DUCT SIZING :
The most widely used method to size ducting is the equal friction method.
The other methods are velocity reduction and static regain.
Equal friction method
The Equal Friction design method sizes the ducts for a constant “Friction Rate”, which describes the average pressure drop per 100 feet of duct in a system. A 22 very common friction rate for a reasonably well designed system is 0.1 in. wc per 100 ft. of duct length. Reduced friction rates of 0.05 in. wc per 100 ft. increases the duct size and costs by 15%, but cuts the portion of the total pressure drop in ductwork by 50%, resulting in fan energy savings of 15% to 20%.
Procedure:
Equal friction method uses a duct slide rule, duct calculator, or friction rate chart to determine the relationship between duct size and air flow, i.e. how much air will come out of a given size duct. The methodology is as follows:
a. Select maximum air velocity in main duct after fan outlet.
b. Enter the friction chart knowing the cfm and velocity to determine the friction rate per 100 feet of duct length and select the diameter (refer to chart below).
c. From the friction rate obtained in the previous step, use the same value to determine diameters for all other sections of the ductwork.
d. The total friction loss in the duct system is then calculated by multiplying the friction loss per 100-foot of length to the equivalent length of the most critical path of the ductwork having maximum resistance.
Advantages of the Equal Friction Method:
Limitations of the Equal Friction Method:
Velocity Reduction Method
b. Find the diameters of the main and branch ducts from air flow rates and velocities.
The velocity in duct can be expressed as: A = Q / v Where,
• A = duct area (ft2 )
• Q = air flow rate (cfm)
• v = air speed (fpm)
c. From the velocities and duct dimensions obtained in the previous step, find the frictional pressure drop for the main and branch ducts using the friction chart above.
Static Regain Method:
Static regain is the process of converting velocity pressure (VP) to static pressure (SP). The Static Regain method of duct sizing is based on Bernoulli's equation, which states that when a reduction of velocities takes place, a conversion of velocity pressure into static pressure occurs. With this method, the duct velocities are systematically reduced over the length of the distribution layout, which allows the velocity pressure to convert to static pressure, offsetting friction losses in the succeeding section of duct. This means there is the same static pressure near all the branches and all the diffusers and, therefore, the system design requires little or no balancing.
Advantages:
1. All duct sections have uniform static pressure at all branches and outlets, thereby simplifying outlet selection and correct air quantity at each outlet.
2. The duct system will stay in balance because the losses and gains are proportional to a function of the velocities. Therefore, it is an excellent method for designing variable air volume systems.
3. The Static Regain method uses less pounds of sheet metal and is less noisy.
Disadvantages:
1. One disadvantage might be seen in networks with a large pressure drop in a section near the fan outlet. The velocity could be reduced to the minimum within a few sections in such a way that all the ductwork downstream would be sized using minimum velocity.
2. Another disadvantage could stem from specifying a very low minimum velocity. Ducts would then tend to be very large at the end of long branch runs. The sizing method does not account for the total mechanical energy supplied to the air by the fan. Oversized ducts can occur at the ends of long branches.
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