Air compressors are an essential component of pneumatic systems. Their simple function of increasing air pressure levels while reducing volume makes them crucial mechanical devices in a wide range of industrial applications. Yet, many engineers and managers are plagued with specifying air compressors tailored to a particular application.

Wrongly specifying a compressor accounts for the majority of the problems that arise in industrial plants with compressed air systems. Undersized compressors cause insufficient airflow to production equipment, whereas oversized compressors cause increased energy consumption and operating costs.

This begs the question: what compressor size is necessary for a particular application? To answer that, there are some key terms engineers and managers must understand.

Understanding flow and pressure

Flow, typically measured in cubic feet per minute (cfm), describes the volume of air that the compressor can produce in a given amount of time and at a given pressure. In contrast, pressure describes the amount of force that the compressor can provide to perform a certain amount of work at any given point in time.

A simple example for understanding pressure in compressed air systems is to imagine moving a 1 lb wooden block across a table using compressed air. The compressor must provide the pressure (force) needed to move the block. So, if the minimum pressure required to move the block is 120 psi, but the compressor cannot provide it, the wooden block won’t move. In contrast, flow is the ability of the air compressor to continuously move the block for, say, a 24-hour period.

Step 1: Determine airflow requirements

Manufacturers of air tools usually specify the average cfm requirements for their products. Sum the cfm ratings of all air tools and equipment to be used simultaneously; this total cfm is the minimum airflow that the compressor must be capable of supplying.

It is important to consider only the air tools to be used at the same time and not the air tools that will be used throughout the workday. This will help to prevent obtaining an inflated cfm number and purchasing an overly large compressor.

Step 2: Compensate for leaks and future expansion

The totaled cfm in the first step is only the minimum cfm requirement. Air compressors are sized not only to supply the present air tools but also to compensate for leaks and future expansion of the facility. As such, it is advisable to increase the capacity by 25% to 50%.

Assuming the total airflow requirement was obtained as 100 cfm, then a compressor with an airflow specification of 125 cfm (25% increase) would compensate for leaks and allow for future expansion of the facility.

Step 3: Determine the duty cycle and needed cfm

The duty cycle describes how long the compressor would run during its cycle time. It is given as:

For example, an air compressor will have a duty cycle of 70% if its cycle time was 20 minutes, and it ran for 14 minutes during that time. The amount of cfm needed to specify the right compressor can then be obtained using:

So, assuming a total airflow requirement of 125 cfm and a duty cycle of 70%, the needed airflow would be 87.5 cfm.

Step 4: Determine the required pressure

Like the airflow requirements, manufacturers also specify the minimum operating pressure requirements for their air tools and equipment. List the minimum operating pressure requirements for all the tools in the facility.

Single-stage compressors are able to handle pressure ratings as high as 150 psi, whereas a two-stage compressor will be needed for higher operating pressure requirements. However, it is important to note that a multi-stage compressor typically will have a lower cfm rating. This is because, in a multi-stage compressor, the air is compressed up to an intermediate pressure and passed on to two or more cylinders.

A major advantage of the multi-stage compressor is its efficiency, and this is due to the fact that the air is cooled between stages.

Step 5: Determine the preferred drive system

The environment in which the compressor is to be used will determine the suitable drive system for the compressor. An electric motor is suitable for use in environments or job sites where there is availability of electric power or hookup of some sort, while a gasoline engine-driven compressor is more suited for job sites where electric power is not always available.

Note that electric-driven compressors are less expensive and easier to maintain compared to gasoline engine-driven compressors. Gasoline engine-driven compressors offer flexibility and portability. In addition, they operate continuously, making them ideal for work where continuous pressure is needed.

If electric driven compressors are to be used, then it is important that the voltage rating and power requirements are assessed. An oversized compressor will result in significant energy losses and profit loss.

Step 6: Determine the receiver (tank) size

Air tank, sometimes referred to as a receiver, play a crucial role in compressed air systems. It acts as temporary storage to accommodate peaks of demand from the system or plant. As such, it helps to reduce the frequency of compressor operations as well as the operating costs associated with excessive starting and stopping of the compressor unit.

The tank volume can be calculated using:

So, consider a scenario where a receiver must supply air to a pneumatic circuit consuming 15 ft3/min for 3 minutes between 120 psig and 100 psig before the compressor resumes operation. Solving for V in the formula, the volume of the receiver will be obtained as 33.075 ft3 (or 247.4 gal).

The diameter and length of standard receivers of a given capacity. Source: Compressed Air and Gas HandbookThe diameter and length of standard receivers of a given capacity. Source: Compressed Air and Gas Handbook

An appropriate receiver size would be 34 ft3, and it would have the dimensions 7 ft in length by 30 in in diameter. To allow for unexpected overload and future expansion of the facility, the size of the receiver should be increased by 25%. Hence, a receiver size of 42.5 ft3 would be satisfactory.

Even after specifying compressors, it is important to adhere strictly to proper practices and maintenance schedules. This will help users enjoy trouble-free operation and get the most life from the equipment.

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