Maintaining Compressed Air Pressure Stability In Food Processing Operations

While compressed air might not be a listed ingredient on food or beverage packages, it can certainly have a huge impact on costs, quality, and scrap rates in their production processes. Here’s an inside look at key factors that impact pressure stability and operating efficiency, how to identify potential drawbacks in an inherited air system, and what to specify for greater efficiency in new or retrofit designs.

Why Care About Compressed Air?

As important as compressed air is for so many food processes, it is often taken for granted. Compressor rooms are typically “out of sight, out of mind” and not a high-priority concern for a lot of people — until problems crop up. Whatever the operating status — normal or problematic — there are financial and functional benefits to an ongoing evaluation of system efficiency.

Every compressed air system is different, and every one changes over time. Even a system that functions reliably can be costly in terms of wasted energy, lost production time, or product being scrapped due to pressure instability. Worse yet, many performance and reliability issues — moisture buildup, delayed servicing of dryer or filtration equipment, imbalances in compressor/dryer sizing, compressors running hot, etc. — can snowball over time.

Causes Of Air Pressure Issues

Many factors can contribute to unstable air pressure dipping low enough to cripple operations — increased demand, increased leakage, deferred maintenance, or the loss of a compressor. Even if an air system was properly sized and designed at the time of installation, performance will change over time as production equipment ages or is changed out and demand for air increases or decreases. Be on guard for these commonly experienced, costly issues:

• Oversized Compressors. Ensuring peak flow capacity is important, but how compressors are sized, configured, and controlled makes a big difference in pressure stability and operating efficiency when responding to fluctuating air demand. Running an oversized compressor at a 50-percent duty cycle for long periods, for example, is not an efficient use of energy (see Rapid Cycling below).

• Rapid Cycling. The two most efficient operating conditions for a fixed-speed rotary screw compressor are at 100 percent full load or “off.” Repetitive loading and unloading in a concentrated span of time can disrupt pressure stability and is definitely linked to higher maintenance costs and shorter compressor life. Evaluate compressor performance and use master control systems to match equipment capabilities to the known fluctuations in the application. Consult with the Compressed Air and Gas Institute (CAGI). CAGI has developed a Performance Verification Program that verifies the information that participating manufacturers publish on standard CAGI Data Sheets.

• Limited Design Options. Just because a specific compressor configuration can satisfy maximum flow demands does not make it the best choice for production schedules that vary day by day. Calculate the impacts of known fluctuations in air demand and evaluate better compressor, piping, and storage alternatives for satisfying that demand. Finally, balance the initial capital expense (CAPEX), long-term operating expense (OPEX), maintenance requirements, system wear, and built-in design concepts that make it easier to accommodate future growth.

• Inadequate Storage. Whether storage receivers are too small or are filled with water due to a malfunctioning drain, a lack of reserve capacity in the compressor station can cause system pressure to fluctuate under heavy demand. Additionally, dedicated storage capacity near demanding processes located far from the main compressor room can have huge benefits for overall pressure stability and performance efficiency.

• Improper Equipment Selection Or Control. Using oversized equipment or running multiple compressors in parallel without proper control can lead to units consistently running at low duty cycles, repetitively loading and unloading multiple machines simultaneously, or experiencing unreliable performance due to excessive wear. The significant rise in popularity of variable frequency drive (VFD) compressor equipment over the past 20 years is a good example of this. Spurred by power-company rebates, some hasty implementations of VFD compressors have resulted in unreliable performance from an otherwise promising technology.

• Dropping Below Threshold Pressure. Dropping below the minimum pressure required at a point of use (sometimes called the “scream” point) cripples operation. Once an air system dips below that pressure, actuators can malfunction or hold-points can fail, resulting in increased scrap. Protective low-pressure sensors can shut equipment down completely, resulting in a total loss of production.

• Artificial Demand. To compensate for fluctuating air demands that are not managed by appropriate system design, many users operate at higher pressures than needed without getting additional work done. This is “artificial demand.” Keep in mind that every 2-psi increase in pressure causes approximately a 1 percent increase in power consumption (Figure 1). Further, any leaks in the system will waste more air at higher pressures.

Figure 1. Using higher pressure to compensate for fluctuating demand can be expensive, increasing the out-of-pocket costs for leaks as line size increases.

Maintaining Pressure Stability At Point-Of-Use

Whether troubleshooting an existing compressed air system or designing a new one, it is important to evaluate performance at every juncture.

Compressed air users like to incorporate a margin for error into their systems, by running at a pressure greater than their highest calculated demand. Some will accept a 10 percent cushion, while others prefer a 20 percent cushion or more. In this case, more is not better. Too high of a cushion means that more air escapes more rapidly through every orifice in the system — every leak, every actuator, every piece of process equipment.

Because generating air pressure requires energy, and energy costs money, and efficiency is a function of system design, it pays to:

• Design For Optimum Pressure Balance. The correlation between flow and pressure impacts air system performance and pressure stability in use. To optimize efficiency, it is important to balance air system design and control to the facts that:

• More available compressed air supply than consumption raises pressure.
• More consumption than available compressed air supply reduces pressure.

• Calculate Pressure Drop. Energy-wise, it is desirable to run at the lowest practical pressure that can satisfy system demands, but some pressure drop is unavoidable. Aim for approximately a 10-psi differential in pressure between the compressor discharge and the most remote point of use in the system. The highest pressure in the station is located at the discharge point of the compressor and is immediately reduced by filters and dryers. Then, depending on pipe length and diameter, you may experience pressure drop farther down the line. Use this pressure drop calculator for designing efficient compressor setpoints needed to accommodate all downstream needs. Whenever possible, size piping to accommodate future growth. Adding compressors is far easier and less costly than re-piping. Finally, routine maintenance on dryers and filters will keep pressure drop low and air quality high.

• Use Storage To Accommodate Pressure Changes. Compressed air storage is an important factor in maintaining pressure stability while accommodating demand fluctuations. Total calculated storage includes all available volumes in the system — pipes, pressure vessels, filter housings, etc. — but large reserves designed to be released during peak demand are typically held in receiver tanks because of the volume needed. There are three typical types of storage to consider:

• Wet receiver tanks are typically installed between compressors and dryers and filters.
• Dry receiver tanks are typically installed in the compressor room right after the last filter prior to the distribution piping.
• Point of use receiver tanks can be located anywhere in the system, all the way to the end of the line, to satisfy demand surges from specific pieces of air-consuming equipment (such as bag houses). Appropriately configured point-of-use (or storage) receiver tanks can be a more efficient solution than installing dedicated compressors around a facility to meet demanding needs.

The Value Of A Master Control Strategy

Modern master controllers that capitalize on newer, faster processors and advanced control algorithms play an important role in pressure stability and operating efficiency. These algorithms, perfected over years of development, provide efficient system control by reading pressure throughout the system and knowing what’s available (total compressor capacity), what’s currently on-line (compressor output), and how the system reacts to changes in compressed air input and consumption. Letting a properly programmed master controller proactively control an air system’s response to changing demands and pressure readings ensures pressure reliability by:

• Avoiding the “Control Gap.” Using combinations of fixed-speed and/or VFD compressors offers excellent flexibility for matching air demand closely — even as it varies with production activity. In those situations, however, it is important to avoid control gap conditions by using a master controller to coordinate compressor transitions across all operational modes — start-up, full load, fluctuating operating speeds, standby, shutdown, etc. (Figure 2).

Figure 2. This chart shows how a master controller running two fixed-speed compressors (one 40 hp, one 75 hp) and one 75 hp VFD compressor can satisfy the continuous demand of an application efficiently, without control gap.

• Optimizing Stability And Efficiency. Beyond avoiding control gap situations, a master controller can also track dynamic operating conditions within an air system and analyze the best ways to maintain pressure stability by:

• Starting or stopping selected compressors.
• Adjusting the output of VFD compressors.
• Managing compressed air storage to satisfy specific peaks in demand.

• Evaluating Performance Efficiency. Master controllers can enable remote monitoring and management of compressor operating status, efficiency, and cost, as well as a detailed history for in-depth analysis. With compressed air consumption representing up to 15 or 20 percent of a facility’s overall electricity bill, the ability to identify declining performance, optimize compressor operation, and document results to plant management can pay significant dividends.

• Factoring In Energy Cost. Fine-tuning compressor efficiency requires balancing two major cost factors — the quantity of energy consumed and peak-demand charges for that energy.

For example, turning on three compressors to pressurize the system at the beginning of a work week can cause a big surge in peak demand charges. A master controller that senses no production demand could save a large part of that cost by scaling back to operating just the smallest or most efficient compressor to build up to the desired operating threshold pressure.

The same master controller can also avoid the inefficiency of cascade pressure control over-pressurizing a system by capitalizing on efficiencies learned from previous demand patterns. Algorithms that utilize historic performance, as well as current system status and demand, help to maintain pressure stability in the most cost-efficient way.

How Striking A Balance Makes A Difference

In conclusion, many factors impacting pressure stability and cost efficiency allow each compressor installation and control system to be tailored for the best overall balance. For example:

• Using a smaller compressor with a master controller to satisfy periods of low demand instead of running a larger compressor at low efficiency during off-peak periods helped one food processor achieve an 18-percent reduction in compressor energy costs without compromising process requirements (Figures 3A and 3B).

Figures 3A and 3B. The highlighted red line in Figure 3A (top) shows the time a 350 hp compressor ran at a fraction of capacity to maintain air system pressure during non-production hours. Figure 3B (bottom) shows how using a master controller to select the appropriate time to run a 50 hp compressor to satisfy off-shift demands eliminated wasteful large-compressor operation to cut overall energy costs by 18 percent annually.

• Heat recovery from air-cooled or water-cooled compressors boosts overall cost efficiency and reduces the overall CO₂ footprint of food plants that require heat for their processes. In these applications, a master controller can even factor in efficient heat recovery as part of its calculations on which compressors to run at which output levels.

• Proper pipe specification is a prime example of pressure stability being affected beyond the compressor room or master controller. Undersized piping — whether improperly specified at the time of installation or resulting from an inappropriate material that corroded over time — can compromise pressure stability over the life of the system.

• Comprehensive air demand analysis can help reduce plant operating pressures. This candy-manufacturing application was able to drop from 125 psi down to 100 psi without compromising process operations. The new solution saved more than 800,000 kWh of energy in a year, enabling the project to pay for itself in less than 10 months.