What Are Mass Flow and Volume Flow?
Mass flow
Essentially, mass flow is the mass of a gas or liquid that flows in a certain amount of time. It is usually measured in units such as kilograms per second (kg/s) or pounds per minute (lb/min). In many industries, mass flow is critical to ensure accurate measurement and control of processes.
One advantage is that when measuring mass flow, mass or weight does not change with temperature or pressure. However, weight will change with gravity, but gravity is the same and does not affect mass flow.
Volume flow
Similarly, volume flow is the volume of gas or liquid flowing in a certain period of time, referred to as flow rate, represented by Q. It is usually expressed in units such as cubic meters per second (m³/s) or gallons per minute (GPM). Volume flow is essential for determining equipment size, monitoring fluid flow, and evaluating system performance.
Volume flow refers to the volume of fluid passing through a cross-section per unit of time. It is an important parameter for measuring the speed of fluid movement and is widely used in fluid mechanics, engineering fields, and the design and selection of flow meters.
In general, volume flow can be calculated by the following formula: Q=AXV. Among them, Q represents volume flow, A represents the cross-sectional area, and ⅴ represents the average flow velocity of the fluid on the cross-section.
Common equipment for measuring volume flow includes the following: Common flow velocity measurement methods include electromagnetic flowmeter, vortex flowmeter, ultrasonic flowmeter, etc.
How Are Mass Flow and Volume Flow Related?
Mass flow is directly related to volume flow and depends on the density of the fluid. To calculate mass flow, simply multiply the volume flow by the density of the fluid: mass flow = volume flow x density. Since density varies with temperature and pressure, it is important to consider the specific conditions of the fluid when converting between mass flow and volume flow.
The volume flow is calculated as follows: Volume flow (Qv) = Average flow velocity in the pipe (u) × Cross-sectional area of the pipe (A). This formula means that volume flow is the volume of fluid that flows through the pipe per unit time, usually in units such as cubic meters per second (m³/s) or liters per minute (l/min).
Volume flow is calculated by dividing the mass flow by the density of the fluid. Volume flow varies with temperature and pressure, while mass flow remains constant when temperature or pressure changes.
In addition, volume flow is different from mass flow. Mass flow is the flow expressed in mass/time, usually in kilograms per second (kg/s), and the calculation formula is mass flow (M) = medium density (ρ) × volume flow (Qv). This means that the mass flow rate depends not only on the flow velocity and cross-sectional area but also on the density of the fluid.
The Difference Between Volume Flow and Mass Flow
Both measure the movement of fluids, but “volume flow” refers to the volume of fluid flowing past a point per unit time, while “mass flow” measures the mass of fluid flowing past a point per unit time.
Essentially, volume flow is about the space occupied by the fluid, while mass flow focuses on the amount of material actually flowing. This makes it more reliable when dealing with compressible fluids such as gases, whose volume can change significantly with changes in pressure and temperature.
Key points of difference:
- What is measured:
Volume flow measures the volume of a fluid per unit time (e.g., cubic meters per second), while mass flow measures the mass of a fluid per unit time (e.g., kilograms per second). - Effects of pressure and temperature:
Volume flow is affected by changes in pressure and temperature because it depends on volume. Mass flow, on the other hand, remains constant even if pressure or temperature changes because the mass of the fluid remains constant. - Application:
Mass flow is often preferred in applications where the amount of actual material needs to be precisely controlled, such as chemical processes or gas handling. Volume flow is suitable for situations where the volume of the fluid is the primary consideration, such as measuring airflow in a ventilation system. - Measurement equipment：
Mass flow meters and volume flow meters are used to measure mass flow and volume flow, respectively. Some common mass flow meters include Coriolis flow meters, thermal mass flow meters, and differential pressure flow meters. On the other hand, volume flow meters may include turbine flow meters, positive displacement flow meters, and ultrasonic flow meters.
How to Measure and Control Mass Flow and Volume Flow?
How do we measure mass flow？
Mass Flow Meter:
This type of meter directly measures mass flow using principles such as thermal mass flow sensing, Coriolis force, or differential pressure. These principles are less affected by temperature and pressure changes than volumetric flow meters.
Based on volumetric flow calculations, if you have a volumetric flow meter, you can calculate mass flow by multiplying the volumetric flow rate by the fluid density.
How to measure volume flow?
Volumetric flow meters:
These meters measure volumetric flow directly using methods such as turbine rotation, vortex shedding, or magnetic flow meters.
Based on cross-sectional area and velocity calculations, you can calculate volumetric flow by multiplying the fluid velocity by the cross-sectional area of the pipe where the flow is being measured.
How to control mass flow and volume flow?
To control mass and volumetric flow, specialized devices called “mass flow controllers” are primarily used, which directly measure and regulate the mass of fluid flowing through a system.
They also adjust for changes in pressure and temperature, providing more accurate and stable flow control than simply controlling volumetric flow. For volumetric flow control, you can use standard flow control valves. However, be aware that their accuracy can be affected by fluctuations in pressure and temperature.
- Mass flow controller: This is the primary instrument used to control mass flow, as it measures the mass of fluid flowing through a system, not just the volume. This makes it ideal for applications that require precise control, regardless of changes in pressure or temperature.
- Volume flow control valve: Standard flow control valves can be used to regulate volumetric flow by adjusting the size of the opening through which the fluid passes. Even with a fixed valve setting, changes in pressure and temperature can significantly affect the actual volumetric flow.
Understanding the key differences between mass flow and volumetric flow and mastering conversion techniques are essential for professionals in industries that rely on accurate flow measurement. By following the guidance provided in this guide, you can enhance your understanding of these key concepts and improve operational efficiency.
At Sino-Inst, we are an experienced manufacturer and supplier of flow meters and process control equipment, offering a wide range of products tailored to your specific needs. We support customization and are committed to providing you with high-quality, reliable solutions
