GB2056653A - Heat exchanger - Google Patents

Abstract

A heat exchanger comprises an inner tube bundle, an outer shell 7 having an inlet opening 8 on to the tube bundle and a baffle grid comprising a plurality of rods or tubes 5, 6 interposed between the inlet 8 and the inner tube bundle (not shown). The baffle grid modifies the flow pattern of fluids entering the heat exchanger before contact with the walls of the tube bundle and reduces erosion. The grid comprises two rows 5, 6 staggered with respect to each other, and set at right angles to the heat exchanger tubes (not shown). <IMAGE>

Description

SPECIFICATION Heat exchanger This invention relates to an improved heat exchanger, more particularly to an improved inlet baffle for a heat exchanger of the tube and shell type. The term "heat exchanger" include condensers within its scope.

Tube and shell heat exchangers are well known and generally comprise a cylindrical shell encasing a tube bundle. The tube bundle comprises a number of tubes, set in rows known as banks, the tubes being kept in position by header plates into which the ends of the tubes are expanded. Recessed head covers are set over the header plates. Inlets and outlets to the shell and head covers are provided in such mannerthat, in use, one fluid passes through the tubes of the bundle and a second fluid passes through the shell and over the tubes of the bundle.

Tube and shell heat exchangers are frequently employed to condense a gas or mixture of gases entering the heat exchanger at a temperature near or within their boiling range. Under these conditions, there can be a two-phase flow of vapour and liquid entering the shell of the exchanger with cooling liquid passing through the tubes. This situation arises particularly in overhead condensers on crude oil distillation units.

Under such conditions of two-phase entry heat exchangers tend to suffer from impingement attack.

Impingement attack takes the form of pitting and cavitation of the tubes at or near their entry.

Such corrosion, although local in its action, is obviously undesirable and may lead to contamination and/or loss of valuable products. It will almost certainly result in expensive equipment lying idle until repairs can be effected.

One approach to the problem has been to insert baffle plates over the entries to the tube bundles but this, while conferring protection in the vicinity of the baffle plates, transfers the impingement attack further downstream.

It is our opinion that impingement attack is due, in part, to the velocity pattern of the fluid impinging on the tubes being non-uniform, that is to say, that some droplets of liquid have an excessively high velocity.

We have now discovered that by inserting a baffle in the form of a grid of rods or tubes at the entry to the heat exchanger it is possible to reduce the extent of impingement attack.

Thus according to the present invention there is provided a heat exchanger comprising an inner tube bundle, an outer shell having an inlet opening on to the tube bundle and a baffle grid comprising a plurality of rods or tubes interposed between the inlet and the inner tube bundle to modify the flow pattern of fluid entering the heat exchanger before contact with the walls of the tube bundle.

Preferably the baffle grid contains two rows of rods or tubes in the region directly in line with the inlet port of the heat exchanger and a single row in the region extending away from that directly in line.

The second row of rods or tubes should be staggered with respect to the first. Most preferably the spacing of the two rows is such that the rods or tubes are on an equilateral triangular pitch.

The two rows combined intersect liquid droplets and at the same time form a resistance which will reduce vapour velocity. In the region away from the inlet only a single row is required since its function is to even out the vapour flow.

Preferably the rods or tubes in the baffle grid are set at right angles to the heatexchangertubes.

In an end entry heat exchanger shell the baffle grid should be situated between the tube sheet and the first transverse baffle. In a central entry heat exchanger, the shell impingement baffles should be placed between the central dividing baffle and the first baffle either side of the centre. For maximum effect the baffle is preferably located about halfway between the top of the shell and the top of the tube bundle.

The invention is illustrated with reference to Figures 1 and-2 wherein Figure 1 is an isometric view of a partial baffle grid and Figure 2 is a section showing the insertion of the grid in a heat exchanger.

The grid comprises supporting plates 1,2,3 and 4.

The plates carry rods in two rows 5 and 6. Row 5 is intended to extend over the whole front of the heat exchanger 7 and row 6 to shield only the area immediately in line with the heat exchanger inlet 8.

Provision is made for a central dividing baffle 9 in the heat exchanger between supports 1 and 4 and 2 and 3.

Erosion is reduced by reducing the velocity of the liquid particles past the tubes. This is done in two stages. Firstly, the particles are slowed or stopped.

Secondly, the maximum vapour velocity is mini mised so that re-entrained droplets are accelerated as little as possible.

Minimising the maximum vapour velocity is equivalent to ensuring that the vapour velocity is as uniform as possible over the whole of the area of entry to the tube bank.

1. A heat exchanger comprising an innertube bundle, an outer shell having an inlet opening on to the tube bundle and a baffle grid comprising a plurality of rods or tubes interposed between the inlet and the inner tube bundle to modify the flow pattern of fluids entering the heat exchanger before contact with the walls of the tube bundle.

2. A heat exchanger according to claim 1 wherein the baffle grid contains two rows or rods or tubes in the region directly in line with the inlet port of the heat exchanger and a single row in the region extending away from that directly in line.

3 A heat exchanger according to claim 2 wherein the second row of rods or tubes is staggered with respect to the first.

4. A heat exchanger according to claim 3 wherein the spacing of the two rows is such that the rods or tubes are on an equilateral triangular pitch.

5. A heat exchanger according to any of the preceding claims wherein the rods or tubes in the

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (6)

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Heat exchanger This invention relates to an improved heat exchanger, more particularly to an improved inlet baffle for a heat exchanger of the tube and shell type. The term "heat exchanger" include condensers within its scope. Tube and shell heat exchangers are well known and generally comprise a cylindrical shell encasing a tube bundle. The tube bundle comprises a number of tubes, set in rows known as banks, the tubes being kept in position by header plates into which the ends of the tubes are expanded. Recessed head covers are set over the header plates. Inlets and outlets to the shell and head covers are provided in such mannerthat, in use, one fluid passes through the tubes of the bundle and a second fluid passes through the shell and over the tubes of the bundle. Tube and shell heat exchangers are frequently employed to condense a gas or mixture of gases entering the heat exchanger at a temperature near or within their boiling range. Under these conditions, there can be a two-phase flow of vapour and liquid entering the shell of the exchanger with cooling liquid passing through the tubes. This situation arises particularly in overhead condensers on crude oil distillation units. Under such conditions of two-phase entry heat exchangers tend to suffer from impingement attack. Impingement attack takes the form of pitting and cavitation of the tubes at or near their entry. Such corrosion, although local in its action, is obviously undesirable and may lead to contamination and/or loss of valuable products. It will almost certainly result in expensive equipment lying idle until repairs can be effected. One approach to the problem has been to insert baffle plates over the entries to the tube bundles but this, while conferring protection in the vicinity of the baffle plates, transfers the impingement attack further downstream. It is our opinion that impingement attack is due, in part, to the velocity pattern of the fluid impinging on the tubes being non-uniform, that is to say, that some droplets of liquid have an excessively high velocity. We have now discovered that by inserting a baffle in the form of a grid of rods or tubes at the entry to the heat exchanger it is possible to reduce the extent of impingement attack. Thus according to the present invention there is provided a heat exchanger comprising an inner tube bundle, an outer shell having an inlet opening on to the tube bundle and a baffle grid comprising a plurality of rods or tubes interposed between the inlet and the inner tube bundle to modify the flow pattern of fluid entering the heat exchanger before contact with the walls of the tube bundle. Preferably the baffle grid contains two rows of rods or tubes in the region directly in line with the inlet port of the heat exchanger and a single row in the region extending away from that directly in line. The second row of rods or tubes should be staggered with respect to the first. Most preferably the spacing of the two rows is such that the rods or tubes are on an equilateral triangular pitch. The two rows combined intersect liquid droplets and at the same time form a resistance which will reduce vapour velocity. In the region away from the inlet only a single row is required since its function is to even out the vapour flow. Preferably the rods or tubes in the baffle grid are set at right angles to the heatexchangertubes. In an end entry heat exchanger shell the baffle grid should be situated between the tube sheet and the first transverse baffle. In a central entry heat exchanger, the shell impingement baffles should be placed between the central dividing baffle and the first baffle either side of the centre. For maximum effect the baffle is preferably located about halfway between the top of the shell and the top of the tube bundle. The invention is illustrated with reference to Figures 1 and-2 wherein Figure 1 is an isometric view of a partial baffle grid and Figure 2 is a section showing the insertion of the grid in a heat exchanger. The grid comprises supporting plates 1,2,3 and 4. The plates carry rods in two rows 5 and 6. Row 5 is intended to extend over the whole front of the heat exchanger 7 and row 6 to shield only the area immediately in line with the heat exchanger inlet 8. Provision is made for a central dividing baffle 9 in the heat exchanger between supports 1 and 4 and 2 and 3. Erosion is reduced by reducing the velocity of the liquid particles past the tubes. This is done in two stages. Firstly, the particles are slowed or stopped. Secondly, the maximum vapour velocity is mini mised so that re-entrained droplets are accelerated as little as possible. Minimising the maximum vapour velocity is equivalent to ensuring that the vapour velocity is as uniform as possible over the whole of the area of entry to the tube bank. CLAIMS

1. A heat exchanger comprising an innertube bundle, an outer shell having an inlet opening on to the tube bundle and a baffle grid comprising a plurality of rods or tubes interposed between the inlet and the inner tube bundle to modify the flow pattern of fluids entering the heat exchanger before contact with the walls of the tube bundle.

2. A heat exchanger according to claim 1 wherein the baffle grid contains two rows or rods or tubes in the region directly in line with the inlet port of the heat exchanger and a single row in the region extending away from that directly in line.

3 A heat exchanger according to claim 2 wherein the second row of rods or tubes is staggered with respect to the first.

4. A heat exchanger according to claim 3 wherein the spacing of the two rows is such that the rods or tubes are on an equilateral triangular pitch.

5. A heat exchanger according to any of the preceding claims wherein the rods or tubes in the baffle grid are set at right angles to the heat exchanger tubes.

6. A heat exchanger as hereinbefore described with reference to the accompanying drawings.