The conditioned air coming from air conditioning equipment must be properly distributed in the utilized area. Conditioned air cannot be directly supplied from air conditioning equipment but it is allowed to flow with the help of diffuser. The return air from room is also supplied from ducts sometimes return air is exhausted to the atmosphere. Successful working of air conditioning equipment needs efficient duct design. Duct design expenses are nearly 35 to 40% of project cost and it is required to minimise.
Factors Affecting Air Conditioning
The main objective of an air distribution system is to produce a proper combination of temperature, humidity air motion in the condition room area. The comfort condition should be maintained perfectly. Temperature fluctuation or improper air motion create discomforts.
Air needs to be properly distributed within utilized area. The velocity of air should be between 10 to 15 m/min. The flow direction of air should be towards the faces of the people. Downward flow is preferred over upward flow. The temperature of conditioned air should not differ to large extent from the temperature of the room. Several arrangements of supply and return air are shown in figure 16.1 (A, B, C, D).
Figure 16.1: Air Distribution Combination
The system shown in the figure A is used mostly for summer air conditioning. The supply and return grilles are located on the same wall. This system is cheaper to construct. The system shown in figure B is used to provide uniform supply and prevent downward reverse current. The supply and the return grilles are located on different walls. This system can create a stagnant space below the supply outlet.
The system shown in figure C is the “pan type” air distribution system. This is efficient and ensures air circulation throughout the space.
The system shown in the figure D is preferred for heating, i.e. winter airconditioning. Here, two supply register and two return grilles are used to provide a cross draft.
Sometimes, perforated ceilings are used and return ducts are placed on floor level. The drawback is the high cost of the system. This system is preferred where ceiling height is less as shown in figure 16.2
Factors affecting air distribution are:
1. Size of the duct
2. Velocity of the air in the duct
3. Location of grilles & diffusers
4. Flow pattern of air in the room
5. Entertainment of room air
Ventilation system is used for spaces where only exchange of air with the surrounding is desired. The application area may be a subway or an underground outlet or a building kitchen. The air in the application area is not conditioned for human comfort. The amount of ventilation air depends on the occupancy of space or the area and the heat load of the space. There are two types of ventilation systems. There are: (1) Natural Ventilation (2) Mechanical Ventilation.
Natural ventilation makes use of wind flowing due to the pressure difference between the application area and the surrounding. This is similar to natural draught in a chimney. There is no control over the flow and dust particles from the surrounding may enter ventilated space. That is why this system is not preferred for commercial application and is only used where low cost is desired.
Mechanical ventilation uses a fan to ventilation space. The quantity of flow and direction of flow is controlled by fan. Mechanical ventilation systems are further classified as extraction, supply or combined supply-extraction system.
In extraction system, a fan sucks out air from the application area. This is the most widely used method. It is similar to induced draught system. The effectiveness of extraction depends on location of air inlets in the ventilated space. In supply systems, a fan forces air to the ventilation area. It is similar to a forced draught system. A positive pressure gradient is maintained in the conditioned space. In combined system, fans suck air from the room and supply air to the room. This is similar to balanced draught. Fresh air can be conditioned in the system. However, the possibility of air short-circuiting the ventilated space exists. Figure 16.3 shows three types of mechanical ventilation systems, i.e. the supply, the extraction and the combined system.
Air Conditioned for a Theatre
Figure 16.4 shows the air distribution system for a theatre. The air distribution shown is the upward air flow system. A downward system is also possible. In the upward flow system, air enters the theatre through inlets near the floor of the theatre. The air flows upwards. The exhaust grill is locked at the top in the ceiling. The advantage of the system is that air enters the theatre, collects heat from the occupants light and equipment becomes hot and rises upwards. The movement upward is aided by lower density of hot air. This system also removes bad odours from the conditioned space. This system is suited for winter or summer airconditioning.
Duct Arrangement for a Commercial Establishment (Shop/Mall)
The duct systems, which carry conditioned air from the conditioning equipment to the conditioned space and back, are designed in specific patterns. Several types of duct layout are available as perimeter loop, radial perimeter system or extended plenum system figure 16.5 shows one such extended plenum system for a shop or a commercial mall. In this system, the conditioner is located in one corner of the mall. Two branch ducts run along the length of the rooms. Four diffuser, usually in a rectangular pattern, are located in four corners of each zone. The return air duct is located in the centre of the zone. The outside air duct is located close to the air conditioning equipment as shown in figure 16.5.