In the 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 or porous system, a vacuum unit must have the capacity to remove more atmosphere or air molecules than are able to leak back into the system.
The following are the most important terms to be considered and understood in any discussion or application using vacuum.
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.
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.
Compressed air — The energy source that drives an air driven vacuum pump, measured in scfm. Compressed air is comparable to the electricity that runs a mechanical vacuum pump. Generated by an air compressor, and supplied via a network of piping at a certain pressure level (psi).
Air supply pressure — The pressure of the compressed-air supplied, usually measured in psi. The measurement used to determine the optimal operating pressure of an air driven vacuum pump. Optimal pressure is obtaining a balance of supply pressure and air consumption to achieve the maximum efficiency level.
Evacuation time — The amount of time it takes to evacuate a given volume to a desired level of vacuum.
Energy consumption — The amount of energy, whether expressed in hp, kW, scfm or any other expression, that a vacuum pump uses to generate a desired amount of vacuum.
Lifting force — The lifting capacity of a suction cup, determined by multiplying pressure times the area.
Volume — The total of all area in a vacuum system from the interior of the pump to, and including, the area of application. These are the most important and primary terms to understand about vacuum. Please refer to the glossary for a more complete listing of terms.
Levels of vacuum
Vacuum is typically divided into three areas of application, dependent upon the level of vacuum required.
- Low level vacuum applications are typically those requiring high flows and low force (inches of mercury). These applications are primarily serviced by blowers. Screen printing on cloth is an application that falls into this range.
- Industrial vacuum falls within the range of 6 to 29.5 in.-Hg. The largest number of applications occur here. Vacuum in the industrial range can consist of anything from pick and place to thermoforming. The largest segment is in the 12 to 21 in.-Hg range.
- Scientific or process is an area encompasses the deepest levels to 29.92 in.-Hg. Vacuum at this level is usually measured in torr. Flow in this range is minimal in transition from viscous to molecular. Examples of applications are ion implantation and space simulation.
- The highest level of vacuum achievable on Earth is 29.92 in.-Hg. A perfect vacuum (i.e. 30 in.-Hg) — a space that contains no molecules or atoms — is purely theoretical. The only possible place where this condition can exist is in space and, even there, a few atoms can be found.