Kettle Reboiler Tube Pitch. 4

[Christine74]

Why do kettle reboilers usually feature an "open" tube pitch, with p/d ratios on the order of 1.5 - 2.0 as opposed to 1.25 for conventional shell and tube exchangers? Vapor blanketing? Pressure drop? Thinner tubesheets?

Thanks.

-Christine

 

[Montemayor]

Christine:

I've never heard about 'an "open" tube pitch'; I've always used a square tube pitch on my kettle reboilers - some of which I designed and built myself.

The reason I always used a square pitch is to enable the cleaning of the external surfaces of the tubes in the usual U-tube bundle. Otherwise, if you resort to using triangular pitch, the cleaning is next to impossible.

Vapor blanketing, Pressure drop, or Thinner tubesheets have nothing to do with the selection of the type of pitch.

 

[rmw]

Everything is a trade off. Art Montemeyer has stated a valid reason for having an 'open' (as in not tight) tube pitch. On the other hand, the more 'open' the tube pitch, the lower the reynolds number of the fluid flowing across the tubes and the more surface required.

Do you want a cleanable bundle, do as Art suggests. Do you want an efficient bundle, tighten (close) up the pitch to the minimum required by TEMA for your tube OD, square or triangular.

rmw

 

[unclesyd]

I'll second Montemayor's post in favoring a square pitch tube layout. The Chem E's will scream and holler that you are taking away their heat transfer area, but in the majority of the time they will have to hid behind there monster computer monitors as the overall efficiency of the square set bundle over time will be better if you made a side by side comparison with a delta pitch set.

My first line of attack where there was a fouling situation of any heat exchange was to try to force opening up the tubes with a square pitch set. At least you could clean them when they fouled.

 

[rmw]

If they do the sizing right the pitch will be taken into account and no heat transfer area will be lost. The area will be larger due to the efficiency loss, and the heat exchanger more expensive, but if that is the trade off for cleanability, then it is a good one. It is the bean counters that will scream the most.

rmw

 

[FireTurnip]

Increasing tube pitch in a kettle reboiler decreases resistance to flow to the inner tubes and thus increases heat flux.