Understanding Compressor Pressure Units

The compressed air in designed flow rate and at designed pressure will be used for a various applications like sand blasting, laser cutting, cement manufacturing and water borehole drilling etc.

 

If we want to choose a suitable air compressor or gas compressor for our factory facilities, we shall also consider properly the discharging pressure of a compressor apart from its flow rate. The air compressor compress air from atmospheric pressure to its designed pressure like 10 barg, or 250 psig etc. The compressed air in designed flow rate and at designed pressure will be used for a various applications like sand blasting, laser cutting, cement manufacturing and water borehole drilling etc. In this article, we will discuss air compressor pressure units in depth so that our client will not be confused about this key aspect.

 

1. Definitions of Some Common Pressure Units

 

To begin with, we shall understand the definitions of the commonly used pressure units. So we can feel easier to make the conversions.

 

Pascal or Pa: 1 Pa means the pressure of 1 newton force evenly on 1 square meter surface. 1 Pa = 1 N/m².

 

Kilopascals or KPa: Kilopascal has a close tie with Pascal, just like Kilometer with meter. 1 Kpa = 1000 Pa.

 

Megapascal or MPa: Megapascal is also derived from the basic pressure unit, Pascal. But it is more frequently used in air compressor or gas compressor field. 1 Mpa = 1000 KPa = 1000000 Pa.

 

Kilogram force per square centimeter or kgf/cm²: This is a unit usually used in China market. 1 kgf/cm² means the pressure of 1 kilogram force on a surface of 1 square centimeter. 1 Mpa = 10 kgf/cm². When purchasing some gas compressors from China, its discharging pressure might be in kgf/cm².

 

Atmosphere or atm: This is defined as the average atmospheric pressure at sea level. 1 atm = 101325 Pa = 101.325 KPa. In centrifugal air compressor technical requirements, we may see this unit frequently used.

 

Bar: This is a widely seen and used unit in air compressor industry. 1 bar = 10⁵ Pa. We will explain further its applications in this article.

 

Millibar or mbar: Milli means one thousand part of. So mbar means one thousand part of bar. 1 bar = 10³ mbar = 10⁵ Pa. This unit is frequently used for vacuum pump to measure its ultimate pressure.

 

Millimeter of Mercury or mmHg: this unit means the pressure generated by 1 millimeter mercury. It also has another name, Torr. Like the above mbar, this unit is also widely used in vacuum technology, because it is a pressure far below 1 atm.

 

Pounds per square inch or PSI: Together with Bar, this unit is another widely seen unit in air compressor industry. It means the pressure generated by 1 pounds force on a surface of 1 square inches. This is an imperial unit while the Pa is a metric unit.

 

Conversions between the above units are listed below:
1 Mpa = 10 kgf/cm2 = 1000 KPa = 10⁶ Pa = 10 bar = 10⁴ mbar = 145.04 psi;

1 atm = 101325 Pa = 101.325 KPa;

1 Pa = 10^-6 Mpa = 10^-5 Pa = 0.1 mmHg

 

2. Air Compressor Working Pressure, Converting Bar to Psi Form

 

On air compressors or gas compressors, the manufacturers normally mark their working pressure by bar or psi, which are the two most frequently used pressure units. We can multiply the Bar value by 14.504 to get Psi value. In the below form, we listed some commonly used pressure values conversions in compressor industry.

 

0.1 bar	1.45 psi	All the pressures below 1 bar are usually used in vacuum pump. Vacuum pump is used, for example, to remove air or oxygen from the packing materials so the food could be stored for a relatively longer period. After the vacuum pump removes air from some space, the pressure in the space will be lower than 1 atm.
0.2 bar	2.90 psi
0.3 bar	4.35 psi
0.4 bar	5.80 psi
0.5 bar	7.25 psi
0.6 bar	8.70 psi
0.7 bar	10.15 psi
0.8 bar	11.60 psi
0.9 bar	13.05 psi
1 bar	14.50 psi	1 bar is normally the suction pressure of compressors.
1.3 bar	18.85 psi
1.5 bar	21.76 psi	Normally, the air blower discharges air pressure below 2 bar(g).
2 bar	29.01 psi
2.5 bar	36.26 psi	For the required pressure below 5 bar, some manufacturers will suggest their normal compressors of 7 bar working pressure and install a pressure reducing valve. This will waste a lot of energy. We manufacture the low pressure air compressor whose discharging pressure could be lower than 5 bar.
3 bar	43.51 psi
3.5 bar	50.76 psi
4 bar	58.02 psi
4.5 bar	65.27 psi
5 bar	72.52 psi
5.5 bar	79.77 psi	Within this range, we call it normal working pressure. Most applications including pneumatic tools and air separation require compressed air pressure within this range. And almost 70% of air compressors on the market are designed for this required pressure range. Our Comair electric oil injected & oil free air compressors are discharging pressure within this range and they are designed to be energy saving. Our diesel portable air compressors use a two stage air end for this pressure range, which makes it save 30% fuel than other diesel compressors on the market. Moreover, for sand blasting operation, we also design our compressors with built-in after cooler & water separator so the treated surface could be totally dry and clean.
6 bar	87.02 psi
6.5 bar	94.28 psi
7 bar	101.53 psi
7.5 bar	108.78 psi
8 bar	116.03 psi
8.5 bar	123.28 psi
9 bar	130.54 psi
9.5 bar	137.79 psi
10 bar	145.04 psi
10.5 bar	152.29 psi
11 bar	159.54 psi
11.5 bar	166.80 psi
12 bar	174.05 psi
12.5 bar	181.30 psi
13 bar	188.55 psi
13.5 bar	195.80 psi	For rotary screw air compressors, they can be called high pressure air compressor if they discharge air within this range. We have our oil free water lubricated screw air compressor to discharge compressed air pressure in this range and could be widely used for various bottle blowing applications; For diesel portable air compressors of this pressure range, they are mostly likely to be used for water borehole drilling, oil & gas well drilling, pipeline leaking testing, geothermal well drilling or shipyard sand blasting etc. Our diesel portable air compressors use world leading Cummins engine, which could have international after sales services around the world. For some applications, we can also suggest oil free piston air compressors if the client insists. Oil free piston air compressor could also discharge totally oil free compressed air.
14 bar	203.06 psi
14.5 bar	210.31 psi
15 bar	217.56 psi
15.5 bar	224.81 psi
16 bar	232.06 psi
16.5 bar	239.32 psi
17 bar	246.57 psi
17.5 bar	253.82 psi
18 bar	261.07 psi
18.5 bar	268.32 psi
19 bar	275.58 psi
19.5 bar	282.83 psi
20 bar	290.08 psi
20.5 bar	297.33 psi
21 bar	304.58 psi
21.5 bar	311.84 psi
22 bar	319.09 psi
22.5 bar	326.34 psi
23 bar	333.59 psi
23.5 bar	340.84 psi
24 bar	348.10 psi
24.5 bar	355.35 psi
25 bar	362.60 psi
25.5 bar	369.85 psi
26 bar	377.10 psi
26.5 bar	384.36 psi
27 bar	391.61 psi
27.5 bar	398.86 psi
28 bar	406.11 psi
28.5 bar	413.36 psi
29 br	420.62 psi
29.5 bar	427.87 psi
30 bar	435.12 psi
35 bar	506.64 psi
100 bar	1450.38 psi
250 bar	3626.00 psi	250 bar is the discharging pressure of natural gas compressors. CNG mother station & CNG daughter station both require natural gas compressor of 250 bar.
300 bar	4,351.20 psi	The highest pressure of a screw air compressor could reach to around 40 bar; For pressures of 300 bar above, we usually suggest our reciprocating type compressor. One application of such high pressure compressor will be used to produce urea, because under high pressure, the carbon dioxide will be easier to react with ammonia.
350 bar	5,076.40 psi
400 bar	5,801.60 psi
450 bar	6,526.80 psi
500 bar	7,252.00 psi
550 bar	7,977.20 psi
600 bar or above	8,702.40 psi or above

 

In the below form, we list some commonly used PSI values and convert them into Bar.

1 psi	0.07 bar	This pressure ranges are frequently used in vacuum applications and industry.
10 psi	0.69 bar
14 psi	0.97 bar	Atmospheric pressure at high altitude locations shall be around this value. Centrifugal air compressor design shall consider the atmospheric pressure at the working site to avoid any compressor surges.
15 psi	1.03 bar	This pressure ranges could be achieved by air blower.
23 psi	1.59 bar
40 psi	2.76 bar	Our Comair specially designed low pressure air compressor could discharge compressed air of this range. So it doesn’t need any pressure reducing valve for energy wasting.
45 psi	3.10 bar
50 psi	3.45 bar
55 psi	3.79 bar
60 psi	4.14 bar
65 psi	4.48 bar
70 psi	4.83 bar
75 psi	5.17 bar
100 psi	6.89 bar	Both Comair Oil Injected Screw Air Compressors and Comair Oil Free Screw Air Compressors discharge compressed air of this range, which is a normal working pressure. Most applications will require this working pressure range, including oxygen generation, nitrogen generation, rock drilling and other pneumatic tools powering etc.
110 psi	7.58 bar
120 psi	8.27 bar
125 psi	8.62 bar
130 psi	8.96 bar
140 psi	9.65 bar
145 psi	10.00 bar
150 psi	10.34 bar
189 psi	13.00 bar
220 psi	15.17 bar	Diesel portable air compressors in this pressure range will be normally used for ground stabilization, water and geothermal well drilling, oil & gas well drilling, and pipeline services.
290 psi	19.99 bar
363 psi	25.03 bar
500 psi	34.47 bar
1000 psi	68.95 bar	Screw air compressors couldn’t discharge this high pressure. We shall suggest either reciprocating compressor or centrifugal compressor.
1200 psi	82.74 bar
1400 psi	96.53 bar
3626 psi	250.00 bar	This pressure is usually a CNG compressor discharging pressure.
5000 psi	344.73 bar	Normally compressors of this pressure range or above will be used for large chemical industrial application, or oil & gas explorations like gas lifting.
10000 psi	689.46 bar
15000 psi	1,034.20 bar
60000 psi	4,136.79 bar

 

3. Gauge Pressure VS Absolute Pressure

 

Understanding the differences between gauge pressure and absolute pressure is of the same importance as understanding the pressure units conversion. When we design the compressors especially centrifugal air compressor, we shall consider more of absolute pressure as well as the local atmospheric pressure at the client location so the ultimate compressor performance could be achieved. Moreover, when we are making some flow rate conversions or some other calculations, we shall also use absolute pressure value. However, on the compressor nameplate, the marked figure is for gauge pressure value.

 

The below picture shows clearly the difference between gauge pressure and absolute pressure:

 

 

Standard Atmospheric Pressure: this is the atmospheric pressure at sea level. It is 1 atm = 101.325 kPa.

 

Local Atmospheric Pressure: at high altitude area, the local atmospheric pressure will be lower than 1 atm, while at extremely cold area, it will be higher than 1 atm. We can see there always exists a variable level between actual local atmospheric pressure and standard atmospheric pressure.

 

Absolute Pressure A: Supposing there is a tank and we use a vacuum pump to pump out the inside air, we will get an absolute pressure which will be much lower than the local atmospheric pressure.

 

Vacuum Degree: Each kind of vacuum pump has its own ultimate vacuum.

 

Gauge pressure: this is signified by a ‘g’ after the pressure unit like 20 psig. Gauge pressure is the pressure value listed on compressor nameplate.

 

Absolute Pressure B: For a compressor, if its discharged air pressure is 10 barg, it means we can use a gauge to measure the compressed air from this compressor and the figure could be 10 barg. But its actual absolute pressure is 11 bara.

 

We are in Shanghai, China now. Let’s analyze all the above by using Shanghai as an example:

Local atmospheric pressure in shanghai is 101.1 kPa, which is lower than the standard atmospheric pressure of 101.325 kPa. The variable level is 0.225 kPa.

 

The below Comair diesel portable air compressor, CDP-45/10 has the working pressure of 10 barg. When it works in Shanghai, the gauge pressure of the compressed air from this compressor is 10 barg while its absolute pressure is 10 barg + 101.1 kPa = 1,101.1 kPa.

 

If there is a vacuum pump working in Shanghai and its ultimate vacuum is 2 Pa, so the absolute pressure A shall be 2 Pa and the vacuum degree is 101.1 kPa - 2 Pa = 101,098.00 Pa.

 

 

4. Common Required Working Pressures

 

We are now in the modern industrial era, in which there are three main basic resources. They are water, electricity and compressed air. Compressed air is used everywhere, while we seldom notice its merits and existence.

 

We drive the car every day or even ride a bicycle. But few of us could know that compressed air is used in such a small and convenient tool as a bicycle. The air is compressed and pumped into the tyre, which makes its pressure far higher than atmospheric pressure.

 

In the following form, we listed some applications, their required pressures and suggested Comair compressor models.

 

Sand/media blasting	90-125 psig	Comair diesel portable air compressor, CDP-13/10 with built-in after cooler & water separator.
Bubble curtains for aquaculture	100-150 psig	Comair centrifugal air compressor for large flow compressed air.
Paint spraying	40-60 psig	Comair centrifugal air compressor for oil free compressed air.
Fermentation process	14.5 psig	Oil free blower, oil free screw air compressor or centrifugal air compressor
Bottle blowing	435~652.5 psig	Comair oil free water lubricated screw air compressor
Laser cutting	90-240 psig	Comair Laser Cutting Compressor
Rock drill powering	87-217 psig	Comair diesel portable air compressor, CDP-10/8 or CDP-5/7
Pneumatic pick
Pneumatic wrenches
Water borehole drilling	217-290 psig	Comair diesel portable air compressor, CDP-22/20, 20 barg discharging pressure
CNG compressor	3625 psig	Comair CNG compressor for mother station & daughter station
Glass manufacturing	80-123 psig	Comair oil injected screw air compressor

 

Different operations may require different pressures. It’s always suggested to contact your equipment manufacturer to get its required compressed air pressure. Or you can contact our Comair expert!