GB2289122A

GB2289122A - Heat exchanger for oxygen/nitrogen liquefaction

Info

Publication number: GB2289122A
Application number: GB9409073A
Authority: GB
Inventors: Edward Joseph Gregory
Original assignee: Denso Marston Ltd
Current assignee: Denso Marston Ltd
Priority date: 1994-05-06
Filing date: 1994-05-06
Publication date: 1995-11-08
Anticipated expiration: 2014-05-06
Also published as: GB2289122B; GB9409073D0

Abstract

In a heat exchanger for oxygen/nitrogen liquefaction, liquid oxygen 8 passes horizontally through the exchanger and is boiled. Nitrogen passes vertically through the exchanger and is condensed. The horizontal passage of the oxygen results in a lower pressure in the liquid oxygen and so a lower boiling point giving better heat exchange. The pressure P5 is higher than P3 due to the head of liquid oxygen. P5 may be reduced to the level of P3 by providing a perforated fin arrangement (10, Figure 3), or by providing multiple headers (7, Figure 4) and controlling the supply of liquid oxygen to each header. <IMAGE>

Description

Plate Fin Heat Exchangers The present invention relates to plate fin heat exchangers and more particularly but not exclusively to plate fin heat exchangers with horizontal boiling channels.

Presently most heat exchangers are installed vertically in a pool of liquid oxygen. The liquid oxygen enters the bottom of the heat exchanger. This liquid oxygen then boils within the exchanger to leave as a vapour-liquid mixture at the top of the exchanger. The pressure at the bottom of the exchanger must be greater than the pressure at the top of the exchanger to force flow through. The difference between these pressures is essentially the summation of the pressure loss due to frictional pressure loss and the pressure exerted downwards by the weight of the vapour-liquid mixture in the exchanger. The effect of the higher pressure at the bottom of the exchanger is to increase boiling temperature at that point which reduces the temperature difference between the condensing nitrogen and boiling oxygen streams.The reduced temperature difference results in the relatively large size of heat exchangers presently found in air separation plants.

Attempts have been made to reduce the weight effect of the vapour-liquid mixture in the exchanger on boiling temperature by arranging for down flow or downward flow of the liquid oxygen. Thus, the vapour-liquid weight problem is eliminated. However, provision of such down flow exchanger distribution arrangements requires add-on features and increases both cost and exchanger complexity.

It is an objective of the present invention to provide a heat exchanger that substantially relieves the above problems.

In accordance with the present invention there is provided a heat exchanger comprising vertical passage layers and horizontal passage layers, said vertical and horizontal passage layers being arranged adjacent to one another and alternately in the exchanger, said horizontal passage layers being coupled to a header arrangement such that coolant means are passed through said horizontal passage layers such that gas passed through said vertical passage layers may be condensed.

Preferably, the header arrangement includes equalisation means to ensure equalisation of coolant pressure across the exchanger. That equalisation means may be a perforated fin structure with varying orifice sizes or shapes across the structure to facilitate equalisation. Alternatively, the header arrangement may comprise a multitude of header elements of limited liquid head height and operated such that the quantity of coolant supplied to each header element is controlled.

An embodiment of the present invention will now be described by way of example only with reference to the accompanying drawings in which: Figure 1 illustrates schematically a prior vertical heat exchanger configuration; Figure 2 illustrates schematically the present horizontal heat exchanger configuration; Figure 3 illustrates schematically a first distribution arrangement for the heat exchanger illustrated in Figure 2; and, Figure 4 illustrates schematically a second distribution arrangement for the heat exchanger illustrated in Figure 2.

Figure 1 illustrates a prior vertical heat exchanger 1 where nitrogen enters through a nitrogen inlet 2 and exits through an outlet 3 passing through fin or passage layers of the exchanger 1. Liquid oxygen boils at the bottom of the exchanger 1 and rises as indicated by the arrows upwards in passage or fin layers adjacent those carrying the nitrogen. Thus, there is heat exchange between the nitrogen and oxygen flows so that the nitrogen is condensed. This condensed liquid nitrogen flows out of the outlet 3.

The difference between the inlet pressure of the boiling system Pl and the outlet pressure P2 is the sum of the frictional pressure drop of the flowing oxygen and the pressure head of the liquid-vapour mixture in the exchanger 1. This pressure difference is balanced by the pressure head of the liquid outside the column. The pressure P1 determines the boiling temperature of the boiling fluid at that end. The pressure P2 determines the boiling temperature at the other end.

Figure 2 shows an embodiment of present exchanger 6 in schematic form. The exchanger 6 is not immersed in liquid oxygen in accordance with traditional method. In the present exchanger 6 the liquid oxygen is pumped to the exchanger 6 and any liquid oxygen leaving the exchanger 6 falls into a pool significantly below the exchanger 6.

The nitrogen flow is still from an inlet 4 to an outlet 5 through vertical layers but the oxygen flow is horizontal through appropriately oriented passages and as indicated by the arrowhead.

As the oxygen flows horizontally the liquid-vapour mixture exerts no downward pressure on the inlet.

The difference in pressure between P3 and P4 is therefore equal to the friction pressure loss only. Since the pressure values P2 and P4 are the same then pressure P3 is substantially less than P1 (Figure 1). Therefore the exchanger 6 in Figure 2 has a larger temperature difference between nitrogen and oxygen streams than does the exchanger 1 in Figure 1. This allows a smaller heat exchanger size or more efficient operation.

A volume of liquid oxygen 8 is present in a header 7 such that downward pressure from liquid oxygen above the header 7 forces oxygen through the exchanger 6.

It will be understood the effect the header 7 will cause a larger pressure P5 at the bottom as compared to P3 at the top of the exchanger 6. The variation of pressure from P3 to P5 will cause alteration both in the boiling temperature down the exchanger 6 and inequality in flow rates from top to bottom.

This variation in pressure from P3 to P5 may be corrected by graduated orifices fixed to the inlet which reduce the pressure range from P3 to P5 into a constant pressure closely equal to P3 just inside the exchanger inlet.

Figures 3 and 4 show two approaches to providing graduated orifices to the inlet of the exchanger 6.

In Figure 3 a perforated fin arrangement known as a hardway 10 is secured about the inlet of the exchanger 6.

This 'hardway' 10 is a line of fins positioned across the line of flow of oxygen with small holes equally spaced in the fins. These holes have the function of orifices. In this way the pressure P5 can be reduced to a value closely equal to P3. Intermediate pressure values in the header 7 are also reduced to the same value (Figure 3 is drawn out of scale for reasons of clarity). In this way the desired pressure difference P3 - P4 is obtained over the majority of the exchanger.

An alternative to that distribution arrangement shown in Figure 3 is shown in Figure 4. Effectively, a multitude of headers 7 are arranged to supply liquid oxygen to the exchanger 6. The quantity of liquid oxygen supply to each header is strictly controlled. Thus, the pressure in each header 7 is roughly equalised as the 'pressure head' on each header 7 is quite short. Supply of liquid oxygen to each header 7 may be controlled by meters.

It will be understood that the effect of the reduction in pressure loss will be an increase-in temperature difference which leads to smaller heat exchanger size with a commensurate decrease in the size of the column in which they are installed.

Alternatively, one may retain the present size of heat exchanger with improved efficiency of plant operation.

Claims

CLAIMS:

1. A plate fin heat exchanger comprising vertical passage layers and horizontal passage layers, said vertical and horizontal passage layers being arranged adjacent to one another and alternately in the exchanger, said horizontal passage layers being coupled to a header arrangement such that coolant means are passed through said horizontal passage layers such that gas passed through said vertical passage layers may be condensed, the coolant exit from the heat exchanger being arranged such that the coolant may flow freely from said exit under the influence of gravity.

2. A heat exchanger as claimed in Claim 1 in which the header arrangement includes equalisation means to ensure equalisation of coolant pressure across the exchanger.

3. A heat exchanger as claimed in Claim 2 in whch said equalisation means includes a perforated fin structure with varying orifice sizes or shapes to facilitate equalisation.

4. A heat exchanger as claimed in Claim 2 in which the equalisation means comprises a multitude of header elements of limited liquid head height and operated such that the quantity of coolant supplied to each header element is substantially the same.

5. A heat exchanger in accordance with the invention described with reference to Figures 2, 3 and 4.