Know-a-Compressor » How to size an air compressor?

# How to Size an Air Compressor?

First it is important to know how the capacity of an air compressor is measured.

1. Volume of Compressed Air Flow upon a particular given time. This means how much volume of compressed air can be generated by an air compressor when it has run for a particular amount of time. This unit is exactly measured as volume of air upon time.
Eg: CFM (Cubic Feet per Minute) – Cubic Feet is a unit of volume and minutes is a unit of time. 1 cubic foot is =  28.317 Liters. Therefore 1 CFM = 28.31 (LPM) Liters Per Minute.

Similarly Liters per Second (L/sec) or Cubic Meter per hour are units of flow of air.

a. Important point to note here is that some brands specify SCFM (Standard CFM). This means that the compressor will deliver air at the speed of the mentioned CFM but because it is standard it is considered as it is measured at a standard reading which is Pressure is 1 Bar(g), Atmospheric Temperature is 20 Deg C, Atmospheric Pressure is at Sea Level and 0% relative humidity in the atmosphere. This is a reading by ISO 1217 which tries to bring all CFM of different brands on one level. But this is not the actual CFM at your location because air expands and contracts at different temperatures and atmospheric pressure has a very important role in how well the compressor sucks air from the atmosphere. Therefore we at CompressorWale plan to bring to you exact information as to discharge rate of the compressors as per Indian conditions at a high pressure.

b. The above same goes for NM^3/Hr (Normal Cubic Meter per Hour).

c. Because of the above the altitude of your factory / site affects the air compressor performance. At a higher altitude compared to sea level there is less atmospheric pressure which is why the compressors suction capacity will reduce therefore the same machine will give much less air. Eg. Places like Shimla, Mussorie etc. In these cases a much bigger machine is required to deliver less amount of air. Similarly temperature of air and humidity in air also affect the performance of the air compressor but their impact on compressor is less.

d. Therefore for now you can keep a basic general thumb rule that you need to add about 25 to 30% to the above figures to get the exact CFM or M^3/Hr of the compressor.

2. Air Pressure: At what maximum pressure can the compressor deliver air or force the air out? Pressure is measured as weight upon a particular area like 1cm x 1cm or 1inch x 1inch.
Eg: Kg/cm^2 (Kilograms per square centimetre) or PSI (Pounds per square inch) or BAR. You will notice that after a unit like this the alphabet “g” is mentioned which means gauge. Therefore Kg/cm^2 (g) means it is a reading mentioned in a pressure gauge.

There is a false notion that some people have regarding these 2 above units of measurement. Some people think that flow of air or CFM and the pressure both need to be multiplied by the number of pneumatic machines or pneumatic equipments. But only the flow of air or CFM needs to be multiplied and not the pressure. The same is cleared in the below example.

Eg: There are 2 pneumatic machines / equipments of the same kind and each requires an air consumption of 20 CFM @ 6 Bar(g). Then your total Compressed air consumption will be 20 x 2 = 40 CFM @ 6 Bar(g). Note that the Pressure does not need to be multiplied with the number of pneumatic machines.

The above 2 units are the only important points to know before purchasing an air compressor.

Please note that air compressors are rated and measured as CFM at a particular pressure. Most brands have an asterisk (# or *) at the bottom of the compressor technical specifications which states that the compressor CFM rating is measured at X pressure but the maximum pressure of the machine is Y. This is important because CFM (Flow of Air) and the pressure are inversely proportional which means when one goes up the other has to come down.

Therefore we will explain this with the help of the below example.

Suppose 15 CFM is required at 10 Bar.

A 5HP reciprocating piston compressor shows 15 CFM but below it says that this CFM is measured at 7 Bar. Therefore when this pressure is increased to 10 Bar the flow will not remain 15 CFM and will drop down. Therefore this 5HP machine will not be sufficient and a 7.5HP will have to be looked into.

Some examples of compressed air consumption:

3. Some people do not know the above points on ways an air compressor is measured and they only talk in term of HP (Horse Power) of the machines. Well over the years a thumb rule is there that 1 HP = 3 CFM of air in reciprocating piston compressors but this cannot always be considered. It is different at different pressures and it is different in screw compressors. The right way to size an air compressor is when you have figured out the Air Flow and Pressure required then you match a compressor accordingly to this and then whatever be the HP of the machine you will have to stick with it. If you are going in for a reciprocating piston type air compressor then it is recommended to consider 1.5 times the CFM actually required so that compressor gets rest when it cycles on and off / load and unload etc.

Now that you know the above measurements of an air compressor then lets get into the steps involved and the procedure to size the adequate compressor for your operation.

STEPS:-

1. Understand the application. The application could be a pneumatic machine, spray painting operation, just to fill up air in a tank which does oxygenation, pharmaceutical application, plastic bottle making through blow moulding etc,

2. Determine the air flow required in CFM, LPM etc. This can be seen in the manual of the pneumatic machines, you can ask your pneumatic machine manufacturers to suggest the CFM, if the application involves filling of air in a storage tank for oxygenation etc then you need to know the volume of this storage tank and with that the time you want it to fill up completely at a desired pressure and we at CompressorWale.com will guide you the CFM value required to fill that storage tank in the required time.

a. Important point to note is that after you have finalized on a particular CFM that is required for your application and suppose you have decided to go for a reciprocating piston type air compressor then you need to take a compressor which is 1.5 times the required CFM. This is because the piston compressors are not designed to run continuously and are for intermittent use only. Therefore a big size compressor will get cycle time to on and off.

3. Determine the working pressure required for the application. This should also be mentioned in the pneumatic machine manuals or can be taken from manufacturer. There are certain thumb rules regarding pressure like pneumatic applications do not require more than 6 Bar(g) and spray painting requires 3.5 to 5 Bar(g) and pet blow of bottles requires high pressure ranging from 12 to 40 Bar depending on the mould and profile of the bottle etc.

a. Important point to note is that compressors come with a load and unload pressure rating or cut on and cut off pressure rating because of the pressure switch installed on the machine. The difference between this cut on and cut off pressure is called pressure delta. Usually these ratings are both adjustable in almost every pressure switch but it is still advisable to keep a bigger pressure delta between the off and on pressure. In piston compressors the minimum pressure delta advisable is 2 bar (g) and in screw compressors it is 1 bar (g). This way the higher the pressure delta the longer the compressor gets to cycle on and off and there is less load on the electric motor. Note that electric motor manufacturers suggest that an electric motor should not be made to start and stop more than six times in an hour. But also to remember that as the pressure goes higher the compressor draws more electricity and more energy to compressor the air. Therefore there needs to be a balance so max cut out pressure can be kept up to 2 Bar (g) higher than the required pressure.

4. Now that we know the above we get into the purity of the compressed air. Like how much oil and moisture content is allowable to your application. This will guide you whether you need a 100% oil Less / oil free machine or a normal oil lubricated machine with filters will be sufficient. Secondly it will also guide you whether you need a dryer to dry the moisture from the compressed air and whether you need a refrigerated air dryer or a desiccant air dryer.  Below are examples of some applications:-

a. In case it is a pneumatic machine little oil carry over in compressed air is ok and sometime better if oil is present because the movement of the pneumatic cylinders requires oil for smooth operation. But moisture present in compressed air is harmful for the pneumatic cylinders as they could rust over time. Here you will need a refrigerated air dryer to remove the moisture down to +3 Deg C PDP(Pressure Dew Point) – {usually all dryer manufacturers are giving this rating} and a general purpose filter of around 5 or 1 micron rating to stop dust particles and liquid water entering the dryer. There will be people who will recommend a glass type moisture separator but this does not remove moisture in form of water vapour which is removed by the dryer.

b. In spray painting of cars the oil in compressed air creates spots in the finished paint product therefore here 3 filters are required to remove oil content down to 0.003ppm (particles per million). You will need all three – if you only purchase the last filter then it will choke up faster not giving you its life worth.

c. In case of pharma application if compressed air is going to come in contact with the end product like medicines and capsules then neither oil nor moisture is allowable to stricter norms. In this case you will need a 100% oil free machine with Desiccant air dryer to remove the moisture down to -20 Deg C PDP(Pressure Dew Point) – {usually all dryer manufacturers are giving this rating} and a set of 3 filters to purify down to 0.003ppm (particles per million) and 0.01 micron. Some cases an oil lubricated compressor is also allowable with the necessary filters – this depends on the oil content allowable and how strict is the inspection authority.

d. If it is a pet bottle blow moulding application then all 3 filters may be required to remove the hydro carbon odours from the compressed air because the bottle have bad smell if not removed.

5. Is noise level a criteria? If you want to place the compressor near operations and on the factory shop floor near your labour and employees then you could go for a silent machine like a screw compressor (more expensive) but if you can place the machine far and do some piping then you can go in for a reciprocating piston type air compressor (economical option).

6. Does your application require a compressor that needs to be portable or stationary? If it requires to be portable and if the requirement is small within 5HP then you can go for a reciprocating piston type air compressor since some brands give free wheels or access to put wheel on the compressor for portability but if you need a big size compressor that is portable then some brands give approximately 300 CFM machines which are portable if you may have seen them sometime on the road side. You get these type of machines in both electric motor driven as well as diesel engine driven based on your requirement. At the moment no brand offers a standard portable compressor from 5 to 30 HP. They may be able to give you a customized trolley or so for portability.

7. If the size of the machine is fitting in 15HP and less then it is recommended to go for a reciprocating piston air compressor but if higher then better to go for a screw compressor.

8. Just before closing on an air compressor make sure that you have electricity supply at your site / workshop in the desired rating to run the air compressor. The ratings used in India are Single Phase (230V / 50Hz) or Three Phase (440V / 50Hz). Single phase is available only in reciprocating piston air compressor and that too only upto 3HP power rating. Single phase is not available in screw compressors in India. Three phase supply is most commonly used.

9. Backup: you should consider taking a backup compressor if an air compressor is very essential for your business. In most factories an air compressor is so critical that the complete plant will be a stand still if the compressor has broken down and per day salaries are very high for this kind of a plant.

10. Consider a machine 10 to 15% bigger than the required air because in future you may want to add some equipment, your piping may have some leaks, your filters may get clogged etc. Having a little spare amount of air is always better.

Now that you know how to size an air compressor. Please read through the commonly used air compressors i.e. screw and reciprocating-piston air compressors and their differences.

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