Unlike rotary compressors that are designed to run continuously without wear or damage due to overheating, piston compressors cannot run continuously for extended periods as their design has a lot of moving parts, and the cooling and lubrication system is basic. Piston compressors thus have a specified percentage of time they can run without causing any sort of damage to the compressor.  

What Is an Air Compressor Duty Cycle? 

To understand the air compressor duty cycle it is important to first know what cycle time means. When it comes to piston compressors – they run on a start/ stop cycle; meaning when the desired maximum pressure is reached the compressor stops and starts back only when it reaches a set minimum level. The time between one stop and start is the total cycle time. 

The duty cycle is generally represented as a percentage. It is the time duration a compressors’s top block runs to provide pressurized air at a defined flow rate relative to the compressor’s cycle time.  

Why Is Duty Cycle Important? 

The duty cycle is important as it helps to understand how efficiently a piston compressor functions by stopping and starting during use. It helps to determine if a given compressor model will fulfil the air needs of the application when sizing the compressor system. 

It is also important for ensuring the optimum life of the compressor, as exceeding the recommended duty cycle can prematurely wear out or damage the internal components.  

Duty Cycle Deciphered  

If a piston compressor’s cycle time is 10 minutes, and it runs for 7 minutes during that time, then it runs on a 70% duty cycle, which means 3 out of 10 minutes the compressor will be resting/cooling down during a cycle.  

A piston compressor that specifies a 100% duty cycle does not mean it can run continuously, but it can provide air at a specific pressure and flow for the time it runs (In the above– said case it is 7 minutes) with the help of a storage tank. 

Why should Duty Cycle be optimum?  

Heat is generated due to friction while the compressor is operating. Primary source, which is the friction between millions of air molecules while getting compressed makes compressed air very hot. The secondary source of friction are the moving parts inside the compressing unit, which raises the temperature of the cylinder as high as 150 to 200 Deg c.  

99% of the industrial piston compressors come with a basic splash lubrication system –here the oil properties play a vital role in ensuring proper lubrication. Excess heat generation deteriorates the oil properties thereby impacting the lubrication.  

The cooling system is not as sophisticated as what is available in other types of compressors. In a piston compressor, the cooling is majorly forced air cooled with fabricated or casted fan pulleys mounted on the compressor shaft and Copper or Aluminium pipes acting as coolers. These require non-operational time to ensure sufficient cooling time and avoid further heat generation between cycles. 

Effects of running at 100% Duty Cycle 

Some of the effects of running a piston compressor at 100% duty cycle are 

• Piston seizure in the piston housing due to excessive heat in the cylinder 

• Vales expand and jam with continuous exposure to a higher temperature, causing catastrophic failure of the compressor 
• Changes in oil properties lead to higher oil carryover 
• Continuous exposure to higher temperatures leads to higher condensate formation in the receiver tank thereby increasing moisture carryover to the downstream equipment and to the end equipment 
• Higher maintenance cost due to quicker wear and tear of internal parts 

Recommended Duty Cycle 

It is critical to know that the 100% duty cycle on a piston compressor does not mean that it can run continuously. Doing so can damage the compressor, resulting in premature wear and higher maintenance costs.  

Different brands recommend different duty cycles based on their design standards. The ideal duty cycle recommended for piston compressors by ELGi is a 70:30 duty cycle.  

Always choose a compressor that best suits the application requirement!  

SCUBA stands for “Self Contained Underwater Breathing Apparatus” and it is the paramount element of marine exploration.  

The major aspect that confines our time and experience underwater is the air that we carry in our SCUBA tanks.  Air, Ah!! It’s simple when we hear it as a term in itself. But when it comes to using air in SCUBA, we must understand its forms first.  

Right from our early schooling we have learnt that the air we breathe is a composition of nitrogen and oxygen majorly, as well as water, carbon dioxide, ozone, and many other compounds in small traces. But little did we know that the same atmospheric air is compressed, to utilize as a fourth form of energy in many industrial applications. The pressure range in which air is compressed depends on the use case. While for SCUBA, normal atmospheric air is compressed and filtered, so the divers can take more of it with on their underwater trip.  

Compressed air is the typical gas used for recreational diving, while Nitrox, Trimix and Heliox gas mixes are used for technical diving in deep waters. Divers use a pressure regulator (in their mouth-piece) to bring back the air from high pressure to atmospheric pressure (when at the surface) or the local underwater pressure, depending on the depth.  

While underwater, compressed air is utilized at varied levels which is a factor of the depth it is breathed at. For us to visualize – If you are a diver descending into the ocean, for each foot you will experience an increase in pressure by 0.44 psi roughly for sea water and this is a tad bit less for freshwater SCUBA as both the air pressure at the water’s surface, and the water pressure, will act.  

The scuba diver uses a mouthpiece with a pressure regulator to retrieve the air from high pressure to atmospheric pressure, or to the required pressure based on the depth. 

SCUBA tank filling air compressors are similar to the ones used to deliver air for firefighters and for oxygen tanks in hospitals but the vital difference is the pressure and compressed air quality output from the compressor. 

1. Pressure

For SCUBA, when compressed air is being used, based on the way air is carried, a low pressure or a high pressure compressor is chosen.  

a. Low-Pressure Compressors 

For surface supplied diving, low-pressure compressors are used. These compressors are lighter in weight compared to the conventional compressors so as to carry in flotation rings that float on the surface of water and they supply underwater divers with air via a hose. Typically pressure range for surface- supplied diving compressors range from 15 psi to 130 psi maximum.  

b. High-Pressure Compressors 

Divers carry compressed air in Aluminium or Steel tanks for breathing underwater, in this scenario air is compressed in ranges between 3,200 and 5,000 psi.  

The compressor’s Free Air delivery/ output flow is a factor of the tank size and number of tanks that needs to be filled. 

Compressed air quality 

For SCUBA it is mandatory that the compressed air is of breathing quality. A compressed air system can contain contaminants that affect the quality of breathing air from sources like compressor oil, system leaks, inadequate filtration and environmental variables. Hence filtration plays a crucial role. 

Mostly the filtration and moisture separation system is separated from the compressor system, hence downstream filters, moisture separator and dryers are preferred, to ensure the compressed air complies with breathing air standards. Apart from regular filtration, the compressed air is passed through a sequence of activated carbon filters that gets rid of hydrocarbons and carbon monoxide (CO).  

We must keep in mind it is important to choose the right compressor that inherently does not pass over excess moisture, oil, dust particles and other contaminants as excess moisture can cause overloading of the drier, and moisture separator. If left unnoticed, it may lead to deprivation of compressor filtration, fault in regulators, corrosion and oxidation of scuba tanks. While excess oil could increase excess CO, CO2 and hydrocarbon levels in compressed air. 

In order for humans to have a safe and fun underwater experience, making sure you have the right tank and air supply is essential for a successful dive. At ELGi, all our compressors are built with best in class features unique to our brand for keeping moisture and oil carryover as much within the limits as possible. We study the application in-depth before suggesting the compressed air system with the suitable air quality and right size.   

Schedule a call with an air expert here: https://www.elgi.com/in/contact-us/  

Author – Gopikrishna 

Having said that, the need for compressed air are many, based on various application in several industries. 

Selection of the right compressor can be a daunting challenge with several technologies and options available in the market. In this article, we attempt to simplify the process of selecting the industry’s most widely used air compressors; reciprocating and screw compressors for any given application.  

But remember, when in doubt, always reach out to our Compressed Air Experts! 

Factors to consider when selecting an air compressor:  

1. Quality of air 
2. Capacity of the compressor 
3. Duty cycle & utilization 

Quality of Air 

Industrial compressors have oil as the medium that cools, lubricates and seals gaps between the moving parts. As a result of this, compressed air is exposed to the oil and a part of it is carried over. 

In specific applications, compressed air comes in direct contact with products such as food and pharma, this is where oil free air is preferred.  

The quality of air based on the amount of this oil carried is represented by ISO 8573:2010. Class 3 allows for < 1 mg/m3 of oil carry over and a higher quality can be achieved through downstream filtration equipment.  

Depending on your application requirement, the selection of the air compressor will vary.   

Capacity of the Compressor Required 

The capacity of an air compressor is a function of volumetric flow of air and the pressure to which it is compressed. This also determines the power rating (typically in kW or HP) of the air compressor.  

Eg. A 22 kW (30 HP) air compressor can provide a flow of about 140 cfm (3.96 m3/min) at 7 bar g. (102 psi) pressure. 

Few ways to determine the capacity requirement of a plant is through the OEM recommendation, past equipment experience or through air audit. Air audits enable companies to get valuable insights into their plant’s compressed air utilization.  

ELGi’s air audits chart out a customized and comprehensive energy management plan for your needs. Reciprocating compressors are well suited for flow below < 50 cfm and for higher flow capacities, screw compressors are better suited, when comparing the two technologies. 

Duty Cycle & Utilization 

Duty cycle refers to the time a compressor is expected to run in a given hour at full capacity. Reciprocating compressors are designed to operate between 60-80% duty cycles and screw compressors are designed to operate at 100% duty cycle.  

Life cycle cost of a compressor constitutes Initial Cost + Maintenance Cost + Energy Cost  

(In a screw compressor, energy cost contributes to over 75% of the life cycle cost over 10 years’ operation time).  

It is critical to understand the life cycle cost of a compressor before making the purchase decision.  

For more understanding on the life cycle cost of a compressor, refer to: 
https://blog.elgi.com/knowledge-center/identify-the-real-life-cycle-costs-of-your-air-compressor/ 

The amount of time a compressor runs in a day influences the energy cost. While looking to select a compressor with optimal life cycle cost, follow these guidelines:  

1. When utilization is < 4 hours/day select a reciprocating compressor 
2. When utilization is between 4 and 8 hours/day, a belt driven screw compressor is preferred 
3. When the utilization exceeds 8 hours/day, a direct driven screw compressor is the best fit.

You can also use the product selector on our website, to match the products for your requirements.  

https://www.elgi.com/product-selector/