unit 103 powerpoint_2_plumbing_science
TRANSCRIPT
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Plumbing scienceWhat you need to know
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Units of measurement
• In the UK we use two main systems of measurement - imperial and metric
• The standard international system is known as SI units
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Mass and weight
• Mass is constant everywhere in the universe. Weight changes from place to place
• A nail will keep the same mass wherever it is: on a workbench or on the moon
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Density of solids
• Solid materials that have the same shape and size can have a different mass
• This lightness/heaviness is known as density
• Density = mass ÷ volume
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Density of liquids
• Liquids and gases also have different densities
• It is important to understand the density of water
• Water is less dense when heated
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Water
At 4˚C water is said to have a density of 1.0
Amount Temp. Mass
1m³ 4˚C 1000 kg
1m³ 82˚C 967 kg
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Relative density (specific gravity)
• Measures the density of a subject compared to water
• 1m³ of water has a mass of 1000 kg• 1m³ of mild steel has a mass of 7700 kg• Mild steel’s relative density (density compared
to water) is 7.7
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Density of gases
• With gases, air is given the relative density of 1.0
• Is helium more or less dense than air?
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Pressure
Force applied by unit area
• (N/m²) also known as (Pa)
Other terms: bar, lbs/in²
• 1bar = 100,000 N/m²
• 1 lbs/in² = 6894 N/m²
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• Pressure is exerted by solid objects in downwards only
• Liquid exerts pressure downward and outward
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Pressure exerted on all sides not just the base
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• You will need to understand the effects pressure has on pipes and fittings
• The internal pressure is affected by what is being carried (water/gas)
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Atmospheric pressure
• The pressure exerted by the earth’s atmosphere changes as you get higher above sea level
• At sea level it is 1 bar = 14.7 psi
• psi = lbs/in² (pounds per inches squared)
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Properties of water
• Water is a chemical compound made from hydrogen and oxygen (H2O)
• Water can dissolve lots of gases and solids to produce solutions
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Hard water
When water falls on ground that has calcium carbonates or sulphates (chalk, limestone or gypsum) in it, they dissolve into a solution called hard water.
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Soft water
Has no dissolved salts like calcium carbonates/sulphates.
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pH value and corrosion
• pH is the level of acidity or alkalinity of a substance.
• Acids and alkalis can cause corrosion, metals are more at risk
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Atmospheric corrosion
• Pure air or water alone are not very corrosive
• Together as moist air they attack ferrous metals (rusting)
• Other things in the atmosphere increase the corrosive effect, eg salt, carbon dioxide, sulphur
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Non ferrous metals• Copper, lead, aluminium
• These have protective barriers (usually sulphates) which prevent further corrosion. This happens after oxidisation.
• This is known as patina
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Corrosion by water
• Ferrous metals are more likely to be corroded by water
(think of black sludge in radiators (ferrous oxide))
• Ferrous oxide and rust combine to make hydrogen
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The difference between heat and temperature
• Heat is a unit of energy measured in Joules (J). It is the amount of heat energy in a substance.
• Temperature is measured in centigrade and is the degree of hotness of a substance.
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• The wire has a temperature of 350˚C
• The water has a temperature of 70˚C
• The wire is far hotter, but has less energy
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Thermometers
• A bi-metallic strip is used to convert a temperature change into movement
• Liquid thermometers use mainly alcohol and mercury
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Specific heat capacity
• The amount of heat needed to raise 1kg of material by 1˚C.• It takes 4.186 kJ to raise the temperature of water by 1˚C, but only 0.385kJ to raise the temperature of
copper by 1˚C.
• Specific heat values vary as the temperature changes
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Specific heat capacity
• Work out the amount of heat needed to increase the temperature of 250g of water from 20oC to 56oC.
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Specific heat capacity
q = m x Cg x (Tf - Ti) m = 250g Cg= 4.18 J oC-1 g-1 (from table above) Tf = 56oC Ti = 20oC
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Specific heat capacity
q = 250 x 4.18 x (56 - 20) q = 250 x 4.18 x 36 q = 37 620 J = 38 kJ
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Thermal expansion and contraction
These are known as coefficient values
Lead has a higher coefficient value than, say, copper ie it will move more when warm.
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Plumbing materials• Plastic has coefficient value of
0.00018• So if we had a 6m long guttering we
can work out how much it will expand at 19°c!
• To do this we need three values
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1 Length (meters)
2 Temperature rise(°c)
(19 degrees is used as standard temp rise)
3 Coefficient value
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So So 6 6 (m) X (Temp rise)(m) X (Temp rise)1919°c°c X X (coefficient value) (coefficient value) 0.00018 =0.00018 =
6 x 19 x 0.00018 = 0.02052m or 6 x 19 x 0.00018 = 0.02052m or 20.52mm20.52mm
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Heat transfer
• Conduction
• Convection
• Radiation
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ConductionThe transfer of heat energy through a
material
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Convection
The less dense air (hotter) rises and the denser air (cooler) falls
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Radiation
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Radiators only radiate 15% of the heat. The rest is done by convection.
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Radiation is the transmission of heat energy by infrared radiation.
In conduction, the excited vibrational states of molecules are transmitted.Convection is the circulation of hot gases.
Radiation