Cleanability of Size Reduction Equipment

Reference ID: MET-F6EA | Process Engineering Reference Sheets Calculation Guide

Introduction & Context

The calculation presented evaluates the cleanability of size‑reduction equipment (e.g., grinders, crushers, or shredders) during a Clean‑In‑Place (CIP) operation. In process engineering, CIP performance is critical for ensuring product safety, meeting regulatory hygiene standards, and minimizing downtime. The model predicts hydraulic residence time, required chemical dosage, pressure drop, and rinse water consumption based on equipment geometry, fluid properties, and operating conditions. It is typically applied during equipment design, validation of cleaning protocols, and routine process optimization.

Methodology & Formulas

The procedure follows a deterministic sequence: convert user inputs to SI units, compute flow characteristics, determine the effective cleaning volume (including dead‑zone allowance), calculate hydraulic and chemical cleaning times, estimate pressure drop, and finally assess rinse requirements. All relationships are expressed algebraically.

1. Unit Conversions

\[ D = \frac{D_{\text{mm}}}{10^{3}} \qquad \varepsilon = \varepsilon_{\mu\text{m}}\times10^{-6} \qquad \mu = \mu_{\text{cP}}\times10^{-3} \]

2. Cross‑sectional Area

\[ A = \frac{\pi D^{2}}{4} \]

3. Reynolds Number (Flow Regime)

\[ \text{Re} = \frac{\rho\,V\,D}{\mu} \]

4. Volumetric Flow Rate

\[ Q = A\,V \]

5. Geometric and Effective Cleaning Volumes

\[ V_{\text{geom}} = A\,L \] \[ V_{\text{eff}} = V_{\text{geom}}\,(1+\phi_{\text{dz}}) \] where \(\phi_{\text{dz}}\) is the dead‑zone factor.

6. Hydraulic Cleaning Time

\[ t_{\text{hyd}} = \frac{V_{\text{eff}}}{Q} \]

7. Chemical Hold Time and Total Cleaning Time

\[ t_{\text{react}} = t_{\text{hold}} \] \[ t_{\text{total}} = t_{\text{hyd}} + t_{\text{react}} \]

8. Chemical Dosage

Chemical concentration expressed as mass per litre: \[ C = \frac{c_{\%}}{100} \] Effective volume in litres: \[ V_{\text{eff, L}} = \frac{V_{\text{eff}}}{10^{-3}} \] Mass of chemical required: \[ M_{\text{chem}} = C \, V_{\text{eff, L}} \]

9. Pressure Drop (Darcy‑Weisbach, Swamee‑Jain friction factor)

Friction factor: \[ f = \frac{0.25}{\left[\log_{10}\!\left(\frac{\varepsilon}{3.7D} + \frac{5.74}{\text{Re}^{0.9}}\right)\right]^{2}} \] Pressure drop: \[ \Delta P = f\,\frac{L}{D}\,\frac{\rho V^{2}}{2} \]

10. Rinse Water Requirements

Rinse flow (assumed equal to cleaning flow): \[ Q_{\text{rinse}} = Q \] Volume per rinse: \[ V_{\text{rinse}} = Q_{\text{rinse}}\,t_{\text{rinse}} \] Total rinse water: \[ V_{\text{rinse,total}} = N_{\text{rinse}}\,V_{\text{rinse}} \]

Validity Checks (Thresholds)

Parameter	Acceptable Range	Warning Condition
Reynolds number (Re)	\(\ge 4000\) (turbulent)	Re < 4000
Surface roughness \(\varepsilon_{\mu\text{m}}\)	\(\le 0.8\ \mu\text{m}\)	\(\varepsilon_{\mu\text{m}} > 0.8\)
CIP temperature \(T_{\!C}\)	60 °C ≤ \(T_{\!C}\) ≤ 80 °C	\(T_{\!C}\) outside 60-80 °C
Chemical concentration \(c_{\%}\)	0.3 % ≤ \(c_{\%}\) ≤ 0.7 %	\(c_{\%}\) outside 0.3-0.7 %
Dead‑zone factor \(\phi_{\text{dz}}\)	0 ≤ \(\phi_{\text{dz}}\) ≤ 0.3	\(\phi_{\text{dz}}\) outside 0-0.3
Reaction hold time \(t_{\text{hold}}\)	\(\ge 3\) min	\(t_{\text{hold}} < 3\) min

Effective cleanability starts with thoughtful design. Consider the following features:

Smooth, non-porous surfaces – stainless steel or food-grade alloys reduce residue adhesion.
Rounded corners and avoided dead-legs – eliminates pockets where product can accumulate.
Quick-release fasteners – enables rapid disassembly of screens, blades, and housings.
Integrated CIP (Clean-In-Place) ports – allow spray or circulation cleaning without full teardown.
Standardized part geometry – simplifies tooling and reduces the number of unique cleaning tools required.

Cleaning frequency is driven by several variables:

Product characteristics – high-fat, sticky, or hygroscopic materials require more frequent cleaning.
Batch changeover – any switch to a new raw material or formulation should trigger a cleaning cycle.
Regulatory requirements – GMP, FDA, or ISO standards may dictate minimum intervals.
Equipment usage intensity – continuous operation versus intermittent runs affects buildup rates.
Risk assessment outcomes – hazard analysis (e.g., HACCP) can identify critical control points that dictate cleaning timing.

The choice of cleaning method depends on the equipment design and material:

Hammer mills – dry-air blow-out followed by a low-pressure water rinse; for sticky feeds, a caustic soak may be required.
Knife or rotor mills – CIP systems with circulating alkaline solutions; blade removal for manual scrubbing when residues are tenacious.
Impact crushers – ultrasonic cleaning of impact plates combined with solvent wipes for oil-based products.
Granulators – steam-clean cycles with subsequent neutralizing rinse to prevent corrosion.

Validation should be systematic and documented:

Residue swab testing – collect surface samples after cleaning and analyze for target contaminants.
Visual inspection with UV-reactive dyes – highlights hidden residues on complex geometries.
ATP bioluminescence assays – provide rapid indication of organic material presence.
Process challenge tests – run a worst-case product through the equipment, clean, then re-run a clean product to detect cross-contamination.
Documentation review – ensure cleaning logs, SOPs, and deviation records align with GMP or ISO 22000 expectations.

Worked Example – Estimating Cleaning Time for a Conical Screen Mill

A pharmaceutical plant needs to verify that a 50 mm conical screen mill can be cleaned-in-place (CIP) within the 20 min window allowed between product campaigns. The mill will first be flushed with 70 °C water containing 0.5 % (w/v) caustic detergent, allowed to react for 5 min, and then rinsed twice with pure water for 2 min per rinse. We need the total cleaning time and the total rinse-water volume.

Knowns

Mill diameter, D = 50 mm = 0.05 m
Effective internal volume (including 20 % dead zone), V_eff = 1.178 L = 0.00118 m³
Cleaning-fluid density, ρ = 983 kg m^-3
Cleaning-fluid viscosity, μ = 0.4 cP = 0.0004 Pa s
Mean velocity through mill, v = 2 m s^-1
Reaction (contact) time, t_react = 5 min = 300 s
Rinse time per rinse, t_rinse = 2 min = 120 s
Number of rinses = 2

Step-by-step calculation

Calculate cross-sectional area of the mill: \[ A = \frac{\pi D^{2}}{4} = \frac{\pi (0.05)^{2}}{4} = 0.00196\ \text{m}^{2} \]
Determine volumetric flow rate: \[ Q = v\,A = 2 \times 0.00196 = 0.00393\ \text{m}^{3}\ \text{s}^{-1} = 3.93\ \text{L s}^{-1} \]
Check Reynolds number to confirm turbulent regime: \[ \text{Re} = \frac{\rho v D}{\mu} = \frac{983 \times 2 \times 0.05}{0.0004} = 245\,750 \gg 4000\ \text{(fully turbulent)} \]
Calculate detergent mass required for 0.5 % (w/v) solution in the effective volume: \[ M_{\text{chem}} = 0.005\ \text{kg L}^{-1} \times 1.178\ \text{L} = 0.0059\ \text{kg} = 5.9\ \text{g} \]
Compute hydraulic residence time during the caustic flush: \[ t_{\text{hyd}} = \frac{V_{\text{eff}}}{Q} = \frac{0.00118}{0.00393} = 0.3\ \text{s} \]
Total cleaning time (contact + hydraulic): \[ t_{\text{total}} = t_{\text{react}} + t_{\text{hyd}} = 300 + 0.3 = 300.3\ \text{s} \approx 5.0\ \text{min} \]
Volume of water per rinse: \[ V_{\text{rinse}} = Q\,t_{\text{rinse}} = 0.00393 \times 120 = 0.471\ \text{m}^{3} = 471\ \text{L} \]
Total rinse-water for two rinses: \[ V_{\text{total rinse}} = 2 \times 471 = 942\ \text{L} \]

Final Answer

Total cleaning time = 5.0 min (within the 20 min window). Total rinse-water required = 942 L for two 2-min rinses at 3.93 L s^-1.

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