Reciprocating Air Compressors – An Overview of Reciprocating Compressors

Reciprocating air compressors are positive displacement piston type air compressors that suck in air into a chamber and compress it. They do so by decreasing the area of the chamber and increasing the pressure in the air with the aid of a reciprocating piston.

What is a Reciprocating Air Compressor?

A dictionary defines the word reciprocate as, the mean to move something alternately back and forth. A typical reciprocating compressor for home or industrial use will contain one or more pistons in one or more cylinders.

Reciprocating compressors contain cylinders with one closed end and a piston that moves inside them. The cylinders are connected to valves that can be located underneath the compressor’s valve pockets. A vacuum of sorts is created during the initial stage of compressing the air, this is done by air moving into the cylinders via suction valve which is initiated by the movement of the piston.

The piston inside the cylinder then reverses its motion, hence, the term reciprocating being used to describe them. This reciprocation starts the air compressing process, increasing the pressure within the cylinder. When the pressure has increased up to its design limit it then pushes the discharge valve open and allows the air to flow out of the cylinder and into the storage tank.

Main Parts of an Air Compressor

• Cylinder: The chamber in which the piston moves back and forth allowing the air to be compressed.
• Piston: The reciprocating pistons creates the air pressure in the system due to their reciprocating motion within the cylinder and are fully responsible for the compression of the air. These parts of the air compressor are under serious amounts of pressure when in motion and therefore require a very high quality lubricant that is also free of contaminates.
• Connection Rod: This is used to move the piston up and down inside the cylinder and they’re able to take a heavy workload. It is the connection between the piston and the crankshaft and they are often highly durable and therefore do not need replacing, unless proper maintenance is not conducted on your air compressor frequently.
• Crankshaft: The crankshaft plays an important part in transporting the the rotary motion from the electric motor or diesel/gas engines to the connection.
• Suction Valve: The suction valve provides the point in the air compressor where the air is brought into the device from atmosphere. These valves require regular inspection and servicing.
• Discharge Valve: The discharge valve allows for the air that has been compressed to flow out of the cylinder and in towards the storage tank

How Does a Reciprocating Compressor Work?

There are 5 key aspects of the air compression processor in a reciprocating compressor that must be understood. As already mentioned, these compressors tend to draw power from a diesel or gas engine via a belt drive or direct-drive system. These systems will run continuously as long as the engine remains on. We will take a look at them briefly to gain a better understanding of the process as a whole.

The Intake Cycle

The first process of the air compression is the intake cycle. This is where the piston is forced downward through the cylinder and reciprocates this motion. The motion itself provides a vacuum at the top of the piston and cylinder, this allows air of lower pressure to enter into the cylinder via a number of inlet valves that are located above the piston head as it moves downwards. The inlet valves remain open throughout this part of the cycle whilst the discharge valves are closed.

When the piston then moves back up the cylinder in its reciprocating motion, the inlet valves are forced into shutting so that all the air is trapped within the cylinder. The further the piston gets to the head of the cylinder the smaller the area that the air occupies therefore, leading to pressurised air. This air pressure then will exceed that of the air in the discharge valve causing it to open and allowing the pressurised air to exit the cylinder to an air receiver tank to smooth out any pulsations acquired during the compression process. This cycle then repeats itself over and over. Ensuring the discharge valve is closed blocks the air from moving back into the low-pressure section of the cylinder after it has been pressurised.

The Unloading Process

The pressure control/switch or sometimes referred to as the governor will sense that the air within the receiving tank has reached the threshold to cut-out and will therefore initiate the air compressor to unload. The governor is typically mounted to the compressor and is the direct mechanism for unloading working to enforce specific amounts of intake and cut-out pressure.

This unloading of the air could be the full amount or even just partial amounts, it all depends on the design of each reciprocating compressor. Now that the air is compressed in the system, the pressure level is gradually reducing and when it falls down to the pre-set load point a signal will be sent out by the control device to re-start the whole compression cycle.

The Duty Cycle

The duty cycle is a very important air compressors basic to ensure you understand them. The duty cycle is essentially how much of a certain time period is the machine fully-loaded/operating. For instance, 100% duty cycle in an 8 hour period would be the machine operating for 8 hour, whilst a 50% duty cycle would result in a 4 hour operating period. Reciprocating compressors are only designed for a 20 to 30% duty cycle and are unloaded or off the rest of the time. In comparison to rotary screw air compressors which operate at a 100% duty cycle.

It is very important to ensure that your compressor is not exceeding its duty cycle limits otherwise you will require maintenance early. The desire to maximise service life is very important, you do not want to push your compressor beyond its limits as it can be costly. Premature wear and component breakdown is certainly avoidable.

Lubrication

It is a possibility that in some but not all reciprocating air compressors that the driving engine pump will share its lubrication with the compressor to keep the whole system functioning properly. This will require manual adjustments from those recommended by the supplier if you require accurate gauges of the reduced lubricant life expectancy as it is being used not only in the engine but the compressor too.

Cooling

Cooling methods in reciprocating compressors are very important to ensure the longevity of the system and its components. Without cooling methods in place, parts are more likely to wear and the danger of exceeding any temperature limits is of course apparent.

The lubrication also acts as the primary cooling source for most air compressor systems. It does so by being cooled in the engine oil cooler and then recycled through the compressor. This engine oil cooler can also eliminate small amounts of heat from the compressor body, removing it from the system along with discharged exhaust air.

Types of Reciprocating Compressors

Depending on the application of your air compressor, you have a number of type possibilities.

Single-Acting

Single acting reciprocating air compressors only have a single side of the piston being used for the compression of air with the other side being connected to the crankshaft and not used for the compression. It is a very basic set up with one-way and spring-loaded inlet and discharge valves. For every turn in the crankshaft, one compression cycle is completed because the valves are only present at one end of the cylinder.

Double-Acting

In this form of reciprocating air compressor, both sides of the piston are used for the compression of air. Suction and compression takes place simultaneously on each stroke of the piston. These types of compressors have discharge and inlet valves and both ends of the cylinder, resulting in two compression cycles for every turn the crankshaft completes.

Double-acting air compressors tend to be extremely efficient, and thereafter are so prevalent within the manufacturing industry. They must operate at a high power which comes with a significantly large carbon footprint that of course, can not always be so practical. It can be difficult to find one that operates at under 100 horsepower, and this tends to lead to a high level of vibration production. Therefore, careful consideration into the placement of the machine must be undertaken which could provide limitations.

Diaphragm

Diaphragm compressors or otherwise known as membrane compressors, use a rotating membrane as such to pull air into its compression system. A diaphragm reciprocating compressor makes use of hydraulic and an air pressure systems with a protective flexible metal diaphragm barrier being between them. They are most commonly used to compress toxic or explosive gases rather than just air.

Single Stage

A single stage reciprocating air compressor uses just a single cylinder. In the first stroke of the piston, air is sucked in from atmosphere and then in the second, the air is compressed and delivered to the compressor storage tank. They are most commonly suited to powering handheld pneumatic tools that require less than 100 psi of pressure.

Double Stage

The double stage compressor works in a similar fashion to the single stage however, it does not make its way to the compressor storage tank after its first compression. Instead, it goes through a second stroke in a smaller piston to be compressed again. Once the air has twice compressed and pressurised, it is then put through the cooling system and sent to the compressor storage tank. Double stage compressors are typically found in large industrial applications as they can produce significantly more air power than other types.

Reciprocating Air Compressor Uses

Reciprocating air compressors are used across all sizes of businesses and industries. Not only that, but they’re very popular within everyday applications which most people may not realise. In your home, your refrigerator and freezer uses an air compressor to cool air within their units and if you have an air conditioner, they also use them as their main source of power for the cooling of your rooms.

The most common industries that utilise air compression are construction, manufacturing, agriculture, energy sector and HVAC (heating, ventilation and air conditioning). Some surprising uses of compressed air can be found in daily life that you may not have otherwise realised, these are in carpentry & furniture, pressure washing, crafting, pumping inflatable and yard work.

Some recreational activities also make use of air compression like paint balling, scuba diving and even within birthday parties for bouncy castles. But to switch the focus back to more business and industrial related uses, we will first take a look at small businesses.

Small Businesses

The following uses for air compressors are just a few of many applications within small businesses:

• Dental tools
• Medical tools
• Sandblasters within factories or plants
• Snow manufacturing for skiing/snowboarding
• Pneumatic drills, nail guns and hammers
• Air guns for cleaning machinery within factories or plants
• Spray painting for vehicles
• Wood or auto body sanding

Agricultural

The agricultural industry commonly uses air compressors for a variety of needs, some of these are:

• Conveyors moving grains between the silos and other destinations
• Spraying the crops with pesticides or fertiliser
• Powering varying types of material handling machinery
• Ventilation systems in massive greenhouses
• Powering different types of dairy machines from milking to transportation

Large Manufacturing Businesses

Air compressors can be found in large manufacturing businesses whether that be for fabrication, assembling parts, refinery and other plants. It is an essential source to power this massive operations. Some notable uses are:

• Cutting equipment
• Welding equipment
• Air tools and automated machines
• Component ejection from moulds
• Moulding gas tanks and plastic bottles
• Production monitoring devices
• Pneumatic finishing and packaging devices
• Positioners, air knives, clamps and chucks

Is a Reciprocating Air Compressor Right for You?

If you are in the market for an air compressor of any sort, spending time examining the huge variety and styles of compressors may not be productive if you do not know the answers to the following questions:

1. What is the flow rate you need for your air tools or to run your plant, now and for potential future expansion?
2. At what air pressure do you need that flow?

Having the answers to these questions upfront will help narrow the range of air compressors you need to look at. And then you will look for units with the following characteristics:

• A compressor that fit your budget
• One that fits into the area where you would like to install it
• Perhaps a compressor with the lowest up-front cost
• Or a style that has the lowest maintenance / operating costs
• You may want a very quiet compressor
• Or one that provides ultra-clean or breathable air

Once you have determined the parameters of your compressor needs, then you will be pointed in the right direction for the right kind of air compressor.