Decoding “efficiency” for mechanical draft fans

Indoor air quality: In the industrial sector, efficiency is a hot topic, and one increasingly relevant to the design of mechanical draft fans

By Nurul “Moni” Talukder, PE, ProcessBarron May 17, 2013

“Efficiency” is a buzzword in today’s economy. We are on a constant quest to improve efficiency in all aspects of our lives. We are obsessed with green energy, energy-efficient cars, the optimization of our power grid systems, and with improving the efficiency of our household appliances. Our daily interactions with everything around us demand that we become energy conscious. In the industrial sector, efficiency is also a hot topic, and one increasingly relevant to the design of mechanical draft fans.

Mechanical draft fans are used in heavy industrial process operations to move fluid medium from one point to another (See Figure 1). They create draft in a process system so that flow medium can be induced, forced, and boosted. These machines consume a large amount of power, so understanding their “efficiency” dynamics is important. 

Efficiency matters

While there is a lot of talk about efficiency improvements, we often lose sight of how this parameter is derived and defined. Oftentimes, project specifications call for “efficiency” and competing fans are evaluated without proper qualifications and constraints. Equipment manufacturers are faced with the dilemma of deciding which efficiency rating to use when quoting to their clients. Often, projects are awarded based on superior efficiency ratings without giving much consideration to the way in which those ratings are derived.

Currently, there are many different types of fan efficiency ratings prevalent in discussions of draft fan engineering. For instance, a centrifugal fan is selected and sized for certain flow characteristics requiring a finite brake-horsepower. For a given point of operation, while the brake-horsepower remains the same, the efficiency may take different forms.

More specification is required, then, and ratings need to be explained and evaluated to see if they are relevant to the projects in question. This article explains each of these ratings and provides some working guidelines for assessing fan efficiency. 

Defining efficiency

Efficiency is a calculated value. A fan’s total efficiency is defined as the ratio of theoretical air horsepower (AHP) to the actual brake-horsepower (BHP) input to the fan shaft. The equation that describes fan total efficiency can be expressed as:

Ƞt = (AHP/BHP) x 100

Losses between AHP and BHP can be attributed to skin friction, turbulence, leakage, and mechanical friction. So, total efficiency can also be expressed as a culmination of hydraulic, volumetric, and mechanical efficiency.

Ƞt = Ƞh x Ƞv x Ƞm

Ƞh = Hydraulic efficiency

Ƞv = Volume efficiency

Ƞm = Mechanical efficiency

Hydraulic efficiency accounts for the imperfection of the flow path. Volumetric efficiency takes into account leakage through shaft seals and recirculation around the inlet cones and fan casing. Mechanical efficiency accounts for mechanical losses in the bearing, coupling, and seals in a fan system.

Total efficiency can be used to calculate another important variable, a fan’s static efficiency, which is defined as the ratio of fan static pressure (FSP) to fan total pressure (FTP), multiplied by the fan total efficiency.

Ƞs = Ƞt (FSP/FTP)

FSP = Fan static pressure

FTP = Fan total pressure

It is important to note the difference between these two efficiencies. Fan total efficiency gives higher number while static efficiency calculates a lower number. Paradoxically, a calculated higher efficiency does not demand a lower horsepower motor. Motor horsepower requirement for a given fan stays the same. The efficiency numbers are really a fluid dynamics phenomenon. The higher total efficiency is a function of total pressure, which combines static and velocity pressure components, whereas static efficiency only accounts for static component.

Deriving efficiency

The power required to drive mechanical draft fans is viewed as parasitic load. Therefore, minimizing input power to the fan will offer direct economic benefit to the plants. The intellectual knowledge base about the power and efficiency is bound to help engineers to properly specify a fan and manufacturers to optimize and design a better fan.

The origin of production or consumption of power for fluid machinery has its roots in the fundamental thermodynamic relation: 

w = -ʃ v dP

w = work

v = Specific volume

dP = Change in pressure

The AHP for a steady one-dimensional streamline flow can be derived from a classical energy equation, the simplified version of which can be mathematically expressed in the following form: 

AHP = ṁws = ρQghS

Q = Volumetric flow rate, ft3/s

ρ = Density, slugs/ft3

hs = Head, ft 

However, the actual input power (BHP) to drive a fan is described by the following mathematical relation:

BHP = (Q x SP x Kp)/ (CONST x Ƞ)

Ƞ = Efficiency, %

Q = Volumetric flow rate, ft3/min

SP = Static pressure, “w.c.

Kp = Compressibility constant

CONST = conversion constant = 6362

BHP = Input power

Draft fan engineers are most familiar with this formula and use it frequently to rate a fan. This equation can also be used for calculating hydrodynamic horsepower in a ducted flow. Rearranging the equation to calculate for efficiency, efficiency then becomes:

Ƞ = (Q x SP x Kp)/ (CONST x BHP)

As a practical expression, this equation shows that fan efficiency is a function of volume, system pressure, and input power to the fan shaft. The other factor that affects this relation is the compressibility (Kp) of the fluid. Compressibility accounts for relative volume change due to a change in pressure inside the fan casing. This number generally varies from 0.90 to 0.99 for mechanical draft fans.

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.

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.