This section includes notes on certain important thermodynamic properties including
enthalpy (h), Specific heat (cv,cp),
gas constant (R), and Entropy (S)
In many thermodynamic fluid process analyses the sum of the internal energy (U) and the product of pressure (P) and volume(V) is present. The combination (U + PV) is called the enthalpy of the fluid. H is a thermodynamic fluid property but is does not have an absolute value(because it includes internal energy U )value and therefore enthalpy changes are generally applied or enthalpy values are identified relative to a fixed state e.g. water at 273 deg.K . It is important to note that enthalpy is simply a combination of properties ..it is not a form of stored energy although for certain applications it can be treated as energy.
H = U + PV ..........(extensive property)per unit mass
h = u + Pv ...........(intensive property)
When referring for water and steam and other fluids at different states in tables the following enthalpy designations are used
The heat capacity of a substance is classically defined as the amount of heat needed to raise unit mass of
a substance one degree Centigrade.
In SI units the specific heat capacity is the amount of heat required to raise 1 kg mass through 1 degree kelvin. (Unit kJ/kg.K)
Note:The specific heat of a substance is the ratio of the heat capacity of a substance relative to a reference substance generally water.
The heat capacity of water is one calorie per degree C (classical) or (4180 J/kg.K ) The specific heat of a substance relative to water will be numerically equal to its heat capacity in classical units, but not in SI units ;
The term specific heat is often used when the heat capacity actually is meant. This page is concerned only with heat capacity (to be called specific heat capacity). Because the heat capacities of most substances vary with changes in temperature, the temperatures of both the specified substance and the reference substance must be known in order to give a precise value for the specific heat.
Four specific heats are for gases are used.
Note: The molar specific heats are mainly used for chemical studies
The specific heat varies with temperature and pressure. The graph below this illustrates this characteristic for cp. for air
Tables below show the variation of cp and cp with temperatures...
Variation of cp (for Air) with temperature and pressure
The latent heat of fusion is the amount of heat required to convert unit mass a substance
from solid to liquid without change of temperature..
The latent heat of vaporisation is the amount of heat required to convert unit mass of a substance from liquid to vapour without change of temperature.
The gas constant R is derived from the equation of state
Pv = RT .. for unit mass of gas
PV = mRT
The gas constant R is different for each gas and has different units depending on the
unit systems used. Typical units are (kJ/kg.K).
The universal gas constant Ru is the same for all gases and is defined by
PV = NRuT
Based on a pressure of 1.032 bar and at 0oC
|Gas||cp||cv||cp / cv||cp - cv|
Carbon Monoxide |