Domestic applications By covering a body with a bad thermal conductor you can reduce the amount of heat that will leave the body.Air is a bad thermal conductor. Therefore if you can trap as much air as possible in the material around the body, it will be well insulated. A lagging jacket around a boiler in a hot press, insulation on an attic floor, foam in the walls of houses and double-glazed windows are all examples of domestic applications of insulation.
U-value
The U-value of a structure is the amount of heat energy conducted per second through 1 m2 of that structure when a temperature difference of 1 K is maintained between its ends. A good insulator has a low U-value. It has a unit of watt per metre squared per kelvin,W m2 K1.
Fig 9.9: Double-glazed windows
Air
Convection
CONVECTION Convection is the transfer of heat through a fluid by means of circulating fluid.
When the fluid is heated, it expands and becomes less dense. It rises through the cooler fluid above it.This movement of fluid is called a convection current.
SCIENCE,TECHNOLOGY & SOCIETY
Domestic hot water and heating system
Convection plays a part in the domestic hot water system. Hot water in a domestic hot water tank rises by convection to the top of the tank. The hot water outlet is at the top of the tank. The hot water circulates through the house by convection currents. In practice a pump is included, as convection currents alone do not cause the water to circulate efficiently.
