Bends and elbows, while they seem simple enough, tend to cause the most issues in pneumatic conveying systems. That’s why it’s important to be able to correctly calculate which size of elbow you need. In this article, we’ll show you how to calculate a 90-degree angle elbow for your pneumatic conveying system.

Getting the right elbow for your system is crucial because if you don’t have the right one you will probably experience greater abrasive wear, increased product degradation, and more frequent system downtime – all leading to increased costs. While the flow of products through bends is complex, there is a fairly simple way to calculate a 90-degree angle elbow for your pneumatic conveying system.

Calculating pipe or tube elbow size

First, it’s critical to get the size right or the elbow simply won’t fit. You can approach this in two different ways:

  1. Find out what elbow you currently have in your conveying system (an easy approach to take if the elbow is a standard 90-degree elbow).
  2. Figure out what will fit in your system and don’t be too concerned about it being an exact replacement. This is much more difficult because measuring chord length on site is a challenge, and you will need to dust off your trigonometry skills.

For the purposes of this article, we’ll only look at 90-degree elbows – but subscribe to our blog to receive more complex calculations in the future!

How to determine the pipe or tube elbow size needed for a replacement 90-degree elbow

To determine the size of elbow you need, you need three pieces of information:

  • The degree
  • The line size
  • The centerline radius (CLR)

This makes a 90-degree elbow simple to calculate. Since you already know it’s 90-degrees, you’re already a third of the way to knowing all the information you need.

Measuring line size

Getting the line size is also fairly simple as it’s an easy measurement to take. All you need to do is measure the pipe or tube’s outer diameter and wall thickness.

Tube sizes are typically whole number measurements such as two inches, three inches, and so on. Sometimes you may find a half-inch measurement or even a quarter-inch measurement, but this is rare in pneumatic conveying systems.

By measuring the wall thickness, you can then calculate the line size. If the wall thickness is 0.120 inches (11 gauge), 0.083 inches (14 gauge), or 0.065 inches (16 gauge), then your tubing or pipe material conforms to tube specifications. It’s important to note that the term ‘pipe schedule’ references its wall thickness. Pipes usually have different wall thickness measurements which change according to the pipe size. A pipe schedule chart can help you figure out the pipe schedule by the wall thickness measurement.

Finding the centerline radius

The more complex part of determining an elbow size is finding its centerline radius (the arc that goes down the center of the elbow). The centerline radius determines whether the elbow has a long arc, which makes it a long radius elbow, or a small arc which makes it a short radius elbow.

The height of the elbow is the centerline radius plus the tangent length, the tangent being the straight length of pipe that extends off the arc. This section of pipe can be used to install compression couplings on the elbow.

On standard-sized pneumatic conveying elbows, the tangent should be twice the nominal line size. For example, a two-inch elbow (pipe or tube) would have a four-inch tangent, a three-inch elbow (pipe or tube) would have a six-inch tangent, etc. Be warned that some sizes can be both a pipe and a tube size, but knowing this should get you close to a correct size.

Additionally, it’s important to note that the centerline radius usually occurs in one-foot increments. Common CLR’s are 12-inch, 24-inch, 36-inch, 48-inch, 60-inch, and 72-inch.

Calculating a 90-degree elbow – an example

Let’s say we have a normal 90-degree elbow in our pneumatic conveying system. The outer diameter of the elbow measures three inches, which means we probably have a tube elbow. The tube wall thickness is about an eighth of an inch thick, so we have an 11-gauge elbow.

Knowing all of this and using the above tangent rule, we can calculate that we have approximately six-inch tangents on each side of the elbow. Standing the elbow on one end, we measure from the center of the end of the elbow to the ground – it measures 42 inches.

Since the height of a 90-degree elbow is the centerline radius plus the tangent length, that means we have a 36-inch centerline radius elbow (42 inches – 6 inches = 36 inches).

And that’s it! We have a three-inch outer diameter, 11-gauge, 90-degree, 36-inch centerline radius tube elbow.

Other considerations in elbow specifying

If you’re having difficulty in determining your elbow angle, consider the following:

  • Elbow height measurements can be uneven, with tangents easily plus or minus half an inch, or the elbow bend may not be completely symmetrical. This is normally due to tolerances in the bending process.
  • Your centerline radius might not be a standard size – some line sizes have 30 or 32-inch CLRs. However, if your tangents are standard, you’ll be able to work this out and identify which elbow you need.
  • If your calculations are incorrect, you may have an elbow with asymmetrical tangents. These are used to get an elbow to fit in a pneumatic conveying system. If this is the case, your elbow will have intentionally been made with one tangent longer than the other.

Bends and elbows are critical to all pneumatic conveying systems, and ensuring you have the right elbows for your configuration is crucial to ensuring smooth processes. If you don’t, your conveying capacity could be limited, and you’ll experience increased product degradation and wear rates.

Contact a Progressive Products expert to have the elbows in your pneumatic conveying system measured.