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What is the difference between volume flow, mass flow and normal flow?
The volume flow rate Qv is the volume of fluid passing through a cross-section per unit of time (m³/s, m³/h, L/min). The mass flow rate Qm is the mass passing through that section per unit of time (kg/s, kg/h): it equals the volume flow multiplied by the density. For a gas, whose volume depends on pressure and temperature, the volume flow is referred to standardised conditions: this is the normal flow in Nm³/h, which is just another way of expressing a mass flow.
Flow rate formulas
Normal flow of a gas from the actual flow measured at absolute pressure P and temperature T:
- Qv: volume flow rate, in m³/s (1 m³/h = 1/3,600 m³/s)
- S: flow area, in m² (S = π D² / 4 for a round duct); V: mean velocity, in m/s
- Qm: mass flow rate, in kg/s; ρ: density at actual conditions, in kg/m³
- Qn: normal flow, in Nm³/s or Nm³/h; Pn, Tn: normal pressure and temperature of the chosen standard
- P, T: actual absolute pressure (Pa) and absolute temperature (K = °C + 273.15) of the gas
Flow units and normal conditions
| Quantity | Equivalent |
|---|---|
| 1 m³/h | 0.2778 L/s = 16.67 L/min = 2.778 × 10−4 m³/s |
| 1 L/s | 3.6 m³/h = 60 L/min |
| 1 m³/h of water | ≈ 1,000 kg/h = 0.278 kg/s |
| Normal conditions DIN 1343 | 0 °C (273.15 K) and 1,013.25 hPa: air ρ = 1.293 kg/m³, 1 Nm³/h = 1.293 kg/h |
| Standard conditions ISO 2533 | 15 °C (288.15 K) and 1,013.25 hPa: air ρ = 1.225 kg/m³, 1 Nm³/h = 1.225 kg/h |
| Compressed air at 6 bar gauge and 20 °C | 1 actual m³/h ≈ 6.44 Nm³/h (DIN 1343) |
Detailed explanations and calculation in Mecaflux
We consider the flow regime is iso-volume or stationary
(that the fluid is incompressible liquid or of constant density, gas)
-
The flow of a fluid may be, volume flow or mass flow
Mass flow =volume flow x density
-
The volume flow can be calculated with the average velocity of the fluid and the pipe section:
Volume flow (m3/sec)= section(m²) x average speed (m / sec)
- The volume flow can be calculated with with the volume of fluid and the flow time:
Volume flow (m3/sec)=volume of fluid passed (m3) / flow time (seconds)
- relations that bind the volume flow and the fluid velocity, to the different sections are set out in the continuity equation
- The head loss are directly related to the fluid velocity, and therefore to the dimensions of the conduits sections.
For compressible fluids (gas or air):
The gas being compressible, it is possible to change a volume of the same amount of gasby compressing or changing its temperature. It becomes very difficult to speak of a quantity of gas volume without giving the pressure and temperature of the gas when the volume was measured. It would be difficult to give a temperature and pressure whenever we speak of a volume of gas. Then the temperature and pressure at the time of measurement of the volume, are normalized at called normal conditions. This is called normal m3.
Attention, there are two standards, and thus potential errors in conversions:
same pressure of 1013 millibars (average atmospheric pressure) for 2 standards:
DIN 1343 : temperature 273.15K (0°C)
ISO 2533 : temperature 288.15K (15°C)
For calculations of head loss, the size of volumes of gas (compressed or not) moved into the ducts must be given in cubic meters (m3).
For conversions or Nm3 Nm3 m3 / h m3 / h:
P1V1/T1 = P2V2/T2
p1 and T1 being normal temperatures and pressures v1 is the normal volume (relaxed)
p2 and T2 being the temperature and pressure gas V2 is the volume of the compressed gas
with:
P = Absolute pressure (gauge pressure + atmospheric pressure)
V = volume
T = temperature Kelvin
A graphical analysis tool based on the flow can test your network in a flow range selected, and create graphs showing variations of pressure, loss, and energy consumption, depending on the flow. This tool is very effective for the choice of material, in terms of annual operating costs.
- 1 select fluid
- 2 enter volume flow
integrated converter mecaflux
The conversion interface normal volume (nm3) and real (m3) converts the normal flow (Nm3 / h) to real (m3 / h)
Frequently asked questions
How do I convert volume flow to mass flow?
Multiply by the density of the fluid at the actual pressure and temperature: Qm = ρ × Qv. For 10 m³/h of water: 10 × 1,000 = 10,000 kg/h. For 10 m³/h of air at 20 °C and atmospheric pressure (ρ = 1.204 kg/m³): 12.0 kg/h.
Why are normal cubic metres used for gases?
Because the volume of a gas changes with pressure and temperature: 1 m³ of compressed air at 6 bar contains six times more matter than 1 m³ at atmospheric pressure. The Nm³ fixes reference conditions and therefore expresses an amount of matter, like a mass flow.
Which standard should I use, DIN 1343 or ISO 2533?
The one your supplier or specification uses. The difference between the two is 5.5 % (273.15/288.15), a classic source of error. In European industry the Nm³ in the DIN 1343 sense (0 °C) is the most common; ISO 2533 (15 °C) is used in aeronautics and by some gas utilities.
How do I convert and check the gas flow rates of my own installation?
The unit converter of Mecaflux Standard goes from actual flow to normal flow and mass flow from the pressure and temperature of your gas, in either standard, and that flow feeds the velocity and head loss calculations of the ducts directly. For a compressed-air, gas or flue gas network to size, Mecaflux carries out the study of your project, or offers training on the software applied to your own installation.