Vacuum Generators: How They Work and Operate
Vacuum pumps are designed to evacuate a specific percentage of air molecules to attain a vacuum pressure that is dependent upon the available atmospheric pressure. For example: a pump that can attain an 80% vacuum will develop 23.9 inHg (760 mmHg) when barometric pressure is 29.9 inHg while the same pump will only develop 20.7 inHg (524 mmHg) at 4,000 feet above sea level where the local barometric pressure will be approximately 25.8 inHg (655 mmHg). Local weather conditions can also reduce vacuum pressure. When barometric pressure drops from 29.9 inHg to 28 inHg during a storm, vacuum pressure will also drop. It is important to realize that vacuum pressure fluctuations are a normal characteristic of all vacuum systems and are not necessarily caused by a problem with the vacuum pump.
To minimize the effect of vacuum pressure variations, we recommend that systems be designed for mid-range vacuum levels of 12 – 18 inHg (305 – 457 mmHg) that are consistently attainable no matter what the weather conditions may be.
Air-powered vacuum pumps are compact and lightweight allowing them to be mounted close to the point of vacuum usage which minimizes the internal volume of vacuum hose and tubing. Vacuum is produced immediately when compressed air flows into the pump eliminating the need to turn the pump on long before contacting a workpiece as is common with electro-mechanical pump systems.
Electro-Mechanical Vacuum PumpsPremature wear will result from frequently starting and stopping an electro-mechanical vacuum pump. They are primarily suited for systems requiring constant vacuum flow. The pump must be powered continuously for this type of use. Most types of electro-mechanical vacuum pumps are also not suited for operating at maximum vacuum and zero flow conditions which causes poor lubrication and over-heating of the pumping mechanisms.
Electro-Mechanical vacuum pumps tend to be noisy, bulky, heavy, and hot. Because of this, they are usually mounted a considerable distance away from the point of vacuum use. In order to be used in a pick and place system, several additional components are often required. Additional components required include a motor starter, vacuum relief valve, exhaust muffler, large diameter vacuum hoses, three-way vacuum control valve, etc. Collectively, these components and the associated assembly labor add substantially to the installed cost of a vacuum system. On top of this, each additional component comes with the inherit risk for potential failure. Operating costs are also increased as electro-mechanical pumps are high-maintenance items and must be overhauled frequently.
Electro-mechanical pumps efficiently convert electrical power into vacuum flow and pressure. Because they must run continuously, they can’t take advantage of the system duty-cycle to reduce overall energy consumption. However, for systems requiring constant, large vacuum flows, they may be the best solution.
Duty Cycle and Energy ConsumptionDuring a pick and place cycle, a vacuum source is turned on for the pick and remains on during the travel to the place location. Once the workpiece is in the correctly location to be placed, the vacuum source is turned off. Vacuum is not necessary for the travel back to the home position nor for the dwell time before the next pick is required. If vacuum is on for 1/4 of the full machine cycle, then the duty cycle is 25%. An air powered vacuum pump consumed compressed air only while it is creating vacuum. In this example, the average air consumption would be reduced to 25% of the cataloged pump air consumption rate whereas an electro-mechanical vacuum pump must run continuously and consumed energy 100% of the time.
Whenever an adequate supply of compressed air is available, especially if the system has an intermittent vacuum requirement or duty cycle, it’s best to consider an air powered vacuum pump.