What is the difference between mass flow rate and mass flux?

Mass flow rate (kg/s) is total mass per time. Mass flux (kg/m²·s) is mass flow rate per unit area. Mass flux = mass flow rate / area. Mass flux is useful for comparing different-sized equipment and for correlations that depend on velocity (since G = ρv, mass flux relates directly to velocity).

Why is mass flux important in heat exchangers?

Mass flux determines Reynolds number (Re = GD/μ), which governs heat transfer coefficient and friction factor. Higher mass flux means higher velocity, better heat transfer (turbulent flow), but also higher pressure drop. Design balances these factors for optimal performance.

What is critical heat flux?

Critical heat flux (CHF) is the maximum heat flux a boiling surface can dissipate before transitioning to film boiling, where a vapor layer insulates the surface causing rapid temperature rise. CHF depends on mass flux—higher G allows higher CHF. Boiler tubes must operate below CHF to prevent burnout.

How do I calculate mass flux from volumetric flow?

Mass flux G = (volumetric flow rate × density) / area = ṁ/A = ρv. For example, water at 10 L/s through a 0.05 m² pipe: G = (0.010 m³/s × 1000 kg/m³) / 0.05 m² = 200 kg/m²·s. This equals 720,000 kg/m²·h.

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About Mass Flux Density Conversion

Mass flux density (also called mass velocity or mass flux) measures mass flow rate per unit cross-sectional area—how much mass passes through each square meter of surface per second. Unlike simple mass flow rate, mass flux normalizes flow by area, making it essential for comparing equipment of different sizes and for heat transfer correlations that depend on velocity. Since mass flux G = ρv (density × velocity), it directly relates to flow conditions inside pipes and channels.

The SI unit is kilograms per square meter second (kg/m²·s). Mass flux density is fundamental to heat exchanger design (determines Reynolds number and heat transfer coefficient), boiler tube sizing (critical heat flux limits), evaporator performance, drying processes, and combustion chamber design. The correlation between mass flux and pressure drop governs the trade-off between heat transfer performance and pumping power in virtually all thermal equipment.

Our converter handles mass flux density units used in thermal engineering, chemical processing, and heat exchanger design.

Common Mass Flux Density Conversions

From	To	Multiply By
kg/m²·s	kg/m²·h	3,600
kg/m²·h	kg/m²·s	2.778 × 10⁻⁴
kg/m²·s	lb/ft²·s	0.2048
lb/ft²·s	kg/m²·s	4.882
kg/m²·s	lb/ft²·h	737.3
lb/ft²·h	kg/m²·s	1.356 × 10⁻³
kg/m²·s	g/cm²·s	0.1
g/cm²·s	kg/m²·s	10
kg/m²·s	tonne/m²·h	3.6
lb/ft²·min	kg/m²·s	0.0814

Mass Flux Density Unit Reference

Kilogram per square meter second (kg/m²·s) – The SI unit for mass flux density. Represents mass flow rate per unit cross-sectional area, directly related to velocity by G = ρv. Typical heat exchanger values range from 50-2000 kg/m²·s depending on application. This unit appears directly in heat transfer correlations like Dittus-Boelter (Nu = 0.023 Re^0.8 Pr^0.4 where Re = GD/μ) and pressure drop equations.

Pound per square foot hour (lb/ft²·h) – US engineering unit widely used for evaporators, boilers, and heat exchangers in North American industry. 1 lb/ft²·h ≈ 0.00136 kg/m²·s. Evaporation rate specifications, cooling tower performance, and US-origin equipment catalogs typically use this unit.

Kilogram per square meter hour (kg/m²·h) – Practical metric unit for processes where hourly rates are more intuitive than per-second rates. Common for evaporation rates, drying process specifications, and coating operations. Water evaporation from pools is typically 0.1-0.5 kg/m²·h; industrial spray dryers operate at 5-50 kg/m²·h.

Gram per square centimeter second (g/cm²·s) – CGS unit occasionally found in scientific literature and older references. 1 g/cm²·s = 10 kg/m²·s. Used in some mass transfer correlations and laboratory-scale experiments.