Truck Mounted Rotary Screw Air Compressors

• The first thing to do is determine the air needs. Pneumatic tools all require different amounts of air and these needs vary quite a bit, even within a single type of tool. This is why it is important to find out what the needs of the tool are.
• Air power is typically measured in two metrics: cfm and psi. Cubic feet per minute (cfm) is the amount of air that’s being delivered. Pounds per square inch (psi) is the amount of force behind that air.
• Most tools are rated to run optimally at 80 to 110 psi, so you’ll want to find an air compressor that can deliver the right cfm at the psi your tools require. The best way to determine your psi and cfm requirements is to review all the tools to be used and check with the manual or manufacturer.
  • If more than one air tool will be used at the same time, the cfm requirements of each tool will need to be added together to determine total cfm requirements. For example, if one tool requires 30 cfm and the second requires 20 cfm, a compressor capable of 50 cfm is needed (30 cfm + 20 cfm = 50 cfm).
  • There will be instances that an end user will want a compressor that provides a psi rating much higher than the tool being used for breaking force.
• A general rule of thumb is to factor a 25% increase above calculated cfm requirements to ensure the compressor is providing enough volume to overcome any inefficiencies while doing the task.
• Calculating cfm requirements is a challenge when inflating tires. The speed in which to fill the tire will be a critical item to understand as that will determine the volume, or cfm, needed. The maximum pressure needed in the tire will be the determining factor for the psi rating, or force behind the air, as the compressor will need to have a higher PSI rating than that of the tire rating.

The difference between cfm and psi are what they measure. CFM measures volume while psi measures pressure. CFM and psi are often used as performance specifications for air compressors. Together, they indicate the maximum air volume and pressure produced by an air compressor to power air tools.

The abbreviation cfm stands for cubic feet per minute, and measures the volume of air in cubic feet for each minute it moves. In the case of an air compressor, cfm indicates how much air can move per minute. For example, an air compressor’s output could be rated for 30 cfm, which means 30 cubic feet of air is flowing per minute.

The abbreviation psi stands for pounds per square inch, and measures how many pounds of pressure (force), is in an area, specifically in one square inch. The force of the air is what gives compressed air its power. For example, an air compressor’s output could be rated for 100 psi, which means that 100 pounds of pressure is delivered per square inch.

The relationship between cfm and psi is an important one that ensures the proper operation of an air compressor. For a tool to operate and perform optimally, both cfm and psi must be sufficient.

• Here is an example: Imagine you have a garden hose, and you turn it on. Water will flow out, and it might reach a few feet past the end of the hose, perfect for filling up your bucket or watering can. If you take the garden hose and restrict the space at the end of the hose with your thumb, to create less room for the water to flow out, the water will shoot out with much more pressure than before. Even though the hose is producing the same amount of water, the extra pressure will allow the water to travel faster and further.
• A second example: There’s a tunnel with wind blowing through it, and the tunnel gets smaller and smaller. As the wind blows into the tunnel, it travels through the shrinking space, and starts to feel stronger and stronger. This is because even though the volume of air (cfm) remains the same throughout the tunnel, the air is being squeezed into a tighter space, resulting in the pressure (psi) increasing.
• Both examples show the relationship between cfm and psi. While having a sufficient volume of air to power air tools is important, it’s also crucial to ensure there’s enough psi (pressure) to give the air power.

Tool cfm requirements give an idea of what a compressor will need to do but it’s not enough to simply match tool ratings with compressor ratings 1:1, what the air tool is and how it will be used matters. For example, during the use of an impact wrench:

• Will the trigger be held continuously while doing work?
• Or will it be used in quick bursts with short breaks in between use, allowing the compressor to catch up?

Questions to ask to help determine cfm needs:

• Are the tools used for lengthy periods of time? In this instance, a compressor capable of 100% duty cycle is going to be needed.
• Are the tools used for only a few seconds here and there? In this instance, a smaller, intermittent-duty compressor with an air tank may be preferred.

Duty cycle is often hard to understand as there are no universally identified characteristics to represent the values among compressor manufacturers. Simply put, a compressor duty cycle is amount of time the compressor will deliver pressurized air within a total cycle time. For example: 50% duty cycle equals 5 minutes of loaded time in a 10-minute window.

• Another consideration is how the compressor is cooled. Reciprocating air compressors are air-cooled so they cannot operate in a continuous duty cycle whereas rotary screw compressors are cooled via an air-cooled oil cooler.
• Reciprocating compressor duty cycle — 50% (per documentation in the manual).
  Rotary screw compressor duty cycle: 100%.
• Two-stage reciprocating compressors do not have a higher duty cycle than single stage versions.

The type of air compressor you will need all depends on the application.

Reciprocating compressors are best suited for applications that are shorter durations of use and don’t require high volumes (cfm) of air for the application.

• Pros: Less expensive investment, easier to service, lower maintenance costs, more forgiving to the environment (dust, moisture).
• Cons: Larger and heavier air ends, requires air reservoirs, intermittent duty, higher discharge temperatures.

Rotary screw compressors are ideal for applications that need air continuously or high volumes (cfm) of air for the application.

• Pros: 100% duty cycle (continuous airflow), higher cfm output, smaller and lighter air ends, lower discharge temperatures.
• Cons: More expensive investment, higher maintenance costs, less forgiving to the environment (dust, moisture), must be operated for a minimum of 20 minutes.

Reciprocating and rotary screw compressor applications:

• Reciprocating compressors are considered “bulletproof” for use in most applications in the service-body arena, especially those that do not need high volumes of air for long durations.
• The rotary screw compressor is a high-dollar investment, between acquisition and maintenance costs, so care must be taken in specifying an application.
  • A mechanic truck application that requires a high volume of air can be a good scenario for the rotary screw, as long as the compressor is allowed to operate longer than 20 minutes each time it is turned on.

Reasons why a rotary screw compressor might not be right for you:

• 100% duty cycle is not needed.
• Upfront purchase cost is too high.
• High airflow (cfm) is not required.
• Maintenance schedule adhered to, or paid for.

Reciprocating air compressors require the use of an air tank. The larger the air tank, the less work the reciprocating compressor must do.

Rotary screw compressors do not require an air reservoir, but:

• For certain applications, a reservoir is a good addition to a rotary screw compressor application to provide a surge of air at initial startup and ahead of the compressor building pressure. A minimum 10-gallon reservoir is recommended for this.
• It is beneficial if the compressor has been shut off but air is needed. If the compressor has not completed its blow-down process, relieving trapped pressure in the system, it cannot be turned back on right away. The air reserve could handle the quick need for air. This would mitigate the opportunity for compressor short cycling.

Important air tank functions:

• Dampens pulsations from the discharge line of a reciprocating compressor.
• Serves as a reserve of air to take care of sudden or unusually heavy demands for air in excess of what the compressor is designed to do.

Benefit to rotary screw compressors in certain applications; mechanics truck use.

• Prevents excess cycling of the air compressor. This is a detriment to rotary screw compressor effective operation.
• Foreign matter removal that may have bypassed the filtration system.
• Removal of moisture prior to the air reaching tools or equipment.

Standard cubic feet per minute (scfm) measures the air output using a standard that takes atmospheric conditions into account.

• The parameters are determined by the American Society of Mechanical Engineers (ASME) and are recognized across many industries. SCFM calculations are based on atmospheric conditions (standard conditions) to measure air mass flow from an air compressor.
• SCFM standard conditions include atmospheric pressure at sea level at 14.7 psia (760 mmHg), relative humidity of 36% and ambient temperature of 68° F (19° C).

Actual cubic feet per minute (acfm) is the true air mass flow given a certain set of real-life conditions and is impacted by atmospheric conditions.

• For example, an air compressor at high elevation will have a lower cfm output than the same compressor at sea level.

Quick calculation reference:

• For every 1,000′ (305 m) of elevation over standard conditions, acfm demand increases approximately 5%.
• For every 20° F (11.1° C) of ambient temperature increase over standard conditions, acfm demand increases approximately 5%.
• For every 20% of humidity increase over standard conditions, acfm demand increases approximately 0.5%.
• Example: A 1″ impact gun has a minimum air requirement of 45 cfm. The jobsite is at 1,211′ (369 m), the atmospheric pressure is 14.2 psia (732 mmHg), humidity is 24% and the temperature is 84° F (27° C). For a compressor to generate the needed 45 acfm in these conditions, the compressor needs to have an scfm rating of approximately 49 scfm.

This does not need to be done for most applications but needs to be considered when working with severe applications, like high-elevation scenarios.

Why does it matter?

• Considering acfm ensures the compressor will be sized to meet the scfm needed due to the environment it is being used in.
• An air tank will help mitigate the impact of the environmental factors due to having a reserve of air that offsets the acfm.

• Every air system has components in it that need to be properly sized to ensure there is no negative impact on the compressor performance.
• Air travels from the compressor to tools through multiple components such as air hose, fittings, filters, regulators, etc. and each of these can restrict the flow of air if not sized properly, impacting the magnitude of the flow.
• The cfm does not change passing through the air system, but the pressure does with restrictions. For example, a tool needs both airflow (cfm) and pressure (psi) to work properly. A long, small hose feeding a high-air-demand tool can cause substantial pressure drop due to the hose size being too small and creating a restriction. The result will be a compressor that is working harder to keep up with demand and if it can’t keep up, the tool performance will be reduced.
• A result of an improperly sized air system is the potential to order an air compressor that is technically larger than the application needs to overcome the system inefficiencies. An investment in properly sizing the air system components would be a better investment to size a compressor accordingly.

For most hydraulically driven air compressors, minimum chassis specifications are not needed as most chassis today have the ability to provide the hydraulic requirements needed.