Mechinopedia For Mechanical Engineers

For summer A/C , the air conditioned space is maintained at a temperature lower than the surrounding temperature. The moisture content may also have to be maintained at a level lower than the atmospheric level. This requires that the heat which is added to the conditioned space is to be removed continuously.  Various Factors like , i) Heat flow, through the exterior walls, ceilings , floors, doors and windows. ii) Heat by solar radiation. iii) Heat received from the occupants. iv) Heat received by infiltrated air etc., Add to the amount of heat into the room and hence , may be considered as a load. The a/c system should be capable of handling this heat load. In this context the heat load estimation assumes importance and has to accurately arrived.

Vapor Compression System : 1. Electric power is needed to drive the system. 2. Wear and tear is more because of moving components. 3. Tonne capacity is low. 4. Charging of refrigerant is simple. 5. More chances for leakage of refrigerant. 6. Mechanical energy is supplied. 7. Performance at part load is poor. 8. Space requirement is more. 9. Energy requirement is low.  Vapor Absorption System : 1. No need of electric power. 2. Wear and tear is less. 3. Tonne capacity is high. 4. Charging of refrigerant is difficult. 5. There is no leakage of refrigerant.  6. Heat energy is supplied. 7. At part loads, the performance is not affected. 8. Space requirement is less. 9. Energy requirement is high. 

 * Lithium Bromide System is another type of  absorption refrigeration uses water as refrigerant and lithium bromide as absorbent.   * This System is mainly used for water - chilling purposes.  * Water to be chilled is passed through tubes. Refrigerant water, after expanding in the expansion  valve  is  sprayed over these tubes.  * Because of this, the tube water gets chilled and the refrigerant water becomes water vapor.  * Lithium Bromide solution , which is sprayed in the absorber, absorbs the water vapor. Thus, the  solution becomes weak.    * The Weak Solution is pumped to the generator. Here, heating of the solution is done. Because of this, some water evaporates. So, the solution becomes strong.   * This Strong Solution is passed through the heat exchanger and sprayed in the absorber. Thus the cycle  repeats.  * Water vapour formed in the generator is passed through a condenser. Water vapor, thus gets  condensed  and becomes liquid water.  * External supply of cooling water wil

In recent years vapor absorption system becomes more interest due to its potential use as part of power saving plant. Also, it uses more environment friendly refrigerants than current vapor compression system. This uses heat energy instead of mechanical energy as in vapor compression system. This System differs from vapor compression system in such a way that compressor is replaced by an absorber and generator. The absorber absorbs the refrigerant and mixes it with the weak solution to produce strong solution. The function of generator is to heat the strong solution to form vapor of  the refrigerant. There are two types of vapor absorption refrigeration systems depending on the refrigerant and absorbent. 1. Ammonium - Water System. 2. Lithium bromide - Water System. 

 I) AMMONIUM WATER SYSTEM : An Ammonium - water refrigeration system uses ammonia as refrigerant with water as absorbent. WORKING OF THE SYSTEM : * After expansion in the valve, the liquid refrigerant enters the evaporator. There, it absorbs heat from the place to be refrigerated. Its phase changes from liquid to vapor. * Then, the vapor reaches absorber where it is readily absorbed in a low temperature absorbing medium. So, it mixes with the weak solution, thus the solution now becomes strong. Some heat being rejected during the process. * This strong solution is pumped to the high pressure and passes through heat exchanger. Here, it takes some heat from the weak solution which is throttled and returning back to the absorber. This    increases the efficiency of  the system. * Generator heats the strong solution, thereby forming Ammonia vapor. During heating, some water  may also be vaporized. This water vapor is separated in the analyzer. * Even after this, if any water vapor is prese

 CENTRIFUGAL COMPRESSOR : 1. Starting torque is low. 2. Isentropic efficiency is around 70%. 3. It is not suitable for multistage compression. 4. More frontal area is required. 5. Manufacturing cost is low. 6. Running cost is low.  AXIAL COMPRESSOR : 1. Starting torque is high. 2. Isentropic efficiency is around 85%. 3. It is suitable for multistage compression. 4. Less frontal area is required. 5. Manufacturing cost is high. 6. Running cost is high. 

 ROTARY COMPRESSOR :    1. It is simple in construction.  2. Speed is high.  3. It is suitable for large rate of flow at low discharge pressure.   4. Maintenance cost is less.  5. There is no balancing problem.  6. Simple lubrication system is used.  7. Small in size for the same discharge as compared with reciprocating compressors.  8. Uniform delivery of air is possible.  RECIPROCATING COMPRESSOR : 1. Construction is complicated. 2. Speed is low.  3. It is suitable for low rate of flow at very high discharge pressure.  4. Maintenance cost is high. 5. Balancing is major problem. 6. More complicated lubrication system is used. 7. Large in size for the same discharge as compared with rotary compressor. 8. Delivery is not uniform.

 In this type of  compressor, air enters axially and leaves axially. CONSTRUCTION : The arrangement of  axial flow compressor consists of casing, rotating drum, rotor blades and stator blades. The stator blades are fixed to the casing. The rotor blades are fixed on the rotating drum. The airflow passage area is gradually reduced from the inlet to the outlet of  the compressor. WORKING : When the power is given to the compressor, the rotating drum rotates and it creates suction in the compressor. Due to the suction, air enters through the compressor inlet and passes through the rotor and stator blades. As the air flows from one set of stator and rotor to another, it gets compressed. The air is also compressed between the casing and the blades. Pressure of the air is further increased due to gradual decrease in area from the inlet to the outlet of the compressor. Finally, The air at a high pressure is delivered to the receiver. APPLICATIONS : The axial flow compressors are most suitable

 In this type of compressor, air enters axially and leaves radially. CONSTRUCTION :  The arrangement of centrifugal compressor consists of  rotating Impeller , a casing and a diffuser. The impeller consists of a disc on which radial blades are attached. The impeller is surrounded by casing. The diffuser is another important part of the compressor, which is used to convert kinetic energy of air into pressure energy. The air coming out from the diffuser is collected in the casing and taken out from the outlet of the compressor. The Impeller of a centrifugal compressor can be run at the speed of 20,000 to 30,000 rpm.  WORKING : When the power is given to compressor, the impeller rotates, and it sucks the air. This air enters axially with low velocity. The velocity and pressure of the air passing through the impeller are partially increased. Then the air is entered into diffuser. In the diffuser, kinetic energy is converted into pressure energy. So, the pressure of air is further increased

CONSTRUCTION : It consists of rotating drum, spring - loaded vanes, inlet and outlet ports and casing. The rotor is located eccentrically inside the casing. The rotor carries a set of  spring - loaded vanes. These spring loaded vanes are made of fiber of carbon. WORKING : When the power is given to a vane blower, rotating drum (rotor) rotates, and the air is trapped between two consecutive vanes. As the rotation takes place the trapped air first expands and then gradually compressed due to decreasing volume between rotor and outer casing. This partially compressed air is delivered to the receiver. When the outlet is opened, there is a back flow of  high-pressure air from receiver will rush back and mix up with the entrapped air. So, partially compressed air pressure is further increased. Finally, high - pressure air is delivered from the receiver. In vane blower, the pressure of air is increased first by decreasing the volume and then by back flow of air.

Construction : The arrangement of Roots Blower , which is simply a development of gear pump. To get higher pressure ratio, three or four lobes are used. It consists of two rotors called lobes which are aligned in different parallel axis. One of the rotors is connected to the drive and the second rotor is driven from the first. The two rotors rotates in opposite directions i.e one in clockwise direction and the other in anti clockwise direction. The lobes of the rotors are of epicycloids, hypocycloids, or involute profile to ensure correct matching. There match must be a small clearance between the lobe and casing to reduce the wear of moving parts.  Working :  When the power is given to the roots blower, rotors rotate and the air at atmospheric pressure is trapped between the lobe and casing. The trapped air moves along the casing and discharged into the receiver. The flow area from entry to exit remains constant. So, there is no developing in pressure. When the exit port opens, some h

Whenever large quantities of air or gas are required at relatively low pressure, rotary compressors are used.  They are classified as 1. Positive displacement compressors. 2. Non - Positive Displacement Compressors. POSITIVE DISPLACEMENT COMPRESSORS : The Rotary Positive Displacement Compressors Are smaller than the reciprocating compressor at a given flow rate. The air is compressed adiabatically but uncooled. In Positive Displacement rotary type compressor, air is entrapped between two sets of engaging surfaces. The pressure rise is either by back flow of air ( as in Roots Blower) or both By variation in the volume and Back flow ( As in Vane Blower) . Examples - 1. Roots Blower  2. Vane Blower. NON POSITIVE DISPLACEMENT COMPRESSORS :  In Dynamic compressors, the air is not trapped in specific Boundaries but in flows continuously and steadily through the machine. The energy from the impeller is transferred to the air as the air flows through the machine the pressure rise is primaril

 In Multi - Stage Compressor, The Compression of the air from the initial pressure to the final pressure is carried out in more than one cylinder. It is used to get high pressure air. In a compressor, when compression ratio exceeds 5 , generally multistage compression is adopted. In Two stage reciprocating air compressor. It consists of  a low-pressure Cylinder (LP), an intercooler and a high - pressure cylinder (HP). Fresh air is sucked from the atmosphere in low - pressure cylinder during its suction stroke at intake pressure p1 and Temperature T1. The air after compression in L.P cylinder is delivered to the intercooler at pressure p2 and Temperature T2. In the intercooler, the air is cooled at constant pressure by circulating cold water. The cooled air from intercooler is then taken to the high - pressure cylinder. In the high pressure cylinder, air is further compressed to the final delivery pressure (p3) and discharge to the receiver.  Assumptions Made In Multistage Compression :

In a  Single stage compressor the compression of the air from the initial pressure to final pressure is carried out in one cylinder only. It consists of cylinder, piston, connecting rod, crank, inlet and discharge valves. When the piston moves downward i .e, during suction stroke, the pressure of air inside the cylinder falls below the atmospheric pressure. So, the inlet valve opens and the air from atmospheric is sucked into the cylinder until the piston reaches the bottom dead center. During this stroke, delivery valve remains closed. When the piston moves upwards both valves are closed. So, the pressure inside the cylinder goes on increasing till it reaches required discharge pressure. At this stage, the discharge valve opens and the compressed air is delivered through the valve. Thus , the cycle is repeated.  

In Multistage Compressors, the compression air takes place from Initial temperature to the final temperature in more than one cylinder. Therefore, this type of compressor also may have one or two delivery strokes according to the type of action of  the air with respect to piston. 

 In Single Stage Compressors, the compression of air takes place in one stage from the initial pressure. So, this type of compressor may have either one delivery stroke or two delivery strokes depending upon the type of action of the air with respect to piston. 

  A compressor in which suction , compression and delivery of air take place on both sides of the piston is Known as Double Acting Compressor. Therefore, it has two delivery strokes for each revolution of the Crankshaft. 

 A compressor in which suction , compression and delivery of air take place on one side of the piston  known as Single Acting Compressor. So, This type of compressor has only one delivery stroke for each revolution of the crank shaft.

 A machine which takes air or gas during suction stroke at low pressure and then compresses it to high  pressure in a piston - cylinder arrangement is known as Reciprocating compressor. 

  1.) According to the design and principle of operation ,      a.) Reciprocating Compressor      b.) Rotary Compressor.  2.) According to the action ,       a.) Single acting Compressors.      b.) Double Acting Compressors. 3.) According to Number of stages,      a.) Single Stage Compressors.      b.) Multi stage Compressors. 4.) According to the pressure hit,      a.) Low Pressure compressors.      b.) Medium Pressure Compressors.      c.) High Pressure Compressors. 5.) According to the capacity       a.) Low capacity Compressors.       b.) Medium Capacity Compressors.      c.) High Capacity Compressors.