Specific Gravity or Relative Density is the ratio of the density of the substance to the density of water.



Simply put, Relative Density tells us the number of times the substance is denser than water.

The Relative density of water is one.



Fluids are classified into


  • Ideal or perfect fluids
  • Real fluids or Practical fluids
  • Newtonian fluids and
  • Non-Newtonian fluids

Ideal fluids
An ideal fluid is one which has density as the only property.  The ideal fluid has not viscosity, surface tension, cohesion or adhesion.

Real fluids
These are also known as practical fluids. Fluids which have viscosity, surface tension, adhesion and cohesion are called real fluids
Eg. water, oil, air

Newtonian Liquids
Newtonian liquids are those which obey Newton's law of viscosity

Non-newtonian liquids
Liquids which do not obey Newton's law of viscosity are called Non-Newtonian liquids.



Specific weight is the ratio of weight to unit volume


The unit of specific weight is kN/m3



Specific Volume is the ratio of volume to unit mass


It can also be described as the reciprocal of density


The unit of Specific Volume is m3/kg



Compressibility is the change in volume due to the change in pressure of a fluid.

It is the ability of molecules in certain fluids to get compacted under pressure.  When the pressure is reduced, the molecules disperse and spread out again, increasing the volume.

Gases can be compressed by increasing the pressure.

Liquids cannot be compressed.  The change in volume due to the change in pressure is very small in liquids.

Gases are compressed by special devices called compressors.




Absolute Viscosity or Dynamic viscosity is the internal resistance of the fluid to flow. This value tell us the force required to make a fluid flow at a certain rate.

Kinematic Viscosity is the ratio of the Dynamic Viscosity to the Density of a fluid.


It is possible for two fluids to have the same dynamic viscosity and different kinematic viscosity.



Surface tension is defined as the force acting tangentially per unit length on either of a line on the surface of a fluid.

Surface tension is caused by the attraction of the molecules on the surface by the rest of the molecules in the fluid.  This force causes the surface area to be as small as possible.

This explains while water droplets are spherical.  The sphere has the minimum surface area for a given volume.



The rise or fall of a liquid inside a capillary tube is called capillarity.  Capillarity is caused by adhesion or the attraction between molecules of different substances.

When a capillary tube is inserted inside a liquid such water, the level inside the capillary tube rises above the level outside.  This is known as capillary attraction.

When a capillary tube is inserted inside a liquid such as mercury, the level inside the capillary tube falls below the level outside.  This is known as capillary repulsion.



When a liquid in a closed vessel is heated, it evaporates.  The vapour formed accumulates in the vessel and exerts pressure inside the vessel.  The pressure exerted by the vapour is called vapour pressure.

The vapour pressure is directly proportional to the temperature.  As the temperature increases, the vapour pressure increases.  When the temperature falls, the vapour pressure also drops.





Density is the mass per unit volume.  Density describes the amount of the substance which is packed into a certain volume.

Solids have higher density than liquids.  Liquids have greater density than gases.


The unit of density is kg/m3

When a fluid is compresssed, the density increases as the volume decreases.





Overheating in boilers occurs usually in the boiler tubes.  This problem is seen when the boiler is first commissioned and a short while later.  It usually does not appear after the plant has been stabilized.

Scale formation in the tubes can be a reason for overheating.  Scale formation prevents heat transfer and can cause localized overheating.   Overheating can also occur if there are changes in the boiler operation such as a change in fuel or any change in any other significant parameter.