System DesignFor fans in process cooling applications, the term system refers to the path through which air is pushed and/or pulled from beginning to end. The system can be as simple as blowing ambient air directly on product or as involved as a multizoned system with dampers, elbows, filters and cooling coils.
In the typical system design, the performance requirements and system resistance are calculated and then used to select the appropriate fan. However, in too many cases, the effects of the relationship between the system components and the fan are not considered in the selection process. For example, the resistance of a given size elbow at a given flow can be determined easily. However, if that elbow is located at the fan inlet or outlet, further immeasurable losses will be imposed in addition to the simple loss through the elbow itself. Most importantly, these losses cannot be measured or even detected with field instruments because they are, in fact, a destruction of the fan's performance.
The four most common causes of system effects are:
- Eccentric flow into the fan inlet.
- Spinning flow into the fan inlet.
- Improper ductwork at the fan outlet.
- Obstructions at the fan inlet or outlet.
Spinning Flow. Unintentionally spinning air into the fan inlet can have the same effect on performance as the intentional prespin produced by a vortex-type inlet damper. The direction the air is flowing when it enters the fan wheel is very important. To produce its rated capacity, the fan works on the air by changing its direction and accelerating its velocity. If the air is spinning in the same direction as the wheel rotation, fan capacity will be diminished. If the air is spinning in the opposite direction of the wheel rotation, the brake horsepower and noise of the fan will increase. Prespinning flow can result from any number of common situations. Two elbows in close proximity to one another can force the air to make consecutive turns in perpendicular planes, forming a cork-screw effect.
Correcting Bad Inlet ConnectionsThe ideal fan inlet connection creates neither eccentric nor spinning flow. Where an inlet duct is required, the best connection is a long straight duct with straightening vanes. However, in the real world, it often is necessary to adapt the system to the available space. When space becomes the limiting factor, two choices are available: Install corrective devices in the duct or increase fan speed to compensate.
Installing corrective devices in the duct is preferable, but increasing fan speed to compensate often is necessary. In many cases, the corrective devices themselves will represent some resistance to flow. A combination of both choices could be necessary to correct extreme field performance problems. Simple or complex turning vanes can be used to minimize the effects of both eccentric and spinning flow (figure 3).
Even well-designed inlet connections, with or without corrective devices, can produce losses in performance. These losses can be difficult if not impossible to predict. Even the inlet box (with all the turning vanes installed) could represent losses of 10 percent to 15 percent of the required flow.
Of course the fan's speed should never be increased beyond the cataloged maximum safe speed.
It is important to note that the increased resistance will not be observed on the system. The pressure increase is only for the purpose of selecting the fan to compensate for the losses associated with the particular system effect.
The fan laws cannot be applied selectively, only simultaneously. According to the fan laws, if the fan speed is increased 10 percent for a given system, the flow through the system will increase 10 percent (RPM2/RPM1)1, the system resistance will increase 21 percent (RPM2/RPM1)2, and the fan BHP will increase 33 percent (RPM2/RPM1)3. This represents an obvious waste of energy. In many cases, such a change would require the purchase of a larger motor as well as a new drive. If the fan is a direct-connected arrangement limited to one fixed motor speed, the solution becomes even more expensive. These considerations and horsepower penalties apply to all the major causes of system-induced performance deficiencies.
If the available space dictates the need for a turn into the fan inlet, an inlet-box supplied by the fan manufacturer, with predictable losses, should be used whenever possible.
Discharge Ductwork. The connection made to a fan outlet also can affect fan performance. An outlet duct ranging in length from 2.5 to 6 fan wheel diameters, depending on velocity, is necessary to allow the fan to develop its full rated pressure. If the outlet duct is omitted completely, a static pressure loss equal to one-half the outlet velocity pressure will result. The system resistance calculation should include this loss as additional required static pressure.
Air is not discharged from a fan with a uniform velocity profile: Air has weight, and it is thrown to the outside of the scroll. Figure 4 shows a typical velocity profile.
In a duct with a uniform cross section, the average velocity will be the same at all points along the duct. However, where velocity distribution changes (such as the duct adjacent to the fan outlet), the velocities typically are not the same. Because velocity pressure is proportional to velocity squared, the average velocity pressure at the fan outlet will be higher than the average downstream. Because total pressure will be virtually the same, the static pressure cannot be developed fully until some point 2.5 to 6 duct diameters downstream.
Although duct turns directly at the fan outlet are not recommended, there are times when they cannot be avoided. In such cases, the turns should follow the same direction as the wheel rotation. Turns made in the opposite direction of wheel rotation can have a pressure drop beyond normal system calculations. Usually the drop is between 0.5 to 1.5 fan outlet velocity pressures.
Inlet or Outlet Obstructions. System obstructions can be as obvious as a cone-shaped stack cap, which can have a pressure drop as high as one velocity pressure, or as subtle as the installation of a large drive sheave directly in front of the fan inlet. Another common situation is to place a fan inside a plenum or near some obstruction and fail to account for the effects on the airflow to the fan inlet. Losses will increase as the velocity increases or as the distance between the obstruction and the fan inlet decreases.
If system-effect situations cannot be avoided, their impact on performance should be estimated and added to the calculated system resistance prior to selecting the fan. Ignoring system effects can lead to difficult process performance problems later and most certainly higher operating costs over the life of the cooling system.