The present invention relates to thermostatic mixing valves and in particular to variable rate thermostatic mixing valves.
Thermostatic mixing valves (TMVs) are well established and serve to provide a fluid (water) supply at a desired temperature. For this purpose, TMVs have a thermostatic device to control the relative proportions of hot and cold fluids passing in the valve to a mixing zone whence the mixed fluids are caused to impinge on a temperature responsive element of the thermostatic device. Control of the hot and cold flows is achieved by the thermostatic device being operatively coupled to a valve member controlling fluid flows through hot and cold inlet ports of the valve. Consequently, when there is an undesirable rise in the temperature of the mixed fluid the thermostatic device expands to cause the valve member to reduce the hot flow via the hot inlet port and increase tile cold flow via the cold inlet port to restore the fluid supply temperature condition to that desired, with a converse operation when there is a fall in the mixed fluid temperature.
It is established practice to have the hot and cold inlet ports (controlled by the valve member) of a substantially 360xc2x0 form, with the valve member constituted by a slide or poppet valve device, and for the provision of a suitable fluid flow to the 360xc2x0 port the TMV usually employs fluid feed ducts extending laterally to the TMV casing and each feeding to a respective annular chamber within the valve casing, appropriate wall formations being present in the casing to contain fluid flow in each clamber. In a known design the galleries surround their respective hot and cold inlet ports of the TMV. GB-A-2096274 and FR-A-1310027 show such an arrangement. This invention is not necessarily limited to such arrangements, however.
Large bore TMVs are used to supply a number of outlets, such as groups of showers, wash basins or baths. The TMVs must be capable of passing substantial amounts of properly mixed water when a number of outlets are being used simultaneously. The internal arrangement of the TMV should be designed such that the high flow rate can be passed without an unduly high pressure drop. This means that the internal passages are relatively large and the annular passage round the thermostat element must also be relatively large to avoid causing any restriction to the mixed water flow under the maximum demand.
There are, however, drawbacks in such arrangements. One well know problem in thermostatic mixing valves of larger bores is that of achieving sufficient mixing of hot and cold water across a range of flow rates.
When there is a low demand for mixed water the velocity of the hot and cold water streams passing through the TMV drops and is insufficient to mix the two streams fully. The result is that the streams may become laminar and mixing of the hot and cold supplies does not take place. If this happens, then the water surrounding the thermostat element is not fully mixed and as a result the thermostat element may receive a false signal. When this happens the thermostat element is not able to expand or contract as required to control the mixed water temperature. The overall result, therefore, is a loss in control of mixed water temperature.
One known approach to this problem is to provide two separate TMVs operating in parallel with each other (as illustrated and described below with reference to FIG. 1). A small bore TMV can be fitted in parallel with a large bore TMV in conjunction with additional equipment such as a pressure reducing valve or some other throttling device on the outlet of the large bore TMV. Thus, when there is a low demand for mixed water the hot and cold streams only pass through the small bore TMV and not the large bore TMV.
There are disadvantages in this approach in that it requires extra hardware in the form of two TMVs and a throttling device such as a PRV downstream from the larger TMV to implement the TMV which adds to costs. Additional installation steps and maintenance may also be required, and temperature regulation is a function of two individual TMV characteristics.
It is the principal object of the present invention to provide a device for use in a TMV which overcomes the problems of mixing fluids over a range of high and low flow rates.
In particular it is an object of the invention to provide a single TMV capable of handling both high and flow rates automatically whilst retaining a sufficiently effective and reliable thermostatic mixing control function.
According to the principles of operation of the invention to be more particularly described hereinafter, a TMV is provided wherein the arrangement is such that hot and cold fluids are mixed, and the temperature of the mixed fluids is sensed by means operatively connected to a valve actuator enabling a desired temperature profile to be maintained within certain parameters by adjustment of the mixing ratio of hot to cold fluids, and wherein there is provided means permitting by-pass of a proportion of the mixed fluids particularly at high flow rates without loss of the temperature control function.
The invention in particular provides a thermostatic mixing valve for operation over a range of fluid flow rates, comprising housing parts and operating parts accommodated within an operating space defined by the housing parts, said operating parts including:
a valve member movable within the operating space so as to define first and second valve ports respectively, said first valve port tending to open while the second valve port is closing and vice versa, depending on the direction of movement of said valve member;
inlet ducting for delivering first and second fluids at different temperatures to said first and second valve ports respectively;
a mixing space within said operating space arranged to receive said fluids from the valve ports and permit mixing thereof;
a temperature-sensitive actuator, at least a sensing part of the actuator being located within a sensing space within said operating space, said sensing space being in communication with both of said first and second valve ports thereby to receive the mixture of fluids, said actuator being arranged to cause movement of the valve member so as to counteract temperature changes of said fluid mixture; and
a discharge space downstream of said sensing space,
wherein further valve means are provided responsive to demand for fluid at said discharge space, so as to force the mixed fluid to flow via said sensing space in close proximity to said sensing part of the actuator at times of low demand, while permitting a portion of said mixture to bypass said sensing space under conditions of high demand, thereby to permit an increased flow rate.
By providing a bypass within the valve which opens only at times of high demand, it can be arranged that the velocity of the fluids where they meet is sufficient for turbulence and mixing, both at low flow rates and high overall flow rates.
In one embodiment the further valve means is a pressure-responsive valve means, responsive to an increased pressure difference between points upstream and downstream of the sensing space to open a conduit between said points. The further valve means closes below a pressure drop and, therefore, flow rate for which fluid mixing within the mixing space would be unable to take place efficiently.
The sensing space and mixing space may be distinct, or may be formed substantially as one space within the housing.
In one embodiment, said operating space and operating parts are generally cylindrical and have rotational symmetry about an axis of the valve. The temperature-sensitive actuator may be arranged centrally, extending along said axis. The valve member may be a cylindrical slide valve moving axially between first and second annular valves seats, thereby to define said first and second valve ports as annular ports. The inlet ducting may comprise an entry to one side of the valve axis, feeding into an annular gallery space surrounding said valve ports.