Air Compressor Buying Guide

If you are looking for an Air Compressor in the market, but confused about which one to buy, then this guide is for you. In this article I have explained all the concepts and terms used in its specifications. This guide is written keeping DIYer’s needs in mind. Having a basic understanding is more important.

I share two guides. First is a very Basic Simple Guide to help you get started with Compressors and the Second is a Complete guide to advance your knowledge.

Simple & Easy Guide To Air Compressors

1. Know the Specifications of Air Tools you run including its Pressure and CFM Ratings.

2. Know the number of tools you use at the same time.

3. Add its CFM Rating. Consider only the tool with the highest pressure requirement. Add 20% to it. 90% of Air Compressors in the market are rated for at least 115 PSI.

4. Since you have both the values, you need to buy a compressor with that Pressure and CFM output rating.

This is explained with 2 scenarios. We consider that single tool is in use at one time.

Scenario 1: If you are an Automotive Mechanic and looking to buy an Air Compressor to run Impact Wrench (320 Nm) with rating 90 PSI and 4 CFM ( 115 Litre/min).

You need to buy an Air Compressor with Output rating of 8 bar (116 PSI) and CFM rating of at least 90 Litre/min (25% less than specified). It is not necessary to have 200 LPM output because you won’t be using an impact wrench all the time. Say, for a minute, you may use it for 20 seconds. In the meantime, the tank will get enough time to refill itself if needed.  

Scenario 2: Assume you are a painter. You use a HVLP (High Volume Low Pressure) spray gun with rating 10 CFM (280 LPM) at 40 PSI.

So here you don’t need more pressure. But you need lots of volume. So, you need to buy an Air Compressor with ~8bar pressure rating and CFM value of 10 CFM iself. The reason you have already understood. You will be painting continuously for about 1-2 minutes with minor stops. Assume, you painted for complete 1 minute, you have already consumed 280 Litres of air from the tank. The compressor motor must be able to refill the tank as fast as possible or you need to buy a large tank to avoid frequent refillings.

If you are confused why to get 8 bar (116 PSI) Compressor instead of 3 bar (45 PSI) for painting? The reason is, in the power tool market most of the Air Compressors are available with a minimum rating of 8 bar itself. You can find compressors sold specifically for painting purpose with less pressure and high volume output. But they are industrial models and cost more than regular ones, still worth the price considering their reliability. In a painters perspective, larger the tank, the better.

The other reason is, the distance between the tool and tank along the hose pipe is also important. Longer the hose pipe, pressure goes on decreasing. This means, if the output pressure is set to 40 PSI at the tank outlet, and a hose pipe of 25 feet is used, if you find the pressure at the end of 25 feet will always be less, around 30 PSI. This also depends on the pipe used. Larger pipes are always better than thin ones as they retain pressure.

So overall, an Automotive Mechanic should worry about the Pressure rating, and a Painter should worry about the Tank Size and how fast does the compressor fill the tank. Cheaper Compressors work fine for Air Impact Wrench but not for Spray Gun. While painting, you cannot wait until the Tank Refills. You will loose consistency between each coats. When the Pressure and Volume gets lower, the Air to Paint ratio will change resulting in darker shades and bubbles.

Complete Guide To Air Compressors

Air Compressor is a simple device used to compress air as its name indicates. They are categorized on their type, that is the mechanism used to compress air.

Basically an Air Compressor has only two components. One, the compression mechanism which is also called a pump unit. And the other is its tank. Whether an Air Compressor has a tank or not, it is still called a compressor itself. Because, a tank is just used to store compressed air. Stationary compressors always have a tank. Portable Compressors may or may not have one. Example, 12 volt Tyre Inflators or Balloon Inflators rarely have a tank. Still, the tank is also an important element. Later you will know why.

Like said above, the compressor compresses air and stores it in the tank.

Now, you should know the below points of an Air Compressor. 

1. How much air is compressed in 1 minute. (in litres/ minute) and,

2. How many times the air is compressed. That means, initially if the air was about 10 litres, and after compression, its volume reduced to 1 litre, then the compressor compressed the air 10 times.

When the air is compressed, its pressure will also increase. And so does the temperature. If your Air Compressor has a tank, the compressor will start filling it. It will fill until the tank has reached its maximum withholding pressure and then it shuts OFF the compressor.

If the tank is full, and the compressor is still running, it doesn’t make any sense. Well, there is one more electro-mechanic mechanism to switch OFF the compressor when an adequate pressure is reached in the tank. 

You connect your tools like wrench, paint sprayer, nail gunner to the tank using a hose pipe and use it. While you use the tools, they consume air. When the pressure in the tank decreases, the compressor will run again to fill it up. And this is how it works. Now let me dive in deeper.

Compressor Pump Basics

The amount of air compressed in 1 minute is called the compressors flow rate. (don’t mix it up with tool’s flow rate)

In general, it can be expressed in litres per minute (LPM) or most commonly called as cubic feet per minute (CFM). 

Remember, 1 CFM = 28.316 LPM. So, CFM is bigger than LPM.

Now, when the air is compressed, it is pressurised in the compressor before entering the tank. So this pressure is denoted as PSI (pounds per square inch). This is the same pressure reading with which you refill your tires. In some countries, PSI is not used and they mention the pressure in terms of bars.

So remember, 1 bar = 14.5 PSI (Standard Room Temperature)

Don’t mix up Pressure readings of Compressor and Tank. They both are different. If a compressor is rated at 90 PSI, the tank will be rated more at around 120 PSI.

As an example, a K28 Pump Compressor has a capacity to compress 662 Litres of Air per minute at 160 PSI if 5.5 HP is supplied at its flywheel shaft at 1290 RPM.

Volume of air compressed by a compressor is not always constant. As the tank fills up, it starts to resist the flow of air into it. So, the compressor has to put in more energy to push the air into the tank. This slows down the process. You need a strong drive unit which will be explained later.

There is one more Terminology used in Air Compressors called as SCFM instead of CFM. SCFM is rated at STP conditions. Both values are different.

Compressor Tank Basics

Compressor transfers the compressed air to the tank. In the beginning when the tank is empty, the compressed air expands again inside the tank. Its pressure reduces and its volume increases. But after continuously pumping air into the tank, the pressure starts to increase gradually. 

Like mentioned above, every tank has its own pressure rating. The compressed air in the tank should not exceed the pressure at which it is rated. If it exceeds, the tank will explode. It is extremely dangerous. In short, a similar explosion if a car tire is over-pressurized. People get permanent hearing disabilities. After all, its just 60 PSI and not 200 PSI.

The pressure rating of the tank will be mentioned in bar or PSI. There is a safety valve mechanism which opens automatically when the maximum pressure is reached and in case the compressor has not auto switched off due to some errors. Your kitchen Pressure Cooker also has a safety valve. Just in Case.

Size of the tank is measured in liters or gallons in some countries. This measurement has nothing to do with compressed air. The size of the tank is nothing but its volume. OR simply, if you start pouring water into the tank (which you shouldn’t), and measure how much it can carry is nothing but its size. If a tank can fill water upto 30 liters, its size is 30 liter. Same as your domestic water tank.

More the tank size, more the air quantity it can hold. In general a 30 liter tank can hold upto 300 liters of air in compressed form, considering its maximum pressure capacity to be 10 bar (145 PSI).

There are 3 pressures in an Air Tank.

1. Cut-In Pressure: Pressure at which compressor starts refilling the tank.

2. Cut-Off Pressure: Pressure at which compressor stops filling the tank

3. Max Pressure which a tank can handle at which Safety Relief valve is set at.

Now. What size of tank do you need?

If you operate tools which consume a lot of air volume, you need a big tank. Typically a professional paint spray gun needs about 300 litres of air per minute (10.6 CFM) and the pressure at which it operates should be around 40 to 60 PSI. It will still work with lesser PSI, but here your Air to Paint ratio changes and you spray more paint than air. More on it later.

If you have a 30 litre tank rated at 145 PSI (10 bar), which is full of air, without the compressor running. Then you roughly can use the spray gun ( 40 PSI, 300 LPM) continuously for about 15 seconds without drop in pressure. 

Tip: If you have a compressor which itself has the capacity to run your tool directly, even without a tank, you should still need at least a small capacity one. Because, the airflow from the reciprocating compressor is itself reciprocating. It is not constant. Hence using a tank will supply air at almost constant velocity to the Tool. Generally the portable compressors for professional use have 1 gallon tank.

Compressor Types

Now you know the specifications of an Air Compressor, it is important to know how they work. People also call a compressor a pump. Generally there are two types of Air Compressor mechanisms.

1. Screw Compressor ( More of industrial use). I won’t explain this.

2. Piston Compressor. ( Widely used in Trucks, Home, Paint Shops and Automotive Garages)

Old school compressors were all Piston Compressors. Even today they are widely used since they are not expensive when compared with screw types. The pressure output you need determines the number of cylinders and the number of stages. 

Number of Cylinders ≠ Number of Stages. Some two cylinder compressors work as single stages. They have two seperate air inlets and a common tank.

In a two stage compressor with two cylinders, the second cylinder again compresses air from the first cylinder before filling the tank. This even increases its pressure again. There are multi stage compressors with multiple cylinders. The first cylinder will be a Low Pressure Cylinder. And the latter ones will be High Pressure Cylinders.

Generally for pressures up to 100 PSI single stage is used. For pressure up to 200 PSI two stages are used. For anything above is multistage. Paint shops should use multi cylinder single stage compressor units. Because they need more air than pressure.

Note: Don’t blindly judge the number of cylinders of a piston compressor. Some look like a single stage but are actually two stage compressors. The arrangement is inline and not V type. K28 Pump is an example of inline dual stage air compressor.

Tip: If there are two cylinders of the same size, it is a single stage model. If the two cylinders are of different size, it is a dual stage compressor.

The Piston Compressors work similar to Motorcycle Engines. They too need regular maintenance which includes, oil change and air filter replacements. The oil drain interval is not that frequent though. But it needs regular checkups. If the oil level drops or its viscosity (thickness) increases, it will not be able to lubricate the cylinder walls which leads to seizure.

The main disadvantage of oil lubricated compressors is that there may be a chance of oil leakage into the tank. If you are using it for painting purposes you should use a separate water and oil filter before it enters the spray gun.

Most of the small capacity home usage compressors under 3 CFM have oil free variants which means there is no maintenance at all. In oil compressors, there are chances for the oil to leak into the tank. Oil free compressors won’t have that issue. But still, professional models always use oil for lubrication. The leakage is not always bad. It will prevent the tank from rusting. 

On the other hand, screw compressors widely are used nowadays to overcome problems from Piston Compressors. They require very little maintenance. Large scale industries use screw compressors. They make very less noise compared to piston types. All comes with an increased cost. I won’t go deep into this. And there is one more type called centrifugal compressor, that’s purely industrial.

Compressor Drive Mechanism

There should be a mechanical force to drive Air Compressors. It depends on where you are. All the stationary and mini portable compressors work on electric motors. Premium portable ones have mini Petrol/ Gasoline engines on them.

Huge Air Compressors of Civil/ Road Construction industries which are coupled with Jack Hammers always use the most powerful tractor diesel engines. Diesel engines produce more torque than Gasoline engines. Air Compressors used in Brakes of all trucks and buses run directly from engine power.

Drive units can be connected directly or by using belts. This depends on the maximum speed at which the compressor works. Home use DIY ones always use direct shaft drive and professional ones use V belt drive. Diameter of Pulleys is dependent on the input RPM of the Compressor Pump.

Now, How much HP do I need?

Here I can’t give a direct answer. It all depends on the type of tool used. I can say, more the HP, faster it will refill the tank during continuous usage. HP matters more for continuous users like Paint Shops, Sanders etc. Air tools which need more air volume need more HP. Nail Guns don’t need more HP. They will run fine as long as their pressure requirements are reached. Overall HP is the capacity to refill the tank. More HP = More LPM.

In order to know the air requirements of different tools and the recommended tank size, I suggest you to read this article on Air Tools for Air Compressor

HP is a measure of Power input needed at the shaft of the Compressor Pump that a Power Source must supply. If your country mentions power in watts, remember 1 HP = 745.7 watts. So, a 2 HP compressor can also be called as a 1500 watt compressor.

At what rate a compressor refills the tank is what you have to trust the manufacturer. Or if the manufacturer has shared the specifications of the compressor pump, you can calculate how fast it will refill the tank.

Air Compressor Safety

1. Never Weld a Compressor tank when it’s loaded. Drain the air first before applying heat to it. By the way, welding a pressure vessel like a tank is not at all recommended as no one can guarantee its strength after weld. Manufacturers have their own safe test methods.

2. All the compressors have a drain plug at the bottom. During winters, the air inside the compressor condensates into water. It is advised to drain off that water at least once a week. If the water is not drained, then the water reacts with the metal initiating rust. Daily drain is more preferable in cold areas. Generally tanks must be Galvanised from inside. Well, who knows whether the tank is Galvanised from inside? What the manufacturer says, we trust.

3. All compressors have a safety Pop-Off Valve. These valves are rated at a specific pressure, if it exceeds that, it will pop open. And that pressure is set for 90% maximum pressure capacity of the tank. Calibrating these every now and then, increases safety.

Choosing an Air Compressor for different scenarios

Assuming you are a painter.

There are different types of paint sprayers. Some work at high volume and some at low volume of air. It depends on the paint type and whether it’s for Automotive jobs or Wall painting. Commonly used professional paint sprayers consume 300 Litres of air per minute ( 10.6 CFM) @ 40 PSI Air Pressure.

Now which tank size should you need?

If you have a 30 litre (8 gallon) tank @ 150 PSI is equivalent to having a 112 litre (30 gallon) tank @ 40 PSI. Since your sprayer consumes 300 litres of air every minute, by the time it runs for 22 seconds, it has already consumed 112 litres. Practically you will be able to use it just for about 15 to18 seconds.

You can still use the compressor, but you are not getting a pressure of 40 bar. This will mess up the painting process. In reality you can use it upto 18 seconds max considering the pressure drop in the Air Hose Pipe at 80% efficiency.

Now if you had a 60 litre tank for the same compressor, you can use the spray gun for at least 36 seconds with 80% efficiency. And for a 100 litre tank, you can use it for about a minute.

Note: All above calculations were made considering the compressor motor was not running.

Assuming you run an Automotive Workshop.

You use impact wrench for Nut and Bolts. Now, a general purpose 600Nm, ½” impact wrench needs 5 CFM @ 90 PSI

If you have 4 CFM @ 90 PSI Pump with a 30 litre (8 gallon) tank @ 150 PSI, is equivalent to having 50 Litres @ 90 PSI. The above mentioned air wrench consumes an average of 140 Litres of air per minute. You may be able to use the tool continuously for 20 seconds. But, the point here is, most of them don’t use impact wrench continuously. So this size compressor would be sufficient for Wrenches and Brad Nailers.

If you plan to use an Air Compressor to run Air Tools like Sanders, Angle Grinders, Die Grinders, you must have more CFM output generally above 5 CFM. For more details, please refer to the article on Air Tools for Air Compressors.

Now Assume that you get the Idea on Air Compressors. But wait, there is one more important topic to know about. It is about the Air Compressor Fittings: Couplers and Pipes, which is explained in separate Article. And this sums up the Article.