GB1595094A - Method and system for cooling electrical apparatus - Google Patents

Description

(54) METHOD AND SYSTEM FOR COOLING ELECTRICAL APPARATUS (71) We, GENERAL ELECTRIC COMPANY, a corporation organized and existing under the laws of the State of New York, United States of America, of 1 River Road, Schenectady 12305, State of New York, United States of America, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following staternent : -.

The recent United States Government ban on the use of polychlorinated biphenols as coolants for medium and high power transformers necessitates the use of expensive silicone based oils for transformer cooling purposes. Since the quantity of oil required for total immersion cooling is in the order of hundreds of gallons, alternative means for cooling transformers have been proposed. One efficient method comprises the use of a "condensable fluid coolant," which term is used hereinafter where a system or method utilizes the vaporization and condensation cycle of a coolant fluid to remove the heat from the transformer surface during the vaporization portion of the cycle and to transfer the heat via a heat exchanger to the outside world during the condensation portion of the cycle. U.S.Patent 3,024,298 discloses an evaporative-gravity cooling system using a condensable fluorochemical fluid as a coolant for electronic devices. FIG. 1 shows one type of prior art device 10 containing a fluid condensable coolant 11 within a tank which is in contact with a transformer 13 to be cooled.

The transformer 13 becomes cool by heating the coolant 11 to its vaporization temperature and causes the coolant 11 to transfer into a heat exchanger 14 which is forced-cooled.

The coolant 11 readily condenses within the heat exchanger 14 and gives up its energy as heat of condensation.

Another prior art method for cooling electronic devices utilizes liquid film cooling to take heat from the electrical device during operation and transfer the heat to a heat exchanger medium. U.S. Patent 2,924,635 discloses electrical apparatus utilizing a fluid dielectric atmosphere for both electrical insulation and as a cooling mechanism for dissipating heat developed during operation of the apparatus. FIG. 2 shows a liquid cooling means 15 containing a tank 16 and a reservoir 18 containing a liquid coolant 17. The device 15 further contains a pump 19 and a plurality of nozzles 9 for spraying the coolant 17 in liquid droplet form over the surface of a transformer 20.The coolant 17 forms a thin liquid film over the surface of the transformer 20 and the liquid film upon contacting the hot transformer surface readily evaporates and becomes transported in a gaseous state into a condenser 21 for transferring the heat obtained from the transformer 20 to the condenser 21 by the heat of condensation. An expansion tank 22 containing a noncondensable gas is often employed with systems of this kind and an auxiliary heat exchanger 23 may be required.

According to a first aspect, the preset invention provides a method for cooling of self-heated electrical apparatus comprising the steps of: providing an air-tight container having a quantity of "condensable fluid coolant" as hereinbefore defined filling a portion of the container; inserting the electrical apparatus within the container and in contact with a portion of the coolant said electrical apparatus having at least one duct extending through a portion of the apparatus for receiving said cooolant; and adding molecular sieve material to the container for removing contaminant from the coolant.

According to a second aspect, the invention provides a fluid cooling system for electrical apparatus comprising: an air-tight container for housing the apparatus to be cooled; a volume of "condensable fluid coolant" as hereinbefore defined within the container and covering at least a portion of the apparatus to be cooled; a heat exchanger for receiving the coolant in vaporized form and for returning the coolant in condensed form; at least one conduit extending through the apparatus for receiving a portion of the coolant in liquid form at one end and discharging the fluid in gaseous form at an opposite end; and a quantity of molecular sieve material within the system for removing water vapor from the coolant.

The invention thus features a cooling system for electrical apparatus using a "condensable fluid coolant," as hereinbefore defined, a heat exchanger, and a molecular sieve material for removing substances that may contaminate the condensable fluid coolant.

Prior art arrangements and embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings in which: Figure 1 is a perspective view in partial section of one cooling apparatus of the prior art; Figure 2 is a front view in partial section of a further cooling apparatus of the prior art; FIGURE 3 is a front perspective view in partial section of a cooling apparatus according to the invention; FIGURE 4 is an enlarged side view in partial section of the molecular sieve trap assembly of the apparatus of FIGURE 3; FIGURE 4A is an alternative arrangement of the molecular sieve trap assembly; FIGURE 5 is a sectional view of a thermal pump for use with the apparatus of FIGURE 3; FIGURE 6 is a graphic representation of the thermal distribution profile within the cooling ducts of transformers for various cooling mediums;; FIGURE 7 is a graphic representation of the cooling rate as a function of liquid cooling level for the percolation cooling system of this invention; FIGURE 8 is a cross-sectional view of a further embodiment of the thermal pump of FIGURE 5; FIGURE 9 is an enlarged cross-sectional view of a further embodiment of the device of FIGURE 8; FIGURE 10 is a top perspective view of a transformer for use with a percolation cooling system according to this invention; FIGURE 11 is a side view of a configuration of cooling apparatus alternative to that shown in FIGURE 3; and FIGURE 12 is a side view of another configuration of cooling apparatus alternative to that shown in FIGURE 3.

FIGURE 3 shows one embodiment of cooling apparatus employing the "condensable fluid coolant" and molecular sieve principles of this invention. The apparatus 24 includes a boiler 25 consisting of a tank 26, a piece of electrical apparatus such as a transformer 27 which generates heat during operation, and a "condensable fluid coolant" medium 28.

System 24 further includes a heat exchanger 29 having an upper manifold 30, a lower manifold 31, parallel headers 40, and a plurality of interconnecting cooling tubes 32.

An expansion tank 33 containing a noncondensable gas is often employed when the coolant 28 provides the dual function of percolation cooling and acting as a dielectric for the electrical apparatus employed. The noncondensable gas provides the dielectric medium for electrically insulating the electrical apparatus when the temperature is insufficient to cause the coolant 28 to become vaporized and to completely cover the electrical apparatus 27. The expansion tank 33 is connected to the heat exchanger 29 at both the upper and lower manifolds 30, 31, and is connected to the boiler 25 by means of duct 34. Duct 34 houses a container 35 of a molecular sieve substance 36, the purpose of which will be described below.Transformer 27 has a feedthrough bushing attached at its lower end for making electrical contact with the transformer 27, as is common with medium and power transformer assemblies.

The duct 34 is shown in enlarged detail in FIGURE 4 where the top of tank 26 is connected to lower manifold 31 by means of duct 34. The container 35 of molecular sieve material 36 contains a contaminant trap 37 at the bottom section thereof. The vapor transmission path for the condensable coolant 28 is indicated by arrows and proceeds as follows. Upon becoming heated and vaporizing, the coolant 28 enters into the inlet 38 to within the top portion 39 of the duct 34 and proceeds down through the molecular sieve material 36, and into the lower manifold 31 by means of manifold inlet 31'. The condensable coolant 28 may include one or more of a vaporizable fluorinated hydrocarbon, or chlorofluorocarbon such as trichlorotrifluoroethane or chlorofluoropropane which may react with water vapor and become disassociated during operation.The type of transformer to be cooled generally contains a wrapping of paper insulation which at the operating temperatures involved may yield an amount of water vapor which cannot be completely removed during the initial thermal treating process. The purpose of the molecular sieve material 36 therefore is to remove any water vapor or other harmful liquid substances that may be evolved from the transformer paper and picked up by the coolant material 28 during its vapor transition process. A particularly effective granular molecular sieve material 36 is a zeolite type 4A made by the Linde Division of the Union Carbide Corporation. It has been determined that the Linde molecular sieve material 36 effectively removes all traces of water vapor from the coolant 28 and that other liquid contaminants remain in the trap 37 provided at the bottom section of the duct 34.

In the absence of the molecular sieve material 36, the coolant material 28 (e.g. trichlorotrifluoroethane) may become cloudy. The coolant 28 when used with the sieve material 36 remains clear during continuous operation. After passing through the molecular sieve material 36 the vaporized coolant 28 transmits into the lower manifold 31 by means of manifold inlet 31' and from there out to the cooling tubes 32 by means of the headers 40. The vaporized coolant 28 readily condenses within the tubes 32 and returns in liquid form by means of a separate return through the lower manifold 31 back into the tank 26 via condensate return pipe 41.

An alternative arrangement of the molecular sieve 36 is shown in FIGURE 4A. The duct 34 shown in FIGURE 4 has been eliminated and the molecular sieve 36 is included in the lower manifold 31. The vapor transmission path for the coolant 28 is indicated by arrows and proceeds as follows - upon becoming heated and vaporizing the coolant 28 enters into the inlet 38 inside the bottom manifold 31 and down through the molecular sieve material 36 into headers 40 and then into cooling tubes 32. The vaporized coolant 28 readily condenses within the tubes 32 and returns in liquid form by means of a separate return pipe 41 into the tank 26. The return pipe 41 extends above the bottom of manifold 31 so that a trap 37 is provided for other liquid contaminants.

In FIGURES 4 and 4A the condensate return 41 extends below the level of the coolant 28 in the tank 26 so that the vapor phase of coolant 28 must enter the inlet pipe 38 (above the liquid level) and proceed through the molecular sieve 36. The molecular sieve 36 can also be effective by proper sizing of the pipes 41 and 38. With the return pipe 41 above the liquid level of coolant 28 the vapor phase of coolant 28 enters both the return pipe 41 and the inlet pipe 38. By proper sizing of the pipes 41 and 38, as is well known in the art of fluid mechanics, sufficient vapor can be made to flow through inlet pipe 38 and sieve 36 to remove water. Since vapor is continuously formed, condensed, and regenerated, it is not necessary to provide 100 per cent vapor flow through sieve 36.Also, by proper sizing of pipe 41 the upward vapor flow through pipe 41 will not interfere with the downward condensate return through 41.

Another heat pipe arrangement 42 is shown in FIGURE 5 and is described as follows. The tank 26 containing the transformer 27 and liquid coolant 28 having a liquid level 43 is particularly arranged in the following manner.

The transformer 27 contains a plurality of transformer windings 45 with a series of passages such as cooling ducts 44 extending from the transformer bottom 46 to the transformer top 47 and a concentrically located core member 48. The transformer thermal pumping assembly 42 is designed to transfer the coolant 28 from the transformer bottom section 46 through the ducts 44 by means of the thermal gradient existing within the plurality of ducts 44. The cooling ducts 44 provide a conduit for the coolant 28 which becomes heated in transit and cools the transformer 27 by the change of state from a liquid to a vapor.The temperature distribution profile within the cooling ducts 44 for a liquid level 43 as shown is such that the temperature of the coolant 28 at the transformer bottom section 46 is lower than the temperature at a point P corresponding to the liquid level 43 since the heating mechanism is the wattage generated by the transformer 27 and the point P is subjected to a smaller transformer cooling surface than for example point ,P' in the vicinity of the transformer, bottom section 46. The temperature at point P is also higher than the temperature at point P" at the transformer top section 47 for the heat pump of this invention to be operative.

Point P" is a lower temperature than point P since point P" is subjected to a greater cooling surface than point P, provided the proper liquid level is maintained. When power is applied to the transformer 27 a plurality of bubbles 49 begin to move in an upward direction within the cooling ducts 44 and become heated to a greater degree as they proceed further within the transformer 27 since the region at point P has a higher temperature as described earlier. As the bubbles 49 continue to proceed to the vicinity of the transformer top section 47 they acquire enough thermal energy to leave the transformer 27 at the vicinity of point P" as vapor droplets 49' in the direction indicated by the directional arrows.Since the droplets 49' force coolant 28 through the ducts 44 to the transformed top section 47, the top section 47 becomes cooled by the process of evaporation of coolant 28. Further bubbles 49 enter into the cooling ducts 44 and proceed through the regions indicated at points P', and P" in a continuous process during the time the transformer is operating. This process is somewhat similar to the percolation effect used for continuously redistributing water in a coffee percolator.

The temperature distribution within the cooling ducts 44 for transformer 27 is shown in FIGURE 6. The temperature in degrees C is plotted as a function of the relative length of the ducts 44 for the same transformer if it were air-cooled A, oil-cooled B, and percolation-cooled C. The air-cooled temperature gradient A shows that the temperature continuously increases from the bottom section of the transformer through the center section to the top of the transformer since the transformer itself is its own source of heat and the air-cooling mechanism of heat transfer is insufficient to cool the entire transformer uniformly. The termperature gradient for an oil-cooled transformer B shows that the temperature gradient from the bottom to the top of the transformer continuously increases at a slower rate than that for the air-cooled transformer A.The temperatures obtained with oil or air cooling for this transformer are greater than permissible with the insulations commonly used. In order to use oil or air cooling, additional cooling ducts must be provided to lower the temperatures. Thus, additional conductor material is required for oil or air cooling as compared with percolation cooling.

The temperature gradient for the percolationcooled transformer C is as follows.

At the bottom region of the transformer 27 at point P' the ends of the windings are exposed to the coolant 28 which is vaporized due to the heat generated by the winding conductors, thus, cooling the bottom of the winding 45. The coolant 28 enters the ducts 44 and vaporizes due to contact with the part of the winding 45 next to the duct 44. Since the bottom of the winding 45 has a greater surface area exposed to the coolant it will be at a lower temperature than the inner parts of the winding 45. The inner parts are cooled by vaporization of the coolant 28 in ducts 44 and by thermal conduction to the cooler ends of the winding 45. Upon vaporization the coolant 28 forms bubbles 49 which rise rapidly upward through the ducts 44 forcing some of the liquid phase of coolant 28 to the top of the ducts 44 and then onto the top end of the winding 45.The liquid coolant 28 is vaporized upon contact with the upper surface of ducts 44 and the upper end of the winding 45. It was, therefore, determined that for the mechanism of percolation cooling, that is, when coolant 28 is provided within cooling ducts 44, and evaporation rapidly takes place at the transformer upper surface 47, the rate of heat transfer away from the transformer 27 is sufficient to cool the top surface 47, of the transformer 27 at a rate that is equal to the rate at which the transformer 27 becomes heated during normal operating conditions. The temperature at point P" at the top surface 47 can be as low as the temperature indicated at point P' at the transformer bottom 46 depending upon the boiling point of the coolant 28, the liquid level 43, and the dimensions of the cooling ducts 44.

The cooling rate for a plurality of heat pumps 42 having differing liquid levels 43 for a fixed coolant composition is shown in FIGURE 7. It was then determined that the liquid level 43 expressed in per cent height relative to the transformer top section 47 in FIGURE 5 could be decreased without seriously effecting the efficiency of the transformer cooling rate for liquid levels down to less than 75% of the maximum dimension indicated.

The cooling rate profile R remains relatively steady down to a 75% liquid level as indicated in thermal units per unit time and begins to decrease for liquid levels less than approximately 60%. For liquid levels between 60 and 50% the cooling rate decreased substantially and below 50 per cent the cooling rate was not adequate. This phenomenon is not as yet well understood, but is believed to depend upon the heat transfer characteristics for the coolant 28, as well as the geometry and number of transformer cooling ducts, and the power rating of the transformer. FIGURE 6 indicates that the temperature at any distance within the percolation-cooled device C is lower than that within the oil-cooled trans former B and the air-cooled transformer A.

FIGURE 8 is a further example of the transformer heat pump 42 of FIGURE 5. In FIGURE 8 the transformer 27 containing the plurality of cooling ducts 44 and vapor bubbles 49 is modified by duct extensions 50 co incident with the ends of the cooling ducts 44 at the transformer top surface 47. The exten sions 50 carry the vapor bubbles 49 to an ex tended height h above the top surface 47 and deposit the liquid 28 within a specially designed distribution tray 51 having a plurality of spaced perforations 52. The embodiment of FIGURE 8 combines the percolation cooling method of this invention with the liqud surface film evaporation of the prior art to further increase the cooling efficiency.The distributor tray 51 collects the coolant 28 in liquid form and redistributes the coolant 28 in the form of droplets 49' which subsequently drop through the perforations 52 onto the transformer top surface 47. To keep a continuous flow of coolant 28 onto the top surface of the transformer coils 45 an upwardly extending dam member 53 is provided at the top surface 47 of the transformer 27 as indicated in FIGURE 8.

FIGURE 9 shows a further use for the extension 50 of the cooling duct 44. The extension 50 is here brought up into close contact with a busbar 54 so that the liquid droplets 49' can contact and cool busbar 54.

The transformer 27 for providing the heat pump 42 is shown in enlarged top perspective view in FIGURE 10. The ducts 44 between the transformer coils 45 at the transformer top section 47 have a width "W" and a length "L" as indicated. The required number of ducts 44 are used in order to maintain the operating temperature below the maximum permissable value. The width "W" is approximately 3/16 of an inch, and the length "L" is approximately 1-1/2 inches. The core 48 is centrally located as is common with transformers of the medium and high power types.

The dimensions, number, and location of the transformer cooling ducts 44 are determinative of the quantity of coolant used to provide the heat pump of this invention and it is anticipated that a substantial savings of transformer coolant fluid can be realized by a proper heat pump design. For oil-cooled transformers having the temperature gradient indicated at B in FIGURE 6 approximately 155 gallons of coolant oil is generally required whereas for the same size transformer 50 gallons of coolant are required to provide the temperature gradient indicated for the percolation-cooled device at C. The heat pump devices described therefore, provide better cooling efficiency than standard oil-cooled total immersion systems at a substantial savings in materials costs.

Further cooling apparatus usable for percolation-cooled transformers are shown in FIGURES 11 and 12. As described earlier a noncondensable gas such as nitrogen, C2F6, C2C1Fb, SF6 is frequently employed as a dielectric for providing insulation between the transformer windings when the transformer liquid coolant is at a low temperature. The noncondensable gas pressure also determines the boiling temperature of the condensable coolant and is adjusted so that the condensable fluid coolant boils within the operating range of the transformer temperature. For the trichlorotrifluoroethane coolant, Freon 113 such as the type manufactured by DuPont for example, can be used within the heat pump 42 of FIGURE 5.A medium power transformer rated at 18000 watts three phase operated continuously at a boiling temperature of 67"C when the nitrogen fill pressure was adjusted to give a condensable vapor pressure of 12 P.S.I.G. It is anticipated that the operating temperature characteristics can be accurately controlled by the duct dimensions and the coolant boiling temperature, as determined by the nitrogen fill pressure, provided a quantity of coolant always remains in the liquid phase.

If the entire quantity of coolant vaporized the pressure within the system would behave as an ideal gas and increase in proportion to temperature. The use of the noncondensable gas generally requires an upper manifold 30 and an expansion tank 33 as shown in FIGURES 3 and 11 for the following reasons. The expansion tank 33 provides a receptacle for the noncondensable gas after the condensable gas has become sufficiently vaporized to displace the noncondensable gas and to expel it from the vicinity of the transformer windings. The upper manifold 30 is generally required when the liquid coolant 28 is heat treated to outgas any residual noncondensable gases such as air absorbed by the coolant during transportation and storage. In order to outgas the coolant 28 the transformer is short circuited to cause the transformer to become heated and to separate the absorbed air from coolant 28.The heated air separates from the coolant 28 in the outgassing process and enters the expansion tank 33 after passing through condenser tubes 32.

The air then readily transmits into upper manifold 30 which is connected to the expansion tank 33 by means of connecting pipe 55 and connecting valve 56. Once the transformer coolant has been completely outgassed from residual air and the outgassed air is contained within expansion tank 33 the upper manifold 30 is isolated from the expansion tank 33 by closing valve 56. The air is then removed from the expansion tank 33 by evacuation with a vacuum pump. A known quantity of the desired noncondensable gas nitrogen, SF6, C2Fr" or C2+ClFs is then added to the expansion tank 33 through a filling valve 59. The valves 56 and 58 are then opened and the noncondensable gas is allowed to flow throughout the system.

During operation the pipe 57 serves to return any condensate of coolant 28 from the expansion tank to the main liquid supply. Condensate of coolant 28 may form in expansion tank 33 due to fluctuations in ambient temperature.

Pipe 57 is connected to the lowest point of expansion tank 33 to minimize condensate hold up in the expansion tank.

The transformer of FIGURE 12 is similar to that of FIGURE 11 except that the upper manifold 30, the connecting pipe 55, the connecting valve 56 and drain valve 58 can be dispensed with. It has been discovered that the transformer coolant can be pre-evacuated by heating and outgassing the coolant prior to filling within the transformer such that the steps indicated earlier for FIGURE 11 are no longer required. Once the transformer coolant is completely outgassed particular care is taken to insure that the outgassed coolant does not reabsorb air in the final transformer filling stage.

Although the cooling system employing the heat pump and molecular sieve of this invention is directed in the above described embodiments to applications involving medium and high power transformers, this is by way of example only. The method and apparatus employed within the examples shown can be applied to cool other type of electrical apparatus, privided that the heat pump transfer design of this invention can be incorporated within the electrical apparatus involved.

It should also be readily apparent that for certain ranges of ambient temperatures and condensable coolants the noncondensable gas may be omitted without affecting the operation of the apparatus as described.

WHAT WE CLAIM IS: 1. A method for cooling of self-heated electrical apparatus comprising the steps of: providing an air-tight container having a quantity of "condensable fluid coolant" as herein defined filling a portion of the container; inserting the electrical apparatus within the container and in contact with a portion of the coolant said electrical apparatus having at least one duct extending through a portion of the apparatus for receiving said coolant; and adding molecular sieve material to the container for removing contaminant from the coolant.

2. The method of Claim 1 including the steps of adding a quantity of noncondensable gas to the container said noncondensable gas being of a predetermined pressure to cause the condensable coolant to boil at the operating temperatures of the electrical apparatus.

3. The method of Claim 1 further including the step of thermally outgassing the

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

Claims (23)

**WARNING** start of CLMS field may overlap end of DESC **. therefore, provide better cooling efficiency than standard oil-cooled total immersion systems at a substantial savings in materials costs. Further cooling apparatus usable for percolation-cooled transformers are shown in FIGURES 11 and 12. As described earlier a noncondensable gas such as nitrogen, C2F6, C2C1Fb, SF6 is frequently employed as a dielectric for providing insulation between the transformer windings when the transformer liquid coolant is at a low temperature. The noncondensable gas pressure also determines the boiling temperature of the condensable coolant and is adjusted so that the condensable fluid coolant boils within the operating range of the transformer temperature. For the trichlorotrifluoroethane coolant, Freon 113 such as the type manufactured by DuPont for example, can be used within the heat pump 42 of FIGURE 5.A medium power transformer rated at 18000 watts three phase operated continuously at a boiling temperature of 67"C when the nitrogen fill pressure was adjusted to give a condensable vapor pressure of 12 P.S.I.G. It is anticipated that the operating temperature characteristics can be accurately controlled by the duct dimensions and the coolant boiling temperature, as determined by the nitrogen fill pressure, provided a quantity of coolant always remains in the liquid phase. If the entire quantity of coolant vaporized the pressure within the system would behave as an ideal gas and increase in proportion to temperature. The use of the noncondensable gas generally requires an upper manifold 30 and an expansion tank 33 as shown in FIGURES 3 and 11 for the following reasons. The expansion tank 33 provides a receptacle for the noncondensable gas after the condensable gas has become sufficiently vaporized to displace the noncondensable gas and to expel it from the vicinity of the transformer windings. The upper manifold 30 is generally required when the liquid coolant 28 is heat treated to outgas any residual noncondensable gases such as air absorbed by the coolant during transportation and storage. In order to outgas the coolant 28 the transformer is short circuited to cause the transformer to become heated and to separate the absorbed air from coolant 28.The heated air separates from the coolant 28 in the outgassing process and enters the expansion tank 33 after passing through condenser tubes 32. The air then readily transmits into upper manifold 30 which is connected to the expansion tank 33 by means of connecting pipe 55 and connecting valve 56. Once the transformer coolant has been completely outgassed from residual air and the outgassed air is contained within expansion tank 33 the upper manifold 30 is isolated from the expansion tank 33 by closing valve 56. The air is then removed from the expansion tank 33 by evacuation with a vacuum pump. A known quantity of the desired noncondensable gas nitrogen, SF6, C2Fr" or C2+ClFs is then added to the expansion tank 33 through a filling valve 59. The valves 56 and 58 are then opened and the noncondensable gas is allowed to flow throughout the system. During operation the pipe 57 serves to return any condensate of coolant 28 from the expansion tank to the main liquid supply. Condensate of coolant 28 may form in expansion tank 33 due to fluctuations in ambient temperature. Pipe 57 is connected to the lowest point of expansion tank 33 to minimize condensate hold up in the expansion tank. The transformer of FIGURE 12 is similar to that of FIGURE 11 except that the upper manifold 30, the connecting pipe 55, the connecting valve 56 and drain valve 58 can be dispensed with. It has been discovered that the transformer coolant can be pre-evacuated by heating and outgassing the coolant prior to filling within the transformer such that the steps indicated earlier for FIGURE 11 are no longer required. Once the transformer coolant is completely outgassed particular care is taken to insure that the outgassed coolant does not reabsorb air in the final transformer filling stage. Although the cooling system employing the heat pump and molecular sieve of this invention is directed in the above described embodiments to applications involving medium and high power transformers, this is by way of example only. The method and apparatus employed within the examples shown can be applied to cool other type of electrical apparatus, privided that the heat pump transfer design of this invention can be incorporated within the electrical apparatus involved. It should also be readily apparent that for certain ranges of ambient temperatures and condensable coolants the noncondensable gas may be omitted without affecting the operation of the apparatus as described. WHAT WE CLAIM IS:

1. A method for cooling of self-heated electrical apparatus comprising the steps of: providing an air-tight container having a quantity of "condensable fluid coolant" as herein defined filling a portion of the container; inserting the electrical apparatus within the container and in contact with a portion of the coolant said electrical apparatus having at least one duct extending through a portion of the apparatus for receiving said coolant; and adding molecular sieve material to the container for removing contaminant from the coolant.

2. The method of Claim 1 including the steps of adding a quantity of noncondensable gas to the container said noncondensable gas being of a predetermined pressure to cause the condensable coolant to boil at the operating temperatures of the electrical apparatus.

3. The method of Claim 1 further including the step of thermally outgassing the

coolant prior to inserting within the container for removing trapped air.

4. The method of Claim 1 wherein the coolant is selected from the group consisting of a fluorinated hydrocarbon, a chlorofluorinated hydocarbon, a chlorofluoropropane and trichlorotrifluoroethane.

5. The method of Claim 1 wherein the apparatus comprises a transformer.

6. The method of Claim 1 wherein the electrical apparatus is immersed from 50 per cent to 100 per cent of its height within the coolant.

7. A fluid cooling system for electrical apparatus comprising: an air-tight container for housing the apparatus to be cooled: a volume of "condensable fluid coolant" as herein defined within the container and covering at least a portion of the apparatus to be cooled; a heat exchanger for receiving the coolant in vaporized form and for returning the coolant in condensed form; at least one conduit extending through the apparatus for receiving a portion of the coolant in liquid form at one end and discharging the fluid in gaseous form at an opposite end; and a quantity of molecular sieve material within the system for removing water vapor from the coolant.

8. The system of Claim 7 including a noncondensable gas occupying a volume within the container and having a pressure such that the coolant boils at the operating temperature of the apparatus to be cooled.

9. The system of Claim 8 including an expansion tank coupled to said container for receiving the noncondensible gas during the operation of said apparatus.

10. The system of Claim 7 wherein the molecular sieve material is located between an outlet for the coolant from the container and an inlet for the coolant to the container for trapping the water vapor and preventing said water vapor from entering the inlet.

11. The system of Claim 7 wherein the, coolant is selected from the group consisting of fluorinated hydrocarbon, a chlorofluorinated hydrocarbon and a chlorofluoropropane.

12. The system of Claim 7 wherein the coolant comprises a chlorofluoroethane.

13. The system of Claim 8 wherein the noncondensable gas is selected from the group consisting of sulfur hexafluoride, hexafluoroethane, chloropentafluoroethane and nitrogen.

14. The system of Claim 7 wherein the electrical apparatus comprises a transformer.

15. The system of Claim 14 wherein the transformer comprises a plurality of coil wind ings around a concentric core and wherein the coolant transfer conduits comprise a plurality of passages extending from one end of the winding to another end thereof and having a width and a length such that a region inter mediate said ends is hotter than a region at either of said ends during operation of the transformer.

16. The system of Claim 15 wherein the coolant enters said one end of the passages and becomes heated within said intermediate region to its vaporization temperature and exits from the other end of said passages in vapor form.

17. The system of Claim 14 wherein the conduit extends above the surface of the transformer for redistributing the coolant over the surface of the transformer.

18. The system of Claim 17 wherein the transformer further includes a retainer around the perimeter cf the top surface of the transformer for collecting and retaining the coolant on the transformer surface.

19. The system of Claim 17 wherein the conduit extension further includes a distribution tray proximate an end of said extension for receiving coolant from the conduit and for redistributing the coolant back over the transformer surface.

20. The system of Claim 19 wherein the distribution tray includes a first plurality of perforations for passage of the conduit extension and a second plurality of perforations adjacent said conduit for receiving the coolant in vapor form through said conduit extension and transmitting coolant in liquid form through said second perforations.

21. The system of Claim 18 wherein the transformer further includes at least one bus bar proximate an end of the conduit extension for receiving coolant from said conduit.

22. The system of Claim 1 wherein the condensable vapor pressure is 12 P.S.I.

(0.84 Kg/cm2) and the fluid operating temperature is 67"C.

23. A cooling method or system substantially as hereinbefore described with reference to Figures 3 and 4, or Figure 4A, or to Figures 3 and 4, or Figure 4A in combination with any other Figure of the accompanying drawings.