Treatment of compressed air involves removing the moisture and impurities in the air after it leaves the compressor.  The air from the compressor can contain many impurities.  These impurities need to be filtered.  Special assemblies are available which can filter the air.

The FRL (Filter Regulator Lubricator) assemblies are examples of such air treatment systems.  The FRL unit consists of a filter which filters the air and removes particulate matter and dust.  The Regulator reduces the air pressure to the desired value while the Lubricator delivers a fine aerosolised spray of oil to the air.

The oil spray in the air may be desired for certain pneumatic tools such as drills and air motors.  The oil serves to lubricate the parts in these equipments.

In compressors, oil is generally used to form a seal in the pockets formed by the rotating lobes and the casing.  The oil serves to prevent the leakage of air and improve efficiency.

However, this results in oil contamination of the air in the outlet.  This becomes an issue if the air is used for medical purposes, food manufacturing, etc.

Oil free compressors do not use oil in the compressing process.  Hence, there is no oil residue in the compressed air.  The casing and the rotor lobes are designed such that the sealing process does not require oil.

Oil will be used in other components of the machine such as the bearings and the gearbox.  However, this oil will not come in contact with the air.

Rotary air compressors are a family of compressors in which air is drawn into spaces formed between the rotating lobes in the rotor and the casing of the compressor.  The lobes are designed that they form an air tight pocket with the casing.  The lobes are usually screw shaped.  The screw shaped rotating lobes mesh with each other to form a pocket in which the gas travels.    The air is pushed towards the outlet of the compressor.  The gas or the air is thus compressed.  

Rotary air compressors are used where large quantities of compressed air are required.  

Unlike the reciprocating compressors, where the compression occurs in cycles, the flow in a rotary compressor is steady and continuous.  This reduces pulsation in the gas.  

A multistage compressor compresses gas or air over a series of stages.  Each stage increases the pressure by a certain amount till the final required pressure is reached.

When the gas is compressed, heat is produced.  This heat causes an increase in the volume.  The pressure thus obtained will be higher than the actual pressure due to the temperature increase.  Heat in the compressor can also damage the components of the cylinder, such as, the gaskets, o rings, etc.

This heat, therefore, has to be removed.  The intercooler is a cooling device which removes the heat as the gas passes from one stage to the next.

The intercooler consists of tubes which are cooled by a cooling medium, usually water.  As the gas passes through the tubes, it gets cooled.

Perfect or Complete Intercooling

If the temperature of the gas leaving the intercooler is equal to the room temperature, the cooling is said to be perfect.  This is known as perfect intercooling or complete intercooling

Imperfect or Incomplete Intercooling

If the temperature of the gas leaving the intercooler is greater than the room temperature, the cooling is said to be imperfect.  This is known as imperfect or incomplete intercooling.

Multistage compressors compress the gas or air in more than one stage.  The gas is progressively compressed over many stages to reach the final desired pressure.

The following are some of the advantages of multistage compressors over single stage compressors
  1. The volumetric efficiency of a multistage compressor is more as compared to a single stage compressor for the same delivery pressure. 
  2. Leakages are reduced
  3. The torque required is uniform.  Consequently, the size of the flywheel is reduced.  
  4. The cost is reduced for medium and large compressors.
  5. Lubrication is easy as the operating temperatures are lesser.

A compresssor draws air into the chamber and compresses it.  The compressed air is then delivered into a storage vessel.  During the process of compression, the gas gets heated.  This heat has to be dissipated.  A single stage compressor is one, which has a single cylinder and a piston or a single centrifugal impeller.

When a large amount of gas needs to be compressed or when the compression ratio is large, it is not practical to use a single stage compressor.  This because of the following reasons
  1. The heat developed during the compression process cannot be dissipated.  This can cause damage to the cylinder and components, such as, gaskets, seals, etc.
  2. It is not practical to use a single stage compressor as the size of the piston and cylinder will become very large for large capacity compressors.

The volumetric efficiency is the volume of gas compressed for a specified displacement of the piston.

For a single stage compressor,

Volumetric efficiency = (Volume of gas entering the compressor per minute)/(piston                                                                  displacement per minute)

In a multistage compressor, there are many cylinders, the gas is progressively compressed across many stages.  In a multistage compressor, only the displacement of the low pressure cylinder is taken into account.  Therefore, for multistage compressors

Volumetric effficiency = (Volume of gas entering the compressor per minute)/(displacement of                                                low pressure piston per minute)

Air compressors compress the air in the atmosphere to a high pressure and deliver it to a storage vessel.  This compressed air is used in a wide range of applications, such as, manufacturing, control systems, medicine, etc.

Compressors can be classified on the basis of different criteria.

They are

based on the mechanism of opertion.

  1. Rotary compressor 
  2. Reciprocating compressor

based on action

  1. Single acting
  2. Double acting

based on the number of stages

  1. Single stage
  2. Multi stage

based on pressure limit

  1. Low pressure compressor (delivery pressure up to 1 bar)
  2. Medium pressure compressor (delivery pressure from 1 to 8 bar)
  3. High pressure compressor  (delivery pressure up to 10 bar)

based on capacity

  1. Low capacity compressors
  2. High pressure compressors

Circuit breakers are used in a wide range of applications.  They are used in many environments and can handle currents and voltages of different ranges.

Circuit breakers are classified on the basis of different criteria.  Some of the classifications are below

Based on the interrupting medium
  1. Air circuit breaker
  2. Oil circuit breaker
  3. SF6 circuit breaker
Based on type of action
  1. Automatic circuit breakers
  2. Non-automatic circuit breakers
Based on the method of control
  1. Locally controlled circuit breakers
  2. Remotely controlled circuit breakers (remote control can be mechanical, pneumatic or electrical)
Based on the type of mounting
  1. Panel mounted circuit breakers
  2. Remote from panel mounted circuit breakers
  3. Rear of panel mounted circuit breakers
Based on location
  1. Outdoor circuit breakers
  2. Indooor circuit breakers
Based on voltage
  1. Low voltage circuit breakers
  2. Medium voltage circuit breakers
  3. High voltage circuit breakers

The relay in a protection system should be sensitive enough to operate when a fault occurs.  A sensitive relay improves the reliability of the system.

When the parameter exceeds the set value, the relay should start operating.

The sensitivity of a relay is mentioned as a ratio of the minimum value of short circuit current to the minimum value of the quantity for the operation.

The sensitivity is indicated by a sensitivity factor Ks

Sensitivity of a Relay

Is is the minimum short circuit current in the zone and
Io is the minimum operating current for the relay.

The sensitivity of a relay is also related to the VA of the input to the relay.  Lesser the VA of the input, greater will be the sensitivity and vice versa. For instance, a relay which has 1 VA as its measuring input will be more sensitive than a relay, which has 5 VA as its measuring input.

A reliable and effective protection system is a crucial part of any power system.  The protection system protects the equipment in the power system, such as generators, motors, transformers, etc from damage to faults.

The protection system also ensures reliability by localizing and isolating the fault and minimizing its impact on the rest of the power system.

The requirements of a power system are as follows

1. To isolate the equipment or component in which the fault has occurred.  The isolation should be quick enough to prevent damage to the component itself.  For instance, a short circuit inside can severely damage a transformer.  The differential relay, in such a situation, should immediately act and trip the transformer.

2. To isolate the smallest possible section of the power system to minimize the interruption.

3. To prevent disturbance or disruption to other parts of the power system.  The fault, if not isolated in time, can cause the upstream breakers to trip.

A well-designed protection system will greatly increase the reliability and performance of a power system.

The Load Flow Analysis is done to determine the flow of real and reactive between different buses in a power system.  It also helps in determining the voltage and current at different locations. 

To conduct a Load Flow Analysis, the components in a power system need to be modelled.  The modelling is done by developing equivalent circuits of the components, such as the generator, transmission lines and line capacitances.

The Generator equivalent circuit is shown below.

The Thevenin equivalent circuit.

This consists of a voltage source and a resistance and an inductance in series with the load.

E = V + IZ

where Z is the steady stage impedance

The Norton equivalent circuit consists of a power source and an admittance in parallel.

INorton = V/Z

INorton = YV


The load is modelled as a resistnce and inductance in a series circuit that is earth

Transmission lines

Transmission lines are modelled as

Short (less than 80 km)
Medium (80 to 250 km) or
Long lines ( 250 km and above)

The faults in a power system can be caused by a wide range of causes.  Below is a list of some of the most probable causes for electrical faults.

Overvoltage is caused due to surges such as lightning or due to switching loads on and off.  In generators, it can be caused due to the failure of the field controller.

Heavy winds which can cause lines to snap or trees to fall on them. This can cause open circuits, earth faults and short circuits.

Ageing which can result in weakening of insulation and failure. This can result in short circuits and earth faults.

Chemical pollution and deposition on the insulators that can result in flash overs

Faults due to wildlife, such as birds, snakes, mice, etc which can cause short circuits and earth faults.

Collision of vehicles on towers and transmission poles can cause the towers to fall.

The table below will give an idea of the percentage of the types of faults in the different equipment in a power system

Overhead lines                      50%
Transformers                         12%
Switch gear                            15%
Cables                                   10%
Miscellaneous                         8%
Instrument Transformers        2%
Control Equipment                 3%