Author: Christian Menke, Dipl.-Ing. (FH), Ventilatorenfabrik Oelde GmbH

The production of fertilizers is not only complex, but also energy-intensive. It is a matter of interest, therefore, that energy savings of sometimes more than 25 % can be achieved with high-efficiency centrifugal fans and their auxiliary equipment which are required for the manufacture of fertilizers, that is, when using components of which many people believed that their technological possibilities had long been exhausted.

Demands made on fans during fertilizer production

High demands are made on process fans used in plants where basic chemical materials such as ammonia or fertilizers such as urea are produced. They must be efficient, safe, long-lived, robust and resistant to outside influences, such as the aggressive ambient conditions present in chemical plants or changeable climatic conditions.

API Standard 560 clearly defines that a fan, including its auxiliaries must have a minimum service life of 20 years and guarantee continuous operation for 3 years, without interruption (API 560, § E.3.1.1). This underlines the high availability demanded from these fans, which as process fans in the manufacture of fertilizers are, indeed, a key component in every plant. They handle air for cooling and drying the product, convey clean air from wet scrubbers into stacks and are part of the dust collection systems.

Whenever a fan fails, the production process usually stops. In a large plant for urea manufacture, a single day of stoppage caused by production downtime results in a loss of several hundred thousand euros.


Efficient fans – work horses with digital precision 

At a quick glance a fan may look the same as it did 50 years ago. The basic principle is, of course, the same. However, shape, geometry and angling of the impeller blades are nowadays aerodynamically optimized using modern computer simulation. This improves efficiency while simultaneously reducing both the tendency to caking and wear from any abrasive particles present in the gas handled.

In a similar fashion, the fan casing must be perfectly adapted to the rotor assembly. An optimized interaction of impeller and casing alone permit higher efficiency.


Improved auxiliaries – Objective without loss 

In general, the power consumption of a fan is determined by the operating parameters: volume flow, pressure increase, gas temperature and density. While volume flow and temperature are almost always fixed process parameters, the pressure generated is, however, only partially required for the process itself.

A not inconsiderable portion of the pressure increase produced by the fan is needed to compensate for the pressure losses caused by components situated on the fan inlet or outlet, such as filters, silencers, control equipment and connecting ducts. In practice, the influence of the inlet and outlet situation is often neglected or simply accepted, although it is here especially that there may be a large energy saving potential.

An ideally designed connecting duct contributes greatly to lowering pressure losses and, therefore, investment costs. It may be possible to use a smaller fan with a lower motor output.

An ancillary effect of the reduced fan power consumption is a lower energy requirement. The plant operator reduces energy costs and the negative environmental impact is lessened.

Basically, when designing a duct, the following must be taken into consideration: as few bends and turns as possible, avoidance of irregular reductions and where there is a smooth widening ensure the angles are ideal. Since, in practice, bends and turns cannot be avoided, their radii should be as smooth as possible, avoiding sharp corners to eliminate flow interruption.

In an ideal duct the gas flows in layers, a so-called laminar flow. Surface friction in the duct causes the velocity to decrease. The velocity increases from layer to layer, reaching its maximum in the center of the duct. A consistent velocity profile is the result.

If the flow is perturbed, for example by obstructions, corners, irregular reductions or regular widening where the aperture angle is too large, flow interruptions and/or dead-spots will occur, where turbulences form. This is called a turbulent flow. The velocity profile of a turbulent flow is very irregular. Turbulences formed by flow interruption cause pressure losses since they require energy which is drawn from the flowing gas.

With the aid of Computational Fluid Dynamics (CFD) the flow characteristics of the components involved in the process, such as fans, ducts and all other parts which come into contact with the media, can be simulated and optimized on the computer. Particular attention should be paid that the flow onto the impeller is the best possible as the effectivity of the fan depends significantly on this.

The following example shows the optimization of the inlet duct on a double-inlet ID fan. Using plant sketches provided by the client, a 3D model was generated on which a CFD simulation was carried out.

Fig. 1 shows the numerical flow simulation on the 3D model of the inlet duct in the original design. Particularly noticeable are the acutely angled transitions and the dead spot at the end of the duct. This also shows up in the analysis of the simulation where flow separation of the flow lines can be seen at the above-mentioned points. The pressure losses are accordingly large: from the entry up to the end of the first branch the pressure loss is about 260 Pa; from the entry up to the second branch it is 430 Pa because of the turbulences seen on the right, caused by flow separation in the dead-spot at the end of the duct.

Fig. 2 shows the duct after optimization. Here the critical points, such as the acute angles on the changes in section, have been shaped so that the changes in section are now smooth, thus allowing the flow to follow the contours without interruption. Furthermore, the dead-spot at the end of the duct, where previously turbulences had formed, resulting in losses in pressure and efficiency, has been removed.

In the case of double inlet fans, particularly, it is important to have similar flow characteristics in both inlet ducts to keep the impeller in balance because axial thrusts cancel one another and, therefore, the bearings are under less stress.

By completely eliminating flow separation it was also possible to reduce flow velocity, which has a positive effect on pressure loss.

In this case, the optimized geometry of the duct results in a pressure loss reduction of more than 75 %! A smaller fan with a lower motor output can be used, thus resulting in a reduction in energy costs.


Variable speed control – Precision energy saving 

Process fans in fertilizer plants are usually designed for a maximum operating point “rated”, whereas the actual operating point “normal” is achieved by reducing the air volume. This can be accomplished by variable speed control, control using regulating equipment, such as louver dampers, or a combination of both.

The “rated“ operating point defines the volume flow necessary for the process plus hypothetical leakages, excess air and also a safety factor, defined by the API Standard or the project specifications. Usually, a margin of 10 – 15 % is added to the volume flow at operating point “normal” (15 % if specified in accordance with API 560, see API 560 § E.3.1.2 b). The result is the “rated” operating point. This is normal practice in order to have a capacity reserve to cope with any process fluctuations, caused, for example, by climatic or process temperature fluctuations in the media handled, or to allow for a future increase in production capacity. Operation at maximum operating point “normal” with control dampers fully open should be avoided, as an increase of the volume flow in such a way is not possible.

For economic reasons variable speed control should be the preferred choice for regulating volume flow because the fan efficiency will then always be within its optimum range.

Because fans possess a quadratic load curve and the consumed power is cubically proportional to the speed, the power required can be reduced by about half if the speed is reduced by 20 %.

However, fans are still commonly operated at a fixed speed, using control dampers for regulation.

The control damper is often only set one single time to reach the “normal” operating point. It is then possible that the fan will run continuously for months or years in this state. This drastically reduces the fan efficiency.

When using variable speed control almost any, even the smallest, operating point can be set. With damper regulation, severely throttled operation should be avoided, because when the set angle of the vanes is too steep (> c. 60°) it can cause flow interruption or have other unwelcome effects. Vibrations, pulsations, increased noise emissions and, in the worst case, damage to the fan may be the result.

The investment costs are lower for a fan with a fixed speed motor and regulating dampers compared to a fan with a variable-speed drive and frequency converter. The follow-up costs are, however, considerably higher because of the greater power demand. The higher initial outlay for a frequency converter is often recouped within a short time.

A variable speed control is not only of interest for new plants. When an upgrade is planned, a variable speed control can easily be fitted even to existing fans to help lower operating costs.

Following example uses a fan downstream of a wet scrubber, operating as a “Granulator Scrubber Exhaust Fan“ in a urea granulation plant, to compare variable speed control and damper control. This example can, of course, be applied to any industry and application where more than a single operating point is needed. In practice the fan takes exhaust air from the scrubber and conveys it to a stack.

The fan is intended to approach two operating points. Operating point “normal” works with a mass flow of 756,300 kg/h. The operating point “rated” has a volume flow reserve of 15 %. The mass flow is, therefore, at this point 869,745 kg/h. The total pressure increase is 8,000 Pa at operating point “rated” and at operating point “normal” 6,080 Pa. The gas temperature at both operating points is 46°C.

The calculation is based on a double inlet high-efficiency centrifugal fan (arrangement according to AMCA “Arrangement 3, DWDI“), with following characteristics:

• Urea granulation plant
• Granulator Scrubber Exhaust Fan
• Impeller diameter: 2735 mm
• Rotating speed: 985 rpm
• Motor output: 2550 kW
• Constructional Standard: API 673

When observing the first case, the volume flow of this fan was equipped with two louver dampers, one mounted on each inlet box. The mass flow is changed by altering the damper vane angle (0° = damper fully open; 90° = damper fully closed).

The characteristic curve (Fig. 4) shows regulation using louver dampers. The louver damper vanes are moved by 49.4° to approach operating point “normal”. Because of the throttling effect the lower characteristic curve “normal” becomes more “steep” in comparison with the characteristic curve “rated”. It tips downwards to the right.

The pressure loss through the louver dampers reduces the efficiency, in this example, by 20 % to 62.1 % in comparison with the “rated” operating point (82.1 %).

The same fan with variable speed control shows at “normal” operating point a completely different characteristic than with damper control (Fig. 5). Here the efficiency of 82.0 %, at a rotating speed of 849 rpm, is almost identical to the “rated” operating point value of 82.1 %. It is clear that with variable speed control the fan characteristic curve is shifted in parallel. Since there are no pressure losses caused by throttling, the efficiency is at a similar level at both operating points with the variable speed control.

If one calculates the operating costs for one year based on actual electricity prices for the above comparison, the potential for savings becomes very clear. Presuming that the fan runs continuously for one year, then the actual operating time is 8,760 hours/year (365 days x 24 hours = 8,760 hours).

The average electricity price for industrial customers in Europe (EU-28) during the last two quarters of 2015 was 0.119 euros per kWh (Source: European Commission/Eurostat). This gives following result:

If the fan used in this comparison is operated with variable speed control instead of with damper regulation, then the annual savings are 518,093 euros(!). The additional outlay for a frequency converter is recouped after a short time. The annual savings will have positive benefits for the company books.

If one considers that there is not only one fan installed in a plant but, for example, in the urea granulation plant there are, depending on the process, eight fans (of which six are large process fans), then one quickly reaches the conclusion that the annual savings potential would be much higher were speed regulation consistently used.

Using variable speed control proves to have many advantages:
 

• Energy optimization
• Precise and stable regulation
• Start-up at high torque with low starting current
• Lower mechanical wear on fan and motor
• Simple assembly as a result of dispensing with mechanical control equipment
• Lower maintenance costs
• Lower operating costs
• Reduction of noise emissions at the partial load point of the fan

Conclusion:

A well-coordinated system consisting of an efficient fan, optimum intake and outlet ducting and highly accurate variable speed control increases the operating efficiency of every plant, reduces investment and maintenance costs and decreases any negative environmental impact.