undefined | unit 5 reciprocating air compressors

Unit- 5

Reciprocating Air Compressor

Q1. Explain the Volumetric efficiency and its effect on compressor performance

A1.

Volumetric efficiency.

It is the ratio of actual volume of charge admitted during the suction stroke at N.T.P. to the swept volume of the piston. Mathematically, volumetric efficiency

Note : The volumetric efficiency may also be defined as the ratio of the mass of actual charge admitted to the swept mass of the charge at N.T.P.

Q 2. Explain the Performance of reciprocating compressors

A2. For a given evaporator and condenser pressures, the important performance parameters of a refrigerant compressor are:

a) The mass flow rate (m) of the compressor for a given displacement rate

b) Power consumption of the compressor (Wc)

c) Temperature of the refrigerant at compressor exit, Td, and

d) Performance under part load conditions.

The mass flow rate decides the refrigeration capacity of the system and for a given compressor inlet condition, it depends on the volumetric efficiency of the compressor. The volumetric efficiency, ηV is defined as the ratio of volumetric flow rate of refrigerant to the maximum possible volumetric flow rate, which is equal to the compressor displacement rate, i.e

Q3. What is the Limitations of single stage compression, Multistage compression and intercooling?

A3.

Ans Heavy moving/working components to compress air to such a high pressure,

There might be some balancing problems due to heavy moving parts,

The power requirement for such heavy parts movement is too high,

There will high torque fluctuations,

To compensate for the torque fluctuations, a heavy flywheel is required.

Better cooling arrangements are required, and

Lubricating oil which does not get vaporized at such high temperatures.

Multistage compression

1.Intercooler required in multistage compressor so construction of multistage compressor is difficult.

2. In multistage compressor intercooler is used and two compressor is required so cost is high required.

INTERCOOLERS

Limitation of these intercoolers is that their efficiency depends on both the ambient temperature and the road speed of the vehicle if the airflow through the intercooler's core is not fan-assisted.

Q4. What is the Air motor?

A4. As a matter of fact, the compressed air (from an air-compressor) is made to enter the cylinder of an air motor which pushes its piston forward in the same way as of a reciprocating steam engine. Now the actual work is done by the movement of the piston. Now consider an air motor working with the help of compressed air

P1 = Pressure of the compressed air,

V1 = Volume of the compressed air.

The theoretical indicator diagram of a reciprocating air motor without clearance, compression and pressure drop at release is shown in Fig. 31.1. The compressed air from the compressor is admitted into an air motor at A with pressure p1 . It drives the piston forward. But after a part stroke is performed, the air supply is cut-off at B and the expansion occurs from B to C. After the stroke is completed, the air which has done some work is exhausted into the atmosphere at a constant pressure p2. We know that work done by the air per cycle.

Q5. Estimate the minimum work required to compress 1 kg of air from I bar 22°C to 16 bar in two stages, if the law of compression is pv = constant and Site intercooling is perfect. Take R = 287 f/kg K.

A5. Given:

We know that for perfect intercooling, the intercooler pressure,

Minimum work required to compress 1 kg of air,

Q6. Find the percentage saving in work by compressing air in two stages from I bar to 7 bar instead of in one stage Assume compression index! 35 in both the cases and optimum pressure and complete intercooling in two stage compressor.

A6.

Given

We know that workdone in compressing air in one stage,

Saving in work done

Q7. A multi-stage air compressor is to be designed to elevate the pressure from I bar to 100 bar such that the stage pressure ratio will not exceed 4. Determine: I. Number of stages, 2. Exact stage pressure ratio.

A7.

Given

1. Number of stages

Let q=number of stages

We know that

Taking log on both sides,

2. Exact stage pressure ratio

We know that

Q8. A two stage air compressor, with complete intercooling, delivers air to the mains at a pressure of 30 bar, the suction conditions being I bar and 27° C If both cylinders have the same stroke, find the ratio of the cylinder diameters, for the efficiency of compression to be, maximum. Assume the index of compression to be 13.

A8. Given :

We know that for complete intercooling, the intercooler pressure.

Ratio of cylinder diameters,

Note : The ratio of cylinder diameters is also given by

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