the supply and return air systems added together cannot exceed the maximum external static pressure that can be supplied by the appliance blower.
f.Determine the required flow rate for each branch of the supply and return air systems. The total airflow rate, by adding the airflow rate of each branch of the supply system, must equal the minimum required airflow rate (refer to part 3, above). Likewise, the airflow rate of each of the branches of the return air system must sum to the required minimum flow rate.
g.Using the selected air flow rates for each component of the duct system and manufacturer’s literature, or published literature on duct system pressure drops, the pressure drop for each component in the duct system can be estimated.
The ASHRAE Handbook – Fundamentals is an excellent source of duct system design principles and pressure drop data. Conversely, for a specified type of fitting, it is also possible to determine the required size or diameter of the component for a specified pressure drop and flow rate.
h.The resistance of the take-off and the outlet register (or return grill) should then be summed together to determine the total pressure drop for each branch. This value should be close to the assumed value for the pressure drop of the system. If it is not close, then flow rates for each branch must be adjusted, or the design of the duct system must be altered, to give the proper pressure drops. Usually, the cross-sectional area of the ductwork should be changed in order to adjust the pressure drop to a suitable value. Refer to Example 2, (page 14) for a sample calculation of how to use this method for sizing the supply side ductwork for a residence.
Table 3 shows the air handling capacities of 100-ft. lengths of circular and rectangular ductwork based on a 0.1 in. W.G. static pressure drop. The first column to the right is the airflow rate and the second is the required diameter for a circular duct. The third column is the required cross-sectional area of the duct and the other columns to the left are rectangular ducts with sufficient cross-sectional area to handle the flow at the specified pressure drop. [For lengths of ductwork less than 100 ft., simply multiply 0.1 in. W.G. by the ratio of the actual duct length (in feet) over 100 ft. for the approximate pressure drop.] Use the supplier’s catalog for proper sizing of outlet air registers and return air grills to insure that they provide the required flow rate at the desired pressure drop.
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