CA1197209A

CA1197209A - In-tube condensation process

Info

Publication number: CA1197209A
Application number: CA000449760A
Authority: CA
Inventors: John A.R. Henry
Original assignee: UK Secretary of State for Trade and Industry
Current assignee: UK Secretary of State for Trade and Industry
Priority date: 1983-03-18
Filing date: 1984-03-16
Publication date: 1985-11-26
Also published as: GB8406204D0; GB2137330B; GB2137330A; GB8307568D0; JPS6036888A; EP0120630A1

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.

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.

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.

Claims

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.