FIRST STAGE COMPRESSOR

ContentsABSTRACT2TITLE3INTRODUCTION3OBJECTIVE5THEORY5EQUIPMENT7EXPERIMENT SET UP7PROCEDURE8DISCUSSION9CONCLUSION10REFERENCES11

ABSTRACT

This experiment is done to analyse the variation of performance with pressure for a two stage reciprocating air compressor. The objective of this experiment is to investigate the effect of delivery pressure on the performance of a reciprocating air compressor.The experiment is conducted by using The Cussons P905 Two Stage Reciprocating Air Compressor Test Set. The experiment was first started with the discharge valve closed and compressor operating at constant speed at 700 rpm. The pressure is varied from 2.0 until 11.0 bar. The force and the output temperature are recorded. Further calculation was made to determine the inlet airflow, power outputs and efficiencies.The relationship between pressure and the compressor performances are determined by the graph plotted. Next, comments about the graph are stated in the discussion part. Finally, a conclusion on the experiment is made.

TITLEFirst stage compressor

INTRODUCTIONCussons P9050 Two Stage Reciprocating Air Compressor Test Set is constructed in two major assemblies- an instrumentation and a control console which straddles on a welded steel base-frame carrying the air compressor system. The test set is a complete and self-contained system requiring only interconnections between the two major assemblies with a three phase main electrical supply, a water supply and a water drain.The two compressors are separately controlled by DC dynamometer. Each compressor is driven by a single vee wedge belt at a variable speed. The advantage of this drive system is it can be used either a two stage machine or a single stage machine with the second stage undrive and bypassed. The delivery air flow can be varied and the capacity of the delivery can be changed by connecting one or both of two air receivers. This test set also can be used as air compressor in the laboratory by using a single stage form. This test set consists of seven parts which is:

1. The first stage compressor The first stage compressor is an industrial twin-cylinder aircooled machine with a bore of 70 mm and a stroke of 50 mm giving a capacity of 385 ml. The inlet valves are fitted with diaphragm-operated head unloaders which unload the compressor by holding the inlet valves open. The unloaders are connected to automatic pilot valves fitted to the lower air receiver which opens when the air pressure reaches a preset value. The delivery valve acts as a non return valve when the compressor is unloaded thus preventing discharge of the air receiver back through the compressor.

2. DynamometerThe dynamometer is a separately excited DC motor with the stator of the machine mounted in self aligning trunnion bearings. The dynamometer is fitted with torque arms, one of which connects to a strain gauge load cell for load measurement. A 100 tooth wheel is fitted at the non drive end of the machine where a magnetic pick-up feeds a pulse train signal to a frequency to voltage convertor for speed feedback/indication.

3. Belt Drive System

The dynamometer is a separately excited DC motor with the stator of the machine mounted in self aligning trunnion bearings. The dynamometer is fitted with torque arms, one of which connects to a strain gauge load cell for load measurement. A 100 tooth wheel is fitted at the non drive end of the machine where a magnetic pick-up feeds a pulse train signal to a frequency to voltage convertor for speed feedback/indication.

4. Water Cooled Intercooler

The water cooled intercooler is a shell and tube heat exchanger which is mounted vertically on a bracket behind the first stage compressor. The heat exchanger is used in a counter flow configuration with the water flowing upwards through the tubes with the air entering the shell at the top and leaving a t the bottom. A three port two-way ball valve at the inlet to the intercooler is provided to allow the intercooler to be bypassed. A moisture separator is fitted at the exit from the intercooler to provide automatic periodic discharge of the collected condensate. The moisture separator has a maximum operating temperature of 80 C.

5. Air Receiver

Two horizontal air receivers each having a capacity of 250 litres are fitted each with a drain valve, safety relief valves, pressure gauge connection and delivery valve. The receivers are arranged so that the lower one is always used while the top one is available to double the system capacity when required.

6. Bedplate Assembly

The bedplate is a welded structure manufactured from standard steel sections. The lower air receiver is directly mounted across one end of the bedplate with the second air receiver mounted on top of the first. A steel plate cross-member carries the dynamometer and load cell whilst two pairs of hollow section square tube support the two compressor slide bases. The bedplate is mounted on six spring mounts which provide vibration isolation from the floor.

OBJECTIVE1. To investigate the effect of delivery pressure on the performance of a reciprocating air compressor.2. As the requirement for the transfer degree program.3. To introduce the student about the basic knowledge of air compressor system.

THEORY Working Equation (appendix)1. Intake air pressure

* h in mm 2. Inlet air flow

3. Volumetric efficiency

4. Dynamometer power output

5. Compressor power input

6. Ideal isothermal power input

6. Ideal isentropic power input

7. Isothermal thermal efficiency

8. Isentropic thermal efficiency

*Nc = 0.3636 Nd

EQUIPMENT

1) Cussons P9050 Two Stage Reciprocating Air Compressor Test Set.

EXPERIMENT SET UP

Set up the system as a single stage air compressor operating into both air receivers by removing the drive belt from the second stage compressor, and then opening the bypass from the second stage compressor. After that, close the bypass around the cooler and open the interconnecting valve to the second air receiver.

FIGURE 1 : Air System Schematic

PROCEDURE

1. The compressor is operated at constant speed ,700 rpm and readings are taken whilst the air receivers charge.

2. At an increments of air receiver pressure of 0.5 bar with constant compressor speed Nc= 700 rev/min, the reading of air compressor discharge pressure Pc, inlet air flow orifice manometer reading h, and the reading of dynamometer load LD are recorded.

3. When the air receiver pressure reaches the pre-set value on the off-loading valve, both inlet valves are opened and compressor delivery is ceased.

4. A reading of the dynamometer load in this condition can then be used as a measure of the drive belt and cylinder friction power, in which can then be deducted from the measurement of dynamics power output to give a measure of the compressor air power input, Wc.

5. For each set of results obtained, the following parameters are calculated (referred to Appendix ):

a) Inlet air flow, QC b) Volumetric efficiency, volc) Dynamometer power output, WDd) Compressor power input, WTe) Ideal isothermal power input, WTf) Ideal isentropic power input, Wsg) Isothermal thermal efficiency, Th) Isentropic thermal efficiency, S

6. Graphs of the discharge pressure P2 against the inlet air flow Qc is plotted and P2 against the power output Wc, WD, WS, WT are plotted. Then, the graphs of vol, S, r against P2 are plotted.

DISCUSSION

From the graph 1, which is the discharge pressure versus inlet air flow, we can see that the inlet airflow, will decrease as we increase the discharged pressure, and this relation is inversely proportional. Based on the theory, the flow will be low at high pressure.

By studying the shape of graph 2,power versus the discharge pressure, it is obviously shown that all the power increases when the discharged pressure, increases. This shows that the relation for power output and power input is directly proportional. For the relation between power and discharge pressure, we can say that when discharged pressure, increases, the dynamometer power output, ,compressor air power input ,isothermal power input , and isentropic power input, also gradually increase.

Based on the graph 3 which is efficiencies versus discharge pressure, we can conclude that when the discharged pressure increases, the isothermal efficiency and isentropic efficiency are slightly increase. Meanwhile, the inversely proportional of graph is shown by volumetric efficiency.

Due to shortage of water during the experiment, we used chiller instead of inter cooler, to supply the water in and to dispose the water out. Therefore, the temperature T and T are slightly different with the normal condition.

The readings are not accurate because of the parallax error. Eye is not meniscus and not straight to the meter. There might be some errors in this experiment due to some technical reasons, for example, the reading shown on the machine is always fluctuating, so different person will read a different reading, which is not very accurate.

CONCLUSION

As a conclusion, inlet airflow, and are inversely proportional to whereas all the power, proportional to discharge pressure, .

REFERENCES

1. http://www.cussons.co.uk/pdf/english/enthermo/P9050.pdf

2. Boles, Michael A., and Yunus A. Cengel. Thermodynamics: An Engineering Approach with Student Resource DVD. New York: McGraw-Hill Science/Engineering/Math, 2006.

3. http:// www.cussons.co.uk/pdf/english/enthermo/P9050.PDF

11