Abstract
The purpose of this experiment is to investigate the effect of delivery pressure on the performance of reciprocating air compressor. The usage of an air compressor is to increase the amount of air in a confined space and use the compressed air to generate power. The Cussons P9050 Two Stage Reciprocating Air Compressor Test Set is used in this experiment. In the beginning the system is set up to a single stage air compressor and it is operated at a constant speed.
Then, by manipulating the air receiver, the dynamometer load, the inlet air flow orifice manometer reading, outlet and inlet temperature are recorded. From the results gathered, the values of air flow, power output and efficiencies are calculated. Three graphs representing the relationship between the performance of the air compressor and the pressure, represented by the air flow, the power output and the efficiencies are also plotted. From the graphs we can see that the discharge pressure, isothermal efficiency, T and isentropic efficiency, S increases whereas the volumetric efficiency and vol decreases.
The errors will be discussed later.
Introduction
Two-stage air compressors are used in a range of industries, such as in the manufacture of chemicals and fertilizers. They supply power to pneumatic tools like ratchet wrenches spray guns and air nailers. They can be found in gas stations and major manufacturing plants. They are usually stationary and can be used to supply power to a variety of tools for a long period of time. A two-stage air compressor is a heavy-duty air compressor that uses two cylinders. An air compressor is a device that increases the amount of air in a confined space.
Confined air builds pressure and generates power for industrial, commercial and personal uses. The two-stage air compressor comes with two pistons. One pumps air from one cylinder to a second cylinder, and this increases the air pressure for generating the amount of power necessary for the equipment. In this experiment, the Cussons P9050 Two Stage Reciprocating Air Compressor Test Set is used. Cussons 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 the interconnections between the two major assemblies with a three phase main electrical supply, a water supply and a water drain.
First Stage Compressor
The first stage compressor is an industrial twin cylinder air-cooled machine with 70mm bore and 50mm stroke having a capacity of 385ml constructed with a “Vee” configuration at 90? between the cylinders, both connecting rods are driven from a single crankpin overhung from the crankshaft, which is supported, by one roller bearing and one ball bearing.
The cylinders and cylinder heads are heavily finned and are air-cooled by a fan integrated within the flywheel pulley. The crankcase acts as an oil reservoir. Lubrication of all working parts is achieved using oil mist generated by a splash pin protruding from the bottom of one of the connecting rods dipping into the sump oil at each revolution. An oil sight glass is also incorporated in the crankcase. Each piston incorporated two oil scrapper piston rings and two pressure piston pressure piston rings.
The inlet and outlet valves are spring leaves or reed type, both are fitted with diaphragm operated head unloaders which unload the compressor by holding the inlet valves open under the control of an automatic pilot unloader valve which is fitted to the lower air receiver when the air receiver pressure exceeds a pre-set value. The delivery valve acts as a non-return valve when the compressor is unloaded, thus preventing discharge of the air back into the compressor. The two cylinders outlet ports are each connected to a 0. liters capacity anti-pulsation chamber located between the two cylinders. Compressor minimum speed is at 300 rpm while maximum speed is at 1300 rpm.
Second Stage Compressor
The second stage compressor is a vertical single cylinder machine with 50mm bore 70mm stroke giving a capacity of 137ml. The detail design and construction of the second stage are identical to the first stage machine except that only one pressure piston ring is used and neither an inlet head unloader nor an anti-pulsation chamber are fitted.
Compressor minimum speed is at 300 rpm while maximum speed is at 1260 rpm.
Dynamometer
The dynamometer is a separately excited direct current motor arranged as a swinging field machine by mounting the stator on a trunnion end frame extensions supported in pedestals fitted with self-aligning bearings. The non-drive end the dynamometer shaft carried a 100-toothed wheel that is used with a magnetic pick up to provide speed measurement. Two torque arms are attached to the stator diametrically opposite to each other on a horizontal centerline.
If necessary, during manufacture, weights can be bolted to these torque arms to provide static balance. Spherical joints to attach the right hand arm, viewed from the driven end of a strain gauge load cell at a radius of 250mm that measures the retraining torque that is required to prevent rotation of the dynamometer stator. Two weight hangers are provided with a set of weights to calibrate the load cell.
Belt Drive System
Each compressor is driven by a single ‘vee’ wedge belt of 13mm x 10mm section; the outer rim of each compressor flywheel is machined to form a 364. 5mm diameter pitch driven pulley.
The drive pulleys are mounted on the dynamometer shaft using taper lock bushes with the 140mm diametral pitch drive pulley for the first stage being inboard of the second stage. Thus allowing easy access to remove and change the second stage drive pulley (three different pulley being supplied of diametral pitch 106mm, 118mm, 132mm). This arrangement also makes removal of the second stage belt easier when converting between single stage and twin stage. Each compressor is mounted on an adjustable slide base, fitted with a single hexagon head adjustment screw of 24mm across flats, allowing easy adjustment of the drive belt tension.
An extended hexagon-locking nut is provided to secure in the position.
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 exchange uses a counter flow configuration of water flowing upwards through the tubes and the air entering the shells at the top and leaving at the bottom. A three port two-bay 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 collected condensate.
The moisture separator has a maximum operating temperature of 80C.
Air Receivers
The two horizontal air receives are welded steel construction of 457mm diameter, and 1524mm long having a nominal capacity of 250 liters each. Both air receivers are fitted with feet, saddle, drain valve, safely relief valve pressure gauge and delivery valve. The lower air receiver is fitted with a non-return valve, a connection from the compressors, an off-loading valve and a moisture separator directly to the drain valve. This arrangement provides lower air receiver to always be used with the upper air receiver available to doubling the receiver capacity.
Bedplate Assembly: The bed plate 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 barriers the dynamometer and load ceil whist two pairs of hollow section square tube support the two compressor slide bases. The bedplate is mounted on six spring mounts that provided vibration isolation from the floor.
Discussion
The purpose of this experiment is to study the effect of delivery pressure on the performance of a reciprocating air compressor.
The value of the air flow, power output and efficiency represents the performance of the air compressor, which are calculated by applying the formulae listed in the theoretical background section.
Reasons
- The isothermal process is characterised by the least specific energy and it is a standard for these compressors. The usage isothermal efficiency, T is to perform criterion for reciprocating and rotary compressor with high rate water-cooling.
- Isentropic efficiency, s is used to perform criterion for axial and centrifugal compressors with low rate cooling. This is backed up by the fact that perfect performance of these compressors is the isentropic process.
- Volumetric efficiency, vol accounts for losses due to gas leakage through clearances at compressors seals.
In this experiment errors and discrepancies exists, some of it are:
- The scale moves about which makes the readings of the data is unstable which may cause inaccurate data.
- Parallax error where the position of the eyes are incorrect and it is not perpendicular to the inlet air flow orifice manometer scale.
- Internal problems of the air compressor, the age of the compressor is very old and its usage is often. This may cause inaccurate data too.
- The assumption of perfect adiabatic process exists cannot be achieved in the compressor because of the exchange of heat between the gas flow and its surroundings cannot be completely prevented by insulation. This causes the air temperature to increase and the isothermal efficiency to be incorrect.
- The limitation of the methods applied. The method used in this experiment is a dynamic test which does not allow steady state conditions to be achieved. As a result, energy balance of the compressor will not attain equilibrium which may cause the volumetric efficiency and power input some error.
Therefore, some precautions need to be taken in order for this experiment to succeed:
- Make sure the eye position is perpendicular to the inlet air flow orifice manometer scale when the reading is being taken.
- Avoid touching the hot parts of the air compressors during the experiments due to safety issues.
- The limitation of method can be avoided by operating the compressor with air being discharged from the air receivers, with the air receiver pressure is controlled in each value in turn as a steady state condition by carefully adjusting the discharge valve.
Conclusion
The performance of the air compressor can be represented by the values of the air flow, the power output and the efficiency. As a conclusion, when the discharge pressure increases,
- The inlet air flow decreases. The volumetric efficiency, vol decreases.
- The isothermal efficiency, T increases.
- The isentropic efficiency, S increases.
- All the power output WC, WD, WS and WT increase.
References
- http://www. ehow. com/about_5103936_twostage-air-compressor. html
- Yunus A. Cengel, Michael A. Boles, Thermodynamics: An Engineering Approach, 6th edition. McGraw-Hill
- Obert, E. F. & Young, R. L. Elements of Thermodynamics and Heat Transfer. New York: McGraw-Hill.
- Experiment Handout.
