B4 Fans

In air-media climate control systems, fans are what move the airflow. They may be axial or centrifugal, as described below. Both may move supply or exhaust air, and their motor drives may be direct or by belt or chain.

Axial fans. In these units the airstream moves parallel to the axis of fan rotation. Axial fans are economical and take up little extra space, but the duct that contains them is usually larger and they are less efficient because part the airflow is deflected outward. They are preferred for low-pressure nonducted airflow, and because they are relatively noisy they are often used in recreational and industrial settings. Where they are exposed

propeluor

Fig. 3-16. Types of fans in climate control systems.

propeluor

Fig. 3-16. Types of fans in climate control systems.

to dirty airstreams, they should have belt drives that keep the motor out of the airflow. There are three types: tubeaxial, vaneaxial, and propeller. The first two are mounted in ducting, while propeller fans are typically mounted in walls, roofs, and unitary forced-air systems. Propeller fans with variable-pitch vanes are often used to satisfy inflow requirements with long periods of reduced capacity operation and/or s.a.p. sensors.

Centrifugal fans. In these units the exiting airstream moves perpendicular to the axis of fan rotation. Centrifugal fans can operate at low speeds, can be adjusted more accurately for specific airflow requirements, and are more efficient where air volumes are large and under high pressure. They are better where the fan is installed inside ducting, the ducting changes direction, inflow/outflow duct diameters are different, hooded exhausts are installed, and quiet operation is a priority. But centrifugal fans require more space, are harder to clean, and are more expensive. There are five kinds, based on blade orientation: forward-curved, radial, backward-curved, backward-inclined, and airfoil. Straight blades are the most economical, butairfoil blades are the most efficient, and they are normally used in high-capacity and high-pressure scenarios where energy savings outweigh high initial costs. As forward blades are more pressure-inducing, they are installed in systems with high pressure requirements or high air friction losses, while backward blades are best where volumes are

CLIMATE CONTROL

high and ducts are short. Radial blades are rarely used, as their advantages lie between forward and backward curves and thus they have no optimal features. Centrifugal fans have a variety of drive arrangements, motor mounts, and discharge orientations.

In addition to primary air fans there are booster fans (they increase s.a.p. in a particular zone), recirculating fans (they increase supply air without increasing primary air), and return air fans (they mix return air and outdoor air in large systems with high air friction losses).

Fan design involves (1) finding the unit's airflow delivery rate; (2) determining air friction losses in any ducting; (3) determining the airflow's total air pressure differential for its zone; (4) computing the fan horsepower; and (5) selecting the fan based on required horsepower, desirable sound levels, allowable space, practical drive arrangements, and feasible discharge orientations. As these calculations are lengthy for a component that normally has little impact on other architectural components aside from choice logistics, they are not included here. Also, optimal capacities of many fans are created onsite by giving them adjustable vanes, variablevolume controls, variable-speed motors, or dampers that regulate incoming or outgoing air. Variable-speed motors are the most efficient, though they cost more, but they are usually more economical over time.

RECTANGLE-TO-CIRCLE TRANSFORMATIONS: 1:4 M1N. FOR LOW-VELOCITY SYSTEMS 1:7 M1N. FOR VAV SYSTEMS

ELBOW 1NS1DE RADIUS = M1N. 6", BUT USE 12" 1F SPACE PERM1TS

RECTANGLE-TO-CIRCLE TRANSFORMATIONS: 1:4 M1N. FOR LOW-VELOCITY SYSTEMS 1:7 M1N. FOR VAV SYSTEMS

ELBOW 1NS1DE RADIUS = M1N. 6", BUT USE 12" 1F SPACE PERM1TS

VANES 1N ELBOWS M1N1M1ZE VORTEXES AT FAN 1NLET

Fig. 3-17. Proper inlet and outlet ducting for fans.

VANES 1N ELBOWS M1N1M1ZE VORTEXES AT FAN 1NLET

' LENGTH = M1N.3X MAX. SECT1ON D1MENS1ONS

' LENGTH = M1N.3X MAX. SECT1ON D1MENS1ONS

Fig. 3-17. Proper inlet and outlet ducting for fans.

An important aspect of an HVAC fan is the nature of its entering and exiting airflow. If either is turbulent, the fan's effective airflow can be reduced by 45 percent. Turbulence is caused chiefly by bends in the ducting just before and after the fan. To promote laminar airflow in these areas, the fan's ducting should be straight for at least 10 diameters on each side. If this is unachievable due to spatial constraints, the elbow should be vaned and given as large a radius as possible. The incoming and outgoing ducts should be concentric with the corresponding fan face, and any branch ducts intersecting the main ducts near the fan should also have their axes coincide, or else capacity-robbing vortexes will develop outside the fan openings. Another promoter of smooth airflow is a splitter located inside the entering duct; this increases the airflow's straight length-to-width ratio, which is the essential strategy behind installing straight runs, turning vanes, and large-radius elbows as mentioned above. Good and bad examples of a fan's connecting ducting are sketched in Fig. 3-17.

Two important fan accessories are vibration isolators that reduce noise transmission and drain pans that collect water condensation. All parts of every fan should be accessible for servicing.

0 0

Post a comment