Page 182 - 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

The coefficient of linear expansion
Most materials expand when they are heated. When copper pipework expands
it can often be heard as a ticking when the central heating is on. The copper
expands in length by 0.000018 mm/°C. This may not seem a lot, but when it is
considered that this figure is for every degree rise in temperature, then the length
of expansion can be significant. On larger installations, it may mean the use of
expansion joints to accommodate the amount of expansion so that damage to
the pipework is eliminated. PVCu expands by a greater amount of 0.0005 mm/°C.

IMPROVE YOUR MATHS

Let’s see by how much copper expands.
20 m of 22 mm copper pipe contains water that rises from 4°C to 85°C. By how
much does the copper expand?
There is 20 m of copper pipe, an 81°C temperature difference and a 0.000018
coefficient of expansion of copper, so:
20 m × 81°C × 0.000018 mm/°C = 29.16 mm

Convection
Convection is heat transfer through a fluid substance, which can be water or air.

Convection occurs because heated fluids, due to their lower density, rise and
cooled fluids fall.

As water or air is heated it expands, which makes it less dense and therefore
lighter. If a cooler, denser material is above the warmer layer, the warmer
material will rise through the cooler material. The lighter, rising material will
release its heat into the surrounding environment, become denser (cooler),
and will fall because of the effect of gravity, to start the process over again.
In a hot water system, this process is known as gravity circulation.

Hot, less dense water
rises through the
water to the top of
the cylinder.

Cooler, dense water
falls back towards the
heat source to be
reheated and the
process starts again.

p Figure 3.17 Gravity circulation in a hot water system
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