Allen Ray, Process Barron, USA
Mechanical draft fans produce some of the largest parasitic loads required in the operation of a boiler. The fans, which provide combustion air and then exhaust the gases through the air pollution control system, can however be optimized to reduce this load requirement. These fans include primary air fans, forced draft fans, over-fire air fans, flue gas recirculation fans and induced draft fans. It is all too common that these fans are operating inefficiently, wasting energy, as well as causing an excessive carbon footprint.
|Assembly to the manufacturer’s specifications is of paramount to performance of boiler draft fans|
TIP 1: PRACTICE GOOD DUCT DESIGN
System losses, other than those of the boiler and ancillary equipment of the boiler, are normally a result of the total pressure losses from friction in the ductwork, elbows, changes in duct cross-sectional area and losses associated with emissions control equipment.
A certain amount of loss is unavoidable; however, losses may be reduced many times by simple and relatively inexpensive modifications such as turning vanes, well-developed and longer sections for diverging and converging flows, redesign of compound elbows and aerodynamically enhancing or removing obstructions in the flow path.
The losses in total pressure as a result of flow through a system are caused by two factors: friction losses due to the viscosity as the air flows along the surface of the ducts and system equipment and dynamic losses due to the turbulent wake caused by changes in direction and separation of flow around obstructions.
The total pressure (Pt) in a system is comprised of two components, static pressure (Ps) and velocity pressure (Pv). Pt is the summation of Ps and Pv [Pt = Ps + Pv]. Ps is that component of pressure that exists by virtue of compression only and Pv is that component that exists due to motion only. These pressure components will change as the cross-sectional area changes. By conservation of energy, the sum of Ps and Pv at any point in the flow system is equal to the sum of Ps and Pv at any other point in the system, plus any losses in pressure occurring between the two points.
With each conversion of energy, there is a loss of total pressure. The more abrupt the change is in cross-sectional area, the greater the loss. Minimizing the number of times the cross-sectional area changes will reduce the system losses. Also, recognizing changes will have to occur, the more gentle the change or the lower the angle of divergence or convergence in the transition section, the lower the losses will be. If possible, maintain a total angle of convergence of 30 º or less and a total angle of divergence of 15 º or less.
There are losses associated with every turn in the direction of flow or duct elbow so the fewer the better. The gas velocity should be limited to 1220 metres/min (4000 feet/min). The aspect ratio should be no less than 1:1 and no more than 4:1. Round duct, having the least perimeter to area ratio, is the optimum section for minimum frictional loss.
Turning vanes installed in the elbow can help reduce the loss as well as guide the flow and keep the velocity profile uniform. If possible, combine elbows. For example, if a duct is making a change in elevation, use two 45 º elbows if possible instead of two 90 º elbows. Also, try to maintain a reasonable turning radius on the elbow. Generally, the smaller the radius of the elbow, the greater the loss will be.
If there are any unnecessary obstructions in the duct system or any unused equipment, it would be wise to evaluate losses and consider their removal. Taking a simple total pressure reading upstream and downstream of the obstruction or equipment will define the losses and their operational costs.
TIP 2: ‘RIGHT-SIZE’ YOUR FAN EQUIPMENT
Poorly operating mechanical draft fans can be a major contributor to excessive parasitic loss in a boiler. Mechanical draft fans have a peak efficiency point on their fan curve normally located just to the right of the peak of the capacity curve. A fan manufacturer will normally size and design the fan such that the performance curve and the system resistance curve intersect at an efficient point. However, this point of operation is often not achieved because of the over accumulation of safety factors in the specification by the owner, the architect/engineer and ultimately the fan manufacturer.
Fans need to be ‘right’ sized. Oversized fans will often require considerable inlet dampening which in itself is inefficient. The fear of many designers is that the fan will be too small and become a limiting factor in the high demand times of operation so larger than needed safety factors are applied. If the volumetric flow rate is overstated, the fan will be selected too wide and then it will operate closer to its peak, creating the opportunity for unstable operation.
If the pressure is overstated, the fan will require excessive dampening to reach its point of operation. Design for too little flow rate or pressure will leave the fan short and unable to provide the draft requirements at peak loads. Therefore, as stated before, is it imperative designers have a good understanding of the volumetric flow rate needed for the process and the pressure requirements of the system for that flow rate so that the draft fans can be properly sized.
Often fans can be tipped or de-tipped to enhance the performance characteristics to be better suited for the actual system they are operating in. Tipping and de-tipping is the alteration of the effective diameter of the fan impeller without changing the effective width of the impeller. The affinity laws or fan laws are different for tip modifications versus a full geometric scale up of the diameter (see Equations 2-4).
If a fan is being dampened by 30 per cent or more at maximum or peak loads, it is a candidate for de-tipping (removing some of the blade tip to decrease the effective diameter). A smaller diameter will decrease the pressure generating capability of the impeller and thus allow the damper to be opened more and lower the required horsepower.
Conversely, if more capacity is needed, increasing the diameter by adding tips and increasing the effective diameter may be a suitable option. It will increase the overall pressure generating capability of the impeller, change the point of rating on the fan curve and generally allow for more volumetric flowrate. Since the impeller is now performing more work, it will require more horsepower. The horsepower varies to the fourth power of the change in effective diameter.
When making an impeller modification, there are other design issues that must be addressed. The rotor will change in weight which will affect the shaft critical as well as the stresses in the blade and surrounding components. Also, the inertia of the rotor (WR2 or WK2) will change which will affect the starting characteristics of the fan. The motor will need to be investigated to be sure that it is capable of accelerating any additional loads. The clearances in the housing, particularly in the cut-off area, will need to be reviewed. As the impeller gets closer to the cut-off, noise can become an issue. Annoying pure tones can become predominant. If the impeller diameter is reduced too much, it can leave it too far away from the cut-off and possibly cause a loss in performance. Generally, you want a gap between the tip of the impeller and the cut-off bar of 6-12 per cent of the effective diameter.
TIP 3: PROVIDE WELL-DESIGNED INLET AND OUTLET CONDITIONS
Another common problem that causes fan performance degradation is system effects. These effects occur because of differences between the fan inlet and outlet connections to the installed system and the standardized connections used in laboratory tests to obtain fan performance ratings.
Fan system effects occur where there is a significant velocity distortion as a result of abrupt changes in cross-sectional area or elbows close to the fan entrance or discharge. Long, straight duct sections on the inlet and outlet more closely approximate laboratory conditions. In most applications, space is limited and these long sections of ductwork may not be possible. To minimize system effects where turns are unavoidable in close proximity to the fan, the ductwork turns should always be in the direction of the rotation of the fan.
By doing this, counter-spin flow situations at the inlet and discharge can be eliminated. Also, use turning vanes in these situations to help make the velocity profile as uniform as possible. To perform properly, a fan requires three equivalent duct diameters in length at the fan discharge to allow for diffusion and the development of a uniform velocity profile. A length of three equivalent duct diameters is adequate for discharge velocities of 900 metres/min or less. For each additional 300 metres/min increase in velocity, add one duct diameter to the required straight length at the discharge.
On double inlet fans, a properly designed duct section that divides the flow is extremely important. This section is often referred to by fan manufacturers as the pants leg because it is shaped like a pair of pants (trousers). The objective is to distribute the flow as evenly as possible to both inlet boxes and maintain the desired uniform velocity profile. However the duct that comes to the ID fan often runs parallel to the shaft and enters from one side of the fan. In these types of situations, a splitter type turning vane should be used that influences the flow upstream of the pants leg and helps feed both inlet box uniformly. By properly dividing the flow, systems effects and thrusting problems on the fan can be avoided.
TIP 4: FANS MUST BE PROPERLY INSTALLED
Proper assembly of fans is paramount to their performance. One of the most critical but least appreciated areas is where the inlet cone is mated to the eye of the rotating assembly. The inlet cones should be set precisely as indicated by the fan manufacturer. Setting the cone farther away from the eye than designated may cause a gap and cause excessive re-circulation. On the other hand, inserting the cone too far into the eye may cause flow separation at the inlet, reducing volumetric flow rate. That is, the fan behaves as if it is narrower impeller design.
Since ID fans handle hot gases, care must be taken to set the cone in a cold condition where it will grow and centre itself. Also, it is important that the rotor and the housing are set square and plum to one another on independent pedestal designs.
TIP 5: THE RIGHT DECISION ON DAMPER OR VARIABLE SPEED OPERATION
Throttling with a damper or using a mechanism such as a fluid drive, two-speed motors or an electric variable frequency inverter to vary the speed controls fan performance. Most often fans are controlled by inlet dampers mounted on the fan inlet box or by radial type vane dampers that are built into the inlet cone. These types of dampers will pre-spin the gas into the inlet of the fan rotor and unload the mechanical work that the fan will do on the gas stream.
On clean applications, it is recommended that radial type dampers be used. They are more efficient than parallel dampers mounted on the inlet box and will reduce horsepower consumption by as much as 3 per cent. For baseload applications with little to no swing in the performance requirements of the fan, dampers would be the recommended choice for control. However, if there are wide variations in the operation of the process and thus significant variations in the operation of the fan, variable speed operation will likely be the better alternative.
There are several good options for variable speed operation. A hydraulic coupling placed between the fan and the electric motor allows the fans to slow down when heavily throttled while the motor continues to run at its full speed. The fan power varies to the cube of the speed change, whereas the power loss in the coupling is directly proportional to the speed. Therefore, depending on the duty cycle, net power savings can be realized.
Two-speed motors with some form of damper control can be an attractive, cost-effective way of reducing power consumption. Pole-amplitude-modulation motors can operate at two adjacent synchronous speeds. The fan is then selected ao that the test block rating is met with the higher speed of the motor and the net or maximum rated continuous point is met with the lower adjacent speed.
Small fluctuations are then controlled with the fan inlet dampers. By far the best option today for variable speed operation, is an inverter duty electric motor with an adjustable frequency drive. There are minimum losses in this type of drive and it can provide ten to one turndowns for a reasonable initial investment.
TIP 6: UPGRADE TO THE APPROPRIATE BLADE DESIGN
Another effective method to improve the capacity or efficiency of a fan is to change the existing impeller design to a different blade design. When evaluating the savings in power consumption, the payback can often be immediate. Typical blade designs include radial, radial tip, forward curved, flat backward inclined, backward curved and airfoil. The static efficiency is different for all of these designs, with radial blades being the least efficient and airfoil shaped blades being the most with approximately 15 per cent difference in the amount of power required to do the same amount of work.
|The ductwork turns should always be in the direction of the rotation of the fan to eliminate counter-spin flow situations|
It is clear that there are potentially large operational savings with a rapid rate of return if you know where to look. Small investments in doing things such as adding turning vanes, changing an elbow design or adjusting the position of the inlet cone in the eye of an impeller can pay huge returns.
Allen Ray is director of Air & Gas Handling for Process Barron, USA, which provides air handling, fuel handling and ash handling solutions for customers worldwide. For more information visit www.processbarron.com.