US2181726A - Apparatus for transferring heat - Google Patents

Description

Nov. 28, 1939. M. E. F lENE APPARATUS FOR TRANSFERRING HEAT Filed April 24, 1937 0 45 so :20 m: :r: m 13! METAL SURFACE TEMPERATURE DEG. C.

Inventor: Marcus E. Fiene,

1 W 5 jwlflq His Attor'n Patented Nov. 28, 1939 UNITED STATES PATENT OFFICE Marcus E. Fiene, Ballston Lake, N. Y., assignor to General Electric Company, a corporation of New York Application April 24, 1937, Serial No. 138,789

2 Claims.

This invention relates to methods and apparatus for transferring heat from a relatively high temperature source to a vaporizable medium under relatively low pressure particularly subatmospheric pressure where the usual and well known laws of heat transmission fail to apply.

The invention provides improvements whereby I heat is transmitted readily to vaporizable mediums at relatively low pressures such as water through heat conducting bodies such as iron, steel or other metals even when the temperature of the heat source is above the critical value at which the water will assume a spheroidal state in which it ordinarily is exceedingly diflicult to transmit heat thereto to vaporize it. With the vaporizable medium in a spheroidal state, the usual and well I known laws of heat transmission no longer apply.

Such phenomenon occurs whenever a drop of water is placed onthe top of a very hot stove and may be termed non-wetting, i. e., the water is prevented from contacting and wetting the surface of the heated body and hence cannot receive any appreciable amount of heat therefrom.

The improvements of the present invention are of particular advantage in evacuated vapor heat transfer systems of the type disclosed in my copending application, Serial No. 25,691, filed June 8, 1935. This evacuated system consists briefly of a vaporizer, condenser-radiator, and thermostatically controlled means for varying the amount of vaporizable liquid active in transferring heat. With the relatively low sub-atmospheric pressure and the relatively small amounts of vaporizable liquid involved in such an evacuated system, the above described nontting ordinarily occurs whenever the vaporizer is raised above the critical temperature. Thus the heat transfer system may function entirely satisfactorily with low pressure and consequently low temperature steam for heating the vaporizer but when high pressure steam, which necessarily is at a higher temperature, is used as the source of heat no substantial heat transfer may be obtained.

It is the primary object of the present invention 45 to provide a method of and apparatus for transferring heat toa vaporizable medium in which the difliculties arising from non-wetting under the unusual heat transfer conditions mentioned above are effectively overcome.

Briefly, the invention consists in conducting heat from the high temperature-heat source to the vaporizable medium through a first heat conducting body having a substantially large heat conducting and storage capacity and then through a 55 body having a relatively slight heat conducting and storage capacity. In actual practice these heat conducting. bodies may be formed of an integral body with a surface on one side adapted to be contacted by the high temperature heat source and a ridged surface on the other side with 5 the ridges of substantially triangular cross-section adapted to be contacted by the vaporizable medium. When the present invention is applied to a vapor heat transfer system of the type noted 10 above, steam will contact with the heat conducting body or vaporizer. With no vaporizable medium in the vaporizer, there will be no appreciable temperature drop in the vaporizer and the walls thereof will be maintained at substantially the 15 temperature of the steam. When a vaporizable medium, such as water, is admitted into the vaporizer part of it will contact the groove formed between the ridges and thereby contact the body having a relatively large heat conducting and 20 storage capacity. The temperature of this body remains substantially unchanged and, when above the critical value at which the water assumes a spheroidal state, non-wetting occurs. However, some of the water will contact the outwardly 25 extending portions such as the apices of the triangular ridges which have only a slight heat conducting and storage capacity. Due to the transfer of heat from the apices of the ridges to the water, the temperature of the apices of the ridges so rapidly drops to a value low enough to-prevent non-wetting, i. e., allows contact between the water and heat conducting body, with the result that heat is transmitted readily to the water and the latter is vaporized.

The above referred to ridges may assume various shapes, as will be disclosed hereinafter, and all forms equivalent to those described are intended to be within the scope of this invention.

A further object of myinvention is so to form 40 the ridges of the portion of the heat conducting bodies having large and small heat storage capac-- lties, respectively, that heat transfer occurs at a substantially constant rate irrespective of the temperature, of the source of heat. In other words, lain-object of this invention is to so proportion the areas that they provide the equivalent of a'steam pressure reducing valve in the conventional'steam heating system.

The portion of the ridged memberin a nonwetting condition will depend upon its shape, the pressure of the vaporizable medium and the temperatureof the source. At relatively low pressures and high temperatures only a small portion will transfer an appreciable amount of heat to the water and at relatively high pressures and low temperatures the area in contact with the water will increase. The rate of heat transfer may, therefore, be designed to remain substantially independent of the temperature. The results described immediately above may be obtained by forming the heat conducting body with ridges over only a portion of its surface. According to this arrangement the ridged portion would be wetted at low pressures and high temperatures and the entire body at high pressures and low temperatures.

Further objects and advantages of my invention will become apparent as the following description proceeds, and the features of novelty which characterize my invention will be pointed out with particularity in the claims annexed to and forming a part of this specification.

For a better understanding of my invention, reference may be had to the accompanying drawing in which Fig. 1 shows, partly in section, an air heating radiator unit deriving its supply of heat through an evacuated vapor heat transfer system embodying my invention; Fig. 2 is a partial crosssection view of the novel heat exchanger of my invention utilized in the system disclosed in Fig. 1;

Fig. 3 discloses another form of the novel heat exchanger of my invention applied to a water heating system; and Fig. 4 shows a series of experimentally determined curves defining pressuretemperature regions in which boiling and nonwetting occur in an experimental set-up designed to approximate actual apparatus.

In order to better illustrate the phenomenon of non-wetting, reference will be made first to Fig. 4 which shows a series of curves obtained when water was dripped onto a heated brass plate under a bell jar in which the pressure could i be varied to a relatively low pressure, an experimental set-up approximating conditions obtaining in a vapor generating system of the usual type. The brass plate was held at an angle of approximately 20? during the course of the experiment. The abscissae represent the surface temperature of the plate and the ordinates represent the absolute pressure under the bell jar in inches of mercury. The curve A indicates the normal vapor pressure curve and the area to the left thereof indicates a region of no boiling, i. e., when the plate was at the temperature indicated by the abscissa and the vacuum as indicated, no boiling took place. The area between curves A and B indicates the area in which boiling was obtained and the surface of the plate was wetted. The area between curves B and C indicates a region in which explosive boiling occurred and the surface was only partially wetted. The area to the right of curve C indicates a region of no boiling and in which non-wetting occurred. Thus the curve 0 represents the critical temperaturepressure range beyond which non-wetting always occurs with very little, if any, conduction of heat between the plate and the water, the effective heat transfer occurring by radiation through the space between the plate and the water.

While I am about to describe my invention in connection with a vapor heat transfer system of j transferredfrom a relatively high temperature source to a vaporizable medium under relatively low pressure. In the illustrated embodiment there is provided a radiator-condenser i l f r heating air within an enclosure indicated at II. This radiator-condenser is preferably formed of suitable pressed metal plates welded together to form a series of interconnected vapor condensing columns I2 with open air circulation passages l3 therebetween' to facilitate dissipation of heat from the radiator to the air. The bottom wall of the radiator is preferably sloped so as to readily drain the condensed vapor into the conduitshaped receiving tube It. Tube I4 is joined with a vaporizing tube l5 having one end thereof sloping downward and extending into the steam chest IS in which live steam is maintained at all times. The downwardly sloping portion I! of the vaporizing tube l 5 within the steam chest is made with a substantially smooth exterior l8 adapted to be contacted by the steam and the interior of the tube is provided with one portion substantially smooth, indicated at [9 and a second portion provided with a series of grooves as indicated at 20. The construction of the grooved portion is illustrated in greater detail in Fig. 2.

Steam is supplied from any suitable boilenor source (not shown) by the steam supply pipe 2| which also serves 'to return the condensed steam to the source. A suitable heat insulating cover 22 effectively prevents any dissipation of heat from the steam chest IS, the steam pipe 2|, as well as from the vaporizing tube l5 except through the operation of the heat transfer system in a manner to be described hereinafter.

A closed liquid control chamber 23 is located remotely from both the vaporizing surfaces of the tube I 5 and the condenser-radiator l0 and has a relatively small size tube 24 communicating with the bottom thereof and the bottom of the condensate receiving tube I 4. This serves to minimize the transfer of heat between the control chamber 23 and the main heat transfer system consisting of the vaporizer tube and the condenser-radiator.

An electrical heating unit 24 of the cartridge type is mounted inside the tube 25 which is sealed into the liquid control chamber 23. The electrical heating unit is of relatively small capacity and is energized at low voltage derived from the secondary of the transformer 26 to which it is connected selectively by thermal responsive means 21 positioned suitably within the enclo-' cooperatively associated therewith and carried by a bimetallic thermal responsive element 29. I

Associated with the thermostat is a preheat coil locally influencing the latter,- such as a resistor 30, which is connected at one end of the bimetallic element and at the other by a conductor 3| to the secondary winding. These 'connectionsare such that the preheat, coil is ener dw ve the contacts are closed by the room thermostat to energize the cartridge heater.

The capacity of the cartridge heater is suchas to expel all the liquid from the control chamber in fifteen or twenty minutesl .Thus after energization thereof a portion of ,the'fluid isexpelled and the longer the heater is energized the: more fluidwill be expelled with the result v. that a H greater area of the vaporizing surface is wetted and the temperatures of the control chamber and condenser, surface rise. After the temperature of the space reaches a desired value the thermo stat, deenerg'izes the cartridge uheaterallowii'ngf the temperature of the control chamber andtliecondenser surface to fall; The. control chamber is so constructed that its rate of heat dissipation under normal conditions will reduce its temperature to approximately room value in fifteen or twenty minutes. Modulated control is obtained by providing the control thermostat with the preheatcoil 30 for locally heating it. Application of preheat to the thermostat whenever the thermostat calls for heat causes the latter to operate at relatively frequent intervals, dependent upon the amount of preheat, intermittently energizing the cartridge heater. Due to this intermittent heating and the heat storage capacity of the control chamber the temperature of the liquid is maintained at an average varying but slightly from that predetermined value at which heat is transmitted to the heated space continuously and at a rate just sumcient to overcome losses due to dissipation.

Referring now to Fig. 2, a more complete description of the novel heat conducting body of my invention will be given. In this figure is illustrated in enlarged cross-section the ridged portion 20 of the vaporizing tube 15. It will be seen from this figure that the outer surface l8 thereof is relatively smooth, while the inner surface is ridged. The individual ridges 33 are of substantially triangular cross-section and terminate in a sharp apex and define between them a plurality of relatively flat sections 34. The grooves in Fig. 2 are illustrated as being threaded in a continuous well known manner, but they may just as well be formed circumferentially. The ridges are also illustrated as triangular in crosssection but they need not be made in this fashion. The criterion is that the ridges should provide bodies adapted to be contacted by the vaporizable medium and when contacted their temperature should be subject to considerable change so that the temperatures of the extremities will decrease below that critical value at which nonwetting occurs. Thus, these grooves may be made substantially triangular in cross-section or even rectangular provided, however, that the projecting portions or ridges 33 shall have a slight heat conducting and storage capacity as compared with the main body of the heat conductor I9.

The figure shows a. preferred method of constructing the grooves but it is obvious that they could be made by forming the ridged portions separately and welding or otherwise securing them to a cylindrical body or uniting a number of bodies through which the heat flows in such a direction that the heat flow will be first through a body having a relatively great heat storage capacity and then through a body having a relatively slight heat storage capacity.

In operation, whenever the temperature within the enclosure ll decreases below a predetermined value, thermostatic element 29 closes a circuit through the heater 24 across the secondary winding of transformer 26 and simultaneously energizes the preheat coil 30. Energization of heater 24 creates a vapor pressure within the control chamber 23 thereby forcing a portion of the vaporizable liquid, such as water, into the vaporizing tube l5. With steam supplied to the steam chest l6 at all times, the vaporizable medium is transformed into vapor and the vapor flows through tubes l5 and I4 to the condenserradiator l where it is condensed and the heat absorbed by the radiating surface and transmitted to the air within the enclosure. The temperature within the enclosure rises and the amount of vaporizable medium in the heat transfer system will be determined by the temperature requirements within the enclosure. The greater the demand for heat, the greater the length of time that the cartridge heater isenergized and consequently the more vaporizable medium in the system. The more liquid in the system, the higher the temperature and pressure to which the medium is raised and consequently the higher the temperature at the condenser-radiator. Thus, the amount of heat available is, dependent upon the demand and a modulated. supply of heat is obtained. Whenever the heating requirements decrease the amount of heat supplied to the cartridge heater is decreased and the temperature within the control chamber and condenser-radiator decreases. The preheat coil acts to shorten the energization period of the cartridge heater to give a closer modulation of the temperature.

In some installations the steam supplied to the steam chest l6 through the supply pipe 2| may undergo considerable variations in temperature. At times it will be below that critical value at which the vapor within the vaporizer is transformed intoa spheroidal condition and at times it will transform the vapor into a spheroidal state and thereby cause non-wetting. However, by the use of a vaporizer constructed in accordance with my invention, the non-wetting of the entire surface is prevented. The mode of operation inthis case is as follows: .Whenever the vaporizable medium is forced into the vaporizer a portion thereof contacts the apices 33 of the ridges and another portion reaches the flat parts 34. The latter is transformed into a spheroidal condition with substantially no heatbeing transmitted to them. The liquid contacting the apices will, however, decrease the temperature thereof to a point where the surface is wetted and heat readily transmitted to the liquidthereon to vaporize it. The extent of wetting on the ridges is dependent upon the temperature of the steam within the steam chest. At very high temperatures the apices-alone will be wetted and with decreasing temperatures an increased area of the ridges will be wetted and at temperatures below the critical value the entire inner surface of the vaporizer will be wetted and heat transmitted to the liquid throughout the whole chamber or through a zone that shifts from the apices I to the main body of the vaporizer. The arrangement therefore provides a heating system which automatically acts as a reducing valve between the source of supply of heat and the vaporizer for the reason that the water may be boiled irrespective of the temperature of the primary source of steam. y

In Fig. 3 I have illustrated a modified form of a vaporizer and a slightly different form of vapor heat transfer system that is particularly adapted for heating a supply of water. The condenserradiator of the system is diagrammatically illustrated as a coil 40 positioned within a heat exchanger tank Ill and connected by means of a vapor inlet tube 42 to the vaporizing chamber 43. The condensate from the condenser-radiator is returned by conduit 44 to a condensate storage tank 45 connected by a conduit 46 to the bottom of the vaporizing chamber 43. The conduit 46 has interposed therein a valve 41 normally biased to its open position by a spring 48 and adapted to be closed by the energization of a solenoid 49 under the control of a thermal responsive means 50 whenever the latter closes a circuit across the secondary winding of a transformer The thermal responsive means 50 is responsive to the temperature of water within the water storage tank 52 which is connected by conduits 53 and 53' to the heat exchanger tank 4|.

Within the vaporizing chamber 43 is the novel vaporizer 54 of my invention consisting, in this modification, of a tube having a relatively smooth liquid contacting portion 55 and a ridged liquid contacting portion 56 so proportioned that each is about one-half the area of the vaporizer. Steam is supplied to the vaporizer through a conduit 51 from any suitable source and the condensed steam is led from the steam chest through a return conduit 58. The heat losses from the vaporizing chamber are kept as low as possible by heat insulating material 59 surrounding the chamber.

The ridged portion of the vaporizer 54 is constructed substantially as illustrated in the modiflcation shown in detail in Fig. 2 with the exception that the ridges in this case are on the outer side of the tube. It should be noted, however, that the ridges are adapted to contact the vaporizable medium as in the previously described modification. The operation of the vaporizer of this modification is, in all respects, the same as that of the previously described modification. When extremely high temperature steam is supplied to the vaporizer only the ridged portion 56 is wetted and when comparatively low temperature steam is supplied to the vaporizer the entire surface is wetted. When the temperature of the water inthe storage tank 52 rises to a predetermined value the thermal responsive means 50 energizes solenoid 49 to close valve 48. The latter cuts off the supply of condensed liquid to the vaporizer and consequently the transfer of heat to the water in tank 4| by the condenser-radiator 40. When the temperature within the tankfalls below the desired value the thermal responsive means deenergizes the solenoid 49 so that the valve may be opened by spring 48 and liquid again supplied to the vaporizer.

In actual practice the portions 33 and 56 have been made in ridges of substantially triangular cross-section having a height of .105", a base of a body of vaporizable liquid under relatively low pressure to said vaporizing chamber and in contiguity to said wall, means for supplying a vaporized fluid at relatively high temperature and pressure to said condensing chamber to maintain said wall at a temperature above the critical nonwetting temperature-pressure range within said vaporizing chamber, and a plurality of projections extending from said wall and having the ends thereof of relatively slight heat conducting and storage capacity for engaging with said body of vaporizable liquid to reduce the temperature of said end portion below the critical pressuretemperature range and thereby provide a wetting heat conducting contact with said body of vaporizable liquid.

2. In combination, a steam chamber having means for supplying steam at variable temperature and pressure thereto, an evacuated heat transfer enclosure having a vaporizer in heat transfer relation with said steam chamber and provided with means 'for supplying a body of vaporizable liquid to said vaporizer under vacuum, said vaporizer having a plurality of projections of slight heat conducting and storage capacity extending from a portion of the surface thereof into engagement with said body of vaporizable liquid for establishing a wetting heat transfer contact therewith when the steam supplied to said steam chamber raises the temperature of said vaporizer above the critical temperature-pressure range.

MARCUS E. .FIENE. 45

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