WO2006018422A1

WO2006018422A1 - Liquid heater

Info

Publication number: WO2006018422A1
Application number: PCT/EP2005/053974
Authority: WO
Inventors: John Nicholas Reid
Original assignee: Vallid Limited
Priority date: 2004-08-14
Filing date: 2005-08-12
Publication date: 2006-02-23
Also published as: GB2434194A; GB0702239D0; GB2417060A; GB0418186D0; GB2434194B

Abstract

A liquid heater is described that is suitable to heater a cupful of water in a beverage dispensing machine. The heater comprises a vertical elongate heating chamber 10, an inlet 12 near the bottom of the chamber for admission of cold liquid into the chamber, an outlet 14a near the top of the chamber for discharge of hot liquid from the chamber, a heating element 24 arranged within the chamber, and valves 16a and 44 for closing off the inlet 12 and the outlet 14 to allow the pressure in the chamber 10 to rise as the liquid temperature rises. In the invention, the heating element 24 comprises resistance wire encased within a metal tube filled with an electrically insulating material.

Description

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LIQUID HEATER

Field of the invention

The present invention relates to a batch liquid heater suitable for use in a beverage dispensing machine to dispense rapidly a quantity of water, or other liquid, at or near its boiling point.

Background of the invention

Many heaters are disclosed in the prior art for heating a constant stream of water. Examples of these are to be found in GB 2270369, GB 2162027, GB 2047510, GB 2340590 and US 4687905. In all such heaters, for reasons of safety, the water is not heated above a temperature at which it could cause scalding and it is certainly not heated to near its boiling point. As a result, the heating elements in such continuous flow heaters are not exposed to the problems that are caused by cavitation.

In a beverage dispensing machine, it is not necessary to provide a continuous flow of boiling water and instead the heater can be a batch heater capable of delivering a limited quantity of water during each dispensing cycle sufficient to fill the dispensing cup. It is, however, a requirement for the heater to be capable of delivering water at a sufficiently high temperature to brew a beverage such as tea or coffee, i.e. at least near or even above its boiling point, and that the time taken to bring a cupful of water to this temperature should be as short as possible.

PCT/GB03/04386, which is believed to represent the closest prior art to the present invention, discloses a liquid heater comprising a vertical elongate heating chamber, an inlet near the bottom of the chamber for admission of cold liquid into the chamber, an outlet near the top of the chamber for discharge of hot liquid from the chamber, a heating element arranged within the chamber, and valves for closing off the inlet and the outlet to allow the pressure in the chamber to rise as the liquid temperature rises. By closing both the inlet and the outlet of the heating chamber, it is possible to raise the water pressure and thereby also raise the boiling point of the water. Consequently, water hot enough to brew tea or coffee effectively can be produced without the water boiling. The chamber is not used to generate steam and is completely filled with water as it is being heated.

During a heating cycle, after the chamber has been totally filled with water, it is closed off at both ends and the heating element is energised to bring the water in the chamber to the boil. At the very onset of boiling, the heating element is switched off. Next, a valve at the top of the chamber is opened and cold water is admitted into the chamber from below. The cold water displaces the heated water out of the chamber and refills the chamber in readiness for the next cycle.

In the latter proposal, the heating element in the heating chamber took the form of a coil of bare resistance wire. This was not only deemed preferable in order to reduce costs but was regarded as essential in order to be able to switch off the energy supply rapidly once the desired temperature has been reached, to avoid damage to the heating element and possibly over-pressurising the heating chamber.

Object of the invention

The present invention seeks to provide an improvement on the above earlier proposal in which it was found that the heating element had only a limited lifetime. Summary of the invention

According to the present invention, there is provided a liquid heater of the type taught by PCT/GB03/04386, in which the heating element comprises resistance wire encased within a metal tube filled with an electrically insulating material.

The invention is based on the realisation that cavitation around the heating element causes local overheating when it is constructed as a bare resistance wire, ultimately resulting in its destruction. To overcome this problem, the invention proposes the use of a heating element resembling that to be found in a conventional electric kettle, which is capable of surviving even if the kettle is allowed to boil dry. Though such heating elements have been used in continuous flow heaters, kettles and immersion heaters, they have not been used to heat a liquid contained in a sealed pressurised vessel that it completely filled with the liquid. Contrary to expectations, it has been found that such a heating element, having a heat output that is high enough to achieve a short cycle time, can still be switched off sufficiently rapidly to avoid an excessive rise in pressure and pressure in the heating chamber when the liquid approaches its boiling point.

Preferably, the outer tube of the heating element comprises a coiled portion and two straight parallel legs at the ends of the coiled portion which pass through the base of the heating chamber, the length of the central conductor encased within the coiled portion being resistance wire and the lengths encased within the straight legs having low electrical resistance so that only the coiled part of the element is heated when electrical current passes through the central conductor. In the case of a water heater, the water will reach a temperature only slightly greater than 100⁰C and a pressure of no more than two atmospheres. Because of this, it is possible and advantageous to form the walls of the heating chamber of a plastics material that can withstand such temperatures and pressures. Plastics materials such as polysulphones or a polyether sulphones can meet this requirement while leaving a considerably margin of safety. Because the heating element has cold legs in the preferred embodiment of the invention, they may safety pass through, and be sealed relative to, the walls of the heating chamber.

The provision of long cold legs on the heating element passing through the base of the heating chamber permits the heated coil portion to be positioned at some distance above the base. As the liquid is heated rapidly by convection, it is found that though the liquid in the top part of the heating chamber is heated the liquid near the base remains cold and its temperature barely rises above the temperature of the liquid entering through the inlet.

In the preferred embodiment of the invention, a pressure sensor is provided beneath the level of the heated coiled portion of the heating element to measure the pressure of the liquid in the chamber. In prior art proposal in which the heating chamber is not totally sealed while the heating element is activated, the pressure within the heating chamber does not change when the liquid reaches its boiling point. Consequently, only direct temperature measurement can be carried out to determine when to switch off the heating element. As is well-known from kettles, it can take several seconds for a temperature sensor to respond after the desired temperature has been reached and such a delay is not acceptable within the context of the present invention as it can give rise to serious damage. The use of a pressure sensor in the preferred embodiment of the invention, not only improves on the measurement accuracy but also allows the sensor to be located in a cold heart of the heating chamber making it inherently more reliable.

Such a pressure sensor thus provides a reliable indication that the liquid in the heating chamber has commenced to boil and its output can be used to terminate a heating cycle by discontinuing the supply of current to the heating elements and opening the valves connected to the inlet and the outlet to permit the heated liquid to be dispensed.

It is further preferred to use the pressure at the inlet of the heating chamber to displace the liquid that has already been heated. As the inlet is arranged at the bottom of the chamber and the outlet is at the top, natural convection (i.e. the greater density of the cold liquid) acts to prevent the two from mixing. Mixing is further avoided if a baffle is provided within the heating chamber so that minimum turbulence is created at the interface between the hot and the cold liquid.

Brief description of the drawings

The invention will now be described further, by way of example, with reference to the accompanying drawings, in which :

Figure 1 is a view of a batch water heater of the invention with internal components of the heater shown in dotted lines, and Figure 2 is a diagram showing the water circuit of the heater shown in Figure 1.

Detailed description of the preferred embodiment

The batch water heater in the drawings comprises a vertical elongated heating chamber defined by a casing 10 made of a plastics material, such as a polysulphone, that can withstand high temperatures and pressures. The casing is made in two parts 10a and 10b that are sealingly mated to one another. To this end, the two parts of the casing 10a and 10b are formed with collars 10c and 1Od that are screwed or otherwise clamped to one another, and a sealing ring or gasket is sandwiched between them to ensure that the chamber remains watertight, even when under pressure.

An inlet 12 is provided to allow water to be admitted into the lower part 10b of the casing and the upper part of the casing has three discharge outlets 14a, 14b and 14c each controlled by a respective solenoid operated valve 16a, 16b and 16c. The purpose of the three separate outlets will be described below with reference to Figure 2.

The lower part 10b of casing 10 is also connected to a chamber 18 which houses a pressure sensor. The pressure sensor is represented schematically in dotted lines in Figure 1 as comprising a piston 20 and a spring 22, the space within the chamber 18 to the left of the piston 20 being vented to the ambient atmosphere. As a result, the pressure in the casing will drive the piston 20 to the left, as viewed, to compress the spring 22, and a switch may be provided to produce an electrical signal when a threshold pressure is reached, as indicated by a given displacement of the piston 20.

Also mounted within the chamber 10 is a heating element that comprises a heated coil part 24 and two cold legs 26a and 26b. The heating element is constructed in the same manner as a kettle element, that is to say it comprises a resistance wire encased within, and electrically isolated from, a metal outer tube. The electrical insulation surrounding the resistance wire is usually a compressed mineral powder that has high thermal conductivity but poor electrical conductivity. The resistance wire is arranged only within the heated coil part 24 of the heating element, the internal wires within the legs 26a and 26b being highly conductive so that the legs are not heated when current flows through the heating element. The legs 26a and 26b pass through and are sealed relative to two tubular extensions 30a and 30b of the casing part 10b and electrical connections are made to the heating element outside the casing.

The legs 26a and 26b of the heating element are also used to locate a baffle 32 within the lower casing part 10b above the water inlet 12. The baffle 32 can be of any suitable shape to ensure a steady rise of water through it and it would suffice, for example, for the baffle to be a solid plate that defines an annular gap around the perimeter of the heating chamber for the upward passage of water.

Referring now to Figure 2, the water circuit of a beverage dispensing machine comprises, in addition to the heater described above, a water tank 40, a pump 42 and a pressure regulating valve 44 connected between the pump 42 and the inlet 12 of the heater. The regulating valve 44 shuts off when the pressure in the heating chamber reaches a level set by a spring 46 and excess water from the pump 42 is then recycled to the tank 40.

At the discharge end of the heater, the solenoid valve 16a controls the flow of water to a beverage sachet within which a beverage is brewed. The solenoid valve 16b is a recycling and safety valve which returns water to the tank 40. Lastly, the solenoid valve 16c controls the flow of water to a jet which is aimed directly into the dispensing cup without flowing through the beverage sachet.

During operation, the heating element serves not only to heat a cupful of water to its boiling point but also to preheat the water in the tank 40 to a temperature of around 5O⁰C. Such preheating of the water in the tank reduces the time required to raise the temperature of the water to near boiling point during a dispensing cycle.

During pre-preheating, the valve 16b is kept open, the pump 42 is turned on and the heating element is operated to heat the water as it circulates in a closed loop. When the water in the tank attains the desired temperature, the heating element is switched off. In the mode of operation, the heater acts as a continuous flow heater, not a batch heater.

To boil a cupful of water, the pump 42 is operated while all three of the valves 16a, 16b and 16c are kept closed. When the pressure in the heating chamber reaches the value preset by the spring 46, the valve 44 is shut off and isolates the heating chamber from the tank 40. The heating element is next operated to boil the water in the top part 10a of the heating chamber. As the water approaches its boiling point, which may be more than 100⁰C on account of the fact that the pressure in the heating chamber may be above atmospheric pressure, the pressure in the heating chamber rises and moves the piston 20 of the pressure sensor against the action of the spring 22.

When the piston 20 reaches a preset position, the heating element is switched off and the valve 16a is opened. As water is discharged to a beverage sachet, the pressure in the heating chamber drops and the valve 44 is again opened.

Water is now pumped into the heating chamber by the pump 44. Water enters the lower part 10b of the heating chamber and rises past the baffle 32. The baffle ensures a smooth flow so that the interface between the hot and the cold water is not disturbed. The rising cold water thus displaces the boiled water, which then flows past the valve 16a towards the beverage sachet. The purpose of the valve 16c is to allow water to enter the dispensing cup without flowing through the beverage sachet. This allows the strength of the beverage to be controlled and directing a jet of water into the dispensing cup can also be used to cause the beverage to froth, which may be desired on occasions.

The valve 16b that is maintained open during preheating of the tank 40 can also be used as a safety valve by relieving the pressure in the heating chamber and returning water to the tank 40 in the event that the pressure or temperature in the heating chamber reaches unacceptably high levels on account of a malfunction.

It is an important advantage of the described apparatus that the pressure sensor is not exposed to the boiling water and its reliability is not compromised as it is only called upon to operate when cold.

An advantage of using a vertical heating chamber is that any sediment resulting from the use of hard water will collect at the bottom of the heating chamber and will not be dispensed with the boiled water. A plug 34 at the bottom of the casing 10 allows any sediment that collects in the chamber to be drained off from time to time.

It will be clear to the person skilled in the art that various modifications may be made to the above described embodiment of the invention without departing from the scope of the invention as set out in the appended claims. For example, in place of a piston 20, the pressure sensor may use a rolling diaphragm to reduce the risk of leakage.

It should also be mentioned that though the invention has been described by reference to a water heater for use in a beverage machine, it can be used in other situations where it is required to bring a cupful of any liquid to its boiling point in a short time.

Claims

1. A batch liquid heater for use in a beverage dispensing machine to dispense a discrete quantity of a liquid at or near its boiling point during each dispensing cycle, comprising a vertical elongate heating chamber (10), an inlet (12) near the bottom of the chamber (10) for admission of cold liquid into the chamber, an outlet (14a) near the top of the chamber (10) for discharge of hot liquid from the chamber, a heating element (24) arranged within the chamber (10), and valves (44,16a) for closing off the inlet (12) and the outlet (14a) to allow the pressure in the chamber (10) to rise as the liquid temperature rises, characterised in that the heating element (24,26) comprises resistance wire encased within a metal tube filled with an electrically insulating material.

2. A heater as claimed in claim 1, wherein the outer tube of the heating element comprises a coiled portion (24) and two straight parallel legs (26a, 26b) at the ends of the coiled portion, the length of the central conductor encased within the coiled portion being resistance wire and the lengths encased within the straight legs (26a, 26b) having low electrical resistance so that only the coiled part of the element (24) is heated when electrical current passes through the central conductor.

3. A heater as claimed in claim 1 or 2, wherein the walls of the heating chamber are formed of a plastics material.

4. A heater as claimed in claims 2 and 3, wherein the legs (26a, 26b) of the heating element pass through the base of the hearing chamber and the heated coiled portion of the heating element is positioned at a distance above the base of the heating chamber whereby the liquid in the lower part of the heating chamber is not heated by the heating element.

5. A heater as claimed in any preceding claim, wherein a pressure sensor (20,22) is provided beneath the level of the liquid heated by heating element to measure the pressure of the liquid in the chamber.

6. A heater as claimed in claim 5, wherein means are provided for terminating a heating cycle by discontinuing the supply of current to the heating elements and dispensing the heated liquid, when the pressure sensed by the sensor exceeds a preset level.

7. A heater as claimed in any preceding claim, wherein a baffle (32) is provided within the heating chamber

(10) above the inlet to minimise turbulence at the interface between the hot and the cold liquid at the end of a heating cycle, allowing the hot liquid to be displaced by the cold without any significant degree of mixing taking place.

8. A heater as claimed in any preceding claim, in which s drainage plug is provided at the lowed end of the heating chamber to allow any sediment in the heating chamber to be withdrawn.