GB2413623A

GB2413623A - Unvented water heating installation

Info

Publication number: GB2413623A
Application number: GB0508311A
Authority: GB
Inventors: Christopher Samuel Powell; Michael Roy Hales
Original assignee: FABDEC Ltd
Current assignee: FABDEC Ltd
Priority date: 2004-04-30
Filing date: 2005-04-25
Publication date: 2005-11-02
Anticipated expiration: 2025-04-25
Also published as: GB0409803D0; GB2413623B; GB0508311D0

Abstract

A hot water installation includes an unvented hot water tank 4 supplied with cold water through an inlet pipe 2. Water flows though the inlet pipe as heated water is drawn off from the tank via hot water outlet pipe 12. To provide for expansion of water during heating, an air space is included within the tank, defined above the water level 'L'. This air space is replenished during cold water filling by an air feed device 6 associated with the cold water inlet pipe. The air feed device can include a venturi tube (8 fig 4) and an associated non-return valve (10 fig 4) communicating with the atmosphere, such that air is drawn into the tank during filling, or the air supply may be provided by a pump. The venturi device may include a variable orifice, defined by a throttle member (18 fig 4) located within the throat of the venturi which is moveable downstream under the pressure of water flowing through the venturi. The throttle member may be mounted on a slider (22 fig 4), and biased in position by springs (20 fig 4) at both ends, such that movement of the throttle member may increase the diameter of the venturi throat. The non-return valve allows air admission and prevents water leakage, and may be connected to a tundish 11 for water collection in the event of valve malfunction.

Description

IMPROVEMENTS RELATING TO WATER HEATING INSTALLATIONS

Installations for supplying hot water for domestic or commercial purposes are known which incorporate a main or unvented hot water tank provided with a heating system. An outlet pipe leading to taps or appliances may be connected to the main tank in its upper region, and a cold water inlet pipe may be connected to the main tank in its lower region. The inlet pipe may be connected directly to the mains or to a header tank. The water in the tank may be heated by a coil of pipe through which hot water is circulated by a boiler or by an electric heater. In either case heating takes place under the control of a thermostat.

In accordance with British Standard 7206: 1990 an external expansion vessel may be coupled to the inlet pipe or outlet pipe to accommodate expansion ofthe water in the main tank as it is heated. An alternative solution is to arrange for a volume of air to be provided above the water level and to serve as a cushion to buffer expansion of the water in the tank. There is a tendency for the water in the tank to absorb air from the cushion, thereby reducing its effectiveness. To overcome this problem, the present invention provides a method of operating a water heating installation incorporating an unvented hot water tank, wherein air is fed into the water inlet pipe as water flows through the pipe, the air being entrained by the water as it flows into the tank, and so replenishing the air cushion. Preferably the air is drawn into the inlet pipe under the action of water flowing through the pipe.

According to another aspect of the invention, there is provided a water heating installation incorporating an unvented hot water tank, the inlet pipe of which is connected to an air supply duct. In accordance with a preferred feature of the invention, the inlet pipe incorporates a venturi which communicates with the outlet of a one-way valve, the inlet of which communicates with atmosphere so that, when open, the non-return valve allows air to enter the venturi but prevents water escaping. When no water is flowing in the installation, the pressure of water on the outlet side of the valve is substantially greater than atmospheric pressure.

The one-way valve may be adapted to open and close with a snap action, in each case triggered by a predetermined differential pressure across the valve. Therefore, as the rate at which water flows through the venturi increases, the pressure of water acting at the outlet of the valve decreases until it becomes less than atmospheric pressure and the trigger condition for opening the valve is reached, whereupon the valve snaps open and air enters the inlet pipe. While the valve is open, the rate of air flow through the valve is proportional to the differential pressure across the valve. Changes in atmospheric pressure or the rate of flow of water through the venturi will therefore influence the rate at which air is drawn into the inlet pipe. Other conditions may also affect operation. Thus, for example, as relative humidity rises, the rate at which air is drawn into the inlet pipe is reduced. When water ceases to be drawn off from the tank the inlet valve to the tank begins to close and the flow rate of water through the venturi falls. The pressure of water on the outlet side of the valve increases and the trigger condition for closing the valve is reached, whereupon the valve snaps shut. The pressure continues to increase until it exceeds atmospheric pressure.

The crack pressure of the one-way valve should be as low as possible, successful results having been achieved with valves having a crack pressure in the region of 0.03 bar. Such valves are readily available commercially. Water pressures of up to 10 bar present in typical hot water installations do not lead to any risk of water escaping when the valve is open. Although under normal operating conditions no water may be expected to escape from such valves when closed, in order to allow for the possibility that the valve may be faulty when installed, or may develop a fault later, the valve may be connected through a pipe to the tundish normally provided for the expansion relief valve and the pressure/temperature relief valve conventionally associated with unvented tanks.

The drop in pressure which takes place within the venturi depends on both the cross- sectional area of the venturi throat and on the rate of flow of water through the inlet pipe during the drawing off of hot water from the tank. A problem which arises in connection with the application of the invention to a range of hot water installations is that some installations have a greater rate of flow of water through the tank than others, depending in particular upon the pressure provided by the mains supply or by the supply from the header tank which may be pumped. In practice the rate of flow of water through the inlet pipe and into the tank may be between 10 and 45 litres per minute, depending on the draw off rate of the installation. In order to ensure that roughly the same volume of air will be introduced into the tank per unit time that water is flowing, it is desirable that the drop in pressure within the venturi should be much the same, regardless of the expected rate of flow of water in any given installation. This may be achieved by installing a venturi which has a throat diameter appropriate to each installation.

To avoid the need to make available a range of venturis having different throat diameters for installations having a variety of different flow characteristics, means may be provided for adjusting the cross-sectional area of the throat of the venturi in dependence upon the rate of flow of water though the venturi. According to another preferred feature of the invention, therefore, a throttle member is provided within the venturi so that the throat is annular in shape. The throttle member is movable in the downstream direction under the pressure of water flowing through the venturi so as to increase the cross-sectional area of the throat. As a result, the available throat area remains proportional to the rate at which water flows through the inlet pipe. In consequence, the rate at which air is drawn into the inlet pipe is substantially independent of the rate of flow of water during normal operation.

The throttle member may be supported on a rod or shaft extending axially though the throat and be biased towards a rest position centrally of the throat by a compression spring or other resilient member. Preferably, at least the upstream end of the throttle member has a tapered or conical surface and is biased into its rest position by springs acting on both of its sides. The throttle member is displaced as water begins to flow through the venturi so as to open up the throat to an appropriate size depending upon the rate of flow, and is restored to its rest position by the spring or springs when water flow ceases. The compression spring or springs or other biasing device acting on the throttle member are chosen so as to have a stiffness appropriate to the water pressures a typical installation will be expected to experience.

Although it is preferred for the non-return valve to be mounted on the body of the venturi, the valve may be connected with the venturi by way of a length of pipe.

In the drawings: Figure 1 schematically illustrates a water heating installation having an expansion vessel and in accordance with BS 7206: 1990, Figure 2 corresponds to Figure I but omitting the expansion vessel and showing the incorporation of a device for feeding air into the cold water inlet pipe in accordance with the I O invention, Figure 3 is an axial section through a f rst embodiment of the air feed device, incorporating a venturi system, and Figure 4 is a view similar to Figure 3 but of a modif ed venturi tube.

Referring to the drawings, those skilled in the art will be familiar with the type of installation represented in Figure 1 and its operation. In this connection reference is directed to BS7206: ] 990 and this known type of installation will therefore not be further described.

In order to carry out the invention in a preferred way, a number of alterations are made to the installation shown in Figure 1. First, as shown in Figure 2, the expansion vessel depicted in Figure 1 is omitted and a device 6 is incorporated into the cold water inlet pipe 2 so as to introduce air into the pipe. The air so introduced enters the tank 4 and rises to replenish an air cushion at the top of the tank. The water in the tank therefore has an upper level L determined by the volume of the air cushion. The hot water outlet pipe indicated at 12 is therefore necessarily repositioned so as to open into the tank below the anticipated position of the level L. The pipe 13 incorporating the temperature/pressure relief valve is similarly repositioned to open into the tank below the position at which the pipe 12 opens into the tank.

Finally, a shut offvalve 7 is positioned between the device 6 and the tank 4. In the event of any problem being encountered with the device 6, both the conventional stop valve and the shut offvalve 7 may be closed to isolate the device 6 from the mains and the water in the tank 4 and allow the removal and replacement of the device 6.

As shown in Figure 3 the air feed device 6 comprises a venturi tube 8 and a non-return valve 10, shown only diagrammatically. The venturi tube narrows to a constricted throat 12 into which opens an air supply duct 14 extending through an externally screw-threaded boss 16 fast with the body of the venturi tube. The valve 10 is screwed onto the threaded boss. The inlet to the valve may open to the atmosphere directly or as indicated in Figure 2 by way of a pipe 15 (Figure 2) leading to a tundish 1 I to collect both leakage from the valve 10 in the event of malfunction as well as water released from the tank by the temperature/pressure relief valve.

The valve 10 is arranged to snap open when the pressure of the atmosphere at its air inlet exceeds the pressure at its outlet by a predetermined amount, conveniently 0.03 bar, and to snap close when the pressure differential falls back to that value or just below that value.

Non-return valves adapted to open at this pressure and capable of preventing water escaping through the air inlet are readily available on the market. As an alternative to fitting the valve to the body of the venturi tube, an air duct may be fitted to the boss 16 and the valve may be fitted to the other end of the duct or part way along the duct.

As water is drawn off through the outlet pipe 12 an inlet valve opens and water flows through the inlet pipe 2 into the tank to maintain the water level constant. As the water flows through the venturi tube it is caused to accelerate and undergo a drop in pressure. When the pressure differential across the valve 10 reaches 0.03 bar (or other crack pressure) the valve snaps open and air flows into the venturi, is entrained by the water flow and enters the tank. The air cushion above the water level is therefore maintained notwithstanding any tendency of air from the cushion to be absorbed by the water. When water ceases to be drawn off from the tank, and the inlet valve to the tank begins to close, the flow through pipe 2 slows, resulting in an increase in pressure at the valve outlet. When the pressure differential across the valve falls to just below 0.03 bar, or as the case may be, the valve snaps shut and the replenishment of the air cushion is terminated.

The venturi tube shown in Figure 3 used in any given installation has a throat diameter chosen in dependence upon the normal rate at which water can be expected to flow through the inlet pipe for the installation in question, this in turn depending upon the draw off rate from the tank. With the typical flow rates of 10 to 45 litres per minute, and for a standard inlet pipe having an internal diameter of 20 mm, the throat diameter is desirably chosen within the range of substantially 3 mm and substantially 9mm.

The modified venturi tube shown in Figure 4 is of universal application and has a throttle member] 8 disposed centrally within the throat in a rest position when no water is flowing through the inlet pipe. The throttle member is biased towards its rest position by compression springs 20 bearing at one end on the throttle member and at the other end against stops in the form of discs 24 located at the ends ofthe venturi tube. The throttle member is hollow and is slidably supported on a shaft 22. The ends of the shaft pass through openings in the discs and are screw-threaded so as to receive retaining nuts 26 bearing against the discs. The throttle member has conical upstream and downstream surfaces, but other shapes may be employed. When water flows through the inlet pipe the throttle member may be displaced in the downstream direction against the restoring force ofthe springs so as to increase the cross- sectional area of the throat available for the passage of water. The position occupied by the throttle member, and hence the available free area for the passage of water, is related to the rate at which water flows through the venturi tube. The stiffness ofthe springs is chosen so that the throttle member takes up such a position as to ensure that the drop in pressure which takes place within the venturi tube, and the rate at which air is drawn in when the valve is open, is roughly the same at the maximum water flow rate in any given installation, whatever it may be. With a standard inlet pipe having an internal diameter of 20 mm, and therefore an internal cross sectional area of substantially 314 square mm, the throttle member may be movable so that the free cross-sectional area of the venturi throat available for the passage of water is variable within the range of substantially 7 square mm to substantially 64 square mm.

The air cushion in the tank allows the water in the tank to expand as it is heated and the regular replenishment of the air cushion through the inlet pipe compensates for any tendency of the water in the tank to absorb air from the air cushion. Appropriate dimensioning of the venturi ensures that the rate of flow of air into the tank will maintain the air cushion at a suitable volume. In the event that so much air is fed into the tank that the air cushion increases to an excessive volume, the excess air will escape through the outlet pipe and the taps during normal draw off of hot water.

Although the installation is depicted as having a heating coil through which hot water is circulated from a boiler, it Will be appreciated that other types of heating system may be employed. Also, although the installation is illustrated as drawing water from a mains supply, a header tank (which may be pumped) may be used instead, provided a sufficient head of water is provided to operate the air feed device. Devices other than a venturi may be employed to introduce air into the water flowing through the inlet pipe, including for example an electrically operated pump connected to the inlet of the non-return valve, the pump being made to operate when the inlet valve to the tank is open.

Claims

1. A method of operating a water heating installation incorporating an unvented hot water tank, wherein air is fed into a water inlet pipe as water flows through the pipe, thereby to replenish an air cushion above the water level in the tank

2. A method as claimed in Claim 1, wherein air is drawn into the inlet pipe under the action of water flowing through the pipe.

3. A method as claimed in Claim 2, wherein air is drawn into the inlet pipe from atmosphere in response to a drop in pressure within a venturi in the pipe.

4. A hot water installation wherein a water inlet pipe for an unvented hot water tank is connected to an air supply duct.

5. A hot water installation as claimed in Claim 4, wherein the air supply duct is connected to a venturi in the inlet pipe.

6. A hot water installation as claimed in claim 5, wherein the outlet from a non-return valve is connected to the venturi, the inlet to the valve communicating with the atmosphere.

7. A hot water installation as claimed in claim 5 or claim 6, wherein the valve operates with a snap-action at a predetermined differential pressure across the valve.

8. A hot water installation as claimed in claim 7, wherein the rate of flow of air through the valve is proportional to the differential pressure across the valve.

9. A hot water installation as claimed in any of claims 6 to 8, wherein the outlet from a non-return valve is connected to the venturi, the inlet to the valve being connected to a duct, the inlet to the duct being open to atmosphere and associated with a receptacle to collect leakage through the valve in the event of a malfunction of the valve.

] O. A hot water installation as claimed in any of claims 5 to 9, wherein a throttle member is located within the throat of the venturi and is movable in the downstream direction under the pressure of water flowing through the venturi so as to increase the diameter of the throat.

A hot water installation as claimed in claim 10, wherein the throttle member is biased towards a rest position within the throat.

12. A hot water installation as claimed in claim I 1, wherein compression springs bear on both ends of the throttle member

13. A hot water installation as claimed in claim 10 or claim I I, wherein the throttle member is tapered in the upstream direction or in both the upstream and downstream directions.

14. A hot water installation comprising an unvented hot water tank, a water inlet pipe opening into the tank, the pipe incorporating a venturi, and a non-return valve having its inlet communicating with the atmosphere and its outlet communicating with the throat of the venturi.

15. A hot water installation substantially as hereinbefore described with reference to and as illustrated in Figures 2 and 3 or Figures 2 and 3 as modified by Figure 4 of the accompanying drawings.

16. A method of operating a hot water installation substantially as hereinbefore described with reference to Figures 2 and 3 or Figures 2 and 3 as modified by Figure 4 of the accompanying drawings.