Page 175 - The City and Guilds Textbook: Plumbing Book 1 for the Level 3 Apprenticeship (9189), Level 2 Technical Certificate (8202) and Level 2 Diploma (6035)

Chapter 3 Scientific principles

formed from hydrocarbons, and has a density of 0.78–0.81 g/cm . It is made
3
from the distillation of petroleum at temperatures between 150°C and 275°C.
The flashpoint of kerosene is between 37°C and 65°C, and it will spontaneously
combust at 220°C. The heat of combustion of kerosene 43.1 MJ/kg, and its
higher heating value is 46.2 MJ/kg.

Lubricants
A lubricant is a substance, often a liquid or grease, introduced between
two moving surfaces to reduce friction, thus improving efficiency and
reducing wear. There are many types of lubricant in use in the plumbing
industry:

l Silicone grease and spray: used for general lubrication of plumbing parts
for water and drainage systems. It is also used when jointing push-fit plastic
pipe systems to lubricate the rubber seals.
l Graphite paste: used for lubrication of gas taps.
l Cutting oils: used when threading low carbon steel pipe. They help to
prevent overheating of the cutting dies.
l Penetrating oils: used to help loosen tight and rusted joints.

The principal applications of gases

In this section of the chapter, we will look at the principal uses of gases in the
building services industry, together with their properties and the scientific laws
that apply to them.

Types of gases
The principal gases in the building services industry are listed below.

l Air: this has limited uses within the plumbing industry.
l It can be used as a heating medium in warm-air heating systems. Here,
the air is warmed by a warm-air heater, usually fired by gas. The warm air
is distributed to the property by means of a fan.
l It can be used as a pressure charge in expansion vessels. These are usually
installed in sealed heating systems and some unvented hot water storage
vessels.
l Air at high pressure can be used to clear blocked drains.
l Steam: once the preferred method of heating, the use of steam has declined
over recent years. However, because of new, more efficient system designs,
steam is being used as a heating medium for:
l new combined heat and power applications – steam can be used to
generate electricity and warm properties in district heating systems
l electricity generation
l hot water production using large hot water calorifiers
l heating systems – the steam is used instead of water in the heat
emitters.

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