Unit of Measurement for Compressed Air.

Abhijit.S.Musale

www.abhijitsmusale.com | June.2025

Keywords

Compressed Air
SCFM
CFM
Volumetric Flow Rate

Summary

Understanding compressed air units can be confusing, especially when equipment catalogs throw around terms like SCFM, Nm³/min, FAD, CFM, and LPM. This article breaks down what these units really mean and why they matter. Since air is a compressible fluid, its volume changes with pressure, temperature, and humidity—making “standard” and “normal” conditions essential reference points. Using practical examples, the article shows how ambient conditions like heat and humidity (common in places such as India) affect compressor performance and sizing. It explains how Free Air Delivery (FAD) is calculated, why humidity reduces air density, and how conversions between SCFM and CFM vary with temperature and moisture content. By demystifying these units, the piece helps engineers and users make more accurate comparisons, communicate effectively with vendors, and avoid costly mistakes in compressor selection.

Introduction

There is often confusion among new engineers regarding the units used to describe the capacity of compressed air system equipment. Catalogues and specifications frequently present different units, and a clear understanding of each is essential for accurate performance evaluation and capacity calculations. The most commonly referenced units are SCFM (Standard Cubic Feet per Minute) and Nm³/min (Normal Cubic Meter per Minute). In addition to these, other terms such as FAD (Free Air Delivery), CFM (Cubic Feet per Minute), ICFM (Inlet Cubic Feet per Minute), and LPM (Liters per Minute) are also widely used.

All of the previously mentioned terms represent units of volume flow rate. Since air is a compressible fluid, its volume varies with changes in pressure and temperature. In contrast, for non-compressible fluids, volume remains nearly constant within a certain range of pressure and temperature. The same quantity of air, measured in moles, can occupy different volumes depending on the pressure conditions. To account for this variability, units such as SCFM (Standard Cubic Feet per Minute) and Nm³/min (Normal Cubic Meters per Minute) include the prefixes “Standard” and “Normal,” which specify reference conditions of pressure, temperature, and humidity.

Standard temperature and pressure conditions are called STP and they have been defined by IUPAC (International Union of Pure and Applied Chemistry). STP Conditions are shown below.

Table 1 : Standard Conditions
Standard Temperature and Pressure Conditions (STP) by IUPAC
Standard Pressure	1 bar (10⁵ Pa, 100 kPa)
Standard Temperature	0^oC (273 K)
Standard Relative Humidity	0%

Normal temperature and pressure conditions are breifly called NTP and they have been defined by NIST (National Institute of Standards and Technology). NTP conditions are shown below.

Table 2 : Normal Conditions
Normal Temperature and Pressure Conditions (NTP) by NIST
Normal Pressure	1 atm (1.013 bar, 101.3 kPa)
Normal Temperature	20^oC (293 K)
Normal Relative Humidity	0%

When converting SCFM to Nm³/hr, the relationship is approximately 1 SCFM = 1.61 Nm³/hr. While most standard institutes and research organizations define these reference conditions in a similar way, slight variations may exist. Unlike SCFM and Nm³/min, other units such as FAD (Free Air Delivery), CFM (Cubic Feet per Minute), ICFM (Inlet Cubic Feet per Minute), and LPM (Liters per Minute) lack prefixes that specify the pressure and temperature conditions under which the volumes are measured. This omission often leads to ambiguity in interpreting the actual volume of air. Factors such as lower atmospheric pressure at higher elevations, increased temperature, or higher humidity can expand the volume of air. Consequently, a compressor rated to deliver a certain SCFM under standard or normal conditions must consume more power to produce the same volume of compressed air under varying ambient conditions. The influence of environmental factors on compressor sizing and selection is an important subject worthy of deeper exploration.

In hot and humid regions such as India, ambient temperature and humidity significantly affect air density and volumetric flow rate. During summer, when temperatures range between 32°C and 44°C, the actual volume of air (expressed in cubic feet or cubic meters) is lower than under standard or normal conditions because of reduced air density. Humid air is even less dense, as water vapor has a lower molar mass compared to dry air. As a result, compressors must intake a larger volume of air to deliver the same amount of compressed air that would be produced under cooler, less humid conditions. Therefore, it is essential to convert air volume measured at site ambient conditions into “Normal” or “Standard” conditions to ensure accurate comparison and effective communication with compressor vendors, enabling meaningful sizing and selection calculations.

An Example

For example let us assume some ambient pressure, temperatures, and volume and try to convert them into the standard and normal conditions. Following is the presssure temperature and Volume relationship.

P_atm x V_atm T_atm = P_std x V_std T_std = P_nor x V_nor T_nor

(1)

Where,

P = Pressure
V = Volume Flow Rate
T = Temperature
P_v = Vapor Pressure of Saturated Air at Given temperature

Suffixes,

atm = Atmospheric Parameters
std = Standard parameters
nor = Normal Parameters
sat = Saturated Condition

Formula below gives the free ambient air flow rate required for compressor at its inlet.

V_atm = P_std P_atm x T_atm T_std x V_std

(2)

Formula above assumes that the inlet air is completely dry. So for example, assume if your compressor have requirement of 100 SCFM (2.83 Standard m³/min), and your ambient Pressure is 1.01325 bar (14.69 psi), ambient temperature is 35 Deg C (95 Deg F), then using above formula the same quantity (100 sCFM) at ambient conditions will be,

V_atm = [ 14.5 14.69 ] x [ 95 + 460 32 + 460 ] x 100

(3)

V_atm = 111.35 CFM = 3.15 m³/min

So, 100 CFM at standard conditions will be equal to 111.35 CFM at atmospheric conditions. In this relationship the volumetric flow rate of ambient air seems to be depending only on the ambient temperature, because we have assumed that the air is completely dry and the atmospheric pressure does not change much close to sea level. So, in this scenario how volumetric flow rate of ambient air changes with the temperature for 100 SCFM flow at standard condition is shown in table below.

Table 3 : Volumetric flow rate comparision at standard and ambient conditions.
Vol. Flow at Standard	Temperature		Vol. Flow at Ambient
SCFM	Deg C	Deg F	CFM
100	5	41	100.50
100	10	50	102.30
100	15	59	104.11
100	20	68	105.91
100	25	77	107.72
100	30	86	109.52
100	35	95	111.33
100	40	104	113.14
100	45	113	114.94

This is how the free air delivery (FAD) is determined. Units of FAD could be CFM or m³/min or any unit of volumetric flow rate from any unit system. FAD is basically quantity of compressed air converted back to the ambient air conditions or inlet air conditions. It means the volume of compressed air that comes out of the exhaust of pneumatic actuators and mixes in to the open air after its use, is same as the volume sucked at the inlet of compressor. The volume that goes into compressor, the same comes out of it. This is true when the air is completely dry. When ambient air is humid the volume of air that comes out of compressed air system is not same as the volume that entered at inlet. Because as air gets compressed the water in the air gets condensed due to high pressure. Also cooling water and cooling air in the inter cooler and after cooler helps moisture to condense. At last at the compressed air dryer the compressed air dried such that its due point goes to about -10 Deg to -40 Deg C (Depending on the Dryer type). Hence, humidity needs to be considered while compressed air demand from SCFM to CFM.

Actual site conditions have air with certain amount of humidity. Presence of humidity in the air also affects the volume conversion. In standard and normal conditions 0% relative humidity has been considered. Where as humidity can vary from 30% at some of the driest places to 95% at some of the most humid places on earth.

We know that Relative Humidity is ratio of partial pressure of water vapor in the ambient air to the partial pressure of water vapor in the saturated air at the given temperature.

RH = P_{v_atm} P_{v_sat@atm}

(4)

Where,

P_{v_atm} = Partial pressure of vapor at atmospheric temperature
P_{v_sat@atm} = Partial pressure of vapor of saturated air at atmospheric temperature.

Hence, equation (2) can be modified by subtracting partial pressure of water vapor from the total air pressure shown in it.

V_atm = P_std - P_{v_std} P_atm - P_{v_atm} x T_atm T_std x V_std

(5)

V_atm = P_std - (RH x P_{v_sat@std}) P_atm - (RH x P_{v_sat@atm}) x T_atm T_std x V_std

(6)

Now, if we consider the same parameters in the example before and consider 85% Relative Humidity. assuming if your compressor have requirement if 100 SCFM (2.83 Standard m³/min), and your ambient pressure is 1.01325 bar (14.69 psi), ambient temperature is 35 Deg C (95 Deg F), then using above formula the same quantity (100 SCFM) at ambient conditions will be,

V_atm = [ 14.5 - (0 x 0.006132) 14.69 - (0.85 x 0.056261 x 14.5) ] x [ 95 + 460 32 + 460 ] x 100

(7)

V_atm = 116.86 CFM = 3.31 m³/min

Thus, 100 SCFM will be equal to 116.86 CFM at the atmospheric conditions mentioned above. Due to presence of humidity in the air the density of air decreases and its volume increases. 100 SCFM is equal to 111.35 CFM of dry air and 116.86 CFM of 85% humid air. It is evident from the formula that the major factors that affects the volume conversion are temperature and relative humidity. Ambient air pressure does not differ much close to the sea level. Table below shows hows conversion of 100 SCFM air to CFM of dry air and CFM of 85% moist air.

Table 4 : Volumetric Flow rate comparision for dry air and moist air.
Vol. Flow at Standard	Temperature		Dry Air Vol. Flow	Moist Air Vol. Flow
SCFM	Deg C	Deg F	CFM	CFM
100	5	41	100.50	101.24
100	10	50	102.30	103.37
100	15	59	104.11	105.63
100	20	68	105.91	108.03
100	25	77	107.72	110.67
100	30	86	109.52	113.57
100	35	95	111.33	116.84
100	40	104	113.14	120.63
100	45	113	114.94	125.00