Method and apparatus for electrically heating a fluid
BRIEF SUMMARY OF THE INVENTION 
     The present invention relates generally to heat exchangers and more particularly to a new and improved fast acting heat exchanger having notable utility in a compact form thereof as a tankless domestic water heater. 
     It is a principal aim of the present invention to provide a new and improved electrically powered heat exchanger useful as a tankless water heater for providing hot water for domestic consumption and/or heating. In accordance with the present invention, the heat exchanger may be compactly made to provide hot water on a continuous or intermittent basis as necessary to meet the domestic hot water requirements. 
     It is another aim of the present invention to provide a new and improved heat exchanger which provides for heating water practically instantaneously. 
     It is another aim of the present invention to provide a new and improved heat exchanger having high thermal efficiency and thermal conductivity. 
     It is a further aim of the present invention to provide a new and improved heat exchanger for heating a moving fluid and providing exceptional heat transfer between the heat source and the moving fluid. 
     It is another aim of the present invention to provide a new and improved electric water heater having a highly concentrated electrical heat source and providing for heating the water to a transient vapor state to substantially increase the heat exchange rate. 
     It is another aim of the present invention to provide a new and improved heat exchanger which can be compactly constructed and which provide long and reliable service free operation. 
     It is a further aim of the present invention to provide a new and improved electric heat exchanger providing a compact and economical water heater for domestic use. 
     Other objects will be in part obvious and in part pointed out more in detail hereinafter. 
    
    
     A better understanding of the invention will be obtained from the following detailed description and the accompanying drawings of illustrative applications of the invention. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings: 
     FIG. 1 is a longitudinal section view, partly broken away and partly in section, of an embodiment of a heat exchanger incorporating the present invention; 
     FIG. 2 is an enlarged transverse section view, partly broken away and partly in section, taken substantially along line 2--2 of FIG. 1; 
     FIG. 3 is an enlarged longitudinal side view, partly broken away, of a twisted tube of the heat exchanger; 
     FIG. 4 is a partial diagram of a space heating system employing the heat exchanger; 
     FIG. 5 is a longitudinal section view, partly broken away and partly in section, of another embodiment of a heat exchanger incorporating the present invention; 
     FIG. 6 is an enlarged cross section view, partly in section, taken substantially along line 6--6 of FIG. 5; 
     FIG. 7 is a longitudinal section view, partly broken away and partly in section, of a further embodiment of a heat exchanger of the present invention; and 
     FIG. 8 is a partial cross section view, partly broken away and partly in section, taken substantially along line 8--8 of FIG. 7. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the drawings in detail wherein like numerals represent like parts, and referring in particular to an embodiment 10 of a heat exchanger of the present invention shown in FIGS. 1-3, the heat exchanger 10 is shown comprising an elongated generally cylindrical inner twisted tube 12 having a plurality of generally helical corrugations providing alternating helical grooves 14 and ridges 16. As best shown in FIG. 3, there are four separate and parallel corrugations providing four separate and parallel grooves 14a, 14b, 14c, 14d and four alternating separate and parallel ridges 16a, 16b, 6c, 16d which run parallel to each other for nearly the entire length of the tube 12. Tubes of this type can be made by methods such as described in U.S. Pats., Reissue No. 24,783 (of Robert P. Humphrey, dated Feb. 16, 1960, and entitled &#34;Apparatus and Method for Making Spirally Corrugated Metal Tubes&#34;), and U.S. Pat. No. 3,015,355 (of Alfred H. Humphrey, dated Jan. 2, 1962, and entitled &#34;Method For Forming Spirally Ribbed Tubing&#34;). The tube 12 has cylindrical end portions 18, 20 of reduced diameter ans enlarged cylindrical end flanges 22, 24. In each turn of each of the grooves 14 is one or more apertures or openings 25. Additional larger apertures or openings 26 are provided in the cylindrical end 18. The twisted metal tube 12 has a substantially uniform outer diameter and a substantially uniform wall thickness from end to end thereof and provides a large heating surface in a small compact heat exchanger. 
     The twisted or spirally corrugated metal tube 12 is disposed inside a tubular metal jacket or casing 30 in a tight fitting manner with the ridges 16 contacting the inner cylindrical wall surface of the casing 30 to provide four helical flow channels indicated at C around the tube 12. Flanges 22, 24 tightly fit within the ends of the tubular casing 30, and closure caps 32, 34 are applied to the ends of the casing. The jacket or casing 30 has lateral openings 36, 38 at its opposite ends to which are connected inlet and outlet conduits 40, 42. The radially extending conduits 40, 42 serve respectively as a fluid inlet and fluid outlet for the heat exchanger. The reduced cylindrical portions 18 and 20 of the tube 12 have a smaller diameter than the inside of the casing 30 to define two annular chambers 43, 44 at opposite ends of the casings adjacent the inlet and outlet conduits 40, 42 respectively. 
     Inside the twisted or spirally corrugated tube 12 is a looped electrical heating rod 45 enclosing an insulated heater wire 46 in a water tight manner. The ends of the rod extend out through the cap at the outlet end of the casing. The end of wires 46 are suitably connected to an external electric power supply to provide for energizing the electrical heating rod 45. The rod 45 is almost as long as the tube 12 and is supported by the inner sides of the grooves 14 and the reduced cylindrical end portions 18, 20. The shown heating rod 45 may be of the type known in the trade as a &#34;Calrod&#34; heating rod, and which does not oxidize at an elevated temperature. 
     Surrounding the metal casing 30 is an electric heating sleeve 50 which may be a rubberized flexible heating mat as shown. This sleeve 50 may be composed of silicone rubber reinforced with fiberglass and having a resistance heater wire or coil 52 embedded therein. The wire 52 terminates in external terminals 54 which can be connected to a suitable external electric power supply. Surrounding the casing 30 and sleeve 50 is a thick body 55 of thermal insulation material such as asbestos, rock wool, fiberglass or the like. 
     Using the heat exchanger as a water heater, the water to be heated is fed into the device through inlet conduit 40. This water enters annular chambers 43. Communicating with chamber 43 is the interior of the twisted tube 12. Also, communicating with chamber 43 are the parallel helical channels C defined by the grooves 14, ridges 16, and the inner surface of the casing 30. The inlet water thus enters both the tube 12 and the four parallel channels C of flow. The water in tube 12 is quickly heated by the heater rod 45 and turned to steam which passes through the lateral apertures 25 and mixes with the water in the channels C to heat this water. The water is driven through and around the twisted tube 12 and channels C under the pressure of the incoming water supply. Hot Water HW containing steam and/or superheated water flows through chamber 44 and out the outlet conduit 42. The twisted tube 12 has no direct outlet and the water and steam in this twisted tube must pass through openings 25 into the parallel channels C to reach the outlet conduit 42. The conversion of water to steam within the tube 12 (and the concomitant large transfer of heat in vaporizing the water) and reconversion of the steam to water in the outer channels C permits using a high watt density heat source and provides a highly concentrated and rapid form of heat transfer from the heater rod 45 to the water flowing through the outer tube. 
     Electrically heated sleeve 50 serves to raise the temperature of casing 30 and to thereby provide additional means (which may be selectively used when desired) for heating the water passing through the channels C. Thermal insulation 55 maintains the thermal effficiency of the device by inhibiting heat loss from the sleeve 50 as well as around the ends of conduits 40, 42 and around casing 30 and end caps 32 and 34. 
     FIG. 4 shows a space heating system 100 employing the heat exchanger 10. A cold water supply 102 is connected to the heat exchanger inlet conduit 40, and the outlet conduit 42 is connected to a distributor valve system 104 having outlets 104a, 104b and 10497 c. Connected to this valve system through outlet 104a is a manifold pipe arrangement 106 which feeds hot water to space heaters 108. The space heaters 108 are connected to an outlet pipe 110 which may be suitably connected to discharge the used water, or preferably, the outlet pipe 110 is connected to the cold water supply 102 as indicated by broken line 112 to provide for recirculating the water. Both electric heating wires 46 and 52 may be connected to the same electric power supply 113. 
     The heat exchangers 10 may be interposed in other systems where a quick acting and practically instantaneous tankless heat exchanger of the character described may be usefully employed to replace prior known boilers heated by combustion of oil, gas, or fossil fuels. Instead of four parallel helical grooves 14 and ridges 16, more or fewer than four helical grooves and ridges may of course be provided. The twisted tube 12 should preferably be made of copper or other metal having high thermal conductivity. Casing 30 may be similarly constructed, or if desired, the casing 30 can be replaced by a jacket made of ceramic, tempered glass or other suitable insulating material. 
     The apertures 25 in the grooves 14 of the twisted tube 12 allow the heating unit to be flooded with water at the same time that the flow of water is passing helically around the outside of the twisted tube 12. It will be understood that water is premitted to flow into the inner twisted tube 12 but no provision is made to let water flow out except through the apertures 25 and therefore primarily in the form of steam generated within the tube 12 by the heater rod 45. In effect, there has been provided a small steam generator which will eject steam into the helically moving water surrounding the tube 12 to heat that moving water stream. 
     Water has generally poor heat transfer, and by breaking the flow into four channels the speed of the heating process is substantially increased. While water has been mentioned, it should be understood that other fluids may be heated with the same benefits and other forms of heaters may also be used in lieu of the electrical heater rod 45. 
     In FIGS. 5 and 6 there is shown another embodiment 10A of a heat exchanger incorporating the present invention which is basically the same as the heat exchanger 10, with corresponding parts identically numbered. In heat exchanger 10A, four straight electrical heating elements or rods 45a are provided and held in a fixed annular array by suitable spaced support members 66 disposed inside the twisted tube 12a. The tube 12a is located in casing 30a which is wholly enclosed in thermal insulation body 55a. The heating elements 45a extend outwardly of end walls 34a, 34b of the casing 30 at opposite ends of the casing 30a and tube 12a. Inside each rod is a resistance wire 46a. Opposite ends of the resistance wires extend through the insulation body 55a and terminate at relay switches 65 located in relay box 64 at the end of the assembly and connected in a suitable thermostatically controlled circuit. The heat exchanger 10A has an outer rectangular metal cover or casing 60 which encloses the insulation body 55a and is closed at its right end as viewed in FIG. 5 by cap 62. Cap 62&#39; at the other end of the assembly closes the relay box 64. The heat exchanger has an inlet conduit 40a and an outlet conduit 42a extending radially of the assembly at opposite ends of tube 12 a. The heat exchanger 10A basically operates in the same manner as described above for heat exchanger 10 so the description of its operation need not be repeated. The relay switches 65 serve to turn on and off the electric heater wires 46a, and the heater coil or wire 52 in heater mat 50 is energized in the same manner as described in connection with heat exchanger 10. 
     In FIG. 7 there is shown a further embodiment 10B of a heat exchanger 10B incorporating the present invention. The heat exchanger 10B is generally similar to the heat exchanger 10A and corresponding parts are identically numbered. However, in heat exchanger 10B there are shown provided two spirally twisted tubes 12a and 12b. In each tube is a set of four straight electrical heating elements or rods 45a held in fixed spaced array by suitable transversely spaced support elements 66 disposed inside the twisted tube. Each tube is surrounded by an electrically heated mat 50. Opposite ends of the resistance heating wires 46a terminate at relays switches 65a located in relay box 64a. The relay switches are adapted to be connected in a thermostatically controlled circuit for operation in the system. Wires 46a extend through the thermal insulation 55b which surrounds both tubes 12a and 12b. The insulation 55b is enclosed in a snug fitting casing or sheath 60a closed at opposite ends by caps 62a and 62a&#39;. The heat exchanger has two water inlet conduits 40b and 40c. Inlet 40b passes cold water into casing 30b. Inlet 40c passes recirculating or captured water into casing 30a&#39;. The heat exchanger has two outlet conduits 42 b and 42c. Outlet 42b passes hot water out of casing 30a&#39;. The hot water which leaves casing 30b is delivered to a hot water discharge outlet 202. Since the heat exchanger provides substantially instantaneous heating, a hot water storage tank is not required. Such enables considerable economic savings to be made in the installation and operation of the system. 
     While I have illustrated and described several preferred embodiments of my invention, it is to be understood that I do not limit myself to the precise construction herein disclosed and that various changes and modifications may be made within the scope of the invention as defined in the appended claims.