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©2016 This excerpt taken from the article of the same name which appeared in ASHRAE Journal, vol. 58, no. 11, November 2016

Armin Hauer, Member ASHRAE

About the Author
Armin Hauer is advanced technology manager at ebm-papst in Farmington, Conn.

Highly efficient air systems require precise fan selection and a method to adjust the fan performance to the load. Fan supplier applications engineers are experienced in identifying the most suitable fan type and fan size for a specified application, whether the fan duty is fixed or continuously variable. Manufacturer-certified performance and electrical power consumption data support competitive performance contracts and ensure minimum energy consumption.

 

Fan Selection

ASHRAE, AMCA Publication 201, and fan manufacturers provide advice for fan selections. Chapter 21 of 2016 ASHRAE Handbook—HVAC Systems and Equipment and AMCA Publication 201 guides the choice of fan types (Figure 1).

Usually, fan manufacturer catalogs show fan power consumption curves or tables for several points of the fan performance ranges. However, buyers would benefit if their focus were also drawn to the fan efficiency maps, which show how a fan’s operational efficiency changes tremendously from its peak to zero. If a product line consists of just a sparse number of fan sizes, inferior energy performance can result. The investment in a highly rated fan is pointless if its actual duty range is far from its best efficiency point. Even if no compelling space limitations exist, fan diameters are often undersized just to keep first cost low (Figure 2).

 

Airside Adjustment

Table 6 in Chapter 21 of the 2016 ASHRAE Handbook—HVAC Systems and Equipment describes adjustable fan inlet guide vanes and variable pitch axial fans. Yet, these two airside performance adjustment methods are rarely seen outside the industrial fan market. A patented centrifugal impeller design provides a continuously variable adjustment of the effective impeller width to align the fan’s best efficiency point to the target flow and pressure. Speed control, however, is often more convenient.

 

Speed Control

The primary fan control method is adjustment of the impeller shaft speed. Adjustment options for mechanical transmissions are belt drives and gear drives. Adjustable sheaves even permit speed changes during fan operation. Especially if the required impeller speed is very low, the speed-and-torque conversion of a mechanical transmission permits use of a high-speed and low-torque motor. This can easily provide such a motor price advantage that it outweighs the added first cost for the mechanical transmission.
Electronic motor speed adjustments use not just voltage controllers or variable frequency drives (VFDs) for ac induction motors but also advanced motor technologies such as electronically commutated motors and switched reluctance motors. Sometimes electronic continuously variable speed controls are combined with mechanical transmissions for using both the maximum permissible motor output and the entire capacity of the electronics.

Once an air system uses speed control, the common predictions of fan power use the third fan law. It postulates that the mechanical power supplied to a given fan impeller is proportional to the cube of the impeller speed.

 

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