THERMAL ENGINEERING-1 (UNIT-5)

UNIT-5

Compressors: Classification –positive displacement and roto-dynamic machinery – Power producing and power absorbing machines, fan, blower and compressor – positive displacement and dynamic types – reciprocating and rotary types.
Reciprocating: Principle of operation, work required, Iso-thermal efficiency, volumetric efficiency and effect of clearance, multi stage compression, under cooling, saving of work, minimum work condition for two stage compression.

Classification of Compressors:

Positive Displacement Type:

• In this type of compressors, air is physically trapped between to relatively moving components and forced to occupy lower volume, thereby increasing its pressure. 
• Most notable example would be, a reciprocating compressor. In which air is trapped between piston and cylinder volume and then literally pressed to increase its pressure.
• Another point of interest is the output of these pumps. Since constant volume is being displaced by the moving part every time, the flow output of a positive displacement pump is always constant. 

Positive Displacement Compressor:

1. Compresses fixed packets of fluid.
2. The velocity of fluid need not be high.
3. Gives constant flow rate against variable output pressure.
4. The fluid is directly imparted pressure energy.

Application:Such types of pumps are basically designed for higher pressure application for example hydraulic fluid power application.

Non-Positive Displacement Type:

• In this type, a rotating component imparts its kinetic energy to the air which is eventually converted into pressure energy. 
• Centrifugal compressors are non-positive displacement type. Rotating impeller imparts K.E to the air which is converted to PE as air passes through the diffuser. 
• Whereas, in non-positive displacement type, the output fluctuates with the speed of rotation of impeller (as there is nothing physically forcing the air through).

Dynamic Compressor:

1. Compresses the fluid in continuous manner.
2. The velocity of fluid must be high.
3. Gives variable flow rate against variable output pressure.
4. The fluid is provided kinetic energy which is converted into pressure energy.

Application:Non positive displacement pumps are considerably used in chemical and oil industries where liquids are required to transport from one location to other

Power Producing Machines & Power Absorbing Machines:

• A power generating turbo machine convert potential/kinetic energy of fluid into mechanical energy and later in electrical energy. 
• On the other hand power absorbing turbo machines uses electric energy to do work on the fluid.

Explanation:

power generating turbo machine is a alternator which converts mechanical energy to electrical energy.power absorbing turbo machine is a motor which converts electrical energy into mechanical energy(torque). A turbine is a power generating power generating and a compressor/pump is a power absorbing turbo machine.Where fan,blower and compressor like elements will utilize electrical energy and work on fluid.