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Domestic Electric Heater/Geyser

Working principle (Physics)
     The Joules heating effect of an electric resistance is used to heat water.

   We use electric geysers at homes or in clinics or shops to heat water.  It is cylindrical shaped and is mounted on walls. It has a capacity of 50 to 250 liters.  See diagram.

   It consists of a water container vessel with an inlet for cold water and an outlet for hot water. The heater contains a thick (tubular) electric conductor (metallic resistance) coil element. It gets heated due to the dissipation of energy when current passes through it. The heat energy is transmitted to water through conduction and convection mechanisms. Usually the heating element is made of Nickel or chromium.

    There is a mechanism to measure the temperature of water using a thermostat. It shuts off, automatically, the current supply to the heating element, when the temperature is above a preset value. This is set in the factory.  There may be LEDs or small bulbs to indicate whether the heater is switched on or off.

   The water container is usually made of a material which does not get corroded or rusted easily.  The water container is protected with insulating material and air gap so that the heat energy is retained inside the container.  The materials for the outer casings are usually stainless steel, plastic and china glass. Plastic is safer to prevent from leakages and shocks.

Power and rating related information:

    The heater is usually based on AC voltage. It is given a rating : power it consumes, current it passes when the rated voltage is applied. Voltage rating V could be 220 V in countries like India and 110 V in USA. Power rating P could be 1 KW to 5 kW depending on size.  

    Heating Power P = V² / Resistance of heating coil.
    If the water tank capacity is L liters, then rate of heating is: 
              = P / (L * density * specific heat )  = P / ( L * 1 * 4181) = 
              = P / (4181 L)  °K / sec

    If we want to heat water from 20° to 50°, then it takes time T = 
                 T = 30 * 4181 L / P = 2000 L / P  minutes.

    If people could wait for 15 minutes for L litres , then
                   Power needed P = 2000 L /  L = 2 kW

Instant Heaters

    We get instant heaters also. They have a small capacity (2 litres). The water is heated at a constant rate as the water flows through the geyser.  There is no storage and not much insulation provided in this type of heaters.  A heating (electric resistor) coil is used in them. The wattage could be around 2 kW.  Care must be taken as if there is not enough water, it may burn the coil.  Flow of water can be decreased or increased to have hotter or colder water.

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