Determination of Vacuum Pump Operational Efficiency By: D K Singhal Chandpur Enterprises Ltd. email@example.com
Why to Check Is vacuum pump sufficient for our need? Do we need to change the vacuum pump, or relocation may be tried?
Existing 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.
Wet & 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 the water vapor (moisture) condenses. So, at exhaust side, you get lesser airflow. Also, at inlet, due to water evaporation, you get lower gauge reading. 4
Points 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. Seal water consumption may be high at such conditions.
Typical Pump Data Take a look at a typical data supplied by a manufacturer. 6
De-rating at Increased Vacuum For normal operating ranges between 200mmHG and 500mmHg, the capacity drop can be linearly correlated with vacuum level. 7
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. This process increases as the vacuum pump gets older. 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. 8
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- 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 Measurement As flow is to be checked at exhaust side, a conventional vane flow anemometer can be used easily. A hot wire anemometer, if used, has to be calibrated properly at operating temperature and humidity conditions. 10
Instruments for Flow Measurement Alternatively, a Pito Tube (Pilot Tube) connected with a differential pressure gauge 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. 11
Vacuum Measurement A calibrated bourdon tube vacuum gauge can be used for better accuracy. A glycerin filled gauge ensures reading stability with no pulsations. Alternatively, a mercury filled manometer can be used. 12
Sample Calculations In this typical case, volumetric efficiency is reducing more at higher vacuum levels.
Operational 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 be 79%. Or, we may consider that vacuum pump is giving 79% of the rated airflow.
Results 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. 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%).