Introduction to Compressors

 
AIR COMPRESSORS
 Application of Compressors
 Introduction
 
A simple definition of a compressor is a device used to pressurize a fluid, including liquids and gases. There are many different kinds of compressors, but typically the main purpose of using a compressor is to raise the pressure of a liquid or gas. Compressors are found in both gas power cycles and vapor compression refrigeration cycles. Figure (1) shows a sketch of a simple       compressor.
A compressor converts shaft power to a rise in enthalpy of a fluid. The fluid, often a gas, enters the compressor at a low pressure (low enthalpy) and exits at a high pressure (high enthalpy), as shown in      Fig. (2). The rotating shaft is attached to a blade assembly. The rotating blades push on the gas and increase the pressure, thereby increasing the enthalpy. Compressors are continuous flow processes, and can be either axial or radial.
 
Isentropic Efficiency
 
Efficiency = (h_2s-h₁ )/(h_2a-h₁ )
 
Where h₁ = enthalpy at state 1.
h_2ACT = actual enthalpy at state 2.
h_2s = isentropic enthalpy at state 2.
 
To calculate these enthalpy changes, you need to know the initial and final states, for example, temperature and pressure, of the working fluid.
 Types of Air Compressors
 

1. Dynamic Centrifugal/Axial – Use a rotating impeller to impart velocity to the air, which is converted to pressure.

2. Positive Displacement

Rotary – Compress air through the action of rotating elements. Most common types are rotary screw, which uses rotating male and female rotors to compress air, and sliding vane, which uses radially moving vanes.
Reciprocating – Compress air through the use of a reciprocating piston.
 
Fig. (1) A sketch of a simple compressor.
 
 
 
Fig. (2) T-S diagram for compressor.
 
Application of Compressors
 
1- Gas Turbine
     The gas turbine is used in a wide range of applications. Common uses include power generation plants and military and commercial aircraft. In Jet Engine applications, the power output of the turbine is used to provide thrust for the aircraft.
In a simple gas turbine cycle, shown in Fig. (3), low pressure air is drawn into a compressor (state 1) where it is compressed to a higher pressure (state 2). Fuel is added to the compressed air and the mixture is burnt in a combustion chamber. The resulting hot products enter the turbine (state 3) and expand to state 4. Most of the work produced in the turbine is used to run the compressor and the rest is used to run auxiliary equipment and produce power.
Air standard models provide useful quantitative results for gas turbine cycles. In these models the following assumptions hold true.

• The working substance is air and treated as an ideal gas throughout the cycle
• The combustion process is modeled as a constant pressure heat addition
• The exhaust is modeled as a constant pressure heat rejection process

In cold air standard (CAS) models, the specific heat of air is assumed constant at the lowest temperature in the cycle.
Brayton Cycle
The Brayton cycle depicts the air-standard model of a gas turbine power cycle.
The four steps of the cycle are:
(1-2) Isentropic Compression
(2-3) Reversible Constant Pressure Heat Addition
(3-4) Isentropic Expansion
(4-1) Reversible Constant Pressure Heat Rejection
The pv and Ts diagrams are shown in Fig. (4).
 
Fig. ( 3) Gas Turbine Cycle.
Fig. ( 1-4) The pv and Ts diagrams for Brayton Cycle.
 EFFICIENCY
 
here  h = Enthalpy     T = Temperature,
Efficiency = Net power output/ heat add
 
Figure (5) shows an actual small gas turbine engine. Air is drawn into the engine through a grill, this helps protect the engine from the effects of foreign objects entering it. Air passes through a rotating impeller which together with a ring of static vanes, forms the compressor section of the engine. The impeller is fitted with curved vanes at the intake area which guide the air into it, these are called rotating inlet guide vanes. As the air passes through the compressor it is accelerated outwards at high speed and then slowed down in the ring of static vanes, this ring is known as the diffuser as shown in Fig. (6). Additional vanes may be used to curve the air flow around a 90 degree bend so that it can enter the combustion chamber. The pressure of the air is raised to as much as four times atmospheric and is consequently heated.
 
Compressors are usually made of aluminum and are coated to reduce corrosion. Resilience to foreign object damage can be increased by making the rotating inlet guide vanes from steel or the whole wheel can be made of titanium.
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Fig. (5) shows an actual small gas turbine engine
 
 
                             Fig. (6)  Compressor and Diffuser.
 
2- Turbocharger
 
Many people reading about the Concept IC Engine would have seen the similarity between this design and that of a turbocharger, so how is it different ? Well there are a lot of differences the most important of which is that the turbocharger taps into the energy from the exhaust after most of the energy has already been expended while the Concept IC Engine uses these energies when they are at near peak levels. Secondly in an ordinary engine using a turbo charger the exhaust gases are dissipated due to valve design , therefore any thrust which might be gained from the ejection of these hot gases is dissipated or directed side wards into the cylinder walls. In the Concept IC Engine , the thrust created by the explosive ejection of hot gases at velocity is enhanced by the use of a venturi or nozzle and also directed so as to gain maximum thrust from the escaping gases in the downward motion of the piston. Thus although , pressure in the piston when the exhaust valve is fully opened at 80 degrees to 90 degrees before BDC might immediately come to zero , it is compensated to a large extent by the thrust generated. This is where recoilless technology comes into play.
 
However valve design and the utilisation of thrust created by exhaust gases escaping at velocity is only the most obvious difference between turbo charger and the Concept IC engine there are several other fundamental differences.
If one examines Fig. (7). Which is a diagram of a turbocharger layout in a typical engine, it is immediately possible to see that energy from the exhaust is not being fully utilised. They have to travel a long distance before actually powering the turbine and also that the poppet valves used do not allow for generation of thrust from the exhaust gases.
 
The other fundamental difference between a Concept IC Engine and a turbocharger is that in a turbocharger the compressor fan and the turbine fan are in separate housings connected by a common shaft. Thus the compressor is used solely is used solely for increasing the air intake of the piston unit. See Fig. (8).
In a Concept IC Engine the compressor is used to bring in air and compress it prior to being heated and used to actually turn the main turbine , so that the turbine in a Concept IC Engine works in an almost identical fashion to the turbine engine as used in car engines or jet planes.
It can therefore be seen that the turbine unit in a Concept IC Engine is capable of developing about 250 horse power which in turn can be used to run a generator which would feed electricity to separate motors at each rear wheel in the manner of a diesel electric engine.
 
The main piston unit can also be used to run a generator and supply electricity, this would mean that the transmission is to a large extent eliminated. In whatever way the power developed by a Concept IC Engine is used , the most important thing to remember is that the power is there to be tapped in anyway in which it is convenient to use. The exhaust from such an engine would have zero pollution because of the large amounts of air to fuel ratio which are used. It is very important that those who wish to gain a true understanding of the working of the Concept IC engine that they read the article on turbine engines.
 
 Fig. (7) Turbocharger
  
Fig.  (8) Compressor and turbine of the turbocharger
 
3- Compression Cycle
 
         The compression cycle is so named because it is the compressor, which changes the refrigerant vapor from low-pressure to high pressure. This pumping causes the transfer of heat energy from the inside of the cabinet to the outside. Since the compression machine transfer heat from one place to another, it may also be called heat pump.
 
A refrigerating system consists principally of a high-pressure side and a low-pressure side, Fig. (9).
 
A refrigeration cycle follows these steps: from the liquid receiver, G, liquid referigerent, at a high pressure, flows through the refrigerant control, A, (a pressure reducer). It moves into the evaporator, B. The evaporator is under a low pressure. Here the liquid refrigerant vaporizes (boils) and absorbs heat.
 
The vapor hen flows into the compressor cylinder. The piston, intake valve, C, back into the compressor cylinder. The piston, D, on the compression stroke, squeezes the vapor into a small space with an increase in temperature.
 
 
Fig. (9) Compression cycle showing the two pressure conditions.
Low pressure side and high-pressure side.