GB2431461A

GB2431461A - Water heating installation

Info

Publication number: GB2431461A
Application number: GB0620843A
Authority: GB
Inventors: Christopher Samuel Powell
Original assignee: FABDEC Ltd
Current assignee: FABDEC Ltd
Priority date: 2005-10-21
Filing date: 2006-10-19
Publication date: 2007-04-25
Anticipated expiration: 2026-10-19
Also published as: GB2431461B; GB0521487D0; GB0620843D0

Abstract

A hot water installation incorporates an unvented hot water tank 4 supplied with cold water through an inlet pipe 2. Water flows through the inlet pipe as water is drawn off from the tank. An air feed device 6 incorporating a venturi tube 8 and a non-return valve 10 allows air to be drawn into the water flow. The air entrained by the flow of water though the pipe 2 enters the tank and replenishes an air cushion above the water level in the tank. The air cushion allows the water in the tank to expand as it is heated. A by-pass passage 40 allows water to by-pass the venturi and maintain water flow through the system at a desired level.

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.

Pipework 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 tank in its lower region. The inlet pipe may be connected directly to the mains or to a header tank. The water in the main 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 to accommodate expansion of the 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. Patent specification GB No 2 271 835A discloses a solution to this problem which uses an air feed device to supply air to the tank. The air feed device according to this specification may incorporate a venturi arrangement in the inlet pipe to the tank. However, the venturi arrangement proposed is found to suffer from the disadvantage that excessive quantities of air may be drawn into the tank at high cold water flow rates. Patent specification GB No 2 316 474A discloses various devices for limiting the flow rate of air into the system, but none has proved satisfactory. Our patent specification GB No 2 413 623A, relates to an air feed device incorporating a venturi which is an improvement on the air feed devices known previously, but which has been found to place a limit on the rate at which water flows into the tank, thereby restricting the rate at which water may be drawn off.

The present invention seeks to overcome the problem associated with the system described in GB No 2413 623A by providing a passage through which water may by-pass the venturi.

Ideally, the by-pass passage is dimensioned to allow water to flow through it at the maximum rate which the system would supply in the absence of the venturi. In one embodiment of the invention an air feed device in accordance with the invention may comprise an outer sleeve housing a core through which the venturi extends, the by-pass passage being disposed between the core and the sleeve. In another embodiment the by-pass passage incorporates a non-return valve. The air feed device is preferably associated with the water inlet pipe but may be associated with an outlet pipe.

In the drawings:- Figure 1 schematically illustrates a water heating installation, Figure 2 is an axial section through an air feed device in accordance with our Patent

Specification GB No 2413 623A,

Figure 3 is a view similar to Figure 2 but showing an air feed device in accordance with the present invention, Figure 4 is a view similar to Figure 3 but of a second embodiment of air feed device, Figure 5 is an end view of the air feed device shown in Figures 3 or 4, Figure 6 is an axial section through two components of the air feed device shown in Figure 4.

Figure 7 is an axial section through an air feed device as shown in Figure 2 with a by-pass duct arranged externally of the device.

Figure 8 is a cross-section through a modification.

Referring to the drawings, Figure 1 illustrates a water heating installation of a type generally in accordance with BS7206: 1990, omitting the usual expansion vessel, but having an air feed device 6 installed in the cold water inlet pipe 2 leading from the mains supply to the tank 4.

The air feed device 6 in accordance with our Patent Specification GB No 2 413 623A is shown in Figure 2 and includes a venturi tube 8 fitted into a break in the inlet pipe 2. 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. A non-return valve 10 is screwed onto the threaded boss. The inlet to the valve may open to the atmosphere directly or as indicated in Figure 1 by way of a pipe 15 leading to a tundish 11 to collect leakage from the valve 10 in the event of malfunction. The tundish may also collect any leakage from other valves in the installation, such as through pipe 13 for example. 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 to 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 from the tank 4 through the outlet pipe 19 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 L 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 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.

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 may be used instead, provided a sufficient head of water is provided to operate the air feed device.

A problem which has been found to arise with the air feed device illustrated in Figure 2 is that the constriction provided by the venturi places a limit on the rate at which water can flow through the inlet pipe, and this in turn limits the rate at which water can be drawn off through the pipe 19. Consequently the maximum rate of flow through the system actually achieved may be substantially below that at which the system would operate in the absence of the air feed device, which is typically 55 litres per minute. This problem is overcome, in accordance with the invention, by the air feed device shown in Figure 3. Operation of the device shown in Figure 3 is as described above, with the variations mentioned below.

In the construction shown in Figure 3, the air feed device includes an outer sleeve 17 which is inserted into the break in the inlet pipe using a convenient fixing arrangement. A core 8 housed within the sleeve 17 has the venturi passage extending through it. A by-pass passage 18 is located between the inner surface of the sleeve 17 and the outer surface of the core 8.

The core 8 has a shallow boss 20 which bears against the inner surface of the sleeve 17 in the region of the valve 10. An air inlet duct 22 extends through the boss 20 from the throat and registers with a port in the sleevel7, the port itself registering with the air supply duct 14.

In operation, water flows through both the venturi and the by-pass passage. The by-pass passage is dimensioned so that the rate of flow through it may correspond to the desired rate of flow through the system as a whole, typically 55 litres per minute, with the result that the restriction on flow caused by the venturi does not impinge on the efficiency of the overall system.

The inlet section 24 of the venturi passage converges at a constant angle throughout its length, preferably in the range of a swept angle of I degree to a swept angle of 30 degrees, and more preferably 10 degrees swept. Experiments have shown that a 10 degrees swept angle is adequate for achieving a pressure drop from 3 bar to 0 bar in order to draw air into the throat.

The air inlet duct 22 may enter the venturi throat at its point of greatest constriction or just downstream of that point, as shown in Figure 3. Performance deteriorates noticeably downstream of this region. A preferred diameter for the air inlet duct has been found to be 2mm. The divergent section 26 of the venturi tube diverges at a constant angle throughout its length at a maximum 10 degrees swept in order to minimise pressure loss on the outlet side of the device, the exit pressure ideally matching the inlet pressure.

The core may be manufactured from a metal blank of circular cross-section having an external diameter which would be a close fit in the sleeve 17. The venturi passage may drilled out and the blank squared-off externally on three sides so as to form three by-pass passages ISa as shown in Figure 5. The fourth side may be left rounded so as to form the boss 20. When inserted into the sleeve 17, the core is held in place by the boss 20 and the two opposite corners of unreduced diameter.

Figure 4 illustrates a second embodiment in which the air inlet duct 22 opens into an annular distribution chamber 28 in the body of the venturi tube, the chamber 28 encircling the venturi throat. Two or more air inlet ducts 32 extend radially inwardly from the chamber 28 and open into the throat, immediately downstream of its narrowest point.

Manufacture of the embodiment shown in Figure 4 is simplified by producing the core in two component parts, 29 and 30 as shown in Figure 6. During assembly the reduced diameter end portion of component 30 is inserted into tubular portion 34 of component 28. The assembled core is then fitted into sleeve 17. Component 29 incorporates portions of the core which form the upstream part of the converging section of the venturi and the boss 20 through which the air inlet duct 22 extends. Component 30 incorporates the portions of the core which form the downstream part of the converging section of the venturi, and the entire diverging section. The component 30 has an annular recess 36 which, when the components are assembled, is covered by a tubular portion 34 of component 29 to create the annular air distribution chamber 28. The components 29 and 30 are squared-off as described above so as to form the three by-pass ducts I 8a when the assembled venturi tube is inserted into the sleeve.

The embodiment shown in Figure 7 incorporates an air feed device 6 corresponding to that shown in Figure 2 with a by-pass passage in the form of a duct or pipe 40 having its upstream and downstream ends opening into water inlet pipes (or extensions of the device 6) upstream and downstream of the venturi. A non-return valve is provided in the pipe 40 and is arranged to open at a predetermined pressure. In a modification the non-return valve may be omitted.

The arrangement of a by-pass pipe 40 as shown in Figure 7 (with or without a valve 42) may be incorporated in the air feed device shown in Figure 4 of patent specification GB No. 2413 623 A which has a variable orifice, defined by a throttle member located within the throat of the venturi and moveable downstream under the pressure of water flowing through the venturi. As therein described, the throttle member may be mounted on a slider and biased in position by springs at both ends, such that the movement of the throttle member may increase the diameter of the venturi throat. A by-pass pipe 40 including a valve 42 may also be incorporated into the air feed device shown in Figure 3 or Figure 4 of the present

specification.

The incorporation of a non-return valve into the by-pass pipe avoids a problem which may otherwise arise at low flow rates, namely that venturi may not permit sufficient water to flow through it such as to achieve the desired air flow. The non-return valve is arranged to provide sufficient resistance to the water flowing through the by-pass pipe as to cause water to flow through the venturi at rate sufficient to produce a satisfactory airflow. Air is therefore drawn through the venturi at low flow rates with top end flow through the by-pass pipe.

Instead of utilising a separate pipe such as that shown in Figure 7, the device 6 may be manufactured with a by-pass duct and valve housing integrated into the sleeve 17, as shown in Figure 8.

Although the air feed device is described as being located in a break in the cold water inlet, it may be located at the end of the pipe where it enters the tank, so that the water discharges from the device direct into the tank.

Claims

I. A hot water installation including an unvented hot water tank, a water pipe opening into the tank, the pipe incorporating an air feed device including a venturi, an air inlet communicating with the venturi, means being provided to allow water in the pipe to by- pass the venturi.
2. An installation as claimed in Claim I, wherein the air feed device includes a sleeve within which is housed a core defining the venturi, the core having therein a passage which communicates with the air inlet.
3. An installation as claimed in Claim 2, wherein the core is assembled from two tubular components, one of which is inserted into the other, each defining a respective part of the venturi passage.
4. An installation as claimed in Claim 1, wherein the by-pass passage incorporates a non- return valve.
5. An installation as claimed in Claim 2 or Claim 3, wherein an additional by-pass passage incorporates a non-return valve.
6. An installation as claimed in any preceding claim, wherein the air feed device is associated with a water inlet pipe.
7. An air feed device for a hot water installation comprising a sleeve within which is housed a core through which extends a venturi passage, a first non-return valve having its inlet communicating with atmosphere and its outlet communicating with the venturi passage, and a second nonreturn valve in a duct by-passing the venturi passage.