Page 488 - 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

IMPROVE YOUR MATHS

How much expansion takes place?
The amount of expansion that takes place will depend on how many litres of
water the heating system contains. As we have found in previous chapters,
water expands at atmospheric pressure by 4 per cent when it is heated but, in
this case, the water is under pressure, so by how much does pressurised water
expand?
To answer this question, we must first calculate the expansion factor, which can be
used to calculate water expansion for a given volume and pressure. If the density
of the cold water and the density of the water at maximum operating temperature
are known, this is a fairly simple exercise. The calculation is as follows:

d1 – d2
d2
Where:
d1 = density of water at filling temperature (kg/m 3 )
d2 = density of water at maximum operating temperature (kg/m 3 )
If the system has 250 litres of water, and the system is filled with water at 4°C
and the maximum temperature is 85°C, what is the expansion factor?
Water @ 4°C has a density of 1000 kg/m 3
Water @ 85°C has a density of 968 kg/m 3
The equation therefore is:
1000 – 968
= 0.0330
968
So, the expansion factor (e) = 0.0330
Now, we must use this in another equation.
To find the amount of expansion of water in a system containing 250 litres of
water operating at a maximum temperature of 85°C, the equation is:
eC
V =
p1
1 –
p2
Where:
V = The total volume of the expansion vessel
C = The total volume of water in the system in litres (250 litres)
p1 = The fill pressure in bar pressure (1 bar)
p2 = The setting of the pressure relief valve in bar pressure (3 bar)
e = The expansion factor (0.0330)
If these are entered into the equation, the equation becomes:
0.0330 × 250
= 12.36
1
1 –
3
As a percentage of 250, 12.36 is:
12.36 × 100
= 4.94%
250
Therefore:
Water under a pressure of 1 bar when cold expands by 4.94 per cent when
heated to 85°C.

476

9781510416482.indb 476 29/03/19 9:03 PM