By Josef Karbassi, Vice President, Automation Div., PIAB
How to reduce your energy consumption and carbon footprint when most electrical power is produced by gas, oil and coal-based power plants.
Carbon dioxide (CO2) is considered to be the largest environmental threat to our world by most researchers. In order to reduce the usage of fossil fuels—the primary cause of increased carbon dioxide levels in the atmosphere—governments worldwide continue to implement increasingly strict legislature on individuals, service companies and the industry in general so that carbon footprints are reduced.
We will look at industries where vacuum material handling systems are used and focus on systems designed for sealed materials, such as sheet metal, plastic and glass. The solution is simple—use the latest available vacuum technology for vacuum handling applications. We will return to this point later in the discussion.
1. Know how to calculate your system energy consumption and CO2 emissions
The most common vacuum technology for handling sealed materials today uses air-driven vacuum ejectors. The handling system is 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 woodworking machines. The energy consumed here 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 formula to theoretically calculate the air consumption for an ejector nozzle is shown in Table 1.
Often, 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 percent is reasonable). Table 2 demonstrates the theoretic value for some common nozzle diameters at varying feed pressures. Calculations are made at a temperature of 10°C (283.16°K).
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 using 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 often the result of blocking the exhaust. The air traveling 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.
In order to calculate the energy consumed, it is required that the compressor efficiency is known. A normal sized compressor, able to create 7-10 bars of pressure, consumes 6-10 kW per produced cubic meter of air depending on size and efficiency. An ejector system’s total air consumption per year can be easily calculated by adding the air consumed by vacuum production and the air consumed by blow-off function in each cycle, and then multiplying by the number of cycles per year. A better solution is to measure the consumption with a flow meter over the course of a number of cycles.
The CO2 emissions per produced kWh of electric power will be as follows, depending on the type of production:
Gas: 0.2 kg CO2/kWh
Oil: 0.27 kg CO2/kWh
Coal: 0.33 kg CO2/kWh
Nuclear, Wind, Hydro: 0.0007 kg CO2/kWh
Re-calculated for compressed air production, the result is 0.02-0.033 kg CO2/m3, if only considering the “dirty” production methods and basing the compressor efficiency on 10 kW per produced cubic meter of air.
2. Consider your ejector’s efficiency
The ejector’s efficiency is obviously an important parameter to focus on when attempting to minimize energy/air consumption. Ejector efficiency is determined by the vacuum performance (flow and speed of evacuation) in relation to air consumption. There are two main types of ejectors used in sealed vacuum handling systems today—single-stage and multistage ejectors. The multistage design is more complex and requires more space, but it will always be 15-50% more efficient (same speed/response time with less energy consumption). Therefore, it is important to use a multistage ejector whenever possible.
When ejector technology entered the market for vacuum material handling of sealed parts (and started to replace electrically driven vacuum pumps), the main reasons were the simplicity and reliability of the products, as well as the ability to easily control the ejectors’ power during operation. At that time, small ejectors were placed on each suction cup, forming a decentralized system. This type of decentralized system is often the most efficient, as it places suction exactly where it is needed. There is no need for over-dimensioned ejectors to compensate for losses and extra volume. There is also a reduced risk of leakage from fittings and couplings.
3. Know what’s new in products and design
When air-saving technology became available for ejectors, a new trend began. So-called compact ejectors (or smart ejectors) with integrated control functions such as valves, vacuum switches and air-saving functions flooded the market. These compact ejectors are centrally placed and serve several suction cups. They are usually located a few meters away from the points-of-suction. The air-saving function turns the ejector off when enough vacuum pressure is created, and turns it back on to compensate for any leakage occurring in the system. A major advantage is that the centralized ejector with air-saving function only works for a short period of time during the vacuum duty cycle, and energy will be saved when compared to the decentralized concept.
With the centralized compact ejectors, factors like operational reliability and safety (one ejector per cup), speed of vacuum generation, and object release must be sacrificed to a certain degree. Speed can be compensated for with a very large centralized ejector, but this means much greater energy consumption.
A new, compact, decentralized ejector unit with two unique features is the fully pneumatic and internal air-saving Vacustat; a new release valve, AQR, uses the ambient atmosphere to quickly release a handled part. The volume of a single suction cup is so low that atmospheric air is all that is needed. In other words, no compressed air is needed for release, and an automatic air-saving device is in place.
This concept offers all the benefits of a decentralized ejector system in terms of reliability, safety and speed (response and release). The air and energy consumed is minimal. There is no compressed air consumption during the release of objects, and the air-saving function does not have to compensate for leakage from multiple fittings and couplings. The volume is so low that the air-saving function will start almost instantly. The time that the ejector must be on before pick-up is also reduced to almost nothing, and there is no need to create a pre-vacuum in the system. Under these conditions, it is possible to reduce energy consumption by 90-99%.