Turbulent flow can often lead to significant and continuing energy dollar waste. It can affect productivity, quality and production equipment integrity. It’s not something you should want.
The basic rule for compressed air piping between the supply and users has always seemed clear:
- Pipe the air to filters, dryers, receivers and regulators and on to the production floor at the correct flow at the correct pressure with minimum pressure loss
- Size the outlet pipe from the compressor to meet the volumetric flow, not to match the size of the compressor discharge port
Plant engineers and operators should be aware that these principles are often overlooked. Most realize that pressure drop is directly proportional to volumetric flow and inversely proportional to line size.
Air system designers often overlook air velocity as a critical factor. Instead, they focus on friction-induced pressure loss. Ignoring air velocity can result in:
- Unplanned backpressure or pressure drop
- Elimination of effective storage for control operation
- Loss of a steady signal pressure for electronic controllers
Each outcome degrades energy efficiency, plant productivity, quality and production costs.
Basic causes of pressure drop
First is pressure loss by friction. Many pressure drop charts indicate the pressure loss for a certain inlet air flow at a continuous pressure. Friction between air and pipe wall causes this loss, which is usually denominated as pressure drop per 100 feet of pipe.
Similar charts express the estimated pressure loss for fittings in terms of “additional length of pipe.” When added to the length of straight pipe, the sum is called “total equivalent feet of pipe.” These charts reflect the basic calculations for pressure loss, which include:
- Air density at a given pressure and temperature
- Flow rate of the air
- Velocity at pipeline condition
- The Reynolds number
- Other factors, including a friction factor based on the size and type of pipe
These calculations and chart data give the probable minimum pressure loss. The pipe’s internal roughness and scale thickness dramatically affect its resistance to flow (friction loss). Resistance increases with time as the inner wall rusts, scales and collects dirt. This is particularly true of black iron pipe. Pressure drop is proportional to the square of velocity. A high volume intermittent demand can suffer dramatic pressure drop during peak periods. Ignoring this affects the processes the header feeds.
For any given size pipe:
- For a constant pressure, the greater the flow, the greater the loss per foot of pipe.
- For a constant flow rate, the lower the inlet pressure, the greater the loss per foot of pipe.
The most overlooked idea in piping layout and design is air velocity. Excessive velocity can be a significant cause of backpressure, erratic control signals, turbulence and other losses. The British Compressed Air Society suggests that a flow velocity of 20 fps or less avoids carrying moisture and debris past drain legs. This is a reasonable limitation for main headers, interconnecting piping and main branches. Other experts feel that the maximum velocity should be 30 fps or less in branch lines shorter than 50 feet in length. Determining air velocity is easy.
Values for Di as a function of pipe schedule are tabulated in various engineering reference books. Although the formula assumes isobaric flow, it’s usually adequate for rough evaluations. Other more complex relationships accommodate flow with pressure differentials.
So what’s wrong with high velocity?
Some argue it’s simply better for air to get there faster. This is true. But what happens when that high-speed air goes through a block valve, crossing tee or other fitting? It produces turbulence, which, in turn, produces backpressure — two factors that degrade system performance. Here are some examples.
Short cycling in a 350-hp, oil-free, two-stage rotary screw compressor with two-step capacity control troubled this plant. The twin 1,500-cfm compressors were configured as shown in Figure 1. Compressor No. 2, the trim unit, was short cycling. The plant installed a blow off valve so the trim unit ran at part load with continuous blow off. This was obviously expensive because both units were at part load in demand, but at full electrical consumption. It operated like this for more than five years.
The air velocity for 1,500 cfm in a four-inch Sch 40 steel pipe at 90 psig was about 40 fps, twice the recommended value. The pressure loss in 200 ft. of four-inch pipe and one 90-degree short radius elbow was 0.89 psig — not very significant. Calculated values suggested this was an excellent piping arrangement.