Today, carbon dioxide (CO2) is considered to be a high environmental threat to our world by many researchers. Heightened levels of carbon dioxide in the atmosphere lead to global warming, which in the long run can jeopardize our future on this planet.
In order to reduce the use of fossil fuels, the primary cause of increased carbon dioxide levels in the atmosphere, governments worldwide continue to implement increasingly strict legislation on individuals, service companies, and industry in general so that carbon footprints are reduced. For example, allowable emission levels will continue to be reduced, while taxes on them will be increased each year.
In this article, we will take a closer look at how it is possible to substantially reduce energy consumption and, in many cases, reduce an auto plant's carbon footprint in a time when most electrical power is produced by gas, oil, and coal-based power plants. We will look at industries where vacuum material handling systems are used and focus only on systems designed for "sealed" materials, such as sheet metal, plastic, and glass.
Calculating system energy consumption and CO2 emissions
The most common vacuum technology for handling sealed materials today utilizes air-driven vacuum ejectors. The handling system is quite often based on a robot equipped with vacuum lifting devices and suction cups. There are also many manual vacuum handling devices designed for sealed objects, as well as dedicated machinery with integrated vacuum handling systems. Examples include sheet-metal presses, water and laser cutters, and glass and wood-working machines.
The energy consumed by these types of vacuum handling systems is defined by how much compressed air the ejector consumes to create vacuum, and often takes into account how much compressed air is needed in the blow function to release the part quickly enough.
The amount of compressed air consumed in an ejector when vacuum is created depends on the number of nozzle rows, the size of the smallest diameter in the (first) ejector nozzle, and the compressed-air feed pressure. The complete formula to theoretically calculate the air consumption for an ejector nozzle is given below:
(For futher reference and background, go to this Primer on Vacuum and Pneumatics Technology.)
It is quite common that the specified air consumption for ejectors will differ from the theoretic value. The actual air consumption should be very close to the theoretic value (the difference of a few percentages is reasonable). The table below demonstrates the theoretic value for some common nozzle diameters at varying feed pressures. Calculations are made at a temperature of 10C (50F, 283K).
The other, and quite often forgotten, energy thief in a vacuum handling system designed for sealed materials is the blow-off function, which is used for quick release of the object. The air consumed during blow-off is determined by the flow capacity of the valve that controls the function and pressure being used.
When utilizing a large centrally placed ejector (i.e. many cups connected to the same source), very high levels of flow are required in order to quickly break the seal on remotely placed suction cups. In this case, flow levels in the range of 200-500 Nl/min at 4-6 bars are standard.
In a decentralized system using one small ejector at each point-of-suction, the release function is, in many cases, the result of blocking the exhaust. The air travelling through the ejector will be forced into the cup so the air consumption will be equal to, or slightly higher than, the air consumption for producing vacuum. An alternative solution is a small blow-off check valve on a decentralized unit, which typically allows 100-200 Nl/min to pass through at 4-6 bars.