Expt. No. :

Date :

Title: Load Test on Reciprocating Air Compressor.Aim: To study the variation of volumetric efficiency vol with variation in charge pressure.

Theory: An air compressor is a machine to compress the air and to raise its pressure with an appreciable increase in its density and deliver it to a storage vessel. Since the compression of air requires some work to be done on it, hence a compressor is a work consuming device and must be driven externally by means of a prime mover.

Air compressors are classified on three bases. They are as follows: On the basis of method of compression as Reciprocating, Rotary, Centrifugal and Axial.

On the basis of number of stages as Single-stage and Multi-stage.

On the basis of the number of sides of the piston used for compression as Single-acting and Double-acting.

Elaborating further, reciprocating compressors are positive displacement machines, which accomplish compression by a piston within a cylinder as the compressing and displacing element. Rotary compressors are compressors wherein compressing action is brought about by rotating motion either by vanes or lobes or impellers or blades. In a single-stage compressor, the air is compressed to the required pressure in a single cycle i.e. in a single stage. Multi-staging is an arrangement in which two or more cylinders are used for bringing about compression. These cylinders are often provided with inter-coolers to remove some of the heat between each stage. This saves power and keeps the compressors internal working temperatures lower. It is used when air at a very high pressure is required with a lesser power input compared to what would have been needed in case of a single stage compressor. Thus work done per kg of compressed air supplied is reduced in multi-staging. Multi-staging thus improves the efficiency of the compressor for a given pressure ratio.

In a single-acting piston compressor, the piston only compresses air in one direction of its stroke.

In a double-acting model, the piston compresses air with both directions of its stroke. Here both strokes perform work hence it is more efficient in moving a volume of air per unit power than a comparable-size single-acting unit.F.A.D.: Free Air Delivery (F.A.D.) is the actual volume delivered at the stated pressure reduced to intake temperature and pressure and expressed in c.m.m. (cubic meter per min).

Volumetric efficiency vol : Volumetric efficiency is the ratio of the actual volume of gas taken into the cylinder during suction stroke to the piston displacement volume ( PD) or the swept volume (Vs) of the piston.

Intercooler: Intercooler is a heat exchanger in which heat is removed from the compressed air during multistage compression. The reason behind the implementation of such heat exchanger device is to keep the compression process during the multistage operation, as close as possible to the isothermal one, to obey the ideal gas laws. In practice, the gas behavior deviates from the ideal process and tends to close to a polytropic process; which is less efficient than the isothermal. With the use of the intercooler, the outlet temperature of the gas is made as close as possible to the same delivered by the ideal isothermal compression process. More the number of stages (and hence the number of intercoolers), better is the resemblance between the actual compression process and the ideal isothermal process.Aftercooler: After-cooler is a heat exchanger used to remove heat from the completely compressed air obtained from the last stage of multi-staging. It is connected between high pressure cylinder and the reservoir. Its priority is reduction in volume of the compressed air so that reservoir can take in maximum air.Procedure :

1. The water circulation is started for the aftercoolers before actually starting the compressor. This is done in order to bring the after-cooler to its requisitely low temperature.2. The compressor is then started, which takes in air from the atmosphere and starts compressing it.3. The temperatures of the compressed air entering the reservoir are noted with the help of a thermometer attached to the reservoir and the pressure of the compressed air is noted down with the help of a Bourdon gauge attached to the reservoir. The time it takes for the pressure to rise by 1 bar gauge pressure is noted down starting from 0 to 7 bar gauge pressure.4. At intervals of 1 bar charge pressure, temperature and time taken (in seconds) are noted down.5. The readings are noted till the gauge reads 7 bar.6. The compressed air is then released back to the atmosphere.

SPECIFICATIONS OF MULTI-STAGE, SINGLE ACTING AIR COMPRESSOR :

Manufacturers: Reavell and Co. Ltd. (England)Type: Two stage reciprocating with intercooler (air cooled) and aftercooler (water cooled), cylinders (air cooled). Speed = 720 rpm

Pressure = 14 kgf/cm2 Diameter of low pressure (L.P.) cylinder = 117.5 mmDiameter of high pressure (H.P.) cylinder = 73 mm

Stroke of both pistons = 89.5 mm

Volume of receiver = 0.303 m3

OBSERVATION TABLE :Pgauge

(KPa)Pabs

(KPa)Volres

(m3)Temp

(K)Time interval(sec)

SAMPLE CALCULATIONS :RESULT TABLE :Mass

(m)Difference in mass

(m)Mass flow rate(actual)

(kg/sec)Mass flow rate(theoretical)

(kg/sec)vol

%

CONCLUSION: