National Clothesline

Sometimes I am at a plant where it is extremely difficult to get a good shirt from a unit with which I am very familiar.

When or if I determine that the cause is poor installation, I am in a very awkward place. I am not an excuse man. I am a problem-solver. I am not going to solve any problems if the solution that I propose is to replace piping that is hanging up in the rafters.

To some, this may even sound like an exercise in finger-pointing. That is, a way of saying “it’s not your employee, it’s not me, it’s not your equipment (all of which I can fix), it’s the installation (which I can’t fix, but I sound like I know what I’m talking about). Lame.

I’m very sorry, but sometimes the problem is the installation, period! Let’s get really technical this month.

There’s no such thing as too large a compressed air line. A common error that I see in compressed air systems, in addition to poor piping practice, is line sizes that are too small for the desired air flow.

This isn’t limited to the interconnecting piping from compressor discharge to dryer to header. It also applies to the distribution lines conveying air to production areas and within the equipment found there.

Undersized piping restricts the flow and reduces the discharge pressure, thereby robbing the user of expensive compressed air power. Small piping exacerbates poor piping practices by increasing velocity — and turbulence-induced backpressure.

Pipe size and layout design are the most important variables in moving air from the compressor to the point of use. Poor systems not only consume significant energy dollars, but also degrade productivity and quality.

How does one properly size compressed air piping for the job at hand?

You could ask the pipefitter, but the answer probably will be, “What we always do,” and often that’s way off base.

Another approach is matching the discharge connection of the upstream piece of equipment (filter, dryer, regulator or compressor).

Well, a 150HP, two-stage, reciprocating, double-acting, water-cooled compressor delivers about 750 cfm at 100 psig through a 6-inch port.

But most 150HP rotary screw compressors, on the other hand, deliver the same volume and pressure through a 2-inch or 3-inch connection.

So, which one is right? It’s obvious which is cheaper, but port size isn’t a good guide to pipe size.

Many people use charts that show the so-called standard pressure drop as a function of pipe size and fittings. This sizes the line for what is referred to as an acceptable pressure drop.

This practice, too, can be misleading because the charts can’t accommodate velocity and flow-induced turbulence.

Think about it. According to the charts, a short run of small-bore pipe exhibits a low total frictional pressure drop, but the high velocity causes an extremely large, turbulence-driven pressure drop.

Then, there’s the question of the meaning of acceptable pressure drop. The answer to this question often isn’t supported by data, such as the plant’s electric power cost to produce an additional psig.

Not knowing the energy cost of lost pressure as a function of line size can lead to a blind decision. Unfortunately, this is what I find in most of the air piping systems installed during the past 30 years. Older systems that were designed with care are often right on the mark, except if they’ve been modified after the original installation.

Some might call pipe sizing a lost art, but I see the issue as a lack of attention to detail, basic piping principles and guidelines.

The interconnecting piping is a critical element that must deliver air to the distribution headers with little pressure loss, if any. This isn’t only an energy question; it also ensures the capacity controls will have sufficient effective storage to allow them to react to real demand and translate less air usage to a comparable reduction in input electrical energy.

The main distribution headers not only move air throughout the plant, they also supply the appropriate local storage that ensures the process feeds have adequate entry pressure and flow.

The main header system represents storage that supports the operating pressure band for capacity control. You want the pressure drop between compressor discharge and point of use to be significantly less than the normal operating control band (10 psig maximum).

The objective in sizing interconnecting piping is to transport the maximum expected volumetric flow from the compressor discharge, through the dryers, filters and receivers, to the main distribution header with minimum pressure drop. Contemporary designs that consider the true cost of compressed air target a total pressure drop of less than 3 psi.

Beyond this point, the objective for the main header is to transport the maximum anticipated flow to the production area and provide an acceptable supply volume for drops or feeder lines. Again, modern designs consider an acceptable header pressure drop to be 0 psi.

Finally, for the drops or feeder lines, the objective is to deliver the maximum anticipated flow to the work station or process with minimum or no pressure loss. Again, the line size should be sized for near zero loss.

Of course, the controls, regulators, actuators and air bags at the presses have requirements for minimum inlet pressure to be able to perform their functions. In many plants, the size of the line feeding a work station often is selected by people who don’t know the flow demand and aren’t aware of how to size piping.

In my opinion, new air-system piping should be sized according to these guidelines. For a system that doesn’t meet the criteria, the cost of modification must be weighed against the energy savings and any improvements in productivity and quality which can easily translate in thousands of touch-up and lost-production dollars annually.

Obviously, the lower the pressure drop in transporting air, the lower the system’s energy input.

Most charts show frictional pressure drop for a given flow at constant pressure. Wall friction causes most of this loss, which is usually denominated as pressure drop per 100 ft. 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 length.

These charts reflect the basic calculations for pressure loss, which include:

• Air density at a given pressure and temperature.

• Flow rate.

• Velocity at pipeline conditions.

• Other factors, including a friction factor based on the size and type of pipe.

The calculations and chart data help to identify only the probable minimum pressure drop. Internal roughness and scaling dramatically affect the pipe’s resistance to flow (friction loss). Resistance increases with time as the inner walls rust, scales and collects more dirt. This is particularly true of black iron pipe.

Any high-volume, intermittent demand produces dramatic pressure drop during peak periods.

For a given size pipe:

• At constant pressure, the greater the flow, the greater the loss per foot of pipe.

• At constant flow rate, the lower the inlet pressure, the greater the loss per foot of pipe.

• At any condition, smooth-bore pipe (copper, stainless steel) exhibits lower friction losses.

Air velocity

The most overlooked idea in piping layout and design is air velocity. Excessive velocity can be a root cause of backpressure, erratic control signals, turbulence and turbulence-driven pressure drop.

A velocity of 20 fps or less prevents carrying moisture and debris past drain legs and into controls. A velocity greater than 30 fps is sufficient to transport any water and debris in the air stream.

Thus, the recommended design pipeline velocity for interconnecting piping and main headers is 20 fps or less, and never to exceed 30 fps. Line drops, feed lines or branch lines less than 50 ft. long work well at a velocity of 30 fps, but velocity here should not exceed 50 fps.

Incorrect pipe sizing all too often causes unsatisfactory performance. Friction is the culprit. As the volume of air passing through the piping increases, the pressure required to deliver the air also increases due to friction (think of it as wind resistance).

I promise not to get this technical on you again. The reason that it is so important these days is because so many plants are replacing their old shirt equipment with new blown sleeve units. Your Unipress BASF-A simply didn’t use much air at all. The air was simply used for control valves. Not so with the new units. They use a lot less steam, but your old air system simply wasn’t designed for that much air use.

Truth be told, your installer may be trying to save you money by recognizing the existing header and figuring that he can connect from there. If, even with the best intentions, he miscalculates, he may cost you more money than you can count. Or, said in another way, if you don’t like his installation quote, you may beat him down and think that you win, when in fact, you lose in a big way.

Similarly, you might opt to reject his quote and install it yourself. You might think that you’ll save a lot of money, when in fact the reverse is true.

If this column seems too technical, then I’ve done my job. Leave it to the pros.

“If you do what you've always done, you'll get what you always got!”