Determination of vacuum pump operational efficiency

Determination of Determination of Vacuum Pump Vacuum Pump

Operational EfficiencyOperational Efficiency

[email protected], March 2014

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By:

D K Singhal

[email protected]

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 Flow VacuumNo Flow Vacuum

� 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 960.0Data Supplied Calculated

DeDe--rating at Increased Vacuumrating at Increased Vacuum

� 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 - 122R2 = 0.9689

0.0

50.0

100.0

150.0

200.0

250.0

300.0

0 100 200 300 400 500 600

Cap

acit

y D

rop

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

Cap

acit

y D

rop


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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 Pito Tube (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 the Vacuum PumpChanging the Vacuum 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.

Thank You.Thank You.


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Thank You.Thank You.

Determination of vacuum pump operational efficiency

Technology

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