Screw vs piston, how to choose compressor?

Update:17 Apr 2020

It is worth discussing whether the screw type or the pi […]

It is worth discussing whether the screw type or the piston type can make it work best in the equipment. First of all, we need to consider the similarities and differences between the two compressors in order to select the most appropriate one.
Similarities and differences between screw machine and piston machine
Compressor has been widely used in various gas fields, but air compression is the most widely used part. At present, the fixed screw air compressor accounts for about 40% of the market share, while the piston air compressor accounts for 21% of the market share (in USD).
Piston compressors are used in many different equipment. However, in the air system with a nominal pressure of 0.7-1.0mpa, the use of piston compressors is rapidly decreasing.
Of course, due to the progress of piston compressor technology, the demand for higher pressure gas equipment and gas compression equipment that can operate in harsh environment, piston compressor has become a feasible and wise choice in many equipment.

22kW or above
Screw compressors almost occupy more than 22KW, and most of the market share of the air system within the nominal pressure of 0.7-1.0mpa. This trend is driven by improved performance and reliability, reduced maintenance and lower initial costs.
Nevertheless, the double acting piston compressor is still the most efficient one. The shape of screw rotor reduces the high efficiency range of screw compressor, so the key factors of screw compressor are better rotor profile, improved processing and innovative design.
For example, a low-speed, direct driven screw compressor can provide air with an exhaust pressure of 0.7MPa, 0.13-0.14m, which is 90-95% of the double acting piston compressor. For most users, except for some occasions where the energy consumption is particularly considerable, due to its high initial investment (purchase price), the double acting piston compressor with higher efficiency is often not cost-effective due to its long investment payback period.
A well maintained screw compressor can provide 10 years or longer operation. At the same time, the control system with fault diagnosis and processing ability can point out the oil change clearance based on the operating temperature, and also improve the reliability and life of the compressor.

For maintenance costs, screw compressors have advantages over piston compressors. The maintenance cycle of double acting piston compressor is shorter than that of screw machine. The gas valve, piston ring and other vulnerable parts on the piston compressor need periodic maintenance.
The maintenance of screw compressor mainly includes oil, oil filter and oil content. Sometimes, it will cost a lot to replace the air and inspection equipment of screw rotor, but they can work for 10 years or even longer.
The standard screw compressor assembly has a microprocessor-based or electrically controlled controller. These controllers enable the screw rotor to maintain load for 100% of the time. One of the main functions of the controller is to regulate the air flow so that the machine can operate with maximum efficiency under full load, partial load and no-load conditions.
Some screw machine controllers have many other useful control functions, such as operation monitoring, shutdown warning and maintenance warning.
The unit equipped with a double acting piston compressor for good operation and maintenance is beneficial to operation. This kind of equipment can coordinate distribution and make use of regular repair and maintenance to become a successful compressed air system.

According to different lubrication conditions, piston compressors can be divided into two types: lubrication type and non lubrication type. In the lubricating unit, the lubricating oil is introduced into the compression cylinder to reduce the friction between the cylinder and the piston ring. In general, a well lubricated piston ring can last for several years. The application of advanced materials can prolong the service life of piston rings in dry-type units to more than 8000h.
The cost between lubricated and unlubricated piston machines is a factor worth considering. In some cases, oil-free compressed air or gas is required. The initial investment of the unlubricated unit is 10-15% higher than that of the unlubricated unit, but the difference between energy consumption and efficiency is not significant. The biggest difference lies in the maintenance cost of the two types of units. The maintenance cost of the unlubricated unit is four times or more than that of the lubricated unit.
The unbalanced force and large weight of the piston compressor are the main factors affecting the installation cost. Generally, the piston unit needs heavy base and thick foundation. Of course, the compressor manufacturer will provide the relevant data needed to build the base.
Although the initial investment and installation cost of the piston compressor is more than that of the screw machine, the service life of the piston machine under good maintenance can be 2-5 times longer than that of the screw machine.
For decades, the piston compressor has become a reliable heavy-duty machine. With the development of technology, the maintenance cost of piston compressor is greatly reduced while providing high quality air. In the unit with the nominal pressure of 0.7-1.0mpa, whether compressed air or other gases, piston compressor is still the best choice.

22kW below
Small air-cooled piston compressors can be traced back to the early 19th century. They are widely used in a variety of equipment, with a maximum pressure of 1.2MPa. All kinds of air-cooled units can adapt to the wild environment.
The most common small piston compressors are single acting. The exhaust temperature can reach 240 ℃, and the operating noise of most units exceeds 80dba.
For the low-power unit, because the initial investment cost is 40-60% less than the screw machine, the piston machine has a higher use value. Other auxiliary equipment, such as secondary cooler, starter and shutdown switch, shall be taken into account here, and these costs shall be included in the total price.
Small piston compressor can provide reasonable high quality compressed air for many equipments in a long life. Simple design, wide operation range and high reliability are their most important strengths.
Although the initial investment of screw compressors is more expensive than piston compressors, they are becoming more and more popular in the power range of 7.4-22kw. One reason is that screw units are usually packaged as modules. In most cases, the standard screw unit modules are packaged with starters, secondary coolers, and compressor controllers with capacity monitoring capabilities.
Screw compressors can also be used in smaller power ranges, from 3.7 to 22 kW. Under the same power condition, compared with piston compressors, they have the advantage of lower exhaust temperature. The screw compressor is designed to work under 100% load cycle, with low lubricating oil and high quality compressed air.

A small piston compressor needs to be equipped with an air reservoir. The air tank is used to store compressed air and reduce the load operation time of the compressor. Some small piston compressors usually operate for approximately 66% of the duty cycle time.
The service life of a piston engine with an air reservoir of sufficient size is particularly important. Regardless of the size of the air tank or the structure of the compressor and the air tank, the installation of the small piston compressor is always easy. Due to the unbalanced force, any piston compressor should be fixed on the ground.
Most screw machine modules are designed to be independent and movable, and their installation foundation can even be placed on the top of the air tank. The discharge of screw compressor has no pulsation. Nevertheless, the system including the air tank is beneficial to the air signal returning to the compressor controller smoothly and the stable operation of the system.
Small screw compressor can provide users with the whole box, which can be used in those compressed air systems that need constant air volume. The operating noise level of most closed screw units is lower than 80dba. The packaged screw machine can be easily installed on the floor, and usually only a single point connection device is used to connect electricity and gas.
Choosing the right installation space is very important for the reliability and long service life of air-cooled compressor. Good air flow through the compressor body is necessary for good operation and long service life of the machine.
Generally speaking, the compressed air quality of screw compressor is better. Even for oil lubricated screw units, the efficient oil-gas separator can reduce the oil content discharged into the compressed air system to 5ppm. At the same time, the inherent lower exhaust temperature of screw machine can further improve the quality of compressed air. The exhaust temperature of most screw units is only about 50 ℃ higher than the ambient temperature.
No oil, dry or lubrication
When users need oil-free gas or air, which compressor should they choose? The bearings and gears of oil-free compressor still have lubricating oil, but the compressor chamber is oil-free.
Oil free compressors are usually screw machines. Due to the lack of cooling, these oil-free units can reach very high exhaust temperature. New technology sprays water into the compression chamber to reduce the exhaust temperature and extend the life of the machine. The screw type oil-free compressor can be used in the equipment with the power from 14 to 180kW and the nominal pressure of compressed air less than or equal to 1.0MPa.
Dry piston compressor is mainly used in 0.7 ~ 11kw equipment. No lubricating oil is used in the whole dry compressor.

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