When deciding on a new air compressor, you may consider both rotary vane compressors and rotary screw compressors for their similar benefits. Compared to other types of compressors, these are both relatively easy to install and maintain. They are also quieter than piston compressors and compact enough to fit in a smaller space. Additionally, both are energy-efficient options that are more affordable than larger compressor units.
But what sets them apart from each other? While the two have similar applications compared to other compressors, they are unique enough that you should consider the benefits and drawbacks of each before deciding on one. This article will outline the difference between how rotary vane and rotary screw compressors work to help you make the right choice for your operation.
A rotary vane compressor is a compressor unit that consists of a cylindrical housing, adjustable rotary vanes on an off-centered drive shaft, an air inlet and an air outlet. Because the drive shaft is off-centered, the blades create airtight chambers of varying sizes as they push against the cylinder wall. When air enters the largest chamber at the inlet, and as the vanes rotate and retract, the chambers become smaller until the walls reduce the air volume. The compressed air then exits at the outlet and powers other processes down the line.
Compared to the rotary screw compressor, the rotary vane compressor has had a long history. The first patent detailing how a rotary compressor works appeared in 1874, eventually evolving into the rotary vane compressor. Specifically, vane compressors have been on the market for around 100 years and remain a reliable and efficient compressor solution.
A rotary screw air compressor is a compressor unit that includes a pair of screw rotors on parallel drive shafts — an inlet and an outlet. How the rotary screw air compressor works is dependent on the contrary motion of the rotors. The rotors intertwine when turned in unison, trapping air in between the lobes and flutes of the rotors. The trapped air reduces in volume as the rotors force it into a smaller space until it exits the outlet as compressed air.
In oil-lubricated screw models, the rotors sit in a pool of oil to cool and lubricate the compressor, which dampens noise and helps to form a seal. Oil filters and dryers downstream from the compressor remove oil contamination from the end product. Oil-free screw compressors use other methods of regulating temperature without adding oil to the air. Some use high-quality water to cool the rotors instead, while dry rotors rely on air cooling and specialized temperature-resistant materials for the rotors.
Although piston, or reciprocating, air compressors are the most widely-used compressor type, rotary screw compressors and rotary vane compressors have several advantages over them in certain areas. Here are some benefits to using a rotary compressor:
While workers use rotary vane compressors and rotary screw compressors in similar situations, the distinctions between them set them apart from each other. Here are five differences between the two types of compressors:
Rotary vane compressors are helpful in various industries that benefit from a small and quiet compressed air solution. For example, the automotive industry frequently uses vane compressors for air tool operation and auto finishing, while the agriculture industry uses them for operating farm equipment like milking machines.
However, a rotary vane compressor may be less appropriate for other industries. Because some oil can potentially leak into the compressed air, applications that require a pristine and oil-free environment may benefit from another type of compressor. Furthermore, in areas with significant air contaminants like a woodshop strewn with sawdust and wood shavings, debris can clog the vane, requiring frequent cleaning.
While oil leakage is still possible in oil-lubricated rotary screw compressors, oil-free models erase the risk of air contamination. This possibility makes them the perfect choice for applications in the food and beverage industry.
Although rotary screw compressors effectively compress air, the rotor design results in areas where air can pass through and escape back to the intake. It is impossible to create a lobe form that perfectly matches the flutes in the sister rotor, and small holes at the edges of the contact points are inevitable. The rotary screw compressor can mitigate this issue by running at high speeds so the air has less time to escape. Lubricant can also act as a barrier to keep the pressurized air from leaking out.
At the beginning of the rotary screw compressor’s life cycle, it experiences minimal clearance and leakage. However, as the compressor ages, wear on the rotors enlarges the size of the clearance gaps, which increases the rate of leakage. Users can reduce the impact of galling over time by maintaining proper lubrication levels.
Conversely, rotary vane compressor leakage can improve over time. As the rotor blades wear down, they settle more comfortably into their stator and rotor slots. Combined with proper lubrication, the blades remain in constant contact with the surface of the cylinder, which creates a nearly perfect seal.
However, rotary vane compressors still have a chance of experiencing leakage. Because the vanes are constantly sliding in and out of the rotor vane slot, both the vanes and stator slot can wear over time, reducing the seal on the air chambers. Operators need to use enough lubricant in the rotor/stator chamber to combat air leakage and reduce friction. Oil changes are required more often to prevent the stator bores from varnishing or sludge build-up.
Because rotary vane compressors rely on centrifugal force to operate, they are limited in their minimum and maximum operating speeds. A vane compressor’s top speed is limited to only a third of a screw compressor’s capacity. Vanes are also most likely to wear at maximum speeds, which results in complications over time.
On the opposite end, if the operating speed falls below 500 revolutions per minute, there is usually insufficient centrifugal force to seal the vanes against the edges of the cylinder. Additionally, when the rotary vane compressor needs to run at half capacity, the machine will still be consuming 80% of its total energy rating.
By comparison, a rotary screw compressor can match fluctuations in compression with minor energy penalties. They can reduce to 15% capacity depending on the operation’s demands. Between lower operating power and oil-cooled motor and compressor elements, the screw compressor can help to reduce energy costs.
Rotary screw compressors work in a compact environment that requires complex bearings to function. Because the parallel rotors are so close to each other, there is little space available for bearings to sit, so the bearings have to be small. Manufacturers typically use angular contact ball bearings to burden the radial and axial load of the mechanism.
The air-end houses between four to six small bearings to support the screw rotor’s high operational speeds. The ball bearings require regular inspection for signs of damage, like flaking, cage damage, smearing, creep and fretting. In addition to preventing further damage, inspecting damaged bearings can help identify improvement areas in the compressor process. For example, smearing is a sign of improper lubricant, which indicates that either lubricant levels need to be adjusted or the type of lubricant needs to be changed.
In contrast, rotary vane air compressors use simple bearings. Vane compressors operate without any axial forces on the bearings, which results in a significantly longer running time before operators need to replace them. They also only require two simple shell bearings instead of several angular contact ball bearings, making it easier to identify bearing damage.
Most of the auxiliary parts of rotary vane compressors and rotary screw compressors, like oil tanks, pressure valves, and thermal bypasses, are similar. Operators will need to service these components just as often, no matter which compressor you choose.
The distinction for repair cost lies in the rotary components. In general, screw compressors cost more to repair than vane compressors. Because of the strain on the screw compressor’s bearings, they need replacing two to three times more often than the bearings on a vane compressor. Combined with the complexity screw compressor bearings, the repair costs associated with bearing replacement are much higher.
Additionally, the rotors of a screw compressor are more prone to wear than those of a vane compressor. If not appropriately lubricated, galling can occur, which shortens the screw’s life cycle. As bearings wear down, rotors can struggle to maintain clearance within the cylinders. The constant motion results in friction, dramatically increasing the rate of wear.
In case of bearing failure, the entire air-end of the screw compressor may need replacing depending on the damages. The screws can make contact, which results in damage to the rotors, shafts and stators. Rotary screw compressors are complex machines and take time to repair, which usually means the operator will need to ship the compressor to the manufacturer or supplier for extended care. Depending on the damages, repairs may cost between 20% and 40% of the cost of a new unit.
By comparison, when an operator needs to replace a vane compressor’s blades, the job typically takes less than two hours. The compressor unit is simple enough to service it on-site rather than send it to a factory. A set of replacement blades for a vane compressor costs much less than complex repairs on a screw compressor, usually less than 10% of the cost of a new unit.
Whether you decide on a rotary vane or rotary screw compressor, Fluid Aire Dynamics has an air compressor to suit your needs. If you still need help deciding on an air compressor, we offer a compressed air system analysis to ensure you will have the best possible air compressor system in place for your business. With 24-hour emergency air compressor maintenance to customers in Pennsylvania, Maryland, Delaware, New Jersey, southern New York and northern Virginia, you can rest assured knowing we will quickly repair your equipment when you need it most.
Fluid Aire Dynamics is proud to carry the top air compressor brands on the market. Choose our rotary screw compressors from Atlas Copco, Boge and Sullivan-Palatek for their versatility and energy efficiency. Screw compressor models from Boge and Atlas Copco include both oil-lubricated and oil-free so you can always find the right air compressor for your workspace. We also carry Mattei lubricated rotary vane air compressors, one of the top-rated and most widely recognized manufacturers in the industry. Choose from a wide variety of Mattei compressors from the ERC, AC, Optima and Maxima series.
If you’re ready to install a new air compressor or upgrade your current equipment, contact Fluid Aire Dynamics today. We will be happy to answer any of your questions!
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