china vacuum pumps,rotary vacuum pumps,vane vacuum pumps,rotary vane pumps,rotary vane vacuum pumps manufacturer & exporter and trade

In principle, industrial vacuum pumps are merely compressors run with the inlet attached to the vacuum system and the outlet open to exhaust. In smaller sizes, compressors and vacuum pumps are often identical machines. However, in the large sizes that might power a plant-wide vacuum system, the machines differ in minor ways that are intended to enhance efficiency for one application or the other. Manufacturers strongly advise that the same machine not be used for both vacuum and compression at the same time. The heavy loads will damage it.
Three criteria control pump selection: degree of vacuum produced, rate of air removal, and power requirement. However, applications such as filtration may subject the unit to the ingestion of foreign material.
The first pump performance criterion is the vacuum it produces. Manufacturers provide a maximum vacuum rating expressed as absolute pressure in mm Hg, or vacuum in in. Hg. Larger units are usually rated only for continuous duty, but smaller units may have a higher vacuum rating for intermittent duty. In smaller units, temperature-rise considerations limit the vacuum that can be produced.
Continuous and intermittent vacuum ratings are determined for standard atmospheric pressure: 29.92-in. Hg. Lower ambient pressures reduce the vacuum that can be produced. The rating is determined from:
Va = ( Vo * Pa ) / 29.92
where Va = adjusted vacuum rating, in. Hg; Vo = original vacuum rating at standard conditions, in. Hg; and Pa = anticipated atmospheric pressure at the application site, in. Hg.
Rate of air removal is the second criterion. Vacuum pumps are flow rated according to the volume of air exhausted with no pressure differential across the pump. Manufacturers provide curves showing free air delivery at rated speed for vacuum levels ranging from 0-in. Hg (so-called "open capacity") to maximum vacuum rating. Some manufacturers also provide curves of capacity at different speeds for a given vacuum.
The last pump criterion is power requirement. Compared with air compressors, vacuum pumps require relatively little power. At low flows, vacuum (or pressure differential) is high; at high flows, vacuum is low. Therefore, power, which is proportional to flow and pressure differential, is generally low.
Power output of the pump can be found from pressure-flow curves provided by manufacturers. Input power and speed requirements are also shown in the data. Overall pump efficiency (including both volumetric and mechanical efficiency) can be evaluated by combining this data. This is done by dividing the free-air capacity of the pump at the required vacuum level by drive power required at that condition. The result is proportional to the product of gage vacuum and air-flow rate and is representative of efficiency.
All three performance criteria -- vacuum, flow and power -- can be affected by pump temperature. At higher vacuum levels, little air flows through the pump, so little heat is transferred to the air. Much of the heat generated by friction must be dissipated by the pump. This heat gradually raises pump temperature and can drastically reduce service life. Temperature excursions are especially important to intermittent-duty pump, which can overheat if on time greatly exceeds off time.
Vacuum pumps are classified as either positive or nonpositive displacement. A positive-displacement pump creates vacuum by isolating and compressing a distinct, constant volume of air. The compressed air is vented out one port, and a vacuum is created at the other port where the air is drawn in. This generates relatively high vacuum, but little flow.
A nonpositive-displacement pump, on the other hand, uses rotating impeller blades to accelerate air and create a vacuum at the inlet port. While nonpositive-displacement pumps cannot produce high levels of vacuum, they provide high flow rates.
Principal types of positive-displacement vacuum pumps include piston, diaphragm, rocking-piston, rotary-vane, lobed-rotor, rotary-screw, and liquid-ring designs.
Reciprocating-piston pumps generate relatively high vacuums -- from 27 to more than 29 in. Hg -- under a variety of operating conditions. Typical pumps of this type have one or more pistons linked to a rotating crankshaft. The alternating piston action moves air past check valves in the cylinder head to create a vacuum at the inlet port. Lubricated piston pumps are quieter, produce less vibration, have a higher capacity, and feature a much longer life than oilless designs, but they are also heavier and more expensive.