TABLE OF CONTENTS The working fluids in HVAC systems are mainly water, steam, refrigerants, natural gas and oil. Pipes and pipe networks are used to appropriately distribute the working fluids throughout the system. Pipes must be correctly sized to ensure that pressures and flow rates are balanced and that the flow rates are maintained at design values. In this chapter the principles and methods of pipe sizing are discussed.
HVAC piping systems are characterized by relatively short pipe lengths and by the presence of many valves and fixtures. As a result the cumulative pressure loss due to valves and fixtures is at least as important as the pressure drop due to friction in the pipes. The pressure drop, the pipe length and diameter, the pipe roughness, the fluid density, the flow rate and the Reynolds number, and the loss characteristics of the individual fixtures used are all interrelated. The easiest computation is calculating the pressure loss for a system for a given flow rate when the fluid and its properties are known, the pipe material roughness, diameter and length are given, fixture types and their respective positions are known. For most other computations an iterative scheme may be needed.
Additional details can be found in the reference material published by ASHRAE, notably the ASHRAE Handbook series.
The pressure drop due to friction in a fully developed fluid flow inside a pipe is described by the Darcy-Weis-bach equation:
where ? P = pressure drop in lb f/ft
