GB2348945A

GB2348945A - Control of micro-organisms in hot water supply systems

Info

Publication number: GB2348945A
Application number: GB9908296A
Authority: GB
Inventors: Douglas R Stammers
Original assignee: DOUGLAS IND Ltd
Current assignee: DOUGLAS IND Ltd
Priority date: 1999-04-13
Filing date: 1999-04-13
Publication date: 2000-10-18
Anticipated expiration: 2019-04-13
Also published as: GB2348945B; GB9908296D0

Abstract

To prevent the growth of microorganisms eg legionella bacteria in hot water supply systems, pasteurised hot water is delivered to a reduced temperature zone 11, including a supply leg 14, draw-off points 16 and a return leg 18, through which the water is constantly recirculated; and a microbiological growth-inhibiting dosing agent is added, at 24, to the return leg. The dosing agent may be chlorine dioxide, or copper or silver ions from an electrolytic ion generator 24. The recirculated water in the reduced temperature zone 11 is maintained at a sufficient concentration of dosing agent. At times of demand, the water in the circuit is replenished with pasteurised hot water from source 20, and with cold water via line 28.

Description

HOT WATER SUPPLY SYSTEMS This invention concerns hot water supply systems, and is particularly concerned with systems that are suitable for the supply of hot water for ablutions, especially in the workplace, in hospitals and nursing homes, in large residential premises, in sports establishments, and in other places where a high volume water supply system is required, where many draw-off outlets are likely to be used, and where the hot water distribution network is likely to be of a substantial size.

In such premises, there is known to be a particular risk of pathological microorganism populations developing in the hot water system if suitable precautions are not taken. An example of such a microorganism is legionella, which can give rise to legionellosis (legionnaires'disease) in the absence of effective precautions.

An acceptable and conventional way of controlling legionella is to store hot water at pasteurisation temperatures, normally 60 C or higher, and to distribute it at not less than 50 C to the point of use. However, to prevent scalding, especially in premises used by the elderly or infirm, it is essential to limit the hot water discharge temperature at wash basins, baths, showers, bidets and the like. Desired maximum temperatures vary with the particular kind of outlet, but in general a maximum of 43 C, sometimes 44 C, is typical. To achieve this, each draw-off outlet must be fitted with thermostatic mixing valves, or mechanical mixing valves or taps which include and appropriate maximum temperature stop.

In large establishments with extensive hot water distribution systems, the maintenance of sufficiently high temperatures to stop the development of legionella can be difficult, especially if long periods pass with no hot water draw-off occurring.

Other known potential methods of microbiological control include dosing the hot water with dosing agents which inhibit microbiological silver and copper ions, and chlorine dioxide. In order to maintain effective concentrations during periods of high demand, delivery systems must have the capacity to maintain high dosing rates into large volumes of water in a short time.

It is an object of the present invention to provide improved hot water supply systems with protection against microbiological growth.

In accordance with the present invention, pasteurised hot water is delivered to a reduced temperature zone, including a supply leg, draw-off outlets and a return leg, through which the water is constantly recirculated ; and microbiological growth-inhibiting dosing agent is added as required to the return leg of the circuit. In this way, with continuous recirculation, all the water in the reduced temperature zone is maintained with a microbiological growth-inhibiting concentration of dosing agent, while it is recirculated, for which purpose a low capacity dosing means is sufficient; and at times of demand, the reduced temperature zone hot water is replenished with pasteurised hot water which does not need to be treated with dosing agent. By virtue of its location in the return leg, the dosing means is not exposed to large volumes of hot water at times of significant demand, but at times of low or zero demand, the dosing means is able to maintain sterile conditions in the recirculating reduced temperature hot water.

It is likely that a given dosing agent will not be effective, and certainly not equally effective, against all possible microorganisms. For the purposes of this invention, references to microbiological growth may be taken to refer to the growth of any particular microorganism, and in a preferred embodiment of the invention, to the growth of legionella. Likewise, growth-inhibiting preferably implies the absoute prevention of microbiological growth, but does also include mere retardation of growth of a microbiological population.

In a specific embodiment, the invention provides a hot water supply system comprising: a source of hot water at pasteurisation temperature; a reduced temperature hot water distribution circuit including an inlet for pasteurised hot water from the source, a supply leg from the inlet to reduced temperature hot water draw-off outlets, and a return leg for reduced temperature hot water from the draw-off outlets to the pasteurised hot water inlet ; means for recirculating reduced temperature hot water through the distribution circuit; and dosing means in the return leg for maintaining microbiological growth-inhibiting concentrations of dosing agent in the water recirculated to the supply leg.

For most practical purposes, the reduced temperature hot water distribution circuit will include a cold water inlet, in order to be able to reduce the temperature sufficiently quickly when large volumes of reduced temperature hot water are demanded. The cold water inlet is desirably located in the return leg of the distribution circuit, and is supplied with cold water through a non-return valve.

The circuit may also desirably include a return water outlet for the return of recirculating water to the source for re-pasteurisation. This allows fresh hot water to be admitted to the circuit even at times of zero draw-off, in order to maintain the desired reduced temperature. The return water outlet is desirably located in the return leg, preferably before any cold water inlet to the return leg, and leads to the source through a flow regulating valve.

The dosing agent preferably comprises metal ions, in particular ions selected from silver and copper ions. The appropriate microbiological growth-inhibiting concentration will depend upon the circumstances, and it is known that the combination of 400 microgram per litre copper ion concentration and 40 microgram per litre silver ion concentration can be effective against planktonic legionella in both hot and cold water systems. In softened water, silver ion concentrations can be reduced to between 20 and 30 micrograms per litre, when copper ions are also present. If chlorine dioxide is used as the dosing agent, concentrations of about, and preferably not less than, 0.5 milligrams per litre are desirable.

The dosing means for metal ion treatment is suitably an electrolytic ion generator of the kind known for this purpose. The dosing means is preferably located in the return leg of the distribution circuit before any cold water inlet to the return leg, and before any return water outlet from the return leg. The preferred location for the recirculating means is in the return leg before the dosing means, so that the recirculating means, typically a pump, does not have to be provided with the capacity to deal with high demand volumes to the draw-off outlets and can maintain a positive flow pressure through the dosing means and to any return water outlet situated after the dosing means.

The hot water in the distribution circuit can be maintained at the correct reduced temperature by providing a water temperature sensor in the supply leg, and control means responsive to the temperature sensor for controlling the pasteurised hot water inlet to maintain the supply leg reduced temperature hot water in a predetermined temperature range, preferably one with a maximum of 44 C. Such control means may also control any cold water inlet to the distribution circuit to maintain the temperature range. Alternatively, the pasteurised hot water inlet may be controlled by a thermostatic mixing valve to maintain supply leg reduced temperature hot water in the predetermined temperature range.

There may be locations such as kitchens where high temperature hot water is required, for which purpose a pasteurised hot water outlet may be provided between the source and the pasteurised hot water inlet to the distribution circuit.

The source of hot water at pasteurisation temperature may be a tank provided with water heating means, and suitably a known form of thermostatic control. A preferred pasteurisation temperature is not less than 60 C, and preferably the pasteurised hot water admitted to the distribution circuit has not fallen in temperature since leaving the source to below 50 C.

One embodiment of the invention is illustrated by way of example in the accompanying drawing, which is a diagram of a hot water supply system incorporating the invention.

In the drawing, a reduced temperature hot water distribution circuit 11 includes an inlet 12 for pasteurised hot water from a source, in the form of a thermostatically heated hot water cylinder 20 at a pasteurisation temperature of at least 60 C ; a supply leg 14 leading from the inlet to a plurality of draw-off outlets 16, and a return leg 18 from the draw-off outlets to the pasteurised hot water inlet. A pump 22 in the return leg provides a means for recirculating reduced temperature hot water through the distribution circuit, and is located before an electrolytic ion dosing module 24 at which silver and copper ion concentrations are maintained in the recirculating water, by electrolytically generating ions to the required concentrations.

Between the ionising module and the hot water inlet 12 is a cold water inlet 26, which receives cold water from storage tanks or a mains supply through cold water supply conduit 28 including isolating valve 30 and non-return valve 32 just prior to the cold water inlet 26. Since the capacity of the cold water supply conduit is normally larger than that of the return leg 18, in order to be able to provide enough reduced temperature hot water when there is high demand at the draw-off outlets 16, the final section of the return leg between cold water inlet 26 and hot water inlet 12 is of larger diameter than the rest of the return leg, and typically of equal diameter to the cold water supply conduit 28 and also equal to the diameter of the pasteurised hot water supply at 12.

Also in the return leg 18, located between dosing means 24 and cold water inlet 26, is return water outlet 34 from which a proportion of the water recirculating in the distribution circuit 11 is bled off through return water conduit 36 and returned to pasteurisation temperature hot water cylinder 20, the proportion of the recirculating water that is returned being controlled by a double regulating valve 38 in the return water conduit.

Hot water inlet 12 to the distribution circuit 11 is supplied with pasteurised hot water by hot water conduit 40 leading from cylinder 20 and including, successively, isolating valve 42, high limited valve 44 and non-return valve 46 immediately prior to inlet 12. Inlet 12 itself is controlled by three-way mixer valve 48, which determines the proportion of fresh pasteurised hot water and recirculating reduced temperature hot water which are admitted to supply leg 14.

Branch hot water conduit 50 between isolating valve 42 and high limit valve 44 allows pasteurised hot water from conduit 40 to be taken at elevated temperatures of 50 C to 60 C to supply kitchens and the like. A return conduit 51 from the elevated temperature kitchen circuit joins return water conduit 36 downstream of double regulating valve 38.

Supply leg 14 is provided with a temperature sensor 52, at least two metres downstream of mixer valve 48, from which temperature data is fed to programmable microprocessor controlled electronic means 54 to regulate the hot water inlet according to demand and temperature, by sending control signals to motor-driven high limit valve 44 and, particularly, motor-driven three way mixer valve 48. Control of these valves inherently controls cold water admitted to the distribution circuit, having regard to the quantities of water being drawn off from draw-off outlets 16 and the quantity returning to the cylinder 20 through return water conduit 36.

A further programmable microprocessor controlled electronic means 56 is provided to control the quantities of silver and copper ions generated in electrolytic ionising module 24, in a known manner.

A pasteurised hot water inlet bypass conduit 60, provided with an isolating valve 58, is present to permit high temperature hot water to be run directly into the supply leg if an alternative mode of water supply is desired. For example, if for any reason thermostatic mixing valves are to be provided as a temporary expedient at draw-off outlets 16, or if the use of the temperature and dosing agent control system according to the invention is to be suspended for any other reason. Similarly, electrolytic ionising module 24 is provided with a bypass conduit 62 including isolating valve 64 in the return leg 18, and with two isolating valves 66,68 to enable the ioniser to be taken out of service temporarily.

As an example, the system described and illustrated may involve the supply of pasteurised hot water through 80 mm diameter pipework over a distance of 150 metres to draw-off outlets, with 20 mm diameter pipework returning to the source. The amount of hot water circulating in the system at any one time maybe as much as one third of a cubic metre. Since some of the water will have been introduced from a cold water supply, microorganisms could be present, although 60 to 70% of the water in the system has been previously at pasteurisation temperature. Such microorganisms may proliferate during circulation of the reduced temperature hot water. The instantaneous demand at the draw-off outlets may be as much as 4 to 5 cubic metres per hour, and any dosing means for chemical treatment of the water prior to the draw-off outlets would require a corresponding capacity. However, in accordance with the invention, a local zone can be established in the region of the draw-off outlets with a reduced temperature hot water distribution circuit, with a pump and ioniser in the return leg of a capacity that need be no more than sufficient to recirculate the volume of water in the distribution circuit two or three times per hour, thereby ensuring that all water in the distribution circuit is treated in each 20 minute period. Reduced temperature hot water is maintained in biologically safe conditions, and the need for thermostatic mixing valves at each draw-off outlet, with the implicit cost and risk of accidental scalding in case of malfunction, is avoided.

Claims

CLAIMS 1. A hot water supply system comprising: a source of hot water at pasteurisation temperature; a reduced temperature hot water distribution circuit including an inlet for pasteurised hot water from the source, a supply leg from the inlet to reduced temperature hot water draw-off outlets, and a return leg for reduced temperature hot water from the draw-off outlets to the pasteurised hot water inlet ; means for recirculating reduced temperature hot water through the distribution circuit; and dosing means in the return leg for maintaining microbiological growth-inhibiting concentrations of dosing agent in the water recirculated to the supply leg.
2. A hot water supply system according to claim 1 wherein the reduced temperature hot water distribution circuit includes a cold water inlet.
3. A hot water supply system according to claim 2 wherein the cold water inlet is located in the return leg.
4. A hot water supply system according to claim 2 or claim 3 wherein cold water is supplied to the cold water inlet through a non-return valve.
5. A hot water supply system according to any one of the preceding claims wherein the reduced temperature hot water distribution circuit includes a return water outlet for the return of recirculating water to the source for repasteurisation.
6. A hot water supply system according to claim 5 wherein the return water outlet is located in the return leg.
7. A hot water supply system according to claim 6 wherein the return water outlet is located in the return leg before any cold water iniet.
8. A hot water supply system according to any one of claims 5 to 7 wherein the return water outlet leads to the source through a flow regulating valve.
9. A hot water supply system according to any one of the preceding claims wherein the dosing agent comprises metal ions as active ingredients.
10. A hot water supply system according to claim 9 wherein the metal ions are selected from silver and copper ions.
11. A hot water supply system according to claim 9 or claim 10 wherein the dosing means comprises an electrolytic ion generator.
12. A hot water supply system according to any one of claims 1 to 8 wherein the dosing agent comprises chlorine dioxide as an active ingredient.
13. A hot water supply system according to any one of the preceding claims wherein the dosing means is located in the return leg before any cold water inlet to the return leg.
14. A hot water supply system according to any one of the preceding claims wherein the dosing means is located in the return leg before any return water outlet from the return leg.
15. A hot water supply system according to any one of the preceding claims wherein the recirculating means comprises a pump in the return leg before the dosing means.
16. A hot water supply system according to any one of the preceding claims comprising a water temperature sensor in the supply leg, and control means responsive to the temperature sensor for controlling the pasteurised hot water inlet to maintain supply leg reduced temperature hot water in a predetermined temperature range.
17. A hot water supply system according to claim 16 wherein the control means also controls any cold water inlet to the distribution circuit to maintain said temperature range.
18. A hot water supply system according to any one of claims 1 to 15 wherein the pasteurised hot water inlet is controiled by a thermostatic mixing valve to maintain supply leg reduced temperature hot water in a predetermined temperature range.
19. A hot water supply system according to any one of claims 16 to 18 wherein the temperature range has a maximum of 44 C.
20. A hot water supply system according to any one of the preceding claims wherein a pasteurised hot water outlet is provided between the source and the pasteurised hot water inlet to the distribution circuit.
21. A hot water supply system according to any one of the preceding claims wherein the source of hot water at pasteurisation temperature is a tank provided with water heating means.
22. A hot water supply system according to any one of the preceding claims wherein the pasteurisation temperature is not less than 60 C.
23. A hot water supply system according to any one of the preceding claims wherein the pasteurised hot water admitted to the distribution circuit has not fallen in temperature since leaving the source to below 50 C.
24. A hot water supply system according to any one of the preceding claims wherein the microbiological growth-inhibiting concentrations of dosing agent are planktonic legionella growth-inhibiting concentrations.
25. A hot water supply system substantially as herein described with reference to and as illustrated in the accompanying drawings.