An enhanced surface tube.

0001This invention relates to improvements in or relating to enhanced surface tubing particularly for use in tube type heat exchangers.

0002Tube type heat exchangers are well known and have a variety of uses, one of which allows for the recovery of high grade heat from refrigeration systems for the purpose of heating water. The present invention will be primarily described with this function in mind although the present invention may be used in any application where heat transfer is required between two fluid phases and a thermal differential exists allowing for energy transfer.

0003In designing energy recovery equipment for use associated with a refrigeration system the normal operating conditions of that refrigeration system must be maintained and such factors as operating compressor head pressure, refrigerant temperature, condensing temperatures and acceptable pressure drops must be designed into the energy recovery system.

0004To assist with this energy recovery enhanced tubes have been developed. For example the tube disclosed in U.S. Specification No.4245697 by Akira Togashi. It is also known to provide a heat exchange tube made up of two or more conduits, see for example U.S. Specification No.3730229, M.L. Onofrio, or U.S. Specification No.2913009, C.H. Kuthe, but there is no disclosure in the known art of a composite tube which acts as a single skinned enhanced surface tube with the added property of having as the inner water contacting surface stainless steel.

0005Nor is there any disclosure of such a tube construction which will provide a venting path between the skins which make up the tube.

0006The enhanced spiral tube according to the present invention has been designed to operate effectively particularly to recover high grade heat from a system and prevent changes in the normal operating conditions of such a refrigeration system.

0007It is highly desirable in any such system to prevent or minimise the possibility of the refrigerant entering the water supply. Clearly such a safety provision is more essential if potable water is being heated. Equally to protect the refrigeration system it is necessary to prevent or minimise the likelihood of water entering the refrigerant. The present invention has been designed to ensure that these criteria are met.

0008Corrosion is one of the dominant factors which can lead to tube failure. The present invention has also been developed to minimise the impact of corrosion in use.

0009Accordingly the invention consists in an enhanced surface tube comprising an inner conduit of stainless steel, an outer conduit in close cooperative fit with said inner conduit, said outer conduit being of a material compatible with said inner condiut and a spiral groove extending in from the outer surface of the twin wall tube and forming a radically inwardly extending spiral protuberance on the inner surface of the stainless steel conduit thus interconnecting the two conduits so that in use they will act as a single enhanced surface tube.

0010One preferred form of the present invention and modifications thereof will now be described with reference to the accompanying drawings in which <ul id="ul0001" list-style="none"><li>Figure 1 is a section drawn through a enhanced tube according to the present invention,</li><li>Figure 2 is a detail of the connection at the end of the enhanced tube to allow for the venting action,</li><li>Figure 3 is a detail of an alternative form of connection for use at the end of an enhanced tube also allowing for venting action, and .</li><li>Figure 4 is a detail of a triple wall enhanced tube.</li></ul>

0011The enhanced surface tube 1 consists of an inner conduit 2 of stainless steel, an outer conduit 3 in close cooperative fit with the inner conduit 2, the outer conduit 3 normally will be a copper conduit but it can be formed from any material compatible with the inner conduit. It would be feasible for the tube 3 also to be formed as a stainless steel tube. Clearly in selecting the tube 3 the operating conditions would need to be considered and a material selected which was also compatible with the fluid phase passing thereover in the heat transfer system. Normally we have found copper to be the most acceptable conduit for use on the outer skin of the enhanced tube.

0012The two tubes should be formed so that there is a sliding fit between the inner stainless steel tube and the outer conduit 3. It is important to ensure that when the tube is completed a path is left which will allow for venting between the tubes to the end of the enhanced tube in what ever system it is fitted. The dimensions required to achieve. this end will vary depending on the operating conditions and the material and by way of example at 20°C the mean free path between the tubes should be between 0.13 mm and 0.25 mm. It is clear that the operating criteria will be controlled at the lower level to ensure that the venting can take place without a very high pressure and at the upper level that the tubes are close enough together so that there is no significant air gap between the tubes. With this in mind an air gap in excess of 0.5 mm should in most circumstances be avoided.

0013The inner tube is preferably provided by a stainless steel of AISA type 302, 303, 304 or 316.and preferably 316 having a wall thickness of .5 mm to 1 mm and preferably .7 mm

0014The outer tube of copper is preferably a copper having a high purity which will ensure that it can be subsequently coiled or otherwise processed without splitting or failing. The copper should have a wall thickness between .7 mm to 1.5 mm and preferably .91 mm.

0015To complete the tube a spiral groove 4 is formed therein using a spiralling head which forms a depression extending in from the outer surface of the outer conduit 3 and by cooperation and pressure against the inner conduit 2 creates a spiral protuberance 5 which is less pronounced than the depression extending in from the outer surface but still will operate to generate turbulence within a flow passing through the conduit in use. In the example illustrated a single start spiral is shown but it would be possible to have a multi start spiral.

0016The formation of the spiral 4 causes the copper wall at the base 6 of the groove to be pressed into cooperation against the stainless steel and also for the copper to be thinned relative to the copper in the remainder of the tube. The gap 7 which is exaggerated in the drawing thus assumes a spiral path providing the venting to the end of the tube. We have found that to achieve venting which is practical in use the formed tube should allow venting to take place at 20°C when a force of 12 to 14 kPa is applied at one end of the tube and the centre conduit is blocked. It will thus be seen that in use in the preferred application with water passing through the conduit 2 and refrigerant over the outer surface of the conduit 3 a failure of the conduit 2 will cause water to leak through the venting path and be visible at the end of the tube and for the refrigerant to leak out to atmosphere resulting in a drop of refrigerant pressure which will be recorded on the refrigerant monitoring and control equipment. A failure in either wall will not allow a mixing of the refrigerant and water and it would be practically extremely unlikely for there to be a failure in both conduits at the same time. In this way the enhanced tube provides a means of heating potable water which will satisfy safety criteria and standards.

0017The efficiency of the present tube is not significantly less than that which may be achieved using a single wall copper tube. In Table I details are set out with the percentage of heat transfer recorded showing a tube according to the present invention against a copper tube have a wall thickness substantially the same as the composite tube.

0018It is considered that while not critical it is desirable for the spiral groove 4 to have a pitch of between 8 mm and 15 mm and preferably 11 mm with the width of the groove between 2 mm and 4 mm and preferably 3 mm and a depth of between 1 mm and 2 mm and preferably 1.5 mm. The dimensions selected for the groove are considered to be of significance in that the flow characteristic of the refrigerant over the conduit 3 has induced therein a sufficient turbulence to create optimum or near optimum temperature conditions adjacent the interface between the two fluids. This condition would not be reproducible where deep grooves were used. It must also be recognised that the tube is required to operate through a wide range of heat differential and overall it is undesirable to have deeper grooves which can create pockets allowing for the accumulation of a lower temperatured refrigerant than might otherwise be available.

0019The internal stainless steel tube has the spiral 5 which because of the physical transmission through the two skins does have a lesser protuberance but it is still sufficient to induce a turbulence in the flow of fluid and normally water through the tube 2 which will prevent coring and again produce the water at the optimum temperature adjacent the wall of the conduit 2 for heat transfer. It is thought the heat transfer using the tube according to the present invention results from the conditions created whereby the refrigerant medium and the water are caused to contact their respective faces of the conduits 3 and 2 at or near the optimum temperatures for heat transfer. This tends to minimise the heat transfer characteristics or impedence which may otherwise be thought to exist as a consequence of the selection of stainless steel and the provision of the air gap over at least a significant percentage of the conduit area. Whatever mechanism operates practical experience has demonstrated as is shown in Table I that the enhanced tube according to the present invention is not significantly less efficient than that able to be achieved using a comparable tube of copper. <tables id="tabl0001" num="0001"><img file="EP0052522A2_D0001.tif" /></tables>

0020It is a characteristic of the tube according to the present invention that it allows for a rapid heat transfer in the desuperheating phase and this results in a longer contact time between the refrigerant and the tube surface for the transfer of latent heat. The extent to which this function occurs is related to the type of refrigerant the operating conditions of the refrigeration system, the water inlet temperature and the water flow rate. For various combinations of these components U factors for the de-super heating and condensing phase have been calculated. The U factors so derived have been found to be some two-fold greater than those expected from theoretical film factors. Typical examples calculated from experiments using refrigerant R 12 in a (7200 kcal/hr (2.4 ton)), coplematic refrigerator system operating at 0°C with water flow rates of 1.9, 5.7 and 11.4 litres per minute and an inlet temperature of 18°C are give in Table II: v

0021<tables id="tabl0002" num="0002"><img file="EP0052522A2_D0002.tif" /></tables>When used experimentally in a refrigeration system of 11,450 kcal/hr capacity in which the surface area of exchange surfaces is at the limit of capacity the following U factors were obtained as illustrated in Table III: <tables id="tabl0003" num="0003"><img file="EP0052522A2_D0003.tif" /></tables>

0022In use the present invention must be connected so that the venting effect as above described can be realised. The way in which this is achieved will depend upon the type of unit in which the enhanced tube according the present invention is used. One such unit would be a spiral form where the outer casing 8 for example a spiral steel casing has the enhanced tube 1 for passing therethrough. At the junction of the two conduits a Y formation 9 is formed with one leg 10 providing the coupling for the refrigerant which passes in the annular space between the tube 8 and the tube 1. The other leg 11 of the Y is connected for example by brazing the leg 11 to the outer conduit 3 of the enhanced tube 1. The inner conduit 2 projects through the outer conduit 3 and has a descaling coupling 12 and a coupling 13 which will allow a water conduit to be connected thereto. The descaling coupling 12 has a plug 14 in the side thereof which when removed can be replaced by descaling equipment so that if the valves are closed restricting the flow through the conduit 2 a descaling fluid can be pumped through the coil for cleaning purposes.

0023The present invention may also be used in a construction wherein the tube is connected between plenum chambers, for example in a shell and tube type exchanger. In this type of unit the conduit according to the present invention 1 is located within a chamber defined by an outer casing 15, a head plate 16 is arranged to receive and have sealably associated therewith the end section of the outer tube 3. A second head plate 17 has the stainless steel inner conduit 2 passing therethrough. A chamber 18 operating effectively as a plenim chamber allows water to pass into the open end of the tube 2. A venting path 19 is provided between the heads 16 and 17 and can either be by a low tolerance fit or grooves formed in these members. It is also necessary to ensure that there is an axial groove or grooves formed in the interface between the conduits between the end 20 of the copper 3 and the point where the copper emerges from the head plate 18. In this way the present invention may be used to give the advantages of the heat transfer as above described while preserving the venting action according to the present invention.

0024In a further embodiment a triple wall enhanced surface tube is provided. The triple wall tube 21 is constructed having an inner core 22 of stainless steel, a middle layer 23 of copper and an outer layer 24 of stainless steel. This_is manufactured by forming the inner conduit from stainless steel, preferably type 316 having a diameter of 12 to 25 mm. The size is not critical although this would be the usual range of size employed with such tubing. The stainless steel conductor tube 22 is covered with an outer sheath of copper tubing 23 of preferably 209 or 0.9 mm conduit and has a sliding fit over the stainless conduit. In practice we have found that a 0.13 mm difference between the internal diameter of the copper tubing and the external diameter of the stainless steel tubing allows for easy working and provides a satisfactory vent to meet potable water regulations. However, a smaller tolerance could be employed but without any significant advantage and a slightly greater tolerance could be used but care would need to be taken to ensure that there is sufficient depth of the inner tube to cause turbulation of the water and reduce the coring effect which can occur during heat exchange. Placed over this copper tube is another conduit 24 of stainless steel, having an internal diameter of approximately 0.13 mm greater than the outside diameter of the copper conduit. The tube 21 is completed by forming a single or multiple start spiral 25 groove by placing the assembled tubes through a roller which presses the spiral groove into the outer surface of the stainless steel. This deforms both the outer stainless steel, the copper and the inner stainless steel tube so there is a spiral protruberance on the inside surface of the tube in the same way as that previously described.