How to select fans and save energy?


During the whole planning process, there are many choices facing HVAC design engineers. But the choice of fan equipment that is a major consumer of energy is of paramount importance. In this article, energy efficiency improvement options are expanded upon in terms of designing an HVAC system and selecting a fan.

Power Consumption

For fan power consumption, it is in direct proportion to static pressure and system flow rate. In order to get premium efficiency, the least power needed to move air against resistance is calculated in the following equation:

AHP = (Q × P) ÷ 6,356


AHP = air horsepower

Q = volumetric flow rate (cubic feet per minute)

P = pressure (inches of water gauge) or resistance

Space type and occupancy predetermines the flow rates. In ASHRAE Handbook— HVAC Applications1, design criteria are provided for different kinds of commercial and public buildings. Such factors as room circulation, noise and filter efficiency are included in the criteria. Room circulation in air changes per hour often decides airflow requirements. It is a primary means for design engineers to reduce energy consumption by minimizing the static pressure required to move air.

System Pressure Loss

AMCA (Air Movement and Control Association) International states system pressure loss as “the sum of the static-pressure losses due to friction, shock, dissipation of velocity pressure at the system discharge, and the static-pressure differences between the entry and discharge openings of an air system.” Many variables determine the static pressure a fan must overcome and the design engineer can only control some of them. The architect decides where the equipment is located and so the engineer has limited options. Duct configuration and fittings for component connection are main factors that contribute to static pressure. System pressure loss can have other sources that include balancing and control dampers, variable-air-volume (VAV) boxes, diffusers, louvers, coils, filters, and other components in an air stream. As velocity pressure is in proportion to velocity squared, pressure loss that occurs in these components is in proportion to the square of velocity. It means the size is a crucial factor in that fluid velocity is dictated by the cross-sectional area. For instance, if the air velocity in a system is reduced by 10 percent, the system static pressure will experience a 20-percent drop.

As the air power is in direct proportion to pressure, this will cause the energy consumption to be reduced by 20 percent. Apart from taking into consideration the static-pressure loss and attaining the room airflow needed, a design engineer must follow Section 6.5.3 of ANSI/ASHRAE/ IESNA Standard 90.1, Energy Standard for Buildings Except Low-Rise Residential Buildings. When the flow requirements are defined, a design engineer shall put a limit on the system static pressure to meet the power limitation.