]

Should I consider an outer pipe with higher nominal pipe size instead to reduce the pressure drop across the annulus?

Judgment call. I guess so.

 

[Nik_2213]

Some-what off-topic...
Given benzene has been specified as one flow, be sure to review the safety and handling details for this known carcinogen, then give a little thought to your heat-exchanger's possible failure modes...

Toluene is not 'safe', but it is significantly less dangerous than its simpler cousin...

 

[AAMAIK]

I forgot to convert the units of the fouling factor from English units to SI units so I changed it and calculated the design overall coefficient, surface area, the required length for the entire path, pressure drops across the annulus and inner pipe. I went to my professor to ask for feedback after making the changes mentioned above he said the values for the pressure drop per hairpin are a bit off from these in the file below (Examples for Mini-project 2).
Annulus- Delta P=1.75 psi
Inner Pipe- Delta P=0.8047 psi
I revised my calculations several times, but I can't spot where I might have gone wrong. Although, the calculated pressure drops are less than the allowable pressure. I have another question, are the given allowable pressure drops per hairpin or for the entire path.

 

[Chestermiller]

Suddenly the example is using degrees F, rather than degrees C. Could this have anything to do with your mismatch? I will look over the PDF later.