The U.S. Department of Energy estimates that a typical industrial facility uses about 10 percent of its electricity consumption on air compression. This makes compressed air a major expense in the USA. Loss of pressure represents money lost. Inefficient systems in production reduce profits, and can even threaten the ongoing financial viability of business. In these days of cutthroat price competition, manufacturers can’t afford to overlook unintended costs. Find out how to detect a compressed air system pressure drop and how to deal with it.
All About Pressure
Pressure is a measure of force per unit of area. In short, pressure builds up when more of a gas, liquid or solid is made to occupy a finite space. The only way that increased quantity can be made to occupy a restricted area is by forcing it in. You have to maintain that force in order to contain the quantity of gas in its chamber. If you release the force, the mass of gas will escape.
Force is also provided by the walls of the chamber in which the gas is held. If the strength of those walls give out, the force applied to maintain the gas in place will just push it through the splits or cracks in the container walls. Thus, the strength of the chamber holding gas under pressure contributes to the force that maintains the pressure.
Although most scientists and engineers refer to “gas,” they are not talking about gasoline or propane. Gas is a property of some substances in nature. Air is defined as a gas, as opposed to a liquid or a solid.
Applying force to build up and maintain pressure requires energy. The purpose of compressed air systems is to transfer energy from one point to another. Energy applied by an air compressor is meant to be used in a connected location. The endpoint of the compressed air releases it in order to drive a mechanism, such as a piston or a paint sprayer.
A pressure drop is not always unintended, but it is undesired. There are two types of pressure drops: natural and unintended.
Drops in pressure in compressed air systems are often due to the design of the network that distributes the air. Pressure is a mechanical property defined in physics. Often, factory and workshop managers fail to calculate and install the optimum system to deliver the generated force needed by each work station. Poorly planned installations result from hurried purchasing decisions and a rush to install.
Even the most efficient compressed air system will lose some pressure. The Department of Energy advises that a properly designed system should have a pressure loss of much less than 10 percent from the compressor’s discharge to the point of use. However, no one expects that a compressed air system can have no drop at all.
The reason behind this is that the pressure occurs in the chamber of an air compressor because more air is forced into it than would normally occupy the space. When you attach a pipe to that chamber, you effectively increase the volume of space available. The pressure then naturally drops because the same amount of force is applied to a larger containment capacity. Adding on more pipes will provide more volume, so the pressure will drop unless you also increase the force.
It takes some time to calculate the amount of pressure needed at all of the end-use points around the shop floor. Simply adding up all of that usage and expecting the compressor to produce that amount overlooks the natural drop that occurs in the pipelines.
The standard compressed air pressure drop equation for calculating the likely pressure difference between the compressor and the user is called the “empirical formula.” This formula is:
dp = 7.57 q1.85 L 104 / (d5 p)
The factors represented by the letters in that formula are as follows:
dp is the pressure drop measured in kg/cm2
q is the air volume flow at atmospheric conditions measured in m3/min
L represents the length of pipe measured in meters (m)
d is the inside diameter of pipe measured in millimeters (mm)
p shows initial absolute pressure in the system measured in (kg/cm2). This is the rating of your compressor, which gives the pressure expected at its outlet valve.
This is a complicated formula and many factory managers find it off-putting. The standard solution is to simply turn up the pressure at the compressor until the end-use equipment receives sufficient force. However, this strategy is wasteful and is the main cause of inefficient energy use in compressed air systems.
The empirical formula seems complicated, but, in fact, it is simplified because its results show a calculation based on straight pipe from compressor to outlet and does not account for bends. The interior of the pipe can also reduce flow, which is another way of saying reduce pressure. The layout and interior surface properties of piping introduce turbulence.
You can sometimes see the wind force air along a street when the air movement passes through a dusty area. Sometimes the dust just flows in a straight line, but often, it eddies and swirls. You might imagine that air just gets pushed along a pipe like a mass of toothpaste and has no direction other than forward. This is often not the case.
The force at the compressor provides power to drive the air along. The most efficient path for that movement is in a straight line. However, if bends and bumps in the pipe cause air to flurry and ricochet, that movement expends energy to travel in tangential directions. This introduces energy waste in the system through unexpected direction and unproductive movement.
Flow direction is difficult to measure in real world compressed air systems. Again, it is something that facility managers prefer to overlook. Whether the air flows in a straight line or not is determined by a factor called the Reynolds number. That factor is derived through the formula:
Re = ρvd/μ
The meanings of these symbols are as follows:
Re is the Reynolds number.
ρ is the air density.
v is the mean velocity.
d is the diameter of the pipe.
μ is the dynamic viscosity.
Once you have the Reynolds number, you are then able to decide whether your air flow is efficient. Air flowing in a straight line is called “laminar flow.” When air is following a convoluted path, it is in a “turbulent flow.” You can label your flow as laminar if your observations resulted in a Reynolds number of less than 2300. If it is greater than 3000, you have turbulent flow.
You might decide to skip the effort of calculating the Reynolds number and just turn up the compressor. This is a common mistake and introduces much greater inefficiency into your power usage. This is because turbulence creates resistance in the flow. Resistance slows down the flow rate and causes a drop in pressure.
You probably think that increasing the pressure in your piping system will ram through the air at a faster speed and overwhelm turbulence. The opposite is true. Higher pressure actually increases turbulence, so overcoming the problem by brute force is not a viable option.
Another major source of resistance as a cause of pressure drop is a blockage. It is unlikely that any of your tubing will become fully blocked, because that would cause unmanageable pressure to build up, resulting in part of the system bursting. You are more likely to have partial blockages. These blockages slow down the flow rate within your system and cause the pressure to drop. Pressure will increase in front of the blockage, so that makes this type of resistance easier to detect.
The biggest sources of blockages in your system are ones that are built in. You probably have pipe connections throughout your system. If these are bolted or welded, you may have metal intruding into the interior of the pipe, reducing its diameter at a particular point. This creates a type of flow control, similar to a valve. The valves you install in your system are another intentional form of blockage. Monitoring equipment, such as gauges and inline sensors, also reduce the volume of the pipe at certain points.
The pressure drop caused by fastenings, valves and sensors are constant and won’t cause sudden pressure changes. In fact, you may not even realize that those devices create points of restriction influencing your flow rate. However, a major reason for progressive reduction in performance is another device that you probably have many instances of throughout your air network — filters. Air filters are there to remove particulates in your air flow, and, in theory, facilitate transfer speeds. However, as the particles build up on the surface of the filter, fewer and fewer gaps are there to let the air through.
Even a clean air filter creates a small amount of resistance. However, clogged filters will cause noticeable resistance and pressure drop that will worsen over time.
Pressure Drop Caused by Actual Pressure Drops
Production managers operating inefficient compressed air systems are subjected to their own forms of pressure — time and cost constraints. Every 1 PSI of excess operating pressure increases air compressor power consumption by about 0.5 percent. So, taking emergency measures without examining your current layout, will just worsen cost and output performance.
Pressure drops reduce the amount of force available at usage points. They slow down operations and increase costs. Quick thinking and emergency actions tend to result from pressure on the manager. In these instances the instant solution is simply to turn up the compressors to full capacity to provide sufficient pressure at end points in the system. This is a self-defeating task, and may even lead to the unnecessary expense of buying and installing additional compressors.
Overloading a system without monitoring pressure levels throughout will cause upstream components to fail. The most vulnerable points in your system are the connectors between different components — these are maintained by valves and screw thread contacts that are kept tight by seals. Ramping up the pressure to get sufficient delivery at the end of a long flow line will require excessive pressure at the early stages of the system. Your seals and gaskets will also wear out faster than planned and screwed joints will slip their threads.
Blowing components results in leaks. You probably think that leaks caused your pressure drop. They probably didn’t. Inappropriate pipe gauges, overextended pipe runs, bends, pipe interior irregularities, blockages and dirty filters probably did that. The leaks resulted from overloading, which was a reaction to unacceptable pressure drop. If you only hunt for leaks, you are looking for the symptoms of your compressed air system problems, not the cause. Detecting leaks will provide evidence of failure. Get them replaced, but then move on to plan a long-term, cost-effective solution to your pressure drop headaches.
When you’re under pressure, you make mistakes. So the first solution to unexpected pressure drops in your compressed air system is to keep calm and commit to yourself that you will not crank up the compressor’s pressure setting until you have fully investigated the cause of the problem. Increasing the force in your system will just introduce leaks and make matters worse.
Consider a timed approach to resolving the overall issue:
Short Term: Check all the seals and connectors in your system for leakages. Clean or replace all filters and turn down the pressure set points on your compressors.
Medium Term: Place pressure monitors at various points of your system, particularly before and after each device. Check for unusual drops in pressure in the system. Institute regular maintenance checks.
Long Term: Draw out a plan of your system and reorganize the shop floor to reduce pipeline runs. Eradicate all bends and replace pipes that demonstrate unreasonable pressure drops.
Above all, you should realize that your pressure drops cost money and getting on top of this problem can turn your company around. Lost money through excessive power usage is just going to make your production uncompetitive. Getting the most efficient compressed air system working for you will enable you to increase efficiency to outperform your competitors, putting you ahead of the pack.