Why mass flow is important for low-pressure applications

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February 3, 2021

Why mass flow is important for Low-Pressure applications

This blog addresses the difference between mass flow and volume flow as these terminologies hold significance for various low-pressure applications like aeration in the effluent treatment plant, fermentation in food & pharmaceutical.

This blog topic is limited to air as the medium.

Volume flow

The volumetric flow rate of a system is a measure of the volume of fluid flowing per unit of time. It may be calculated as the product of the cross-sectional area of the flow and the average velocity of the flow.

This term predominantly indicates the volume flow rate of compressed air in cubic feet per minute (cfm), cubic meter per minute (m3/min), cubic meter per hour (m3/hr), etc. which are the commonly used units of volume flow.

Volume flow rate is important when the end application includes:

Operating a pneumatic cylinder
Operating a solenoid valve
Operating a control valve
The air required for conveying a powder material etc.

Now you may have the immediate question - why do people specify flow rates as NM3/Hr and SCFM?

Well, for that you have to understand the basic nature of air which is a gas, Therefore, it behaves differently at various temperatures and pressure.

Hence when you talk about airflow rates, it is important to mention it with the associated temperature and pressure for clarity.

NM3/Hr can be simply explained as a cubic meter per hour (m3/hr) at normal temperature and pressure conditions (shortly referred to as NTP conditions).

In NTP conditions

Temperature is: 0-degree Centigrade
Pressure is: 1.013 bar(a)
RH: 0%

SCFM can be simply explained as cubic feet per minute (cfm) at standard temperature and pressure conditions (shortly referred to as STD conditions).

In STP conditions

Temperature is: 15.6-degree Centigrade
Pressure is: 1 bar(a)
RH: 0%

Hence choosing the right air compressor that delivers a required flow rate at all temperature conditions is important.

Example

Let’s take an example of an automobile plant that requires a total compressed air flow rate of 1000 NM3/Hr Free Air Delivered (FAD). The compressor should be able to build a pressure of 6 bar(g). This requirement is generally arrived at by adding the compressed air flow required by all the equipment (cylinders, sovs, control valves, paint guns, etc.). Let us assume that the plant has situated at Pune, India wherein the ambient temperature varies between 10 - 40 degrees and the Ambient Pressure in Pune is 0.9465 bar(a). Now using the FAD formula, the volume flow rate becomes 1227 m3/hr at 40 degree C ambient temperature or 1109 m3/hr at 10 degrees C ambient temperature. That's why it is essential to choose the right compressor with a maximum FAD capacity of more than 1227 M3/Hr so that the plant functions without any problem at peak summer. During winter, the oversized compressor should not run into unloading and waste power. Hence it is always better to have a variable speed drive (VSD) compressor (or) a combination of fixed and variable speed compressor to address these challenges. The implications can be more when the airflow rate requirement is more, say for example 5000 NM3/Hr.

Top Tip: Our air experts recommend calculating the flow rates at various temperatures and then size the optimum compressor or a combination of compressors accordingly.

Mass flow

The mass flow rate of a system is a measure of the mass of fluid passing per unit of time. It may be calculated as the product of the density of the fluid and the volume flow rate. The picture depicts how mass flow varies with temperature. Considering air as medium, the mass flow rate for 1000 NM3/hr volume flow rate at 20 deg C is 1294 Kg/hr, and for the same volume flow at 45 deg C, the mass flow rate changes to 1193 Kg/hr. This means the number of oxygen molecules available per cubic meter of air is less at higher ambient temperatures. Now to connect this example with two critical applications, the importance of mass flow rate can be understood.

Compressed air requirement in the aeration tank of an ETP or STP (Effluent or Sewage treatment plant).
Compressed air requirement inside a bioreactor in the fermentation process.

In both cases, the number(or mass) of oxygen molecules that enter the tanks are critical for the growth of the micro-organisms. Therefore, the Dissolved Oxygen (DO) which is a critical parameter, is maintained at the required level for an efficient process. The above example clearly illustrates the fact that for applications like fermentation and aeration, merely improving the volume flow rate will not help. You have to equally consider the mass flow rate especially when it maintains the dissolved oxygen (DO).

Conclusion

The understanding of mass flow and volume flow rate is critical for designing of right air compressor or root blowers for various applications.

As you know the flow rates are directly impacted by temperature. Therefore, the low-pressure compressors (or) root blowers should be installed in such a way that the cool air directly enters the compression chamber, without picking up heat from any other parts of the air compressor or air blower for efficient working and high plant productivity.

Visit our compressed air wiki for volume flow rate, work, and power tool for measurement.

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