As they’re intended for continuous duty, rotary screw air compressors are not for intermittent usage situations such as construction sites or hobiest. They are best suited for industrial applications that need a constant supply of compressed air. However, there’s no one-size-fits-all product. Instead, the compressor must be matched to the demand for air. This rotary screw compressor buyer’s guide explains what to consider.
Why Choose a Rotary Screw Compressor?
There are various ways of producing compressed air. Small systems can get by with reciprocating compressors. (Picture a piston moving up and down in a cylinder.) For large volumes of compressed air (greater than 6,000 cfm), a centrifugal compressor (e.g., turbine) may be the answer. For most industrial applications, though, a rotary screw compressor is the best choice as well as the most common
The heart of every rotary screw compressor is a pair of helical screws. Meshed, these draw air as they turn, forcing the air into an ever smaller volume until it’s released as compressed air. It’s an efficient design that delivers a constant supply of compressed air. Unlike reciprocating compressors, the rotary screw design is quiet and durable. Together, these advantages make it ideal for industrial usage.
Compressors are usually specified in terms of horsepower, which is broadly equivalent to air volume. Rotary screw compressors are available with outputs from 5to 600 horsepower and more. Kaishan Compressor USA is one of the largest manufacturers of this type of compressor. Their efficiency and long run time make it the preferred choice for most industrial applications.
Choosing the Best Rotary Screw Air Compressor for Your Application
Start by deciding what’s important for the application. Factors to consider include:
1. Stationary vs. Portable
Portable compressors are used where air-powered equipment may be needed in different places, as well as electrical power is not readily available. Portable compressors can be diesel engine or electric motor driven. Portable compressors are commonly built on a chassis with wheels, but not always. Use caution when applying a diesel driven compressor as they often do not come with aftercoolers and can overload dryers with hot wet air – causing damage to downstream equipment and tools.
2. Indoor Use vs. Outdoor Use
A compressor used outdoors will encounter a wider range of ambient air temperature and humidity, and perhaps higher dust levels. Outdoor installation generally provides easier access to cooling and avoids the need for noise mitigation, but may require additional environmental protections.
3. Drive Type System
There are three options available, belt, gear or direct driven. Belts and gears provide versatility in manufacturing, but incur more transmission losses – lowering the overall efficiency of the air compressor system. Many compressors now come with the option for a variable frequency drive to better manage motor power consumption under varying load conditions.
4. Air Volume
Air delivery rate is specified in cubic feet per minute (CFM). As CFM is related to ambient temperature and atmospheric pressure, to enable comparisons, many specifications reference Standard CFM (SCFM).
This is specified in psi. Most air-powered equipment requires 70-90 psi. Be mindful of machines that require a higher pressure than 100psig – this means there are internal regulators.
While not correlating exactly with maximum CFM, horsepower is an indicator of compressor size.
7. Electrical Requirements
Although it varies by country, in the United States most industrial compressors require a 480 volt 3 Phase electrical connection, but 208 & 230V volt 3 phase can be found under 50 hp. And when you get down to the smallest of the rotary screw compressors: 5 & 7.5 hp units – you can find 230 volt single phase. Sufficient sized electrical distribution and breakers are required for reliable operation. Always consult your local licensed electrician.
8. Tank Size
The tank is the reservoir of compressed air. Systems where air demand can fluctuate widely need a larger reservoir to prevent excessive compressor cycling. There are many rules of thumb in regard to compressor sizing, you will hear anything 4-6 gallons per hp. But honestly, there are many factors to take into account when sizing a receiver tank – from budget, floor space, control type of compressors in the system, whether they are networked vs working off of pressure band, as well as the size of the compressors. You need the tank large enough, to keep a critical system pressure in your plant, while allowing your backup compressor to come online – if something were to happen to your primary compressor. Think of it like a water tank, you need your backup pump to spin up and start pumping water up to the tank before the tank runs dry. A compressed air audit (link to audit article) will help you better understand the demands of your compressed air system – in order to plan for such an event.
The price of a compressor is related to capacity and features. Note that over the life of a compressor, energy costs will be far greater than the purchase price. For this reason, consider paying more for higher efficiency. Don’t be afraid to ask for a preventive maintenance quote when looking at a new compressor.
Sizing the Rotary Screw Compressor
Three factors to consider are:
- Total volume of air required
- Compressor power source
- Compressor cooling
Volume of Air Required
This depends on how many tools and other pieces of equipment will be running at the same time. Most tools are designed for air delivered at 90 psi.
The following table indicates average air consumption for common tools:
Few of these run continuously. The average consumption figure given on the tool usually assumes a 25% duty cycle. When calculating the maximum volume of air needed, determine how many tools could be in use at the same time and use that to arrive at the total.
Many industrial facilities will have other pneumatic equipment not listed here. Some, such as conveyors, will run continuously while other items such as actuators may only cycle intermittently.
Compressor Power Source
Industrial users typically prefer electrically powered compressors over engine-driven alternatives. Electricity is the most efficient energy source and avoids the need to refill fuel tanks. Most factories will have 480V three-phase power available, which is what larger industrial compressors require. Situations where only 230V power is available will limit the size of compressor that can be used.
A byproduct of compressing air is heat; larger compressors generate more of it. To avoid maintenance and reliability issues, the compressor should be sited where there is ample room for cooling. A bigger unit will need a bigger space. Alternatively, consider using multiple smaller compressors in various locations.
Features to Consider
Take the following features into account when choosing a compressor:
1. Materials & Components
Check which manufacturer makes core components. Some manufacturers source everything externally while others, such as Kaishan, produce most themselves ensuring quality in each step of the process. Watch out for manufacturers who cut corners on what they feel are “non critical” parts. An example of this is the use of rubber hoses with an expected life of 1-3 years. A compressor system is only as good as it’s weakest component. Check the parts manual and see the recommended rebuild intervals and spares – these are usually indicators of how confident a manufacturer is on the life expectancy of their components. Some manufacturers will go as far as calling the airend ( the heart of the compressor) an element – so you relate it to an oil filter element or air filter element – which needs to be replaced on some interval. A quality rotary screw airend – if serviced properly – should easily last over 10 years.
Just like in the automotive industry controls can go from very basic, from pressure switch and gauges to PLC and microprocessor controlled. The more advanced the controls – the more features which can increase the life of the equipment – by being pro-active with maintenance and monitoring additional inputs that can protect the equipment. Advanced controls can also contribute to energy savings – by networking multiple compressors – making them work together like an orchestra conductor – vs the alternative where each are trying to respond to system changes independently.
The interface on the controller should be intuitive and easy to use – but also protect critical parameters from being accidentally changed. The latest generation of controllers even offer internet connectivity so that the compressor can be monitored via the web as well as send out alerts whenever a problem is detected.
3. Noise Reduction
Rotary screw compressors are quieter than other designs but don’t run silently. A large contributor to sound is the style of fan that is in the package – an axial fan (like the prop of an airplane) are usually louder, where as a centrifugal fan (like you find in most home air conditioning systems) are much quieter. Ask what noise mitigation features are incorporated and consider soundproofing the compressor enclosure or room.
4. Cooling System
In most rotary screw designs, the lubricating oil also carries away heat, with fins and fans playing a role. As discussed above the type of fan determines the amount of backpressure allowed on the cooling air of the package – a centrifugal fan allows for better ducting – if you need to remove the heat from the compressor room. Ask the manufacturer how it ensures the compressor is cooled sufficiently.
5. Safety Features
The compressor should have sensors and controls to protect against conditions that would shorten its life or create risks elsewhere. Ensure the compressor meets the required electrical certification like UL (Underwriter’s Laboratory). All moving components are properly guarded, so no accidentally contact is possible.
The choice of compressor is influenced by your industry. Those requiring a constant supply of air — perhaps for conveying powders or granular materials — will benefit from a rotary screw design. Packaging halls, and machining and assembly lines where air is needed all the time are other applications.
Some processes and industries need air only intermittently. Paint spraying, sand blasting and automotive repair are examples. Here, a reciprocating compressor may be a better choice for meeting fluctuating air demands.
Get Help With Compressor Selection
Kaishan manufactures a range of high-quality rotary screw compressors that meet industrial needs. Our product specialists can help you decide which one is right for your application. Contact us to start the discussion.