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Of the tests required to evaluate the performance of the compressor are given below:

 Pump - up or capacity Test

This test determines the pumping capacity of the compressors in terms of free air delivery (FAD) i.e. air pumped at atmospheric conditions. To conduct this test, the air receiver of the compressor should be isolated from the system i.e. only compressor should be with the receiver. Drain the air receiver completely. Now, switch on the compressor and observe the time taken by the compressor to maintain the working pressure in air receiver. In other words, note down the time required by the compressor to fill the air receiver up to required pressure. Minimum three readings are required to calculate an average value of time in minutes. Feed this average value in minutes and other values such as receiver diameter, length or volume in the formula to determine the pump-up capacity of the compressor.

The following are different formulas used to evaluate the performance of Compressor :

FAD = V x (P2 - P1) / (Po x T)

Where

FAD - Free Air Delivery (m3/min)

V - Volume of the receiver (m3) + volume of the pipeline connected from compressor to air receiver (m3)

Po - Atmospheric Pressure

P1 - Initial Pressure of the receiver (Kg/cm2)

P2 - Final Pressure of the receiver (Kg/cm2)

T - Average Time taken (min)

Specific Power (KW / 100 CFM) = Actual Power x 100 / FAD (CFM)

Where

KW - Actual Power drawn by compressor

Compare this value this design value of FAD. If the difference is more than 20%, it is high time to look at piston rings, cylinder bores etc.

 Leakage Test

To conduct this test, close all the valves at the equipments where compressed air is in use. Drain the air receiver completely and start the compressor. Note down the time taken by the compressor to maintain the system pressure i.e. up to compressor unloads. This is compressor on load time in seconds. Due to the leakages in the systems (if present), the pressure in the receiver drops to the cut off pressure and again compressor starts. Note down the time taken by system pressure to drop up to cut off pressure. This is compressor off load time in seconds. The readings should be taken minimum three times and the average values are to be used to determine leakages in the lines. Feed these two values in seconds in the formula to determine leakages and its potential.

Leakages (m3/min) = FAD x T1 / (T1 + T2)

Where

FAD - Actual free air delivery of compressor (m3/min)

T1 = Average on load time of compressor (min)

T2 = Average off load time of compressor (min)

Power wasted in Rs. / Annum = 5.54 x L x Operasting hours per annum x Rs./KWhr

Where

L - Leakages in m3/min

Conversion Factors :

M3/hr = NM3/hr x 1.17

Scfm (Standard Cubic foot per minute) = M3/hr x 31.81

M3/min = cfm x 35.3

While large leakages can be detected easily due to hissing sound produced, there are large number of small leakages that are difficult to detect. Small leakages can be detected by applying soap solution on pipelines, joints etc. or with the help of the ultrasonic testing equipments available in the market. Some of the most susceptible points are:

Underground pipelines

Threaded pipe joints

Flange connections

Valve steam

Traps and drains

Filters

Hoses

Connectors

Operating valves on pneumatic devices

Check valves

Relief valves

End use machines or tools

Pressure drop in Compressed Airline

In most of the industries, compressed air is supplied from a central compressor room. As per the requirement, the number of compressors list goes up. Finally most industries are saddled with a battery of compressors at one location while end use points are spread over large area. Hence considerable losses take place in the distribution of pipe lines, joints, bends, valves, hoses, couplings etc. Proper sizing of the pipelines and hoses and selection of appropriate type of valves and couplings are essential to ensure efficient operations.

The pressure in a pipeline can be calculated using following formula:

Pressure Drop, Kg/cm2 = 7.57 x (Q^1.85) x L x (10^4) / (d^5) x p

Where Q = Air flow in cu.m/min (FAD)

L = Length of pipeline (m)

d = inside diameter of pipe (mm)

P = Initial pressure, kg/cm2

^- indicates 'to the power of'

 The points to be kept in mind while designing a distribution system are :

·         Low pressure drop between the compressor plant and the end use point.

·         Minimum leakage

·         Minimum number of joints, bends, fillings in the pipeline.

·         Proper design and layout of the pipeline

A general guide for selection of pipe size as recommended in IS:6202 is that the pressure drop should not exceed 3 kg/sq.cm at the farthest end of the line. For plants, covering large area, the pressure drop up to 0.5 Kg/sq.cm may be acceptable.

The required pipe diameter and length of the pipe line are given in report if the pressure drop exceeds the allowed pressure drop. The pressure drop can be reduced either with the increase in diameter or with reducing the effective length of the pipe but not with both.

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