One of the first questions people ask when painting their own car is how big an air compressor they’ll need. “Big” can mean anything from tank capacity to motor output to volume potential. And while each of these is important, it’s good to understand how one affects the others. Below is an explanation of each of these factors.
Airflow
Modern HVLP spray guns require a lot of airflow to perform at their best compared to older designs. The “HV” part of that term stands for high volume, which is measured in cubic feet per minute, or cfm. And that’s where you should start when looking at compressors. Most HVLP guns use between 6 and 20 cfm of air at a pressure of around 30-40 psi. At the bare minimum, your compressor should be able to match the performance needs of the gun and nozzle sert you’re spraying with.
Capacity
Beyond that, your storage tank must have sufficient volume to supply that airflow even when the compressor motor isn’t running. One cubic foot is roughly equal to 7.5 gallons. On the low end of the scale (at 6 cfm), an HVLP gun can consume as much as 45 gallons of air each minute. On the high end, that’s as much as 150 gallons, assuming continuous use.
Duty Cycle
Admittedly, you may not always spray in such a long, continuous flow. Nevertheless, the tank’s capacity is important for reducing the frequency at which the compressor runs. This is known as the duty cycle, expressed as the percentage of time the compressor can run in a 60-second span. A machine with an 80% duty cycle can operate for 48 seconds out of every minute, for example. Some compressors, like those that use a scroll-type design, can even run full time, or at 100% duty cycle.
The more frequently it runs, however, the hotter it operates. Excessive heat generates more moisture in the compressed air, and moisture in the lines can ruin a paint job. So clearly the less frequently a compressor runs, the less likely you are to encounter moisture issues and pressure variation. And that’s where storage capacity comes in.
Horsepower
Of course, to supply enough capacity and airflow while still running cool, you need a powerful enough motor to run it all. Naturally, that’s expressed as horsepower or hp. Depending on the overall specification of a given compressor, output can range from as little as perhaps 1/3 hp for a small pancake compressor to 7-1/2 hp for a larger industrial unit. For the most part, you won’t have much say in horsepower when choosing a compressor. That factor will be determined by a given machine’s capacity and airflow.
The Right Compressor for Painting
Choosing the ideal compressor for your painting needs will largely be determined by your painting equipment. Let’s assume your HVLP gun requires 10 cfm at 40 psi. Assuming you only have the trigger pulled 50% of the time, you’ll consume about 37.5 gallons of air per minute. If your compressor has an 80% duty cycle, you will need at least 30 gallons (80% of 37.5) of compressed air in reserve to maintain steady flow. This is effectively the bare minimum capacity most people will recommend for painting. And your compressor will be operating at its maxmium duty cycle, so you’ll need to manage moisture closely.
The best option is to use a compressor that has more than enough capacity to ensure you have consistent air delivery. A 50- or 60-gallon compressor would be sufficient for most people. A high-duty-cycle compressor can fill the gap by allowing the compressor to run non-stop without adverse effects. You can also search for a low-cfm paint gun depending on what you’re spraying.
In the end, the right tool for the job is always the right answer. Know what you need before you shop.