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Trends in the magic world of compressed air – where are we going? 

Elvira Rakova, PhD

Welcome to the first article in a series addressing the evolution of compressed air optimization.

Part 1: Energy optimization, trends, and standards – actuator application. 

Is the electrification of each process an answer to the rising prices of energy?

The question of choice of the right application has been influenced by rising electricity prices and a new Green Deal. The question of electrification comes up most frequently when designing a new industrial machine.  Policymakers and companies already understand the issue and bring forward the standard on the efficient choice of pneumatic systems. 

A new ISO standard will cover the optimization of the full compressed system starting from the actuator including the TCO analysis that allows the comparison of electro-mechanic drives. 

Electric and pneumatic drives are widely used to perform a variety of handling tasks. While electric drives are highly dynamic, very precise and demonstrate tailored control concepts for position, velocity, and force, pneumatic drives are low in installation and maintenance costs and offer both a flexible and robust design. The bottom line is that compressed air is an expensive source of energy, whereas electric drives present high investment costs. To find the balance between cost, efficiency and functionality, various factors have to be considered, including fulfilment of the task regarding velocity and forces, expected energy consumption, life cycle costs and the payback period over the lifetime of the system.

The major challenge with the high-energy consumption of pneumatic drives is the absence of a defined sizing approach. 

The choice of the pneumatic cylinder according to its task promises high energy savings in comparison to conventional methods that use high safety factors. Additional energy-saving measures offer a lower total cost of ownership in comparison with electro-mechanics.

Optimization of design parameters – An Example

Chosen application is: pneumatic press with a supply pressure 5 bar, with the press capacity of 500 N down and back with the mass of 2 kg.

Application of energy saving measured to a cylinder drive can be summarized in the following decision-making matrix:

Figure 1: Decision-making diagram for cylinder choice

Cylinder diameter recalculation

The first step is to check whether it is possible to change the cylinder and, if so, to check if the diameter of the cylinder is chosen correctly. Commonly, cylinders are oversized leading to unnecessary energy consumption and bigger dead volumes (e.g. cylinder and tubes), impacting further on losses in dynamics, higher mass of the system and increased acquisition costs. 

The definition of the ideal diameter of a cylinder is indeed crucial for achieving optimal energy consumption with respect to the expected task. In that sense, it is important to consider parameters such as required forces, pressure level in the cylinder inlet, required velocity, time, type of mounting and also friction forces.

In practice, the correct choice of the cylinder diameter does not impact its functionality but it does have an impact on the energy consumption. Indeed, calculations demonstrate that poorsizing of a cylinder’s diameter can lead to an increase in energy consumption of up to 40 %. 

ISO-cylinders are represented by standard numbers. The higher the number, the bigger is the gap between current and next size (see: 40, 50, 63, 80, 100). Some companies offer intermediate non-standard diameters in their efficiency program cylinders, for example 45 mm, that can save up to 25 % of energy consumption. 

Reduce supply pressure

In the case of already installed cylinders, users can still save energy on their systems by reducing the pressure levels. 

Figure 2: Energy consumption calculation for a standard cylinder, reduced pressure option and “oversized” drive. 

This solution, however, is not optimal as it requires an additional pressure reducer, and the energy saving is lower in comparison to a correctly sized cylinder. Moreover, dead volumes will influence the studied application where the velocity of a bigger drive with reduced pressure is lower. The reason is mainly dead volumes which are about 10 % of the piston volume. The value of dead volume gains with each cylinder size, increasing filling time, and therefore motion cycle. Additionally, the tube diameter also plays a role because usually it is chosen according to supply pressure and the total cylinder volume. In these cases, the tube with the diameter of 8 mm was chosen for the cylinder size up to 40 mm and a pipe of 10mm diameter for bigger cylinder. In all cases the length of the pipe is 1.5m. Presented results show the importance of the correct sizing of the actuator at the concept phase, where the smaller cylinder with higher pressure level consumes less energy than the bigger cylinder with the lower supply pressure level. 

Figure 3: Simulation results of circuits with cylinder 40mm under 5 bar and reduced pressure by 4 bar, but bigger cylinder diameter of 50 mm.  

However, this approach works for multi-axis systems within one machine or even on the level of the production plant. Adjusting the pressure level can also be applied to the group of actuators, creating a multi-pressure level network. 

Another opportunity for pressure adjustment is the reduction in one direction only, as required by many applications.  

Reduced pressure return

In many applications, cylinders do not require any effort for the return stroke. Usually, lower pressure is sufficient to ensure the displacement. For the demonstrated application, for example, effort is required only to lift the mass while descending it, then lower pressure is needed. Reducing pressure level for descending guarantees saving in energy consumption up to 28 %.

Figure 4: Energy consumption of reduced pressure return

Energy-saving control with intelligent pneumatics

Alongside parameters optimization such as diameter and pressure level, there are ways to reuse exhaust energy and use expansion energy for the motion. In this section, we take a look at a bridge circuit and the use of an air reservoir. 

The use of the bridge circuit and the premature disconnection of the compressed air supply from the pneumatic actuator makes the energy savings potential particularly evident in comparison to the standard circuit. The bridge circuit consumes less than 50% of the amount of compressed air per double stroke in contrast to the standard circuit. The curves of the chamber pressures show that the short-term ventilation time of the cylinder chambers limits the pressure level in the present case to approximately 4 bar. After the movement has started, the chamber pressure drops during extension. This phenomenon occurs because the movement of the piston increases the volume of the chamber but the compressed air supply is interrupted. The pressure in the end positions is significantly lower than in the standard circuit whereby a secure holding of the position is uncertain. 

An exhaust air recovery presents a simple air spring set at a certain level which offers up to 40% of energy savings. For this application, the size of the air reservoir has to be identified and recalculated for each application and requires additional space for the installation. 

Figure 5: Test results of standard circuit (left-hand side), alternative control and energy consumption calculation

In the end, comparing both pneumatic and electro-mechanic drives, the TCO analysis shows that cost per cycle of a pneumatic cylinder calculated over 6 years is lower than electro-mechanics for load application before 20 kg.

Figure 6: Pneumatics vs electro-mechanics, TCO analysis

Pneumatic vs electro-mechanics is no longer a battle, but a matter of a choice. 

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