HVACR416 - Design

Duct Fittings and Components

Elbows

23.4.6• Elbows are 90° bends (unless otherwise specified) in ducts.

• Airflow is diverted from a straight line using elbows. • Air has inertia. Once set in motion, air tends to continue in

the direction it is moving.

• Air wants to flow in straight line. It takes energy to make airflow change direction.

Elbows

23.4.6

Fans

• Airflow is generally produced by some type of fan that is usually located in the inlet of the air conditioner.

• Air is moved by creating a positive or negative pressure. The air inlet to the fan is a negative pressure. The exhaust of the fan is a positive pressure.

Fans

23.4.7• Air feed into the fan is called induced draft. Exhaust from the

fan is called forced draft.

• Fans are constructed of metal or plastic.

• There are several types of fans. The two most popular are:o Axial flow (propeller).o Radial flow (centrifugal flow).

• Air flowing along the direction that the is axle is pointing is called axial flow.

• Air flowing at a right angle to the axle (radius) is called radial flow.

Fans

23.4.7

Fans

23.4.7

• This is a three-blade axial flow fan with a clockwise rotation.

• Axial flow fans are usually direct-driven by mounting the fan blades on the motor shaft.

• Handle blades carefully. If bent or twisted, the blades must be replaced.

Fans

23.4.7

• Radial flow fans are most often used on large installations.

• Maybe belt or direct drive.

Fans

23.4.7

Fan Designations

Low pressure, Class I Less than 3 3/4" (9.5 cm)Medium pressure, Class II 3 3/4" to 6 3/4" (17 cm)High pressure, Class III 6 3/4" to 12 1/4" (31 cm)High pressure, Class IV Greater than 12 1/4" (31 cm)

Radial Flow Fans

23.4.7

• Two designs:o Backward-inclined

blades (for large units).

o Forward-inclined blades (for small units).

Radial Flow Fans

23.4.7

• Static pressure increases as the square of the ratio of increase in the volume of airflow. For example:

o 1000 cfm to 2000 cfm is two times more flow. Therefore, 22 = 2 X 2 = four times more static pressure.

• Static pressure increase is the square of the ratio of the change in rpm.

Radial Flow Fans

23.4.7• Ways to measure performance of fan:

o Pressure difference across fan—Inlet velocity pressure equals outlet velocity pressure (due to conservation of mass).

o Static pressure.

o Velocity pressure.

o Static pressure difference.

o Some technicians measure pressure relative to room air pressure.

Radial Flow Fans

23.4.7

• Formula for determining fan capacity for the furnace: Btu/hr furnace output cfm = X air temperature rise in °F 1.0505

• Air temperature rise in °F is determined by measuring the warm air supply plenum temperature and subtracting the cold air return temperature. The 1.0505 factor is the total external static pressure.

• For cooling, use 400 cfm/ton of capacity.

• Total pressure drop in ducts should be about 0.2" of water column. For a furnace with cooling unit, the fan should provide .10 of water column.

Radial Flow Fans

23.4.7

• To provide more efficient belt drive, belt tension should be on lower belt section.

Radial Flow Fans

23.4.7

• The belt tension is correct when the belt can be pushed out of line a distance equal to its width.

• Fan speed may be varied using adjustable (variable pitch) pulleys.

Radial Flow Fans

23.4.7• All fans collect dirt and lint, which

reduce efficiency. Every six months, remove the fan and scrape, rub, or vacuum the dirt from the blades.

• Oil each bearing with one or two drops of oil each year. If bearings are oiled too much, the oil may coat the fan blades.

Special Duct Problems and Duct Maintenance

23.5• If ducts are not concealed, they may need painting.

o Ducts made of galvanized (zinc-coated) steel must be treated prior to painting.

• Duct systems should have no “dead ends” where dust and stagnant air may collect.

• Reversing the airflow may loosen some of the dust that can then be caught in the filter.

• Periodic duct cleaning should occur to maintain proper airflow.

• Aluminum-to-steel joints encourage corrosion due to electro-chemical action. Joints between dissimilar metals should be avoided.

Smoke Detectors

23.5• City and state codes may require smoke detectors in residential and commercial structures.

• Detectors must be properly located.

• High-velocity air from ducts may keep detectors from working. If a long horizontal run of duct ends at an outlet grille that is not concealed, the smoke detector should be hung on the underside of the duct just behind the outlet grille.

• In residential systems, smoke detectors should be as far away from grilles as possible.

• Never place detectors in corners where there is almost no airflow.

Noise

23.5.1• Two common problems with air distribution systems are noise, produced by movement or vibration, and drafts.

• Three factors that affect noise are the noise source, carriers, and noise amplifiers or reflectors.

• Noises may be:

o High pitch sound. Usually caused by air velocity that is too high. Possibly due to sharp metal edges.

o Low pitch rumble. Usually caused by fan and motor sounds traveling along duct system.

o Popping sound as unit starts or stops. Caused by expansion or contraction of the duct.

Noise

23.5.1• To locate the source of a high-pitched sound, remove the grille or diffuser. If the noise stops, it was due to sharp edges. Locate and correct the problem.

• Decibel meters measure noise level.

o Decibels (dB) refer to the frequency of pressure fluctuations in the air and the amplitude, or size, of these vibrations.

o Airborne sound is expressed in cycles per second (cps).

Noise

23.5.1• Sources of noise:

o Fans and motors.

o High-velocity air traveling through ducts and causing turbulence.

o Sucking and throbbing noises produced by compressors.

o High-velocity refrigerant flow, especially at sharp bends in the piping.

• Noise caused by high-speed air is often the result of an undersize unit or duct.

Noise

23.5.1

• Rigid structures are noise or vibration carriers.

• Hard, smooth surfaces in the conditioned space may reflect or amplify sound.

• Soft fabrics, such as drapes, curtains, and fabric-covered furniture, are noise absorbers. Felt-lined, soft-insulation-lined, and covered ducts absorb noise.