IMPORTANCE OF PIPE SIZING
Pipe size is a critical aspect in designing a hydronic system for cost, longevity, and noise reduction. The inside pipe diameter should be determined after the optimal flow rate is calculated. The pipe diameter is calculated with a maximum velocity limit. This velocity limit is critical to prevent damages from occurring. For HVAC applications, Hays recommends using maximum velocity of 7 fps for design. Fluid velocity (V) is directly proportional to the fluid flow (Q) by equation 1 (ASHRAE, 2013).
Q =VA (eq. 1)
where Q=Flow, ft3/sec V= Velocity, ft/sec A=Area, ft2
Problems can occur when a maximum velocity is not taken in consideration. From the flow equation above, as the flow increases if the pipe area does not increase this will create a higher fluid velocity. Flows at higher velocities can create undesirable noise levels, high erosion levels, and even higher pumping costs.
Noise in piping systems are a result from turbulence, cavitation, release of entrained air, and water hammer (ASHRAE, 2013). The Reynolds Number (Re) is a key indicator when looking at turbulent or laminar flow. Any value when calculating the Reynolds number over 10,000 is considered fully turbulent in that system. The Reynolds number is a dimensionless number found using equation 2 (ASHRAE, 2013). From this equation, the Reynolds Number is directly proportional to the fluid velocity. Any increase in velocity and turbulence increases with it creating unwanted noise.
where Re=Reynolds Number, dimensionless D= inside diameter of pipe, ft ρ=fluid density, ft2 μ= dynamic viscosity, lbm/ft-s
Erosion occurs in hydronic systems by sediment, and water bubbles. Erosion in piping systems at velocities lower than 10 ft/sec. is not substantial. Sediment in the system at high velocities is where erosion transpires at a quick pace. Putting a strainer in piping systems is always a good idea to reduce sediment build up.
Water hammer is another design characteristic to be considered. The water hammer phenomenon is triggered when any moving fluid though a system stops abruptly. Large pressure spikes can be observed when fluid is moving at high velocities. Equation 3 (ASHRAE, 2013) shows that fluid velocity has a direct correlation to the pressure rise in a hydronic system. Keeping the velocity at a reasonable speed can reduce damage during any situation where water hammer occurs.
∆ph=(ρcs V)⁄gc (eq.3)
where ∆ph= Pressure rise caused by water hammer, lbf/ft2 cs= Velocity of sound in water, ft/sec gc = Gravity constant, ft/sec2
In summary, by properly sizing pipes based on flow through the system will prevent unwanted issues. Below is a chart of recommended pipe sizes given a certain flow rate. Use Hays Velocity Calculator for Pipe Sizing (Hays Fluid Controls, 2016).