Trim Down Costs by Increasing Compressor Count

We always think of trimming in the context of removing or reducing something. Whether it’s getting your hair trimmed or trimming down for summer, the end goal is always less than the starting point. So it might seem counter intuitive that reducing total operating costs sometimes requires adding machinery to your compressed air system.

What is a trim compressor?

Operating compressors above their optimal capacity can consume excess power and overwork machines, leading to overheating, premature machine wear and increased maintenance needs. The maintenance, energy bill, part replacement and potential downtime can be extremely costly for operators. Trim compressors solve that problem. A trim compressor is a supplemental compressor that runs most efficiently at part-load. It picks up the extra workload when compressed air demands increase during different shifts, while allowing main compressors to remain base loaded (meaning they are running at full capacity and pressure). This ensures the main compressors do not get over-worked when air demand increases.

Methods of Trim Control

There are multiple methods of trim control. Each plant or facility should determine its trim needs based on the types of compressors in the plant and their control systems. A few of the most common methods are listed below:

• Start/Stop: The simplest form of control, start/stop uses a pressure switch to sense system pressure. When a predetermined pressure threshold or ceiling is reached, a signal is sent to the main motor to start or stop the trim compressor. Operators can adjust the pressure switch to determine the upper setting and make the lower setting a fixed pressure or a differential between the highest and lowest pressures. This control method requires an air receiver to avoid frequent starting and stopping and is typically used with compressors rated 30 hp and lower.

• Load/Unload: Unlike start/stop controls, the load/unload control does not operate within a pressure range. Instead, valve unloaders hold open inlet valves so that air is drawn in and expelled without any compression taking place. Because of this, load/unload controls can only operate at 0 or 100 percent, unless a clearance pocket is added. A clearance pocket is a secondary volume that can be opened temporarily to create five steps of control, allowing the compressor to work at 0, 25, 50, 75, or 100 percent capacity.

• Inlet Valve Modulation: IVM works when a regulating valve senses discharge pressure over a certain range. When the discharge pressure reaches within 10 psi (some newer models operate within 3 psi) of this point, a proportional pressure is sent to modulate or adjust the inlet valve, causing a pressure drop and increasing the compression ratio. This type of control can only be used with lubricant injected rotary air compressors and is usually limited to operating in the range of 40 percent to 100 percent of rated capacity.

• VDC: Variable displacement control (VDC) adjusts to match demand by varying the displacement of rotors. Inlet valve modulation and load/unload control accompany VDC after the initial rotor regulation. There are three types of VDC: slide valve, spiral valve, or turn valve. Lubricant injected rotary air compressors can also use VDC in the form of step control valves or poppet valves, both of which use discreet steps of percentage capacity.

• VSC: Variable speed control is the principle behind VSD compressors. In this process, variable speed can be accomplished through variable frequency ac drive or switched reluctance dc drive. In this system, air end displacement is directly proportional to rotor speed, but air end efficiency depends on male rotor tip speed. Most VSD operate at or above the ideal male rotor tip speed. Because of this, VSD compressors achieve the greatest energy savings when running long hours in the mid to low capacity range, making them best suited for trim duty.

VSD Benefits

The initial cost of purchasing a VSD compressor for trim purposes is high compared to other types of compressors. But when properly applied for trim use, VSD saves more money over the life of the machine. When VSD compressors are running at part load, they are much more energy efficient than compressors with inlet valve modulation or load/unload, as these typically lose efficiency below 90 percent of capability.

When they are running above 80 percent of capacity VSD compressors do not offer significant energy savings, and once they approach 100 percent of capacity can become more costly to run than standard fixed speed compressors. Because of this, VSD compressors are ideal for trim use. The benefits of a VSD trim compressor are even more significant in areas with high electric rates. Before purchasing a VSD compressor, check to see if there are any rebates offered by local electric utilities in your region. This could further reduce costs and amplify savings.

Lowering air compressor costs may require adding or replacing a machine. Using a trim compressor can shield your main compressors from rapid instrument wear, shortened machine life and unnecessary power consumption brought on by overexertion. Pioneered by Atlas Copco, VSD compressors offer significant savings over the lifetime of the machine. Contact an Atlas Copco representative today to find the best VSD compressor for your application.