Vacuum generators operate based the venturi principle, Figure 4. Filtered, non-lubricated compressed air enters through inletA. A diffuser orifice (nozzle), B, causes the air stream to increase in velocity, thereby lowering its pressure, which creates a vacuum in channel C. The air stream exhausts to atmosphere through muffler D.
Vacuum generators offer several advantages. They are compact and lightweight, so they often can be mounted at or near the point of use. They are inexpensive, and because they have no moving parts, do not require the maintenance associated with mechanical vacuum pumps. They do not need an electrical power source because they generate vacuum by tapping into an existing compressed air system. However, if retrofitted into a machine, capacity of the existing pneumatic system may have to be increased. Heat generation, which often is the limiting factor with mechanical vacuum pumps, is of little concern with vacuum generators.
Mechanical pumps most often are specified to provide a machine with vacuum on a continuous basis. But many of these machines actually use vacuum only intermittently at many different locations. In cases like this, vacuum generators can provide a practical alternative by supplying vacuum intermittently at each source rather than continuously for the entire machine.
Vacuum generators are controlled simply by initiating or terminating compressed air flow to the nozzle. Vacuum generators have been used for decades, but relatively recent improvements have led to nozzle designs that provide higher operating efficiencies.
Another development using venturis is the multi-stage vacuum generators. In this configuration, two or more vacuum generators are piped in series to produce greater vacuum flow without using more compressed air. Essentially, the exhaust from the first nozzle (which determines the maximum attainable vacuum level) serves as input for a second stage. Exhaust from the second stage then serves as input for a third stage. This means that a multi-stage generator evacuates a given volume faster than a single-stage generator does, but they both will eventually pull the same vacuum level.
Selecting a vacuum generator depends on the lifting force required and the volume of air that must be evacuated. Lifting force depends on the vacuum level the generator can pull — which, in turn, depends on the air pressure supplied — and the effective area of the vacuum cup. In most applications it is important that a generator be able to pull the required vacuum in as short a time as possible to minimize air consumption.
How Long Does it Take to Reach Maximum Vacuum?
When choosing among several vacuum pumps, an important factor may be how long it takes each pump to reach the needed vacuum.
In general, a small capacity pump and a large capacity pump with equal maximum vacuum capabilities will both produce the same vacuum. The smaller pump simply takes longer. How much longer depends on the capacity of the pump and the size of the system. But simply dividing system volume by open pump capacity won't produce the proper answer.
During pump-down, the higher a vacuum becomes, the fewer air molecules remain in the closed volume. Therefore, fewer molecules can be removed by each pump stroke. As a result, there is a logarithmic relationship when approaching a perfect vacuum. The time required to pump a system down to a certain vacuum level can be approximated using this formula:
t = (V×n) ÷ 4q,
where:
t is time, min
V is system volume, ft3
q is flow capacity, cfm, and
n is a constant for the application.
For exact applications, n can be determined by using a natural logarithm. For most purposes, the following will suffice:
n = 1 for vacuum to 15 in.-Hg
n = 2 for vacuum >15 but ≤ 22.5 in.-Hg., and
n = 3 for vacuum ≥ 22.5 and up to 26 in.-Hg.
One additional complication: pump capacity in the equation is not the open capacity (capacity at atmospheric pressure) usually cataloged by manufacturers. Instead, it represents the average capacity of the pump as system pressure drops to the final vacuum level. This value is not readily available but can be approximated from manufacturers' pump performance curves. These curves plot pump capacity at various vacuum levels.
To mesh these curves with the equation, simply substitute values in the equation using pump capacity readings from the curve at various vacuum levels at 5-in.-Hg increments, up to the desired level. Then total these times.
Finally, note that this pump-down time is based on all system components operating at optimum levels. A 25% additional time allowance is recommended to compensate for system inefficiencies and leakage.
How High Altitudes Can Impact Vacuum
Atmospheric pressure determines the maximum vacuum force that can be achieved. And standard atmospheric pressure at sea level is 29.92 in.-Hg. But what happens at locations a mile above sea level?
The maximum vacuum that can be achieved in locations above sea level will be less than 29.92-in.-Hg. The force will be limited by the ambient atmospheric pressure. Vacuum pumps have maximum vacuum ratings based on sea level conditions and must be re-rated for operation at higher elevations.
First, determine the local atmospheric pressure. A rule of thumb is that for every 1000 ft. of altitude above sea level, atmospheric pressure drops by 1 in.-Hg. Using rounded-off figures, for a city at an elevation of 5,000 ft, the atmospheric pressure is about 25 in.-Hg.
To adjust a pump rating, think of that rating as a percentage of atmospheric pressure at sea level. If a pump is rated for 25 in.-Hg, it can achieve 83.4% (25 29.92) of a sea level perfect vacuum. At a 5000-ft elevation, that same pump can achieve 83.4% of 25 in.-Hg - or a vacuum of 20.85 in.-Hg.
