| Application Considerations 
Integral Face-and-Bypass Dampers.
(Figure A-23) This custom option, offered in horizontal and vertical coil tube
arrangements, encloses the coil fins between dual
dampers. If heat is needed, these dampers open; otherwise, they remain closed to prevent
"coil wiping" on the leaving side of the coil
and avoid unwanted heat pickup.
Application considerations:
- Vertical coil tube arrangements promote condensate removal and afford better
freeze protection.
- Extra distance is required downstream of this module to promote proper air distribution and reduce stratification.
Fan Module
Fan Type. Designers can choose from five fan types to tailor the air handler’s performance to application requirements;
Table A-4 summarizes and compares the characteristics
and application considerations of these fans.
When evaluating the merits of each fan type, consider
the application’s volumetric rate of airflow,
static pressure, required sound characteristics and available space, as well as the nature of the load and how the
occupied space is used.
To verify that a specific fan’s performance will
satisfy design requirements, use the Modular Climate
Changer selection program or, if you prefer, see catalog supplements CLCH- CS-7A
and CLCH-CS-7C. The fan curves published there include
casing effect. These supplements also discuss how
ductwork connections, air density, fan/ motor heat, drive losses and use of "high-performance" (> 65-percent efficient)
filters affect fan performance.
Fan Control Method. Depending on the control
method used, the air handler fan will provide either a
constant- or variable volume of supply air. In a
constant-volume (CV) system, the fan delivers a consistent amount of air; cooling and/or heating devices adjust the
temperature of that air foroccupant comfort. Since the
fan runs at constant horsepower under all load
conditions, system operating costs are higher than
those of a variable-volume system.
A variable-volume (VAV) system provides
occupant comfort by delivering a fluctuating amount of
constant-temperature air; usually, supply duct static pressure determines how
much air the fan provides. Varying fan horsepower with
building load can offer substantial energy savings and
reduce the building’s airflow requirements. Airflow modulation is accomplished
in one of two ways: with a variable-frequency
drive (VFD) that adjusts fan speed and airflow
by varying motor speed, or with fan- mounted inlet guide vanes (IGV) that limit the amount of air entering the fan.
Table A-4 — Fan Summary Chart
| |
FC Fan |
BI Fan |
AF Fan |
Model Q™ Fan |
Plug Fan |
| Fan Type |
Centrifugal, housed |
Centrifugal,housed |
Centrifugal, housed |
Vaneaxial |
Unhoused |
| Inlet |
Double |
Double |
Double |
Single |
Single |
| Airflowdirection |
Radial |
Radial |
Radial |
Axial |
Pressurized,all directions |
| Optimal Pressure Range |
Low to Medium (0—5in.wg) |
High (4—8in.wg) |
High (4—8in.wg) |
Medium (2—5in.wg) |
Medium |
| First Cost (Relative) |
Low |
Medium |
Medium(2—5 in. wg) |
High |
High |
| Operating Cost (Relative) |
Medium |
Medium-high |
Medium-high |
Low |
Medium |
| Typical RPM range |
400—1,200 |
1,000—2,600 |
1,000—2,600 |
1,400—2,900 |
600—1,200 |
| Blade shape |
Curved |
Flat |
Airfoil |
Airfoil |
Airfoil |
| Acoustical characteristics |
Significant air turbulence that quickly abates; little
blade-tone noise. |
Significant air turbulence; strong blade tones in
250-Hz octave band. |
Significant air turbulence; strong blade tones in 250-Hz
octave band. |
Little turbulence; at hard-to-attenuate low frequencies. 1 |
High inlet noise, but lower discharge noise than FC, BI
and AF fans due to uniform discharge velocity and "plenum effect." |
| Suggested source attenuation |
Add a discharge plenum. |
Avoid using inlet vanes. |
Avoid using inlet vanes. |
Avoid using inlet vanes |
Add a discharge plenum or"Q-Plus" silencer. Add
a discharge plenum or inlet silencer. |
| Space required directly upstream of fan ind raw-thru
arrangements |
None |
None |
None |
Medium blank module. |
Medium blank module |
| Space required directly downstream of fan in blow-thru
arrangements |
Medium blank or diffuser module. |
Medium blank or diffuser module. |
Medium blank or diffuser module. |
Two turning modules |
Medium blank module. |
| When to use? |
Low-to - High-pressure applications. |
High-pressure applications. |
High-pressure applications. |
Quiet applications, especially 6,000-50,000 cfm. |
Multiple-duct-arrangement applications. (1, 2) |
Notes:
1. Add duct takeoff losses from the plug/plenum fan
module to external static pressure; see catalog supplement CLCH-CS- 7A for details.
2. The plug fan’s uniform discharge velocity may
eliminate the need for supply-air ductwork turns and diffusers, suiting it for
installations with limited equipment room space. |
