Industrial Ventialation

Does this help?

design of the ductworks in ventilation systems are often done by using the

• Velocity Method
• Constant Pressure Loss Method (or Equal Friction Method)
• Static Pressure Recovery Method

The Velocity Method

Proper air flow velocities for the application considering the environment are selected. Sizes of ducts are then given by the continuity equation like:

A = q / v (1)

where

A = duct cross sectional area (m²)

q = air flow rate (m³/s)

v= air speed (m/s)

A proper velocity will depend on the application and the environment. The table below indicate commonly used velocity limits:

Type of Duct Comfort Systems Industrial Systems High Speed Systems Main ducts 4 - 7 m/s 8 - 12 m/s 10 - 18 m/s Main branch ducts 3 - 5 m/s 5 - 8 m/s 6 - 12 m/s Branch ducts 1 - 3 m/s 3 - 5 m/s 5 - 8 m/s

Be aware that high velocities close to outlets and inlets may generate unacceptable noise.

The Constant Pressure Loss Method (or Equal Friction Loss Method)

A proper speed is selected in the main duct close to the fan. The pressure loss in the main duct are then used as a template for the rest of the system. The pressure (or friction) loss is kept at a constant level throughout the system. The method gives an automatic velocity reduction through the system. The method may add more duct cross sectional changes and can increase the number of components in the system compared to other methods.

The Static Pressure Recovery Method

With the static pressure recovery method the secondary and branch ducts are selected to achieve more or less the same static pressure in front of all outlets or inlets. The major advantage of the method are more common conditions for outlets and inlets. Unfortunate the method is complicated to use and therefore seldom used

Thank you for reply.

I was looking is somebody can explain me how to calculate amount fo air that need to be exhausted in room (shop area), and if I should use hoods above each working bay.

Shops will be similar to autoshops, there is no welding or painting inside.

thank you

It depends on the envisaged final useage of the area and the standards or building codes of the country. some codes will specify the maximum number of people which can occupy a certain area of a building per sq meter or sq foot and then specify the required incoming fresh air in CFM or L/s or cu meters/hr, per person and from that you can calaulate the fresh air quantity. In other areas such as factories, warehouses etc:- they will specify an air change rate, which of course can only be calculated from volume Viz:- volume x rate/hr = air quantity/hr. Hope this helps Phil L

There are standards which give the number of air changes per hour. These vary dependant on the use dependant on wether they are offices shopping malls workshops or whatever. Contact your local authority council and they will advise you on the air changes per hour required. After that it is just simple maths