Question

Explain the ‘downward system’ of air distribution with a neat sketch.

Hint:

Describe supply from the ceiling level with downward flow and return at floor level. Draw the room, supply duct, outlets, return grill and air handling unit.

Answer 1

Step 1: Understanding the Concept:
An air distribution system carries conditioned air from the air handling unit to the rooms through ducts. The downward system is one type of such a system. It is also called the overhead system.

Step 2: Working of the downward system.
The main supply duct runs along the roof or ceiling of the building, or in the false ceiling.
Branch ducts are taken down from the main duct to the rooms. The conditioned air leaves through outlets (diffusers) placed at a high level.
Cool air is heavier than warm air, so it falls down by itself. It settles in the occupied zone.
The return air grills are placed near the floor level. Warm air is drawn from the bottom and goes back to the air handling unit through the return duct.

Step 3: Neat sketch (what to draw).
Draw a room with a main supply duct under the ceiling and a fan with a cooling coil (air handling unit) at one side.
Draw branch ducts coming down from the main duct, with arrows pointing downward from the ceiling outlets.
Draw the return air grill near the floor, with arrows going to the return duct and back to the unit.
Label: air handling unit, supply duct, ceiling outlet, conditioned space, return grill, return duct.

Step 4: Advantages and use.
1. Cool air falls naturally, so the fan power needed is less.
2. Air spreads well in the room and the temperature is even.
3. Ducts run in the ceiling, so floor space is saved.
The system suits theatres, cinema halls, auditoriums and large halls where cooling is the main need.

Final Answer:
In the downward system, conditioned air is supplied from ceiling level and falls down, while return air is taken out near the floor.