Energy-efficient suction and filtration systems

Modern suction systems are expected to be particularly economical with the electrical energy. With regard to the current and future expected energy price situation, this demand is more than understandable.

In principle, it should be said that essentially 2 factors determine the energy demand of a plant:

1. The required amount of air in connection with the total pressure requirement.

2. The plant design.

 

1. Required amount of air and total pressure required

The biggest energy consumers of an exhaust system are the fans (fig. 1). The power requirement of a fan is determined by the factors:

  • exhaust volume flow,
  • total pressure requirements and
  • efficiency.

As a result, it is possible to find out which individual objectives must be followed during the plant planning to achieve the overall objective of achieving maximum energy efficiency:

  • the conveyed air volume should be adapted to the respective requirements during all operating phases.
  • the total pressure requirement of the plant should be minimised. Since the machine resistances of the exhaust system are specified (specifications of the machine manufacturers), the possibilities are limited to the dimensioning of the pipework system as well as the return-air system and the layout of the filter system.
  • the fan is optimally designed so that a high fan efficiency degree is achieved. Depending on the plant design, fans with efficiencies of between 55% (overpressure system) and 85% (vacuum system) are used. The comparison between the efficiency levels, i.e. with the same air quantity and the same total pressure, shows that the efficiency degree differences cause an advantage of 35% in favour of a vacuum system.

The total air quantity to be exhausted is determined by the number of machines operated simultaneously due to the production process.

First, based on the machine layout and the information provided by the machine manufacturer a list of machine-related exhaust volume flows is prepared.     

Together with the operator, the number of machines to be exhausted at the same time is determined and the necessary extraction air quantity is calculated based on the volume flow list. A characteristic size is the concurrency factor, which indicates the ratio of the actually necessary exhaust air quantity in relation to the total air volume. From practice it is known that these concurrency factors are between 40% and 100%.

In many cases, the exhaust system can be operated with lower air quantities caused by operational processes. This can be expressed by the so-called "load level". The load level, not yet an introduced technical term in the industry, provides information about the daily routine of operations, while the concurrency factor represents a pure planning size.

One aspect that can also influence the amount of air to be exhausted is to ensure the transport speed in the piping system. In order to be able to transport the chips and dusts extracted at the machines through the piping system, i.e. without material deposits in individual pipe sections, minimum air speeds must be observed. This aspect must be checked as part of the dimensioning of the pipework system.

The second size, which determines the power requirement, is the total pressure demand. After the determination of the pipeline path, the selection of the filter system and the planning of the return-air system the required total pressure requirement can be determined with help of the pipework system calculation.

The pressure losses in the individual plant elements are mainly depending to the air speeds and the resistance coefficients or the pipe frictional value. Higher air speed means, on the one hand, smaller pipe diameters or on the other hand, higher pressure losses.

It is known that an increase in air speed from e.g. 20 m/s to 24 m/s is equivalent to an increase in pressure loss of 44%. The energy demand increases even around 73%.

In the context of plant planning, it is therefore necessary to dimension the pipework system and other plant components in such a way that an optimum is achieved, whereas the factors:

  • investment costs,
  • energy costs,
  • flexibility of use and
  • operational safety

are regarded as a variable.

With the required total air volume and the total pressure, the parameters for selecting the exhaust fans are determined.

Depending on the plant design (overpressure system or vacuum system), the fans are laid out to meet the required power requirements.

 

2. plant design

The plant design essentially determines the overall efficiency degree of the plant and thus the energy needs. The following plant-technical characteristics are decisive:

  • vacuum system or overpressure technology,
  • controlled or unregulated exhausting capacity.

It is a matter of course that other factors also have a role to play in the design of the plant. Where the following are to be mentioned:

  • the investment budget,
  • the local conditions,
  • the material quantities,
  • decentralized or centralized filter arrangement,
  • direct or indirect air guiding,
  • etc.

However, in relation to the energy efficiency of an installation, the two aspects mentioned above are decisive, so that the rest of the other factors mentioned in this article should not further be highlighted.

- overpressure technology or vacuum system

Overpressure and vacuum systems are different in the arrangement of the exhaust fans.

If the exhaust fans are arranged in the direction of flow in front of the filter system, this is an overpressure system. An arrangement in the clean-air chamber of the filter system or after the filter system is a vacuum system.

Depending on the task, fans with efficiency levels between 55% and 75% are used. The woodworking machines to be exhausted are usually divided into several groups, the assignment of which depends on the work processes, the air volumes and the machine resistances.

The required negative pressure for the individual woodworking machine can, for example, vary considerably, so that group-related larger differences in the total pressure demand can result. The fans of the individual suction groups are calculated - as explained - using the value pair “amount of air/total pressure demand” with the nominal working point in or in the range of the efficiency degree maximum on the fan characteristic curve. The adjustment of the fan to the plant data is usually reached by a corresponding selection of the transmission ratio of the belt drive. Compared to the direct drive the belt drive is an additional source of loss, which can be between 10-15% in case of unfavourable conditions.

Overpressure systems can thus be easily adapted to the different pressure demand of individual woodworking machines.

If plant components are operated differently from this nominal operating point, the energy consumption only changes in the context of the throttle effect, which is the result of closing the exhaust connection of a non-operated woodworking machine. For the remaining operated woodworking machines, this means that the available exhaust capacity increases and thus lies above the nominal exhaust capacity (layout condition). This excess suction power leads to excessive energy consumption.

As already stated above, in the vacuum systems, the exhaust fans are arranged in or after the filter system. The achievable efficiency levels are up to 85%. In this arrangement the fans nearly have clean-air conditions, so that closed high-performance running-wheels equipped with backwards curved blades can be used. The achievable efficiency levels are 85%.

The adjustment of the suction power to the demand is made by using several fans, which are switched on or off on demand. The fineness of the adjustments depends on the number of fans.

For the dimensioning of the pipework system and the selection of the exhaust fans the same criteria as for the overpressure technology are applied.

A greater effort is generated when the machine resistances vary greatly. This aspect must be considered for the planning. In extreme cases on the raw air-side fans are used to compensate the higher pressure requirements, so that the overall power demand for the complete system is at a similar level. The exhaust fans installed downstream the filter system can be designed to the lower level in relation to the total pressure.

-uncontrolled or controlled exhausting capacity

The possibilities of adapting the exhaust power to the needs by using several fans were already explained in the descriptions about the overpressure and vacuum technology.

It is certainly easy to understand that a large savings effect is achieved if the capacity is continuously adapted to the respective requirements. This possibility exists in an almost ideal manner with speed-controlled exhaust fans.

By powerful frequency converters, combinable with standard motors (standard efficiency category IE3, optional IE4) this functionality can be achieved. By the frequency converter when changing the fan speed, the corresponding pressure increase as well as the required amount of air is adjusted. The regulatory principle is based, for example, on pressure stabilizing at a fixed place in the conveyor system. The static pressure on the measuring location is determined via a pressure measuring sensor and passed on to the PID controller integrated in the frequency converter. The PID controller evaluates the signal and performs a target-performance-comparison. Depending on the deviation, the speed of the fan motor is changed via the frequency inverter so that the pressure level specified as set value is reached.

As already mentioned, the pipework system and the fan must be based on the conditions of this operation process in order to ensure the savings potential of the respective use case to the fullest extent possible.

The following points must be considered:

  • compliance with the minimum conveyor speeds to avoid deposits in the pipes,
  • determination of the fans in a specific characteristic range,
  • pressure balance of the pipe system (e.g. by using support fans).

Another advantage of the speed control is the constancy of the exhausting power regardless changing operating conditions of the system.

During the operation of filtration systems and/or after longer operating times, the resistance of the filter elements change, so that within an operation cycle the volume flow changes and/or in the longer term results in a steady reduction of the extracted volume flow. In the case of plant planning, these influences must be considered for the uncontrolled systems by providing appropriate reserves for the fans. This means that such systems are operated averagely with a performance surplus, which results in a higher power consumption.

In the case of speed-controlled systems, these disadvantages are avoided by continuously adjusting the operation to the current conditions by means of the pressure monitoring already mentioned in connection with the speed control. The increasing resistance of the filter elements in an operating cycle is compensated by the increase of the fan speed, so that the extracted volume flow at the individual processing machines remains almost constant. Conversely, after cleaning the filter elements and thus reducing the system resistance, the speed of the exhaust fan is reduced so that unnecessary power surpluses are avoided.

In summary, there is no lump-sum concept for each individual case regarding the variety of tasks, with which the desired goal of the highest possible energy efficiency of the entire plant is achieved. The previous versions should have shown that in very many cases, especially with strongly fluctuating load levels, the speed-controlled vacuum system is unbeatable in terms of optimal energy efficiency. The practice has shown that electricity savings of up to 50% are possible, with other significant advantages of this system:

  • lower heat losses, especially for waste-air operation,
  • reduced wear and
  • reduced noise emissions,
  • lower risk potential.

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