WO1998030844A1

WO1998030844A1 - A water pre-heating unit for a hot water tank

Info

Publication number: WO1998030844A1
Application number: PCT/IL1998/000011
Authority: WO
Inventors: Haim Hazan
Original assignee: Negev Soltech Ltd.
Priority date: 1997-01-09
Filing date: 1998-01-09
Publication date: 1998-07-16
Also published as: IL119984A0; AU5494198A; IL119984A

Abstract

The invention provides a water pre-heating unit (10) for insertion in an insulated hot water storage tank (12), the unit comprising a housing (14) adapted to be positioned on the bottom surface (16) of the tank (12), an electric water immersion heater (18) positioned in the housing (14), a cold water inlet transfer conduit (26) having at least a portion (28) of its length positioned within the housing (14), a first opening (30) in the conduit (26) extending at least above the mid-height of the housing (14) for receiving tank water and a second opening (34) in the conduit (26) located within the housing (14) for discharging the tank water into the housing (14), the water within the conduit (26) being pre-heated while in the conduit as the conduit is heated by water within the housing (14), and a hot water outlet transfer pipe (36) extending upwardly from inside the housing (14) and having a length sufficient to position the outlet (38) of the pipe (36) in an uppermost volume of the water storage tank (12) when the housing (14) is inserted in the tank (12).

Description

A WATER PRE-HEATING UNIT FOR A HOT WATER TANK

Technical Field

The present invention relates to a water pre-heating unit for assembly in a hot water tank. More particularly, the invention provides means for electrically heating part or all of the water held in a storage tank of the type typically used within an apartment, on the roof of a building, or as a part of a solar water heating system. When only part of the stored water is heated, heating is fast and hot water is available for use, even when the remainder of the stored water is cold or nearly so. Background Art

Water heaters are widely used .in industry and homes, tank storage capacity being related to the use intended. The most widely used hot water tanks serve a single residence, and in Israel are heated on most days by solar energy. Capacity of domestic water tanks in the U.S. ranges from 20 to 120 gallons, while in Israel the typical capacity is in the range of from 120 to 200 liters. Smaller, electric-only tanks may be used to serve a part of a single residence, for example, the kitchen; such tanks may be as small as 30 liters.

Hot water storage tanks are insulated to preserve heat from the time of its generation until the hot water is required for consumption.

A disadvantage of the conventional type of electric water heater is that almost all of the tank contents are heated before a small quantity of hot water becomes available for use, for example, for providing a shower. In Israel, heater elements are usually limited to 2000 or 2500 watts so as not to overload the apartment electric supply; consequently, 2 to 4 hours are required to heat a full tank of water.

A common modification to the convential water tank which attempts to alleviate this difficulty is the provision of some form of central sleeve having a 5-20 liter capacity, for receiving the electrically-heated water. Such sleeve prevents the dispersal of heat from such water to some degree. Various other devices are known, each having advantages and drawbacks.

A second problem with conventional water heaters is that cold line water directly enters the bottom of the housing holding the immersion heater. Such water contains small quantities of rust flakes, sand, small stones and other solids, which in time accumulate in the housing, obstruct the free flow of water, and may cause burnout of the heater element. A further disadvantage of feeding line water directly into the immersion heater housing is that more electricity is needed to heat the cold incoming water than would be necessary to heat the warmer water which is already available near the mid-height of the storage tank.

Another problem arises occasionally during a failure of the water supply, when the tank will empty and the heater element, whether immersion or dry coil type, will rise above its correct operating temperature when powered. Burn-out ensues. In an attempt to prevent such an occurrence, and to maintain water at least around the heater element, some supply lines are fitted with a hermetic one-way valve. However, here a new difficulty arises. The valve will cause substantial pressure fluctuations as the tank water is heated and expands, and as water is withdrawn. Such pressure fluctuations lead to the premature failure of the tank or its connector pipes, or to the solar absorber panels connected thereto, due to stress-corrosion. Furthermore, failure of the thermostat during extended use of the heater element can cause steam generation inside the tank, and if a one-way valve prevents pressure release through the water inlet, a tank explosion can result.

A still further problem with conventional water heaters is that each time a consumer opens a hot water faucet, cold water enters the water tank in the vicinity of the thermostat, which subsequently turns on the heating unit. When such numerous, unnecessary temperature fluctuations occur, the lifespan of said thermostat is substantially decreased; an additional disadvantage is the high electricity consumption resulting from such an arrangement. In preferred embodiments of the present invention this problem is solved by not placing the thermostat in the vicinity of the cold water inlet. Disclosure of the Invention

It is therefore one of the objects of the present invention to obviate the disadvantages of the prior art electric water heaters and to provide an electric heater and a water tank containing same, which are suitable for the fast, efficient and safe heating of a portion of the water contained in the tank.

It is a further object of the present invention to provide a water heater wherein the immersion element is protected against premature failure.

A yet further object of the present invention is to provide a water heating system which operates without requiring a hermetic one-way valve and which is protected from severe pressure fluctuations.

The present invention achieves the above objectives by providing a water preheating unit for insertion in an insulated hot water storage tank, said unit comprising a housing adapted to be positioned on the bottom surface of said tank, an electric water immersion heater positioned in said housing, a cold water inlet transfer conduit having at least a portion of its length positioned within said housing and extending alongside said immersion heater, a first opening in said conduit extending at least above the mid-height of said housing for receiving tank water and a second opening in said conduit within the lower volume of said housing for discharging said tank water into said housing, the water within said conduit being pre-heated while in said conduit as said conduit is heated by water in proximity to said immersion heater within said housing, and a hot water outlet transfer pipe extending upwardly from inside said housing and having a length sufficient to position the outlet of said pipe in an uppermost volume of said water storage tank when said housing is inserted in said tank.

In a most preferred embodiment of the present invention, there is provided an electric water heater comprising an insulated hot fluid storage tank; a housing positioned on the bottom surface of said tank; at least one water inlet pipe positioned between the tank outer wall and the housing outer wall, providing water to the storage tank; an electric water immersion heater positioned inside said housing; a cold water inlet transfer conduit having at least a portion of its length positioned within said housing; a first opening of said conduit extending at least above the mid-height of said housing for receiving tank water and a second opening of said conduit located within said housing for discharging said tank water into said housing, the water within said conduit being pre-heated while in said conduit as said conduit is heated by water within said housing; a hot water outlet transfer pipe extending upwardly from inside said housing and having a length sufficient to position the outlet of said pipe in an uppermost volume of said water storage tank when said housing is inserted in said tank, and a consumer water outlet pipe having a first opening in said uppermost volume of said water storage tank for drawing water therefrom and a second opening extending to the ouside of said tank to form a heated water consumption outlet.

In especially preferred embodiments of the present invention, the water within said conduit is preheated by at least 3°C while in said conduit, and is preferably preheated by at least 5°C while travelling between said first opening and said second opening in said conduit.

In another preferred embodiment of the present invention, said second opening is oriented for directing water passing through said conduit in a downward flow within said housing, to effect the further heating of said water while it flows downwardly within said housing.

Further embodiments of the invention will be hereinafter described. In Israel Patent No. 59575, Glazer describes and claims an electric water heater unit for domestic water tanks, comprising a heating element enclosed within a pre-heating chamber, said chamber having an inlet for tank water provided at a lower section thereof. An outlet for pre-heated water is provided at an upper section of the chamber. A heated water consumption outlet pipe is connected at one of its ends to said pre-heated water outlet, extending therefrom to the uppermost section of the tank, wherein the pipe is provided with openings for the flow of pre-heated water out of the pipe. Such flow is mixed with tank water for consumption. The consumption outlet pipe extends outside the tank. An inlet pipe is connected at one end to said chamber inlet, and extends up to about half the tank height.

In the drawings accompanying said patent, the cold water inlet transfer supply pipe is seen to be heat-insulated, and the water therein will therefore not be preheated by the heater element. In contradistinction thereto, the present invention provides pre-heating of incoming water by passing such water downwards in an uninsulated conduit within the water heating unit.

Due to striction 30 in the cold water transfer pipe, seen in Fig. 1 of the Glazer patent, hot water cannot be drawn from the heating element chamber at normal flow rates. A demand for hot water will be met by partially heated tank water entering the openings in the consumption outlet pipe. Solid debris in the tank water may cause a blockage in striction 30, and such blockage will be difficult to clear.

Furthermore, in the Glazer heater, the control thermostat is positioned outside the chamber holding the heater element and in proximity to the cold water transfer pipe. Thus, the thermostat will not react to the high temperature water within the heating chamber. Consequently, the water will evaporate out of said heating chamber, thus endangering the heater element. In contradistinction thereto, in the present invention the thermostat is inside the heater housing and will react to excess heat caused by loss of water by shutting off the power to the element.

Also, as shown in the drawings accompanying Israel Patent 59575, the outlet has openings which will collect water from locations lower than the top of the tank, and thus will fail to collect the hot water available at the top of the tank. In contradistinction thereto, the present invention provides a consumer water outlet pipe having an inlet opening only in the uppermost volume of said water storage tank, so as to draw therefrom the hottest water available.

Finally, it is clear from only a preliminary perusal of the drawings that the Glazer device is not adapted for installation in pre-existing heating tanks, and thus the manufacturing cost of the Glazer device will substantially exceed the cost of the device described in the present invention.

It will thus be realized that the novel water heater of the present invention will heat water for a shower within a short time span and will have a longer operating life than existing devices. Furthermore, the water pre-heater unit of the present invention can be retrofitted to existing prior art hot water tanks.

An electric water heater tank constructed in accordance with the present invention was tested. For the purpose of the test, a vertical-axis tank was fitted with thermocouples at seven evenly-spaced heights on the tank wall for recording water temperature during use of the electric heating element. It was found that temperature measured in the top seventh layer of water rose from 23 °C to 60°C within 18 minutes of operation. The tested heater tank, having its heating element working around the clock, has to date produced 250 m³ of hot water (a quantity consumed by an average family during several years), and so far the amount of sediment is negligible in quantity. More importantly, said amount of sediment has not interfered with the proper operation of the tank.

The invention will now be described in connection with certain preferred embodiments with reference to the following illustrative figures and examples, so that it may be more fully understood.

With specific reference now to the figures and examples in detail, it is stressed that the particulars described and shown are for purposes of illustrative discussion of the preferred embodiments of the present invention only, and are presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of the invention. In this regard, no attempt is made to show structural details of the invention in more detail than is necessary for a fundamental understanding of the invention, the description and examples, taken with the drawings, making apparent to those skilled in the art how the several forms of the invention may be embodied in practice. Brief Description of the Drawings Fig. 1 is a sectional, elevational view of a preferred embodiment of the heater unit according to the present invention; Fig. 2 is a sectional, elevational view of a second embodiment of said heater unit; Fig. 3 is a detailed view of a pre-heating unit fitted with an adjustable height inlet transfer conduit; Fig. 4 is a sectional, elevational view of a complete water tank according to the present invention, and Fig. 5 is a sectional, elevational view of a further embodiment of the heater unit of the invention. Description of Preferred Embodiments

Referring now to the drawings, there is seen in Fig. 1 a water pre-heating unit 10 for insertion in an insulated hot water storage tank 12, shown in phantom outline. The unit 10 is advantageously manufactured, particularly the flange and attachment screw positioning, according to dimensions suitable for prior art water tanks. The unit 12 can thus be supplied as a retrofit item to existing hot water tanks.

A vertically-oriented housing 14 is adapted to be positioned on the bottom surface 16 of tank 12. An electric water immersion heater 18 is attached within housing 14. Heater terminals 20 are seen below the lower face 22 of housing 14. A control thermostat 24 is also positioned inside housing 14, both for protecting immersion heater 18 and also for preserving electricity when housing 14 and its surroundings have reached the desired temperature.

It will be noted that housing 14 is provided with a water-retaining lower face 22, pierced only by the connections to immersion heater 18 and to thermostat 24. Consequently, water is retained in housing 14 around immersion heater 18, irrespective of failure of the line supply to the tank, and irrespective of whether or not a one-way valve is installed in the supply line.

A cold water inlet transfer conduit 26 is provided to bring water from the tank 12 to housing 14. A portion 28, and as shown, preferably a major portion, of its length is positioned within housing 14 and extends alongside immersion heater 18. When heater 18 is powered, the water in portion 28 is pre-heated by the surrounding hot water.

A first opening 30 in conduit 26 for receiving tank water extends above housing 14 at mid-height of the housing, or, as shown in the drawing, advantageously above the upper face 32 of housing 14. The conduit's second opening 34 is located within a lower volume of housing 14, for discharging water into the housing. A hot water outlet transfer pipe 36 extends upwardly from inside housing 14 and has a length sufficient to position the outlet 38 thereof in an uppermost volume of tank 12, when inserted therein. The hottest water is thus transferred directly to its point of storage, that is, to the highest point of the tank 12. With reference to the rest of the figures, similar reference numbers have been used to identify similar parts.

Referring now to Fig. 2, there is seen a water pre-heating unit 44 wherein the second opening 46, that is the outlet, of cold water inlet transfer conduit 48 is located in proximity to the mid-point height of the immersion heater housing 50.

Fig. 3 illustrates a detail of a water pre-heating unit 52. The cold water inlet transfer conduit 54 is attached to the upper face of housing 56 in a manner allowing its height to be adjusted, thus affording control over the degree of water circulation inside storage tank 12. Such height adjustment is also of use where a retrofit heater unit is being installed and the tank height is not known beforehand.

In addition, the ability to adjust the height of conduit 54 is advantageous for the optimization of two parameters: one being the time/speed of heating, and the other being the amount of heated water. In the case of a. higher positioned conduit, the water above opening 30 will heat up quickly but in a smaller quantity; the remaining tank water will be heated in a slower process, assisted by the heat conductivity of the conduit walls. In the case of a lower positioned conduit, the results will be the opposite. It should be noted that transfer pipe 36 could also be positioned at different heights, the maximum height attainable being limited only by the underside of the top of the heating tank.

The conduit 54 is inserted into a collar 60 welded to the top of housing 56. The upper portion 62 of collar 60 is split and slightly distorted to grip the pipe 54. No screwed parts are thus required, and the collar 60 thus has an acceptable life in the hostile environment prevailing inside tank 12.

Seen in Fig. 4 is a complete electric water heater 64. An insulated hot fluid storage tank 66 is of cylindrical shape, the cylinder axis 40 being shown as vertical. A vertically-oriented housing 68 is positioned on bottom surface 70 of tank 66. An water inlet pipe 72, positioned between the tank 66 outer wall and the housing 68 outer wall, provides water to tank 66. Such supply may be cold water from a municipal supply line, or warm or hot water which has passed through solar energy collectors, (the inlet and outlet pipes of which are not shown). Water inlet pipe 72 is provided with an inlet baffle 76, so as to prevent the unwanted mixing of hot and cold water held in the tank 66.

As stated above, water enters the bottom 70 of the tank outside the housing 68 holding the immersion heater 74. The supply water contains small quantities of entrained rust flakes, sand, small stones and other solids, which settle harmlessly on the tank bottom 70 and do not enter the housing 68, where they would obstruct the free flow of water and might cause the burn-out of immersion heater 74.

Housing 68 is fed by a cold water inlet transfer conduit 78. A portion 80 of its length is positioned within housing 68 and extends alongside immersion heater 78. A first inlet opening 42 in conduit 78 extends above the mid-height of housing 68 for receiving tank water, while a second outlet opening 82 is located within a lower volume of housing 68 for discharging water into housing 68. The water held in portion 80 is pre-heated while in the conduit, due to its proximity to the immersion heater 74. Such pre-heating decreases the temperature differential between the water entering and exiting the system, subsequently creating slower circulation of water and permitting the water to undergo a longer heating time, resulting in hotter water output. A hot water outlet transfer pipe 84 extends upwardly from inside housing 68, and is of a length sufficient to position its outlet 86 in an uppermost volume of the water storage tank 66. Said outlet can be formed with a vertex and two side openings (not shown) to enable said outlet to be placed flush against the underside of the top of said tank 40, without interfering with the exit of water therefrom.

Water is a very poor conductor of heat, and as long as there are no convection currents, the hot water at the top of tank 66 loses a neglible amount of heat to the colder water on which it rests. There is, however, some downward heat loss via the metallic wall of tank 66, which is reduced in the course of time as the inner surfaces of the tank walls become coated with mineral deposits.

A consumer water outlet pipe 88 has a first, inlet opening 90 in the uppermost volume of water storage tank 66, to draw therefrom the hottest water available. Such hot water may have been produced by the immersion heater, or by a solar energy system, or by a combination thereof. The second, outlet opening 92 of outlet pipe 88 extends to the outside of tank 66 to form a heated water consumption outlet. The outlet opening 92 shown is at the base of the tank 66; however, in a further embodiment (not shown) the outlet is at the side of the tank.

The electric water heater 64 is advantageously arranged to operate at supply line pressure, such pressure being maintained at all times. Sharp pressure fluctuations are thus avoided, leading to extended life for the tank 66 and its connected components. Any volumetric expansion of heated water is relieved by slight reverse flow from the tank into water inlet pipe 72. Pressure drop due to water withdrawal for consumption is immediately compensated by the entry of fresh supply water into tank 66. The use of a one-way valve in the inlet pipe is thus neither necessary nor desirable.

Fig. 5 shows a further embodiment of a heater unit 96. The cold water inlet transfer conduit 98 is formed from a wall portion 100 of housing 102 and a heat- conductive partition 104 positioned within said housing. When hot water is withdrawn through pipe 106, cold water enters inlet 108. Heat is transferred from the water in proximity to the immersion heater 110 through partition 104 to pre-heat water held in conduit 98. Such pre-heated water eventually contacts the immersion heater 110 after passing through port 112.

EXAMPLE 1

A test was carried out with an electric water heater according to the present invention in which the pre-heating unit, as illustrated in Fig. 2, was modified so that conduit 48 extended only 8 cm inside the housing, which constituted about one- quarter of the housing height.

In this test, the housing height was 32 cm; the cold water inlet transfer conduit was a polypropylene pipe having an outer diameter of 21 mm and a wall thickness of 3 mm. The temperature at the first opening of the conduit, the temperature at the second opening of said conduit, and the temperature at the top of the tank were measured and recorded at 10-minute intervals for 1 hour, and the results are set forth in Table 1 below. TABLE 1

EXAMPLE 2

The experiment of Example 1 was repeated, with an electric water heater according to the present invention in which the pre-heating unit, as illustrated in Fig. 1, was modified so that conduit 28 extended 20 cm inside the housing, which constituted about two-thirds of the housing height. All other parameters of the experiment remained the same as in Example 1, and the results are set forth in Table 2 below.

TABLE 2

It should be noted that the heat conduction coefficient of polypropylene is 0.22, while that of iron is 60; of copper, 386; and of brass, 111, from which it is obvious that using a metallic inlet conduit will result in an increase in the pre-heating of the water passing therethrough.

As will be noted, even when the conduit extends only 8 cm within the housing, there is already achieved a pre-heating of at least 3°C between the inlet and outlet of the conduit, which results in a temperature elevation of 29°C within ten minutes at the top of the tank, and a total temperature elevation of 37°C within one hour. When the conduit extends 20 cm within the housing, there is achieved a pre-heating of at least 6°C between the inlet and outlet of the conduit which results in a temperature elevation of 31°C within ten minutes at the top of the tank and a total temperature elevation of 44°C within one hour.

It will be evident to those skilled in the art that the invention is not limited to the details of the foregoing illustrated embodiments and that the present invention may be embodied in other specific forms without departing from the spirit or essential attributes thereof. The present embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.

Claims

1. A water pre-heating unit for insertion in an insulated hot water storage tank, said unit comprising: a housing adapted to be positioned on the bottom surface of said tank; an electric water immersion heater positioned in said housing; a cold water inlet transfer conduit having at least a portion of its length positioned within said housing; a first opening in said conduit extending at least above the mid-height of said housing for receiving tank water and a second opening in said conduit located within said housing for discharging said tank water into said housing, the water within said conduit being pre-heated while in said conduit as said conduit is heated by water within said housing, and a hot water outlet transfer pipe extending upwardly from inside said housing and having a length sufficient to position the outlet of said pipe in an uppermost volume of said water storage tank when said housing is inserted in said tank.

2. A water pre-heating unit according to claim 1, wherein said second opening of said cold water inlet transfer conduit is located in a bottom half of said housing.

3. A water pre-heating unit according to claim 1, wherein the water within said conduit is preheated by at least 3°C while in said conduit.

4. A water pre-heating unit according to claim 1, wherein said second opening is orientated for directing water passing through said conduit in a downward flow within said housing, to effect the further heating of said water while it flows downwardly within said housing.

5. A water pre-heating unit according to claim 1, wherein said cold water inlet transfer conduit is attached to said housing in a manner allowing height adjustment thereof when said unit is inserted in said tank, whereby the degree of water circulation inside said storage tank can be controlled.

6. A water pre-heating unit according to claim 1, further provided with a control thermostat positioned inside said housing.

7. A water pre-heating unit according to claim 1, wherein said housing is provided with a water-retaining base pierced only by connections to said immersion heater and a thermostat, whereby water is retained in said housing around said immersion heater irrespective of failure of the line supply, and irrespective of whether or not a one-way valve is installed in said line supply.

8. A water pre-heating unit according to claim 1, wherein the major length of said cold water inlet transfer conduit is positioned within said housing.

9. A water pre-heating unit according to claim 1, wherein said first opening of said cold water inlet transfer conduit is located above the upper face of said housing.

10. A water pre-heating unit according to claim 1, wherein said cold water inlet transfer conduit is formed from a wall portion of said housing and a heat-conductive partition positioned within said housing.

11. An electric water heater, comprising: an insulated hot fluid storage tank; a housing positioned on the bottom surface of said tank; at least one water inlet pipe positioned between the tank outer wall and the housing outer wall, providing water to the storage tank; an electric water immersion heater positioned inside said housing; a cold water inlet transfer conduit having at least a portion of its length positioned within said housing; a first opening of said conduit extending at least above the mid-height of said housing for receiving tank water and a second opening of said conduit located within said housing for discharging said tank water into said housing, the water within said conduit being pre-heated while in said conduit as said conduit is heated by water within said housing; a hot water outlet transfer pipe extending upwardly from inside said housing and having a length sufficient to position the outlet of said pipe in an uppermost volume of said water storage tank when said housing is inserted in said tank, and a consumer water outlet pipe having a first opening in said uppermost volume of said water storage tank for drawing water therefrom and a second opening extending to the ouside of said tank to form a heated water consumption outlet.

12. An electric water heater according to claim 11, wherein said inlet conduit is provided with an inlet baffle.

13. An electric water heater according to claim 11, said tank being arranged to operate at supply line pressure, any volumetric expansion of heated water being relieved by slight reverse flow from said tank into said inlet conduit.

AMENDED CLAIMS

[received by the International Bureau on 30 ϋune 1998 (30.06.98); original claims 1 and 3 replaced by amended claim 1; remaining claims unchanged (1 page)]

1. A water pre-heating unit for insertion in an insulated hot water storage tank, said unit comprising: a housing adapted to be positioned on the bottom surface of said tank; an electric water immersion heater positioned in said housing; a cold water inlet transfer conduit having at least a portion of its length positioned within said housing; a first opening in said conduit extending at least above the mid-height of said housing for receiving tank water and a second opening in said conduit located within said housing for discharging said tank water into said housing, the water within said conduit being pre-heated by at least 3°C while in said conduit, as a result of said portion of said conduit positioned within said housing being heated by water within said housing, and a hot water outlet transfer pipe extending upwardly from inside said housing and having a length sufficient to position the outlet of said pipe in an uppermost volume of said water storage tank when said housing is inserted in said tank.