Electric continuous flow heater
FIELD OF THE INVENTION 
     This invention relates to a continuous-flow water heater of the type having a tank in which an eletrical heating unit is disposed, and to which are connected an inlet for the cold water to be heated and an outlet for heated water. 
     BACKGROUND OF THE INVENTION 
     It is proposed in West German Patent Application No. P 3218863.3 that a continuous-flow water heater be in the form of a vertical cylindrical tank containing a concentric array of electrical heating elements, and, that heated water be withdrawn from an upper end of the tank. In order to increase the rate of heat exchange between the heaters and the water to be heated, the water is caused to move within the tank in a cyclic path by introducing the water tangentially into the tank at positions spaced vertically of the tank. By so doing, the formation of steam bubbles in the tank is reduced, as is the mechanical noise produced by such steam bubbles. 
     SUMMARY OF THE INVENTION 
     It is an object of this invention to further increase the rate of heat transfer from the heating elements to the water, and, to further minimize the formation of steam bubbles, and, in consequence, to permit a reduction in size of the water heater for a given output rate of the heater. 
     This is accomplished in the heater of the present invention by inducing a forced recirculatory flow within the heater utilizing energy derived from the entering water flow in supplement to thermally induced convection flow within the heater. 
     According to the present invention, a continuous-flow water heater includes a tubular baffle within the tank and which separates a first compartment from a second compartment. The heating unit is located in the second compartment. The two compartments are open to each other at both of their ends. The water inlet opens into the second compartment at its lower end and is in the form of a vertically directed high velocity jet of water that induces a vertical recirculatory flow within the tank between the first and second compartments. 
     This assures, that the flow along the heating unit or units is increased to a maximum, resulting in diminshed steam bubble formation, and also in diminishing noise. 
     The enhanced water of ciruclation leads to a uniform temperature distribution in the tank. This permits making the volume of the tank small, thereby contributing to a compact construction of the continuous-flow heater. 
     Another advantage of the invention is that build up of lime deposits in the tank is greatly reduced. 
     In a preferred embodiment of the invention, the two compartments are open toward each other at the top and bottom, and the inlet discharges into the tank at the bottom. It is achieved thereby that the thermal convection caused by the heating unit, and the direction of flow deriving from the water in flow into the second compartment are codirectional. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     The invention will now be described with reference to the accompanying drawings, which illustrate preferred embodiments of the invention, and in which: 
     FIG. 1 shows a schematic sectional view of one embodiment of continuous-flow heater; 
     FIG. 2, a section along line II--II in FIG. 1; 
     FIG. 3, a schematic sectional view of a further embodiment of continuous-flow heater; 
     FIG. 4, a partial section of a preferred embodiment of continuous-flow heater; 
     FIG. 5, a plan view of the heater of FIG. 4; 
     FIG. 6, illustrates a support for the heater of FIG. 4; 
     FIG. 7, shows a bottom flange of the tank of FIG. 4, in section; 
     FIG. 8, shows the flange of FIG. 7 in a sectional plane perpendicular to FIG. 7; 
     FIG. 9, shows a spacer for the heating units of FIG. 4; 
     FIG. 10, shows an outlet of the heater of FIG. 4; and 
     FIG. 11, shows an alternative to the outlet of FIG. 10. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An inlet 2 leads into a cylindrical tank 1 at the bottom thereof. At the top, an outlet 3 is connected to tank 1. 
     The inlet 2 lies approximately on the axis of the tank 1. Around the axis of the tank are grouped several electric heating units 4, which extend parallel to the axis of the tank. 
     In the embodiment according to FIG. 1, the inlet 2 is designed as a water jet pump, and includes an injector nozzle 5 and a diffusor tube 6. Located at the lower end of the diffusor tube 6 is a conical suction orifice 7. 
     A cylindrical baffle 8 is disposed between the heating units 4 and the tank 1, and forms a first compartment 9 of annular cross-section which is in open communication at its ends with a second compartment 9&#39; interiorly of the baffle 8 and in which the heating units 4 are arranged. 
     In FIGS. 3 to 11, the inlet 2 is in the form of a venturi nozzle 10. To the latter is connected a differential pressure switch 11, which switches the heating units 4. The venturi nozzle 10 has a throat 12. When cold water flows through the venturi nozzle 10, a higher pressure builds up before the throat 12 and a lower pressure prevails in the region of the constriction 12. The switch 11 is switched by the pressure differential. 
     A diffuser 13 follows the throat 12, and corresponds in function to the injector nozzle 5 of FIG. 1. In the embodiments of FIGS. 3 to 11, the suction orifice 7 is provided by the spacing between the diffuser 13 and the lower edge of guide tube 8. 
     As shown in FIG. 4, tank 1 is provided with an upper closure 14 and a lower closure 15. Supported by the upper closure 14 are U-shaped electric heating units 4. As shown in FIGS. 4 and 9, spacers 16 are supported on the heating units 4 by means of rings 17. The spacers 16 are provided with noses 18 which hold and support the guide tube 8. In addition, flow directing vanes 19 are provided on the spacers 16 to induce helical rotation of the water flow around the central axis. 
     As shown in FIGS. 4 and 5, heating units 4 are electrically delta-connected to poles R,S and T of a three-wire supply system by means of contact strips 20. 
     The upper closure 14 is secured to the tank top by use of a compression ring 21 which reacts against a shoulder 22 at the tank top. The shoulder internally supports a sealing ring 23. 
     Tank 1 is supported within a housing 24 by means of a lug 25 (FIG. 6) provided on the compression ring 21, and which is received within a seating 26 provided on a backwall of housing 24. The lower closure 15 is secured to the housing 24 by a bracket 27 attached to housing 24 and secured to the lower closure 15 by a bolt 28. 
     The lower closure 15, has bores 29 and 30 connection of the differential pressure switch 11. The cold water line is to be connected to a lateral port 31. An adjustable by-pass 32 permits adjustments of the pressure differential applied to the switch 11. By-pass 32 opens into the second compartment 9. 
     As shown in FIGS. 4, 10 and 11, the outlet 3 is arranged on the side of tank 1 adjacent the top thereof. In FIG. 10, a threaded bushing 33 is secured about an opening 34 of tank 1 by means of a flange 39. An outlet pipe 35 has a bead 36 formed by upsetting. The latter is pressed into bushing 33 by a threaded collar 37 screwed into bushing 33 and seats on a sealing ring 38 reacting against the flange 39. 
     In FIG. 11, a first collar 40 is attached to the side of the tank 1 by means of a flange 39. An outlet pipe 35 has a bead 36 formed thereon by upsetting, and which is engaged by a second collar 41. The bead 36 is urged into seating engagement with a sealing ring 38 interposed between the bead 36 and the flange 39, by screws 42 which extend through the collar 41 and are threaded into the collar 40. 
     A perforated plastic insert 43 produces a back pressure in tank 1. 
     The continous-flow water heater descibed operates as follows: 
     The direction of water flow within tank 1 is indicated by arrows in FIGS. 1 and 3. When cold water passes into tank 1 through inlet 2, a pressure drop occurs at the suction orifice 7. Water already in tank 1 is thereby entrained with the water injected through inlet 2. The temperature of the resulting mixed stream is thus intermediate the temperature of the injected cold water and that of the heated water drawn in through orifice 7 from the first compartment 9. The heating units 4 add further heat to the mixed stream. Owing to the high flow velocity of the mixed stream as well as the greater volume thereof as compared with the volume of water entering through inlet 2, steam bubble formation at the heating units 4 is reduced. 
     Part of the upwardly directed flow of heated water leaves tank 1 through outlet 3. The remainder continues by convection flow and is drawn into the compartment 9, where it is drawn downwardly towards the suction orifice 7. Hence the same volume of water traverses the heating units 4 several times. 
     In conventional continuous flow heaters, the volume of cold water entering through inlet 2 is the same as the relatively small volume of hot water leaving through outlet 3, and the flow past the heaters is relatively quiescent. The present invention provides for a greatly increased volumetric flow past the heating units 4, by virtue of the internally circulatory flow, this having the further advantage that lime or scale settling out at the bottom of tank 1 is automatically flushed out of the tank and cannot accumulate. 
     The volume of tank 1 is made sufficiently large enough so that residual heat in the heating units 4 does not result in boiling of the water after water flow has been turned off and the heating units thus de-energised. 
     Numerous modifications are within the scope of the invention. Thus it is possible, for example, to eliminate the diffuser tube 6 or to connect the diffusor tube 6 directly with the guide tube 8, so that the bottom of the annular space 9 merges directly into the suction orifice 7. It is not necessary to design the inlet 2 as an injector nozzle or venturi nozzle. The desired suction effect can be obtained also with a simple pipe end. Instead of the guide tube 8, a flat wall is sufficient provided that it creates in the tank 1 a compartment 9 remote from the heating units and through which preheated water can be returned to the lower end of the second compartment 9&#39;. 
     Further it is possible to provide the guide tube 8 with appertures through which partial streams of water can circulate between the compartments 9 and 9&#39;.