Understanding the Units of Air Flow
When we are choosing a suitable air compressor, there are always some key aspects we shall consider, which includes discharging pressure, flow rate, motor or engine power, portable or stationary, oil content or pressure dew point. In this article, we shall mainly discuss about the compressor flow rate.
When we are choosing a suitable air compressor, there are always some key aspects we shall consider, which includes discharging pressure, flow rate, motor or engine power, portable or stationary, oil content or pressure dew point. In this article, we shall mainly discuss about the compressor flow rate.
1. Compressor Flow Rate
Flow rate of a compressor is measured by the volume of air flowing per unit of time. It is calculated as the result of the cross-sectional area of the air flow and the average flow velocity.
In air compressor industry, a compressor air capacity on the catalogs is the Free Air Delivery (FAD). It is the value of discharged air volume converted to suction condition, which is of 1 bar(a) and 20 ℃. Comair diesel portable air compressor of CDP-13/10 is used as an example to further illustrate on the Free Air Delivery (FAD).
However, for air treatment equipment including air dryers and line filters, the air treatment volume on the catalog is always described and measured by Nm3/min, which is different from the compressor specs. Nm3 means normal cubic meter at 0℃, with relative humidity of 0% and under atmospheric pressure, which called Normal conditions.
2. What Is M3/min And Nm3/min?
The m3/min(cubic metre per minute) is the most frequently used unit to descide a compressor air capacity. The above Comair diesel portable air compressor has the air capacity of 13 m3/min.
The m3/min is a unit to measure the air flow in a given period of time. Cubic meter is a metric unit used to measure the volume of space. Cubic meter per minute refers to the volume of air that flows through a cross-section of 1 meter x 1 meter in one minute. The symbol is “m3/min”.
Standard cubic meters per minute (Nm³/min) is a unit in the filed of air treatment and scientific research fields.
The only difference between m3/min and Nm3/min is the temperature. M3/min is a unit under 20℃ while Nm3/min is a unit under 0℃. They can be converted by the below formula:
P1*V1/T1=P2*V2/T2
For our above Comair diesel portable air compressor, its air capacity is 13 m3/min;
P1=1 bar(a), V1=13 m3/min, T1=273+20=293K;
P2=1 bar(a), T2=0+273=273K;
So when it is converted into Nm3/min, it shall be 12.11 Nm3/min.
We can also easily understand that from the temperature of 20℃ to 0℃, the temperature drops so the air shall contract.
3. What Is CFM, ACFM And SCFM?
CFM (Cubic Feet per Minute) is a commonly used unit by clients outside Chinese market for measuring air flow. For Comair diesel portable air compressor below, Chinese users will usually say it discharges air flow of 10 m3/min while the clients from English speaking countries may say it is of 353 cfm. Or the clients from English speaking countries will normally send inquiries in CFM, because it is an imperial unit.
CFM refers to the volume of air passing through a cross-section of 1 foot x 1 foot in one minute, that is, CFM is the volume unit of air flow. We shall know that CFM is measured under actual working temperature and pressure conditions. So CFM can also be called ACFM, which are the same. Like m3/min, CFM is also a unit for measuring free air delivery in accordance with ISO 1217 : 2009, Annex C: Absolute intake pressure 1 bar (a), cooling and air intake temperature 20°C.
SCFM is measured under standard conditions. It is mainly used in research fields and comparisons between different products.
The standard ambient temperature range for calculating SCFM flow rate is 60℉ (15℃) to 68℉(20℃), sea level pressure is 14.7 psi (760 mmHg), and relative humidity is about 36%.
SCFM and CFM changes with different temperature and pressure. Below chart will further help you understand how to convert under various conditions:
|
Condition |
SCFM (Standard) |
Equivalent CFM |
Notes |
|
20℃ @ 14.7psi |
2 SCFM |
2 CFM |
The same condition; SCFM=CFM |
|
37℃ @ 14.7psi |
2 SCFM |
~2.12 CFM |
Warmer air expands, so CFM higher |
|
0℃ @ 14.7psi |
2 SCFM |
~1.86 CFM |
Colder air contracts, so CFM lower |
|
20℃ @ 12 psi |
2 SCFM |
~2.45 CFM |
Lower pressure, more volume per minute |
|
20℃ @ 16 psi |
2 SCFM |
~1.83 CFM |
Higher pressure, less volume per minute |
The above calculation and understanding is of great importance, when the client is at high altitude area or at locations of extreme temperatures. If the compressed air consumption equipment requires air by SCFM, we shall make careful conversions. So we can choose correct compressors for the clients.
4. What Is Other Flow Rate Units?
In air compressor industry, there are also other units used regularly. However, they are quite easy to understand if we master the above units. The below are their details:
M3/hr, cubic meter per hour, is a unit to measure the flow rate of a compressor hourly. Since 1 hour equals to 60 minutes, 1 m3/min equals to 60 m3/hr.
L/min means liter per minute. 1 cubic meter = 1000 liters. And 1 liter = 0.001 cubic meter. So 1 L/min=0.001 m3/min.
L/s means liter per second. Since 1 minute equals to 60 seconds, 1 L/s=60 L/min=0.06 m3/min.
In3/min means cubic inches per minute. 1 foot equals to 12 inches. So 1 CFM=1728 in3/min.
Gallon is also another unit for measuring the volume or size of some thing, especially for liquids like milk, water or petroleum. So we shall also know its conversions into our familiar units. 1 UK Gallon = 4.546 liters. However, 1 US Gallon = 3.785 liters.
So 1 UK Gallon/min = 4.546 L/min; 1 US Gallon = 3.785 L/min.
In air compressor industry, we normally have 80 gallon air compressor, 20 gallon air compressor, or 30 gallon air compressor. They are small air compressor and are mounted on a air receiver tank of 20 gallon, 30 gallon, 60 gallon or 80 gallon. Their air capacity is within 20 cfm.
5. Flow Rate Units in Gas Compressors.
For gas compressors, especially the reciprocating natural gas compressors, flow rate units might be a little more complicated than regular screw air compressors. For other industrial gas applications, we may even use mass flow rate for precise data collection and compliance with regulations, because mass flow rate will not directly be affected by gas pressure and temperature. However, once we are clear of each unit’s condition, we will not be confused among them.
To measure gas flow, common mass flow rate unit is kg/hr and lb/hr. However, almost all the gas compressor and air compressor manufacturers on the market use volumetric flow rate units, which are MMSCFD, Nm3/hr, Sm3/hr, and SCFD.
Standard cubic foot unit, namely SCF, is under 15.6℃ and 14.7 psia; And normal cubic meter unit is under 20℃ and 101.3 kPaA.
By conversion, 101.3 kPaA almost equals to 14.7 psia. So the only difference for the above units is the temperature.
1 m = 3.28084 feet;
1 m3 = 1m * 1m * 1m = 3.28084ft * 3.28084ft * 3.28084ft = 35.3147 ft3;
In a day, we have 24 hours;
So the above gas flow rate units could be converted as below:
1 MMSCFD = 1000000 SCFD = 41666.67 SCFH = 1179.88 Sm3/hr (SCMH) = 1197. 87 Nm3/hr.
Conversion between mass flow rate unit and volumetric flow rate unit can be also complicated. If a client wants to set up a CNG mother station and its required flow rate is in mass flow rate unit, we shall also need to get the natural gas density so we can convert from the mass flow rate unit into volumetric flow rate unit.
6. Air Flow Conversion: CFM to M3/min.
For ordinary air compressors including diesel portable air compressor, oil injected screw air compressor, oil free scroll air compressor, oil free screw air compressor and centrifugal air compressor, the most frequently used flow rate unit is CFM and m3/min. 1 m3/min = 35.3147 CFM.
In the below form, we listed some commonly seen conversions between the two units and the corresponding Comair compressor models:
|
10 CFM |
0.28 m3/min |
Under normal working pressure of 7 bar to 13 bar, oil free scroll air compressor or oil injected screw air compressor of less than 18 kW will deliver air capacity within 100 CFM. |
|
20 CFM |
0.57 m3/min |
|
|
30 CFM |
0.85 m3/min |
|
|
40 CFM |
1.13 m3/min |
|
|
50 CFM |
1.42 m3/min |
|
|
60 CFM |
1.70 m3/min |
|
|
70 CFM |
1.98 m3/min |
|
|
80 CFM |
2.27 m3/min |
|
|
90 CFM |
2.55 m3/min |
|
|
100 CFM |
2.83 m3/min |
|
|
150 CFM |
4.25 m3/min |
Suggesting 30 kW compressor, COI-30 |
|
180 CFM |
5.10 m3/min |
Suggesting CDP-5/7, smallest diesel portable air compressor |
|
200 CFM |
5.66 m3/min |
Suggesting 37 kW compressor, COI-37 |
|
250 CFM |
7.08 m3/min |
Suggesting CDP-7/7, diesel portable air compressor for rock drilling, & jack hammer etc. |
|
280 CFM |
7.93 m3/min |
Suggesting 45 kW compressor, COI-45 |
|
300 CFM |
8.49 m3/min |
Suggesting 55 kW compressor, COI-45 |
|
350 CFM |
9.91 m3/min |
Suggesting CDP-10/13, diesel portable air compressor for sand blasting, cable blowing & shotcrete blasting. |
|
400 CFM |
11.33 m3/min |
Suggesting 75 kW compressor, COI-75; or CDP-11/10, for quarrying blasting. |
|
450 CFM |
12.74 m3/min |
Suggesting CDP-13/10 with built in after cooler & water separator, sand blasting compressor in oil & gas industry |
|
500 CFM |
14.16 m3/min |
Suggesting CDP-15/18 diesel portable air compressor, for water borehole drilling or mining drilling blasting. |
|
600 CFM |
16.99 m3/min |
Suggesting 90 kW compressor, COI-90 |
|
650 CFM |
18.41 m3/min |
Suggesting 110 kW compressor, COI-110 |
|
750 CFM |
21.24 m3/min |
Suggesting CDP-21/10 with built in after cooler & water separator, sand blasting compressor in oil & gas industry |
|
800 CFM |
22.65 m3/min |
Suggesting CDP-22/20, high pressure for water borehole drilling & oil well drilling |
|
900 CFM |
25.49 m3/min |
Suggesting CDP-27/10 with built in after cooler & water separator, sand blasting compressor in oil & gas industry |
|
1000 CFM |
28.32 m3/min |
Suggesting CDP-32/10, a renting required compressor in oil & gas industry |
|
1200 CFM |
33.98 m3/min |
Suggesting CDP-34/25, high pressure for water borehole drilling & mining drilling |
|
1600 CFM |
45.31 m3/min |
Suggesting CDP-45/10, a large flow air compressor in oil & gas industry |
7. Gas Flow Conversion: Nm3/hr, SCMH, SCFH & MMSCFD.
Gas compressors, especially the natural gas compressor, usually have very large flow rate. So their units are usually by each hour. Commonly seen units include Nm3/hr, SCMH, SCFH & MMSCFD.
Please see below form with some conversions between these units.
|
500 Nm3/hr |
492.5 SCMH |
17392 SCFH |
0.417 MMSCFD |
|
600 Nm3/hr |
591.0 SCMH |
20870 SCFH |
0.501 MMSCFD |
|
700 Nm3/hr |
689.5 SCMH |
24348 SCFH |
0.584 MMSCFD |
|
800 Nm3/hr |
788 SCMH |
27827 SCFH |
0.668 MMSCFD |
|
900 Nm3/hr |
886 SCMH |
31305 SCFH |
0.751 MMSCFD |
|
1000 Nm3/hr |
985 SCMH |
34785 SCFH |
0.835 MMSCFD |
|
1200 Nm3/hr |
1182 SCMH |
41740 SCFH |
1.00 MMSCFD |
|
1500 Nm3/hr |
1477 SCMH |
52176 SCFH |
1.25 MMSCFD |
|
2000 Nm3/hr |
1970 SCMH |
69567 SCFH |
1.67 MMSCFD |
|
6000 Nm3/hr |
5900 SCMH |
208333 SCFH |
5 MMSCFD |
|
12000 Nm3hr |
11800 SCMH |
416666 SCFH |
10 MMSCFD |
