Unit 37 Ductwork

15

A _ temperture difference from the inside to the outside of the duct is considered the maximum difference allowed before insulation is necessary.

Components of Forced Air System

Blower, ductwork, return-air system, grilles and registers

_ are fastened to the main trunk by a takeoff-fitting, which encourages the air moving down the duct to enter the takeoff to the branch duct.

Branch ducts

Flexible Duct

Comes in sized up to 24 in. in diameter. Can be either insulated or uninsulated. Keep duct runs as short as possible. Has more friction to airflow than metal duct. Should be stretched as tight as possible.

Zoning with a multispeed compressor and variable-speed blower

Computer controlled panels store information regarding the air requirements for each zone. Examples of data used: return and supply air temperature, airflow requirements for each zone, percentage of airflow for each zone as a percentage of total system airflow

Conditioning Equipment

Cooling coil, heating device, humidifier, or device to clean air

Squirrel cage blower

Creates more pressure from the inlet to the outlet. Has a forward curved blade and a cutoff to shear the air spinning around the fan wheel. Can be used in very large, high pressure systems.

The Supply Duct System

Distributes air to the terminal units, registers, grilles, or diffusers in the conditioned space

Common duct problems

Excessively long flexible duct runs. disconnected duct runs, closed dampers, collapsed flexible duct, loose insulation in the duct, blocked grills and or registers, end of duct issues

Infiltration

Fresh air enters the structure by _ or by ventilation from a fresh air inlet

Natural-draft system

In a _ the air passes naturally over the conditioning equipment (becoming obsolete)

Spiral Metal duct

More commonly found on large systems. Comes in rolls of flat, narrow metal. Can purchased in various diameter or can be made at the job site. Popular in Commercial applications.

Types of Duct Systems

Plenum system, extended plenum system, reducing plenum system, perimeter loop

Water column

Pressure in the ductwork is measured in units called inches of _

The blower

Provides the pressure difference needed to force the air into the duct system, through the heat-transfer surfaces, through the grilles and registers, and into the room

larger

The return air duct is normally sized slightly _ than the supply duct.

Friction

The smoother the duct's interior surface is, and the slower the air is moving, the less _ there willl be.

Manometer

Tool used to measure air pressure in the ductwork

Air friction chart

Used by systems designers to size ductwork and duct systems

Balancing Dampers

Used to balance the flow of air in various parts of the system. Dampers should be located as close as practical to the trunk line. As close to the trunk as possible is the best place to balance airflow. Handles allow the dampers to be turned at an angle to the airstream.

Static pressure

air pressure in the duct

Aeroseal

alternative method for duct cleaning

Direct-Drive Assemblies

does not have belts and pulleys. Multispeed motors are often used. Speeds can be changed by switching wires at the motor terminal box.

Fiberglass duct styles

flat sheet or round prefabricated cut

Ductwork materials

galvanized sheet metal, aluminum, fiberglass ductboard, spiral metal duct, flexible duct

Belt-Driven Assemblies

have two bearings on the blower shaft and two bearings on the motor shaft. Motor pulley and fan mtor pulleys can be adjusted to change blower speeds.

Air moving in a duct has _, meaning it wants to move in a straight line

inertia

Flow hoods

measure air volume at supply registers; total airflow is measured at the main duct

Velometer

measures how fast air is moving past a particular point in an air duct

Zone damper

open and close automatically in response to changes in room temperature, depending on whether or not the temperature requirements for the zone are satisfied

extended plenum system

popular choice for use in a long structure. Extends the main plenum closer to the farthest supply register. ducts called branches complete the connection to the terminal units.

Velocity pressure

pressure generated by the velocity and weight of the air

Reducing plenum system

reduces the sie of trunk duct size as the volume of air the duct decreases, accomplished with duct fittings called transitions

codes

state or local known standards

plenum system

suited for a job where the room outlets are all close to the unit. Desirable when the duct runs are short.

Total pressure

sum of static pressure and velocity pressure

Gauge

the measurement of the thickness of galvanized steel duct

Friction losses in ductwork are due to...

the rubbing action of the air against the side of the duct and the turbulence of the air while moving down the duct

Using turning _ will improve the air-flow around corners

vanes

perimeter loop system

well suited for installation in a concrete floor in a colder climate. Warm air is in the whole loop when the furnace fan is running.