WO2012140260A1

WO2012140260A1 - Heat exchanger apparatus

Info

Publication number: WO2012140260A1
Application number: PCT/EP2012/056917
Authority: WO
Inventors: Stig GRAFSRØNNINGEN; Atle JENSEN
Original assignee: Universitetet I Oslo
Priority date: 2011-04-14
Filing date: 2012-04-16
Publication date: 2012-10-18
Also published as: GB201106416D0

Abstract

A natural convection heat exchanger apparatus comprises: a tube bundle for flow of a first fluid inside the tubes (1 to n) in a heat transfer relationship with a second fluid outside the tubes; wherein the second fluid is a liquid and, in use, moves over the tubes by convection in a flow direction; and wherein the tube bundle comprises tubes (1 to n) spaced apart from one another in the flow direction with at least one tube (1; 1, 2; 1, 2, 3) being spaced apart from the adjacent tube (2; 2, 3; 2, 3, 4) by a greater distance (S) that the distance (T) between subsequent tubes (2 to n; 3 to n; 4 to n).

Description

HEAT EXCHANGER APPARATUS

The present invention relates to a heat exchanger apparatus.

A heat exchanger is used to transfer heat from one medium to another. Often this will involve heat exchange between two fluids. Heat exchanger devices are widely used, for example in heating systems, refrigeration, air conditioning, power plants, chemical plants, petrochemical plants and petroleum refineries.

Many heat exchanger designs make use of an array of tubes with a fluid running through the inside of the tubes and another fluid running over the outside of the tubes. The tubes are arranged in a tube bundle, and they may be straight or curved. The tubes of the tube bundle can terminate in inlet and outlet plenums or "water boxes".

An example of such a heat exchanger is a "shell and tube" heat exchanger, where tubes for flow of a fluid sit within a shell, and another fluid flows through the shell in contact with outer surfaces of the tubes. Baffles can be used to guide the fluid in the shell in a meandering path across the tube bundle, in order to ensure maximum contact with the tubes. The shell can be a pressure vessel and so shell and tube heat exchangers can be used for high pressure operations. Water, sea water for example, is often utilised as one of the fluids, typically the fluid in the shell, with the process fluid, which is cooled or heated by the water, being in the tubes.

Some heat exchangers use natural convection to move liquid across the tubes of a tube bundle. In this type of heat exchanger the tubes may simply be placed in a body of liquid without a containing shell. Alternatively a shell or housing may be placed generally around side edges of the tubes in order to permit flow of a liquid in and out of the shell and across the tubes vertically by convection.

Viewed from a first aspect, the present invention provides a natural convection heat exchanger apparatus comprising: a tube bundle for flow of a first fluid inside the tubes in a heat transfer relationship with a second fluid outside the tubes; wherein the second fluid is a liquid and, in use, moves over the tubes by convection in a flow direction and wherein the tube bundle comprises tubes spaced apart from one another in the flow direction with at least one tube being spaced apart from the adjacent tube by a greater distance that a distance between subsequent tubes.

Thus, within the tube bundle the spacing between the tubes is not constant, but it varies along the direction of the convection flow path of the second fluid. This enables improved flow of liquid across the tube bundles and hence improved efficiency for the heat exchange process. Typically the flow direction is a vertical direction and so the spacing between the tubes is a vertical spacing when the heat exchanger is installed in its in use configuration. The term 'vertical' is used below in this sense, referring to an in use orientation of the heat exchanger. The flow of fluid can be fluid circulating through the tube bundle and passing through each tube in sequence. This could be a one-way circulation (i.e. passing once through the tubes without recycling). The "subsequent tubes" are those tubes that are downstream in the direction of convection flow of the second fluid. Thus, tubes at the start of the convection flow are at a greater spacing than tubes further downstream in the convection flow.

Natural convection heat exchangers of the type defined herein use movement of the liquid coolant in convection as the mechanism for generating flow of coolant across the heat exchange tubes. When the heat exchanger is used to transfer heat from the first fluid to the second fluid, with the second fluid thus cooling the first fluid, the second fluid in contact with the tubes is heated and flows upward. An upward convection current is generated starting from a first tube that is the lowest tube. In contrast, if heat is transferred from the second fluid to the first fluid such that the first fluid cools the second fluid then the cooled second fluid will generate a downward convection current starting at a first tube that is the highest tube. The heat exchanger apparatus may include a shell enclosing the tube bundle. In some circumstances this can improve the fluid flow. Alternatively the tube bundle can be placed in water without a shell or housing.

The first fluid may be a single phase fluid such as a gas, or a multi-phase fluid, comprising combinations of gas, liquid and condensate. The second fluid is a liquid such as water or sea water. The heat exchanger makes use of turbulent effects generated by the induced convection flow as it is initiated by the first tube of the heat exchanger in order to enhance the heat transfer efficiency when the convection flow passes the subsequent tubes of the heat exchanger. This provides increased heat exchanger between the first and second fluids whilst enabling the size of the heat exchanger to be reduced.

Preferably, the heat exchanger is arranged such that the vertical distance between at least two tubes in the tube bundle that are in an upstream position in the direction of the convection current is greater than the vertical distance between at least two tubes in the tube bundle in an downstream position in the direction of the convection current. Hereinafter, the direction of the convection current will be referenced when describing the tubes by using terminology referring to the first tube as the tube in the tube bundle that is furthest along in the upstream direction of the convection current and referring to the last tube as the tube in the tube bundle that is furthest along in the downstream direction of the convection current. Thus, in a preferred arrangement of the type described above the at least two upstream tubes may comprise the first tube and an adjacent tube. Also or alternatively, the at least two downstream tubes may comprise the last tube and an adjacent tube. In some preferred embodiments the first and second tubes are spaced apart by a first vertical distance and the remaining tubes are spaced apart by a smaller vertical distance. In other preferred embodiments the first, second and third or first, second, third and fourth tubes are spaced apart by a larger distance than the remaining tubes. The vertical distance of spacing between the remaining tubes may be constant. The larger vertical distance of spacing for the first pair of tubes and subsequent pairs of tubes may be constant.

Alternatively it may vary, decreasing in the direction of the convection current.

It should be noted that the tube bundle may comprise multiple sets of tubes arranged at the same vertical heights, or alternatively in a staggered manner, and spaced across the horizontal width of the shell, such that the first tube is one of a set of first tubes and correspondingly the last tube might be one of a set of last tubes.

By spacing the first tube and adjacent tube at a greater distance apart a greater distance is permitted to enable laminar flow of the convection current to develop into turbulent flow. The distance necessary to allow the flow to undergo a transition from laminar to turbulent flow depends on the Rayleigh number. The distance decreases for increasing

Rayleigh number. This distance will vary depending on the heat transfer and the diameter of the tubes

Preferably the distance between the tubes is a distance determined as sufficient to promote or enable the transition from laminar flow to turbulent flow. Such a configuration may advantageously increase the heat transfer from the second tube and subsequent tubes by up to 35% compared to heat transfer from the first tube. The distance S between tubes can be usefully considered as a multiple of the diameter D. If the distance S between the first tube and the second tube is too small, for example below S = 3D for typical heat transfer conditions and a tube diameter of about 5 cm, the heat transfer from the second tube may be reduced compared to the first tube. Furthermore, if the distance between the first tube and the second tube is too large, for example above S = 6D for similar conditions, the increase in heat transfer from the second tube compared to the first tube may be small since there is less influence from the first tube. There is thus an optimal range for the distance between the first tube and the second tube.

Above the first tube the kinetic energy increases with downstream distance and it is therefore highly beneficial when the distance S between the first tube and second tube is set large enough to ensure turbulent flow. The flow above the second tube spaced apart from the first tube by one distance is however inherently different than the flow above the first tube. The kinetic energy, turbulent kinetic energy in particular, is considerably larger above the second tube than above the first tube. It is therefore possible for the distance between the second tube and the third tube (and for subsequent tubes) to be less than the distance between the first tube and the second tube whilst still maintaining an increased heat transfer for all the tubes in the tube bundle when the distance between the first tube and the second tube is set to promote a transition to turbulent flow.

The required normalized distance between the two tubes at the start of the convection flow to allow the flow from to lowermost tube to undergo a transition to turbulent flow may be approximated by the following implicit equation:

gpqw

where S is centre to centre distance, D is tube diameter, GrQ,_turb = 2 ^χ 10⁹ is an

experimentally determined constant, p, C_p, v and β are density, heat capacity, kinematic viscosity and coefficient of thermal expansion of the ambient liquid (second fluid)

respectively, q is heat flux (W/m²) from the tube, and g is gravity. If unknown, the heat flux q can be determined from the Nusselt number Nu and the Rayleigh number Ra_D using empirically determined correlations such as, for example:

AT.. _ i n n _j ^Q.387i½y' ₂

2V a — u .uu -t- _(_ ø .559/p_r) 9/ i6 ] 8/27 j

in which Pr is the Prandtl Number.

These calculations and the above empirically determined correlation enable the distance between the tubes to be set at a distance sufficient to enable the transition from laminar flow to turbulent flow. Thus, preferably the distance S between the first and second tubes is set to be within 15% of the value for S determined by the above formulae, more preferably within 10% and/or may be set to be larger than this S value.

By way of example, when the heat flux q from is 3.7 kW/m², the tube diameter D is 0.054 m, the ambient liquid (second fluid) is water and has a temperature of 20°C, then the calculation above gives an optimal calculated separation distance of S = 4.55D. Thus, the distance S between first and second tubes may, for example, be set to be in the range of 4D to 5D. This will promote the formation of turbulent flow after the first tube and before the second tube, and hence increase heat transfer for the second tube.

As noted above, an increase in heat transfer also occurs for subsequent tubes even if the spacing between these subsequent tubes is smaller. Thus, in some cases the third tube can be spaced from the second tube by a smaller distance with subsequent tubes also spaced at a smaller distance, whilst still taking advantage of increased heat transfer for all tubes as a result of the transition to turbulent flow before the second tube. In other cases it may be advantageous to have a larger spacing between several pairs of tubes, followed by a smaller spacing between subsequent tubes. For example there may be a larger spacing S between the first and second tubes and also the second and third tubes, followed by a smaller spacing T for subsequent tubes.

The distance T between the subsequent tubes may be less than 75% of than the distance between the first and second tubes, more preferably less than 50%, and may be less than 25% of than the distance between the first and second tubes. The distance T between the subsequent tubes depends on the parameter S. If S is large, say 8-10D, then T may be 4-5D. However, if S is small, for example 3D, then T may be 2D. For even larger S, say 15-16D, T may be 6-7D. It is preferred that the distance T between subsequent tubes is not less than about 2D.

The variable distance between the tubes hence ensures that the heat exchanger apparatus can take full advantage of turbulent effects induced in the convection flow across the heat exchanger, i.e. inducted by the lower hotter tubes when the heat exchanger dissipates heat, whilst also avoiding unnecessary increases in the size of the heat exchanger by having an unnecessarily large space between subsequent tubes in the tube bundle. The majority of the tubes in the heat exchanger can be more densely packed whilst still taking advantage of an increased heat transfer for those tubes, which results from the turbulent flow initiated by the larger spacing of the first tubes. If the number of tubes is large the overall height of the heat exchanger may be reduced considerably due to the dense stacking of the tubes. It is advantageous to keep the heat exchanger as small as possible.

In preferred applications where the first fluid is used as a coolant and where the heat exchanger is used to keep the first fluid cool by dissipating heat into the second fluid then the first tube would be the lowest tube and the last tube would be the highest tube, low and high referring to the orientation of the heat exchanger when in use.

The tube bundle may comprise straight tubes, for example tubes extending in a straight line between a input plenum and output plenum. The tube bundle may comprise a single meandering pipe, for example comprising straight tube sections joined by pipe bends.

The variable distance configuration has advantages both for in line and staggered tube bundles and hence may be applied to either in line or staggered tubes.

There may be a housing or shell containing the tube bundle. The housing may comprise walls enclosing the tube bundle, the walls being arranged to be about sides of the tube bundle when the heat exchanger is in use with openings in the base and top for flow of liquid. The walls may comprise vertical walls placed around the tube bundle, the walls being vertical when the tube bundle is in use. This can promote generation of a steady vertical convection current through the housing and across the tubes.

Natural convection heat exchangers can be submerged in the sea or in a lake or other quiescent or near quiescent body of water, which hence provides a source of water at the upstream end of the tube bundle and an outlet for heated water at downstream end of the tube bundle. Near quiescent includes sea water currents or similar, but excludes fast flowing rivers.

The heat exchanger apparatus is hence preferably a submergible natural convection heat exchanger for submerged use.

In one possible embodiment, the heat exchanger is within a subsea gas compression apparatus, for example for subsea gas compression as part of an Increased Oil Recovery (IOR) measure. The invention may hence provide a subsea heat exchanger apparatus, preferably a subsea gas compression apparatus incorporating the heat exchanger apparatus. The heat exchanger apparatus or multiple heat exchanger apparatuses may provide inlet and intermediate gas cooling of the compressor train, well head cooling or export pipeline cooling.

Certain preferred embodiments of the invention will now be described by way of example only and with reference to the accompanying drawings, in which:

Figure 1 (a) is a cross-section through a single tube bundle with increased spacing for the lowest two tubes;

Figure 1 (b) shows an alternative arrangement with increased spacing for the three lowest tubes; and

Figure 1 (c) shows an alternative arrangement with increased spacing for the four lowest tubes.

The heat exchanger may comprise multiple tube bundles arranged in a horizontal manner, either in line or staggered.

The Figures illustrate a tube bundle arrangement for a convection heat exchanger where the liquid, in this example embodiment, flows upward. Thus the liquid outside the tubes is used to remove heat from the fluid inside the tubes. The tubes could be connected at their ends to plenums or similar. The tubes may be part of a meandering pipe.

For intermediate Rayleigh numbers (such as Rayleigh numbers typically encountered in heat exchangers with water or seawater as ambient cooling medium) the flow by convection across the tubes will undergo a transition due to shear instabilities a distance above the lowermost first tube 1 in the tube bundle. To take full advantage of this effect it is essential that the second lowermost cooler tube 2 is placed sufficiently far from the lowermost tube 1. This increased spacing is shown in Figure 1 (a). The distance S between the lower tubes is calculated using the equations set out above.

If the turbulent intensity induced from the lower cooler tube is sufficiently high, the flow downstream the two first cooler tubes, i.e. above the 2nd lowermost tube 2, will be fully turbulent. The turbulent kinetic energy above the second tube, compared to above the first tube, is significantly higher. Hence, for this embodiment the remaining tubes 2, 3, 4, 5, 6,... n-2, n-1 , n have equal spacing T. This allows for a more dense stacking of the tubes in the upper part of the heat exchanger.

For lower Rayleigh numbers it can be necessary to have a larger distance S between the first three or four cooler tubes to allow the flow to develop into a fully turbulent state before reducing the distance T between cooler tubes in the upper part of the heat exchanger. It is of crucial importance that the distance S between the lowermost cooler tubes is sufficiently large. This is since weak perturbations are dampened by the upper cylinders if they are spaced too closely together, and thus the advantageous turbulent effects are lost. Figure 1 (b) shows an arrangement where the spacing between the three lowest tubes 1 , 2, 3 is increased compared to the spacing between the remaining tubes 3 to n. Figure 1 (c) illustrates the arrangement where the spacing between tubes is larger for the four lowest tubes 1 to 4 and smaller for the remaining tubes 4 to n.

The use of differing spacing allows for an overall reduction of the size of the heat exchanger. The total number of cooler tubes may be reduced due to increased local heat transfer from each tube bundle due to turbulent effects. If the number of cooler tubes is large the overall height of the heat exchanger will also be reduced due to the dense stacking of the upper cooler tubes.

The Figures show a single tube bundle comprising n tubes. Multiple tube bundles could be used together. The multiple tube bundles would be offset from one another horizontally and either arranged in line, with tubes all at the same heights, or staggered, with tubes in adjacent bundles offset vertically from each other.

Claims

CLAIMS:

1. A natural convection heat exchanger apparatus comprising:

a tube bundle for flow of a first fluid inside the tubes in a heat transfer relationship with a second fluid outside the tubes;

wherein the second fluid is a liquid and, in use, moves over the tubes by convection in a flow direction; and

wherein the tube bundle comprises a plurality of tubes spaced apart from one another in the flow direction with at least one tube being spaced apart from the adjacent tube by a greater distance that the distance between subsequent tubes.

2. A natural convection heat exchanger apparatus as claimed in claim 1 , wherein the natural convection heat exchanger is arranged such that, in use, it generates a convection current in a vertical direction and the vertical distance between at least two tubes in the tube bundle that are in an upstream position in the direction of the convection current is greater than the vertical distance between at least two tubes in the tube bundle that are in an downstream position in the direction of the convection current.

3. A natural convection heat exchanger apparatus as claimed in claim 2, wherein the first tube and an adjacent second tube are spaced apart by a greater distance than distance between the last tube and an adjacent tube.

4. A natural convection heat exchanger as claimed in claim 3 wherein the first tube and second tube are spaced apart by a greater distance than the distance between all of the remaining tubes of the tube bundle.

5. A natural convection heat exchanger apparatus as claimed in claim 3, wherein the first, second and third or first, second, third and fourth tubes are spaced apart by a larger distance than the remaining tubes.

6. A natural convection heat exchanger apparatus as claimed in claim 4 or 5, wherein the remaining tubes of the tube bundle are all at the same spacing.

7. A natural convection heat exchanger apparatus as claimed in any of claims 2 to 6, wherein the increased distance between the upstream tubes is a distance determined as sufficient to enable a transition from laminar flow to turbulent flow in the liquid flowing across the tubes.

8. A natural convection heat exchanger apparatus as claimed in any preceding claim, comprising a housing or shell containing the tube bundle.

9. A natural convection heat exchanger apparatus as claimed in claim 8, wherein the housing comprises walls enclosing the tube bundle, the walls being arranged to be about sides of the tube bundle when the heat exchanger is in use, with openings in the base and top for flow of liquid.

10. A natural convection heat exchanger apparatus as claimed in any preceding claim, wherein the heat exchanger apparatus is a submergible natural convection heat exchanger for submerged use.

11. A subsea gas compression apparatus incorporating a natural convection heat exchanger apparatus as defined in any preceding claim.

12. A natural convection heat exchanger apparatus substantially as hereinbefore described with reference to the accompanying drawings.