Performance Evaluation Of Vacuum System General Electric Air Conditioner AS_10 Researchpaper\Performance Pump Down Time

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International Journal of Scientific & Engineering Research, Volume 2, Issue 11, November-2011 1

ISSN 2229-5518

IJSER © 2011

http://www.ijser.org

Performance Evaluation of Vacuum System:

Pump-down Time

Vishal D. Chaudhari, Avinash D. Desai

Abstract— Vacuum system finds applications in number of industries like process industry, pharmaceutical industry, petroleum industry,

material handling industry, etc. In most of its applications it is used as vital part of the system. A suboptimal performance in vacuum system

may result in inferior overall system performance. The performance of vacuum system is practically gauged in terms of time required to

achieve the requisite low pressure. This time is referred as pump-down time. Among the number of factors affecting pump-down time

important are pressure to be achieved and configuration of the system (length and diameter of tubing used to connect vacuum tank and

pump). In this paper theoretical procedure for calculating pump-down time is explained for the given pressure and configuration of the

system. These theoretical calculations are compared with the actual observations taken from the system. Graph of evacuation pressure vs

time is drawn for theoretical and actual observations and it is analyzed.

Index Terms— Vacuum, vacuum pump, pump-down time, performance, conductance, pumping speed.

——————————  ——————————

1 INTRODUCTION

Vacuum pump finds application in many number of indus-

tries as a vital part of the system. But very few things are

known by the engineers regarding vacuum pump and vacuum

system performance

Vacuum is defined as the space devoid of mater. For general

sense it is considered as pressure below atmospheric pressure.

Vacuum is broadly divided in the following categories

Pressure Range

Pressure in mbar

Low vacuum

103 - 100

Medium vacuum

100 – 10-3

High vacuum

10-3 - 10-7

Ultra high vacuum

10-7 – 10-12

The different types of vacuum pumps are available for dif-

ferent pressure ranges of vacuum. Noramally the manufactu-

rere provides the characteristics curve showing relation of

pumping speed versus pressure for each vacuum pump. This

characteristic curve helps to determine the pump-down time in

ideal situation. Pump-down time is the time required to to

achieve the required pressure.

But the characteristics curve has limitations when actual sys-

tem consisting of tank to be evacuated, piping of differeent di-

mensions with bends, is there. In this paper theoretical analysis

for finding pump-down time of a simple system is explained

followed by the experimental results for the same system.

2 Theoretical Calculation for pump-down time

The theoretical analysis for punp-down time calculation for a

vacuum system consists of following procedural steps:

Step 1: calculation of Knudsen Number to determine whether

the flow is viscous or molecular.Knudsen number is the ratio

of mean free path of molecule to the diameter of pipe. For Kn

< 0.01 the flow is regarded as viscous and for Kn > 0.03 the

flow is regarded as molecular.

Step 2: Conductance calculation: the pumping speed of a va-

cuum pumping station is reduced upto the recipient through

intermediate line, components such as valves and bellows. The

longer the lines and smaller the cross section, the grater are the

losses.

For determination of losses, in practical applicatons, conduc-

tance C is used. In addition to length and diameter, conduc-

tance also depends upon the type of flow of pumped down

material. For vacuum technology mainly viscous and molecu-

lar flow are suitable. In the range of viscous flow the conduc-

tance is dependent on pressure while in molecular flow range

it is independent of pressure. The conductance for round pipes

is calculated universally for all pressure ranges and for all gas

types as:

For air the con-

duc- tance is calculated

as:

————————————————

 Vishal D. Chaudhari, currently pursuing masters degree program in me-

chanical engineering in Pune UniversityIndia, E-mail: vishaldchaudha-

ri@gmail.com

 Prof. Avinash D. Desai, Vice-Principal and Head of Mechanical Engi-

neering Department, Modern College of Engineering, Pune















 M

T

pr

l

r

Cm30

.

039.0

*6.3 3

















 95

.

039.0

*6.3 3



m

pr

l

r

C

International Journal of Scientific & Engineering Research, Volume 2, Issue 11, November-2011 2

ISSN 2229-5518

IJSER © 2011

http://www.ijser.org

For air in the laminar flow range, the second term in the pa-

ranthesis can be omitted, yielding a simplifies formula,

In the molecular flow range the first term in the paranthesis

can be omitted, yielding the formula for air as;

For a system having number of valves, joints and bends, for

calculating conductance equivalent length is to be considered

instead of pure length of pipe.

Step 3:

Assuming mean pressure the value of effective pump-

ing speed is calculated by the following formula:

Step 4:

The effective pressure can be calculated using equa-

tion;

The value of assumed pressure should match with the

pressure calculated afterwards.For this purpose iteration me-

thod is to be used to find out the proper value of effective

pumping speed.

Step 5:

Using the value of effective pumping speed from step

4 the value of pump-down time is calculated using the formu-

la;

Using above procedures the pump-down time for a

particular pump is calculated theoretically to achieve a specific

pressure.

The experimental set-up for the system studied is as follows:

As shown in experimental set-up, a system is studied in

which it is required to evacuate a tank of 98 litres using pump

for which the rated pumping speed is 10 m3/hr upto a pres-

sure of 0.1 mbar. The tank is connected to the vacuum pump

with standard pipe for which internal diameter is 30mm. The

characteristic curve for the pump is as shown below:

hrm

l

pr

Cm/

.

7750 3

4



hrm

l

r

C/

.340 3

3



SC

SC

SC

Seff 





.

11

1

S

SP

PeffEff .



hr

P

P

S

V

t

eff 

















2

1

ln

International Journal of Scientific & Engineering Research, Volume 2, Issue 11, November-2011 3

ISSN 2229-5518

IJSER © 2011

http://www.ijser.org

The

abov

e

char

acter

istics curve for the pump selected indicates that pump speed

of 10 m3/hr is fairly constant for pressure upto 0.1 mbar. For

pressure lower than the pressure from the curve deviates from

linearity, the volume rate curve is divided in several partial

pressure ranges of small volume flow rate. From this

characteristics curve it is possible to calculate the pump-down

time using standard pump-down time formula but in that case

limitations of practical system such as configuaration of actual

system are not considered.

Using the conductance formula for air, conductance is calcu-

lated. For calculating conductance the initial guess for mean

pressure is taken as 0.096 mbar.

From this value of conductance , effective pumping speed is

caluculated. Theoretical pumping speed is taken as 10 m3/hr.

Then pressure is calculated as:

This pressure is compared with the assumed mean pressure

and the error is found out. Iteration method is used to reach

maximum correct value. Microsoft excel tool can be used to

acieve the maximum correct value of pumping speed.

For the above given conditions using bisection method of ite-

ration the value of effective pumping speed comes to be 4.4645

m3/hr with an error of 1.5 %. Using this pumping speed val-

ue, the time required to evacuate tank of the given size is cal-

culated as given below.

Hence the time required to evacuate the tank of 98 litres con-

sisting of tubing of length 5 m using the given vacuum pump

is 12.1 minutes.

The calculations are repeated for pressures ranging from

atmospheric pressure to the final low pressure of 0.1 mbar and

graph of pressure vs pump-down time is plotted on semi-

logarithmic scale.

3. Results and Discussion:

The following observations are recorded from the above

graphical analysis:

1. The pumping speed varies with pressure to be

achieved and system configuaration. Hence it is not

advisable to calculate the evacuation time from the

characteristics curve of vacuum pump alone.

2. As pressure decresres, the time required for evacuat-

ing the tank increases and this increase is more for

very low pressures.

3. The variation of actual evacuation time from theoreti-

cal evacuation time may be attributed to leakages

through the various joints in the sytem.

4 CONCLUSION:

When a vacuum system is to be designed to get a par-

ticular pressure in predecided time, pressure should not

be the only criteria for selection of vacuum pump. Evacua-

tion time also depends on length and diameter of tubing.

So before installing any vacuum system proper study of

overall configuaration of system with pump is necessary

so that pump-down time can be optimized using proper

length and diameter of tubings in the system.

5 ABBREVIATIONS:

C-Conductance in m3/hr

S- Pumping speed for pump in m3/hr

hrmSeff

Seff

SC

SC

Seff

/95759.4

1083178.9

10*83178.9

*

3











mbarP

P

S

SP

Peffeff

04958.0

10

95759.4*1.0

*







min1.12

min60*

1.0

1013

ln

4645.4

098.0

















t

t

 

hrmC

hrmC

/83178.9

/95096.0*5.1*2150

500

5.1*6.3

3

3

3





International Journal of Scientific & Engineering Research, Volume 2, Issue 11, November-2011 4

ISSN 2229-5518

IJSER © 2011

http://www.ijser.org

Seff – Pumping speed at tank in m3/hr

Pm – mean pressure in mbar

r – Radius of tubing in cm

l – Length of tubing in cm

T – Absolute temperature in K

M – Molecular mass of gas

η – Viscosity of gases in Pa-s

t – Pump-down time in min

V – Volume of tank in m3

P1, P2 – Pressures at respective ends in mbar

ACKNOWLEDGMENT

The authors express their sincere thanks to all the staff of

Modern College of Engineering for their kind support.

REFERENCES

[1] Yasuhiko Senda, ―Theoretical analysis of vacuum evacuation in viscous flow

and ita applications‖, SEi Technical Review (October 2010)

[2] Robert E. Pearson and gary M. Atkinson,‖Teaching vacuum technology using

spreadsheet calculations‖, IEEE (September 2003)

[3] GE Xiaohong, Huang Hongwu, Li Hui and Li Yadan, ― Design and

verification of an auxiliary systemfor high vacuum die casting‖, Chi-

nese Journal of Mechanical Engineering, China (August 2010)

[4] Douglas J. Reinemann,‖the history of vacuum regulaton technology‖

MNC Annual Meetings Proceedings.(2005)

[5] David M. Hata,‖ Vacuum system laboratory development‖, CERN

[6] John F. O’Hanlon, A user’s guide to vacuum technology, John Wiley

and sons, Inc

[7] Vacuum pump manual, Pfeiffer Vacuum Gmbh.

[8] D.J. Hucknall and A. Morris, Vacuum Technology- Calculations in

chemistry, RSC publishing house, Lonon.

[9] Dorothy Hoffamn, Bawa Singh, John H. Thomas III, Handbook mof

Vacuum Science and Technology, Academic Press, London.

[10] Dr. Walter Umrath, Fundamentals of Vacuum Technology, Wiley

Publication.