What is Vacuum?
Vacuum refers to pressure that is less than atmospheric pressure. Atmospheric pressure is 14.7 pounds per square inch because the weight of the air over square inch is 14.7 lbs at sea level. When we reduce the pressure below atmospheric, this lower pressure is refereed to as a vacuum. Vacuum is expressed in many different measuring systems that can sometimes cause communication errors. These include millibar (usually describing weather) inches of mercury gauge (most common)) inches of mercury absolute, torr, and mills.
There is a theoretical absolute zero pressure.
Pressure is the force generated when a surface is struck by molecules of a fluid. The theoretical zero is used for the zero of absolute measuring systems. In gauge systems, zero inches is atmospheric pressure. Zero inches gauge equals about 29.92 inches mercury absolute at sea level. This can lead to confusion; Care must be exercised when communicating vacuum levels.
SCFM vs ACFM
Vacuum systems pump gases from low pressure to higher pressures (usually atmospheric). Since gases expand and contract in volume with changes in pressure, the description of the flow depends whether it is measured a the higher discharge pressure from the pump (usually atmospheric) or at the lower expanded pressure at the pump inlet.
Discharge volume is usually expressed in SCFM, which denotes the flow at sea level in 70 Deg F. air. ACFM flows express the gas flow at the expended volume at the pump line. As an example a 10 SCFM flow is about equal to a flow of 20 ACFM at 15” mercury gauge. In the previous example changing inlet conditions to 7.5” mercury gauge results in doubling the ACFM at the pump inlet to 40 ACFM. A change of 7.5” mercury deeper vacuum doubles the volume the pump must move. Asking for deeper vacuum than needed can be very expensive.
What’s in the gas stream!?
In a vacuum pumping system, the gas that is being evacuated passes through the pump. In some cases the gas can do damage to the pump. Rotary screw and vane pumps require relative clean air free from particulate and liquid contaminates as most anything can affect their close tolerances and oil lubricated surfaces. Blower vacuum system generally can tolerate light liquid entrainment in the gas stream as long as it is not corrosive. Piston systems can tolerate small amounts of contaminates. All the above pumps cannot work with a corrosive gas stream.
Liquid real sealed pumps can tolerate contaminated gas streams. There are three basic types of liquid ring pumps.
The first uses water a sealing fluid and just passes the water through and then down the drain. Since the gas is only in contact with the pumps housing and water (not the bearings or other close tolerance parts of the pump), it can handle some trash and contaminates. Making it from stainless steel or other metal can offer corrosion resistance in extreme applications. Please remember that the water leaving these pumps can present an environmental impact in the gas stream contains hazardous materials. The next type recovers part of the discharge water and recirculates it back to the pump. This can reduce water usage. The final type is an oil fluid sealed unit that uses oil a sealing fluid. These pumps are less tolerant to contamination but since the oil is not used to lubricate bearings and constantly being filtered, it can be used in moderately contaminated gas streams. Oil sealed liquid ring should not be used in most corrosive gas streams.
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