Final answer:
The critical factors for selecting the sizes of lines in an air distribution system are flow rate and pressure loss. These determine the duct size needed for the required volume and efficiency of air distribution.
Step-by-step explanation:
The two factors that are critical to selecting the actual sizes of the lines used in an air distribution system are flow rate and pressure loss. The flow rate determines the volume of air that needs to be distributed within a specific time frame, and it is directly related to the cross-sectional area of the ducts. On the other hand, pressure loss, which results from friction and other factors, affects how effectively the air can move through the system. By calculating these two factors, engineers can determine the appropriate duct sizes to ensure efficient operation of the air distribution system.
For example, if an air conditioning system requires a certain flow rate to maintain a comfortable temperature, the diameter of the ducts must be large enough to allow for that volume of air to pass through with minimal resistance and pressure loss. Similarly, if the goal is to save on costs by using smaller ducts, one must also consider that the resistance would increase, potentially requiring more energy for the distribution system to maintain the desired flow rate.