WO2008125947A1 - Steam generator - Google Patents

Abstract

A steam generator (11) of a food steamer is provided. The steam generator (11) has multiple power settings and a steam utlet (18).The steam outlet(lS) comprises a variable orifice and a movable valve member (30). The valve member (30) is directl responsive to boiler pressure to vary the vent capacity of the outlet (18).

Description

STEAM GENERATOR

This invention relates to a self-contained food steamer of the kind comprising a steam generator and a cooking chamber, and is particularly concerned with improving efficiency of steam generation.

A typical self-contained food steamer is electrically powered and comprises a water boiler with one or more food vessels stacked above. Steam from the boiler permeates through apertures in the base of the vessel(s) to cook food placed thereon.

A flash or instant boiler, for example a tubular boiler, is typically provided; such boilers are efficient and can be arranged to direct steam flow to a precise target.

One problem with food steamers is to reduce the start-up period during which the food vessels are brought up to temperature by steam generated in the boiler. During this period cooking is not very predictable because steam may condense on cold food vessels. Furthermore the presence of food in the vessels during this period may also introduce uncertainty.

Ideally the start-up period should be as short as possible, and for this purpose a powerful boiler may be utilized. However on reaching working temperature, the required rate of steam production falls significantly, so that a powerful boiler will waste energy by producing too much steam. Excess steam does not speed-up cooking and so ideally the steaming rate during this period should be quite low. Moreover, excess steaming has the significant disadvantage of rapidly depleting the water reservoir. A large reservoir increases cost of manufacture in addition to increasing the start-up time if the boiler is of the immersed type.

One solution to this problem is to utilize a thermostat to control steam production. However steaming ideally requires a steady and continuous supply of steam, whereas a thermostatic control inevitably results in cycling of temperature according to the hysteresis characteristic. Frequent on/off boiler cycles may shorten boiler life and cause the boiler to produce a mixture of steam and water at each energization. These factors are undesirable.

Another solution is to provide dual boilers so as to ensure increased steam volume during the start-up period, one boiler being switched off by a suitable sensor when the steamer reaches a predetermined temperature. This solution results in increased costs due to additional components, and may also be somewhat difficult to arrange within the water boiler.

Another problem with the dual boiler solution is that the steam outlet must be sized for steam flow at full power. It will be understood that when one boiler is switched- off the cross-sectional area of the outlet is too large to maintain the desired pressure within the boiler, and accordingly a mixture of steam and water tends to be emitted.

Conversely an outlet sized for a single boiler will likely cause excessive internal boiler pressure if both boilers are operating, with a consequent increase in temperature and triggering of the usual high temperature cut-out. The same applies if a single boiler is driven at increased power.

A better solution is required, and preferably a solution which provides continuous steaming without reliance on electrical sensors and the like.

According to the invention there is provided a steam generator for a food steamer, said generator comprising a water boiler having a variable power setting, said boiler having a steam outlet, and said outlet having a moving element restrictor therein, wherein said outlet comprises a duct, and said moving element comprises a valve member reciprocal within said duct, wherein said duct comprises a tube, and said valve member comprises a piston within said tube, and wherein said tube is in use vertical, and said piston is urged by gravity to a minimum vent condition, in use said moving element being directly responsive to boiler pressure to control the venting capacity of said outlet, and thereby to maintain boiler pressure between predetermined limits. By instant boiler we mean a boiler which instantaneously raises a small volume of water to boiling temperature, rather than raising the entire volume of water to boiling, as in a kettle. A typical instant boiler is of the electrical tubular type with a water entry at one end and a steam outlet at the other end. The tube comprises a heating element having a large surface area. Tubular boilers can operate continuously and are adapted to raise steam very quickly.

The moving element comprises a reciprocal piston which increases the vent area of said outlet in response to an increase in boiler pressure, and reduces said vent area in response to a reduction in boiler pressure.

The invention thus allows a steam generator to be operated at high power, with a correspondingly large area outlet, during a start-up phase; and at a lower power, with a correspondingly smaller area outlet, during a steady state cooking phase. Steam pressure within the boiler is regulated by the moving element to a value large enough to give high quality steam, but not so large as to cause over-pressure in the boiler, or overheating.

In one embodiment the water boiler includes two electric heating elements, one of which is switched-off during the steady state cooking phase.

Other features of the invention will be apparent from the following description shown by way of example only in the accompanying drawings in which: -

Fig. 1 is a schematic representation of a conventional food steamer.

Fig. 2 is a transverse cross-section through a steam generator incorporating the invention.

Figs. 3-5 are transverse cross-sections through a steam valve of the invention in various conditions of use.

With reference to Fig. 1, a schematic food steamer 10 comprising a steam unit 11 on which is stacked a plurality of cooking vessels 12, 13, 14 and a lid 15. Within the steam unit is a boiler 16 immersed in a water chamber 17 and having a steam outlet 18 projecting into a steam space 19 below the lowermost vessel 12. Steam permeates upwardly through apertures in the base of each vessel 12, 13, 14 (as indicated by arrows), and excess steam exits via openings in the lid 15.

Fig. 2 illustrates a typical steam unit having a water chamber 17 containing dual electrically heated flash boilers 20, 21 and a single steam outlet 18 from which steam issues in use as illustrated by arrows 22.

Figs. 3-5 show a valve embodying the invention and located in the steam outlet 18.

The upper wall 31 of the casing of the water chamber defines an upright tubular vent 32 having an integral cap 33 and circumferential openings 34 immediately adjacent the cap 33. As illustrated the openings are radial, but the edge of the cap is relieved at the outer side to permit generally upward exit of steam, as indicated by the acute angle 35a.

Within the tubular vent is a piston-like valve member 30 movable axially between a lower abutment 36 and the cap 33. The valve member 30 is of two-part construction and comprises a cylindrical body 37, of e.g. stainless steel, having feet 38 for contact with the abutment 36, and a tubular cap-like insert 39 of plastic at the end opposite the feet, which defines a plurality of radial outlets 41. These outlets 41 are of similar form to the openings 34 being adjacent the capped end and relieved to allow generally upward exit of steam from within the body 37 via the outlets 41 and openings 34. The two part construction of valve member 30 allows a good response to changes of pressure, and is convenient for manufacturing.

Fig. 3 shows the rest position of the valve member 30, in the lowest position with feet 38 resting on the abutment 36. In this position steam from within the boiler can exit through a fan-like gap indicated by the angle 35a.

As heat builds up in the boiler, for example due to operation of a single heating element, the restrictive exit defined by angle 35a is insufficient to allow all steam to escape. Steam pressure thus lifts the valve member 30 to the position of Fig. 4 in which the exit is less restricted, as indicated by fan-like acute angle 35b.

Further build-up of pressure, for example due to operation of a second heating element causes the valve member 30 to rise to the position of Fig. 5, in which the steam exit is still less restricted, as indicated by fan-like exit angle 35c.

It will be appreciated that the exit angle 35a, 35b, 35c opens progressively according to the position of the valve member 30, and that the valve member 30 rises in proportion to increasing steam pressure. Thus is provided a self-regulating steam valve whereby pressure within the boiler can be maintained at a substantially constant level regardless of whether one or two heating elements is energized.

It will be understood that the circumferential extent of the opening which defines the steam exit may be varied to suit the required steam volume and generation capability, this opening being approximately proportional to required steam volume for a given boiler pressure.

Variation of the steam exit configuration is of course possible within the scope of the claims of the present invention, and the particular configuration illustrated in Figs. 3-5 should be considered illustrative. Other kinds of self-compensating moving element restrictor are envisaged; however the illustrated embodiment is presently preferred.

The preferred embodiment is described with two electrical heated flash boilers; however the skilled man will realise that the invention is not limited to two heaters, but can be adapted to a single heater or to more than two heaters, within the scope of the claims appended hereto.

Claims

1. A steam generator for a food steamer, said generator comprising a water boiler having a variable power setting, said boiler having a steam outlet, and said outlet having a moving element restrictor therein, wherein said outlet comprises a duct, and said moving element comprises a valve member reciprocal within said duct, wherein said duct comprises a tube, and said valve member comprises a piston within said tube, and wherein said tube is in use vertical, and said piston is urged by gravity to a minimum vent condition, in use said moving element being directly responsive to boiler pressure to control the venting capacity of said outlet, and thereby to maintain boiler pressure between pre-determined limits.

2. A generator according to claim 1 wherein said piston comprises radial apertures adjacent the crown thereof.

3. A generator according to claim 2 wherein said apertures extend partially through said crown.

4. A generator according to any of claims 1 to 3 wherein said tube comprises radial vents adjacent the end thereof.

5. A generator according to any of claims 1 to 4 wherein said tube includes a stop at the open end thereof to define the maximum vent condition of said piston.

6. A generator according to claim 5 wherein said stop comprises a crown of said tube.

7. A generator according to any preceding claim and comprising one or more electrical heaters associated with said boiler, and control means to energize one or more of said heaters on demand.

8. A generator according to claim 7, and having dual electrical heaters.

9. A food steamer comprising a steam generator according to any preceding claim.

10. A steam generator of a food steamer substantially as described herein with reference to the accompanying drawings.