GB2137330A

GB2137330A - In-tube condensation process

Info

Publication number: GB2137330A
Application number: GB08406204A
Authority: GB
Inventors: John Alexander Richardso 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: 1984-03-09
Publication date: 1984-10-03
Also published as: JPS6036888A; GB8406204D0; EP0120630A1; GB2137330B; CA1197209A; GB8307568D0

Description

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GB 2 137 330 A

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SPECIFICATION

In-tube condensation process

5 This invention relates to a heat exchange process of the kind in which a vapour is caused to flow in parallel paths through a number of tubes, so as to transfer heat to an external fluid flowing over the outer surface of the tubes. The fluid within the tubes thus condenses as 10 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 15 process of this kind that theflow of vapour may not be evenly distribted so that sometubes take a greater flowthan others. This may result, for example, from differing pipefriction in different tubes, from different tube lengths, from differing flow conditions overthe 20 external surfaces of individual tubes, etc.

Whateverthe reason, the result can be that in some tubes, all of the vapour is condensed before it reaches thefarend of thetube. In other tubes, condensation may be incomplete so that a mixture of vapour and 25 condensed liquid issues from thefarend, and enters the outlet manifold. Such vapour may partially condense on supercooled liquid issuing from other tubes. Some of the vapour which has failed to condense may also, however, enter other tubes, in 30 which condensation is complete before reaching the far end. This latter vapourthen travels along such tubes in the reverse direction, and condenses. There will thus be a point in such tubes where vapourflows meet from both directions. This leads to a severe problem, 35 in that a small proportion non-condensible gas is inevitably present in the vapour. Because this gas is caught between two flows, it is not swept out of the tube, but tends to accumulate at the meeting point, so that eventually a substantial length of the tube 40 becomes occluded by an immobile body of non-condensing gas. This length of the tube thus becomes ineffective for condensing vapour, and the thermal efficiency of the heat exchanger is thus substantially reduced. Furthermore, the condensate flowing 45 through this length of tube continues to be cooled, and in some cases may freeze leading to total occlusion of the whole tube. The problem is particularly acute where the vapour is at less than atmospheric pressure, since any leaks will result in an increase in 50 the proportion on non-condensible gas present.

In the past, the only real solution to the problem has been to ensure that reverse flow into the vapourtubes did not occur, by supplying excess vapourto all tubes. A mixture of vapour and condensate is thus caused to 55 issuefrom each tube, and each tube operates at maximum thermal efficiency. However, the separation and recirculation of the uncondensed vapour poses a difficulty, and creates undesirable complication in the design of the heat exchanger. 60 The present invention provides a different solution to the problem.

Accordingly,thepresentinvention providesa heat exchange process comprising the steps of —causing a condensing vapourto flow in parallel 65 paths through a plurality of tubes;

—causing a fluid coolantto flow overthe external surfaces of the tubes;

— providing a fluid flow restrictor atthe outlet end of each tube; and

— ensuring that the mass flow rate of the condensing vapourth rough the tubes is sufficient to maintain the restrictor in each tube substantially full of condensate.

The restriction provided by the fluid flow restrictors should normally not be substantially more severe than necessary in orderto 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 effect of increasing the pressure drop in each tube, which can have a beneficial effect on flow distribution in the tubes.

Preferably, the restrictors are provided in theform of removable inserts. Cleaning ofthe vapourtubes is thus facilitated.

The invention will now be described by way of example only with reference to the accompanying drawings, in which.

Figure 1 is a simplified schematic view of an air-cooled heat exchanger in accordance with the invention, and

Figure 2 is a detailed view of a part of Figure 1, showing a flow restrictor in place, and showing a flow of condensate therein.

As shown in the drawings, an air-cooled heat exchanger comprises a plurality of vapourtubes 1, through which a vapourto be condensed flows from a common inlet manifold 2 to a common outlet manifold 3. Although only a single row of tubes 1 is shown in Figure 1, it will be appreciated that the heat exchanger may have several such rows, all connected to the same inlet and outlet manifolds 2,3.

Asupply of coolant fluid, in this instance ambient air, is caused to flow over and around the exterior surfaces ofthe 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 for constraining the coolant flow.

Each tube 1 is provided with a flow restrictor 4, in the form of a removable insert positioned in the downstream end of each tube.

The inserts 4 are all identical, and the size ofthe restriction therein is such that forthe intended conditions of operation ofthe heat exchanger, the flow rate of vapour in each tube 1 will result in a flow of condensate at the downstream end just sufficient to fill the restrictor substantially completely with condensate 5 (i.e. across its entire cross-section). If the degree of restriction is insufficienttheflow of condensate will not be enoughtofillthe restrictorand vapour will then be able to flow back down the tube concerned in the reverse direction with the disadvantages noted hereinbefore. Agreater degree of restriction can be tolerated more readily, but should be avoided as far as possible, in that any undue restriction of theflow is undesirable.

Of course, if appropriate to the flow conditions, different sized restrictors can be used in different tubes.

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GB 2 137 330 A

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Claims

1. A heat exchange process comprising the steps of

—causing a condensing vapourto flow in parallel pathsthrough a plurality oftubes;

5 —causing a fluid coolantto flow over the external surfaces ofthetubes;

—providing a fluid flow restrictor atthe outlet end ofeachtube;and

—ensuring thatthe mass flow rate of theconde-

10 nsing vapourthrough the tubes is sufficient to maintain the restrictor in each tubesubstantially full of condensate.

2. A heat exchange process according to claim 1 wherein the restrictors are provided in theform of

15 removable inserts.

3. A heat exchange process according to claim 1 and substantially as hereinbefore described.

4. A heat exchange process substantially as hereinbefore described with reference to Figure 1 and 2

20 ofthe accompanying drawings.