Top factors to consider when deciding between a reciprocating air compressor and a rotary screw air compressor for mobile applications
When deciding whether to install a new engine-driven air compressor or change your existing design, it’s essential to understand the operating characteristics, capabilities, and limitations of the two alternative air compressors commonly considered.
To help you understand how this decision affects your vehicle engine and application, we’ve put together this review of the most common differences between reciprocating air compressors and rotary screw air compressors.
First, let’s talk about how each compressor works, and then we’ll break down the benefits of reciprocating compressors and rotary screw compressors.
For a more general performance comparison, please visit Rotary Screw vs. Reciprocating Air Compressors.
The Reciprocating Air Compressor
On typical reciprocating compressors used on mobile equipment, the compressor is driven by a stand-alone diesel or gas engine or is hydraulic-driven and operates continuously while the engine is running. Reciprocating compressors may also be called piston air compressors.
Benefits of Reciprocating Air Compressors
- Simple maintenance
- Capable of high pressures
Disadvantages of Reciprocating Compressors
- Interrupted flow rates
- 20 to 30% duty cycle
- Low life expectancy
- Maintenance costs
- Excessive heat
Piston compressors are typically driven by an engine either by a direct drive through an auxiliary engine port or by a belt drive system and operate continuously while the engine is running.
The compression cycle is typically controlled by an unloading mechanism and—in some cases—a governor. If a governor is used, it is most likely mounted on the compressor. The function of the governor is to maintain the system air pressure and specific cut-in and cut-out pressures.
The Intake Cycle
As the piston is drawn down in the cylinder, a vacuum is created between the top of the piston and the cylinder head. This action causes lower pressure atmospheric air to enter the cylinder through inlet valves above the descending piston head. During this cycle, the inlet valve is open, and the discharge valve is closed.
As the piston moves into an upward stroke, the inlet valve closes, trapping a volume of air in the cylinder. This area in the cylinder is steadily reduced as the piston moves, creating air pressure. Once the air pressure in the cylinder overcomes the spring resistance on the discharge valve, the valve lifts off its seat, allowing the pressurized air to evacuate the cylinder, and the cycle repeats.
When the discharge valve is closed, it acts as a non-return check valve and blocks pressurized air from returning to the lower pressure area within the cylinder.
The air that leaves the discharge port has a low-frequency pulsation that must be smoothed out before the compressed air is usable in downstream equipment, which is accomplished using an air receiver tank.
Unloading the Compressor
Once the air receiver tank’s air pressure has reached the high-pressure cut-out point, the pressure control device sends a signal to the compressor to unload. The compressor will unload fully or partially, depending on the compressor design.
As compressed air is used downstream, the pressure in the receiver tank will start to fall. As it falls to the cut-in (load) point set on the control mechanism, the compressor returns to the compression cycle and delivers air under pressure to the air storage tank to rebuild the tank pressure.
The Duty Cycle
The compressor’s duty cycle is the difference between the time it is fully or partially loaded and the time it runs fully unloaded or shut off entirely. Most reciprocating compressors are designed to run fully loaded between 20 – 30 percent of the time and unload the rest.
Compressors designed with these duty cycles can experience high degrees of increased maintenance or outright failure should the designed duty cycles frequently be surpassed. Most of the damage is heat-related.
Some of the reasons for exceeding the duty cycle would be under-sizing the compressor or air receiver tank for the application and excessive air leaks anywhere in the system (this would be equivalent to additional load).
If the compressors do not unload as recommended, the compressor lubricant can break down due to high temperatures, causing excessive wear on the many components.
In some designs of vehicle engine driven reciprocating compressors, the drive engine sump provides a portion of its lubricant supply to the compressor. This supply becomes an integral part of the engine lubrication system.
The lubricant recommended change-out schedule would change from the standard recommended by the engine supplier without the compressor’s addition. The engine supplier would have to consider the compressor’s heat load to the engine lubricant to determine the reduced life expectancy of the engine lubricant.
Typically, in an engine-driven air compressor application, the primary cooling source is the portion of the lubricant that’s cooled by the engine oil cooler and then recirculated to the compressor. Secondarily, the airflow from the engine oil cooler fan would tend to remove some of the radiated heat from the compressor’s body and carry it away with the exhaust air.
Without these cooling methods, it would be challenging to maintain air temperature limitations within the factory specifications for the compressor and the engine cooler if the application exceeds the compressor’s designed duty cycle.
Increased temperature of the lubricant beyond recommendations could lead to a premature breakdown of the lubricant resulting in more frequent lubricant changes or, failing that, premature failure of moving parts within the engine and the compressor.
Rotary Screw Air Compressors
A variety of methods can power rotary screw compressors. They can be driven via a belt or coupler from an engine, driven by hydraulics, or via the vehicle’s transmission.
Rotary screw compressors are oil-injected with a self-contained lubricating system and do not share oil from the engine, and therefore avoids the complexities and risks of reciprocating compressors.
Benefits of a Rotary Screw Compressor
- Continuous airflow
- 100% duty cycle
- Better energy efficiency
- Longer lifespan
- Lifetime Warranty*
- Larger volumes of air
- Higher CFM per hp
Disadvantages of Rotary Screw Air Compressors
- More expensive upfront
- Requires skilled maintenance
*VMAC’s rotary screw compressors include the VMAC Lifetime Limited Warranty.
Oil-injected Rotary Screw Compressor Components
Rotary screw compressor systems have a closed-loop oiling system that performs four main functions:
Compressor – compresses air via oil injected rotary screw compressor
Oil separator – separates the oil from the air via 1st and 2nd stage separation
Oil cooler – cools the oil via oil cooler, either liquid to liquid or air to liquid heat exchangers
Oil filter – filters the oil
These components can be combined assemblies or separate components
Intake and Compression of the Air
Atmospheric air enters the screw compressor through the inlet air filter.
The air then travels through the inlet valve, which is either a load-no-load or modulating poppet type.
Air enters the rotor assembly, and compression begins as the rotors mesh at the air inlet end of the compressor housing. Air is drawn into the cavity between the male rotor lobes and the female rotor flutes. As rotation continues, the rotor lobes pass the edges of the inlet ports, trapping the air in a cell formed by the rotor cavities and the cylinder wall.
Further rotation causes the male rotor lobe to roll into the female rotor groove, reducing the volume and raising the pressure. Oil mist is injected at a point in the compression cycle to seal the rotors’ clearances and between the rotors and the casing, which results in higher compression.
This oil mist also removes the heat of compression and lubricates the rotors and bearings. Compression continues until the rotor lobes pass the discharge port’s edge and release the compressed air and oil mixture.
The rotary screw compressor has a self-contained lubrication system. The compressor oil has three functions:
- Lubricates the compressor’s bearings, gears and rotors.
- Cools the compressor as it carries away the heat of compression. The oil leaving the compressor is very hot and must be cooled before being re-injected again.
- Seals the rotors.
The oil starts in the oil separator/ reservoir tank and is forced by air pressure through the lubrication system.
When the compressed air-oil mixture leaves the compressor, it flows into the separator/reservoir tank, where the first stage of air-oil separation occurs. This tank also acts as an oil reservoir.
From the air-oil separator/reservoir, the oil will flow through an oil cooler (either liquid to liquid or air to liquid). The oil cooler is required because oil absorbs the heat from compression and needs to be cooled before it is injected into the compressor.
At some point, the oil flows through an oil filter to help remove contaminants. In some systems, the oil will pass through the oil filter on its way back to the compressor after cooling; in others, the oil is filtered before cooling.
Secondary air-oil separation occurs through a coalescing filter, which removes the remaining oil mist from the air. Oil removed (scavenged) from the air by the coalescing filter is returned to the compressor via a small oil return line or scavenge line.
The pressure line helps control the inlet valve/governor to control airflow in the compressor system.
*There are numerous air-oil separator/ oil filter configuration options available; this image shows one variation.
The Duty Cycle
As mentioned above, the compressor’s duty cycle is based on the difference between the time it spends fully or partially loaded to the time it runs fully unloaded or shut off entirely.
Due to the rotary screw compressor design and the very low comparative oil temperatures vs. a reciprocating compressor, the screw compressor is designed as a 100% duty cycle compressor. For applications requiring over a 25% duty cycle, a rotary screw compressor is recommended.
As shown, the reciprocating and rotary screw compressors are quite different, although they ultimately perform the same job of delivering compressed air. When choosing, installing, or sourcing a compressor for your application, considering these differences may help determine the best option for you.
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