Air distribution system

Standards for satisfying conditions

No condition will satisfy everyone.Air velocity will be sensed by a person exposed to it as a feeling of coolness.If air temperature is 1F higher for each 15fpm increase in velocity, most persons feel equally comfortable.Variations upto 4 F in temperature and Velocities b/w 15fpm and 50fpm are considered acceptable. Neck region is twice as sensitive. Temperature changes or high velocities when subjected to this region will be effective

ConclusionsLimit the temperature variations to maximum 4 FMaintain an air motion b/w 15 to 50 fpmIn industrial environment higher air velocities are sometimes permissible or even favourable.

Types of Jets The proper understanding of various types of jets is very important for the selection and location of different types of supply outlets.

Isothermal Jet

The figure shows a axial jet of air exiting from an outlet with an air temperature equal to that of surrounding air temperature.

Non isothermal Jet

There is some difference between isothermal and non isothermal jet. There is difference in temperature for cooling and heating between the air sent and space and so is the density of the air. The density of cold air is high compared to that of hot air.

Effect of obstructions on jet

The maximum velocities will remain close to the surface, creating a low pressure region between the jet and the surface. This effect rises when the outlet is within two feet of the surface. The closer the jet to the surface , more the effect.

It is very healthy practice to consider this effect before selecting the supply outlets. Since this tends to keep the total air envelope out of the occupied zone. Thus improves the overall room air motion throw and drop for confined space

Impact of opposing jets

Ducting The conditioned air has to be supplied to different parts of the conditioned space. The duct systems convey the conditioned air from the conditioned equipment to the proper air distribution points or air supply outlets in the room and carry the return air back from the room to the conditioning equipment for reconditioning and recirculation.

Thus it is necessary to design the air duct system in such a way that the capital cost of ducts and the cost of running the fans is lowest.

Classification of Ducting Supply air duct:The duct which supplies the conditioned air from the conditioned equipment to the space to be conditioned is called Supply air duct. Return air duct:The duct which carry the recirculating air from the conditioned space back to the air conditioning equipment is called return air duct.

Fresh air duct: The duct which carry the outside air is called fresh air duct. Low pressure duct: When the static pressure of the duct is below 50 mm of water gauge, the duct is said to be a low pressure duct.High pressure duct: When the static pressure is from 150 to 250 mm of water gauge, the duct is said to be a high pressure duct.

Low velocity duct: When the velocity of the air in the duct is upto 600 m/min, the duct is said to be low velocity duct.High velocity duct: When the velocity of the air in the duct is more than 600 m/min, the duct is said to be high velocity duct.

Different Parameters for Duct design.Head loss should be 0.1.Main duct velocity should be 1200 fpm to 1500 fpm.For branches around 750 fpm.For throw velocity should be around 500 fpm.Aspect ratio should be 1 to 4.

Aspect ratio is defined as the ratio of width to height. Higher the aspect ratio, higher the friction and more sheet metal usage. For side throws, we should maintain grill or slot diffusers for supply and return.Acoustic and thermal insulation should be done.Gauge of sheet decreases, then thickness increases.

Different Duct Shapes Duct may be round, square or rectangular. Round duct carry more air with less friction loss than ducts of any other shape with the same cross sectional area. Square duct will have less friction loss compared to that rectangular ducts. Aspect ratio of rectangular ducts should be kept as low as possible.

Despite all these considerations, the demands of the building construction, more often then not dictate the use of rectangular ducts with aspect ratio from 2:1 to 6:1, frequently the depth of space available for duct work, such as ceiling heights, will limit the duct to 8, 10 or 12 and the duct must be sized with depth, as a fixed and limiting dimensions.

High static pressure and velocity will result in noise. But the low velocity system will result in less fan motor static and duct size, resulting in less noise. Now flexible ducts made up of aluminum foil are also available in the market.

Types of Duct design

1)CFM method 2)VRM method 3)SRM method

Duct Size & No. of sheets Calculation

Duct Routing

Duct insulation

(Thermal & Acoustic)

Selection of insulation1.Low thermal conductivity.2. Impervious to watervapour.3. Resistance to fire.4. Mechanical strength.5. Resistance to fungus and bacteria.6. Easiness of laying.

The different types of materials used are glass wools, expanded polystyrene, nitrile foam, polyurethane, cork, etc.

Inlet and Outlet componentsDiffusersGrillsVCD-Volume Control DamperCollar damperFire and smoke damper

How to Select supply and return openingsLocate diffusers uniformly.Diffusers need not be in the centre of the area being served by particular diffuser as 1,2, & 3 way patterns Return air openings should never be too close to the supply air openings.

Total airflow cfm is a very important parameter in selecting the size of the supply and return air opening.Type of application should be also a criteria for the selection as well.Height of the ceiling.If horizontal air throw is required, we chose grills or slot diffusers.