Choosing the Right Compressor

Six Steps to Selecting the Right Size Compressor

In order to select a compressor unit that will handle their compressed air requirements, one must first determine what air volumes and pressures are required on site, and then choose a unit to suit. To follow is a ‘Six Step’ Method in selecting the right sized compressor for your customer’s air requirements.

1. Separate the equipment into two categories.

This should be based on both Continuous and Intermittent use. Make a list of all the air tools and equipment intended for use with the compressor. Hand triggered tools (air impact wrenches, etc.) are generally considered Intermittent. Air motors or sand blasters, on the other hand, create a steady demand on the compressor all the while they are operating, and can therefore be considered Continuous.

2. Determine the total CFM requirements for each type of equipment being used.

Using the chart on page 39, list the free air consumption for the items in each category and multiply the CFM required for each tool by the number of similar tools to be used.

Example: If there are two different stations, each using a 5-inch body sander, the total consumption would be 2 sanders x 5 CFM each = 10 CFM

3. Total the CFM for the items in both the Intermittent and Continuous categories.

Example:

Intermittent Use:

ItemCFMQtyTotal

1/2″ Impact414

Body Sander5210

Spray Gun10110

Total Consumption:24 CFM

Continuous Use:

ItemCFMQtyTotal

3/32″ Sand Blaster717

Total Consumption:7 CFM

4. Combine the total consumption for Intermittent and Continuous Use.

For Intermittent use, the total consumption is generally multiplied by a factor of .40 to reflect an approximate 40% intermittent use of the various tools. Using our example in ‘3’, 24 CFM x .40 = 9.6 CFM

For continuous use, you must use the full value of the various tools. The total consumption for the compressor would then be as follows:

9.6 CFM (Int.) + 7.0 CFM (Cont) = 16.6 CFM

Note: If a number of intermittent air tools are to be used simultaneously in a continual demand situation, replace the 40% (.40) in the formula with a figure that better represents the actual percentage of operation, i.e. 60% (.60) or 70% (.70).

5. Divide the ‘Total Consumption for the Compressor’ as determined in ‘4’ by .75 .

In order to allow the Compressor to cool properly, we must allow for downtime between cycles. The dividing of the Total Consumption by .75 allows for this.

6. Compare the total CFM requirement with the compressor manufacturer’s specifications.

Ensure that the maximum output pressure of the compressor Unit is sufficient to handle any of the tools used. In an instance where two tools are used by the same operator, but never at the same time, list only the tool with the higher air consumption. Adjusting the formula to suit individual shop requirements is reliant on a first-hand knowledge of the total shop operation. If care is taken to include all air consuming equipment, a good assessment of the total consumption can be made.

On-Site Electrics

Air Compressors are available in a variety of electrical voltages and in both single and three phase units. It’s important to know the shop’s electrics to choose the correct compressor to suit.

Electrical Installation Tips

1) Though single-phase units are available, use three phase units when on-site electrics allow. Single-phase units require much more power (and are therefore more costly) to both start and operate.

2) Single-phase units are normally only available in small and medium power units. If you must use single-phase power and need more output, use multiple compressors.

3) Over the course of a day, on-site voltage may vary depending on demand. Motor manufacturers are aware of this, and allow a variance of the nameplate motor voltage +/- 10%.

Example:

Nameplate voltage: 460 volt

Maximum allowable: 506 volt (460 + 10%)

Minimum allowable: 414 volt (460 – 10%)

When choosing a suitable compressor unit for certain on-site electrics, ensure that the building voltage (at the unit) is within the +/- 10% of the motor voltage variance as noted above.

Example:

On-site voltage: 208-60-3 (208 volt, 60 cycle, 3 phase)

Suitable Motor: 200 volt (200 + 10% = 220 max allowable)

Note: Though a 230 volt motor will handle the 208 on-site electrics, should the building voltage drop below 207 volts (230 – 10%), premature Motor failure could occur. Use a 200-volt motor.

Motor failure due to incorrect on-site voltage is normally not covered by the manufacturer’s warranty.

4) All electrical work should be carried out by a licensed electrician, and done in such a manner that it meets all applicable Codes and Regulations.

Should you have any questions concerning electrical componentry required, local Codes or Regulations contact your electrician, local governing authority, or the compressor manufacturer.

5) Many compressor units require a magnetic starter to provide overload protection to the electric motor. Starters are often bought separately but can be ordered from the compressor manufacturer.

6) Check with you local power authority to determine if you need special approvals or site inspections for your installation. Inspections required on site by a local authority are normally done at the customer’s expense.