IMAGE COURTESY OF FESTO
simplest sense, vacuum is pressure that is lower than atmospheric. At sea level, atmospheric pressure is usually 14.7 psia. Therefore, any pressure lower than that constitutes a vacuum. In a vacuum system, the difference between atmospheric and vacuum pressure creates the ability to lift, hold, move and generally perform work. The essence of vacuum generation is the reduction of molecular impacts within a system. In effect, vacuum is the pressure differential produced by evacuating air molecules from a system. There are two types of vacuum applications: sealed, or non-porous, and open, or porous. In a closed system, removing air progressively decreases the air density within the confined space. This causes the absolute pressure of the remaining gas to drop, causing a vacuum. To achieve a vacuum in an open system, a vacuum unit must have the capacity to remove more atmosphere or air molecules than are able to leak back into the system.
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The following are the most important terms to be considered and understood in any discussion or application using vacuum. IMAGE COURTESY OF PIAB
VACUUM That pressure or any pressure lower than atmospheric. As previously discussed, at sea level atmospheric pressure is usually 14.7 psia; any pressure lower than that constitutes a vacuum. VACUUM FLOW The rate at which atmospheric pressure is removed from a system, or the amount of outside atmosphere that flows through a pump. Usually, this is measured in standard cubic feet per minute (scfm). The significance of vacuum flow is that it determines the speed of evacuation of a system, or the ability to compensate for leakage in a system. As the level of vacuum (vacuum force) in a system increases, the flow rate decreases because there are fewer molecular impacts.
FLUID POWER WORLD
6 â€˘ 2016
FREE AIR CAPACITY The amount of outside atmosphere a vacuum pump can displace at 0 in.-Hg, or wide open. This expression is commonly used by vacuum pump manufacturers as an indicator of size and performance. It is confusing in that it does not tell us what type of performance to expect in a given range (level of vacuum force). Free air capacity is analogous to the size of an engine in a car; it is a starting point, but tells us very little about specific performance or efficiency. VACUUM FORCE This term is most commonly defined as the level of pressure within a system, usually measured in inches of mercury. The amount of vacuum force created usually determines the lifting capacity of a suction cup or measures the amount of atmosphere left in a system. Vacuum force can not overcome porosity in a system, nor can it speed the evacuation time of a given volume. The higher the level of force, the longer it takes to achieve.
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Fluid Power World Handbook 2016