Reducing carbon footprint with the right vacuum system

Today, carbon dioxide (CO₂) is considered to be the largest environmental threat to our world by most researchers. Heightened levels of carbon dioxide in the atmosphere lead to global warming, which in the long run can jeopardise our future on this planet.

In a time when most electrical power is produced by gas-, oil- and coal-based power plants, it is possible to substantially reduce energy consumption and, in many cases, reduce a plant’s carbon footprint, where vacuum material handling systems are used. This can be achieved by using new vacuum technology designed for sealed materials, such as sheet metal, plastic and glass.

Calculating vacuum system energy consumption and CO₂ emissions

The most common vacuum technology for handling sealed materials today utilises air-driven vacuum ejectors. The handling system is quite often based on a robot equipped with vacuum grippers. 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 also 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 a 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:

  

It is quite common that the specified air consumption for ejectors will differ from the theoretical value, but the actual air consumption should be very close to the theoretical value (a difference of a few percentage points is common). Figure 1 demonstrates the theoretical value for some common nozzle diameters at varying feed pressures. Calculations are made at a temperature of 10 °C (283.16 K).

Figure 1: Air consumption for different ejector nozzle diameters.

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 utilising a large centrally placed ejector (ie, 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 decentralised 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 decentralised 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 metre of air depending on size and efficiency. The total air consumption for an ejector system per year can easily be calculated by adding the air consumed by vacuum production and the air consumed by the blow-off function in each cycle, and then multiplying by the number of cycles per year. Even better is to measure the consumption with a flowmeter over the course of a number of cycles.

An accepted fact is that the CO₂ emissions per produced kWh of electric power will be as follows, depending on the type of production:

Gas:	0.2 kg CO2 per kWh
Oil:	0.27 kg CO2 per kWh
Coal:	0.33 kg CO2 per kWh
Nuclear, wind, hydro:	0.0007 kg CO2 per kWh

Recalculated 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 metre of air.

How to reduce vacuum system carbon footprint to a minimum

Figure 2: Single-stage ejector nozzle.

Figure 3: Multi-stage ejector nozzles mounted as a cartridge.

The ejector’s efficiency is obviously an important parameter to focus on when attempting to minimise air and energy consumption. Ejector efficiency is determined by the vacuum performance (flow and speed of evacuation) in relation to air consumption. Basically, there are two main types of ejectors used in sealed vacuum handling systems today — single-stage ejectors and multistage ejectors. The multistage design is more complex and requires more space, but it will always be 15-50% more efficient (same speed and 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 decentralised system. In many cases, a decentralised system such as this is the most efficient system, 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. However, 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 metres 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. One major advantage of this system is that the centralised ejector with an 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 previous decentralised system.		Figure 4: Decentralised system.
		Figure 5: Centralised system with compact ejector and air-saving function.

With the centralised compact ejectors, factors like operational reliability and safety (one ejector per cup), and speed of vacuum generation and object release must be sacrificed to a certain degree. Speed can be compensated for with a very large centralised ejector, but this means much greater energy consumption.
Another issue in utilising centralised compact ejectors is that the blow-off function has to be very powerful in order to release parts quickly enough. This is because pipes are long and often restricted, leading to large amounts of air consumption during the time needed for blow-off. Figure 6 shows a typical work cycle in a sealed vacuum handling application utilising a compact ejector with an air-saving function.

Figure 6: Cycle analysis of a centralised compact ejector.

Air consumption occurs during the following phases shown in Figure 6:

Dark blue — A vacuum is started in the system before the actual pick to increase pick-up speed. Blue — Enough vacuum is made in the system to compensate for leakage from fittings and couplings. A few recoveries per cycle are not unusual due to leakage. Red — The object is released using positive pressure blow-off. Dark red — Excessive blow-off time. It is obvious that even with an air-saving function in place there will be a great deal of compressed air consumed during each cycle. Can air consumption and carbon footprint be substantially reduced? A compact decentralised ejector unit has now been developed to substantially reduce air consumption and therefore the carbon footprint of a vacuum system. The system comprises a decentralised vacuum pump with an air-saving function built in, and an automatic quick-release valve (AQR) that uses the ambient atmosphere to quickly release a handled part. Because the volume of a single suction cup is so low, atmospheric air is all that is needed. In other words, no compressed air is needed for release, resulting in further air savings. In addition, the AQR eliminates the need for double hoses for each decentralised unit, resulting in easier plumbing and no cost for an extra control valve. This concept offers all the benefits of a decentralised ejector system in terms of reliability, safety and speed (response and release). The air and energy consumed is virtually nonexistent. 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.		Figure 7: Vacustat-COAX featuring AQR technology.
		Figure 8: AQR (atmospheric quick-release valve) used in a decentralised vacuum system.

As Figure 9 highlights, the pump is only working for an extremely short period of time.

Figure 9: Cycle analysis of a Vacustat-Coax with AQR.

Typical application

Consider a typical sealed vacuum handling application with the following conditions and requirements:

• 10 s cycle time
• 6000 working hours per year
• 5 s vacuum duty cycle
• Four 75 mm diameter cups
• 0.1-0.2 s maximum response time
• Release time less than 0.1 s

Decentralised solutions use approximately 25,000-40,000 m³ of air per year (depending on whether single- or multi-stage ejector technology is used), while a compact ejector with air-saving function will reduce air consumption to approximately 15,000–20,000 m³ of air per year (depending on whether single- or multi-stage ejector technology is used).

In contrast, a Vacustat-COAX and AQR solution will use about 1000 m³ of air per year. Under these conditions, it is possible to reduce energy consumption by 90-99% by simply using the latest available technology.

Returning to our previous equation, we can calculate that 15,000–40,000 m³ of air corresponds to approximately 450–1200 kg of carbon dioxide emissions if the electrical power is supplied by a coal, oil or gas plant. This is based only on a single application or station. A typical automotive plant can have up to 400 of these applications in operation. The carbon footprint of vacuum handling in these plants can be between 180,000 and 480,000 kg when utilising traditional vacuum technology (based on the conditions described above). When utilising a Vacustat-COAX with AQR technology, the carbon footprint can be reduced to only 12,000 kg.

In comparison, the average amount of CO₂ emitted from one car is 180 g/km. The reduced carbon footprint an automotive plant can achieve per year by using the latest vacuum handling technology corresponds to between 933,333 and 2,600,000 km of driving.

Piab Vacuum Technologies Pty Ltd
www.piab.com.au