WO2009041872A1

WO2009041872A1 - Steam generator for ovens

Info

Publication number: WO2009041872A1
Application number: PCT/SE2008/000520
Authority: WO
Inventors: Martin Holmer; Francies Williams; Örjan RING
Original assignee: Power Steam Generator I Göteborg Ab
Priority date: 2007-09-24
Filing date: 2008-09-21
Publication date: 2009-04-02
Also published as: EP2205098A4; EP2205098A1

Abstract

Steam generator, in particular for ovens, for the generation of steam with essentially atmospheric pressure, comprising a pump (2), a cylindric water reservoir housing (16) and a heater (4), connected in series in a loop. The heater has a smaller water space than the reservoir and has a heating power sufficient to heat the water circulated by the pump to steam generating temperatures, but less than what would be required for turning all the water passing through into steam. The flow cross section for the water inside the heater is small enough to provide a high speed of the water in the heater in order to prevent or essentially eliminate particles from settling in the heater.

Description

Steam generator for ovens

This invention is concerned with the generation of steam, in particular for ovens.

Steam generators in ovens are used to prevent the dehydration that otherwise frequently occur when heating, frying or grilling food. The food will in this way be juicier and more easy to eat. The water fed to these steam generators and vaporized there is ordinary mains water, which often contain calcium that with time is deposited in the steam generator in particular on the objects with the highest temperature. Finally the steam generator will be clogged and has to be exchanged and already before this its efficiency will be lessened due to impaired flow through and in particular the reduced heat transfer from the heating elements to the water through the deposited minerals, in particular limestone. The object of the steam generator in accordance with the invention is to reduce the risk for deposits of lime or other substances dissolved in the water.

In accordance with the invention the above object is solved by, pumping the water passed or through the heat generating or transferring element or elements at high speed.

This allows a balancing of pumping rate and heating rate such that the water circulate many times more than the evaporation rate. This allows both a high flow rate and keeps elements and tubes clean - and ensures that the water does not evaporate in he tubes. High heating density can be used without significant evaporation in the tubes blocking the pumping action.

The fast flow of water and some steam efficiently pull along the particles emanating from the heating of the water that could otherwise settle on the heating element or elements.

The particles in the water is then removed or collected from the advantageously circulating water and the generated steam fed to the interior of the oven.

Preferably the water is heated in tubes that may advantageously be helical in shape. The heat energy may be generated inside the tubes, by means of for instance an electric heating cable that runs along the tube. Alternatively the heat can be provided from the outside by a burner or by electricity using resistance, induction or micro- waves. It is also possible to use tubes that are integrated with electric heating.

The water flow should in relation to the heating energy be sufficient to prevent the water from entirely being turned into steam. Overheating may otherwise occur destroying the electric heating and also the flushing away of the mineral particles will become less efficient.

In reality the amount of water passing in a given time should be many times the amount that in the same time can be evaporated by the heating elements.

Below the above indicated upper power limit however the heating power may be considerable, in particular in relation to the amount of water present in the tubes. This means that in the few seconds it take for water to pass through a tube a limited percentage of water will turn into steam before or at leaving the tube, but since the amount of water passing is considerable a high steam production will be obtained. This also means that steam generation can be initiated very quickly. The delay time is essentially caused by the inertia in the heating of tubes and heating devices. This is a great advantage since you only have to wait seconds before you have steam, which can be compared with known oven devices where you have to wait minutes.

Also the generation of lime particles will be concentrated to the heated tubes which simplify collection and removal of these. In its simplest form particles of for instance lime can be allowed to sink to the bottom of a combined sedimentation tank and water reservoir that at intervals can be cleaned. In an improved further development of the invention separation of water, lime and steam is enhanced by means of powerful swirling motion of the water in the top of the water reservoir. This rapid swirling motion of the water cause the heavier particles as lime to gravitate towards the wall of the reservoir and the steam toward the center thereof. The water outlet to the pump is preferably arranged in the center of the reservoir and a distance above the bottom of the reservoir so that the amount of lime particles in the water delivered to the heating will be small. The steam is led off through an opening in the top of the reservoir.

In a very advantageous development of the invention said combined reservoir and separating sedimentation tank is shaped as a cylinder and arranged in the space inside of the heating elements, which may be constituted by tubes. This gives a short distance for the water to travel from the heater to the separator. When the water from the tubes enter into the reservoir through essential tangential inlets the water will experience a pressure drop freeing the steam an effect that is believed to be enhanced by the swirling motion in the reservoir. At the same time already precipitated particles in the water as well as particles precipitated when entering into the top of the reservoir will simultaneously with the steam evaporation travel towards the wall of the reservoir. This means that lime particles and the like will be removed from the water almost instantly before they have a chance to pollute the generator as a whole unit. As the water sink towards the bottom the swirling subside and the lime particles fall along the wall down towards the bottom of the reservoir. Further developments of the invention are apparent from the subclaims and the following description of embodiments of the invention, with reference to the drawings. In the drawings fig. 1 depicts a steam generator without outer heat insulation, fig 2 same view as fig 1 but with some parts of the heater tubes removed and fig 3 a steam generator that is slightly different on the inside. The steam generator shown in figs 1 and 2 is provided with three helically shaped tubes 4 that in their ends are provided with T-shaped connectors 14 through which cable like heat elements 3 extend into the tubes, through these and out in the other ends. The T-shaped connectors also connect the tubes to a pump 2 in the lower end and to a separation device arranged centrally inside the tubes. The T-connectors 14 in the upper ends of the tubes are connected to entrance tubes 15 that open 7 tangentially into the upper end of a central cylindric reservoir housing 16. These entrance tubes 15 may instead of being tangential relative the inside of the cylindrical housing extend into the housing and be bent slightly downward and tangentially along the inside of the housing. From a top of the cylindrical housing 16 a short, vertical, central cylinder 9 extends a short distance into the cylindrical housing, ending shortly below the inner ends of the entrance tubes and serves as an outlet for the steam to the oven. Sieve like elements 12, 13 or perforated discs are arranged in the cylindrical outlet and serve to collect surplus water from the steam. When the water together with entrapped lime particles and steam exit the entrance tubes

15 it will with high speed follow the inside of the cylinder in a swirling or rotating movement that slowly descends downward. During this movement the entrapped steam, being lighter than water will emerge on the inside of the swirl, while the lime stone particles that have a higher density than water will end up close to the inside wall of the cylinder. Gradually the speed of the water is reduced as it descends toward the bottom of the cylinder, the inside of the water body having, where the swirl is, a concave parabolic shape in the top.

The speed with which the water enters the above device is higher than that provided by the pump since the successive generation of steam in the helical tubes 4 increase the volume of the mixture, and thus its speed increase continuously. This high speed will not only provide good separation but will also increase the ability of the water to sweep away particles. At least the majority of the lime particles sink down along the inner wall of the cylinder into an annular space between a central outlet tube 5 and cylinder wall.

The pump 2 is a centrifugal pump with three evenly spaced outlets, one for each helical tube providing the same flow through each tube even if the flow resistance vary, making certain that the flow in all the cylindrical tubes is sufficient to prevent overheating and destruction.

To compensate for the water that leaves the steam generator in the shape of steam fresh water has to be added. This fresh water is when needed intermittently added through one or several inlets 11 in the annular space between the wall of the cylindric housing 16 and the central outlet 5 to the pump via a valve from a water mains. The rather frequent but low speed adding of fresh water will continuously slightly stir the collected limestone particles preventing the sintering of these. The adding of water is controlled by a sensor device 6 arranged a distance up in the cylinder. The sensor device may for instance be of a resistance measuring type.

The inlet or inlets may be directed tangentially or a shield may be arranged above to secure a repeated limited stirring of the lime particles but a minimum of lifting of the lime particles. In the same annular space as the water feed inlet a valve provided water outlet 10 or several are arranged. To the inlet an additional valve with higher capacity or unrestricted flow is arranged from the water mains. Every now and then when heating and circulation are turned off the lime sludge in the bottom is flushed out by opening the large inlet valve and the outlet valve. A separate inlet for flushing may also be an alternative. In fig 3 an alternative embodiment is shown, where no tubular steam outlet extends down into the reservoir, instead an inner spiral flange 19 is arranged on the inner wall of the cylindric reservoir housing 16 leading water in a spiral downwards.

If an increased separating centrifugal force is desired one can at the top of the reservoir provide a swirling space with smaller diameter, for instance in the shape of a lid or insert integrated with the inlet tubes.

The invented steam generator does not only alleviate the problem with scaling but provide also a very rapid heating up eliminating the need to heat all water in the entire generator and maintain it at 70⁰C as in the state of the art.

Instead of using tubes at the heating channels with narrow and continuous cross section channels can be made with an inner cylindric core provided with external helical groves and two halves of a surrounding mantle, likewise provided with corresponding groves that coincide with the grooves in the central core. Heating can be with one cable in each channel or by an external heating of the channels.

Claims

1. Steam generator, in particular for ovens, for the generation of steam with essentially atmospheric pressure, characterized in comprising a pump, a water reservoir and a heater, connected in series in a loop, the heater having a smaller water space than the reservoir.

2. Steam generator according to claim 1, characterized in having heating power sufficient to heat some of the water circulated by the pump to steam generating temperatures, but less than what would be required for turning all the water passing through into steam, and that the flow cross section for the water inside the heater is small enough to provide a high speed of the water in the heater in order to prevent or essentially eliminate particles from settling in the heater.

3. Steam generator according to claim 1 or 2, characterized in the heater comprising one or several tubes or cylindrical channels within which heating elements in the shape of heating cables are arranged.

4. Steam generator according to any of the claims 1 - 3, characterized in comprising tubes or channels that are heated from the outside by combustion or electricity.

5. Steam generator according to any of the preceding claims, characterized in the reservoir having tangential inlets from the heating elements and a cylindric shape at the inlets and a distance below causing the entering water to swirl or rotate causing a separation of steam and lime particles from the water inward and outward respectively.

6. Steam generator according to claim 5, characterized in the reservoir having a central cylindrical tube like outlet for water extending a distance up from the bottom of the water reservoir and an annular space between said cylindrical outlet and the reservoir wall for the collection of lime particles.

7. Steam generator according to any of the preceding claims, characterized in a water inlet for the providing of water to replace the water that is turned into steam, said water inlet being arranged in the bottom of a collection space for lime particles so that the fresh water slightly stirs the lime, preventing this from clogging.

8. Steam generator according to any of the preceding claims, characterized in comprising valve provided inlets and outlets for a flushing of the steam generator in order to flush out accumulated lime particles.

9. Steam generator according to any of the preceding claims, characterized in the heating tubes or channels being wound around the reservoir.

10. Steam generator according to any of the preceding claims, characterized in comprising several tubes and with one outlet from the pump to each tube or channel.