Determination of vacuum pump operational efficiency

Determination ofDetermination of
Vacuum PumpVacuum Pump
Operational EfficiencyOperational Efficiency
deveshksinghal@gmail.com, March 2014
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By:
D K Singhal
deveshksinghal@gmail.com

Why to CheckWhy to Check
Is vacuum pump sufficient for our need?
Do we need to change the vacuum pump, or
relocation may be tried?
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Existing TrendsExisting Trends
No standard available information on efficiency
determination on operational conditions.
Standard methods use standard conditions, which
generally are entirely different than the mill environment.
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No FlowVacuumNo FlowVacuum
This is also called as “peak vacuum” of the pump.
Some mills check the level of vacuum developed with
inlet valves closed.
A drop in gauge vacuum is considered a drop in vacuum
pump efficiency.
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pump efficiency.
Yet, if you are operating at low vacuum levels, the result
obtained by this may be misleading.

Wet & Dry OperationWet & Dry Operation
Routine tests account only for dry air.
The air is sucked in at 100% RH with some
entrained water.When the pressure of the
same increases in the vacuum pump, some of
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same increases in the vacuum pump, some of
the water vapor (moisture) condenses. So, at
exhaust side, you get slightly lesser airflow.
Also, at inlet during normal operation due to
water evaporation, you get lower gauge
reading.
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Points to RememberPoints to Remember
Exhaust airflow reduces with increase in vacuum.
An old vacuum may give (almost) same airflow as rated
at low vacuum levels, but at high vacuum levels, capacity
reduces rapidly.
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reduces rapidly.
Seal water consumption may be high at such conditions.

Typical Pump DataTypical Pump Data
Take a look at a typical data supplied by a manufacturer.
mmHg Inlet Flow Capacity Loss
0 3000 0.0
50 2990 10.0
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50 2990 10.0
100 2970 30.0
200 2940 60.0
300 2900 100.0
400 2800 200.0
500 2700 300.0
600 2400 600.0
660 2040 960.0
Data Supplied Calculated

DeDe--rating at IncreasedVacuumrating at IncreasedVacuum
For normal operating ranges between 200mmHG and
500mmHg, the capacity drop can be linearly correlated
with vacuum level.
300.0
350.0
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y = 0.82x - 122
R2
= 0.9689
0.0
50.0
100.0
150.0
200.0
250.0
300.0
0 100 200 300 400 500 600
CapacityDrop
Vacuum Level, mmHG

Why does the vacuum reduce?Why does the vacuum reduce?
• The drop of capacity is mainly due to two reasons-
▫ At higher vacuum level, some part of exhaust air gets short
circuited to inlet air, no matter how good is sealing.
Furthermore, some part of sealing water gets evaporated due
to vacuum.This process increases as the vacuum pump gets
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to vacuum.This process increases as the vacuum pump gets
older due to wear and tear.
▫ At higher vacuum levels, there is a lot of moisture in process air
to be handled.This moisture condenses in vacuum pump, and
exhaust volume is lower than expected. Due to this, dry
checking of vacuum pump is generally not satisfactory to
process persons.
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What to Check?What to Check?
Obtain existing performance data on vacuum pump.
Preferably, it should be at exhaust side, and the inlet side
may be calculated as under-
y = 0.82x - 122
R2
= 0.9689
0.0
50.0
100.0
150.0
200.0
250.0
300.0
350.0
0 100 200 300 400 500 600
Vacuum Level, mmHG
CapacityDrop
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Inlet flow = (760-mmHg)*Outlet Flow / 760
Vacuum levels can be changed by throttling the inlet
valves, while the machine clothing is running in wet
condition.

Instruments for Flow MeasurementInstruments for Flow Measurement
As flow is to be checked at
exhaust side, a conventional
vane flow anemometer can
be used easily.
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be used easily.
A hot wire anemometer, if
used, has to be calibrated
properly at operating
temperature and humidity
conditions.
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Instruments for Flow MeasurementInstruments for Flow Measurement
Alternatively, a PitoTube (Pilot Tube)
connected with a differential pressure gauge
can be used. But, the major problem is relative
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can be used. But, the major problem is relative
low accuracy in case of oversized piping or in
case of high vacuum, low exhaust airflow
condition.
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Vacuum MeasurementVacuum Measurement
A calibrated bourdon tube vacuum gauge can
be used for better accuracy.
A glycerin filled gauge ensures reading stability
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A glycerin filled gauge ensures reading stability
with no pulsations.
Alternatively, a mercury filled manometer can
be used.
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Sample CalculationsSample Calculations
In this typical case, volumetric efficiency is reducing more
at higher vacuum levels.
Vacuum Exh. Flow Inlet Flow Inlet Flow Volumetric
mmHg (Measured) (Calculated) (Design) Efficiency
0 2950 2950 3000 98%
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0 2950 2950 3000 98%
50 2700 2890 2990 97%
100 2370 2729 2970 92%
200 1900 2579 2940 88%
300 1500 2478 2900 85%
400 1050 2217 2800 79%
500 650 1900 2700 70%
550 250 1188 2400 49%
600 - - - -

Operational EfficiencyOperational Efficiency
As we can see that the operational efficiency is reducing
rapidly with increase in vacuum levels.
For practical purposes, the efficiency at the operational
levels can be considered. For example, if the pump has to
be operated at 400mmHg vacuum, the efficiency would
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be operated at 400mmHg vacuum, the efficiency would
be 79%.
Or, we may consider that vacuum pump is giving 79% of
the rated airflow.

ResultsResults
On the basis of this analysis, we may conclude something
like this-
“This vacuum can be used for low vacuum applications
such as Low-Vac boxes of wire part etc. as the efficiency
is better at lower vacuum levels.”
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is better at lower vacuum levels.”
“On an 400mmHg operating vacuum, the pump has been
de-rated by 21%. So, if it does not serve the process
requirement, it should be replaced.”
“If it is serving the process right now, we may replace the
same with a new pump of smaller capacity and save
power (same 21%).”

Changing theVacuum PumpChanging theVacuum Pump
Many mills do not want to replace vacuum pump with a
new one, as the present one is OK.
In case, there is a drop in efficiency, please do consider
the cost of electricity loss due to reduction in efficiency.
If payback period suits you, talk to the best vacuum pump
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If payback period suits you, talk to the best vacuum pump
suppliers, negotiate well, but, replace the vacuum pump at
the earliest.

ThankYou.ThankYou.
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ThankYou.ThankYou.

Determination of vacuum pump operational efficiency

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