GB2261610A - Controlling liquid temperature - Google Patents

Abstract

Apparatus is provided to supply a liquid, for example water, at a constant temperature by mixing two supplies of the liquid, one hot and the other cold. Control of the output temperature is effected by measuring and controlling the flow rate in one or both of the supplies by way of flow meters having a rapid response time and motor driven valves. Mixing takes place without the provision of a dedicated mixing chamber. In the case where the flow rates in both supplies are measured and controlled it is possible further to control the output flow rate in addition to the output temperature, also by controlling the flow rates in the supplies. Further accuracy may be achieved by sensing the input supply temperatures in addition to the flow rates. The invention is applicable to a domestic shower.

Description

Fluid Flow and Temperature Control Apparatus The present invention relates to fluid flow and temperature control apparatus. In particular it is concerned with apparatus for providing liquid, typically water, to a user at a required temperature and/or flow rate where this is achieved by mixing in varying proportions two supplies of the liquid at different temperatures, designated "cold" and "hot respectively.

The apparatus is of general application but is best explained in relation to the control of a domestic shower which is supplied with both hot and cold water.

A prime requirement of such a system is that the output be kept at a required constant temperature regardless of input conditions. A number of arrangements have already been proposed to effect such control which utilize a motorized mixing valve under the control of a microprocessor. In most cases the input parameter for the control system is the temperature of the water output and the control system acts to keep this constant by adjusting the mixing valve to allow, for example, more hot water into the outlet in the case that the outlet temperature falls. The overall flow rate in such a system is typically determined em. pirically by the user.

In some arrangements, there is provided means for storing preferred flow and temperature settings for future use. This is the case in US 4682728 which monitors the angular position of the valve settings and in US 4696428 which uses inter alia a flow meter in the output from the mixing valve. Both of these prior art systems monitor the temperature of the mixed outlet water.

Problems are caused to this type of system when the input conditions change. For instance if the pressure in one of the water supplies to a shower were to fall suddenly then the proportion of water from the other supply in the outlet would increase and the outlet temperature would change. Control systems are designed to account for such variations and to ensure that the outlet temperature remains constant. However, it will be appreciated that the speed of response of the control system, and hence the length of time for which the outlet is not at its required temperature, is dependant on the speed of response of the sensors used to provide data to the control system.

It is conventional for temperature sensors to be the prime means of controlling a shower and that is the case in the above identified US patent disclosures.

However, temperature sensors are typically slow to react to rapid temperature changes and thus systems utilizing such sensors are prone to supplying water which is not at the required temperature for comparatively long periods of time while re-adjustment is made.

There is thus still a need for apparatus for the supply of liquid at constant temperature which will react quickly to changes in the input conditions in order to maintain the required output temperature and/or flow rate.

According to our EP,91304146.3 this is achieved by the use of a flow meter in one or both of the supplies to sense the flow rate and to supply this data as the input parameter to the control system. The advantage of this is that flow meters react to changes in condition faster than temperature sensors and this enables the control system of the present invention to react to changes in condition fastet than known systems.

The operation of the present invention is based on the following equation, which is essentially an energy continuity equation for the mixer valve: Mc STc + Mh STh = Mo STo ,...... (1) where Mc, Mh, Mo designate the mass flow rates in the cold supply, hot supply and outlet respectively, S designates the specific heat capacity of the liquid being supplied, and Tc, Th, To designate the temperatures of the cold supply, hot supply and outlet respectively.

Given that Mo = Mc + Mh, it will be appreciated that equation (1) can be re-arranged to give: To= Mc Tc + Mh Th .......(2) Mc + Mh Thus it can be seen that outlet temperature, To, can be controlled by sensing and controlling only input parameters. Of the input parameters, the mass flow rates, Mc and Mh, are the most likely to be subject to rapid change and these parameters can be sensed by the flow meters which have a fast response time. The input temperatures typically vary quite slowly and only a small amount and thus this variation may be sensed accurately using conventional temperature sensors, in spite of their relatively slow response.

Alternatively in some cases it is reasonable to assume that the supply temperatures remain constant, in which case it is necessary only to sense the flow rates in order to maintain a constant output temperature.

Further it may be the case that one supply has a guaranteed flow rate, in which case this parameter need not be sensed.

Flow meters typically measure volume flow rate, which is related to mass flow rate by the equation m = pv, where D designates density and V designates volume flow rate. This may substituted in equation (2) to give: To = 80r Vr To + Mh Vh Th ....... (3) /Oc Vc +,vh Vh In many situations to which this invention is applicable, #c # #h due to the small change in liquid density over the temperature range concerned and thus equation (3) reduces to: To = Ve Tr + Vh Th (4) Ve + Vh Alternatively the control system may be adapted to take account of the different densities in the hot and cold supplies.

All of the embodiments envisaged are described in relation specifically to domestic showers, but as mentioned above, this invention is equally applicable to any situation in which it is required to supply liquid at a given temperature by mixing hot and cold supplies in varying proportions.

All of the described embodiments use volume flow meters and it is assumed that #c = #h in these applications. Therefore equation (4) above is used as the basis for discussion of these embodiments.

Before describing the apparatus in detail, it is considered helpful to explain the background of the problems which are encountered with showers. The typical problem which a user faces when showering is that for example, if someone else turns on a hot water tap in the house, then the person showering finds that his shower will go cold due to the reduction in pressure in the hot supply to the shower. Thermostatic mixer showers are made to overcome this problem but virtually all of these suffer from the slow speed of response of temperature sensors discussed above and from insufficient range of adjustment. It can take more than fifteen seconds to bring the water back to something approaching the original temperature and considerably longer actually to recover the original setting.It will be appreciated, that if the hot water tap were switched off rather than switched on, a conventional thermostatic shower should deliver uncomfortably hot water for at least fifteen seconds. This effect is due to the fact that the temperature sensing divices are not sufficiently responsive to enable rapid accurate temperature control on the basis of temperature sensors in the output alone. In fact most temperature variations which are desired to be removed are a result of variations in flow rate in the supplies which result in variations in output temperature and flow. The present invention is therefore designed to maintain temperature by monitoring and adjusting where necessary input flows in accurate manner.

Our earlier EP-91304146.3 specifically described an arrangement utilizing a mixing chamber. We disclaim from this application the use of a specially fabricated mixing chamber. The present invention envisages the use of conventional plumbing parts where mixing can take place, such as a Y-coupling or the outlet pipe of a shower or in the shower head itself.

It is also envisaged that mixing could take place in a container separate from the water supply apparatus or indeed in free space.

It will be appreciated from the above that the present invention is of wide applicability and that mixed fluids at a precise temperature can be achieved quickly and simply.

Subject to the above disclaimer, the following claim define the scope of the present invention.

Claims (11)

CLAIMS:

1. Apparatus for providing liquid at a defined temperature from a mixture of a first supply (1) of liquid having a temperature higher than said defined temperature and a second supply (2) of liquid having a temperature lower than said defined temperature, the apparatus comprising: flow sensing means (5) for sensing the flow rate in one of said first and second supplies, adjustment means (6) arranged to control the flow rate in said one of said first and second supplies, and control means responsive to said flow sensing means (5) to control said adjustment means (6) whereby to adjust the flow rate and maintain the defined temperature of the mixed liquid.

2. Apparatus as claimed in claim 1 wherein said flow sensing means (5) comprises a flow meter arranged to provide an electrical signal functionally related to the volume flow rate through said one supply, and said control means comprises an input arranged to receive said electrical signal.

3. Apparatus as defined in claim 1 or 2 wherein said adjustment means (6) comprises a valve and a stepper motor, said stepper motor being arranged to receive control signals from said control means and to open and close said valve.

4. Apparatus as defined in claim 1, 2 or 3, further comprising: further flow sensing means (7) for sensing the flow rate in the other of said first and second supplies, and further adjustment means (8) arranged to control the flow rate in the other of said first and second supplies, and wherein the control means is further responsive to said further flow sensing means (7) and is operative to control sad further adjustment means (8) whereby to adjust the flow-rate and maintain the defined temperature of the liquid.

5. Apparatus as defined in claim 4 wherein said further flow sensing means (7) comprises a flow meter arranged to provide an electrical signal functionally related to the volume flow rate through said other supply and said control means comprises an input arranged to receive the electrical signal from said further sensing means (7).

6. Apparatus as defined in claim 2 or 5 wherein said flow meter comprises a turbine arranged to be rotated by flow through the meter and a light source and a photosensitive device arranged such that a light beam from said light source falls on said photosensitive device and is alternately interrupted and restored during rotation of said turbine.

7. Apparatus as defined in claim 4, 5 or 6 wherein said further adjustment means (8) comprises a valve and a stepper motor, said stepper motor being arranged to receive control signals from said control means and to open and close said valve.

8. Apparatus as defined in any of claims 4 to 7 wherein said control means is further adapted to control said first mentioned and said further adjustment means (6,8) whereby to maintain the flow rate of the mixed liquid at a determined magnitude.

9. Apparatus as defined in claim 4 further comprising first and second temperature sensing means (15,25) arranged to detect the temperature of said first and second supplies respectively.

10. Apparatus as defined in any preceding claim wherein said control means comprises a microprocessor.

11. Apparatus as defined in any preceding claim arranged to provide water to a domestic shower.