Grills and Register
A decorative converting for an outlet or intake is known as grill and grill provided with damper is known as register. Grills improve the aesthetics of the air-conditioning system Grills are the rectangular duct outlet having vertical and horizontal vane which may be fixed type or adjustable type.
Grill can be located in the floor or in walls or in the ceiling. Grills usually contains a series of slotted vane fitted in a frame. The vanes can be operated by a damper driven by a motor. Usually, ceiling to floor air distribution is used to eliminate drafts and provide proper air circulation in the room. Generally one or two grills are provided for each room depending upon the size of room The throw and air velocity can be calculated.
L = kQ/√ab
Where,
L = Throw in meters
Q = Discharge of air in m.cu/min
k = Constant
a, b = Length and width of grill
Diffusers :
It is the supply device for the supply air duct. Diffusers also improve aesthetics of the air-conditioning system. They are usually are at same level as false ceiling, which is provided to conceal duct three-way supply diffuser.
Diffuser are responsible for proper supply of conditioned air to the room. Usually they are made of rectangular plates attached one above the other. The air is supplied in four direction through gaps between the plate. The four way and two way diffuser are also available. Round diffuser are also available.
Construction and Application of Supply Duct, Return Duct and Makeup Duct
● Duct is a passageway for supply and return air. The material used for construction of ducts are galvanised iron sheet, Black sheet and Aluminium sheet metal.. Generally galvanized iron sheet is preferred because zinc coating prevent rusting, and avoid cost of painting.
● The thickness of galvanized sheet is varies from 26 gauge (0.55 mm) to 16 gauge (1.6 mm). Aluminium sheets are lighter weight and resistance to moisture.
● Nowadays resin bounded glass fibre duct are used because they are strong and easy to manufacture according to mquired shape and size. These are used for low velocity 600 m/min. Cement asbestos duct used for underground air distribution and exhausting corrosive materials.
(1) Supply air duct : The duct which supplies the conditioned air from the air conditioning equipment to the space to be conditioned.
(2) Return air duct : Duct which carries the recirculating air from the conditioned space back to air conditioning space or equipment.
● There are several basic types of supply and return duct systems. Any one of the system types, or a combination of different types cn be utilized to fit the needs of a particular structure. The general types of supply duct system as explain in earlier section.
A duct system located in an attic must be insulated and must have a vapor barrier installed to prevent condensation on the exterior of the ductwork. Condensation can cause corrosion and rusting of the duct system and possible structural damage to the ceilings. All joints and side seams must be sealed to prevent duct leakage. The air handler should be located where the shortest duct runs possible are attained. The shorter the duet runs are, the lower the resistance to air flow and the lower the heat gains and heat losses. A flood area must extend at least 3 ft on all sides of the unit to provide a platform for service work. It can be used with upflow, downflow, or horizontal air handlers and furnaces.
The six basic locations for supply duct systems are as follows:
1. attic installations
2. basement installations
3. between floors of multistory structures
4. crawl space installations
5. conditioned space installations
6. embedded in concrete slab.
The concept of the reducing plenum system is that when the air velocity lost to the branch runs reaches approximately 50%, the plenum size is reduced to regain the velocity in the remaining portion of the plenum. This reduction also improves the air flow characteristics at the branch ducts that are closest to the air handling unit.
The delivering the primary air into the unoccupied zone of the room and mixing it with the secondary air, the designer ensures that the primary airstream does not come in direct contact with the room occupants. Of the primary airstream is allowed to come in contact with the room occupants, the result will be complaints of drafts and the overall comfort of the occupants will suffer.
Return Air Duct System :
The return air duct system is just as important as the supply system. In many cases, the return system consists of one or two improperly sized openings connected back to the air handler through improperly sized return ducts or through wall, floor, and ceiling framing cavities. Although panned framing members can be used for return systems, they generally should be avoided. Air leaking into the system can cause additional sensible and latent loads system and lead to comfort complaints from the Occupants of the structure.
If duct leakage occurs in the attic space during the cooling season, warm, moist air can be pulled into the return, resulting in an increase in both the latent load and the sensible load. This infiltration into the return system will be aggravated if the return system is undersized.
Central retum systems can be applied in smaller structures with success. These systems consist of one or more return most strategically located within the structure and sized to retum the total amount of the supplied air volume to the indoxxi Mowe. Both the inlet box and the return connecting ductwork should be sized where the room air will flow back to the blower really low velocity. The return duct system must be free of leaks
Air delivered to all rooms of the structure should have an unrestricted path back to the return grille. This means that closed doors must not isolate a room
The construction is to undercut the doors, thus allowing air to pass under a door if it is closed. The amount of the undercut is determined by using the same methods as those used to size the return grille or the connecting return duct, when the door is closed.
Factors to be Considered for Construction of Duct System :
The following points must be considered for construction of duct system.
1.The duct pressure drop and the filter pressure drop added together must be less than 0.15 inches of water.
2. Noise Attenuation Provide some means of acoustical dampening with no "line-of-sight" between the opening of the air handler and the grille.
3. Duct, size is dependent on duct length, air flow, and allowable static pressure drop
4. Metal Duct. Metal duct can be used provided it is lined with an acoustical lining,
5. If the duct system is installed in an unconditioned area, such as an attic or crawlspace
6. The filter size is dependent on the type of filter and the amount of airflow. Typically, a disposable filter has the lowest pressure drop and an electrostatic filter has the greatest pressure drop.
7. Avoid creating a negative pressure zone near gas-fired equipment
