Mechanical Engineering Principles

HEAT ENERGY AND TRANSFER 215

changes are reversible processes. When heat energy
flows to or from a substance and causes a change
of temperature, such as betweenAandB, between
CandDand betweenEandFin Figure 19.1, it is
calledsensible heat(since it can be ‘sensed’ by a
thermometer).
Heat energy which flows to or from a sub-
stance while the temperature remains constant, such
as betweenB and C and betweenD andE in
Figure 19.1, is calledlatent heat(latent means con-
cealed or hidden).

Problem 9. Steam initially at a temperature

of 130°C is cooled to a temperature of 20°C

below the freezing point of water, the loss of

heat energy being at a constant rate. Make a

sketch, and briefly explain, the expected

temperature/ time graph representing this

change.

140

130

120

100

Steam

AB

CD

Water

Ice

80

Temperature /

°C

60

40

20

− 20

0

Time

Figure 19.2

A temperature/time graph representing the change
is shown in Figure 19.2. Initially steam cools until
it reaches the boiling point of water at 100°C. Tem-
perature then remains constant, i.e. betweenAand
B, even though it is still giving off heat (i.e. latent
heat). When all the steam at 100°C has changed to
water at 100°C it starts to cool again until it reaches
the freezing point of water at 0°C. FromCtoD
the temperature again remains constant until all the
water is converted to ice. The temperature of the ice
then decreases as shown.

Now try the following exercise

Exercise 98 A further problem on change

of state

1. Some ice, initially at− 40 °C, has heat
   supplied to it at a constant rate until
   it becomes superheated steam at 150°C.
   Sketch a typical temperature/time graph
   expected and use it to explain the differ-
   ence between sensible and latent heat.
   [Similar to Figure 19.1, page 214]

19.5 Latent heats of fusion and

vaporisation

The specific latent heat of fusion is the heat

required to change 1 kg of a substance from the solid

state to the liquid state (or vice versa) at constant

temperature.

Thespecific latent heat of vaporisationis the

heat required to change 1 kg of a substance from a

liquid to a gaseous state (or vice versa) at constant

temperature.

The units of the specific latent heats of fusion and

vaporisation are J/kg, or more often kJ/kg, and some

typical values are shown in Table 19.1

The quantity of heatQsupplied or given out

during a change of state is given by:

Q=mL

wheremis the mass in kilograms andL is the

specific latent heat.

Thus, for example, the heat required to convert

10 kg of ice at 0°C to water at 0°Cisgivenby

10 kg×335 kJ/kg=3350 kJ or 3.35 MJ

Besides changing temperature, the effects of sup-

plying heat to a material can involve changes in

dimensions, as well as in colour, state and electri-

cal resistance. Most substances expand when heated

and contract when cooled, and there are many prac-

tical applications and design implications of thermal

movement (see Chapter 20).

Problem 10. How much heat is needed to

melt completely 12 kg of ice at 0°C?

Assume the latent heat of fusion of ice is

335 kJ/kg.