By MARCEL KAMUTZKI, P.ENG., AMCA Industrial Manufacturers Market Group
Specifiers of electrical equipment used in industrial environments must take into account any number of factors, including the presence of hazards, such as explosive gases, corrosive and erosive materials, and dust and other particulate; maintainability; and how the equipment will affect the surrounding environment. This article will discuss important considerations in the development of specifications enabling proper selection and construction of fans for industrial applications and the information necessary for an effective fan proposal.
Communication Is Key
Proper fan selection and construction requires good communication—in the form of a specification—between the consulting engineer or user and the fan manufacturer concerning:
- General information, including overall project scope. How many systems are being considered, and how many fans per system? Should all fans be delivered at once, or is staggered delivery preferred? Extended project timelines can make cost estimating difficult. To mitigate risk—for the purchaser and the manufacturer—the purchaser may need to consider escalation terms. What is the application and required service life of the fan? Industrial users often require a fan to have a service life of 20 years or more.
- Basic details, such as site elevation, fan location (inside or outside), and ambient conditions, which can have a significant impact on fan and driver sizing.
- The purchaser’s experience successfully operating and maintaining certain fan types or arrangements. How does the purchaser wish to control fan performance? Will the fan be operating near design conditions most of the time, in which case dampers may be the most cost-effective means of flow control? If the fan will be operating mostly at a lower flow and pressure, variable speed may yield significant power-saving opportunities.
- The required operating points and load cycle. Certain load cycles make variable-speed control impractical, as a fan must provide a minimum pressure for a system to operate effectively. In such cases, damper control may be the best option. Perhaps the cycle changes so frequently that controlling the fan with variable speed alone becomes difficult. For example, a centrifugal fan designed for high flow and low pressure may be ideal in a single-inlet configuration for constant-speed operation, but wholly unsuitable if frequent speed changes are anticipated. In such a case, a double-inlet impeller with narrower flow passages and lesser operating stresses may be a better fit.
- The load cycle (time at each load condition) and frequency of fan stoppage and restart. Is efficiency an important consideration? If it is, how will the cost of power be evaluated at different operating loads? Fan performance varies by fan and load condition. Efficiency at all relevant operating points should be considered.
- Operating-speed ranges, if the fan is to be variable-speed. Every structure has an infinite number of natural frequencies that, if excited, can cause severe damage to a fan and surrounding equipment. Fans can be designed so no harmful natural frequencies are within defined operating-speed ranges.
- The items to be supplied by the fan manufacturer. Typically, flow-control dampers are in the manufacturer’s scope, as they directly impact fan performance. What about ancillary items, such as inlet and outlet transitions and silencers? How will the fan be driven—electric motor or turbine—and who will provide the driver?
- Details of the application. What are the dust load and particle size at the fan inlet?
- The likelihood of naturally occurring outside excitation, such as earthquakes. If the probability is high, manufacturers will consider deformation to ensure there is no undesirable mechanical contact or failure during a seismic event.
Of course, the purchaser must specify the required fan operating points. Usually, at least one load point is well-defined, including the required flow, pressure, operating temperature, and gas density.
If the gas is a mixture, component quantities or the overall specific gravity must be specified to enable the manufacturer to calculate the inlet density and specific heat ratio of the gas stream.
Density is such an important consideration because most processes are designed on a mass-flow basis. Fans, on the other hand, are volumetric devices, providing the same volumetric flow rate against a constant (density corrected) pressure. For example, a cooling-system fan that provides 10,000 cfm at sea level, where the density is 0.075 lb per cubic foot, also will provide 10,000 cfm at a higher elevation, such as in Denver, where the density is approximately 18-percent less. Mass flow over the cooling coils, therefore, would be reduced by 18 percent. Meanwhile, the fan flow rate would need to be increased to provide the required cooling.
If a fan has inlet or outlet duct connections, what are the sizes of the ducts? Is the fan manufacturer required to supply transition pieces from the duct to the fan and from the fan to the duct (evasé)? What about the losses for those transition pieces? Are they accounted for in the operating points? How much space is available for transitions? Although short transitions tend to be less efficient than long ones, modern flow-analysis tools can help to improve the efficiency of short transitions. This tends to be more beneficial for large high-flow applications, in which small improvements in fitting efficiency can yield large returns in the form of energy savings.
Does the customer have a preferred operating speed? Faster fans usually are smaller and less expensive, but also can be less efficient. Sometimes, space permitting, a slower, larger fan can be an economically viable option.
Construction and Special Requirements
Are there any special requirements for the fan rotating assembly? Which type of fan is preferred, axial or centrifugal? If centrifugal, is there a type of blading that is preferred or required? Is efficiency of paramount importance, or is the fan handling a dirty gas stream, in which case long-term reliability likely is to be preferred? If the gas stream is dirty, is the particulate abrasive or sticky? Are there corrosive elements?
Does the customer have preferred limits or design guidelines? Is there a requirement for minimum first critical speed of impeller and shaft? Does the fan manufacturer also need to take oil-film-bearing or roller stiffness into account?
What about bearings? Is there a preferred bearing type, such as anti-friction or sleeve? Is there a required manufacturer? Is grease lubrication or oil lubrication preferred? What are the ambient- and process-temperature limits? How will the bearings be cooled? With water? With air? With circulating oil? Will the bearings require special shaft seals? Will there be any instrumentation requirements for monitoring bearing temperature and vibration? Is local display or simply transmission of a signal to a control room required? Which communications protocol is required?
Is a full local control panel required, or are instrument-mounted transmitters sufficient? What functions will be present?
Once a fan has been specified for performance, what should be considered for static components, such as bearing pedestals and sole plates and fan casings? Does the fan have an inlet box? If it does, is the inlet box integral to the fan casing or bolted to the fan inlet in Arrangement 1 or 9 configuration? Are there any special material requirements for temperature extremes? Are corrosive or explosive atmospheres a potential hazard? Is there a minimum material-thickness requirement? If the flow is abrasive, are wear-protection plates required? Does the site have good experience with a specific type or style of protection?
Will the fan be exposed to high temperatures? If it will, should the fan casing be insulated to address personnel-protection concerns or reduce the amount of noise being transmitted to the surrounding environment? Should the vendor provide insulation mounting studs, or should that be left to the site installation contractor?
What about temperature excursions? Will the fan be exposed to even higher temperatures during upset conditions? If it will, how frequently and for how long, and what are the temperature extremes? How quickly will the gas-stream temperature change? Fans typically are suitable for gradual temperature changes, of, say, 15°F per minute. Some applications experience much more rapid temperature changes.
Has a preliminary plant layout been completed, and is the fan rotation, discharge, and inlet-box orientation known? Industrial fans usually can be customized easily to satisfy any inlet- and outlet-angle requirements.
Is there a preferred form of coupling? Is maintenance-free a preference or a requirement? Certain couplings are ideal for variable-speed applications because they are torsionally soft and can be tuned to avoid natural frequencies. Are there any special guarding requirements? Certain industries require non-sparking guards. These can be aluminum, brass, or plastic.
What sort of driver is required for the fan, and who is responsible for providing it? Will the fan be motor- or turbine-driven? Will it have a gasoline engine or a diesel engine? What utilities are available for the driver? For a motor, what is the voltage, phase, and frequency? For a turbine, what is the steam pressure and quality? Are any controls required for the driver? If yes, are they local or remote?
Various other accessories, such as turning gears, which can be used to help start a high-inertia fan or keep a high-temperature fan rotating while it cools down, also should be specified. Will the fan require frequent cleaning to remove buildup? If yes, will this be done online while the fan is running or during a maintenance period? What is the cleaning medium? Air? Steam? Water?
Does the application or installation location necessitate special protection for fan components? Perhaps the fan is located in a humid environment or near a seashore with a high concentration of airborne salt. If that is the case, special paint may be necessary to protect the fan, which should be made clear to the fan manufacturer.
Are there noise limits that must be observed? The purchaser should consider general Occupational Safety and Health Administration limits on sound levels within and outside a plant environment. Defining acceptable noise limits to satisfy various criteria is beyond the scope and expertise of most fan manufacturers, so the purchaser is encouraged to work with a qualified sound engineer. Generally, if limits are not specified, fan manufacturers will assume the end user has a plan to mitigate sound once a fan is installed. Achievement of required sound levels is possible with minor design modifications, so it is beneficial to address issues at the time fans are quoted.
Typical noise-abatement strategies include inlet and/or outlet silencers to reduce noise being transmitted through ductwork and insulation to reduce noise transmitted through fan casings. Attenuation can be improved by increasing casing-material thickness or adding a fan-shaft seal.
Testing and Quality Assurance
What sort of testing is required to validate a fan? Smaller fans often can be run-tested in the factory at minimal expense, providing assurance the fan will operate trouble-free once installed.
For larger fans, performance testing at the factory is possible, but may not be practical. If a fan is going to be tested, the acceptance criteria should be specified or agreed upon prior to design and construction.
Are there any special analytical requirements and acceptance criteria, such as low-cycle fatigue or torsional critical speed? The results of these analyses can affect material selection.
Does the site require spare parts for the new fan? Having items such as bearings and shaft seals on hand can be helpful in the event of a minor upset during first run. Though this may add a small cost, it will ensure startup is not delayed.
Will the fan be installed and started right away, or will it be stored? If it will be stored, for how long and under what conditions? Will it be stored inside of a heated warehouse or in an outdoor “laydown” area? Answers to these questions will impact packing requirements. If the fan is assembled, ongoing maintenance may be needed to ensure it remains in ready-to-run condition.
To Learn More
For further information on preparing fan specifications and guidance on evaluating proposals, see AMCA Publication 801, Industrial Process/Power Generation Fans: Specification Guidelines.
About the Author
Marcel Kamutzki, P.Eng., is vice president, engineering and automation, for Daltec Process Fans and chair of AMCA’s Industrial Manufacturers Market Group.