feed more than 3 radiators together off 15mm pipe. I used to ask “Why? What happens if I have 3 large double radiators?” I was just told “just do it, that is how it WHAT CIPHE INSURANCE is”. SERVICES As I progressed towards being a CAN DO FOR YOU Chartered Engineer, I started to realise how dangerous these rule of thumbs can be, especially for apprentice plumbers. Lets not forget!
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CPD – CONSULTATION; THE KEY TO BOILER SELECTION
January/February 2017
“theory alone is useless” “practical alone is dangerous” Imagine your customer complaining about excessive noise within the heating system that you have just fitted, or the radiators or underfloor heating are not delivering enough heat to the For plumbing & heating room. Well it could actually be down to engineering professionals incorrectly sized pipework! “Why is this?” Pipework, fittings and other ancillary equipment that are fitted within a system create a resistance to the flow of water, which in turn creates a pressure loss through friction. Dependant upon the amount of flow required to deliver a specific amount of heat, the correct selection and design of pipework is critical to ensure that the heat emitter (e.g a radiator or floor heating system) is supplied the correct volumetric flow of heated water. In addition to the effect on the delivery of heat, incorrect or undersized pipework can lead to excessive flow velocities which in turn creates noise. These high velocities can also cause cavitation at elbows, pumps and valves, which can ultimately lead to the system failure.
Formula
V T h Cp
ρ
V=
h (Cp * ρ * T)
(1)
(1)
Volumetric flow rate Temperature difference (oC) Heat flow rate (kW) Specific Heat Capacity (kJ/kg.K) Density (kg/m3)
techtalk −
take the ‘pressure’ out of pipe design
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© CIPHE
The amount of volumetric flow rate required to deliver a specific amount of heat is calculated using the simple formula (1) .
Question Calculate the required volumetric flow rate based upon the following system parameters; (1) 18 kW heat load (2) Specific heat capacity of water 4.19 kJ/kg.K (3) Temperature difference of 10 o C. (4) Density of water 0.977 kg/m3.
Answer 0.44 l/sec =
18 (4.19 * 0.977 * 10)
Tip When designing a heating pipe system, the plumbing and heating engineer must design the pipework to ensure water velocities do not exceed recommended levels. Pipe Diameter (mm) 15 - 50
Velocity (m/s) 0.75 - 1.15
Table 1: Typical water velocities for small bore pipework (BSRIA)
A starting point could also be to limit the pressure loss per metre of pipe
to between 250 to 350 Pa m. However, you must understand the theory before making such assumptions Pipe sizing is an easy process when you know how, although it is important to understand some simple housekeeping rules. Before you calculate the pipe flow velocity, it is important to ask the manufacturer for the “internal” diameter of their pipework.
The CIPHE is putting the spotlight on professionalism within the industry, with a view to attracting more school leavers to recognise the life-long career opportunities presented by apprenticeships in © CIPHE the/ 25.6mm sector ID 28mm OD
Once the internal pipe diameter is Page 08 known, the flow area is then calculated with formula (2).
Formula
Area (m2) = 3.142 * r 2
(2)
r = Internal pipe radius (m)
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