GB2329236A

GB2329236A - Boiling point detector for hot water dispenser

Info

Publication number: GB2329236A
Application number: GB9720729A
Authority: GB
Inventors: Paul Geoffrey Phillips
Original assignee: Redring Electric Ltd
Current assignee: Redring Electric Ltd
Priority date: 1997-09-12
Filing date: 1997-10-01
Publication date: 1999-03-17
Anticipated expiration: 2017-10-01
Also published as: GB9719482D0; GB2329236B; GB9720729D0

Abstract

A hot water dispenser heated by an electric heating element 3 controlled by a thermistor 4 and logic circuit 17 (Fig 2) is subjected to a boiling point calibration process, so that boiling water can then be supplied efficiently, regardless of the atmospheric pressure at which the dispenser is operating. The calibration process consists of switching on the element 3 to heat the water, measuring via the thermistor 4 the temperature as it increases, comparing subsequent values, noting when the increases cease, and storing the value in memory as representative of the boiling point. In subsequent operation of the water dispenser, the stored value is used to control the heating element 3. The calibration process does not take place if the float switch 8 is closed indicating water level is low.

Description

Liquid Heating Svstem The present invention relates to a liquid heating system and more particularly to a liquid heating system having a boiling point detector.

Many water heaters, such as those used to dispense hot water for catering, are desired to be able to dispense water at its boiling point as well as water at a lower temperature for use in hot beverages and the like. Such heaters generally have a manually controllable water outlet valve, an automatically controlled water inlet valve, and a control mechanism to control the heating of the water by an electric heater such that it remains within a small temperature range around a desired temperature.

However, the mechanisms provided for controlling these water heaters to provide water at its boiling point have been unsatisfactory. Some such heaters are calibrated during manufacture to have a maximum temperature at just below 100"C, so that operation at a lower than usual atmospheric pressure, such as at altitude, does not cause the water heater to boil continually and emit excessive steam. However, it is not possible to obtain water at its true boiling point from these water heaters at pressures greater than that for which the heater was calibrated as being the minimum expected pressure. Other water heaters heat continually the water and use an in-built condenser to return evaporated water to its liquid phase. Such heaters tend to be energy inefficient.

In accordance with a first aspect of the present invention, a liquid heating system includes a container for holding a liquid; a heating element; means for detecting increases in temperature of a liquid being heated by said element; and means dependent on said increases for determining the boiling point of said liquid when said increases cease.

The system may further comprise means to store in a memory a value indicative of the boiling point and may further comprise means to control the heating element with reference to said value.

Preferably, the system further comprises a liquid inlet valve and a liquid outlet valve and the boiling point detection means are inoperative to determine the boiling point when the liquid inlet valve is open.

In accordance with a second aspect of the present invention, a water heater comprises a system in accordance with the first aspect.

In accordance with a third aspect of the present invention, a boiling point detector comprises: temperature sensitive means to provide a signal representative of a liquid temperature; and means to determine whether the temperature representative signal remains substantially constant for a predetermined period of time thereby being indicative of a boiling point having been reached.

Here, the boiling point detector may further comprise means to store in a memory a value corresponding to a temperature representative signal so determined.

The invention will be further described by way of example with reference to the accompanying drawings, in which, Figure 1 shows schematically in section a water heater in accordance with the present invention in the form of a hot water dispenser; Figure 2 shows a control device of the Figure 1 water heater; and Figure 3 shows a boiling point detection operation performed by the Figure 2 control device.

Referring firstly to Figure 1, a hot water dispenser 1 comprises a body 2 for containing a liquid, an electric heating element 3, and a thermistor arrangement 4, which comprises a discrete thermistor encased in a copper pocket. Both the heating element 3 and the thermistor arrangement 4 are located below the level 5 of water 6, which is maintained by control of a solenoid valve 7 by an unshown electrical circuit incorporating a float switch 8 mounted in the body 2. This circuit is described further below with reference to Figure 2. The solenoid valve 7 is fitted with a flow regulator 9.

Heat insulation 10 separates a volume 11 above the water level 5 from a condenser 12.

Condensed steam is able to return from the condenser through a channel 13 in the insulation 10 by the force of gravity. In use, hot water is extracted from the body 2 by the manual opening of a valve 14 fitted with a water filter 15. A vent 16 is provided for release of excess steam from the volume 11 to the atmosphere.

A control device 17 is mounted on the outer surface of the body 2 so as to be in electrical connection with the heating element 3 and with the thermistor arrangement 4.

Referring to Figure 2, the control device 17, shown schematically, comprises an electrical power terminal 18, having live, earth and neutral terminals 19, 20 and 21 respectively, a power supply PCB 22, float switch connection terminals 23, solenoid valve connection terminals 24, heating element connection terminals 25 and a logic PCB 26 and thermistor connection terminals 30.

The power supply PCB 22 is connected to the logic PCB 26 by parallel lines 27, which conduct electrical power to the logic PCB 26 and conduct control and information signals in both directions. For the purpose of this invention, these signals comprise heater on and off control signals from the logic PCB 26, which are used by the power supply PCB 22 to control a relay 28.

The float switch 8 of Figure 1 is chosen such that it is closed when the water level 5 is low. Thus, the series connection of the float switch connection terminals 23 and the solenoid valve connection terminals 24 shown in Figure 2 provides for water to flow into the heater body 2 when the water level 5 is low, without the need for electric control.

An unshown live detection circuit is included in the power supply PCB to detect a voltage on a connection line 29. This voltage is indicative of the state of the float switch 8 and is used by the unshown live detection circuit to generate a float switch state signal for communication to the logic PCB on one of the lines 27.

Thermistor connection electrodes 30 are connected directly to an unshown interface of the logic PCB 26. In this way, the logic PCB 26 is able to apply continuously a voltage to the thermistor in the arrangement 4 and to measure the current flowing therethrough, which is dependent on and representative of the temperature of the thermistor, with simple interface circuitry.

The boiling point detection operation 35 shown in Figure 3 will now be described. The operation 35 starts on power up 36 of the heater 1. A check 37 is then made on the state of the float switch 8 by way of the unshown live detection circuit in the power supply PCB 22. The operation 35 remains in a loop 38 until the water level 5 is sufficient to open the float switch 8. In this manner, cold water entering the heater 1 cannot cause an incorrect determination of the boiling point to be made. When the float switch 8 is determined to be open, the heating element 3 is switched on 39. An alternative operation would be to switch on 39 the heating element immediately after power on, and then perforrn the float switch state check 37. This alternative may provide a more rapid boiling point detection from power up 36, especially if the water level 5 is initially low.

A measurement of the current flowing through the thermistor arrangement 4, indicative of the water temperature, is then made and the result placed 40 into an unshown first-in first-out eight segment buffer. All eight segments of the buffer are then read 41 and compared with each other 42. If two or more segments of the buffer are found to contain different values, the operation 35 returns to the float switch state determination 37.

If each of the eight segments are found to contain the same value, it is determined that the boiling point has been reached since the temperature has ceased to increase. The heating element is switched off 43 and the buffer value recorded 44 in an unshown memory as being indicative of the measurement obtained from the thermistor arrangement 4 when the water 6 is at boiling point. This value can be described as a calibration number.

At this time, the boiling point determination operation 35 is terminated and normal heating control operation is commenced 45. This normal operation will be known by the skilled person, so is not described here. However, it is preferred that water temperatures that the heater 1 is desired to provide, such as a HOT mode temperature, are calculated from the calibration number determined above, instead of from some calibration scheme at the time of manufacture.

It has been found that, for reliable boiling point determination, the resolution of the thermistor arrangement current measurement is such that one increment in value stored in the unshown buffer equates to approximately 0.5 C and that the buffer is added to 40 approximately every 4 seconds.

The boiling point detection operation 35 is preferably performed in a microprocessor on the logic PCB 26, although a programmable logic array could equally well be used. In either case, additional circuitry or processing capacity may be provided for other control or monitoring functions such as normal operation, boil-dry detection and the like. It may be desirable that, when in normal operation BOIL mode, the heater 1 is controlled to provide water at 98% or 99% of the detected boiling point. This feature will provide assurance that the heater 1 will not boil continually if, for example, atmospheric pressure decreases after the boiling point detection operation has been performed.

Advantageously, to provide improved electrical efficiency, the control device further comprises means, operative when in a BOIL mode, to detect non-utilisation of the heater 1 and to switch the heater 1 to a HOT mode if not utilised for, for example, thirty minutes. Thus, if no water is drawn off for a period of thirty minutes whilst in the BOIL mode, the heater 1 is switched to the lower temperature HOT mode. Said detection may involve input from unshown control switches and the float switch 8.

Claims

1. A liquid heating system including a container for holding a liquid; a heating element; means for detecting increases in temperature of a liquid being heated by said element; and means dependent on said increases for determining the boiling point of said liquid when said increases cease.

2. A system according to Claim 1 further comprising means to store in a memory a value indicative of the boiling point.

3. A system in accordance with Claim 2 further comprising means to control the heating element with reference to said value.

4. A system in accordance with Claim 3 in which the heating element is selectably controllable to maintain the liquid at a temperature a predetermined amount below, or a predetermined fraction of, the boiling point.

5. A system in accordance with Claim 4 further comprising means to detect non-utilisation of the system, and to control the heating element to maintain the liquid at said temperature if not utilised for a predetermined period of time.

6. A system in accordance with any of Claims 1 to 5 further comprising a liquid inlet valve and a liquid outlet valve, in which the boiling point detection means are inoperative to determine the boiling point when the liquid inlet valve is open.

7. A system in accordance with Claim 6 in which a float switch associated with the liquid inlet valve is used to disable the boiling point detection means.

8. A water heater comprising a system in accordance with any of Claims 1 to

9. A boiling point detector comprising: temperature sensitive means to provide a signal representative of a liquid temperature; and means to determine whether the temperature representative signal remains substantially constant for a predetermined period of time thereby being indicative of a boiling point having been reached.

10. A boiling point detector in accordance with Claim 9 further comprising means to store in a memory a value corresponding to a temperature representative signal so determined.

11. A water heater comprising a boiling point detector in accordance with Claim 9 or Claim 10.

12. A liquid heating system having a boiling point detector substantially as shown in or as described with reference to Figure 3 of the accompanying drawings.

13. A water heater having a boiling point detector substantially as shown in or as described with reference to Figure 3 of the accompanying drawings.