Unit 103 powerpoint_2_plumbing_science

Plumbing scienceWhat you need to know

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

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

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

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

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

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

Density of gases

• With gases, air is given the relative density of 1.0

• Is helium more or less dense than air?

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

• Pressure is exerted by solid objects in downwards only

• Liquid exerts pressure downward and outward

Pressure exerted on all sides not just the base

• 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)

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)

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

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.

Soft water

Has no dissolved salts like calcium carbonates/sulphates.

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

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

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

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

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.

• 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

Thermometers

• A bi-metallic strip is used to convert a temperature change into movement

• Liquid thermometers use mainly alcohol and mercury

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

Specific heat capacity

• Work out the amount of heat needed to increase the temperature of 250g of water from 20oC to 56oC.

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

Specific heat capacity

q = 250 x 4.18 x (56 - 20) q = 250 x 4.18 x 36 q = 37 620 J = 38 kJ

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.

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

1 Length (meters)

2 Temperature rise(°c)

(19 degrees is used as standard temp rise)

3 Coefficient value

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

Heat transfer

• Conduction

• Convection

• Radiation

ConductionThe transfer of heat energy through a

material

Convection

The less dense air (hotter) rises and the denser air (cooler) falls

Radiation

Radiators only radiate 15% of the heat. The rest is done by convection.

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