Field of the invention

The invention relates to a cooking pot.

Description of the related art Cooking pots are commonly used for cooking and/or frying of food. Cooking pots have in most cases a bottom being surrounded by ring like wall structure. In some cases a lid may be positioned on top of the wall structure, closing the cooking pot. When cooking heat from a heat source, typically a cooking plate has to be transferred via the bottom to the food, e.g. a soup, vegetables or meat, inside the cooking pot. In particular if used on an electrical cooking plate it is essential that the lower side, i.e. the cooking plate surface of the cooking pot remains planar, to provide good thermal contact between the cooking plate and the cooking pot. However, due to thermal stress the bottom of a cooking pot tends to bend like a dome (or an inverted dome) and accordingly the thermal contact between the cooking pot and the cooking plate is reduced. To reduce this bending, sandwich bottoms have been suggested. The layers of the bottom have been chosen on the one hand to provide a good thermal conductivity, e.g. by using copper or aluminum layers. Stability has been accounted for by steel, in particular stainless steel layers. In most cases the stainless steel layers form the two boundary layers of the sandwich bottom. The layers have been combined, such that the bending forces of the different layers compensate each other. A method for combining different layers to a sandwich bottom of a cooking pot is disclosed in

EP 2 468 156A1

US 2013/056474A1 discloses a cooking pot with a bottom to which a double lay- er wall structure is attached. The space between the two layers of the double layer wall structure is evacuated to reduce energy transfer from inside the cooking pot to its surrounding.

Q.U reports in the US-patent application US 6,132,823 a hollow rod like heat conduction constructional element having an extremely high thermal conductivity. The interior of the rod is coated with three basic layers, the first layer being a combination of sodium, beryllium, a metal such as manganese or aluminum, calcium, boron and dichromate radical; the second layer formed over the first layer and being a combination of cobalt, manganese, beryllium, strontium, rhodium, copper, 13-titanium, potassium, boron, calcium, a metal such as manganese or aluminum and the dichromate radical; and the third layer formed over the second layer and being a combination of rhodium oxide, potassium dichromate, radium oxide, sodium dichromate, silver dichromate, monocrystalline silicon, beryllium oxide, strontium chromate, boron oxide, 13-titanium and a metal dichromate, such as manganese dichromate or aluminum dichromate. A theoreti- cal understanding for the extremely well heat conductivity of the heat conduction constructional element was not obtained yet.

Summary of the invention

The problem to be solved by the invention is to provide a cooking pot having an improved heat transfer between the cooking plate facing side and the up facing side of its bottom.

The invention is based on the observation that the coating according to Qu's method imposes difficulties as there is an incompatibility between the typical cooking pot material (stainless steel) which is to be coated and the coating layers. The problem is solved by the cooking pot of claim 1. Advantageous embodiments are subjects of the dependent claims. The cooking pot has as usual a bottom, to which a wall structure is attached. The wall structure encloses a space above the bottom. Food may be put into the space for heating it. Different from the prior art cooking pots the bottom of the cooking pot according to the invention preferably has a hollow evacuated com- partment. Evacuated means here that the pressure inside the hollow compartment is lower than ambient pressure and preferably lower than 10 ⁴ Pa. Preferably, the inner surfaces enclosing the compartment are coated with inorganic compounds to thereby obtain an optimized heat transfer.

For coating, an aqueous solution of several inorganic salts may be inserted into the compartment. The aqueous solution is also referred to a coating liquid. Subsequently the compartment may be evacuated and closed, i.e so to speak sealed and preferably heated.

The aqueous solution may preferably comprise (per 100ml of water (H ₂ 0)) at least one of the following groups of ingredients: II. First group of ingredients (List 1):

Ingredients Formula Amount

1 Sodium peroxide Na ₂ 0 ₂ 0.12g

2 Beryllium oxide BeO O.Olg

3 Potassium

K ₂ Cr ₂ 0 ₇ 0.2g

dichromate

4 Calcium dichromate CaCr ₂ 0 ₇ -3H ₂ 0 0.2g

5 Boron oxide B ₂ 0 ₃ O.lg

List 1

III. Secc >nd group of ingredien ts (List 2) Ingredients Formula Amount

6 Cobaltous oxide Co ₂ 0 ₃ O.OOlg 7 Strontium chromate SrCr0 ₄ O.lg

8 Potassium

K ₂ Cr ₂ 0 ₇ 0.6g

dichromate

List 2

VI. Third group of ingredients (List 3)

Ingredients Formula Amount

9 Sodium dichromate Na ₂ Cr ₂ 0 ₇ 0.2g

10 Silver dichromate Ag ₂ Cr ₂ 0 ₇ O.OOlg

11 Potassium

K ₂ Cr ₂ 0 ₇ 0.6g - 0.8g

dichromate

An example for preparing a coating liquid, i.e. an aqueous solution for subse- quent coating of the hollow compartment of the cooking pot is set out below:

(1) Dissolve in the given order and one after the other the ingredients of list 1 in the given amount in preferably ultra pure water (resistivity preferably p >18.2 ΜΩ cm at 25°C).Use for example an Erlenmeyer conical flask. After adding each ingredient to the water, stir the water for at least lOmin. (2) After adding all ingredients of list 1, keep stirring for at least 30min.

(3) Dissolve in the given order and amount and one after the other the ingredients of list 2 into the solution. After adding each ingredient to the water, stir the water for at least 5 min.

(4) Dissolve the ingredients of list 3 one after the other in the given amount and order to the solution under permanent stirring. After dissolving the ingredients, keep stirring for at least 40min. (5) Store the obtained solution, subsequently referred to as coating solution, in an airtight sealed container under ambient temperature (temperature 5-30°C). Avoid contact of the liquid to air and in particular to oxygen.

The such obtained aqueous solution may be used for coating the hollow com- partment as set out below:

1. Prepare the compartment inside the cooking pot's bottom made of a metal like aluminum, stainless steel (e.g. material number 1.4301/ composition: X5CrNil8-10 or material number 1.4404 /composition

X2CrNiMol7-12-2) or carbon steel, such that is cleaned from dust, oxides oil and the like. This can be accomplished for example by first cleaning disc like layers, being spaced by an as well cleaned ring like spacer, and subsequently weld or bond the disc like layers and the spacer to form a disk like bottom. Thus one obtains a closed hollow compartment inside the cooking pot's bottom.

2. Prepare at least one injection opening having for example a diameter of e.g. about 3 to 8mm. The diameter can vary (e.g. 1mm to 5cm) dependent on the volume of the compartment being enclosed by the inner surface and the size of the inner surface. The injection opening is preferably prepared before cleaning the inner surface and before closing the profile, to avoid that drilling chips reside inside the intermediate product. The injection opening can be drilled for example into the spacer before the cleaning step.

3. An injection pipe may be connected to the opening by e.g. welding or bonding.

4. Evacuate the compartment, i.e. the compartment to a pressure of about 10 ⁴ Pa or lower. To this end a vacuum pump can be connected to the pipe. Now the pump and the compartment are in fluid communication. After the evacuation of the space, the pump can be disconnected from the pipe.

5. Inject the coating liquid prepared as explained above into the compart- ment, for example via the pipe. The volume of the injected liquid should be between 1000ml and 800ml per m ² of the surface of the compartment. For example using a cylindrical bottom with diameter of 30cm and height of 1cm has an inner surface of about 0.865m ² . One should thus insert between 865ml and 692ml of the aqueous solution. To this end one may use an injector being connected to the pipe. The pipe may preferably be connected to a valve. A very simple, but efficient possibility is to connect a rubber hose to the pipe and clamp it to keep the vacuum. The injector may be coupled to the rubber hose and the clamp can be opened. Thereby, the coating liquid is drawn into the compartment due to the low pressure inside the compartment.

6. After inserting the coating liquid, one may optionally evacuate the compartment again to at least about 10 ⁴ Pa.

7. Seal the injection opening preferably permanently, for example by clamping the root of the pipe using mechanical tools which may be actuated hydraulically or pneumatically. One may close the whole as well by welding.

When first heating the such prepared thermal conductor, the liquid inside the compartment coats the surface of the compartment. Description of Drawings

In the following the invention will be described by way of example, without limitation of the general inventive concept, on examples of embodiment with reference to the drawings. Figure 1 is a flow diagram explaining preparation of the coating liquid.

Figure 2 is a flow diagram illustrating the method for manufacturing the cooking pot.

Figure 3 is a sketch of a cooking pot according to the invention.

An example for preparing a coating liquid is explained with respect to Fig. 1. In a first step (Step 1), 100ml of ultra pure water (resistivity preferably

p> 18.2 ΜΩ cm at 25°C) is filled in an Erlenmeyer conical flask at standard laboratory conditions. Subsequently the chemicals (ingredients) as listed in List 1 are dissolved under stirring one after the other in the given order as listed and in the given amounts. After having inserted a chemical and before insertion of the sub- sequent chemical of List 1 the solution is stirred for at least 10 min.

Subsequently the obtained solution is further stirred for about 30 min.

In Step 2, the chemicals (ingredients) as listed in List 2 are dissolved under stirring one after the other in the given order as listed and in the given amounts. After having inserted a chemical and before insertion of the subsequent chemical of List 1 the solution is stirred for at least 5 min.

In the next step (Step 3), the chemical as listed in List 3 are dissolved in the given order under continuous stirring in the given amounts.

In the last step the flask is sealed and the coating liquid can be stored an ambient conditions (5°C≤ T≤30°C). The procedure of manufacturing a heat conductor using the coating liquid as prepared according to the above example is explained with respect to Fig. 2.

The procedure starts with assembly of the bottom of the later cooking pot. First the an upper disc 11 a lower disc 12, and a ring like spacer 10, all e.g. of stainless steel are cleaned. The spacer 10 may have a whole 14 to which an injection pipe 16 may be attached. After cleaning, the two disks 11, 12 and the spacer 10 may be assembled and fixed to each other by e.g. welding or bonding, to obtain an airtight connection, i.e. the compartment is sealed and exchange of fluids is possible only via said whole 14. The wall may be attached to the bottom e.g. by welding or boding, in particular by induction welding.

An injection pipel6 is connected by welding or bonding to the opening 14. The other end of the pipe 16 is connected via a valve to a vacuum pump via tube 19 and the compartment of the cooking pot is evacuated to at least 10 ⁴ Pa. Now, the fluid communication of the tube 19 and the pipe 16 is closed by the valve. In- stead the compartment is connected to a container 20 (e.g. a beaker) with the coating liquid 30 until a predefined amount of the coating liquid 30 is sucked into the compartment. For an compartment having a diameter of 30cm and height of lcm m one should add about 800ml of the coating liquid. The connection between the container 20 and the pipe 16 is disconnected using the valve and the compartment is again evacuated to at least 10 ⁴ Pa by connecting the tube 19 with the pipe 16 using the valve. Subsequently the compartment of the rod 10 is sealed for example by clamping or welding the pipe, if possible close to the disc 12. This is symbolized in the detail by arrows. Subsequently the cooking pot should be heated. Figure 3 shows a further cooking pot example. In this example, not only the bottom, but as well the wall 5 has a double layer structure, i.e. bottom and wall enclose a compartment 13. The compartment 13 may be manufactured by adding an inner shell 31 and an outer shell 32, which are connected by e.g. welding at their respective upper rims 34. To enhance rigidity, supports 15 may be positioned between the two shells 31, 32. The compartment 13 between the inner shell 31 and the outer shell 32 is preferably coated with inorganic salts, in particular a coating liquid as prepared according to Fig. 1. The coating procedure is similar to the procedure as explained with regard to Fig. 2. The later inner surfaces of the two shells 31, 32 should be cleaned, prior to mounting them to form the cooking pot. For ease of coating the cooking pot has an inlet 24 to which a pipe like 16 can be attached. As already explained above in more detail, the compartment 13 is evacuated, a corresponding amount of coating liquid is filled in the compartment, the compartment may be evacuated again and is subsequently closed. Closing of the inlet 24 is rather simple as it is hidden behind a handle after mounting of the handle. This means, the handle is preferably mounted after closing the inlet 24. Like the cooking pot of Fig. 2 the cooking pot should be heated after the inlet 24 was closed, to ensure that the water evaporates and the inorganic compounds of the coating liquid attach to the inner surfaces of the compartment 13.

List of reference numerals

1 cooking pot

5 cooking pot wall

10 spacer

11 upper disc

12 lower disc

13 compartment

14 opening

15 support structure

16 pipe

18 tube to container 30 with coating liquid 20

19 tube to vacuum pump

20 container (e.g. beaker)

30 coating liquid

31 inner shell

32 outer shell

33 handle

34 rim