ASHRAE Journal - May 2014 - 56

Column EnginEEr's notEbook
Kent W. Peterson

Face Velocity Considerations
In Air Handler Selection
By KEnt W. PEtErSOn, P.E., PrESidEntiaL MEMBEr/ FELLOW aSHraE

Fan energy can account for 30% to 40% of building HVAC system energy. In addition, fan
energy use is directly proportional to the pressure drop. Therefore, the more restrictive the supply system, the higher the pressure drop, and the higher the fan energy use.
Most air-handling units are still selected at the typical 500 fpm (2.5 m/s) rule-of-thumb,
regardless of application. However, energy efficiency proponents say that lower velocity is better for high-performance operation. This month's column evaluates methods
to reduce fan energy from internal pressure drop along with the increased initial costs
and reduction in annual operating costs.1
air-Handling Unit applications

Cooling Coil Considerations

Applications for using air-handling units in HVAC
systems can range from providing ventilation and
controlling temperature and humidity in a typical
office building to providing precise temperature
and humidity control in 24/7 facilities. To determine the system's air-handling unit requirement,
the designer must consider the function and physical characteristics of the space to be conditioned.
Specific design parameters must be evaluated to balance initial cost, operating expense, maintenance,
and noise.
Various applications can vary significantly in load
density, hours of operation and annual load profiles.
Constant volume units use significantly more energy
than variable volume units. All of these factors impact
the life-cycle-cost considerations when selecting airhandling units.

Most everyone involved in HVAC unit selection is
aware of the 500 fpm (2.5 m/s) rule for sizing cooling
coils and generally that is sufficient to keep water droplets from leaving the outer edge of the discharge side of
the coil (carryover). It is also good practice to use coils
with a maximum of 8 to 10 fins per inch (fpi), as higher
fpi can have moisture carryover at 500 fpm (2.5 m/s),
higher air-side pressure drop and not allow adequate
space between fins for coil cleaning.
The required length of the drain pan extending past
the leaving edge of a cooling coil at 500 fpm (2.5 m/s)
also increases as fin count goes up. A 12 fpi coil typically requires a drain pan that extends 18 in. (457 mm)
past the cooling coil while an 8 fpi coil needs only 12 in.
(305 mm). Reducing the coil face velocity to 400-fpm
(2 m/s)and using an 8 fpi coil can result in the drain
pan extending only 6 in. (152 mm) past the coil face and
reducing air handler cabinet length.
Guiding Principles
Taylor provided good guidance on coil selections at
The air handler system power consumption can be
500-fpm (2.5 m/s) for maximizing chilled water ∆T.2
estimated by the following equation.
Lowering cooling coil face velocity allows more resident
time in the cooling coil and typically lowers the rows
Airflow(cfm) Pressure Drop (in.w.g.)
Fan Power (kW) =
×0.746 and/or fins per inch resulting in lower coil pressure
6, 345 × Efficiency (%)
drop.
Note that the efficiency is the product of the fan, motor,
belt, and where equipped, variable frequency drive
Kent W. Peterson, P.E., is chief engineer/COO at P2S Engineering in Long Beach, Calif.
He is former chair of Standard 189.1.
efficiencies.
56

ASHRAE JouRnAl

ashrae.org

M ay 2 0 1 4

ASHRAE Journal - May 2014

Table of Contents for the Digital Edition of ASHRAE Journal - May 2014