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Compressor Basics

Compressor Basics
HP Rating:

The first criteria many people use when they want to purchase a compressor is horse power rating. Most people assume the higher the horsepower rating, the better the air compressor. However, all horse power ratings are not created equal and can easily be misrepresented. You go to your local giant hardware retailer to pick out a new air compressor to run your impact wrench. They have a 5 hp unit that is priced really cheap. Why is that 5 hp industrial unit cost so much more? 5 hp is 5 hp, right? Not necessarily. Let me explain. Look at how much power the hardware store unit draws. It probably needs around 10 amps from a normal 220 Volt circuit. At this rating, you are really getting only 2 hp. The 5hp rating on the box is inflated. To really produce true 5 hp you need at least 20 amps from 220 volt circuit to get it. If you are looking for 5 hp electric compressor, buy the industrial unit and stay away from the cheaper unit at your local hardware store. I am not saying that every air compressor at your local hardware is junk. A lot of times one of those units is all you need. Just be aware of the true horsepower rating. DIY vs. Industrial.

Compressor Basics
Pressure:

The next consideration when purchasing a new air compressor is, "How much pressure will I need?" PSI or Bar. For the those that don’t know, PSI is short for "pounds per square inch" In Europe, you will see them measured in Bar. In the USA, PSI is used. Bar is the one most commonly used in South Africa. If needed the formula is 1 Bar = 14, 7 PSI.
Most of the commonly used air tools require about 7 Bar to operate correctly. However, you will still need a compressor with a higher shut-off pressure. It must produce a minimum of 7 Bar, and will shut off at 8 to 9 Bar. Most air compressors that you find at the local hardware giant shut off at 7 to 8 Bar. Don't let that fool you. You might think all you need is 7 Bar, so that should work just fine. Generally, these light duty compressors shut off at 7 Bar, and don't forget about pressure loss in the line. The little light duty compressor will barely run an impact wrench. It might be fine for light duty DIY- use, but if you really intend use your air tool, more is definitely better.

Compressor Basics
Volume:

The next rating criteria for air compressors are CFM. CFM stands for "cubic feet per minute” 1 CFM = 28.328 l / minIt is a measurement of volume. Basically it is how much air is being moved. Air tools require a certain amount of air volume to run on. Bar is just part of equation. Don't be confused by different CFM ratings at different pressures. Every manufacturer is trying to make their product look better by giving higher CFM ratings at different pressures. The only real concern is how much CFM you will get at 7 Bar. Remember 7 Bar is what most air tools require to operate. To find out what your air tool needs to run, just look on the box for the manufacturer’s specs. Generally, air tools require 4 - 6 CFM. A good rule of thumb on air compressors is you should get 3 -4 CFM per real HP at 90 PSI.

Compressor Basics 
Tank size:

The final factor to consider is tank size. Tank sizes are generally stated in Litres. For example, 100L is a common tank size. So the question remains what size tank do I need? First of all, don't confuse a large tank with more run time for your air tools. If you use your compressor intermittently, a large tank is fine. However if you have need for continuous use, you will need a small tank with big enough pump and motor. If the pump and motor are powerful enough, you shouldn't run out of air. You can save some Rands by purchasing an air compressor with a large tank and smaller motor for intermittent use. If you need to run a 1" impact wrench (about 20 CFM) intermittently, and have a small compressor with a large tank, you might have enough air stored in the tank to do the job. However, if you are constantly running your air tool, you will need to invest in a more powerful air compressor to do the job.

Compressor Basics 
Market perception:

The common perception in the market, is that a compressor is measured by its tank size. Some customers might even measure the size by looking at the size of the motor. The correct way to measure the size of the compressor is to refer to the pressure and volume it produces.Schulz:Weg MotorGerman engineeringLong HistoryCast Iron Pumps (Complete pumps)Stainless steel valve platesCE certified – Hydrostatically tested tanksGenuine ratingsASME safety valveNo warranties to date !!!