Unit No. 61 — Engineering Thermodynamics Essay Example


Summary on reciprocation of air compressors

Air compressor is a machine used to compress air molecules for them to be carefully packed. The compression is done with the success of the presence of pressure as well as his volume of the gas needed. The reciprocation is done for a large amount of gas to me packed in a smaller container under higher pressure, and thus the volume collected is less than the one used (Basinski, 2006).


Theory of a pressure-volume diagram for a single stage reciprocating air compressor cycle.

Task 1. A. Neglection of clearance.

In the air compression with Neglection of the volume removal, it is theoretically predefined that the process can occur in either of the three compression types. The types include the compression isothermally, compression adiabatically as well as compression polymorphically (Basinski, 2006).

Therefore, the line represented by a and bare the suction strokes. Air in this cylinder is adiabatically compressed through line bc. It is then pushed out of the tin with the temperatures kept constant under the process labeled cd. Therefore, the process from point a to d shows the whole process of the air compression. Below is the diagrammatic representation of the process.

Unit No. 61 -  Engineering ThermodynamicsUnit No. 61 -  Engineering Thermodynamics 1

Diagram: 1.1 Diagram showing compression without clearance

  1. B. Reciprocation with clearance.

As the reality proves, pistons are not in a position to release all the gas. Therefore volume is necessary to be available and located in between the cylinder as well as the pistol’s heads. As a result, little amount of the gas that has been compressed is trapped at the point labeled 3.as the piston gets displaced from its position, on the head of the cylinder, there is the expansion of the gas that has been compressed.

The development occurs under a theory of pVN = C until the current pressure level gets to that of the inlet pressure. The efficiency of the volume is the predefined as nvol – (volume induced ÷volume swept). The presence of leaks in the pistons worsens the efficiency of the amount. In equation presentation, the clearance is said to be (C= volume of clearance ÷ volume swept). The processes labeled 4-1 and 2-3 are found to be isothermal whereby there is the lack of change in the temperatures when inducing and expelling is occurring. Therefore, the effects of clearance on the pressure-volume diagram are as shown below. (Kradjan, 2005, pp. 512)

Unit No. 61 -  Engineering Thermodynamics 2

Diagram: 1.2 Compression of single-stage cycle in the absence of clearance

Task 1. C. Real pressure-volume compressor cycle

In this section of air compression, there is a record of an increase in the temperatures as a result of the cylinder’s warmth. Therefore from point 4 to point 1 is where induction is taking place. Therefore, at P1, the valves restriction is lower than as it is in P4. Also, there is a tendency of movement of the valves thus the reason behind the shape of the actual pressure-volume drawing on uni-staged compressor cycle.

Unit No. 61 -  Engineering Thermodynamics 3

Diagram: 1.3 Actual drawing of uni-staged compressor cycle.

Task 1. D. Multi-staged compressor cycles, pressure-volume drawing

In the cycle of a multi-staged air compressor, we have several cylinders of different diameter in the system. The first stage has the air taken in compressed. Then there follow the passage of the air to the cooling chamber with the aim of attaining temperatures close or equal to that of the ambient air. After that the air that has been cooled moves to a stage which is intermediated, where further compression and heating takes place. (Basinski, 2006)

Later the air is cooled to temperatures close to that of ambient air. At the final stage, the air is moved to, where the air is compressed again to the pressure that is required and then delivered to the receiving chamber after being passed through an after-cooler. The after-cooler ensures the air is sufficiently cooled. The diagrammatic representation of the process was shown below

Unit No. 61 -  Engineering Thermodynamics 4

Diagram: 1.4 Multi-staged air compressor cycle

Task 1. E. (i) Cycle’s clearance volume

The clearance volume is defined as the vacuum left after the stroke is done in a cylinder of a compressor. The vacuums are comprised of the spaces left between the ends of the pistol or a bottle as well. After the stroke is completed, there is the trapping of gas that is compressed in the vacuum. (Petrovsky, 2005)

E. (iii) Isothermal efficiency

The effectiveness of the processes of isothermal compression ensures that the temperatures in the system are always kept constant. Therefore, there is a necessity of heat exchange together with the gases as well. There was to need ensures there is no inflation and deflation of temperatures during charging and discharging respectively. The heat exchange, which can also be referred to as «intercooler” among the compression stages as well as the steps in the tank and the regulator help in the obtaining of heat exchange. Besides, as a way of reducing an excessive loss of energy, optimization of the intercooler is an important task to take purposely for the small pressure decrease and the increase in the heat transfer.

Approximation of the isothermal compression in absentia of the intercooler can also be made by the compressor as well. It is due to the increased ratio of the surface area of the size of the chambers where compression occurs. As a result, we have high amounts of heat that are dissipated from the walls of the compressor. Perfect results on the isothermal air compressor are a source of proving behind the process being reversible thus heat transfer between the gas and the environment surrounding are required. (Basinkski, 2006)

The importance of isothermal air compressor.

It is in the isothermal compressor where we have the exchange of heat taking place. Therefore it is from the intercooler, and the process gives a positive result. The isothermal compressor ensures that the diffusivity of temperatures is high as compared to the velocity of the compression.

E. (ii) Volume efficiency

The effectiveness of the capacity is said to be the ratio of the density mass of a given volume air that is at the intake to the density mass of the mixture of air and fuel at the atmospheric pressure in a cylinder. In simpler terms, we can say that volumetric efficiency is the ratio of the volume of material processed by a machine or a device about the total amount that the machine has. (Kradjan, 2005, pp. 518)

A Proper understanding of the definition of ideal compressors is a necessity in the tackling of the efficiency based on size. The conventional compressor is known for its’ execution of the compressions which are isentropic. The state of thermo-dynamicity of a suction line which is located at the inlet’s compressor is equal to the air moving into the cylinder. Besides, the ideal compressor assumes the existence of the volume of clearance when the location of the piston is the dead center at the top of the system. As a result, the volumetric efficiency is measured by diving swept volume from the volume induced in the project. (Petrovsky, 2005)

Importance of volumetric efficiency

Volumetric effectiveness in the big engines is required to improve the engineers who have been atomized to come up with smaller designed engines which are more powerful and preservative to the economy of fuel used to power them as well as reduce the handling cost. The process also saves the weight of the machines or engines.

E. (v) Importance of cooling air during compression

In the process of air compression cooling was necessary before proceeding to the next chamber as a way of preventing elongation of dying to the heat along the pipes. Letting the air to cool when the compressor went off would result in leakage of air in the joint of the system thus a reduction of the efficiency of the process. Cooling if also preferable to the release of the air directly to the receiver of the air from the compressor.

E. (v) Importance of intercooler

Intercooling brings about the increase in the system’s induction through the reduction of the heat produced by the turbocharger. Therefore, it opens a change for further heating. It also takes away the use of resource wasting methods to lower the temperature taken in through imposition of more than required fuel into the cylinder that is in the chamber where the air is being inducted. (Petrovsky, 2005)

E. (iv) Effects of volumetric and isometric efficiencies’ ratio.

Both the two ratios work towards reducing the errors applicable in the process as well as being economical. They are aiming at reducing heat loss and excessive use of fuel in the process thus bringing an economic impact in the process.

Task 2. (i) Safe operations of reciprocating air compression

Use of a bursting disc is necessary since it is made of copper discs which are the safest to us when the pressure exceeds the predefined value. The increase can be brought by the leakages available in the air tubes in the cooler areas. Use of fusible plugs is another safety operation of reciprocating air compression. The plugs are majorly located at the side where the discharge occurs during compression. The fusion happens if the temperatures of the air are more than those of the operation. The plugs ought to be made of materials that under high temperatures melt.

Sometimes the intercooler fails in operation. Therefore the compressor overheats. As a remedy to the problem, trips of high water temperatures are activated to curb the excessive heat absorption to the compressor.

Besides, the presence of a lobe oil pressure alarm for signaling when the pressure is low is an important device since its’ alert prevents the shaft from cranking as well as saves the bearing from getting damaged. When there is a failure in the functioning of the pumps attached, or when there is insufficient water in the intercooler, there is a failure in work of the internal parts of the compressor as a result of the increased temperatures. To solve this, a no-flow trip is induced, and the flow of water monitored continuously. As my last safety risk, an overlap has to be fitted in the system when there is high currents absorbed as the compressor is being started or when it’s running, a process that increases risks of the motor getting damaged.

Task 2. (ii) Principle causes of faults in air reciprocation

There are several faults about the exchange of air. At times the compressor fails to start. The possible problem is the unloader valve. It is brought about by the descending blow of the valves as the compressor turns off. Also, the compressor tends to produce noise as it is operating. The parts that bring about the sound are the rubber dumping feet’s or the compressor itself. Sometimes the noise produced has a knocking sound. Therefore it means that the problem is the rod bearing or the main bearing of the compressor.

Severally many compressors are found to consume much oil. There are various reasons behind the same. Either there is oil leakage in the compressor or oil has been compressed together with air. In the case of oil leakage, repair of the leakage joint is done with immediate effect. If there is oil compression with they are, the probable causes are to be sought. They include the usage of the wrong oil for the machine, high level of oil in the oil tanks, the surface of the cylinders worn, piston ring worn or incorrect installation of the disk as well as extremely high temperatures which are known for reducing the velocity of oil thus more oil carried out.(Kradjan, 2005, pp. 513)

In other incidences, the compressor may fail to construct pressure as well as air blows getting out of the inlet filter. The problem could be brought about by dusty, dirty or broken channels. The up-stroke in the system makes the air to push through the filter rather that posting it to the receivers. At times it is due to the deliberate creation of pressure.

Task 3 .Isothermal efficiency

n (iso) = isothermal work= Unit No. 61 -  Engineering Thermodynamics 5

Task 6. (a) Rankine’s cycle of steam power plantUnit No. 61 -  Engineering Thermodynamics 6

Task 6. (b) Discussion of the Rankine’s cycle

In the cycle, there are several processes taking place, with each stage involving the change in the state of the fluid working. From process 1-2, we have the fluid working being pumped from the level of low pressure to the high pressure thus small amount of input energy is required for the pumping. Process 2-3 has the liquid under high pressure entering the Boiler. Heating is done at a constant temperature with the use of heat from an external source, thus the fluid becoming a wet or a dry vapor. (Basinski, 2006)

Process 3-4 has the dry saturated vapor inflating through the turbines in the system which acts as a power generator. As a result of the lowering of pressure and temperature of the steam, condensation is probable to occur. The final step. 3-4, we have the wet, saturated vapor entering the condenser for condensation that occurs at a constant temperature and pressure as well. In conclusion of the process, the pressure and temperature of the condenser are changed by the cooling coil’s temperatures while the fluid is going through Phase-change.

Task 6. (a) Basic steam power plant circuits’ diagram (Basinski, 2006)

Unit No. 61 -  Engineering Thermodynamics 7

Task 6. (c) Reason behind preferring Rankine Cycle to Carnot’s Cycle

Rankine’s cycle is more convenient to the user with no full range of possible processes as compared to the Carnot’s cycle. Therefore, the Rankine’s process is more economical and gives the best and preferred results to the researchers.

Task 7. (b) Sketch of a T-enthalpy chart.

Unit No. 61 -  Engineering Thermodynamics 8

Task 7. (c) Cycle efficiency calculation

Unit No. 61 -  Engineering Thermodynamics 9


The Carnot’s cycle that represents is discussed under single-staged and multi-staged air compression efficiency of air. The various factors that have been a leading problem to the functioning of the compressor have been highlighted respectively. Thermodynamic1st law is a determinant of the rate at which heat is lost to the cylinder.


In the compressor, there must be a coolant and an intercooler which are functional for the success of the project. Besides, aiming at producing an economical process in the air compression should be the key objective. Use of the cheaper source of fuel in the system should be implemented as a way of cutting down the compression cost.

Reference list

Basinski, E.M., Martin, R.L., Meece, M.W. and John, H.B.I., Carrier Corporation and Thomas Industries Incorporated, 2006. Air compressor. U.S. Patent 5,515,769.

Kradjan, W.A., and Lakshminarayan, S., 2005. The efficiency of air compressor-driven nebulizers. Chest, 87(4), pp.512-516.

Petrovsky, J. and Roloff, H., Daimler-Benz Aktiengesellschaft, 2005. Reciprocating piston air compressor. U.S. Patent 4,498,848.