Mechanical Engineering Principles

Part 3 Heat transfer and fluid

mechanics

19

Heat energy and transfer

At the end of this chapter you should be
able to:

• distinguish between heat and temperature


• appreciate that temperature is measured on
  the Celsius or the thermodynamic scale


• convert temperatures from Celsius into
  Kelvin and vice versa


• recognise several temperature measuring
  devices


• define specific heat capacity,cand recog-
  nise typical values


• calculate the quantity of heat energyQ
  usingQ=mc(t 2 −t 1 )


• understand change of state from solid to
  liquid to gas, and vice versa


• distinguish between sensible and latent
  heat


• define specific latent heat of fusion


• define specific latent heat of vaporisation


• recognise typical values of latent heats of
  fusion and vaporisation


• calculate quantity of heatQusing
  Q=mL


• describe the principle of operation of a
  simple refrigerator
  
  
  • understand conduction, convection and
    radiation
  
  
  • understand the construction of a vacuum
    flask
  
  
  • appreciate the use of insulation in conserv-
    ing fuel in the home
  
  
  
  

19.1 Introduction

Heatis a form of energy and is measured in joules.

Temperatureis the degree of hotness or coldness

of a substance. Heat and temperature are thus not

the same thing. For example, twice the heat energy

is needed to boil a full container of water than half a

container — that is, different amounts of heat energy

are needed to cause an equal rise in the temperature

of different amounts of the same substance.

Temperature is measured either (i) on theCelsius

(°C)scale (formerly Centigrade), where the temper-

ature at which ice melts, i.e. the freezing point of

water, is taken as 0°C and the point at which water

boils under normal atmospheric pressure is taken

as 100°C, or (ii) on thethermodynamic scale,in

which the unit of temperature is thekelvin (K).The

kelvin scale uses the same temperature interval as

the Celsius scale but as its zero takes the ‘abso-

lute zero of temperature’ which is at about− 273 °C.

Hence,

kelvin temperature=degree Celsius+ 273

i.e. K=(°C)+^273