SUPPLY DEVICES AND ROOM AIR DISTRIBUTION
One way to condition air is to use a fan to move the air over the conditioning space. The components that make up the forced-air system are the blower (fan), the air supply system, the filter, the balancing damper, the return air system, grilles and registers where the circulated air enters the room and returns to the conditioning equipment.
TYPES OF FANS
The fan or blower as it is sometimes called, can be described as a device that produces airflow or movement. The fan provides the pressure difference to force the air into the duct system, through grilles and registers and into a room. Several different types of fans produce this movement such as propeller fan, axial-flow fans and centrifugal fans.
The propeller fan is used in exhaust-fan and condenser-fan applications. It will handle large volumes of air at low pressure differentials. The propeller fan can be cast iron, aluminum or stamped steel and is set into a housing called a venturi to encourage airflow in a straight line from one side of the fan to the other. The propeller fan makes more noise than the centrifugal fan so it is normally used where noise is not a factor.
The centrifugal fan has characteristics that make it desirable for duct work. It builds more pressure from the inlet to the outlet and moves more air against more pressure. This fan has a forward curved blade and a cutoff to shear the air spinning around the fan wheel. The centrifugal fan is very quiet when properly applied.
SUPPLY AIR DUCT SYSTEM
The supply duct system distributes air to the terminal units, registers or diffusers into the conditioned space. Starting at the fan outlet, the duct can be fastened to the fan housing directly or have a fireproof vibration eliminator between the fan and the ductwork. The duct system must be designed to allow air moving toward the conditioned space to move as freely as possible but the duct must not be oversized. Duct systems can be plenum, extended plenum, reducing plenum or perimeter loop.
It has an individual supply system that makes it well suited for a job in which the room outlets are all close to the unit. This system is economical from a first cost standpoint and can be installed easily with minimum experience. The supply diffusers are normally located on the inside walls and are used for heating system with very warm or hot air as the heating source. The return air system can be a single return located at the air handler which makes materials economical.
Extended Plenum System
This system can be applied to a long structure such as the ranch-style house. This system takes the plenum closer to the farthest point. The extended plenum is called the trunk duct and can be round, square or rectangular. The system uses small ducts called branches to complete the connection to the terminal units.
Reducing Plenum System
The reducing plenum system reduces the trunk duct size as branch ducts are added. This system has the advantage of saving materials and keeping the same pressure from one end of the duct system to the other when it is properly sized. This ensures that each branch duct has approximately the same pressure and velocity pushing air into its takeoff from the trunk duct.
Perimeter Loop System
The perimeter loop duct system is particularly well suited for installation in a concrete floor in a colder climate. The loop can be run under the slab close to the outer walls with the outlets next to the wall. The loop has a constant pressure around the system and provides the same pressure to all outlets.
RETURN AIR DUCT SYSTEM
The return air duct is constructed in much the same manner as the supply duct except that some installations are built with central returns instead of individual room returns. Individual return air systems have a return air grille in each room that has a supply diffuser. The individual return air system will give the most positive return air system, but they are expensive. The return air duct is normally sized at least slightly larger than the supply duct so there is less resistance to the airflow in the return system than in the supply system.
The purpose of an air filter is to free the air of as much of the airborne contaminants as is practicable. The main types of filters are :
- Dry : in which the contaminants are collected in the filter medium
- Viscous or Impingement : in which the contaminants adhere to a special type of oil.
- Electrostatic : in which the contaminants are positively charged with electricity and collected on negative earthed plates.
These use materials such as cotton wool, glass fiber, cotton fabric, treated paper, foamed polyurethane as the cleaning medium. The efficiency of the filter depends largely upon the area of medium offered to the air stream and for this reason the filter can be arranged in a 'V' formation which increases the area
In the case of an automatic roller type filter, when the filter is dirty, a pressure switch will switch on an electric motor which will turn the dirty spool and allow clean fabric to enter the filter chamber.
These are of dry fabric type and are very efficient in moving even the smaller particles from the air. This high performance is obtained by close packing of a very large number of small diameter fibers but this unfortunately results in a high resistance to air passing through the filter.
These have a large dust-holding capacity and are therefore often used in industrial areas where there is a high degree of atmospheric pollution. The filter medium is coated with a non-inflammable, non-toxic and odorless oil, which the contaminants adhere to as they pass through the filter. There are two types of viscous filters such as cell type and automatic type
These types of filters have three main components : ionizer, metal collector and electrostatic power pack. The various air contaminants are given a positive electrostatic charge by an ionizer screen which is the first part of the filter. The screen consists of a series of fine wires possessing an electrostatic charge produced by a direct current potential of 13 kV. The wires are spaced alternatively with rods or tubes which are at earth potential.
The air containing these positively charged contaminants then passes through a metal collector, which consists of a series of parallel plates about 6mm apart, arranged alternatively so that one plate which is earthed, is next to a plate which is charged with a positive direct current potential of 6 kV. The positively charged air contaminants passing through the collector are repelled by the plates of similar polarity ( which are positive) and are attracted by the negative earthed plates.
A well-designed system will have balancing dampers in the branch ducts to balance the air in the various parts of the system. Balancing the air with dampers enables the technician to direct the correct volume of air to the correct run of duct for better room temperature control.
The dampers should be located as close as practical to the trunk line, with the damper handles uncovered if the duct is insulated. The place to balance the air is near the trunk so if there is any air velocity noise, it will be absorbed in the branch duct before it enters the room. A damper consists of a piece of metal shaped like the inside of the duct with a handle protruding through the side of the duct. The handle allows the damper to be turned at an angle to the air stream to slow the air down.
ROOM AIR DISTRIBUTION
The supply air enters a room either through a grille, register or diffuser. A supply grille has adjustable vanes for controlling the direction of the air entering a room. A register is a grille that also has a damper behind it so that the amount as well as the direction of the air entering a room can be controlled.
The location of supply air outlets is very important for the comfort of the occupants. The goal is to gently circulate all of the air in a room so that there are neither stagnant nor draftly areas. Make sure too that beams or other objects do not block the air supply from reaching all parts of a room. Where heating is the major problem, the outlets should be paced lower. However, to prevent short-circuiting of the air, do not place return openings right next to supply outlets. Also avoid floor return grilles because dirt and small objects fall into them.