Screw air compressors (SACs) are famous in manufacturing, process, or construction industries because of their high efficiency, long life, and maintenance-free operation. The screw air compressor is a desirable alternative to generic reciprocating compressors. In addition, these screw compressors are the only compressor type employed for compressing air, refrigeration fluids, and other gases. Screw air compressors are not only adaptable to any industrial need, but they are efficient and reliable. As a result, the current high demand for SAC has made its market flourish many folds.

In this article, we will

• Review the two basic types of air compressors: positive and dynamic displacement compressor
• Review how screw air compressors work, their benefits and industrial applications
• Discuss different types of screw air compressors along with their pros and cons.
  • oil-free screw air compressors
  • lubricated screw air compressors
• Answers frequently asked questions related to screw air compressors.

What Is an Air Compressor?

The compressor automatically collects air from the environment (inlet supply) at atmospheric pressure and converts it into high-pressure air (outlet supply) through some mechanical action. The outlet compressed air is either stored or distributed to end-use applications through cylinders or piping. 

Air compressors are necessary for any manufacturing, process, or construction industry. For example, a large chunk of industrial pneumatic machinery depends on an efficient compressor one way or the other. It is a must-have tool to accomplish blowing, stirring, and sorting. Air compressors can also act as a power source for air-driven tool actuators. 

Compressed air demand of any facility can be continuous or intermittent. Continuous air demand is called the base load while varying intermittent demand represents the air demand variance. 

Powering an air compressor constitutes almost 25-30% of the energy usage by a manufacturing facility. 

Therefore, selecting and choosing a compressor that saves recurring energy costs and reduces downtime and maintenance are the prime objectives when investing in compressors for any facility.  

Types of Air Compressors

There are two basic types of air compressors positive displacement and dynamic displacement type. The positive displacement compressor (PDC) increases the pressure by reducing the fluid volume. In contrast, the dynamic displacement compressor (DDC) increases the fluid pressure by transferring the energy into the fluid by impeller action.

Centrifugal compressors achieve compression by collecting air in a high-pressure chamber while an internal impeller accelerates the air at high speed. Although DDC can achieve very high pressure but PDC are preferred and widely used because of their simplicity, efficiency, and reliability.

Principal of Positive Displacement Compressors

The volume of the intake air in all the positive displacement compressors is gradually reduced by reducing the volume of the container cavity, thereby increasing the air pressure and potential energy.

For any production facility, a large chunk (usually 15-25% or more) of energy is used to generate compressed air. Therefore, energy-efficient compressors are always in demand.

In addition, modern positive displacement compressors offer high-efficiency air compression with less downtime compared to other alternatives. PDCs have shown impressive acceptance in the process and construction industry because of their efficiency, reliability, small footprint and flexibility.

The most widely positive displacement compressors are:

1. Reciprocating air compressors
2. Rotary screw air compressors

Compressing the air by the repeated movement of a piston within the cylinder forms a reciprocating compressing action, which is suited for applications requiring an intermittent and low-throughput (less than 6000CFM) of compressed air.

For continuous air supply, centrifugal or rotary screw compressors are preferred.

Screw Air Compressors (SACs)

How does a Screw Air Compressor work?

A pair of helical screws (or rotors) intertwine to draw the air into the small cavity between the screws. The male and female rotors (screws) are turned in the opposite direction, reducing the cavity’s volume while developing pressure with each turn. As the cavity volume decrease, the trapped air starts to compress and gain potential energy. The screw length, pitch, shape of its discharge port, and rotation speed are intuitively designed to achieve a particular pressure ratio. Modern screw air compressor adapts the working pressure according to the designed pressure ratio, thus achieving high efficiency.

Furthermore, the mechanical assembly of dual rotors allows the rotary air compressors to operate without suction and discharge valves. As a result, rotary air compressors have a much more comprehensive operating range, thus making them suitable for various applications.

Why should you invest in rotary screw compressors?

1. Energy Saving & Efficiency

Energy saving is the prime objective of any industrial process; therefore, choosing screw air compressors can provide additional benefits and energy efficiency.

Moreover, unlike piston compressors, rotary screw compressors can operate for continuous and long hours without generating much heat. Therefore screw compressors are preferred since a compressor with an unlimited duty cycle is preferable in industrial processes to achieve seamless production targets.

In addition, because of low mechanical losses, rotary screw compressors are more efficient and reliable than piston compressors. The screw air compressors induce low mechanical losses and retain their capacity or throughput over an extended period, thus having a long life.

Moreover, installing the screw air compressors with variable speed drives (VSD) with a permanent magnet motor system saves even more energy costs compared to a fixed speed compressor. The flexible operation of a rotary screw compressor with VSD allows the compressor speed to slow down if the compressed air demand is reduced or vice versa. However, the compressed air flow rate remains constant, so the compressors’ running costs can be saved during reduced production demands or breaks.

2. Small Footprint & Low Noise Vibrations

Furthermore, since there are no inlet/outlet valves or impeller-based compressing, the screw air compressors are immune to vibration noise. As a result, they can operate at significantly high speeds, thus generating more compressed air for small dimensions compared to the generic piston-based compressors.

Rotary screw compressors are compact machines, and their footprint can even reduce with “full-feature” variants, having an inbuilt refrigerant dryer making the dewpoint at the outlet to +4°C.

Modern screw air compressors are available in a power range of 2-500 kW and can be customized to advanced designs, e.g., close-coupled or vertical configurations.

Another advantage of oil-less SACs is that the output compressed air contains very little oil (<3 ppm or less), which is suitable for many critical industrial processes.

3. Reliable with Minimal Maintenance Necessary

In addition, compared to other compressors, the implicit design of rotary air compressors consists of a fixed and straightforward gear drive and motor assembly. So, there is no requirement for frequent mechanical adjustments. As a result, the compressor has significantly less wear and a maintenance-free operation.

In addition, rotary air compressors are inherently safe to operators as no inspection is necessary during an extended period.

4. Instant & Continuous Compressed Air Production

Screw air compressors produce compressed air instantly by rotating the helical screws rapidly. However, rapid screw rotation raises the overall temperature of the mechanical assembly. Therefore, an internal cooling system may be required for temperature compensation.

The Advantages of Screw Air Compressors in Summary

The advantages of long life, low maintenance, and high efficiency make them an ideal choice for any industry requiring a screw air compressor in the power range of 5-500HP.

Some reasons to invest in screw air compressors for your industry are listed:

• High Efficiency
• Low vibrations and noise
• Low maintenance and high reliability
• Small size
• Long duty hours and long life (up to 80,000 hours)
• Low oil contamination in output compressed air (< 3ppm)

Rotary Screw Compressor Types

Rotary screw compressors are divided into oil-free screw air compressors (dry screw type compressors) or lubricated screw-type compressors.

The oil-free screw type rotors are synchronously driven through timing gears ensuring minimum clearance between the rotors.

On the other hand, in the lubricated-screw type, oil is the lubricant, and the primary rotor is the male rotor which drives the female rotor.

Oil Free Screw Air Compressor

The oil-free screw air compressors or dry-screw type air compressors do not need oil lubrication and have asymmetric screw movement. An external timing gear assembly is utilised to synchronise the rotors’ movement.

In addition, primary and secondary screws never come in contact with each other or their housing. Thereby eliminating friction, wear, tear, and lubrication costs. This mechanical setup results in low internal leakage, high efficiency, and reliability.

The outlet air pressure of a single-stage compression using an oil-free rotary screw compressor is limited because of the temperature difference between the inlet and outlet discharge. However, multiple-stage compression with interstage cooling can achieve a high-pressure discharge.

Applications

The common application of oil-less, oil-free or dry-type screw air compressors is in critical processes where even minor oil contamination cannot be tolerated, such as Food, Healthcare, Pharmaceuticals, and semiconductor manufacturing.

As a result, there is severe competition among rotary screw compressor manufacturers to capture the market. However, some compressor builders stand out for their reliable products. For example, the Japanese KOBELCO Air Compressor and ELGi Air Compressor are renowned compressor builders offering excellent sales service and spare parts.

Below are some of the oil-less screw air compressors available from these manufacturers.

Oil Lubricated screw air compressor

The primary male rotor drives the secondary female rotor, and there is no requirement for timing gear assembly to synchronise both rotors. The compression chamber of the lubricated-type screw air compressor is flooded with lubricant oil, which serves many purposes, i.e., lubricating, sealing the mechanical parts, and cooling the rotors and the mechanical assembly.

As a result, the outlet discharge temperature is lower than in oil-free screw compressors, while the air leakage from the compression chamber is also minimised.

In addition, the intrinsically cooled compression in the lubricated-type SAC allows attaining significantly high pressure (15-17 bars) through single-stage compression.

However, the low efficiency, high leakage, cooling system, and lubricating oil costs are some of the disadvantages of these compressors.

Applications

Oil-lubricated screw air compressors are utilised in standard industrial processes where minor oil contamination is not an issue. They can provide high flow throughput (CFM) with a 100% duty cycle.

Some of its applications are Pneumatic conveyor belts, paint production lines, and robotic and CNC manufacturing. KOBELCO Air Compressor and ELGi Air Compressor also build Oil lubricated screw compressors.

The following figures and the links to their products are given below.

How to choose your rotary screw compressor?

Making a prudent choice when selecting a compressor can be rewarding in terms of saving recurring energy and maintenance costs.

Positive displacement compressor choices are

• piston-based compressor
• oil-lubricated rotary screw compressor
• oil-less rotary screw compressor

The compressor selection primarily depends upon outlet airflow (cubic feet per minute), air pressure (bar), and air quality. Next, the efficiency, duty cycle, noise/vibrations, and cost are compared.

Finally, the operating environment (outdoor, indoor, offshore), ambient conditions, storage, and piping system are also important considerations.

Repair, maintenance and servicing of the screw compressor

Schedule compressor maintenance provides many long and short-term benefits. As a rule of thumb, the scheduled inspection and maintenance is after six months or 1000 hours of operation.

1. An unplanned compressor downtime is undesirable for any industry. However, preventive and scheduled maintenance significantly reduces downtime and saves repair and spare parts costs.
2. Efficiency is maintained over a more extended period for the compressors which receive scheduled servicing. For example, only a reduction few PSI of output pressure of a compressor raises overall energy consumption by 1%.
3. Longer life is guaranteed for a well-maintained rotary air compressor.
4. Regular maintenance of the rotary compressors enhances personnel, equipment, and facility safety.

Contact our specialist today to conduct your regular preventive maintenance.

Rental of Screw Air Compressors

Operating a screw air compressor for a limited duration for a one-time project can be costly. Therefore, Air to Air Engineering offers a rental service for the screw air compressor you require at a nominal cost. In addition, we provide excellent inspection, repair, and maintenance services. Some of the maintenance tasks for your rotary screw compressor can be delegated to us, which include:

• Inspection and repair of drive train assembly and belts.
• Control system inspection and adjustment.
• Oil inspection, recommendation, and replacement.
• Repairing inlet/outlet valves, filters, and piping.
• Servicing and lubricating motors
• Repairing and servicing the cooling system.
• Draining water and oil.

Eager to know more about our air compressor rental services? Contact us today, and our dedicated specialist will follow up with you as soon as possible.

FAQs

1. What is a screw-type air compressor?

Screw air compressors produce compressed air by the rotation of two helical screws rotating in opposite directions. The ambient air is trapped in the cavity between the rotors; as the rotors turn in the opposite direction, the volume of the cavity reduces to compress the trapped air.

2. What is a screw compressor used for?

Screw air compressors are essential industrial equipment. Depending on the application requirement, oil-lubricated and oil-less screw air compressors are used in process manufacturing, chemical plants, pharmaceutical, food, and semiconductor industries.

3. How does a screw-type air compressor work?

A pair of helical screws (male and female) are assembled to form a cavity. The volume of the cavity gradually reduces when the screws are turned in the opposite direction, thus compressing the air trapped in the cavity.

4. Should I choose a piston (reciprocating) compressor or a screw compressor?

Piston compressors provide higher air pressure ( >150psi ) with an intermittent duty cycle, while rotary screw compressors are recommended for applications operating under 150psi with a continuous duty cycle. Oil-less rotary screw air compressors are the only choice for critical pharmaceutical, food, and healthcare processes. Industrial mixing plants, robotics manufacturing, semiconductors, and CNC machines utilise screw air compressors extensively. In addition, screw air compressors are suitable for industrial tooling like blowing, stirring, cleaning, and sorting.

5. Do screw compressors need a tank?

Rotary screw compressors are versatile equipment capable of operating without a storage tank. The tankless option is preferred when the compressor operates in small bursts without wasting time in tank filling. Compared to piston compressors, the tankless operation of screw air compressors has a significant time and cost-saving advantage. However, the end user always has an option either to go tankless or with a tank.