Gas water heater

Abstract

A domestic gas storage water heater utilizes a plastic non pressure tank to deliver mains pressure hot water. The heater may be external or internal of the tank and has a heat out put of about 4-10 Mega joules per hour. A pressure transfer module uses the mains pressure of the incoming cold water to deliver hot water from the tank at mains pressure. The tank may be of larger storage capacity than conventional gas heaters and the arrangement ensures that the heated water in the tank is stratified into layers of different temperature. This enables an uninterrupted supply of hot water to be achieved.

Description

BACKGROUND TO INVENTION

[0001] Gas water heaters that are commercially available and deliver mains pressure water are characterized as having a stainless steel tank or a vitreous enamel lined mild steel tank with a high rating gas burner. These heaters don't have a large capacity water storage but can heat the water quickly.

[0002] The heat efficiency of such heaters is not high and the output of Carbon dioxide, a green house gas, is quite high. Usually the heat exchange is achieved by passing the hot combustion gases up a flue through the centre of the tank or over the external surface of the tank. Australian patent 720062 is an example of this approach. This has the disadvantage of having to heat the total volume of water in the storage tank to the desired temperature.

[0003] U.S. Pat. No. 4,503,810 discloses a combustion gas water heater in which the hot combustion gases do not exchange heat with the stored water but rather a small volume of water is quickly heated in direct exchange with the combustion gases in an external heat exchanger. Cold water is drawn from the bottom of the storage tank heated in the external heat exchanger and then returned to the upper or middle portion of the storage tank by convection.

[0004] Another problem with water heater storage tanks is corrosion of the tank which occurs with unlined and also vitreous enamel lined tanks. The use of non-corrodible stainless steel tanks is an expensive solution to the problem. Plastic tanks are cheaper but these are unsuitable for storing mains pressure hot water.

[0005] One proposal has been to use plastic lined metal tanks as proposed in U.S. Pat. No. 4,338,888. The combustion chamber has water walls for heat exchange. Water from the bottom of the tank is heated in the water walled combustion chamber and the heated water is returned to the lower portion of the tank.

[0006] Another approach for electric heated tanks is USA patent 4437484 which uses an un-pressurised plastic tank and a pump.

[0007] It is an object of this invention to provide a storage water heater which overcomes these problems.

BRIEF DESCRIPTION OF THE INVENTION

[0008] To this end the present invention provides a combustion water heater which includes

[0009] a) a non-pressurised water storage tank having a cold water inlet in the lower portion of said tank and a heated water outlet in the upper portion of said tank

[0010] b) a low heat output combustion chamber having a flue for the egress of combustion gases

[0011] c) a heat exchange surface associated with said flue

[0012] d) means for contacting water from the lower portion of said water storage tank with said heat exchange surface

[0013] e) means for conducting water heated at the heat exchange surface to the upper portion of said water storage tank

[0014] f) pump means associated with the heated water outlet to increase the pressure of water from the non pressurised tank.

[0015] By using a non-pressurised tank a plastic tank can be utilised with the resultant advantages of longer service life due to less corrosion. By returning the heated water from the heat exchanger to the upper portion of the tank, temperature stratification of the water in the tank is established. This has the benefit that water extracted from the upper portion of the tank is always the hottest water in the tank. The plastic used can be any structural polymer that is heat resistant and suitable for the application. Stratification of heated layers of water is achieved by introducing the hot water to the top portion of the tank and separating any hotwater being heated in the bottom of the tank from cold water by using a separate chamber or a shroud.

[0016] The use of a low joule burner in combination with a heat stratified tank means that the stored water can be cost effectively heated over a longer period of time. Because the hottest layers are adjacent the outlet there is little risk that the supply of hot water will be interrupted. The low joule burner is preferably rated at 1 to 2 kw.

[0017] The pump adjacent the outlet ensures the that the water pressure from the hot water taps will be at a pressure equivalent to mains pressure. Preferably the pump is a multi compartment pressure transfer module that utilises the mains particularly high density polypropylene. The tank can be formed by any suitable moulding technique although for high density poly propylene blow moulding is preferred. The storage volume of the tank may be greater than that which is conventional for gas heaters but equivalent to that of electric storage heaters intended to supply similar daily volumes. A domestic size tank would be 315 litres storage capacity but smaller or larger sizes are feasible. A large tank is required when the power rating of the combustion chamber is selected to provide a time to heat the whole tank of about 12 to 20 hours. The dimensions of such a tank are typically about 660 mm diameter and 1700 mm height. For smaller tanks a higher rating burner could be used to provide a larger proportion of the tank with hot water.

[0018] The tank 1 is provided with an outer skin 2 and a layer 3 of insulation to limit heat losses from the tank. The outer skin is preferably of a hard wearing, tear resistant and weather resistant plastic such as high density polyethylene, low density polyethylene, ABA, ABS or even sheet metal. Sheet metal has the advantage of having lower thermal expansion characteristics and may also be less expensive. The insulation material can be any suitable material with low heat conductivity with polyurethane foam being the preferred material.

[0019] The removable lid 4 also has an insulation layer between an inner and outer skin. Because the tank is not intended to operate at mains water pressure the seal between the tank and the lid need not be particularly strong. This means that access to the interior of the tank for manufacturing and maintenance purposes is much easier than for mains pressure tanks. The lid 4 is arranged to seat on the tank above the operating water level 9 of the tank.

[0020] The pressure transfer module 5 is of the type described in WO97/46805. The contents of that patent are incorporated herein by reference. Cold water passes into the module through mains pressure cold water inlet 10 and after being reduced in pressure the water flows through cold water outlet 12 into the lower portion of the tank.

[0021] The PTM is either positioned in the top of the tank or draws hot water from the top layer of water in the tank. Hot water is drawn into the PTM 5 and exits at mains pressure through the hot water outlet 11.

[0022] In the embodiment shown in FIG. 1 cold water from the bottom of the tank is drawn into the heat exchanger 7 and passes up the heated water delivery pipe into the upper section of the tank. The external heat exchanger 7 comprises a gas flame 6 mounted in a chamber with an associated air inlet. The hot combustion gases are caused to circulate around pipes or water walls in the heat exchanger and then to exit via the flue outlet 8. In FIG. 1 the flue is shown as balanced with the flue outlet 8 being adjacent the air inlet for gas flame 6.

[0023] The embodiment of FIG. 2 is a first version of a tank with the gas flame 6 and heat exchanger 7 disposed in the lower portion of the tank. A shroud or convection flume 13 confines the cold water entering the bottom of the tank through inlet 12 so that it is in heat exchange relation ship with the external surface of the heat exchanger 7. The arrangement of heat exchanger 7 within the shroud 13 may be in accordance with the well known principals of heat exchangers in increasing the heat transfer surface area without impeding the flow of flue gases from the flame 6 to the flue outlet 8. In FIG. 2 the flue outlet 8 is disposed in the lid 4 of the tank. To maximise the heat transfer the hot water delivery pipe 14 surrounds the vertical flue 16 up into the upper portion of the tank to terminate just below the water level 9.

[0024] In the embodiment of FIG. 3 which is a variation of the embodiment of FIG. 2 the flue is balanced with the flue outlet 8 being adjacent the air inlet for flame 6. As shown in FIG. 3 the heat exchanger 7 provides a path for the flue gases which traverses the base of the tank and then doubles back . The arrangement of heat exchanger 7 can be designed to provide maximum heat transfer area between the conduits for the combustion gases and the water contained within shroud 13. In the embodiment of FIG. 3 the hot water delivery tube 14 can be of smaller diameter.

[0025] In all three embodiments the operation of the gas flame may be fan assisted to ensure complete combustion of the fuel and to assist in the positive flow of the flue gases to the outlet 8. In the embodiment of FIGS. 2 and 3 the fan and gas mixing valve may be situated on the top of the tank to avoid having them attached to the external wall of the unit which would increase the floor imprint of the unit. This has the additional advantage of keeping the fan away from dust accumulation near the bottom of the unit. The fan and gas valve can be a unit in which the speed of the fan controls the gas inlet valve to maintain the correct air/gas mixture required for clean combustion. A pre-mix burner with a DC fan is preferred. In a simple unit with on temperature sensor a one speed fan would be suitable. A two speed fan would need two temperature sensors one higher than the other.

[0026] The burner used with a 315 litre tank in accordance with any of the above 3 embodiments is rated at 5-6 Mega-joules/hour which given the efficiency of the stratification in the tank and the insulation on the tank gives a heating out put of about 1.1-1.3 Kw. The efficiency rating is about 80%. The burner may be a simple flame into the heat exchanger or may be a flame supported on a thimble like mesh shroud or even a longer mesh burner to extend within the tank for the embodiments of FIGS. 2 and 3.

[0027] The burner can be controlled by a control unit which includes one or more temperature sensors within the tank. With one sensor placed near the bottom of the tank a temperature above the set temperature for the tank would trigger the switching off of the burner. With two sensors one near the top and one near the bottom variable heating rates can be utilised. A low temperature at the upper sensor would trigger a higher input rate for the fan to increase the heat out put until the upper sensor exceeded the set temperature.

[0028] For domestic dwellings, the water heater of this invention provides an uninterrupted supply of mains pressure hot water at low energy cost and low green house emissions. In addition the tank has a longer useful life than conventional gas water heaters of equivalent capital cost.

Claims

1. a combustion water heater which includes

a) a non-pressurised water storage tank having a cold water inlet in the lower portion of said tank and a heated water outlet in the upper portion of said tank

b) a low heat output combustion chamber having a flue for the egress of combustion gases

c) a heat exchange surface associated with said flue

d) means for contacting water from the lower portion of said water storage tank with said heat exchange surface

e) means for conducting water heated at the heat exchange surface to the upper portion of said water storage tank

f) pump means associated with the heated water outlet to increase the pressure of water exiting from the non pressurised tank.

2. A water heater as claimed in claim 1 in which the water in the tank is stratified by temperature with the hottest water adjacent the hot water outlet.

3. A water heater as claimed in claim 1 wherein the heat exchange surface is disposed within the water storage tank.

4. A water heater as claimed in claim one wherein the pump means is a multi compartmented pressure transfer module adapted to release hot water at near mains pressure by utilising the existing mains pressure of the cold water inlet.

5. A water heater as claimed in claim 1 wherein the heat exchange surface is disposed within the lower portion of the tank and the tank incorporates a shroud which separates the water in the lower portion of the tank, which surrounds the heat exchanger, from the water in the remainder of the tank, and a water outlet allows water from within the lower portion to rise to the upper section of the tank.

6. A water heater as claimed in claim 1 or 2 in which the energy out put of the combustion unit is 1 to 2 kilowatts.