Improve your air distribution with a demand-side outlook

What you can do at the end of the air distribution system to improve efficiency.

By Frank Moskowitz

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Energy is one of the most fundamental parts of our universe. We use energy to do work, to light our cities, to power our vehicles, trains, planes and rockets. Energy warms our homes, cooks our food, plays our music and gives us pictures on television. Energy powers machinery in factories. Energy is the ability to do work.

Energy is one of two types, depending on whether it’s moving or stored. Stored energy is called potential energy. The moving variety is called kinetic energy.

Air, when compressed, represents potential energy that can be used in a variety of ways. Humans began to compress air and apply it to widespread and diverse uses about 100 years ago. Early compressors were steam-driven, but in other respects were essentially the same as today’s models, differing only in refinement of materials and tolerances. The compressor then, as now, delivers air under pressure into a storage vessel or directly into the piping network for distribution to points of use. The controlled release of air pressure on pistons, against rotating blades, and through orifices represents the kinetic energy that powers the air components and instruments common throughout industry.

When applying compressed air as a utility in your process, the desired result usually is some form of work (as in force through a distance). The work that’s available through the use of compressed air is a function of the weight of the air. It’s weight that does the real work.

The point is you can’t use the average flow as a guideline for sizing components.

– Frank Moskowitz

Let’s say you have a compressed air receiver that’s loaded to 100 psig. It was work produced by various types of energy that compressed the air, now representing potential energy. If potential energy is to do work, it must be transformed to other forms of energy. The transfer occurs when the force of air pressure is applied to a tool or pneumatic device. Force, the product of mass (weight) times acceleration (movement in the direction of force), is the key word here. The tool or device is more concerned about how many pounds per minute (weight) are being delivered.

The first and foremost complaint from an operator or production area is, “I don’t have enough pressure.” The problem is actually a flow restriction that manifests itself as low pressure. There are many reasons certain areas of a plant might lack sufficient air pressure. At many facilities, however, some production machines require large volumes of air to actuate large, fast-acting cylinders. The more that air cylinders are working in tandem, the greater will be the peak flows that most point-of-use piping and valves simply can’t handle.

If point-of-use components such as filter/regulator/lubricators (FRLs), quick disconnects, piping and hoses aren’t sized properly for the end-use tool, the result is a restriction to the mass flow and a poorly performing tool or a production machine that fails to produce consistently. The quick fix is to increase the end-use pressure by cranking the regulator fully open to increase the available mass. If the performance still is unacceptable, the knee-jerk reaction is to raise the pressure in the entire air system.

Air, when compressed, represents potential energy that can be used in a variety of ways.

It’s true that both of these solutions increase the available air mass and, thus, offer the appearance of a good resolve. But increasing the system pressure carries significant penalties in operating cost. In most rotary screw compressors, for example, every 1 psi change from rated pressure changes the required brake horsepower by 0.5%. Increase system pressure by only 10 psi and the power consumption goes up 5%.

Also, deviations in discharge pressure from the compressor’s rated pressure change the overall compression ratio. Compression ratio shifts change the compressor’s volumetric efficiency, which results in changes in capacity. A good guideline is that increasing the pressure by 10 psi causes a reduction in capacity of about 0.4%.

Reduce pressure drop

Excessive pressure drop degrades system performance and leads to excessive energy consumption. Minimizing pressure drop requires a systems approach in both design and maintenance. On the supply side, select air treatment components such as aftercoolers, moisture separators, dryers and filters that exhibit the lowest possible pressure drop at specified maximum operating conditions. On the demand side, verify that point-of-use filters, regulators, lubricators, hoses, quick disconnects, hose reels and valves have the proper rating to handle peak flows. Also, pneumatic equipment, supply side and demand side, deserve the recommended best-practices maintenance procedures.

A typical high-speed production machine might contain many fast-acting cylinders. Air cylinders often are the culprits that cause the large point-of-use pressure drops because their flow ratings typically are engineered for consumption during a minute. The real issue is rate of flow when the cylinders are moving. Peak flows could be as much as five to 10 times the published average flow. Because many people size for air consumption per minute, most cylinder control circuits and associated FRLs are undersized and cause high pressure differentials. The following is an example of determining the flow rate into a cylinder.

Beware average flow

The flow rate into a cylinder is based on the amount of air needed to move the piston load at a specified speed and to force exhaust air out of the other side of the cylinder. Because a specific mass of air is required to perform the work, the flow rate can be denominated in terms of standard cubic feet per minute (scfm). A standard cubic foot of air is defined as air at a barometric pressure of 29.92 inches of mercury (we will use 14.5 psia) with a temperature of 68°F and a relative humidity of 0%, and weighs 0.0750 pounds.

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