help me

Exactly how much detail do you want?

Do this help from wikipedia?

http://en.wikipedia.org/wiki/Reboiler

Or how about this except from Perry's Chemical Engineer's Handbook?

Kettle Reboilers

It has been generally assumed that kettle
reboilers operate in the pool boiling mode, but with a lower peak heat
flux because of vapor binding and blanketing of the upper tubes in the
bundle. There is some evidence that vapor generation in the bundle
causes a high circulation rate through the bundle. The result is that, at
the lower heat fluxes, the kettle reboiler actually gives higher heat transfer
coefficients than a single tube. Present understanding of the
recirculation phenomenon is insufficient to take advantage of this
in design. Available nucleate pool boiling correlations are only very
approximate, failing to account for differences in the nucleation characteristics
of different surfaces. The Mostinski correlation [Eq.
(5-102)] and the McNelly correlation [Eq. (5-103)] are generally the
best for single components or narrow-boiling-range mixtures at low
fluxes, though they may give errors of 40 to 50 percent. Experimental
heat-transfer coefficients for pool boiling of a given liquid on a given
surface should be used if available. The bundle peak heat flux is a
function of tube-bundle geometry, especially of tube-packing density;
in the absence of better information, the Palen-Small modification
[Eq. (5-108)] of the Zuber maximum-heat-flux correlation is recommended.
A general method for analyzing kettle reboiler performance is by
Fair and Klip, Chem. Eng. Prog. 79(3), 86 (1983). It is effectively limited
to computer application.
Kettle reboilers are generally assumed to require negligible pressure
drop. It is important to provide good longitudinal liquid flow
paths within the shell so that the liquid is uniformly distributed along
the entire length of the tubes and excessive local vaporization and
vapor binding are avoided.

Also, here is a short page on reboiler selection:

http://www.distillationgroup.com/questions/question011.htm