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How Do Air Compressors Work? - E234B - 09-18-2021
How Do Air Compressors Work?
How Do Air Compressors Work?
Air compressors are an invaluable tool for both industrial work and DIY at home, and there are several different types to choose from depending on the job you need doing. Air compressors have a number of uses, such as to fill gas cylinders for industrial purposes and scuba diving, to create the power needed to run pneumatic tools and spray guns, for pumping up automotive tyres, and within heating and air conditioning systems.
As we’ve touched on here, there are myriad uses for screw air compressor both in commercial and domestic environments. Within the category of air guns, there are several types, each of which is suitable for a different job. We’ve compiled a guide to all the major types of air compressor, how they work and how they differentiate from one another.
Whether you’re an engineering manager or in charge of facilities for your company, being informed about how air compressors function and what they’re used for is handy and can help you make the proper decisions for your business and industry.
Get all the information you need to know about air compressors, complete with the infographic below, with our comprehensive guide. We’ll address the benefits of using premium air compressors in your industry, as well as the questions of ‘what are compressors?’ and ‘how does a compressor work?’, covering all the essentials in one convenient place.
The infographic below shows how a Hydrovane series rotary sliding vane air compressor works
Since their invention in the 19th Century, mechanical, automated air compressors have continued to be one of the most widely used tools in industrial settings. Air compressors provide a continuous stream of power that is safer and cooler than many other forms of energy. For many industries, such as metal work and mining environments, air compressors are an absolutely essential tool. After the basic utilities of water, gas and electric, compressed air is actually considered to be the fourth utility.
Air compressors are also an affordable choice of tool for many manufacturing jobs, as they are durable, and high quality types require minimal maintenance and repairs.
Between the two main categories of compressor – the scroll (piston) compressor and the rotary screw (reciprocating) compressor, you have a tool for every type of industrial and commercial setting, as well as various domestic uses.
The most common types of regular screw air compressor are single and dual phase, both of which operate in the same fundamental way, only dual phase has one more step involved in the compression process. In a single phase compressor, there is one chamber and the air is compressed a single time; in a dual phase, there are two chambers and the air is put through compression twice.
Be careful not to confuse single and dual phase compressors with the number of cylinders a compressor has. Both types of compressor use two cylinders; one-cylinder compressors are less common, because air balancing is made easier with two cylinders. The difference between single and dual stage compressors is that in the former the cylinders are both the same size; in the latter they are different sizes.
How Do Single and Dual Phase Compressors Operate?
Single phase compressors, also referred to as piston air compressors, works in a relatively simple and straightforward way. First, air is drawn into the cylinder; from here, it is compressed once by a single piston movement within a vacuum system.
The power of this compression is measured in PSI (pounds per square inch) or Bar – the higher the PSI/Bar, the more power the compressor has. In a single stage air compressor, the air is typically compressed at a rate of around 120 PSI (8.2 Bar). After the air has been compressed, it is sent into the storage tank from where it is dispelled into various tools as a source of energy.
Dual phase compressors operate the same way, except there are two stages of compression, rather than just one. After the first round of compression, the air is sent into a second chamber, where it is compressed for the second time, at a rate of around 175 PSI (12.1 Bar). After this, the air is sent to a storage tank in which it is cooled down and ready for application.
Both types of compressor are typically powered by either an electric or petrol motor, which drives the piston and causes the compression to happen.
Single Phase and Dual Phase Applications
Both function in fundamentally the same way and can be used for similar tasks, such as operating a pneumatic drill or other high-powered tools such as those found in a manufacturing plant.
Single phase compressors tend to be used within domestic settings for smaller workshop jobs done with handheld tools, such as woodwork, metal work and general DIY.
Dual phase compressors, on the other hand, are better for larger scale work in operations such as operations needed in vehicle repair shops, pressing factories and other plants where parts are manufactured.
Oil-Free vs. Oil-lubricated Air Compressors
Another way to compare variable frequency air compressor is to look at whether they use oil or not – there are oil-free and oil-based / lubricated compressors and both are suited to slightly different jobs. For the air to be drawn into the chamber safely and effectively, the piston needs to be in top working order. To work properly, the piston must be lubricated with oil.
With regards to lubrication, there are two main types of compressor to choose from: oil-free and oil-based. The oil is used on the cylinder to ensure the compression goes smoothly.
The Difference Between Oil-Free and Oil-Based Compressors
Oil-free air compressors already have a lubricated cylinder (often with a non-stick material such as Teflon) and therefore require no further maintenance to work properly. Oil-based compressors require oil to be added to the piston area and changed regularly. Just how often you need to change the oil will be outlined in the manufacturer’s manual that came with your compressor.
On the whole, oil-free compressors tend to weigh a lot less than oil-based compressors, as not only do they not have the weight of the oil, but they are more compact machines, requiring fewer separate parts to make them work. Oil-free compressors, being less complex in design, also tend to be more affordable than oil-based compressors.
However, although they’re more weighty and expensive, oil-based compressors have their benefits. For one thing, they are strong and durable, and usually have a longer lifespan than their oil-free counterparts. This is usually because over time the greasing material (usually Teflon) begins to wear down and lose its lubrication abilities.
Another important factor that should be considered when choosing between an oil-free and oil-based compressor is that the oil-less version tends to heat up faster and to a higher temperature than those which use oil. Compressors without oil also make a lot more noise than those with, so if you want a less noisy workplace, this is a factor to consider too.
Oil-Free and Oil-based Compressor Applications
Oil-free compressors are a great option for those in need of a lightweight, low maintenance tool for home use. Oil-based compressors are better suited to heavy duty jobs and commercial and industrial use, as although they’re generally heavier and require more maintenance, they are also more robust and versatile.
For industrial purposes and extensive, day-long use, oil-based compressors are by far the best option. If you’re looking to invest in quality compressors for your business, opting for oil-based machines is almost certainly the best route to take.
Within an industrial or commercial setting, there are numerous uses for oil-based PM screw air compressor, including:
Vehicle painting and repairs
Sanding and woodwork
Creating snow banks in ski centres
Tools within dentistry and other medical environments
Pneumatic construction tools such as nail guns
Air cleaning tools such as blowguns
Oil-free compressors can be used for domestic use, such as small-scale jobs like blowing up balloons, home workshop and DIY jobs. They are also largely used in industries where there is a need to avoid the product or consumer coming into contact with oil: food and beverage, pharmaceutical and dental, for example. In these sensitive applications, the consequences of having oil contamination in the air are too high to risk, so having an oil free compressor is a must. There is compressed air quality testing from the International Organisation for Standardisation (ISO) which oil-free technology can help you achieve.
Fixed vs. Variable Air Compressors
Another factor to consider when choosing the right type of air compressor for your industry is whether they’re equipped with fixed or variable speed. Let’s take a closer look at what these different types of compressor can do for you.
The main difference between air compressors that use a fixed speed system and those that use variable speed is the manner in which the motor gets its power. The compression element is much the same across all machines, but the way the motor operates has an effect on the usability, efficiency and lifespan of the machine.
How Do VSD Compressors Work?
Variable speed compressors (often referred to as VSD (Variable Speed Drive) or VFD (Variable Frequency Drive) – compressors) operates by automatically adjusting the motor speed in accordance with the demand for air. This happens through a system that converts voltage from the mains power supply into a variable frequency.
Power is drawn through a converter, inside which it is converted twice. First, it converts AC power into DC power using diodes. A capacitor then cleans the AC, and then converts it to DC using a transistor, which acts as switches. These switches control the frequency of power sent to the motor, which in turn controls the speed of the motor.
A VSD air compressor contains this technology, allowing the speed of the motor and the amount of air compression used to be closely controlled. There are pros and cons to both variable speed compressors and their counterpart, fixed speed compressors – check them out below to decide which one is best for your industry.
Fixed Speed Pros and Cons
Fixed speed air compressors send a consistent, continuous stream of power into the motor, which gives you a reliable frequency in all your air compression jobs. The initial cost of fixed speed compressors is lower than that of variable speed compressors, is easier and cheaper to maintain and is a must-have within industries where the power demand is continuous.
This type of compressor is, however, less efficient than its variable speed counterpart. It is also less efficient at saving energy, and can therefore be more expensive to run, with fewer incentives available too.
Variable Speed Pros and Cons
Variable speed air compressors allow you to control the voltage and the frequency of the power in the motor, giving the user more agency over the tool. This type of compressor is more efficient when it comes to power usage, as you can easily control your output and only use what you need. Many industries will find that this type of air compressor is ideal, as it can be used in line with the demand of the job. When less power is needed, you can easily adjust the speed of the compressor, saving money and protecting the environment at the same time.
The downsides of a variable speed compressor are that there is a higher initial capital cost to pay, and maintenance and repairs are more expensive too. They are also not well suited to applications that require a continuous stream of power.
Piston Compressors, Scroll Compressors & Rotary Screw Compressors
So far in this article, all the compressors mentioned are operated using pistons, which is a generic system for one type of compressor – the scroll compressor, also known as a reciprocating air compressor. The other type is the rotary screw compressor, which doesn’t have pistons and operates in a slightly different way.
Here’s a quick overview of how scroll compressors and rotary screw / reciprocating compressors work, and what the unique benefits are of each of them.
Scroll Compressors
Scroll compressors are a type of piston compressor, and are also called reciprocating compressors. These are the most common type of two stage air compressor, due to affordability and general availability. But how does a reciprocating air compressor work? The piston system works by having a piston travel downwards, decreasing the pressure inside its internal cylinder through the creation of a vacuum. The sudden change in pressure causes the door of the cylinder to be forced open, and draw air in. When the piston travels up again, the air is forced out of the cylinder at a much higher pressure point. This continues in a reciprocating, ‘scroll’ pattern, hence its name.
Pros and Cons of Scroll Compressors
Pros:
Cools down quickly
More efficient use of energy
Cons:
Higher initial capital cost
More complex parts – harder to maintain
Rotary Screw Compressors
Rather than using pistons, rotary screw compressors rely on rollers to do the compression. Rollers are positioned just off the middle of the central shaft to ensure that one side of the roller is always in contact with the wall. The rollers are powered, rotating extremely fast, causing the same result as scroll compressors: they draw in air at a low pressure and force it out at a higher pressure.
Pros and Cons of Rotary Screw Compressors
Pros:
Good power capacity
Lower initial capital price
Easy to maintain
Cons:
Limited cooling abilities
Require continuous maintenance checks
How Do Low Noise Air Compressors Work?
One complaint often heard around air compressors is that some of them are noisy due to friction and a power source, which can be distracting and inconvenient for both commercial and domestic use. The industry has risen to deal with these complaints however, and there are several low noise air compressors to choose from, offering a decibel rating of around 40 dB.
40 dB is considered low noise; 60 dB is considered the safest decibel level for compressors; anything over 85 dB could cause long-lasting damage to your hearing, and you should avoid being exposed to this level for extended periods of time.
Low noise and silent air compressors are available in both oil-free and oil-based versions, so just how is the silencing effect achieved?
As well as choosing compressors with low decibel levels, one aspect of this technology that makes a big difference is the inclusion of an acoustic chamber for containing the noise. Another tip is to opt for electric-powered compressors, rather than gas-powered, as the former type of power source makes for a quieter machine.
The Bottom Line
Which type of air compressor you go for depends on a variety of factors. You first need to consider what application the compressors will be used for? Do you need it for continuous, ongoing use in large industrial machinery, or will it be used sporadically? Your answers to these questions will help you decide on whether to go for variable or fixed speed compressors.
Another question that you must factor into your decision is: how much do you want to spend? Remember that the initial price of a quality compressor is not the only expense associated with a compressor; consider also the cost of the power needed to run the machine, plus any related ongoing maintenance and repair costs. How much time and energy will investing in high quality compressors save your company? These are all important calculations that should be made before making your decision.
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