Efficient Flow Regulation- Variable Speed Drives for Fan Systems

Fans often serve over a wide range of operating conditions, many of which can be below the maximum design capacity. This requires the regulation of flow during times of low demand. Various approaches can be used for flow regulation, including fan speed control, use of inlet vanes, and use of inlet and outlet dampers. These approaches have their respective advantages and drawbacks, and optimal choice depends on the operating conditions prevailing in the system.

Dampers provide flow control by changing the restriction in the path of an airstream. By increasing system resistance, dampers force fans to operate against higher back-pressure, which reduces their output forces them to operate away from their best efficiency points. These conditions increase both operation and maintenance costs. However, use of dampers can als help improve system efficiency by isolating parts of the system, or by providing more favorable start-up conditions for fans.

Fan rotational speed adjustments provide the most efficient means of controlling air flow. By reducing fan rotational speed, less energy is imparted to the air- stream, which means less energy must be dissipated by the system airflow-control devices. Variable speed drives (VSDs) are one of the two methods that can be used to control fan rotational speed – the other one being less flexible and in times more costly option of using multiple-speed motors.

VSDs allow fan rotational speed adjustments over a continuous range, avoiding the need to jump from speed to speed as required by multiple-speed fans. VSDs include several different types of mechanical and electrical systems. Mechanical VSDs include hydraulic clutches, fluid couplings, and adjustable belts and pulleys. Electrical VSDs include eddy current clutches, wound rotor motor controllers, and variable frequency drives (VFDs). VFDs are by far the most popular type of VSD, largely because of their proven effectiveness in reducing energy costs. VFDs decrease energy losses by lowering overall system flow. By slowing the fan and lessening the amount of unnecessary energy imparted to the airstream, VFDs offer substantial savings with respect to the cost-per-unit volume of air moved.  However, use of VFDs may not be suitable (e.g. high static pressure requirements) or may not offer the best solution certain conditions (e.g. systems with small and infrequent demand fluctin) (DOE, 2003. p.44-46)

Development Status Products

Efficient Flow Regulation- Variable Speed Drives for Fan SystemsCosts & Benefits

Parent Process: Fan Systems
Energy Savings Potential

This measure is estimated to offer following improvement potentials (UNIDO, 2010):
• 35% for low efficiency base cases;
• 20% for medium efficiency bas cases;
• 8% for high efficiency base cases.

Electrical and primary energy savings potential of this technology for different countries is estimated as following (UNIDO, 2010):

  Final (GWh/y) Primary (TJ/y)
US 5487 60245
Canada 1007 12543
EU 3870 36363
Thailand 507 5098
Vietnam 232 3311
brazil 927 10341
CO2 Emission Reduction Potential

Total emission reduction potential, as kt CO2/y for different countries are estimated as (UNIDO, 2010): 

US 3306
Canada 501
EU 1688
Thailand 263
Vietnam 114
brazil 135


Estimated typical capital costs of this measure for different system sizes (S) are:
• US $8000  for S < 37 kW;
• US $15000 for 37 kW < S < 75 kW;
• US $30000 for 75 kW < S < 150 kW;
• US $80000 for 150 kW < S < 375 kW;
• US $150000 for 375 kW < S < 745 kW; (UNIDO, 2010)

Cost of conserved energy, expressed as US $/MWh-saved, in different countries are estimated as following (UNIDO, 2010):

US 65.6
Canada 64.8
EU 69.7
Thailand 35.3
Vietnam 35.3
brazil 37.2

Efficient Flow Regulation- Variable Speed Drives for Fan Systems Publications

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24, 64-76

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