This article takes an in depth look at industrial air compressors.
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An industrial air compressor is a machine designed to increase the pressure of gases, with air being the most frequently compressed. Various types of compressors are employed across industries to perform tasks such as:
Similar to pumps, compressors are classified into two main types: positive-displacement and centrifugal (sometimes referred to as kinetic or dynamic). While pumps are generally centrifugal, most compressors fall under the positive-displacement category.
Industrial compressors come in a wide range of sizes, from compact models that can fit in a glovebox and are used for tasks like inflating tires, to large-scale turbo compressors employed in pipeline operations.
When air is compressed, it is subjected to pressures greater than atmospheric pressure, utilizing energy in the process. As the compressed air seeks to return to its original state, it releases energy as it expands back to atmospheric pressure. Air compressors thus increase air pressure, harnessing the potential energy. Unlike other power sources, compressed air does not require energy conversion at the application point. Pneumatic devices, which use compressed air, are known for their high power-to-weight or power-to-volume ratio.
While compressed air may not match the speed of electricity or the force of hydraulics, it offers a versatile range of applications. It is frequently chosen for its cost-effectiveness and efficiency.
The primary benefit of compressed air is the enhanced control it provides, along with the absence of electrical shock hazards or fire risks associated with oils. Nonetheless, it's important to adhere to safety codes and regulations when handling compressed air.
Compressed air exists in two types: active air and energy air.
Active air refers to compressed air that interacts directly with products. It is commonly used in food and beverage preservation, as well as in electronics, pharmaceuticals, and chemical industries.
Energy air is compressed air utilized to transfer and store energy for mechanical tasks. It is commonly employed to power pneumatic tools.
Given their different uses, active air requires higher purity compared to energy air. Contaminants such as dust, water, or oil must be filtered out. For instance, the water used for powering a turbine or flushing a toilet does not need to meet the same quality standards as drinking water.
Air compressors that are of 30 horsepower size or below include rotary and reciprocating compressors. These all compress air in diverse ways.
Reciprocating or piston compressors utilize the back-and-forth motion of pistons to compress gas within cylinders. The compressed air is then expelled through valves into high-pressure storage tanks. Typically, the compressor and tank are assembled together in a skid or frame as a single packaged unit.
In addition to supplying compressed air for energy, piston compressors are also used by pipeline operators to transport natural gas. When choosing piston compressors, factors such as flow rate (scfm) and required pressure (psi) are considered. These compressors can be either oil-lubricated or oil-free.
Key types of reciprocating compressors include:
The main types of rotary air compressors are:
Reciprocating single-acting compressors can be configured as either one-stage or two-stage compressors. A single-stage compressor draws air from the atmosphere and compresses it to the final pressure in a single stroke. These compressors are typically used for pressure ranges between 70 psi and 135 psi.
In contrast, the two-stage compressor compresses the air to an intermediate pressure in the first stage. Two stage units can be used to provide higher pressures than single stage compressors provide. The compressed air that goes into the second stage first passes through an intercooler. The intercooler eliminates some heat generated in the first-stage cycle.
Compressors with multiple pistons operate in a duty cycle rather than continuously. This cycling allows the heat produced during operation to be largely dissipated through air-cooled fins.
Single-stage and two-stage reciprocating compressors are usually oil-lubricated, though oil-free versions are also available.
The rocking piston compressor is a variant of reciprocating piston compressors. It creates pressure through the reciprocating motion of a piston connected by a single-piece rod, which allows the piston head to rock during operation. This type of compressor uses low-friction, non-metallic rings and often does not require lubrication. Rocking piston compressors are generally smaller and have lower pressure capabilities.
The reciprocating diaphragm compressor generates pressure through the oscillation of a flexible diaphragm, driven by an eccentric mechanism. A motor-driven eccentric causes the diaphragm to move, thereby varying the volume of the compression chamber as it expands and contracts.
Similar to a diaphragm pump, the flexible diaphragm isolates the drive mechanism from the process fluid, preventing any lubricant from coming into contact with the gas. Consequently, these designs eliminate the need for sliding seals between moving parts, meaning they operate without lubrication.
Diaphragm compressors are ideal for applications where contamination of the output air must be avoided, such as in laboratories or hospitals. Due to their limited output and pressure capabilities, they are typically used for light-duty tasks.
In rotary sliding vane compressors, a rotor is mounted eccentrically within the housing. As the rotor turns, centrifugal force pushes the vanes outward until they seal against a lubricating film on the stator wall. This design prevents metal-to-metal contact, as the vane tips glide on the lubricant. The rotating motion of the rotor decreases the volume of the spaces between the sliding vanes, resulting in air compression.
Single-stage rotary vane compressors are widely used in industries where pressure ranges from 60 to 200 psi and typically feature oil injection. Multi-stage rotary vanes, used in pressure ranges from 60 to 150 psi, employ flow-through lubrication, which consumes the lubricant. These compressors are commonly used for moving bulk materials, such as concrete.
As much as rotary vane blowers as well as vacuum pumps can be oil-free, the rotary vane compressors are not free of oil. Thus, rotary vane compressors are not best at providing oil-free air. However, they can provide compressed air free from pulsations. They use bushes instead of bearings and are thus less prone to contaminating the environment when compared to screw compressors.
Rotary vane compressors offer several advantages, including:
Rotary helical screw compressors feature two intermeshing helical rotors housed in a twin-bore casing. In a single-stage design, the inlet is typically positioned near the drive shaft end at the top of the cylinder. Air is drawn into the cavity between the secondary rotor grooves and the main rotor lobes as the rotors unmesh at the cylinder inlet. As the rotors continue to turn, the air is trapped between the rotor cavities and the cylinder wall. The main rotor lobes then roll into the rotor grooves, reducing the volume and increasing the pressure, which leads to compression as rotation proceeds.
Once the cell closes, oil is injected to seal gaps and dissipate compression heat. As compression progresses, the rotor tips pass the discharge port, resulting in a mixture of compressed air and oil. Rotary helical screw compressors can be configured as single-stage or multi-stage and may use water, oil, or be oil-free for lubrication. The key advantages of rotary helical screw compressors include:
They are commonly preferred for trailer-mounted applications, including road construction and other building projects.
Rotary scroll compressors compress air through the interaction of a fixed scroll element and a helical scroll that orbits, progressively compressing the air. This continuous motion results in a steady, pulse-free flow of compressed air. With fewer moving parts, these compressors require less maintenance. Rotary scroll compressors can be either lubricated or oil-free.
Compressors need controls to manage their operation according to the demand for compressed air. Various applications and requirements necessitate different types of controls. For steady air attributes, constant-speed controls are used to ensure continuous operation.
Most air compressors are powered by:
Power transmission in air compressors typically involves gears, V-belts, or direct drive systems. These methods are described in more detail below.
There are two primary performance specifications for air compressors, which are:
Having more than one discharge pressure at varying flow rates can reduce the compressor volumetric efficiency. This occurs when there is increased system pressure expressed in pounds per square in gauge (psig). Therefore, careful consideration should be given when selecting the compressor’s maximum operating pressure.
Different compressors offer varying capabilities and technologies. Choosing the right air compressor for your specific needs is crucial.
The oil-injected compressor is less expensive and can be used in environments such as the manufacturing environment where there may be no need for immediate oil-free compressed air.
The oil-free compressors are more expensive. However, they can produce the right quality of compressed air fit for the pharmaceutical sector or food production. Thus, oil-free compressors are best for active air, and oil-injected compressors are best for energy air.
The piston compressors are less costly and can be maintained with ease. These are applied in environments where a lot of compressed air is needed part of the time e.g in garages. They have the drawback of being noisy and thus making them not fit for environments such as laboratories. They, however, hold well when operated in dirty environments. The piston compressors are prone to passing air into the supply of compressed air, known as carryover. A high amount of heat is generated by piston compressors in operation; they are thus sized based on duty cycle, which has a baseline of 75% run and 25% rest.
Rotary screw compressors come in two versions: fixed speed and variable speed. Fixed speed compressors are ideal for applications requiring a consistent air flow, while variable speed compressors are suited for applications with fluctuating air demands. Although variable speed models have a higher initial cost, they are energy-efficient and can provide long-term savings through reduced energy consumption.
While rotary screw compressors involve a significant initial investment, they offer several long-term benefits. They can operate at high speeds, producing more compressed air efficiently. These compressors are also quieter, more energy-efficient, and have a compact footprint, making them ideal for continuous operation. It is crucial for rotary screw compressors to maintain an operating temperature to ensure effective compression due to the precise tolerances between the rotors. When sizing rotary screw compressors, careful consideration of air usage is necessary, whereas oversizing is less critical for piston compressors.
In environments with constant air use, such as an auto body shop for painting, a rotary screw compressor with lower carryover rates and continuous operation is often preferred. Conversely, for environments with sporadic air usage and less stringent requirements for air cleanliness, such as a general auto repair shop, a piston compressor may be more suitable.
Both rotary and piston compressors are available in oil-free and oil-injected versions, offering a broad range of options for businesses to select the most suitable industrial compressor for their specific needs.
Regardless of the compressor type, compressed air is usually cooled, dried, and filtered before being distributed through pipes. When designing plant-air systems, it is important to select components based on the system’s size and to include filter-regulator-lubricators at supply points.
Rotary screw compressors with engine drives are commonly used in larger site compressors and are typically mounted on trailers. They are designed for continuous operation, even when air is being discharged.
Scroll compressors are gaining popularity in lower-end air compressors and refrigeration systems. They are ideal for applications requiring Class 0 clean air, such as in food production, pharmaceuticals, electronics, laboratories, cleanrooms, and medical settings.
For compressing hazardous gases, sliding-vane or diaphragm compressors are recommended. In situations where large volumes need to be compressed, kinetic compressors are more suitable.
Beyond the factors discussed earlier, additional parameters to consider when specifying an air compressor include:
The pressure capability, measured in psi, should match the requirements of the equipment using the compressed air. Most air tools operate at standard pressures, but certain applications, like engine starting, require higher pressures. For daily tools, a single-stage unit with up to 135 psi may be adequate, but higher-pressure applications may necessitate a two-stage compressor.
Volumetric capacity refers to the amount of air a compressor can deliver per unit time, typically measured in cubic feet per minute (cfm). This can vary by manufacturer, with the standardized measurement being standard cubic feet per minute (scfm). Another measurement is actual cubic feet per minute (acfm), which represents the amount of compressed air delivered to the outlet and is usually less than the compressor's displacement due to blow-by losses.
This is the power needed to drive the air compressor and is determined by the pressure and volume considerations. In determining the compressor capacity, it is also vital to consider the system losses such as pressure drops through filters and dryers, and piping losses etc. Considerations of the drive also need to be made, such as motor direct drive or belt, diesel drive or engine gas etc.
Additional factors to consider when selecting an air compressor include:
The primary function of oil in compressors is to dissipate heat generated during compression. In some designs, oil also serves as a seal.
Manufacturers employ various methods to create oil-free compressors. For example, piston compressors may feature one-piece piston-crank assemblies with eccentric bearings that ride on the crankshaft, allowing reciprocating pistons to move without a wrist-pin bearing. Additionally, self-lubricating materials can be used for sealing rings and cylinder liners. In rotary screw compressors, tighter clearances between screws can eliminate the need for oil sealant.
Employing these methods to produce oil-free air compressors can involve trade-offs, including challenges with heat management, increased wear, more frequent maintenance, and potentially reduced capacity.
When defining compressor capacities for setups that operate jackhammers continuously, factors such as the number of users, the cfm requirements of the tools, and environmental conditions must be considered. In such cases, a helical screw compressor designed for continuous operation may be recommended, as it can run on a single tank of fuel for up to 8 hours.
For a small shop, compressor capacities should account for both continuous and intermittent air tools. Intermittent tools, such as a ratchet wrench, and continuous tools, like a paint sprayer, have different air demands. Consumption charts can help estimate the needs of various tools, allowing for the determination of overall compressor capacity based on whether the tools are used continuously or intermittently.
In manufacturing facilities, compressor capacities are determined using similar principles. For instance, a packaging line might use compressed air for tasks such as blowing off devices and actuating cylinders. Equipment manufacturers typically provide consumption rates for each machine. Alternatively, the air consumption of each actuated device can be calculated based on factors like bore size, stroke length, and cycling rate.
Larger process plants and manufacturing operations require substantial amounts of compressed air. In these scenarios, ensuring continuous availability of compressed air often justifies the cost of multiple systems. This approach helps prevent costly production line stoppages.
During compression, atmospheric air can contain heat and sometimes oil, which can lead to moisture if the intake air is not dry. The presence of these additional elements may impact the end-use and performance of the compressed air, depending on the specific operations.
Compression generates heat, which must be managed by collecting the air in a tank to allow it to cool and condense some of the moisture. Compressed air tanks are equipped with valves for draining accumulated water, which can be either manual or automatic. An aftercooler can further reduce heat, while desiccant and refrigerant dryers can be added to the air supply line to remove additional moisture. Filters are used to eliminate entrained lubricants and any other particles that may have entered with the intake air.
The compressed air is distributed to various points, where it is best practice to install a filter, regulator, and lubricator (FRL) at each drop. The FRL units adjust the air to meet the specific requirements of each tool and provide necessary lubrication where needed.
For piston compressors, the most common control method is start/stop control. This system maintains pressure within a specified range by turning the compressor on when the pressure drops to the lower threshold and running it until the upper threshold is reached.
An alternative method, called constant speed control, enables the compressor to continue running for a set time after reaching its upper setpoint. This helps to manage higher-than-normal air usage by reducing the frequency of motor starts during peak demand periods.
Systems with more than 10 horsepower often feature a selectable dual control system, which allows operators to switch between two different modes of operation.
In addition to start/stop and constant speed controls, helical screw compressors may include other control options such as inlet valve modulation, load/unload control, sliding valve, variable speed drive, and automatic dual control. Multi-unit installations can also utilize compressor sequencing for optimized performance.
Selecting the appropriate control scheme involves balancing the cost of meeting demand against the costs of equipment idling and accelerated wear. The goal is to find the optimal trade-off between efficiency and maintenance expenses.
Air compressors are widely utilized across various industries. They power pneumatic tools, inflate tires, apply paint, and clean surfaces. They are essential in processes such as metal fabrication, woodworking, painting, coating, and construction.
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