9 replies to this topic

[chemE610]

I want to know the difference in the design equations for a shell-and-tube heat exchanger and a kettle reboiler.  I know that the LMTD is not applicable for reboilers, why is this the case?

[Bobby Strain]

A kettle is a shell and tube exchanger. The calculations are different because of the open vapor space.

 

Bobby

[Art Montemayor]

A kettle reboiler undergoes submerged boiling (or latent heat transfer).  This type of reboil involves convection currents and much complex heat film coefficients.  An LMTD is not applicable due to the latent heat transfer - but more importantly because of the effects of nucleate boiling.  A heat flux is more applicable.

 

A shell and tube heat exchanger is normally designed for efficient heat transfer using high(er) Reynolds Numbers and higher film coefficients.

 

SRfish can probably give a much better explanation.

[fallah]

chemE610,

 

Kettle type reboiler is a type of shell and tube heat exchanger with differences in design basis some of which are as follows:

 

1) As a submerged bundle equipment, it's a pool boiling device which has to be designed based on the data for nuclear boiling.

2) The tube arrangement , triangular or square pitch, don't have a significant effect on relevant heat transfer coefficient.

3) The shell should be designed so that there would be adequate space for the disengagement of the vapor and liquid.

[srfish]

Art Montemayor and Fallah give a good basic explanation on kettle reboilers. Also a  kettle has two basic flow patterns. There is a vertical circulation around the bundle and a longitudinal flow toward the weir.

 

In a shell and tube heat exchanger there there will be a forced-convection effect. There can be up to six different flow patterns in horizontal flow and six in vertical flow. The highest heat transfer is when there is an annular flow pattern.

[shan]

Functionally, kettle reboilers return only vapor to the column while forced circulation/natural circulation/thermosyphon reboilers send both vapor and liquid back to the column.

[chemE610]

Thank you for your explanations.  They really helped me to understand the exchangers much better.  I really appreciate everyone taking the time to answer my question!  

[breizh]

Hi ,

 

You may find good information in the document below.

 

Good luck,

 

Breizh

[Padmakar Katre]

I want to know the difference in the design equations for a shell-and-tube heat exchanger and a kettle reboiler.  I know that the LMTD is not applicable for reboilers, why is this the case?

 

Hi,

From where you got the information that LMTD is not applicable in case of kettle reboilers? Just the boiling curve and you will come to know delta T is essential for boiling and is a governing factor in reboiler designs. Kettle is a pool boiling mechanism with vapor only at the exit whereas in other shell & tube exchangers which are mainly theromosyphon these are flow boiling mechanism and exit is 2-phase.

 

All these are shell and tube heat exchangers only it's just the TEAM shell type is different,

1. Kettle reboiler - K type shell (shell side boiling) with very high re-circulation of shell side fluid

2. Vertical thermosyphon - E type shell (with tube side boiling-1 tube pass)

3. Horizontal thermosyphon - H, G, J12, X type shells (shell side boiling)

4. Horizontal thermosyphon - tube side boiling (special applications - 2 tube pass)

 

In case of boiling, the dominant coefficient is boiling hence less effect of circulation rate and boiling coefficient is function of wall super heat (delta T or MTD), boiling liquid critical pressure and surface tension etc

 

Keep in mind that vapor fraction in case of kettle is 1 at exit nozzle however it's always less than 1 at exit of tube bundle, any entrainment of liquid drops is taken care by vapor space provided above bundle.

[srfish]

If the boiling is for a single component at constant temperature, the standard LMTD can be used. For a mixture boiling over a temperature range,it gets complicated. To be safe, use the outlet temperature of the boiling mixture in order to calculate the LMTD.