Electric in line water heating apparatus

Abstract

In line water heating apparatus comprising a cylindrical heating element positioned within a length of vertical helically corrugated tubing is compact in size, low in cost and very efficient in transferring heat to the water. The outer surface of the cylindrical heating element is uniformly spaced from the innermost portions of the corrugations along its entire length by a slight distance therefrom which is less than the depth of the corrugations. The internal helical tube corrugations cause a portion of the water entering at the bottom to tend to travel in a spiral, thus lengthening its contact path with the heater element before it is drawn off at the top of the apparatus. However, their main purpose is to induce turbulent flow conditions in the water and thus enhance the rate of heat transfer. Since the corrugated tube is spaced from the heater core, the water can also circulate and maintain its temperature when the valve is closed. A dual mode temperature control permits the small supply of water which is always present in the device to be maintained at a temperature somewhat less than the desired temperature so that water leaving the apparatus can very quickly be brought to its maximum and so that randomly drawn off single cups of water will be at a sufficiently hot temperature.

Description

BACKGROUND OF THE INVENTION

This invention relates to in line water heaters and particularly to heaters which are adapted to heat small quantities of water in an instantaneous manner, such as for use in coffee brewing apparatus in aircraft.

Various heaters for heating fluids are known in the prior art, as exemplified by U.S. Pat. Nos. 2,245,085; 2,307,924; 2,577,269; 2,792,487; 2,802,089; and 3,584,194. Each of these heaters can be characterized as having spirally shaped heater elements or spirally shaped internal walls which cause the fluid to flow in an extended path. Although British Pat. No. 398,589 shows an engine heater having a corrugated outer tube, most of the engine coolant would not flow through the tube since the tube is merely connected to a coolant carrying tube by a tee fitting with any flow in the heater tube being caused by gravity due to variations in density. Any heating that takes place in such a heater would be very slow and inefficient.

SUMMARY

It is among the objects of the present invention to provide a compact, in-line water heater that is simple to construct and maintain and which is capable of continuously, and almost instantly delivering hot water at a temperature very close to boiling.

The heater apparatus comprises a short length of braid reinforced, helically corrugated tubing having a water inlet and flow control valve at its lower end and a water outlet at its upper end. A tubular sealed heating element is mounted in the lower end and extends upwardly for almost the entire length of the tube. The smooth outer surface of the heating element is spaced from the tubing corrugations by a distance slightly less than the depth of the corrugations. The spacing causes turbulence to be induced in the water when it is flowing so as to increase heat transfer. The spacing also permits the water remaining within the apparatus when water is not being drawn off to circulate and thus be maintained at a relatively uniform idle temperature. A temperature control positioned in the water within the tubing at a point above the heating element operates to maintain the water between an idle temperature of about 180.degree.F and a use temperature of about 195.degree.F.

To prevent the water pressure within the corrugated metal tubing from expanding the tubing, a flexible metal braid is placed around the tubing and is firmly affixed to metal ferrules attached to the ends of the corrugated tubing. A temperature control at the exit end of the heater senses the outlet water temperature and controls the operation of the heating element. An overheat thermostat mounted within the heater core serves to shut off the heating elements in case of an excess heat malfunction while a pressure release valve mounted near the outlet of the apparatus provides additional safety protection in the event the water pressure becomes excessive.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a front plan view of the heating apparatus with portions broken away in section; and

FIG. 2 is a bottom end view of the apparatus of FIG. 1;

FIG. 3 is an enlarged view of a portion of FIG. 2.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The in line heater apparatus shown generally at 10 includes an upper end member 12 and a lower end member 14, each of which members have a bracket member 16 attached to it for mounting the apparatus 10 in a vertical position. A thin, corrugated metal tube 20 is affixed at its ends, preferably by welding, to the upper and lower end members 12, 14. A braided wire metal sleeve 22 contacts the outermost surface portions of the corrugated tube 20 and is also welded to the end members 12, 14. The braided sleeve not only prevents elongation of the corrugated tube 20 under pressure but also increases the maximum pressure which may be present inside the apparatus 10. The various aforementioned metal elements are preferably formed of stainless steel. Although the apparatus 10 could be used any place there is a need for a continuous flow of hot water, it is particularly useful in a coffee brewing apparatus for an aircraft. Since the invention relates to the internal design of the apparatus 10 rather than to the switches and other electrical circuit elements which control the flow of water through it, these circuit elements will be only generally described. Although not shown the entire unit 10 is preferably wrapped in a blanket of fiber glass insulation having an outer foil faced surface.

In operation, water enters the inlet valve member 26 which is threaded into the end member 14. The entering water is heated by contact with the surface of sealed heating unit 30 which is mounted into the end member 14 by pipe threads 32. Heat is generated inside the heating unit 30 by three heater wires 34 connected in a three-phase delta arrangement. The heater unit 30 is protected against being damaged by overheating, such as might occur if the unit were operated without water being present, by means of an overheat thermostat (now shown). Wires 36 connected to the overheat thermostat lead out of the heater unit 30 to a control box 35. As the water which enters the unit 10 moves upwardly through the unit a portion of it tends to travel in a spiral through the corrugations 37 while the remainder of it moves in a generally axial direction through the space 38 between the heater unit 30 and the inside walls of the corrugations 20. The innermost portions of the corrugations 20 comprise ridges 20' which induce turbulence in the water flowing through the space 38, the space 38 having a width w less than the depth d of the corrugations, so as to enhance the rate of heat transfer. A water outlet fitting 40 is threaded into the upper end member 12. Water leaving the outlet fitting 40 preferably passes through an electrically actuated solenoid type valve (not shown) which controls the flow out of the unit. A pressure release valve 42 is mounted in the upper end member 12 as a safety feature in the event that excess pressure develops within the unit. A temperature control assembly 44 also is threaded into the end member 12. The temperature control unit 44 includes a temperature sensing probe 46 which senses the temperature of the water above the top of the heater unit 30. The probe 46 is preferably of stainless steel which surrounds a column of mercury. As the mercury rises with an increase in temperature it first closes the contacts of an idle mode switch which deactivates a heater control relay (not shown). If the apparatus is in a full heating mode, the heater control relay will remain actuated until the mercury rises further and closes the contacts of a heating mode switch. As the temperature of the water sensed by probe 46 changes, the heater 30 will be cycled on and off as needed. A plurality of wires 48 extend from the switches within the temperature control 44 to the control box 35 where they are connected to the aforementioned relay (not shown) which is in circuit with the heater wires 34. The circuit is arranged so that during standby or idle operation, the water within the heater is controlled by control 44 and heater unit 30 so as to be maintained at an idle temperature of approximately 180.degree.. As soon as water flow is commenced, such as by actuation of the "brew" button on a coffee brewer (not shown) to which the unit 10 may be attached, the temperature control 44 actuates the heater unit 30 to cause the water to be additionally heated so that water leaving the outlet 40 will reach and will maintain a temperature of about 195.degree.F, a temperature at which it is held until delivery of hot water is no longer desired. By maintaining the unit at an idle temperature it is possible to add sufficient heat even to the first cup of water drawn off so that the unit may be used to provide either a single cup of water such as for making tea or may be operated continuously such as to fill a coffee pot. The water inlet valve 26 is preferably of the type which compensates for changes in inlet pressure to assure a constant flow of water matched to the capability of the heating elements. The constant water flow rate is also important when the unit is controlled by a time delay relay which permits water to flow for a fixed period of time to brew a pot of coffee when a brew cycle is started.

In a test unit incorporating the invention and utilizing a 3,000 watt heater (Watlow type T8Ax6A), a Micropump Incorporated flow control valve type 89ZSS, and a Philadelphia Scientific Glass Corporation Model 5-09319 temperature control, a flow of about 20 oz. per minute was achieved at a temperature of 195.degree.F. The efficiency of the design was compared to a unit which was identical except that the corrugations 20' touched the outer surface of the heater unit 30 so as to force all of the water to follow a spiral path. The design was also compared to a unit wherein the corrugated tube 20 was replaced by a smooth wall tube having the same internal diameter. In each instance, the inlet water temperature, the inlet pressure, the length of the brew cycle and the quantity of water in the pot were held constant. The design of the present invention wherein a space 38 is provided between the heater 30 and corrugated tube 20 provided a 3.degree. to 5.degree.F higher pot temperature in every test run.

Claims

I claim as my invention:

1. In line heater apparatus for providing an instantaneous supply of hot water comprises a length of helically corrugated tubing positioned in surrounding, spaced relation to an axially positioned cylindrical heater element mounted on one of first and second end members positioned at each end of said tubing and integrally attached thereto, means on said first end member for attaching the apparatus to a water supply line, means on said second end member for attaching the apparatus to a valve controlled dispensing line, temperature control means mounted in said second end member for controlling the operation of said heater element, said cylindrical element extending for substantially the entire length of said corrugated tubing and the outer surface of the cylindrical heater element being uniformly spaced from the innermost portions of the corrugations along its entire length by a slight distance therefrom which is less than the depth of the corrugations, the spacing being sufficient that water flowing from one end of said tubing to the other will flow in a generally helical path through the corrugations and in a generally axial direction in said space and in turbulent contact with said cylindrical heating element.

2. The heater apparatus of claim 1 wherein a braided metal sleeve surrounds said corrugated tubing and is welded to said first and second end members, said braided sleeve serving to prevent elongation of the tubing due to increases in the pressure of the fluid within the system.

3. The heater apparatus of claim 1 wherein the tubing, end members and braided sleeve are each formed of stainless steel.