Optimisation: what does it mean?

In the age of the conscious energy use, of decarbonization and of operating costs reduction, optimisation can be defined as a process which reduces the gap between the actual system performances and the technical limits of the applied technology. The applied technology is obviously supposed as BAT (Best Available Techniques) which, in case of dust emission control, but not only, are included in the Bref document (EU Industrial Emissions Directive). The concept of optimisation can be in any case applied to every industrial process and could be roughly considered the way to permanently solve system issues impacting on OPEX, being these last one the costs to keep the business sustainable.

How is it possible to measure the gap between the present operation level of a system respect to the optimised one and decide how to proceed? How far is it convenient to go with the optimisation of a system?

An omogeneous flow distribution of the bags is basical in the optimisation process of a bag filter. This in general put in evidence the limits of the system and the gap between the present operation and what is the expectations at the end of the optimisation process.

In our view optimisation is related to business sustainability, which it is not depending just on costs: today a business is sustainable when is respectful of the environment, and of the health and the safety of the workers. Machine and plant optimisation is functional if we think of technological obsolescence and is necessary when changes have occurred over time that today negatively impact the performance of the overall plant, we will explore the systems surveyed by “small” nuisance dust collectors and then we will move to process filters, which performances have a high impact on the environment and on OPEX. Moreover, the optimisation of an industrial system is also the binding step for any upgrading, as in case of WHR systems.

Small nuisance doesn’t mean not relevant

Applications of nuisance dust collectors cover grinding, drying, separating, material handling and bulk loading and unloading. Today, the most restricted regulation for the diffused emissions and for health and safety of the workers, obliges to extend the necessary attention also to filters not directly involved in the production process. Problems or malfunctioning of such filters, normally are not a direct cause of production loss and this can determine a poor attention to this equipment.

Nuisance dust collectors are frequently overloaded, undersized, and poorly maintained. Old filter type, such as mechanical shaking, reverse air etc, have been all substituted today by pulse jet type filters; if they are in good structural and mechanical condition, the complete system can be optimised working on the design of the whole system. Filter controls can be optimised as well: all work in synergy, but all begins from the design.


The location, the distance and the number of dedusting points are all factors impacting on the efficiency and the effectiveness of the venting system. When approaching an assessment of existing installation as well as the design of a new system, the recommendation is to not exceed 18 ÷ 20 m distance from dedusting points and filter.

Another point to examine is the position of the pick-up points, capturing the dust introduced in the dedusting system. Normally they are considered points where it is necessary to suck dust as much as possible, but it is true the contrary: maximum dust suppression with minimum dust collection. The venting system is not a material conveyor!

Material falls from equipment must be controlled in a different way, repairing holes, improving covering etc.

Figure 1

Figure 2

Example is given in Figure 1 and 2 where wrong and recommended pick-up point design and location are reported. Recommended design is such that reduces the amount of dust captured and conveyed to the filter, with benefit in terms of reduction of the cleaning frequency, saving of compressed air, increasing of bag life, filter availability and, in general, reduction of maintenance cost.

The design of the pick-up point must involve also parts of the vented machine; wide enclosures must be provided as sort of “calm room” to give dust the chance to settle down and convey only a small amount to the filter. Skirts must be provided as well to reduce the free area where the air is sucked into the system, preventing dust escaping; air velocity at free opening area, impacting on the total amount of air to be sucked, must be kept 1 ÷ 1,1 m/s.

The desired velocity at the hood opening should be maintained in the range of 1,3 m/s to limit the amount of dust conveyed to the filter.
After the pick-up points design, the focus must be put on the ductwork. The air velocity inside ducts must be in the range of 18 ÷ 20 m/s; this is the requirement to allow dust to be transported into the duct, keep under control wear and limit the energy needed for moving the air into the system.
Horizontal duct must be avoided because they will fill up with the dust. Transition must be properly designed as well: elbows, joints and branches have an impact on duct wear and dust transported by the air and are places where pressure drops are concentrated, impacting on the energy needed to run the system.

Bag filter

The bag filter is the last part of the design, but it cannot solve issues determined by a wrong design of upstream equipment and system.
The general criteria used for bag filter must be considered:

  • Reduce as much as possible the amount of dust reaching the bags, adopting pre-settling chamber
  • Optimise the gas distribution, considering also a possible side approach of the bags when length is more than two meters.

It is evident that the implementation of such criteria is probably not convenient in small filter, but for units properly sized for more than 10.000 m3/h, savings in terms of maintenance cost and energy could be quite relevant, considering that in a cement plant are normally installed more that 60 nuisance dust collectors.
The size of the filter must be made considering a filtering velocity in the range of 1,2÷1,5 m/1’; the selection is normally made trying to balance the necessary reliability with the available space for the installation. The material used for the bags is commonly polyester felt of 550 g/m2; different material, such as acrylic felt or polyamide can be considered in case of considerable water content in the air flow.

Filter controls

All the different kind of filter control panels today available on the market includes, in their features, the control of the cleaning frequency as function of a set point of the measured bags pressure drop. This is the starting point to efficiently manage the operation of a nuisance dust collector, optimising bags life and compressed air consumption.
For big size of dust collectors, the possibility to control the fan suction can be an interesting option to preserve from a premature bag set deterioration. In facts the fan, provided with a manual control valve, runs at fixed speed and is designed to deliver the requested flow at the max allowed filter DP. When bags are new, the pressure drop is low and the damper must be partially closed to maintain the air flow within the design parameters, avoiding bags overcharge and premature failure.
The fan damper must be progressively open as the DP of the bags increases; this operation is normally manually executed and requires frequent inspection by the operator. An automatic controller, which monitors the pressure at filter inlet and controls the fan damper, can easily manage the filter operation, allowing a better control of the operating costs.

Process stands out

As well as malfunctions, the results of the adoption of optimisation concepts are in general more evident on process bag filters for a simple reason: they have a direct impact on the production, they are big and savings on energy consumption and maintenance are relevant on the OPEX.

As for nuisance, process filters optimisation cannot start if the process up stream is not already optimised as well; in general, a not optimised process means unstable conditions, not allowing a clear definition of targets for the optimisation of the downstream bag filter.
In facts, a not efficient gas conditioning in the cooling tower, poor gas mixing among streams coming from kiln, raw mill by-pass, raw mills and even a not optimized clinker cooler process, could render ineffective actions taken for the optimisation of the filters installed downstream. The above cases are applicable to cement plants, but the concept can be extended to every industrial process.

Process data

For the above reasons, the optimisation of a process bag filter cannot start before the conclusion of an assessment of the process data, including also possible fluctuations and flushes, in order to build a complete picture of the filter operating condition.

Filter controls

As said, devices for the control of bag filters operation have reached today a high level of reliability; with the application of digital technologies, they can become “smart”.

With the installation of smart controllers, the optimisation on process filters can reach high levels because the system can automatically follow variations of operating conditions and find the best setting related also to the normal deterioration of the bags performances.
Of course, the best results are obtained if the smart controller is installed on a system with a high grade of optimisation.

Bag filter

Once process conditions and fluctuations at filter inlet are well defined and consolidated, the assessment of the bag filter can start defining the targets of the optimisation process.
Since we are speaking of optimisation, the original operation of the filter is supposed to be in any case acceptable in terms of dust emissions and failures affecting the operational continuity; interventions addressed to solve situation of poor abatement efficiency, frequent bags failures, very high DP and reduced bag life must be normally solved as soon as possible because cause of production loss and is not a “pure” optimisation action considering this last one as something to be taken to reduce OPEX mainly.

Optimisation, for process filters, are actions taken to introduce ameliorations with a pay back; of course, every kind of intervention taken to ameliorate the present operation of a system can be considered optimisation, but a real added value can be introduced only optimising a process already at a minimum level of acceptability, which already allows the sustainability of the business.
Dealing with process bag filter, the most common request for optimisation concerns a better DP management and reduction; this is normally connected to the extension of the bag life, if emissions are in any case under control. The final target is obviously the reduction of the OPEX in terms of maintenance cost and energy cost.
The optimisation involves in general the following topics:

  • Flow distribution on the bags
  • Cleaning system efficiency

Other optimisations can come adopting cages of new design and bags of different materials or concept.

Do not waste anything

Can WHR system be subjected to some optimisation process as well?
Identify margins for optimisation in a waste heat recovery system, in terms of production increase, especially for recent installations, is not always possible; it normally depends on the accuracy used during the assessment of the operating condition made before the investment decision.

Margins are in any case limited by the size of the turbogenerator installed which gives the max power potentially generable.
The optimisation in such case is more focused in the integration of the WHR in the process respect to increase the instant power production, resulting in a higher power average value; adopting this approach, it could be possible to individuate situations in which with minor changes in the way to operate the system upstream the WHR, could be maybe possible to sustain a higher power production.
Possible optimisation of WHR system in terms of components or whole system as well, can be explored in the adoption of solutions that allow a longer life of thermal oil with innovative solutions concerning the oil system itself and improve the efficiency of the boilers through a more effective cleaning system.

Facing the reality

Reduce OPEX, save money and, in this modern age, also to reduce power consumptions to be active part in decarbonization process, can be dreams for every plant manager; of course, every optimisation process must be based on a trustable business plan, because the requested investment must return in an acceptable period.
To think about the optimisation of a system or equipment, and plan possible actions, it is necessary to find answers to the following questions:

How is it possible to measure the gap between the present operation level of a system respect to the optimised one and decide how to proceed? How far is it convenient to go with the optimisation of a system?

The first question reflects the technical aspect; the answer can be found only through a deep knowledge of the process, the systems and equipment and of the most updated applicable technology as well, to individuate the technical limits. CTP Team has recently established the new OTP division, which means Optimisation Technology and Performances, specialized in the optimisation of process Filters and Waste heat recovery systems. Collecting skills and expertise of manufacturer and clients, OTP’s mission consists in reaching higher performances, reliability and savings by providing technical analyses of the existing scenario and study optimized solutions.
The second question is about economic aspects, since every enthusiastic optimisation process requires investments; this is the point where dreams impact with the reality.

Optimisation is “optimised” if it is sustainable.

Key value of the optimisation path with OTP division

Optimisation path with OTP drive higher performances, lower internal consumptions, low risks for health and safety.

Components and Technologies available nowadays for bag filter’s cleaning system, give to the designers many options for the upgrading of existing system, with immediate results in terms of OPEX consumption.

An omogeneous flow distribution of the bags is basical in the optimisation process of a bag filter. This in general put in evidence the limits of the system and the gap between the present operation and what is the expectations at the end of the optimisation process.