GB2284882A - Coated finned tube heat exchanger - Google Patents

Abstract

A method of manufacturing a heat exchanger comprises the steps of: coating a heat conducting conduit with a metallic coating; and securing one or more metallic heat dissipating elements to the conduit. The heat disipating elements are preferably secured to the tube by plastically deforming the tube from within by driving a bullet through the conduit. The conduit is preferably steel coated with lead, tin/lead or copper. The heat disipating elements are preferably made of aluminium, copper or steel and may be coated. The heat disipating elements may be made of stainless steel. The metallic coated conduit and/or the heat disipating elements may be further coated with plastics. <IMAGE>

Description

HEAT EXCHANGER This invention relates to heat exchangers.

Heat exchangers, in particular those of the "fin and tube" type, are commonly used either to transfer heat from a refrigerant gas (condenser) or to absorb heat by means of refrigerant evaporation (evaporator direct expansion) to or from the atmosphere in domestic or industrial refrigeration plants. For example, fin and tube heat exchangers may be mounted on the roofs of large retail stores, to dissipate heat to the atmosphere which has been extracted from cold rooms or stores.

Fin and tube heat exchangers comprise one or more heat conducting tubes, for carrying the refrigerant, bonded or attached to a series of heat conducting fins or plates. Heat is conducted from the tube to the fins, and is then dissipated to the atmosphere, from the relatively large surface area of the fins when the heat exchanger is being used for condensing, and when used as an evaporator heat is drawn from the atmosphere via the large area of fin into the tube. Previously, the tubes have been made from, for example, copper, cupro-nickel or brass.

Recently, concerns have been raised over possible damage caused to the atmospheric ozone layer through the use of chloro-fluoro-carbons ("CFCs") as a refrigerant. These concerns have developed into international treaties and recommendations to cut the use of CFCs dramatically over a period of a few years. This proposed reduction will be particularly onerous to developing countries which rely heavily on the use of CFCs in refrigeration plants. However, the main alternative refrigerants which are available, namely ammonia or carbon dioxide (CO2), ammonia not being chemically compatible with copper bearing tubes in a fin and tube heat exchanger.

One previously proposed solution to this problem is to employ a stainless steel tube and aluminium fin heat exchanger being of thin wall. This method of manufacture has proved to be costly with regard to basic material costs and the high skill factor required to produce the heat exchangers. The heat exchangers also suffer large variances in their heat transfer quality because a mandrel-induced method generally used to bond the fin to the tube can be erratic. They are also not recommended for external use because the aluminium fins can very quickly be removed or damaged by corrosion.

Another established method of manufacture is to employ large diameter thick wall mild steel tubes, inserted into steel plate fins.

This whole assembly is then zinc coated by means of hot dipped galvanising. Compared to copper tube aluminium finned heat exchangers and stainless steel aluminium finned heat exchangers its heat transfer capability is very poor.

Attempts to increase its efficiency by reducing tube wall and fin thickness have met with considerable warping when the structure is hot dipped galvanised.

The use of hot dipped galvanising also substantially increases the weight of the heat exchanger. The increased weight of the heat exchanger can be a particular problem when the heat exchanger is mounted on the roof of a retail store, requiring extra roof strengthening to support the unit. The method of hot dipped galvanising also limits the materials that can be employed for manufacture. The technology involved in producing either a stainless steel or hot dipped galvanised heat exchanger is too complex for use in, for example, developing countries.

This invention provides a method of manufacturing a heat exchanger, the method comprising the steps of: coating a heat conducting conduit with a metallic coating; and spring-back bonding one or more metallic heat dissipating elements to the conduit.

The invention addresses the above problems by providing a heat exchanger in which the conduit is coated with a metallic coating (in particular, a soft malleable metallic coating is advantageous), and then one or more heat dissipating elements are spring-back bonded to the conduit.

The coating may be selected to provide resistance against external chemical attack or corrosion, for example, by rain water and ice. However, since the method does not require the coating to be applied to the heat dissipating elements, the weight penalty is substantially reduced in comparison to the hot dipped galvanised heat exchanger described above. Also the coating can be changed to enhance the heat exchanger's corrosion characteristics simply by using a material coating more compatible with the finned heat dissipating elements. A series of different coatings may be applied to give the same effect.

As described below, the coating on the conduit can improve the bonding between the conduit and the heat dissipating elements; the closer the material characteristics are to those of the coating of the dissipating elements, the better the mechanical bond.

The heat dissipating elements are bonded to the conduit by spring back bonding. In this process, the conduit is inserted into a closely fitting hole in the element or elements, and a mandrel (e.g. a bullet mandrel) is driven through the conduit. The passage of the mandrel through the conduit expands the conduit to beyond its elastic limit, so that the conduit remains in an expanded state.

Other heat conducting materials might be envisaged such as, for example, steel conduit coated with lead (Pb) or tin lead (SnPb) or steel conduit tube that has been electroplated with copper (Cu). Also the metal coating can be treated, for example, with polyester or vinyl.

Also the specification of the primary metal coated conduit tube may be varied to suit different applications, for example high pressure CO2.

The invention is applicable to various types of heat exchangers, such as so-called "shell and tube" heat exchangers. These heat exchangers can consist of a steel shell with conduit tubes passing through its centre, then welded into a plate at each end of the shell.

Liquid or gas refrigerant or water is used indirectly to remove heat.

By extending the surface area of the metal coated conduit tubes by spring bonding dissipating elements to them, the performance or duty of the shell and tube heat exchangers can be increased.

Preferably the heat dissipating elements are formed from a material selected from the following list: (i) aluminium, or aluminium with any coating; (ii) copper, or copper with any coating; (iii) stainless steel; and (iv) steel, or steel with any coating.

The coatings applied to the materials in the above list may be other metals or may be, for example, a plastics or vinyl material, for protection against external corrosion.

In a preferred embodiment, and for ease and low expense of manufacture, the conduit is made of steel.

Viewed from a second aspect this invention provides a heat exchanger comprising a heat conducting conduit, the conduit being coated with a metal coating, and one or more metallic heat dissipating elements bonded to the conduit.

Viewed from a third aspect this invention provides refrigeration apparatus comprising a heat exchanger as defined above or manufactured according to the method defined above.

An embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings, throughout which like parts are referred to by like references, and in which: Figure 1 is a schematic diagram of a fin and tube heat exchanger; Figures 2a to 2d illustrate stages in the construction of a fin and tube heat exchanger; Figure 3 is a sectional view of part of a fin and tube heat exchanger; Figure 4 is a further schematic view of part of a fin and tube heat exchanger; and Figure 5 is a schematic diagram illustrating the bonding between a fin and a tube in a fin and tube heat exchanger.

Referring now to Figure 1, a fin and tube heat exchanger comprises an array of metal fins 10 and a tube 20 for carrying, for example, a refrigerant. The tube 20 is arranged to double back through the array of fins 10 several times, in order to increase the amount of heat passed from the refrigerant, through the tube 20, to the fins 10 and ultimately to the atmosphere or heat from the atmosphere back to the refrigerant.

In the present embodiment, the tube 20 is made of galvanised steel. The fins 10 are made of one of the following materials: 1) aluminium or aluminium with any coating; 2) copper or copper with any coating; 3) stainless steel; or 4) steel or steel with any coating.

The refrigerant is ammonia or carbon dioxide.

The fins 10 are bonded to the tube 20 by so-called spring back bonding. This process involves inserting a plurality of straight tubes into closely fitting holes in the fins 10. A bullet mandrel is then driven through the tubes to bond the tubes to the fins. The passage of the mandrel through each tube expands that tube to beyond its elastic limit, so that, after expansion, the tube remains in an expanded state.

The corresponding hole in the fin 10 is also expanded, but because of a slight clearance (of the order of 0.3mm) between the tube 20 and the fin 10, the fin 10 does not exceed its elastic limit and elastically contracts the heat dissipating elements onto the tube.

The tube 20 is made of galvanised steel. The use of a zinc coating on the tube 20 provides protection against corrosion. On passage of the mandrel through the tube the soft and malleable zinc coating adjusts itself to the fin 10 giving a more efficient spring back bond.

Once the plurality of straight tubes have been fitted to the fins 10, U-shaped return bends 30 are welded, soft or hard brazed, or soft soldered to the straight tubes to form the looped shape of the single tube 20.

Figures 2a to 2d illustrate stages in the construction of a fin and tube heat exchanger. Figure 2a illustrates one of the straight tubes 40 (from which the tube 20 is ultimately formed) before attachment to the fins 10. The tube 40 has an initial length of Y1 and, as illustrated in Figure 2b, an initial outside diameter of X1 and wall thickness of T1.

The fins 10 are placed onto the tube 40 and a bullet mandrel 50 is forced through the tube 40 (Figure 2c). The head 60 of the mandrel 50 is shaped to cause the tube 40 to be expanded beyond its elastic limit. This causes a bonding between the fins 10 and the tube 40 as described above.

The expansion process does not substantially change the thickness of the tube 40 after expansion. In other words, the wall thickness T2 after expansion (Figure 2d) is substantially identical to the thickness T1 (Figure 2b). However, after expansion the diameter X2 of the tube is greater and the length Y2 of the tube 40 is shorter.

Figure 3 is a sectional view of part of a fin and tube heat exchanger, and Figure 4 is a corresponding view along the direction of one of the tubes 40.

Figure 5 is a schematic diagram illustrating the bonding between a fin and a tube. Figure 5 illustrates a thin layer of zinc on the outer surface of the steel tube 40. Also illustrated is the cross sectional shape of the fins 10 which comprise an outwardly projecting fin portion 110 and a cylindrical bonding portion 120 formed by punching through the fin material. The cylindrical bonding portion 120 allows an extended bonded area 130 to be formed bonding the fin 10 to the zinc coating 100 of the pipe 40.

Finally, as mentioned above, the U-shaped return bends are welded, soft or hard brazed, or soft soldered to the tubes 40 to form the single tube 20. A mechanical joint is first formed, by pushing a smaller diameter return bend into the tube 40, by flaring either the tube 40 or the return bend and engaging one with another, or simply by pressing the two together to form a tube to tube butt joint. The joint is then welded, or brazed, or soft soldered. If necessary, the zinc coating may be pared off the tubes 40 in the region where the joint is formed.

The soft solder (if used) can give protection against corrosion.

Other heat conducting materials might be envisaged such as, for example, steel conduit coated with lead (Pb) or tin lead (SnPb) or steel conduit tube that has been electroplated with copper (Cu). Also the metal coating can be treated, for example, with polyester or vinyl.

Also the specification of the primary metal coated conduit tube may be varied to suit different applications, for example high pressure CO2.

The above techniques are applicable to various types of heat exchangers, such as so-called "shell and tube" heat exchangers. These heat exchangers can consist of a steel shell with conduit tubes passing through its centre, then welded into a plate at each end of the shell.

Liquid or gas refrigerant or water is used indirectly to remove heat.

By extending the surface area of the metal coated conduit tubes by spring bonding dissipating elements to them, the performance or duty of the shell and tube heat exchangers can be increased.

Claims (12)

1. A method of manufacturing a heat exchanger, the method comprising the steps of: coating a heat conducting conduit with a metallic coating; and spring-back bonding one or more metallic heat dissipating elements to the conduit.

2. A method according to claim 1, in which the conduit is of steel coated with lead or tin lead.

3. A method according to claim 1, in which the conduit is of steel electroplated with copper.

4. A method according to claim 1, in which the heat conducting conduit is coated with a series of two or more different coatings.

5. A method according to claim 4, in which the metallic coating is further coated with polyester or vinyl.

6. A method according to any one of the preceding claims, in which the heat dissipating elements are formed from a material selected from the group consisting of: (i) aluminium, or aluminium with any coating; (ii) copper, or copper with any coating; (iii) stainless steel; and (iv) steel, or steel with any coating.

7. A method according to claim 6, in which the heat dissipating elements are coated with a plastics or vinyl material.

8. A heat exchanger comprising a heat conducting conduit, the conduit being coated with a metal coating, and one or more metallic heat dissipating elements bonded to the conduit.

9. Refrigeration apparatus comprising a heat exchanger according to claim 8.

10. A method of manufacturing a heat exchanger, the method being substantially as hereinbefore described with reference to the accompanying drawings.

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

12. Refrigeration apparatus substantially as hereinbefore described with reference to the accompanying drawings.