Vacuum Pumps and systems are widely used in the chemical process industry for various applications such as drying, solvent recovery, distillation, short path distillation (Molecular distillation), concentration etc. It is therefore, essential that the vacuum principles are understood which can be employed to maximize process throughputs, product purity and quality & minimize power consumption.
Success has been achieved in many industries such as food product, essential oils, aromatics, solvent recovery and steam jet replacements. Wide range of pumps and vacuum equipment is being used in the industry to achieve the desired vacuum and pumping speeds. The understanding of their advantages and limitations can result in optimizing their performance.
What is Vacuum?
Vacuum is simply a pressure below atmosphere. To create vacuum in a system, a pump is required to remove mass (gas/vapor) from the system. The more mass is removed, lower is the pressure that exists inside the system. Various vacuum levels are defined depending upon the ultimate vacuum as:
Coarse Vacuum 10 – 760 Torr
Medium Vacuum 0.001 – 10 Torr
Fine Vacuum 10^-3 – 10 ^-7 Torr
Ultra High Vacuum < 10^-7
Generally, the chemical industry operates in Coarse and Medium vacuum range. In this range the vacuum is generally measured in mm Hg gauge or Torr (absolute pressure). Measurements from datum as atmosphere are gauge reading, whereas the measurements referred to absolute zero are expressed in Torr. For example at sea level (atmospheric pressure 760mmHg), a system maintained under vacuum of 700mmHg, as indicated by vacuum Boudorn gauge, is said to have absolute pressure of 60 Torr. Vacuum gauges, mercury manometers, transducers etc. indicate gauge pressure and their reading when subtracted from atmospheric pressure gives absolute pressure. It is important to under stand the above since all vacuum principles and calculations are based on absolute pressure units.
Pumping speed: It is the volumetric rate of exhausting, generally expressed in Lts/min., m3/hr or cfm. It is the rate at which the inlet of the pump actually removes the gas / vapor load. It should not be confused with Displacement of the pump. Displacement of a pump is the geometric volume swept by the pump per unit time at rated operating speed. For most of the pumps, pumping speed is close to displacement value at no load conditions (FAD-Free air delivery) and changes with inlet pressure, reaching to zero where the pressure attained is said be pumps
Ultimate pressure: The Curve below, gives pumping speed for different type of pumps.
It is evident from the curve that pumping speed drops with drop in pressure. This must be taken into consideration while selecting a pump. The inlet pressure at which the pump’s speed falls to zero is termed as “Ultimate pressure or Blank-off pressure” of the pump. It is a pump characteristic, dependent on the type of pump/ pump construction. The ultimate pressure/Blank off pressure of a pump can be easily checked by measuring the inlet pressure, with inlet of the pump blanked off. At Blank-off pressures, the effective pumping speed of the pump is zero. This means that a process can never achieve vacuum better than the blank off vacuum of the pump. While selecting a pump, desired process vacuum and that achievable by a pump must be verified. Ultimate vacuum is the pump type characteristic and general conception that using a bigger Pump (of the same type) would yield better vacuum is false. The process engineer’s should establish desired process vacuum and the selection of the pump should be made accordingly. To get better working vacuum and higher pumping speed, Boosters are invariably used in combination. In most cases much higher speed and lower pressure can be achieved with a fraction of extra power, when Booster combination is used.
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