Kaishan USA | May 13, 2021 | Air Compressor Information
As they are intended for continuous functioning, rotary screw air compressors do not suit intermittent usage situations such as construction sites or hobbyists. They are best suited for industrial applications that need a constant supply of compressed air. However, there is no one-size-fits-all product. Instead, the compressor must match the demand for air. This rotary screw compressor buyer’s guide explains factors to be considered while choosing a compressor.
There are various ways of producing compressed air. Small systems can manage 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 as well as most common choice.
The heart of every rotary screw compressor is a pair of helical screws. Meshed together, these draw air as they turn, forcing the air into an ever smaller volume, until it is released as compressed air. It is 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 HP, which is broadly equivalent to air volume. Rotary screw compressors are available with outputs ranging from 5 to 600 HP and more. Their efficiency and long run time make them the preferred choice for most industrial applications. Kaishan Compressor USA is one of the largest manufacturers of this type of compressor.
Start by deciding what is 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 and electrical power is not readily available. Portable compressors can be diesel engine or electric motor driven. Portable compressors are commonly (but not always) built on a chassis with wheels. Exercise caution when using diesel driven compressors, as they often do not come with aftercoolers and can overload dryers with hot, wet air, ultimately 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, thereby lowering the overall efficiency of the air compressor system. Many compressors now come with the option for a variable frequency drive in order to better manage motor power consumption under varying load conditions.
4. Air Volume
Air delivery rate is specified in CFM. As CFM is related to ambient temperature and atmospheric pressure, several specifications reference Standard CFM (SCFM) for enabling comparisons.
This is specified in PSI. Most air-powered equipment requires 70-90 PSI. Be mindful of machines that require a higher pressure than 100 PSIG; this means there are internal regulators.
While not correlating exactly with maximum CFM, HP 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; however, 208 and 230 volt 3-phase connections can be found under 50 HP. When you get down to the smallest of the rotary screw compressors of 5 and 7.5 HP units, you can find 230 volt single phase. Sufficiently sized electrical distribution and breakers are required for reliable operations. 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 several rules of thumb in regard to compressor sizing; you will require at least 4-6 gallons of storage capacity per HP. But honestly, there are several factors to take into account when sizing a receiver tank, including budget, floor space, control type of compressors in the system, whether networked or working off a pressure band, as well as the size of the compressors. You need the tank to be 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 its capacity and features. Note that during the lifetime of a compressor, energy costs will be far greater than the purchase price. For this reason, consider paying more for higher efficiency. Do not hesitate to ask for a preventive maintenance quote when looking at a new compressor.
Three factors to consider here include:
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 tools 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 figure.
Several 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.
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 3-phase power available, which is what larger industrial compressors require. Situations where only 230V power is available will limit the size of the 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 situated where there is ample room for cooling. A bigger unit will need a bigger space. Alternatively, consider using multiple smaller compressors in various locations.
Take the following features into account when choosing a compressor:
1. Materials and Components
Check which manufacturer makes core components. Some manufacturers source everything externally while others, such as Kaishan, produce most themselves—thus ensuring quality in each step of the process. Watch out for manufacturers who cut corners on what they feel are “non-critical” parts. A compressor system is only as good as its weakest component.
Check the parts manual and see the recommended rebuild intervals and spares–these are usually indicators of how confident a manufacturer is about the life expectancy of their components. Some manufacturers will go as far as calling the airend (the heart of the compressor) an element, so that you relate it to an oil filter element or air filter element, which needs to be replaced at more frequent intervals. A quality rotary screw airend, if serviced properly, should easily last over 10 years.
Just as in the automotive industry, controls can go from very basic, with pressure switch and gauges, to PLC and microprocessor controlled. The more advanced the controls, the more features available to increase the life of the equipment. These features are proactive in maintenance and monitoring additional inputs that can protect the equipment. Advanced controls can also contribute to energy saving by networking multiple compressors; that is, making them work together like an orchestra conductor—instead of each one trying to respond independently to system changes.
The interface on the controller should not only be intuitive and easy to use, but also protect critical parameters from being accidentally changed. The latest generation of controllers even offers internet connectivity, that way 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 they do not 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) is usually louder, whereas centrifugal fans (like the ones you find in most home air conditioning systems) are much quieter. Analyze the noise mitigation features 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 in the cooling air of the package. Ask the manufacturer how they ensure 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 certifications, such as Underwriter’s Laboratory (UL). All moving components are properly guarded, so no accidental 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 needing rotary compressors.
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.
Kaishan manufactures a range of high-quality rotary screw compressors to meet industrial needs. Our product specialists can help you decide which one is right for your application. Contact us to start the discussion.
As they are intended for continuous functioning, rotary screw air compressors do not suit intermittent usage
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