10 replies to this topic

[Mohamed_Mamdouh]

Hello,
I need help with a certain concept of heat transfer.

Can someone help me understanding the difference between the tube fouling resistance based on tube ID and tube OD ?

Also, which is better to use on process datasheets or softwares like EDR ?

Also why they have different values or they should be the same ?

Appreciate your help

[latexman]

The fouling on the tube ID comes from the tubeside fluid.  The fouling on the tube OD comes from the shellside fluid.  If the tubeside fluid and shellside fluid are markedly different, it would not be surpising that the tubeside fouling and shellside fouling are markedly different, also.  Conversely, if the tubeside fluid and shellside fluid are identical, it would not be surpising that the tubeside fouling and shellside fouling are identical, also.

[Bobby Strain]

You appear to be a student.

 

Bobby

[Mohamed_Mamdouh]

The fouling on the tube ID comes from the tubeside fluid. The fouling on the tube OD comes from the shellside fluid. If the tubeside fluid and shellside fluid are markedly different, it would not be surpising that the tubeside fouling and shellside fouling are markedly different, also. Conversely, if the tubeside fluid and shellside fluid are identical, it would not be surpising that the tubeside fouling and shellside fouling are identical, also.

Does this mean that the fouling based on the tube OD similar to the shell side fouling ?

[latexman]

Yes, more than similar, exactly.  The fluid on the OD of the tube IS the shellside fluid.

[Mohamed_Mamdouh]

Yes, more than similar, exactly. The fluid on the OD of the tube IS the shellside fluid.

Perfect ! I actually asked this question because I received a shell and tube HE datasheet from a client where the shell side fouling is specified as well as the fouling of tube side based on the OD ;yet, they are not the same value ?

Is there an explanation for this or there is something wrong here ?

[Vegeta]

I believe this is related to the mathematical definition of the overall heat transfer coefficient "U".

[breizh]

Hi,

Agree with Vegeta.

The global heat transfer coefficient depends on the area of reference (inside or outside tube):.

*if the reference is the inside pipe diameter:

 1/Udi=1/hi+1/hdi +e/ (lamba *(Di+Do)/(2*Di)) +1/(hde*Do/Di) +1/(he*Do/Di)

 

*if the reference is the outside pipe diameter:

 1/Ude=1/(hi*Di/Do) +1/(hdi*Di/Do) +e/ (lamba *(Di+Do)/(2*Do)) +1/hde +1/he

 

e=thickness of the pipe; lambda =thermal conductibility of the pipe ;  hdi,hde : fouling coefficient internal and external 

Di internal pipe diameter; Do external pipe diameter.

 

Breizh

[latexman]

Agree with Vegeta and breizh.  In your S&T HE, the tube side fouling factor and shell side fouling factor are combined to obtain the overall heat transfer coefficient referenced to the heat transfer area based on tube OD.  Well done, guys!

[Mohamed_Mamdouh]

Hi,

Agree with Vegeta.

The global heat transfer coefficient depends on the area of reference (inside or outside tube):.

*if the reference is the inside pipe diameter:

 1/Udi=1/hi+1/hdi +e/ (lamba *(Di+Do)/(2*Di)) +1/(hde*Do/Di) +1/(he*Do/Di)

 

*if the reference is the outside pipe diameter:

 1/Ude=1/(hi*Di/Do) +1/(hdi*Di/Do) +e/ (lamba *(Di+Do)/(2*Do)) +1/hde +1/he

 

e=thickness of the pipe; lambda =thermal conductibility of the pipe ;  hdi,hde : fouling coefficient internal and external 

Di internal pipe diameter; Do external pipe diameter.

 

Breizh

 

Thank you Breizh for comment.

 

If you can see attached, the datasheet specifies both ID and OD in the tube side fouling.

 

But why they didn't just put the 0.00021 in the shell side if it is based on the tube OD ? 

Attached Files

•  Fouling.JPG   116.2KB   0 downloads

[Pilesar]

The tubeside fouling resistance is converted to an outside diameter basis for convenience and consistency. Tubes are sized with consistent outside diameter for nominal sized tubing even though the wall thickness may change. Heat exchanger surface area is traditionally based on the outside surface of the tube. So resistance is also stated based on the outside surface of the tube. Even though there is heat transferred from the inside of the tube to the tube wall, through the tube wall, then from the outside surface of the tube to the shell side fluid, it is traditional to just use the tube outside surface area in calculations. As long as everyone understands the same basis, there is no additional inaccuracy in practice. Because there is a different metal surface area inside the tube compared to outside the tube, converting tube resistance to an O.D. basis eliminates dealing with two surface areas in calculations.