Two Stage Air Compressor

p-v diagram

Volume


Pressure



University of Birmingham
Mechanical Engineering
Fluid flow, Thermodynamics and Heat Transfer
Course Work






T2: Two Stages Air Compressor







Submitted by: Stelio Michael Machado Antonas (Student number: 1315848)
Group 2
Date of submission: 08/04/2014







Abstract
The purpose of this experiment was to investigate the effect of the intercooling process on the performance of a two stage reciprocating air compressor. The usage of an intercooler is to decrease the amount of work done by the machine. The GT103 two-stage air compressor test set was used in this experiment. In the beginning of the experiment, the machine was turned on and driven by electrical motor. Then, by manipulating the air receiver pressure, the dynamometer load, the outlet and inlet temperature were recorded as same as the voltage and current. From the results obtained, the values of air flow rate, air indicated power, efficiencies and indexes of compression were calculated. Three graphs representing the relationship between the performance of the compressor and the delivery pressure, represented by the air flow rate, power and the efficiencies were plotted. From the graphs it can be noticed that the efficiency of the machine, the electrical input power and the air indicated power increases as the delivery pressure increases whereas the free air delivery decreases. The errors will be discussed later.

1. Introduction
A range of industries, such manufacture of chemicals and fertilizers, uses compressors to supply power to pneumatic tools like ratchet wrenches spray guns and air mailers, they can also be found in gas stations and they can be used to supply power to a variety of tools for a long period of time, those are called two-stage air compressor. This report will examine the main advantages of a two-stage intercooled compression process as compared with a single stage process. The report is divided into four main sections. It will first explain the materials used during the experiment and how the experiment was conducted. It will then show some calculations that were done to achieve the results. It will then go on to show the results of the experiment in tables and chats and some discussion of the result. And finally some conclusion will be provided as to explain the errors and the advantages of the two stage air compressor

2. Material and methods
The machine that was used is a GT103 two-stage compressor test set. The machine consist of two industrial compressors, one with a low pressure cylinder and other with a high pressure cylinder, an intercooler between the two stages and an air receiver. The piston of each cylinder is driven by a prime mover electrical motor, and linked with a dynamometer. The machine also has two analogue meter for each compressor instrument group that measure the voltage and the current applied to the dynamometer, which all can be seen in figure 2 in the lab sheet.
In the beginning of the experiment, air is drawn into the compressor through a filter and compressed in the first stage with a low pressure cylinder that reduce the volume and increase the temperature of the air. It then passes to through an intercooler to reduce the temperature in a constant pressure process by a counter current water flow heat exchanger. The air then is compressed in the second stage with a high pressure cylinder before the air goes to the receiver

2.1. Calculation
Before any calculations could be done, all readings of pressure were converted from gauge to absolute pressure. Calculation for the 10 bar receiver pressure have been made. For the free air delivery, it was first calculated the mass flow rate ma by substituting the measured value of p, the values of Cd, E,A, given in table 1, see appendix A, and the calculated value of ρ into equation (1), see appendix B, giving 0.00437 kg/s. It was then substituted the calculated value of ma, the given value of Rair, T and P into equation (2), see appendix B that gave 0.0035635 m3/s of volumetric flow rate. It was next calculated the compressor efficiency by substituting the calculated value of the Air indicated power of the system and the Electrical input power to the system into equation (3), see appendix B, which gave 90.84% of efficiency. It was then calculated the index of compression for stage 1, 2 and overall by substituting the measured values of T1, T2,P1 and P2 into equation 4 that gave 1.43 for the first stage, 1.308 for the second stage and 1.222 for overall. Next was calculated the air indicated power for each stage by substituting the calculated value of ma, n and PR for each stage, the given value of Rair and T into equation (5), see appendix B, which gave 799.3 W for the first stage and 944.8 for the second stage.

3. Results
The indexes of compression of stage one, two and overall are different and one smaller than another due to differences of pressure and temperature of stage 1 and 2, as shown in table 1, see appendix A.
Table 2, see appendix A, shows the data collected during the experiment and the calculated value of the Free Air Delivery and the calculated value of the mass flow rate as well. It can be noticed that as the delivery pressure decrease the Free Air Deliver experience a small increasing as well as the mass flow rate.
It can be noticed a significant improvement of the volumetric efficiency when the pressure of the gas is increased by passing the air through the intercooler after the first compression (low pressure cylinder) as shown in table 3.

Simultaneous readings were taken at the compressor when pressures from 5 to 10 bar were obtained. It can be noticed that as the Free Air Deliver experience decreasing when the delivery pressure increases as shown in figure 1. The figure is easy to understand and quite straight forward since the volume flow rate (Free Air Delivery) is inversely proportional to the pressure as shown in equation (2), so the bigger the pressure the smaller the volume flow rate ( Free Air Delivery).
The electrical power input increases as the delivery pressure increases, since is required more power to deliver a bigger pressure. The electrical power in stage 1 experience a small increase since the air passes through a low pressure cylinder that does not use a huge amount of power. The electrical power in stage 2 experience a large increase, since the air passes through a high pressure cylinder that use a large amount of power, as shown in figure 2.
It can also be noticed in figure 2 that the indicated power of the compressor increases as the delivery pressure increases since the pressure ratio increases as the delivery pressure increases, and the indicated power is directly proportional to the pressure ration as shown in equation (5), see appendix B.
There is an increasing in the efficiency of the machine with respect to pressure due to intercooler. There is 90.84% of efficiency when the pressure in the receiver is 10 bar, which is very good since the machine use almost the entire input power and since the efficiency is directly proportional to the air indicated power of the system and inversely proportional to the electrical input power of the system as shown in equation (3), see appendix B, the efficiency of the system is bigger when the difference between the air indicated power of the system and the electrical input power of the system is small which means that the machine uses a large amount of power and reject a small amount of power.
Figure 1: relation between free air delivery and delivery pressure.


Figure 2: relation between electrical input power and air indicated power of 2 stages with respect to delivery pressure.


Figure 3: Relation between the overall efficiency with respect to delivery pressure.


Figure 4: p-v diagram for a two-stage compression cycle with intercooling.


3.1. Discussion

From the results obtained, it is observed that the work required to the compressor is reduced by passing the air through the intercooler which has a counter clock water flow which leads to low temperature and volume which reduce the input power required by the compressor as shown in figure (4), were the black line is a single stage air compressor with points shown in table 4, see appendix B, and the red is for the two stage air compressor with the points shown in table 4, see appendix B.

4. Conclusion
From the intercooling processes carried out in two stage air compressor experiment, it can be concluded that the work required to compress the air is less than the single stage compressor. So the power required to drive the two stage air compressor is also less with respect to the single stage compressor. From all the results of intercooling processes, it can be concluded that the use of the intercooler result in large volumetric efficiency. The polytropic index of compression n of this machine is usually between 1.2 and 1.3, and the calculated value of the overall polytropic index of compression, n, is 1.222, which means that the value obtain from the experiment is between these limits, we can conclude that the experiment for this part was successfully carried out because, the value obtain was in the limits.
There could be possible error that took place during the experiment conducted. Since the pressure and temperature through the rig have to be in stable conditions and requires a lot of time, so this may have caused errors during taking the measurement. There might also have error due to the instrument used to carried out the experiment, the machine used in this experiment maybe not functioning well, thus, it affect the measurement. In order to overcome these error and to get the accurate value of the experiment, things need to be put into consideration. Firstly, the person doing the experiment need to wait until the machine reaches stable conditions, the observer must be 90ᵒ perpendicular to the reading of A and V and before the experiment is conducted the machine used must be ensure to be functioning well.

Appendix A

Table 1: detail of the machine and indexes of compression.
Cd
d
D
E
A
Index of compression
stage 1
Index of compression stage 2
Index of compression overall
0.62
12.7
25.4
1.0328
0.000127
1.43
1.308
1.222


Table 2: data collected during the experiment and calculated values of free air delivery and mass flow rate.
Receiver pressure
air at inlet
air to intercooler

air from intercooler
air at delivery
1st stage
Delivery pressure
2nd stage delivery pressure
A1
A2
V1
V2
FAD
Mass flow rate
10
24
99

38
187
1.1
10
6
10
120
120
0.0035635
0.00437
9.5
24
99

37
185
1.1
10
6
10
120
120
0.0035635
0.00437
8
23
98.5

37
177
1.1
8
6
9
115
120
0.0035914
0.0044
7
24
97.5

37
168
1
7
6
8.5
115
120
0.0036053
0.00442
6
24
96.5

35
158
1
6
6
7.5
115
120
0.0036053
0.00442
5
25
95

34
150
1
5
6
7
115
120
0.0036467
0.00447



Table 3: calculated values of power and efficiency.
Receiver
pressure drop
shaft speed 1
shaft speed 2
Electrical input power stage 1
Electrical input power stage 2
Electrical power overall
Indicated power stage 1
Indicated power stage 2
overall indicated power
Efficiency
(%)
Volumetric efficiency
(%)
10
127
750
750
720
1200
1920
799.3
944.8
1744.1
90.84
74.09
9.5
127
751
745
720
1200
1920
799.3
944.8
1744.1
90.84
73.99
8
129
752
747
690
1080
1770
683
815
1498
84.63
74.47
7
130
750
745
690
1020
1710
648.1
770.6
1418.7
82.97
74.96
6
130
752
747
690
900
1590
573.3
683.1
1256.4
79.02
74.76
5
130
751
747
690
840
1530
497.8
592.1
1089.9
71.24
75.71

Appendix B

Equation (1): V=maRairTP

Equation (2): ma=CdEA2 pρ

Equation (3): η=Air indicated power of the systemElectrical input power to the system

Equation (4): T1T2=P1P2n-1n

Equation (5): Wi=maRairT0nn-1(PR)(n-1)n-1

Table 4: Points of the p-v diagram.
Volume (m³)
Pressure (bar)
1.96E-04
9.87E-01
1.16E-04
2.09E+00
1.05E-04
2.09E+00
2.94E-05
1.10E+01
2.83E-06
1.10E+01
3.84E-06
2.09E+00
6.48E-06
9.87E-01
3.64E-05
1.10E+01

FREE AIR DELIVERY VS DELIVERY PRESSURE

Delivery pressure (bar)


Free Air Delivery (mᵌ/s)



Power vs Delivery pressure

Delivery Pressure


Power




Overall efficiency vs Delivery pressure

Delivery pressure (bar)


Efficiency (%)