Mechanical Engineering Principles

218 MECHANICAL ENGINEERING PRINCIPLES

(ii) The metal of a radiator of a central heating
system conducts heat from the hot water inside
to the air outside.

Convection

Convectionis the transfer of heat energy through a
substance by the actual movement of the substance
itself. Convection occurs in liquids and gases, but
not in solids. When heated, a liquid or gas becomes
less dense. It then rises and is replaced by a colder
liquid or gas and the process repeats. For example,
electric kettles and central heating radiators always
heat up at the top first.
Examples of convection are:

(i) Natural circulation hot water heating systems

depend on the hot water rising by convection

to the top of the house and then falling back

to the bottom of the house as it cools, releas-

ing the heat energy to warm the house as it

does so.

(ii) Convection currents cause air to move and

therefore affect climate.

(iii) When a radiator heats the air around it, the hot
air rises by convection and cold air moves in
to take its place.

(iv) A cooling system in a car radiator relies on
convection.

(iv) Large electrical transformers dissipate waste
heat to an oil tank. The heated oil rises by con-
vection to the top, then sinks through cooling
fins, losing heat as it does so.

(v) In a refrigerator, the cooling unit is situated

near the top. The air surrounding the cold

pipes become heavier as it contracts and sinks

towards the bottom. Warmer, less dense air is

pushed upwards and in turn is cooled. A cold

convection current is thus created.

Radiation

Radiationis the transfer of heat energy from a hot
body to a cooler one by electromagnetic waves. Heat
radiation is similar in character to light waves — it
travels at the same speed and can pass through a
vacuum — except that the frequency of the waves
are different. Waves are emitted by a hot body, are
transmitted through space (even a vacuum) and are
not detected until they fall on to another body. Radi-
ation is reflected from shining, polished surfaces but
absorbed by dull, black surfaces.

Practical applications of radiation include:

(i) heat from the sun reaching earth

(ii) heat felt by a flame

(iii) cooker grills

(iv) industrial furnaces

(v) infra-red space heaters

Vacuum

Inner silvered

surface of

thin glass walls

Cork stopper

Liquid

Outer case

Figure 19.3

19.8 Vacuum flask

A cross-section of a typical vacuum flask is shown

in Figure 19.3 and is seen to be a double-walled

bottle with a vacuum space between them, the whole

supported in a protective outer case.

Very little heat can be transferred by conduction

because of the vacuum space and the cork stopper

(cork is a bad conductor of heat). Also, because

of the vacuum space, no convection is possible.

Radiation is minimised by silvering the two glass

surfaces (radiation is reflected off shining surfaces).

Thus a vacuum flask is an example of prevention

of all three types of heat transfer and is therefore

able to keep hot liquids hot and cold liquids cold.

19.9 Use of insulation in conserving

fuel

Fuel used for heating a building is becoming increas-

ingly expensive. By the careful use of insulation,

heat can be retained in a building for longer periods

and the cost of heating thus minimised.