Volumetric efficient of a compressor, introduction, calculation, procedure, conclusion and results

1. VOLUMETRIC EFFICIENCY OF A
COMPRESSOR
EXPERIMENT NO.:
INSTRUCTED BY:
GROUP MEMBERS:
NAME :
COURSE : BSc. Engineering
INDEX NO. : 150
GROUP :
DATE OF PER. :
DATE OF SUB. :

2. INTRODUCTION:
In most of the manufacturing processes use compressed air or liquid. They are used in various
processes. As an example, to work with pneumatic tools, need high pressure air, can provide using
compressor. The use of compressor can be categorized under three headings, power service, process
service and control.
In power service, air or liquid is used to produce motion or to develop force or both in same time. In
actuators that using air, pneumatic tools are some of the examples. The meaning of the power service
is an application in which air or other gas enters into a process itself. As an instance combustion use
air to produce power itself. Control applications that use are control triggers, start stop modulates or
direct machine processes.
Mainly compressors are used to increase the pressure of the air in mechanically means. Most of the
industries are used compressed air but some of the special purpose industries are used Nitrogen,
Oxygen as the requirement of them.
There are various types of compressors. Mainly they divide in to two categories, positive displacement
and dynamic. Again, positive displacement compressors are divided in to two categories, rotary and
reciprocating. And also, dynamic compressors are divided in to two categories as centrifugal and axial.
Positive displacement compressors are usually reciprocating piston type. They can be used to supply
small amount of air with high pressure. In centrifugal compressors increase kinetic energy of the gas
using impeller, and convert energy into increase pressure in a divergent outlet passage. To compress
moderately large amount of gas to moderate pressures, use centrifugal compressors. In axial
compressors gas flow through series of blades and when it passes through various blades velocity
fluctuation happens and when increase the kinetic energy that convert in to pressure energy. These
types of compressors used in mainly jet engines, gas turbines.

3. THEORY:
By definition;
Volumetic Efficeincy (ƞ) =
Volume of free air deleivered per min (Vinduced)
Volume swept by the piston per min( Vswept)
Free air delivery = K ×
t1 + 273
P1
+ √
H × M2
t + 273
K = 7.52 × 10−2
(Nozzle coefficient)
t = Nozzle temperature
H = Observed head in mm of H2O (upstream)
M2 = P1 −
h
13.6
in mm of Hg
P1 = Barometer reading in mm of Hg
h = Observed head in mm of H2O (downstream)
t1 = Atmospheric air temperature
Vswept =
π d2
4
× l × V × N
(Seattlr pi, 2017)
D = Diameter of the bore
l = Stroke length
V = motor speed (450 rpm)
N = number of cylinders.

4. PROCEDURE
• The valves of cooling water supply and discharge lines are fully opened.
• Then the motor is fully unloaded and started.
• The air compressor is started and the unit is allowed to function at 300rpm for about ten
minutes.
• Then the test is commenced and required readings are taken.
• The pressure at air receiver is kept constant at the required value while taking the reading and
the constant speed of 300 rpm is maintained.
• Note down the height of the liquid columns to calculate the pressure
• Change the speed of the motor and repeat the practical for another value.

5. CALCULATIONS:
Data:
No: of cylinders : 2
Bore : 101.6 mm
Stroke : 101.6 mm
Calculations are done and shown for the test 2
𝑃𝑑𝑒𝑙𝑖𝑣𝑒𝑟𝑦 = 10 𝑝𝑠𝑖
𝑇𝑑𝑒𝑙𝑖𝑣𝑒𝑟𝑦 = 30.5 ℃
H = 92 mm
h = 24 mm
𝑀2 = 762.5 −
24
13.6
= 760.735 𝑚𝑚𝐻𝑔
Free air delivery = 7.52 × 10−2
×
30 + 273
762.5
× √
92 × 760.735
30 + 273
= 0.4542 m3/min
Vswept =
π × (101.6 × 10−3
)2
4
× 101.6 × 10−3
× 300 × 2
= 0.4942 m3/min
Volumetic Efficeincy (ƞ) =
0.4542 m3/min
0.4942 m3/min
× 100 %
= 91.9 %

6. RESULTS:
For compressor 300 rpm
For Compressor 400 rpm
Test P (delivery)
(Psi)
T (delivery)
(℃)
H
(mm)
h
(mm)
Free air
delivery
(m3/min)
Swept
Volume
(m3/min)
Efficiency
(%)
1 10 30 92 24 0.4542 0.4942 91.9
2 20 30.5 57 19 0.3582 0.4942 72.47
3 30 30.5 83 20 0.4322 0.4942 87.45
4 40 30.5 56 14 0.3551 0.4942 71.85
5 50 31 53 16 0.3460 0.4942 70.01
Test P (delivery)
(Psi) T (delivery)
(℃)
H
(mm)
h
(mm)
Free air
delivery
(m3/min)
Swept
Volume
(m3/min)
Efficiency
(%)
1 10 30.5 179 50 0.6337 0.659 96.17
2 20 31 71 21 0.4003 0.659 60.75
3 30 31 122 21 0.5248 0.659 79.64
4 40 21 99 16 0.4729 0.659 71.75
5 50 31 115 32 0.5092 0.659 77.28

7. For compressor 500 rpm
Test P (delivery)
(Psi)
T (delivery)
(℃)
H
(mm)
h
(mm)
Free air
delivery
(m3/min)
Swept
Volume
(m3/min)
Efficiency
(%)
1 10 30.5 221 54 0.7040 0.8237 85.47
2 20 30.5 92 21 0.4550 0.8237 55.24
3 30 31 57 33 0.3585 0.8237 43.52
4 40 31.5 176 42 0.6307 0.8237 76.57
5 50 32 242 59 0.7402 0.8237 89.86

8. Volumetric efficiency vs delivery pressure
For 300 rpm
Load (psi) Volumetric Efficiency (100%)
10 91.9
20 72.47
30 87.45
40 71.85
50 70.01
For 400 rpm
Load (psi) Volumetric Efficiency (100%)
10 96.17
20 60.75
30 79.64
40 71.75
50 77.28
For 500 rpm
Load (psi) Volumetric Efficiency (100%)
10 85.47
20 55.24
30 43.52
40 76.57
50 89.86

9. Free air delivery vs rpm
For 10 psi
rpm Free air delivery
300 0.45
400 0.63
500 0.70
For 20 psi
rpm Free air delivery
300 0.36
400 0.40
500 0.46
For 30 psi
rpm Free air delivery
300 0.43
400 0.52
500 0.36
For 40 psi
rpm Free air delivery
300 0.36
400 0.47
500 0.63
For 50 psi
rpm Free air delivery
300 0.35
400 0.51
500 0.74

10. DISCUSSION
• Usefulness of the lab exercise and engineering application
Piston engines are used in many applications in engineering world. So that we have to get maximum
efficiency from these engines and reduce wastage of the energy. In order to get some knowledge,
we are going to get the understanding of the volumetric efficiency varies according to load.
Through that results we can get an idea in which load we have to use and the running speed of the
machine that deliver maximum efficiency. Mainly these results are used in automobile industry.
And also, refrigeration and ac, pneumatic tool industries use these results. In other words, we can
say that these results are going to help all the industries that use piston engines, gas turbines and
industries that use high pressure liquids and gases.
Mainly in designing piston engines they are going to consider how much air is going to compress
and the pressure increase inside the cylinders. With the practical testing, they come to know that
what are the best compression ratios that the material of the piston engine can withstand. In the
boiler making industry also same. Most of the industrial applications are based on metallurgical
limits of the components and if they can use maximum limits of the properties of the gases or
liquids, it comes to cost effective. So that these practical results are going to design components as
well as increase cost effectiveness of the industry.
• Volumetric efficiency of an air compressor
Volumetric efficiency is the ratio between the volume of free air delivered to the compressor per
unit time and volume swept by the piston per unit time. More specifically, volumetric efficiency is
a ratio (or percentage) of the quantity of air that is trapped by the cylinder during induction over the
swept volume of the cylinder under static conditions. In the case of forced induction volumetric
efficiency can exceed 100%.
Basically, volumetric efficiency is reduced than that appears because of the clearance volume, that
have to take for account. That causes the difference between theoretical and practical values that
comes from the volume of air compressed inside the cylinder. When gas is compressed and delivered
through the discharge valves, all the gas doesn’t discharge and remain some amount, that causes the
clearance volume. When piston goes down, suction valve is not open immediately with the
movement of the piston because of the high pressure inside. That makes clearance volume. So that
gas used in suction stroke goes unused and reduces the volumetric efficiency of the compressor.
• Factors that affect volumetric efficiency
1. Induction and exhaust system flow friction – because of the flow friction, pressure drops occur
in the inlet across the air filter.
2. Induction system heat transfer
3. Valve time effects –
a. At low engine speed, amount of mixture that entered the cylinder during the intake is
pushed back into the intake by moving piston, during the compression stroke prior to
inlet valve closing
b. At high engine speed, high momentum of the air in the intake manifold towards the
cylinder set up during the intake stroke forces additional air into the cylinder while the
intake valve remains open
c. During the valve overlap period between inlet valve opening and exhaust valve closing,
both valves are open. The amount of overlap, and engine speed, affect the residua
friction

11. 4. Valve flow friction
5. In cylinder heat transfer – incoming air is cooler than the cylinder walls. So that heat transfer
from walls to air decrease the density of the air.
6. Intake tuning
7. Delivery pressure
8. Clearance volume of the piston
9. Number of stages in the compressor
10. Speed of rotation of the compressor
11. Operating temperature
12. Valve operating speed
13. Method of cooling
14. Method of closing isothermal processes.
• Meaning of free air delivery
That means actual quantity of compressed air converted back to the inlet conditions of the
compressor. Actually, this measures the internal capacity of compressor. That is the actual air
compressed at the inlet condition of the compressor. In good compressor, free air delivery should
be higher to make the cooling process much more effective.
• According to the steady flow energy equation, the possible variation of pressure and
temperature along the line from the compressor outlet to the orifice meter.
Temperature Variation
There are many ways of temperature varying along the compressor outlet to the orifice meter. Such
as,
a) Temperature drop at the outlet pipe.
The outlet pipe line is long to a certain extent and because of that heat transfer is happened
between the pipe line and the atmosphere.
b) Temperature drop at the pressure controlling gate
The turbulence created by the opening and the closing mechanism will help to destroy some
kinetic energy of the flow, hence resulted in drop of temperature.
c) Temperature drops at the storing tank.
Some amount of heat energy will be wasted to the environment in the storage tank, which
resulted in reduction in temperature.
Pressure Variation
In the practical pressurized air is stored in the tank and then released in to the orifice meter. So that
there should be pressure drop at the outlet of the tank and also inlet of the tank from the
compressor. And also, - there are several gates and valves that air must go through, causes some
pressure drop.
• Accurate method of measuring the absolute pressure upstream of the orifice meter.
We use u tube to get the pressure at the orifice. To get accurate results we have to increase the height
of the liquid columns. That reduce the reading errors. Here we have to use liquid that has density lower
than the liquid that we are using in the practical.

12. And also, if we can use calibrated pressure sensors rather than U tube, we can directly measure
pressure accurately and reduce human errors that will affect.
• Discrepancies observed between the experimental values and the expected values
Mainly these changes accrue because of the human errors and equipment errors. Rather than that,
the liquids and gases are not ideal one because they are real and have deviations from the ideal gas
behavior. We develop our system using ideal behavior of the substances. So that makes an error.
And also, there are heat losses in the system. Compressor test rig is consisting of longer steel tube,
that dissipate heat to the outside.
The speed of the engine is not constant during the practical so that the final values will deviate
from the others. And also, temperature of the atmosphere will change. That also affect the
practical.
We use some rounded numbers because we have to convert some values into another measuring
systems. That will make small errors.
• Suggestions to minimize errors.
Tubes and other steel parts must be covered to avoid heat rejection to the outside.
Have to use keen eye to read the heights of the liquid columns
Temperature and heights of the liquid columns must be read in eye level
Readings must be taken as fast as possible because the state will change rapidly.
References
Global, M. (2017). Expert air compressors. Retrieved from Air compressors:
https://www.expertmarket.co.uk/air-compressors/What-is-the-Volumetric-Efficiency-of-Air-
Compressors?question_page=4
K., A. (2011, march 27). Bright Hub Engineering. Retrieved from Mechanical Engineering:
http://www.brighthubengineering.com/hvac/33373-introduction-to-compressors/#imgn_2A
Seattlr pi. (2017). Retrieved from What is volumetric efficiency:
http://education.seattlepi.com/volumetric-efficiency-5041.html