Decoding "efficiency" for mechanical draft fans

05/17/2013


Efficiency ranges

The efficiency and power requirements of a draft fan depend on the type and style of the blades used in a particular application (See Figure 2). These two variables have an opposite relationship to each other. For example, a fan with a low horsepower demand will have a high calculated efficiency.

Figure 2: Centrifugal fan characteristics curve. Courtesy: ProcessBarron

Classically, fan blade designs are categorized in three major groups with relative efficiencies and degrees of power:

Radial: 

  • Straight radial blade (RB) – lowest efficiency (65% to 72%) and therefore highest horsepower demand
  • Radial tip (RT) – low efficiency (72% to 78%) and therefore high horsepower demand

Forward curved (FC) -  lower efficiency (72% to 76%) and therefore higher horsepower demand

Backward inclined (BI)

  • Single thickness backward inclined flat (BF) - high efficiency (79% to 81%) and therefore low horsepower demand
  • Single thickness backward curved (BC) – higher efficiency (80% to 82%) and therefore lower horsepower demand
  • Dual thickness airfoil (AF) – highest efficiency (83% to 88%) and therefore lowest horsepower demand 

Efficiency variations

A mechanical draft fan’s efficiency is dependent on the system pressure used in each calculation. System pressure requirements can take many different forms, leading to an array of ratings. The primary variations are defined below:

  • Static pressure rise: Static pressure differential between the fan outlet and inlet: SPR = SP2 – SP1 (SPR = Static pressure rise; SP1 = SP2 = Static pressure at fan inlet and outlet)
  • Fan total pressure: Total pressure differential between the fan outlet and inlet: FTP = TP2 – TP1 (FTP = Fan total pressure; TP1 = TP1 = Total pressure at fan inlet and outlet)
  • Fan static pressure: This represents the pressure difference between the fan total pressure and the velocity pressure at the fan outlet: FSP = FTP – VP2 (FSP = Fan static pressure; VP2 = Fan outlet velocity pressure)

There are corresponding efficiencies associated with each of these pressures:

  • Static efficiency (ȠSPR) is calculated utilizing static pressure rise. The relative comparison with other efficiencies can be mathematically expressed in the following form: ȠSSPR < Ƞt
  • Fan total efficiency (Ƞt) is calculated utilizing fan total pressure and bears the following relationship to static efficiency and fan static efficiency:  Ƞt > ȠSPR> ȠS
  • Fan static efficiency (ȠS) is calculated utilizing fan static pressure: ȠS < ȠSPR < Ƞt 

The Air Movement and Control Association (AMCA), an association of air system equipment manufacturers, provides rating guidelines to ensure a common platform for the engineers, manufacturers, and users. However, each project is unique and ultimate project execution goals are subject to the interpretation of the players involved. 

Evase effects

When calculating efficiency, static pressure regain due to an evase at the fan outlet may also be accounted for.

Evase output = Static pressure + (VP1 – VP2) x Conversion efficiency

Conversion efficiency may range from 60% to 80%. This has a dramatic effect on calculated efficiency and power input for some mechanical fan applications. Evase effectively reduces a fan’s working pressure requirements and thus reduces input power to the fan shaft. In order to avoid any confusion and to support fair comparisons, project specifications need to be abundantly clear if evase is to be considered when evaluating fans for efficiency. 

Conclusion

Understanding the concepts of power and efficiency and their appropriate use in evaluation of the mechanical draft fans will undoubtedly help plants and engineers to clearly define the requirements and in return they will receive better response from the manufacturers. Evaluation of mechanical draft fans based on “efficiency” may not offer the desired benefit to the clients.

As discussed in this article, an apparent higher efficiency does not necessarily mean a lower input power to the fan. The clients are the rate payers and they pay for the actual power used to drive the fans. So, for relative comparisons, “input power to the fan” should be evaluated rather than the calculated efficiency. 

Nurul “Moni” Talukder, PE, is a chief engineer in the Air Handling Group at ProcessBarron in Pelham, Ala. ProcessBarron is a leader in the design, manufacturing, installation, and maintenance of air and material handling equipment for heavy industry. Moni can be reached at: mtalukder@processbarron.com.


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