1.1 Heta Transfer System
A thermal system used for heating flow of water is shown below.
Electrical heating element is provided in the tank to heat the water. The tank is insulated to reduce heat to the surroundings.
The necessary simplifying assumptions are:
1) There is no heat storage in the insulation.
2) All the water in the tank is perfectly mixed and hence at a uniform temperature.
θ = Surrounding temperature
qi = Rate of heat flow from heating element in J/sec
qt = Rate of heat flow through tank insulation.
C = Thermal capacity in J/oC, R= Resistance of thermal insulation.
The rate of heat flow from the water to the surrounding atmosphere through insulation is,
As per the heat transfer principles,
substituting equation (1) and (2)
Neglecting the term θ/R from the equation ( 4) this is because the variation of water temperature θo is over and above ambient temperature θω.
i) Thermal resistance : In the heat transfer system, the thermal resistance is defined as the ratio of change in temperature between the two substances to the change in heat flow rate.
It may be expressed in oC/J/ min.
ii) Thermal capacitance : The thermal capacitance is nothing but thermal capacity of the thermal system. It is the ratio of rate of heat storage to the rate of size of temperature of the tank.
It is the product of mass in kg and the specific heat of liquid in J/kg oC hence the thermal capacitance is measured in J/oC.
1.2 Thermometer
Consider a thermometer placed in a water bath having temperature , as shown in Fig.2.
θo is the temperature indicated by the thermometer. The rate of heat flow into the thermometer through its wall is,
The indicated temperature, rises at a rate of,
