Compressed Air vs Air Blowers

Air Blowers and compressed air machines are the two main devices used by manufacturers to create high-speed air streams for industrial use.

Q: What is the difference between an air compressor and an air blower?

A: The main difference between compressors and blowers is the pressure ratio. Compressors operate at a high-pressure ratio, and blowers a low-pressure ratio.

Air compressors cram air into a small space which makes the air denser. Air blowers move air/gas with a moderate increase of pressure. The direction of the air being pushed by the blower can be changed by making changes to the blade angle of the blower.

Intro

Air Blowers and compressed air machines are the two main devices used by manufacturers to create high-speed air streams for industrial use. In contrast to air compressors, Air Blowers have a much lower pressure ratio than compressors, making them cheaper and more energy-efficient to run.

  • Typically, compressed air is used in applications that either require intermittent bursts of air, or air that has been compacted.
  • In contrast, Air Blowers are used in applications where air is required to be continuous, or when a high-velocity air stream is needed.

Industrial uses for air

Air, and the movement of air, is often an integral process in manufacturing. How devices that move air are defined and built depends on three key principles - the flow, which is the measure of air output usually measured in volume per unit of time, the velocity, which is how fast the air is moving usually measured in distance per unit of time, and the pressure, which is the measure of force applied on an area usually measured in pounds per square inch (PSI) or Bar (metric).

Pressure ratios

Blowers and compressors are devices commonly used to create pressurised air streams for manufacturing processes. In general, they are classified by measuring the pressure ratio, which is the outlet pressure divided by the inlet pressure. 

The main difference between compressors and blowers is this pressure ratio. Compressors have high-pressure ratio, and blowers low-pressure ratio. This is because compressors compact air into small spaces making it denser and highly pressurised, whilst blowers move air meaning that only a moderate increase in pressure is needed. This has implications for performance, as well as cost and energy usage, and should be considered when deciding the best air system for an application. 

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    Bernoullis Principle

    Bernoulli's Principle

    Bernoulli's Principle is a key principle in fluid dynamics that sets out the relationship between flow, velocity and pressure. It states that as the speed of a fluid, such as air, increases, the pressure on the fluid decreases  - and if the speed decreases, the pressure increases.

    How Air Blowers work

    What is an Air Blower?
    An Air Blower is a device that increases the speed and volume of air. Air Blowers come in different forms (regenerative blowers, positive displacement blowers for example), but the most common are centrifugal blowers. The air generated by Air Blowers is highly effective for cooling, cleaning, blow-off, removing/regulating moisture, and Air Blowers are known in particular for their energy efficiency. 

    What are the parts of a Centrifugal Air Blower?
    Centrifugal Air Blowers typically include fan housing, inlet and outlet ducts, driveshaft, drive mechanism, and crucially, impellers. Impellers are rotors that are used to increase air pressure and air flow, and in the Air Blower are powered by electricity to make them spin. As air enters the centre of the impellers it is divided between the impeller blades.  This process increases the volume of the air stream, and as the impeller spins, it accelerates this air using centrifugal force, and the high-velocity air is slowed/diffused in the blower housing to create pressure. Impellers are basically the opposite of turbines, which decrease pressure and extract energy from fluids such as air.

    Industrial Centrifugal Air Blower (SolvAir: PX 1500)

    Air volume
    Centrifugal Air Blowers therefore use the kinetic energy of their impellers to increase the volume of an air stream. In short, they work by transferring energy between their impellers and the air. When designing an Air Blower, engineers consider the angle, length and speed of the impeller blade rotation, as these variables control how much air moves through the blower, along with the air speed, and air volume.

    Air velocity and mass flow rate
    The impeller creates a vacuum type effect that pulls air through the impeller blades, pushing it out the other side. So, whilst the air velocity - the measure of the distance and speed air is travelling (usually defined in meters per second) - is fixed, the mass flow rate - the mass of air passing a point in the system (usually defined in kilograms/pounds per second) -  is not. This means that blowers can use lower flows to create higher velocity, and so operate at a low-pressure ratio.

    Local/Cumulative Pressure
    Pressure, and maintaining low pressure, is a key feature of Centrifugal Air Blowers. Blower systems are tested using two different pressure measurements - local pressure - the pressure at a specific point - and cumulative pressure - the total pressure from the entire system. 

    Cumulative pressure is used as an indicator of how well the blower is functioning. Components, such as piping or air knives, will increase cumulative pressure, meaning the volumetric flow rate of the blower decreases, and so this is considered by engineers when designing a system using an Air Blower. The cumulative pressure reading is also an effective way of finding faults - for example,  if there are blockages, cumulative pressure increases, and a total blockage will mean no air flow (see Bourneill's Principle). 

    In contrast to Air Blowers, compressed air machines operate under a high pressure to volume ratio, as they manage air through a different method.

    How Compressed Air works

    What is compressed air?
    Compressed air is standard, breathable air that has been reduced in volume, and is then kept at this smaller size in a pressurised chamber. Compressed air is typically used in two main ways: as an energy source, and as air that is used for blowing. 

    How is compressed air created?
    Compressed air is created by applying pressure - and then maintaining that pressure - to atmospheric air (air that is at “atmospheric pressure” - the pressure within the atmosphere of the Earth). This is done through the atmospheric air being trapped in a container, and then the space in this container being reduced. This then reduces the volume of the atmospheric air by reducing the space between the molecules in it, forcing them together (it is important to note that compressed air is made up as the same molecules as atmospheric air, just in a smaller space). Pressure is then maintained on the container to keep air in this reduced space.

    Air compressors
    The device that reduces the volume of atmospheric air, to create compressed air, is known as an air compressor. Air compressors are available in a range of functions, for example low, medium and high pressure, ranging from 150psi to over 1000psi (PSI - Pounds per Square Inch), 10 to 70 bar.

    Air compressors typically reduce the volume of atmospheric air in two ways - positive displacement, where the volume of air in a chamber is decreased, and dynamic displacement, where centrifugal force is applied to accelerate and decelerate air in a chamber. 

    A modern industrial air compressor

    Positive displacement air compressors
    Positive displacement air compressors typically use pistons to reduce the space in chambers. The most typical example of a piston positive displacement air compressor is a bicycle pump. Positive displacement piston air compressors typically use either a single-stage, where the piston compresses the air in one stroke, or a two-stage process, where the piston compresses the air and then moves it to a second, smaller cylinder, where another piston compresses it again. 

    Positive displacement air compressors are also available using a rotary screw design, where helical screws (one male one female) are used as rotors that mesh together to trap air, and then to reduce the volume of this air as it moves down through the rotors.  

    Typically, positive displacement piston air compressors use a crankshaft driven by an electric motor or gas powered motor, that is connected to a rod and piston, cylinder, valve head and air tank to hold the air. A pressure switch is activated when the tank reaches the pressure limit, and this stops the motor. 

    Dynamic air compressors
    In contrast to positive displacement air compressors, dynamic compressors compress atmospheric air by increasing the velocity, and then decelerating the flow to create pressure. They are sometimes called “turbo compressors” because of their ability to create highly pressurised air (such as in jet engines).

    Dynamic air compressors work at constant pressure where the air flow is continuous, using a rotating impeller design to transfer energy to the air. The most common type of dynamic air compressors are centrifugal compressors, where air is accelerated by one or more rotating impellers. 

    Centrifugal compressors are similar to centrifugal blowers, however centrifugal compressors run at higher speeds than blowers, and create greater pressure rises. 

    Pressured air
    In both positive and dynamic displacement, the air trapped in the chamber is pressurised. 

    The process of pressurising air forces the molecules within atmospheric air closer together, making them move more rapidly. This means that the temperature of compressed air is greater than atmospheric air, and that the process of compressing air generates heat. Compressed air can get so hot it becomes dangerous, and so a cooling system is often included in air compressor systems to deal with this. Aftercoolers also reduce the amount of water in the air stream, which is increased as a result of the air temperature rising as it is compressed.

    Oil or no Oil?
    Many models of air compressors use oil to cool or seal their pressurised chamber. This can be problematic as it means small droplets of oil escape with the ejected air, which can be highly problematic for some applications. Oil free compressors are therefore available, but they do not tend to last as long as air compressors that use oil, because of the wear and tear on the non-lubricated system.

    Air Flow
    Once compressed air is released from the pressurized chamber of an air compressor, as this additional pressure is no longer being exerted on the air to keep it compact, it quickly expands back to atmospheric pressure and is ejected from the chamber as air flow. The nature of this air flow is determined by the discharge pressure (Boyle’s Law). 

    Boyles Law

    Boyle’s Law

    Boyle’s Law states that when gasses (such as air) are pressurised, their volume contracts, and when you decrease the pressure, the volume increases.

    Typical use cases

    Overall then, air compressors deliver air with a high pressure to volume ratio, and Air Blowers deliver a larger volume of air with lower pressure.  This means that they tend to be used for specific and different applications by industry.

    Typically, compressed air is used in applications that either require intermittent bursts of air, or air that has been compacted. This includes pneumatic drills, power tools, air cylinders for automation, air brakes and sandblasting.

    In contrast, Air Blowers are used in applications where air is required to be continuous, or when a high-velocity air stream is needed. This includes ovens or furnaces, water management, removal or regulation on objects, creating sterile environments for research.

    Benefits / Downsides

    Due to the variance in pressure, with air compressors having a high-pressure ratio, and Air Blowers operating at a low-pressure ratio, both systems have implications for applications, cost and energy management.

    Application
    Cleaning & Drying Can be the best system if intermittent air bursts required. Concentrated air flow, able to run continuously at low cost and with energy efficiency.
    Blow-off Ineffective at blow-off for certain industries. Low pressure, high-velocity air flow suitable for blow-off applications.
    Contamination The cooling process needed to maintain compressed air means some fluid (oil and/or water) is ejected in the airstream, affecting drying applications. Generates clean air - no air or oil needed in the system, meaning expelled air is dry. Able to create “air curtain” protective barriers.
    Moisture regulation High cost to run continuously, difficult to ensure precision needed for moisture regulation. Air flow able to be easily directed, to create an “air curtain” laminar air flow that adheres to surfaces, meaning it can remove or regulate moisture on a surface as needed.
    Large products The size and energy requirements needed to deliver air over longer lengths are disproportionately large. Easily supports large product drying/blow-off/moisture and contamination control.

    "The kWh price of compressed air is ten times greater than the price of electricity so it makes sense to identify areas where compressed air could be replaced with a less energy-intensive alternative”

    Carbon Trust

    Maintenance & Efficiency

    Maintenance of air systems
    Compressed air systems also need a great deal of maintenance. For example, filters need to be regularly changed, bearings need to be greased, air/oil separators and heat exchangers/coolers cleaned, and drains and cooling water need to be monitored. A 3mm leak in a compressed air system will generate considerable costs over a year in wasted electricity!

    In contrast, Air Blowers require relatively little maintenance, primarily because no oil or water is required in the system. This means that Air Blowers are able to run without the need for human supervision or regular monitoring.

    Air system efficiency
    Compressed air systems can, however, still be the best solution for specific air needs. For example, if you need air to be blown intermittently, then air compressors provide the control to be able to perform this. For example, a solenoid valve can be installed on an air compressor system to start and stop the air supply. 

    However, if you need air to be blown continuously, then compressed air systems are typically an inefficient and expensive way to manage air. As a result of the process needed to compress air, a great deal of electricity is needed to compress, cool or dry, store and use it. It is estimated that the overall efficiency of a compressed air system can be as low as 10-15%. To address this, some compressed air systems try and recover the heat that is generated by the compression process, for example by seeking to use this heat for energy.

    Summary

    As a result of the precision, energy efficiency and low cost to run Air Blowers, many industries are now switching away from air compressors and are using Air Blowers for their applications. 

    Examples include where cooling, aspirating, mixing, inflating, drying, debris removal and cleaning are needed. For example, in the food and beverage industry, Air Blowers are now widely used to dry bottles and cans prior to labelling, as this eliminates all moisture and oil contamination and prevents spoiled goods. 

    Using Industrial Compressed Air For Blowing?

    Our free consultations and site visits often highlight poorly configured and inefficient use of compressed air (examples shown below) where Air Blowers offer both substantial long-term cost savings and increased performance.

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    What are Air Knife Systems used for?

    Air Blowers & Knives are amazingly versatile and are used for a wide variety of applications in manufacturing, such as:

    air-knife-drying-system-bottles

    Drying and removing liquid films from surfaces

    Controlling surface coating

    Blowing off dust and debris

    Conveying, holding down or cushioning materials with air

    Cooling/heating materials

    Providing air curtains/barriers