Reference ID: MET-A18B | Process Engineering Reference Sheets Calculation Guide
Introduction & Context
The surface finish of mixing equipment—quantified by the arithmetic mean roughness Ra—directly affects cleanability, product contamination risk, and power consumption in agitated vessels. In pharmaceutical, food, and fine-chemical processes, a finish smoother than the empirical limit reduces bio-film adhesion and cleaning-cycle time, while a finish that is too smooth (< 0.2 µm) may be economically unjustified and outside validated correlations. The worksheet provides a single-point check that the measured Ra lies within the validated Reynolds-number regime and satisfies both a conservative “target margin” and a contractual “work-order” specification.
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Reynolds-number validation
The impeller Reynolds number must exceed the empirical lower bound for the roughness correlation to hold:
\[
Re_{\mathrm{impeller}} = \frac{\rho N D^{2}}{\mu}
\]
where ρ fluid density (kg m-3) N impeller speed (s-1) D impeller diameter (m) μ dynamic viscosity (Pa·s)
Surface-finish acceptability window
The measured Ra must fall inside the correlated range:
Parameter
Lower bound
Upper bound
Unit
Ra
Ra,min valid
Ra,max valid
µm
Outside this window, the literature correlation is not applicable.
Compliance criteria
The as-built finish is compared against two internal limits:
Criterion
Symbolic limit
Meaning
Target margin
Ra ≤ Ra,target margin
Conservative process requirement
Work-order rule
Ra < Ra,spec limit
Contractual procurement limit
Both must be satisfied; the worksheet returns PASS only when the measured value is at or below the target margin.
A 0.4 µm (16 µin) Ra mechanical polish followed by electropolishing is the industry norm. This finish:
Eliminates micro-pits where product or microbes can hide
Reduces cleaning time and chemical consumption
Meets ASME-BPE and cGMP requirements for wetted parts
Rougher finishes create a thicker boundary layer, lowering heat-transfer coefficients by up to 15 %. A 0.4 µm Ra electropolished surface:
Minimizes fouling resistance
Maintains design U-values, shortening batch cycle time
Reduces hot-spots that can degrade product
2B (cold-rolled, annealed, pickled) is acceptable only for non-critical food contact zones. For product-contact surfaces, upgrade to:
2B + 120-grit polish (≈ 0.8 µm Ra) minimum
Electropolish to pass cleanability tests under FDA 21 CFR 117
Use a combination of:
Surface-profile stylus (Ra, Rz) on representative 150 mm coupons welded into the vessel
Replica tape tests for field welds
Visual comparison to ASTM A967 photo standards for electropolish
Worked Example: Verifying Impeller Surface Finish Acceptability
A 5 m³ open-top vessel is used to blend a low-viscosity aqueous solution at ambient conditions. The 300 mm diameter pitched-blade impeller was supplied with a “0.8 µm Ra max” specification. After fabrication, the measured average roughness is 0.55 µm. Determine whether the surface finish is acceptable for the intended Reynolds-number regime.
Compare measured roughness with the specification limit: Ra,meas = 0.55 µm ≤ Ra,max,spec = 0.8 µm → within specification.
Check against the recommended target margin: Ra,meas = 0.55 µm ≤ Ra,target = 0.6 µm → satisfies the tighter margin.
Conclude acceptability: the impeller surface finish is acceptable for the duty.
Final Answer: The measured Ra of 0.55 µm is below both the 0.8 µm specification limit and the 0.6 µm target margin; therefore, the surface finish is acceptable for the 15,000 Reynolds-number mixing duty.
"Un projet n'est jamais trop grand s'il est bien conçu."— André Citroën
"La difficulté attire l'homme de caractère, car c'est en l'étreignant qu'il se réalise."— Charles de Gaulle
