IN-TUBE CONDENSATION PROCESS

METHODE DE CONDENSATION SOUS TUBE

English Abstract

ABSTRACT
Title: In-Tube Condensation Process
A heat exchange process of the kind in which a condensing
vapour flows in parallel paths through a bundle of tubes (1),
while coolant fluid flows over the exterior of the tubes. Each
tube is provided with a flow restrictor (4) at its outlet, the
degree of flow restriction being such in relation to the
fluid mass flow rate, that the restrictors are maintained full
of condensate (5) across their entire cross-section. A
reverse flow of vapour via the outlet manifold (3) into the
outlets of certain tubes is thus prevented by the presence of
condensate filling the restrictors and thermal inefficiency
resulting from consequent occlusion by non-condensing gas of
certain tubes is thus avoided.

Claims

- 4 -
I claim:
1. A heat exchange process comprising the steps of
- causing a condensing vapour to flow in parallel
paths through a plurality of tubes;
- causing a fluid coolant to flow over the external
surfaces of the tubes;
_ providing a fluid flow restrictor at the outlet
end of each tube; and
- ensuring that the mass flow rate of the condensing
vapour through the tubes is sufficient to maintain the
restrictor in each tube substantially full of
condensate.
2. A heat exchange process according to claim 1 wherein the
restrictors are provided in the form of removable inserts.

Description

119~7~09
In-Tube Cond~nsatio11 Process
This invention relates to a heat exchange process of the kind in
which a ~apour is cau~e~ to flow in parallel paths through a number
of tubes, .so as to transfer heat to an external fluid f]owing over
the outer surface of the tubes. ~he fluid wi-thin the tubes thus
condenses as it gives up latent heat to the external fluid. This
arrangement is common in air-cooled or shell and tube condensers
which are often used, for example, in chemical plants.
It often happens in operation of a heat exchange process of this
1cind that the flow of vapour may not be evenly distributed so that
some tubes take a greater flow than others. I'his may result, for
example, from differing pipe friction in different tubes, from
different tube lengt11s, from differeing flow conditions over the
external surfaces of individual -tubes, etc.
Whatever the reason, the result can be that in sorne tubes,
all of the vapour is condensed before it reaches the far end of tile
I tube. In other tubes, condensation may be incomplete so that a Mi~ture
of vapour and condens~d liquid issues from the far end, and enters the
outlet manif`old. Such vapour may partially cQndense on supercooled
liquid is.suing frorn o~er tubes. Sonne of the ~apour ~1hich has failed
to conden~e may also, however, enter other tubes, in which condensation
is complete befor~ reaching the far end. This latter vapour then travels
along such tubes in the reverse direction, a11(3 condense.s. There wil~
thus be a point in such tubes where vapour ,lows meet from both
directions. This leads to a severe problem, in that a small proportion
non-condensible gas is inevitably present in the vapour. Because this
gas is caught between two flows, it ic not swept out of the tube, but
tends to accumulate ~t the meetinp point, so that eventua]ly a sub--
stantial length of the -tube becomcs occluded hy an immGbile body of
non-condensin~ gas. rhis length of the tube thus becomes ineffective
for condensing vapour, and the thermal ef~iciency of the hea-t exchanger
is thus substantially reduced. Furihermore, the conden ate flowing
through this length of tube conti~ues to be cooled, and in some cases
may free~e leading to -total occlusion of the w}-1ole tube. The
problem is particularly acute where -the V3pOli~" iS ~t less than
atmospheric press1lre r æince any lea1is will r~sult in an inCl`ec!se in the

ll9~Z()9
proportion of non-condensible gas present.
In the past, the only real solution to the problelll has been
to ensure that reversc flow into the vapour tubes did not occur~
by supplying excess vapour to all tubes. A mixture of vapour and
condensate is thus caused to issue from each tube, and each tube
operates at maximum -thermal efficier.cy. l1owever, the separation and
recirculation of the uncondensed vapour poses a difficulty, snd creates
undesirable complication in the design of the heat excllanger.
The present invention provides a different solution to the
problem.
Accordingly, the present invention provides a heat exchange
process comprising the steps of
- causing a condensing vapour to flow in parallel paths through
a plurality of tubes;
- causing a fluid coolant to flow over the external surfaces of
the tukes;
- providing a f`lui~ flow restrictor at the outlet end of each tube,
and
- ensuring that the mass flow rate of the condensing vapour through
the tubes is sufficient to maintain the restrictor in each tube
æubstan-tial]y full of condensate.
The restriction provided by the fluid flow restrictors should
normally not be substantially rnore severe than necessary in order to meet
the objective. This will have the effect of preventing any reverse
flow of vapour into a tube from the outlet Manifold. The restrictors
will also have the e~fect of increasing the pressure drop in eaeh tube,
which can have a beneficial effect on flow ~istribution in the
tubes.
Preferably, the restrictors are provided in the form vf removable
inserts. Cleaning of the vapour tubes is thus facilitated.
The invention will now be described by way of example only wi-th
reference to the accompanying drawirlgs, in which.
Figure 1 is a sirnplified schernatic view of an aix-cooled
heat e>;changex in caccordance Wi th the invention, and
F:igure 2 is a det;ailed view of a part of Figure 1,
showing a flow restrictvr in place, and sho~:ing a f3vw
of condensate therein.

1197Z09
As show~ in the drawings, an air-cooled heat exchanger comprises
a plurality of vapour tubes 1, through which a vapour to be condensed
flows from a co~mon inlet manifold 2 to a common outlet manifold 3.
Although only a single row of tubes l is shown in Figure 1, it will
be apprecia~ed that the heat exch~nger may have several such rows, all
connected to the same inlet and outlet manifolds 2, 3.
A supply of coolant fluid, in this instance ambient air, is
caused to flow over and around the exterior surfaces of the tubes 1,
in the direction indicated in Fig 1 by the arrow ~A'. This can be
arranged, for example by means of a fan, or by natural convection,
and the tubes 1 can if desired be positioned within a duct ~or
constraining the coolant flow.
Each twbe 1 is provided with a flow restrictor 4, in the form
of a remova~le insert positioned in the dowr.stre~ end of each tube.
The ins~rts 4 are all identical, and the size of the restriction
therein is such that for the intended conditions of operation of the
heat Rxcha~eer, the flow rate of vapour in each tube 1 will result in
a flow of c~ndensate at the downstream end just sufficient to fill
the restrictor substantially completely with con~ensate 5 (ie across
its entire cross-section). If the degree of res~riction is
insufficien~ the flow of con~en~ate will not be e.~ough to fill the
restrictor and vapour will then be able to flow back down the tube
concerned in the reverse direction with the disad~antages noted
hereinbefore. A greater degree of restriction ca~ be tolerated
more readily, but should be avoided as far as pos~ible, in that
any undue restriction of the flow is undesirable~
Of course, if appropriate to the flow conditions, different
sized restrictors can be used in different tubes.
MAR/96