Introduction and definitions
The wide use made of various gases in the field of engineering makes it necessary to predetermine their reactions when they are heated, cooled, expanded or compressed.
When a process takes place, the changes which will occur in the properties of volume, absolute pressure and absolute temperature of the gases are related by the gas laws.
When solving problems utilising the gas laws, both pressures and temperatures must be expressed in absolute terms. We define these absolute terms in the following pages.
Absolute pressure
Pressure gauges are commonly used to measure pressures in vessels and pipelines and read pressures normally above atmospheric pressure. If a gauge shows a zero reading it means the pressure is atmospheric.
If the pressure in a vessel is increased above atmospheric to a gauge pressure pg, the actual or absolute pressure in the vessel is given by:
i.e. Absolute pressure p = gauge pressure + atmospheric pressure.
In most practical problems, listed pressures will be in absolute terms unless otherwise stated.
The SI unit for pressure is the Nm-2 or Pascal (Pa)
The bar = 105 Nm-2 = 100 kNm-2 is also commonly used.
Standard atmospheric pressure = 1 atm = 1.013 bar = 1.013 x 105 Nm-2.
When working through problems, the stated or calculated values for pressure are often high numbers. In order to express multiples of SI units concisely, the prefixes in the table below are often used:
Multiplication factor |
Prefix |
Symbol |
1,000,000 or 106 |
Mega |
M |
1,000 or 103 |
Kilo |
k |
For example, 7,200,000Nm-2=7,200kNm-2 or preferably 7.2MNm-2
Absolute temperature
In problems involving the gas laws, the temperature of any gas is measured from absolute zero, which has been determined to be 273°C below the zero point on the Celsius scale, i.e. absolute zero. At this point the internal energy of the substance is also zero.
Absolute temperature is the temperature above absolute zero and is determined by adding 273 to the Celsius temperature scale reading.
i.e. Absolute temperature = Celsius temperature scale reading + 273.
Hence 27°C = 300 K.
Absolute temperature takes the SI base unit, the Kelvin (K), and has the symbol T. Note: A change in temperature of 1°C = a change in temperature of 1 K.
Other quantities
Other significant quantities we will encounter in our thermofluids studies are defined below.
Mass
This is usually defined as the quantity of matter in a body.
Symbol: m (small letter).
Unit: kg (small letters).
Volume
Symbol: V (capital letter).
Unit: m3.
The recommended unit is the cubic metre. Subdivisions such as the cm3 or litre (l) are also used, but as a general rule it is safer to convert data to give volumes in m3 to avoid errors in calculations.
Specific Volume
Symbol: v (small letter).
Unit: m3kg-1.
This is the volume per unit mass and is the reciprocal of density.
i.e