Reference ID: MET-6358 | Process Engineering Reference Sheets Calculation Guide
Introduction & Context
In-line blending ratio control is the systematic adjustment of two (or more) process streams so that their combined composition meets a specified target. In water treatment, food & beverage, specialty chemicals, and pharmaceuticals, it is often necessary to dilute a concentrated additive or to fortify a lean stream without using an agitated vessel. By metering the flows in the correct proportion, the blend is achieved inside the pipe itself, saving space, reducing hold-up, and avoiding off-spec product.
🚀 Skip the Manual Math!
Use our interactive In-Line Blending Ratio Control to compute these parameters instantly online, or download the offline Excel calculation.
Overall mass balance
The total mass flow is conserved:
\[ \rho Q = \rho Q_{1} + \rho Q_{2} \]
where \( Q \) is the blended volumetric flow and \( \rho \) is the common density (assumed identical for both streams).
Component mass balance
For the component of interest (expressed as mass fraction or wt %):
\[ Q C = Q_{1} C_{1} + Q_{2} C_{2} \]
Eliminate \( Q \) with the overall balance and solve for the required ratio:
\[ \frac{Q_{2}}{Q_{1}} = \frac{C - C_{1}}{C_{2} - C} \]
The numerator and denominator are protected against division-by-zero by enforcing a minimum value \( \varepsilon \).
Convert volumetric flow to SI units
\[ Q_{1} \, [\mathrm{m^{3}\,s^{-1}}] = \frac{Q_{1} \, [\mathrm{L\,min^{-1}}]}{1000 \times 60} \]
Pipe cross-section
\[ A = \frac{\pi D^{2}}{4} \]
Mean velocity of stream 1
\[ v_{1} = \frac{Q_{1}}{A} \]
Reynolds number for stream 1
\[ \mathrm{Re}_{1} = \frac{\rho v_{1} D}{\mu} \]
where \( \mu \) is the dynamic viscosity.
Operating regime guidelines
Reynolds number
Flow regime
Mixing quality
Re < 2 300
Laminar
Poor; avoid for in-line blending
2 300 ≤ Re < 3 000
Transitional
Marginally acceptable
Re ≥ 3 000
Turbulent
Good; recommended
Practical ratio limits
Ratio \( Q_{2}/Q_{1} \)
Interpretation
0–0.1
Very low additive rate; check metering precision
0.1–5
Typical industrial range
> 5
High additive rate; verify economics and pipe sizing
The concentration ordering must satisfy \( C_{1} < C < C_{2} \) (or the reverse) for a physically realisable ratio; otherwise, the target is unattainable with the supplied streams.
A well-tuned in-line blending skid with Coriolis-based flow meters and a fast PID controller can hold component ratios within ±0.2 % of set-point (2σ) at steady state. Transient deviations during grade changes are usually <1 % for <30 s. Achieving this requires:
Mass-flow meters calibrated to 0.1 % of rate
Control valve stiction <0.5 % and dead-time <300 ms
Line velocity >1.5 m s⁻¹ to keep mixing length short
Use a rate-limited set-point generator tied to header volume. Configure the ramp so the total volumetric change never exceeds 10 % of header volume per minute. If header level or pressure deviates by >3 %, freeze the ramp and let the level controller recover before continuing.
Coriolis meters with entrainment detection (micro-burst drive) give the best ratio stability under 5 % GVF. If gas volume fraction can spike higher, install an inline degassing vessel or switch to a low-frequency ultrasonic meter calibrated for two-phase correction.
Prove each component meter monthly if the blend is sold by specification, or quarterly for internal transfer. Use a compact prover installed in recycle so the line stays packed; a full prove takes <10 min and keeps uncertainty ±0.15 %.
Worked Example: Setting the Water-to-Additive Ratio in an In-Line Blender
A beverage plant needs to dilute a concentrated flavour additive (Stream 2) with treated water (Stream 1) to obtain a 10 °Brix product. The blender uses two positive-displacement pumps feeding a static mixer. Determine the required additive flow rate when the water line is fixed at 100 L min-1.
Convert water flow to SI units:
\[ Q_1 = \frac{100}{1000 \times 60} = 0.00167\ \text{m}^3\ \text{s}^{-1} \]
Apply the in-line blending mass balance for the target concentration:
\[ C_1 Q_1 + C_2 Q_2 = C_{\text{target}}(Q_1 + Q_2) \]
Since the water contains no additive, \( C_1 = 0 \). Rearranging gives the flow ratio:
\[ \frac{Q_2}{Q_1} = \frac{C_{\text{target}}}{C_2 - C_{\text{target}}} \]
Insert the known concentrations:
\[ \frac{Q_2}{Q_1} = \frac{10}{50 - 10} = \frac{10}{40} = 0.2 \]
