Electric kettles are irreplaceable helpers in making tea or coffee, quickly getting boiling water and solving a number of household tasks. However, the danger of a heater fire makes many users vigilantly monitor their work. They additionally double-check the disconnection, make sure there is water, or unplug the plug. How safe are modern electric kettles and how their manufacturers provide peace of mind to users, we will consider in this article.
How does a thermostat work and what is it for?
Today, it is not necessary to watch the kettle at the time of boiling, so that it does not overheat from prolonged boiling and so that the ten is left without water. Instead of a vigilant user, this function is performed by a thermostat, which reacts to a certain temperature and cuts off the voltage supply to the heating element.
Electric kettles use a relay based on a bimetallic breaker. Structurally, it consists of a flexible plate fixed in a polymer body. The plate is brazed from two metals with different coefficients of thermal expansion.
When metals are heated, they expand, but each type of metal will increase in a different way. For example, there are two plates of copper and steel of the same length 10cm. At temperatures from 0 to 25 ° C, both segments are equal in length. Then we heat both plates to a temperature of 100 ° C, as you can see in the figure, the copper plate grows much more. The difference in the length of the heated plates Δx is due to the different coefficients of thermal expansion for the steel and copper plates.
For a thermal relay, plates of two metals are fastened together by soldering or riveting, forming a bimetallic plate. When it is heated to 100 ° C, the copper plate expands more by the length Δx, but the steel plate practically does not change. As a result of their attachment to each other, the bimetallic plate is deformed under the force of the elongated copper and will bend towards the metal with a lower coefficient of thermal expansion.
In the thermostat, the plate is clamped and rigidly fixed on one side. The second edge remains movable, and when the bimetallic element is heated, it moves to a distance Δy. It is the movement of the free end that performs the logical function of responding to temperature changes in the electric kettle.
The principle of operation of a thermal relay in an electric kettle is as follows:
Teng leads to heating of the liquid and surrounding parts. Heat energy spreads first through the water to the upper layers, and then with the rising steam.
The heated coolant gradually heats up all the parts of the kettle with which it interacts during movement.
The thermostat plate, which is in contact with the coolant, is also heated to the set temperature.
Upon reaching 100 ° C, the bimetallic plate bends sharply, turning the edge towards the least elongated metal.
The mechanical force exerted by the plate moves the lever out of the dielectric and switches the relay contacts.
The logical element goes from the on state to the off state.
As the bimetallic breaker cools down, the copper plate gradually decreases in length to its original dimensions. Steel, bent like a spring, straightens and returns the entire structure to its original state. And the relay plate is ready for heating again.
The kettle button is mechanically moved to the off position, so the return of the bimetallic disconnector to the initial position does not lead to reapplying voltage to the heating element.
Types of thermal protection in a kettle
The main task of an electric kettle is to heat the liquid, so in normal mode it can withstand 100 ° C or more. This range is normal and does not affect the physical state of the elements in any way and does not affect the taste of the water until its steady boil. But beyond the established limit, the process is pointless to continue, and the kettle itself (rubber seals, plastic parts and a heating element) will fail faster.
Thermal protection, which regulates heating processes in an electric kettle, includes two levels. The first interacts with hot steam rising from the upper layers of water to the lid. Its task is to control the state of the boiling liquid. The second is installed under the heating element and reacts to the heating element overheating.
Water heating control
This level of thermal protection is located directly at the on / off button and is represented by a thermal relay, the main component of which is a bimetallic plate matched to a certain temperature.
thermostat for water heating control
Structurally, it is installed in the upper part of the handle, where steam is supplied from under the teapot lid. When water boils, steam intensively affects the bimetallic plate and heats it up to 100 ° C.
the location of the thermostat in the handle of the kettle
The thermostat is triggered every time the kettle boils and ensures normal operation. The bending plate throws the button from the on position to the off position, where it is fixed.
Heating element overheating control
Most electric kettles also have a thermostat or two at once. It is located near the contact group and is connected to the supply loops.
The bimetallic plates of the thermal relay are in contact with the disc heater or the fifth heating element. In the normal mode, all the energy from the heating element is transferred to the heated liquid. Heat energy due to convection is redistributed in the container from the lower layers to the upper ones, making room for cold liquid near the heating element. The process continues until the water boils in the electric kettle. In this case, the maximum water temperature will be near the teapot lid, and the minimum at the bottom. And the heat energy from the heating element will be transferred to the liquid.
If water was not poured into the kettle or it had time to completely boil away, then heat energy cannot be quickly transferred from the heating element. This is due to the thermal conductivity of water and air.
Water is an excellent heat carrier because it quickly receives and gives off heat energy. Air is an excellent heat insulator – it does not accept heat well and gives it away just as badly.
The same situation is observed in the case of accumulation of a large amount of lime deposits. Teng will give off significantly less thermal energy due to poor thermal conductivity of the “sand” layer. The emergency situation will develop as follows:
The heat accumulating in the heating element will cause it to overheat.
Rubber gaskets and plastic parts that come into contact with the heating element will begin to deform.
In an incandescent heating element, the filament breaks or the ceramic layer breaks down onto the heating element body.
To prevent overheating and further ignition of the electric kettle, thermal protection against overheating of the heating element comes into play. Its bimetallic plates are in contact with the heating element and normally withstand the temperature to which the heating element is heated in normal mode.
If the temperature of the disc heater exceeds the set limit, then the bimetallic plate is deformed even before the heating is critical for the kettle. Its edge acts on the pusher and moves the thermostat contacts to the off position.
the bimetallic plate acts on the dielectric pusher
The power circuit will disconnect the heating element from the contact group and stop supplying voltage to the terminals of the heating element. The kettle will stop heating and the heating element will gradually cool down to a safe temperature.
Modern electric kettles equipped with thermal relays have “foolproof” protection. Therefore, if you plug them into the network without water, nothing threatens you or your property. Thermal protection will cut off the power in a timely manner, and the kettle will stop heating the air inside the container.
However, note that if you find an empty electric kettle plugged into the outlet, in no case rush headlong into pouring water into it. Reckless actions will not help the heating element of the device, but a sharp temperature drop will cause compression of the red-hot heating element. From this, both the heating element and the gaskets can crack, which will lead to depressurization of individual units and a reliable assistant will quickly fail.