Page 176 - The City and Guilds Textbook: Plumbing Book 1 for the Level 3 Apprenticeship (9189), Level 2 Technical Certificate (8202) and Level 2 Diploma (6035)

The City & Guilds Textbook: Plumbing Book 1

l LPG: liquid petroleum gas (LPG) can be used for heating appliances such
as boilers, cookers and fires. It is also used with plumbers’ blowtorches for
soldering capillary fittings. There are two basic types:
1 butane – used mainly as a camping gas
2 propane – the most widely used LPG in the building services industry.
l Natural gas: the most widely used fuel in the UK, natural gas has many
applications, both domestic and industrial. It is used as a fuel for:
l gas fires
l cookers
l room heaters
l condensing central heating boilers
l water heaters
l electricity generation
l industrial heating and processes.
l Carbon dioxide: used as a freezing agent with pipe-freezing kits, and is also
used in fire extinguishers.
l Refrigerant gas: see the section on refrigerants (pages 161–2).

Gas laws
Gases behave very differently from the other two states of matter we have
studied so far: solids and liquids. Gases, unlike solids and liquids, have neither a
fixed volume nor a fixed shape. They are moulded completely by the container
in which they are held. There are three variables by which we measure gases.
These are as follows.

KEY POINT Pressure
Pressure is measured as This is the force that the gas exerts on the walls of its container; it is equal on
force per unit area. The all sides of the container. For example, when a balloon is inflated, the balloon
standard SI unit for pressure expands because the pressure of air is greater on the inside of the balloon
is the pascal (Pa). However, than the outside. The pressure is exerted on all surfaces of the balloon equally
in plumbing it is more and so the balloon inflates evenly. If the balloon is released, the air will move
likely that pressure will be from the area of high pressure (inside the balloon) to the area of low pressure
measured in bar pressure (1 (outside the balloon).
bar = 100 kPa) or millibar
(1 mbar = 100 Pa). Volume
The volume of gas in a given container is affected by temperature and pressure.
Pressure is constant if temperature is constant. If temperature is increased, then
both the volume and pressure increase.

Temperature
An important property of any gas is its temperature. The temperature of a gas
is a measure of the mean kinetic energy of the gas. The gas molecules are in
constant random motion (kinetic energy). The higher the temperature, then
the greater the kinetic energy and greater the motion. As the temperature falls,
the kinetic energy decreases and the motion of the gas molecules diminishes.

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