1. What is centrifugation?
Centrifugation is a mechanical separation process that uses centrifugal force to separate solids from liquids (or liquids of different densities).
Works on the principle of density difference.
Faster and more efficient than gravity settling.
Commonly used for solid–liquid separation in chemical and pharma industries.
2. Types of centrifuges used in industry.
Basket (Vertical / Horizontal) centrifuge
Peeler centrifuge
Decanter centrifuge
Disk-stack centrifuge
Tubular bowl centrifuge
Pusher centrifuge
Inverting filter centrifuge
Selection depends on product nature, particle size, feed slurry, and moisture requirement.
3. Working principle of a centrifuge.
Centrifuge rotates at high speed, creating centrifugal force.
Dense solids move outward and form a cake on the wall.
Clarified liquid moves inward and drains out as filtrate.
The separation efficiency depends on G-force, feed rate, and particle size.
4. Difference between filtration and sedimentation centrifuges.
Filtration Centrifuges:
Use filter cloth / screen.
Suitable for solid–liquid separation with distinct particles.
Sedimentation Centrifuges:
No cloth; rely on settling under centrifugal force.
Used for fine particles or liquid–liquid separation.
Provide higher clarity.
5. What is centrifugal force?
The outward force generated when a mass rotates.
Drives particles toward the centrifuge wall → faster separation.
Higher rotational speed → higher centrifugal force → better dewatering.
Formula:
F = m · ω² · r
m = mass
ω = angular velocity
r = radius
6. Define RCF (Relative Centrifugal Force).
RCF is the effective G-force applied inside a centrifuge.
Indicates how many times the applied force is greater than gravity.
Determines separation efficiency and cake dryness.
Formula:
RCF = 1.118 × 10⁻⁵ × r × (RPM)²
r = radius (cm)
RPM = rotational speed
7. Formula for RCF.
Use the standard copy-friendly formula:
RCF (× g) = 1.118 × 10⁻⁵ × r × (RPM)²
Where:
r = radius in cm
RPM = speed in revolutions per minute
Higher RCF → better solid–liquid separation.
Important parameter for scale-up.
8. What is G-force in centrifugation?
G-force = multiple of gravitational force applied during centrifugation.
Determines how fast particles move outward.
Higher G-force → faster separation, lower moisture.
Formula:
G = (RCF value) × g
g = acceleration due to gravity (9.81 m/s²)
9. Difference between batch and continuous centrifuges.
Batch Centrifuges:
Operate cycle-wise (charge → spin → wash → discharge).
Used in pharmaceuticals and fine chemicals.
Precise control over quality and washing.
Continuous Centrifuges:
Run continuously with constant feed and discharge.
High throughput for bulk chemicals, salts, minerals.
Lower labor and higher productivity.
10. What is a basket centrifuge?
A perforated rotating basket lined with filter cloth.
Solids form a cake on the wall; liquid passes through holes.
Suitable for crystals, wet cake, and granular solids.
Offers good washing efficiency and low moisture.
Available in manual, semi-automatic, and automatic versions.
11. What is a decanter centrifuge?
A horizontal, continuous centrifuge used for solid–liquid separation of slurries.
Has a rotating bowl and a scroll conveyor that moves solids toward the discharge.
Handles high solid content and delivers continuous separation.
Suitable for wastewater, chemicals, pharma intermediates, and slurries.
12. What is a peeler centrifuge?
A batch-type centrifuge with a perforated basket.
After filtration, a mechanical blade (peeler) removes the cake from the basket wall.
Provides excellent washing and low moisture.
Ideal for crystalline products in chemical & pharma industries.
13. What is a disk-stack centrifuge?
High-speed centrifugal separator with conical disk plates.
Used for liquid–liquid or solid–liquid separation with very fine particles.
Delivers high clarity due to large settling area created by disks.
Common in biotech, enzymes, fermentation, edible oils.
14. What is a tubular bowl centrifuge?
Very high-speed centrifuge with a long, narrow bowl.
Provides excellent separation of very fine, low-density particles.
Used for pharmaceutical extractions, herbal extracts, biological fluids.
Offers high clarity but low throughput.
15. Applications of centrifuges in chemical/pharma industries.
Solid–liquid separation for crystals and slurries.
Dewatering of wet cake.
Liquid–liquid separation (oil-water separation).
Purification of intermediates.
Washing of product crystals.
Used in API manufacturing, salts, pigments, dyes, resins
16. What is mother liquor?
The liquid phase that passes through the filter during centrifugation.
Contains dissolved solute, impurities, and solvent.
Often recycled to improve recovery.
Quality of mother liquor directly affects product purity and yield.
17. How does centrifuge cake formation occur?
Solids migrate outward under centrifugal force.
They form a compact layer on the basket wall called cake.
Cake thickness depends on feed solids %, particle size, and cycle time.
Permeability of the cake determines filtration rate and moisture.
18. What factors affect separation efficiency?
Particle size & distribution
G-force (RCF)
Feed rate
Viscosity of mother liquor
Cake permeability
Basket speed
Cloth selection (pore size, mesh type)
Proper control ensures clear filtrate and low moisture.
19. What is filtration rate in centrifuges?
Speed at which liquid passes through the cake and filter cloth.
Higher filtration rate → faster cycle time.
Depends on:
Cake resistance
Cloth resistance
Particle size
G-force
Viscosity
Cake thickness
Formula (basic form):
Filtration Rate ∝ (Applied Force) / (Total Resistance)
20. What is cake washing?
Process of spraying clean solvent over formed cake to remove impurities.
Improves purity of final solid product.
Depends on:
Cake permeability
Wash volume
Wash uniformity
Essential in pharmaceutical and high-purity chemical production.
21. What is spin-drying?
Final stage of centrifuge cycle where the basket runs at higher RPM to remove moisture.
Uses centrifugal force to extract residual liquid from cake.
Reduces drying load on downstream dryers.
Critical for achieving low final moisture in pharma-grade solids.
22. How to reduce moisture in centrifuge cake?
Increase G-force / RPM.
Optimize spin time.
Use slower feed rate to avoid wet cake.
Improve cake permeability (crystal size optimization).
Use multiple wash cycles to remove sticky impurities.
Select correct filter cloth with proper pore size.
23. What is differential speed in decanter centrifuge?
Difference in speed between bowl and scroll conveyor.
Controls solids conveyance rate toward discharge.
Low differential → thicker cake, better dryness.
High differential → thinner cake, higher throughput but lower dryness.
24. Explain feed rate importance.
Too high → poor separation, wet cake, high vibrations.
Too low → underutilization, longer cycle time.
Must be optimized to maintain:
Stable cake formation
Good filtrate clarity
Balanced load on motor and bearings
25. Why vibration occurs in centrifuges?
Unbalanced load from uneven cake distribution.
Cloth damage causing uneven filtration.
Worn bearings or shaft misalignment.
Foreign material stuck inside basket.
Excessive vibration may cause structural damage, must be corrected immediately.
26. Balancing methods for centrifuge rotor.
Static balancing – removes imbalance when rotor is stationary.
Dynamic balancing – corrects imbalance during rotation.
Ensures smooth operation, reduces noise, vibration, and bearing wear.
Essential safety requirement for high-speed centrifuges.
27. What is critical speed of centrifuge?
Speed at which the centrifuge’s natural frequency matches vibration frequency.
Causes severe vibration if crossed.
Centrifuges quickly accelerate through critical speed and operate well above or below it.
Avoided to ensure mechanical safety and long equipment life.
28. Safety interlocks in centrifuges.
Lid/cover interlock → prevents opening during rotation.
Overspeed protection → trips motor above safe RPM.
Vibration sensor → shuts down during high imbalance.
Nitrogen purge interlock (for solvent handling).
Brake interlock → prevents sudden stopping that damages bearings.
Critical for operator safety and explosion prevention.
29. What is inerting? Why is nitrogen used?
Inerting = replacing oxygen with an inert gas (mostly nitrogen).
Prevents explosion during handling of flammable solvents.
Nitrogen keeps O₂ concentration below Lower Explosive Limit (LEL).
Protects product from oxidation and ensures safe operation.
30. Why is CIP important in centrifuges?
CIP = Clean-in-Place without dismantling equipment.
Ensures GMP compliance in pharma manufacturing.
Prevents cross-contamination between batches.
Maintains filtrate clarity and cake quality by avoiding deposits.
Reduces manual cleaning, downtime, and exposure to solvents.
31. What is cloth choking?
Blockage of filter cloth pores due to fine particles or sticky solids.
Reduces filtration rate and increases cycle time.
Causes wet cake and poor clarity.
Prevented by:
Proper cloth selection,
Pre-coat,
Regular CIP,
Optimizing particle size during crystallization.
32. Causes of poor cake discharge.
Overdry or over-wet cake.
Sticky or compressible solids.
Wrong cloth type or clogged cloth.
Insufficient basket speed during discharge.
Improper scraper/peeler blade settings.
Incorrect cake thickness.
33. What is solids recovery?
Percentage of solids retained in cake compared to solids in feed.
Higher solids recovery → less loss to filtrate.
Affected by G-force, cloth pore size, particle size, feed rate.
Formula:
Solids Recovery (%) = (Solids in Cake / Solids in Feed) × 100
34. Centrifuge operating parameters.
RPM / G-force
Feed rate
Cycle time
Cake thickness
Wash volume and wash time
Differential speed (for decanter)
Temperature and viscosity
All must be balanced for high purity, low moisture, and clear filtrate.
35. Difference between perforated and solid bowl centrifuges.
Perforated Bowl:
Basket has holes + filter cloth.
Filters solids → forms cake.
Used for solid–liquid filtration (crystals, salts).
Solid Bowl:
No perforations; relies on sedimentation.
Used for fine particles or liquid–liquid separation.
36. What is basket speed?
Rotational speed of the centrifuge basket (RPM).
Controls G-force, filtration rate, and moisture removal.
Higher speed → drier cake but risk of crystal breakage.
Must be optimized based on product properties.
37. Effect of RPM on separation.
Higher RPM → higher centrifugal force → better solid–liquid separation.
Results in lower moisture, faster filtration.
Excessive RPM causes vibration, cloth damage, crystal breakage.
Must balance RPM with feed solids, viscosity, and particle strength.
38. What is residence time?
Time slurry spends inside the centrifuge before discharge.
Longer residence time → drier cake but lower throughput.
Short residence time → wet cake and poor separation.
Formula:
Residence Time = (Mass of Solids Held) / (Solids Discharge Rate)
39. Importance of feed clarity.
Clean feed improves filtrate clarity and cake quality.
Reduces cloth choking and prolongs cloth life.
Prevents imbalance and vibration.
Pre-filtration helps remove large particles or impurities.
40. What is cake permeability?
Ability of filtrate to pass through the cake.
High permeability → fast filtration, lower moisture.
Low permeability → slow filtration, wet cake.
Influenced by crystal size, shape, compressibility, and solids loading.
51. What is cake thickness and how is it controlled?
Cake thickness = depth of solid layer formed on basket wall.
Controlled by:
Feed quantity / feed time
Solids concentration
Basket speed (RPM)
Differential speed (in decanter)
Proper thickness ensures uniform drying, low vibration, and stable discharge.
52. What is acceleration time in centrifuges?
Time taken for centrifuge to reach set RPM from rest.
Affects cycle time and mechanical load.
Too fast → risk of vibration or overload.
Must be optimized for smooth startup.
53. Explain deceleration / braking system.
Braking reduces basket speed to safe RPM before discharge or opening.
Types:
Mechanical brake
Hydraulic brake
Dynamic/electrical brake
Prevents shock loads on bearings and avoids unsafe handling.
54. What is a scraper mechanism?
Mechanism used to remove cake from basket wall after drying.
Mostly in peeler centrifuges or automatic discharge machines.
Scraper blade must be adjusted to avoid cloth damage.
55. Difference between manual and automatic discharge.
Manual Discharge:
Operator removes cake by hand tools.
Used for small batches or sticky products.
Automatic Discharge:
Discharge by scraper, plough, pusher, or peeler.
Suitable for continuous or high-volume operations.
Reduces operator exposure and improves GMP compliance.
56. Why basket opening size matters?
Opening (perforation) size controls filtration rate.
Large openings → fast flow but risk of solids leakage.
Small openings → clear filtrate but slower cycle.
Must be matched with crystal size and cloth mesh.
57. What is filtration area in a centrifuge?
The effective cloth-covered area through which filtrate passes.
Higher filtration area → faster dewatering and lower cycle time.
Depends on basket diameter, height, and perforation design.
Critical for scale-up and capacity calculation.
58. How do you calculate cake moisture?
Formula:
Moisture (%) = [(Wet Cake Weight − Dry Cake Weight) / Wet Cake Weight] × 100
Moisture indicates drying efficiency.
Controlled by RPM, spin time, particle size, wash quality, and viscosity.
59. Effect of particle size on centrifuge performance.
Larger particles → high permeability → fast filtration → low moisture.
Finer particles → clog cloth → slow filtration → wet cake.
Very fine particles may pass into filtrate → poor clarity.
Crystal engineering during upstream crystallization is critical.
60. Why do fine particles pass into filtrate?
Cloth pore size too large.
Particle size too small or broken due to high RPM.
High viscosity preventing proper capture.
High feed pressure or feed rate pushing solids through cloth.
Can be corrected by:
Finer cloth mesh
Pre-coat
Optimizing agitation/crystallization upstream
61. What is clarified liquid?
The liquid portion separated from solids during centrifugation.
Should be clear and free from suspended solids.
Quality depends on cloth pore size, particle size, and G-force.
Used for recovery, recycling, or further processing.
62. What is sediment?
The solid layer that settles at the bowl wall in sedimentation centrifuges.
Formed by fines or small-density-difference particles.
Removed manually (in batch machines) or by scroll conveyor in decanters.
Indicates separation efficiency.
63. Explain separation of immiscible liquids.
Centrifugal force separates two liquids of different densities.
Heavy liquid moves outward; light liquid remains inward.
Used in oil–water separation, solvent recovery, extraction.
Disk-stack and decanter centrifuges are commonly used.
64. What is weir height in decanter centrifuge?
Height of the liquid overflow weir in discharge section.
Controls pond depth and separation zone.
Higher weir → deeper pond → better separation but lower dryness.
Lower weir → shallower pond → drier solids but lower clarity.
65. What is pond depth in decanter centrifuge?
Depth of the liquid layer inside rotating bowl.
Determines settling distance for solids.
Deep pond → better liquid clarity.
Shallow pond → better solids dryness.
Adjusted by weir plates.
66. Function of scroll conveyor in decanter centrifuge.
Moves settled solids gently toward the solids discharge zone.
Runs at a differential speed compared to bowl.
Prevents solids buildup and keeps operation continuous.
Controls cake dryness based on solids residence time.
67. Why scroll differential speed is critical?
Controls the thickness and dryness of the solids cake.
Too low → solids accumulate, may overload or choke.
Too high → solids expelled too fast → wet cake.
Must be balanced based on feed solids and viscosity.
68. What is torque overload?
Excessive resistance to scroll movement due to high solids load.
Causes increased torque on the gearbox.
May trigger automatic shutdown to prevent damage.
Indicates issues in feed rate, solids concentration, or pond depth.
69. What is basket fatigue failure?
Cracking or deformation of basket due to cyclic stresses at high RPM.
Caused by:
Long-term vibration
Imbalance
Corrosion or pitting
Prevented by NDT inspections, balancing, and controlled RPM.
70. Importance of vibration monitoring.
Detects imbalance, bearing failure, misalignment, or uneven cake.
Helps prevent catastrophic mechanical failure.
Ensures stable separation and reduces wear.
Modern centrifuges use online vibration sensors for safety interlocking.
81. Why pre-warm or pre-cool feed sometimes required?
Controls viscosity for better separation.
Prevents thermal shock to basket or housing.
Ensures consistent crystal size and filtration rate.
Helps maintain constant process conditions across batches.
Reduces moisture by improving filtrate flow.
82. What is cloth pore size?
Diameter of openings in the filter cloth mesh.
Determines what particle sizes are retained.
Smaller pore size → clearer filtrate but slower filtration.
Larger pore size → faster flow but risk of solids passing through.
Critical for matching crystal size distribution.
83. Why use different mesh sizes in filter cloth?
To optimize filtration rate vs clarity.
Fine mesh for small particles → improved clarity.
Coarse mesh for large crystals → faster dewatering.
Used based on product properties, viscosity, and particle size.
Helps control cloth choking and cycle time.
84. How to prevent cloth tearing?
Avoid overloading the basket.
Use correct mesh size and cloth material.
Avoid sharp or abrasive solids.
Maintain correct blade/scraper settings.
Ensure proper CIP and washing to prevent buildup.
Replace cloth at scheduled intervals.
85. Why feed solids concentration matters?
High solids → faster cake formation but risk of imbalance and choking.
Low solids → longer cycle time and wet cake.
Optimal solids concentration ensures stable cake formation and good filtrate clarity.
Critical for maintaining consistent batch operation.
86. What is heel formation?
Residual layer of solids that remains after cake discharge.
Occurs due to cloth adhesion, sticky solids, or poor scraping.
Excess heel leads to imbalance and contamination.
Controlled by heel wash, optimized RPM, and scraper settings.
87. Factors affecting cake discharge efficiency.
Cake stickiness and compressibility.
Cake dryness (over-wet or over-dry).
Proper scraper/blade angle and clearance.
Cloth condition and lubrication.
Basket speed during discharge.
Smooth internal surface finish.
88. Why do we use nitrogen blanketing?
Prevents explosion when handling solvents (reduces oxygen level).
Protects product from oxidation.
Reduces formation of static charge.
Maintains safe operating conditions in ATEX environments.
89. Why inert atmosphere required for solvents?
Many solvents have low flash points and can ignite.
Nitrogen keeps oxygen concentration below LEL (Lower Explosive Limit).
Prevents vapor ignition during spin-drying, braking, or cake discharge.
Mandatory for GMP and safety compliance.
90. What is ATEX certification?
ATEX = European safety standard for equipment used in explosive atmospheres.
Ensures centrifuges are safe for flammable vapors, gases, and dust.
Covers design features such as:
Flameproof motors
Anti-static materials
Sealed bearings
Grounding/Earthing
Mandatory for solvent-handling centrifuges in chemical & pharma industries.
91. Why operator safety is critical in centrifuges?
High-speed rotation → risk of mechanical failure.
Handling of flammable solvents → explosion hazard.
Risk of vibration, imbalance, and noise.
Operators must follow PPE, interlocks, SOPs, and lockout-tagout (LOTO).
Ensures GMP compliance and accident-free operation.
92. What are common GMP documentation needs for centrifuges?
SOPs for operation, cleaning, and maintenance.
Batch manufacturing records (BMR).
Cleaning validation records.
Calibration & maintenance logs.
Change control and deviation reports.
Ensures traceability and regulatory compliance.
93. Cleaning validation requirements for centrifuges.
Verify that no residual product or solvent remains after cleaning.
Swab & rinse tests for microbial and chemical residues.
Acceptance criteria based on carryover limits.
Documentation and periodic re-validation required for GMP.
94. What is CIP nozzle position importance?
Ensures spray reaches all internal surfaces.
Prevents dead zones where solids may remain.
Improves cleaning uniformity and reduces manual cleaning.
Critical for pharma-grade centrifuges.
95. Why slow-speed rotation used during CIP?
Helps cleaning solution reach all surfaces uniformly.
Prevents splashing at high RPM.
Ensures better cloth soaking and basket cleaning.
Reduces wear on mechanical parts.
96. What is flooding of basket?
Condition where liquid level rises above basket height.
Causes poor separation, wet cake, and imbalance.
Happens due to high feed rate, clogged cloth, or low RPM.
Must be corrected immediately to avoid overflow or vibration.
97. How to avoid slurry channeling?
Ensure uniform feed distribution.
Maintain optimum cake thickness.
Use correct cloth permeability.
Avoid overloading the basket.
Channeling reduces separation efficiency and increases moisture.
98. What is filtrate foaming issue?
Excess foam in filtrate due to high surface-active impurities.
Causes overflow, poor clarity, and pump cavitation.
Fixed by adding anti-foam agents, controlling feed rate, or adjusting temperature.
99. Why anti-foam agents are added?
Reduce formation of foam in filtrate system.
Improve filtrate clarity and flow.
Prevent overflow from filtrate tank.
Ensure stable pump operation and prevent air locking.
100. What is solids carryover in filtrate?
Presence of fine particles in filtrate.
Caused by:
Torn or oversized cloth
High feed pressure
Very fine particles
High viscosity
Reduced by pre-coat, finer cloth, slower feed, or adjusting RPM.
101. What causes vibration during startup?
Uneven cake heel from previous batch.
Improper balancing of basket.
Foreign objects stuck in the bowl.
Worn bearings or misaligned shaft.
Machine must be checked before ramping to full RPM.
102. What is bearing temperature monitoring?
Sensors monitor bearing heat during operation.
High temperature indicates lubrication failure, overload, or misalignment.
Prevents catastrophic failure of rotating assembly.
103. What is dynamic balancing?
Balancing rotor while it is rotating to remove uneven mass distribution.
Ensures smooth operation, reduces vibration, and extends component life.
Mandatory after basket repair or replacement.
104. Why rubber dampers used in centrifuges?
Absorb vibration and shock loads.
Reduce noise and protect structure.
Improve stability during startup and braking.
Increase bearing and motor life.
105. What is noise isolation?
Reducing noise caused by high-speed rotation and vibration.
Achieved using acoustic enclosures, damping pads, and insulation.
Protects operators and meets occupational safety standards.
106. Why high-speed centrifuges need enclosure?
Contain broken parts in case of basket explosion.
Prevent operator exposure to solvent vapors.
Reduce noise and dust.
Mandatory for GMP and ATEX compliance.
107. Explain nitrogen purging process.
Replaces oxygen inside centrifuge with nitrogen gas.
Ensures oxygen stays below LEL (Lower Explosive Limit).
Prevents explosion during operations with volatile solvents.
Often interlocked with machine startup.
108. What is solvent recovery from centrifuges?
Collecting filtrate (mother liquor) for recycling or distillation.
Increases process economy, reduces waste.
Requires high filtrate clarity and minimal contamination.
109. Why jacketed housings used?
Maintain controlled temperature during operation.
Prevent crystallization or viscosity increase inside housing.
Reduce risk of solvent vapor condensation and product sticking.
Essential for temperature-sensitive products.
110. Importance of feed pump type.
Pump must provide steady, pulsation-free flow.
Positive displacement pumps give consistent feed rate.
Avoid high shear pumps that break crystals.
Directly affects cake formation, clarity, and balance.
111. What is feed pressure control?
Maintains constant feed flow into the centrifuge.
Prevents overloading and avoids flooding of the basket.
Ensures uniform cake formation and stable filtrate clarity.
High feed pressure can push fines through the cloth.
112. Role of throttling valve in centrifuge operation.
Adjusts flow rate of feed slurry.
Prevents sudden surges that cause vibration and imbalance.
Helps maintain constant pressure at the feed inlet.
Essential for achieving consistent cycle time.
113. Why differential pressure monitoring is needed?
Indicates cloth choking or buildup of solids.
High ΔP → slow filtration, wet cake.
Sudden drop in ΔP → torn cloth or channeling.
Helps plan cleaning, cloth replacement, and troubleshooting.
114. What is cake cracking?
Formation of cracks in the cake during drying/spin.
Caused by uneven drying or rapid acceleration.
Leads to uneven discharge, poor washing, and contamination.
Controlled by optimizing spin speed, wash rate, and cake thickness.
115. How to avoid cake blow-out?
Maintain correct cloth pore size.
Avoid overfilling and excessive cake thickness.
Control acceleration and RPM.
Ensure uniform feed distribution.
Prevent overpressure and high feed solids.
116. What is heel washing?
Washing the remaining heel layer after main cake discharge.
Improves purity and removes trapped mother liquor.
Common in pharma GMP to avoid product cross-contamination.
Done at low RPM to avoid splashing.
117. Why multiple wash cycles used?
To remove maximum impurities from cake.
Useful when crystals trap significant mother liquor.
Improves purity and drying efficiency.
Typical in API crystals and high-value products.
118. What is a siphon basket?
A perforated basket with an extra siphon chamber for enhanced filtration.
Creates vacuum-like suction to increase filtrate discharge.
Results in faster filtration and lower moisture.
Ideal for fine crystals and difficult-to-filter products.
119. Difference between top discharge and bottom discharge centrifuges.
Top Discharge:
Cake removed manually from top.
Used in small/medium batch pharma operations.
Bottom Discharge:
Cake drops automatically through bottom opening.
Faster cycle time, more automation.
Used in bulk chemicals and continuous plants.
120. What is plough discharge?
A mechanical plough/arm pushes cake out from basket.
Used in inverting or horizontal centrifuges.
Suitable for sticky or compressible solids.
Ensures low residual heel and faster turnaround.
121. Why scroll pitch matters in decanter centrifuge?
Scroll pitch = distance the solids travel per revolution.
Large pitch → faster movement → wet solids.
Small pitch → slower movement → drier solids.
Must match feed properties (viscosity, solids %, particle size).
122. What is G-force zonation?
Variation of centrifugal force across bowl radius.
Higher radius → higher G-force → stronger separation.
Allows multi-zone clarifying and dewatering in one machine.
Critical in disk stack and decanter centrifuges.
123. Difference between drying zone and clarification zone.
Clarification Zone:
Separates solids from liquid.
Located near feed entry.
Drying Zone:
Removes remaining moisture from solids.
Located near the discharge end.
Ensures final product dryness.
124. Effect of feed temperature on viscosity
Higher temperature → lower viscosity → better filtration and clarity.
Lower temperature → higher viscosity → slow filtration, wet cake.
Must balance temperature for safety (solvent vapor) and product stability.
125. Why do crystals break in centrifuge?
Excessive RPM or acceleration rate.
High blade/scraper force.
Abrasive particles causing attrition.
Pump shear from improper feed pumping.
Broken crystals → fines → cloth choking and poor filtration.
126. What is abrasion on basket?
Wear on basket surface caused by hard, sharp particles.
Reduces basket life and leads to cracks/pitting.
Requires:
Proper material selection (SS316L, Hastelloy)
Regular inspection and UT testing
Lowering abrasive solids load
127. What is shaft misalignment?
Rotor shaft not aligned correctly with motor or bearings.
Causes high vibration, noise, and bearing wear.
Detected via vibration analysis and corrected by skilled technicians.
Must be checked after any major maintenance.
128. What is out-of-round basket?
Basket becomes oval or uneven due to fatigue or damage.
Causes severe vibration and imbalance.
Must be repaired or replaced immediately.
Common in old or overloaded centrifuges.
129. How often to perform UT thickness testing?
UT = Ultrasonic Thickness testing.
Recommended every 6–12 months depending on usage.
Detects corrosion, pitting, erosion, and metal thinning.
Ensures basket integrity and prevents catastrophic failure.
130. What is NDT testing in centrifuges?
NDT = Non-Destructive Testing.
Includes UT, dye-penetrant, magnetic particle, and radiography.
Used to detect cracks, defects, and structural fatigue without dismantling.
Critical for safety and preventive maintenance.
131. What is passivation?
Chemical treatment (usually with nitric or citric acid) to create a protective oxide layer on stainless steel.
Prevents corrosion, pitting, and product contamination.
Essential for GMP centrifuges handling APIs and solvents.
132. Why SS316/SS316L preferred in pharma centrifuges?
Superior corrosion resistance due to molybdenum content.
SS316L has low carbon, preventing weld corrosion.
Ensures cleanability, GMP compliance, and long service life.
Suitable for solvents, acids, and high-purity applications.
133. What is LOD (Loss on Drying) in centrifuge cake?
Measures residual moisture content in the cake.
Indicates efficiency of spin drying.
Lower LOD means better drying and less load on dryers.
134. Why moisture uniformity matters?
Ensures consistent downstream drying and quality.
Prevents agglomeration and sticking during handling.
Essential for tablet compression in pharma.
Affects weight consistency in packaging.
135. What is the rotating assembly?
Includes basket/bowl, shaft, bearings, motor coupling, and seals.
Must run smoothly with minimal vibration.
Proper alignment and balancing are essential for safe operation.
136. What is filter aid?
Material added to improve filtration (e.g., Diatomaceous Earth, Perlite).
Prevents cloth choking, improves clarity.
Forms a porous pre-coat layer for better filtration.
Helps handle fine or sticky solids.
137. Why pre-coat improves filtrate clarity?
Provides an initial uniform porous layer.
Captures fine particles that would pass through cloth.
Protects cloth and increases cycle consistency.
Reduces risk of cloudy filtrate.
138. What is cloth blinding?
Permanent blocking of cloth pores by fine, sticky, or oily solids.
Leads to slow filtration, wet cake, and long cycle time.
Requires cloth replacement or chemical cleaning.
Prevented by proper upstream crystallization control.
139. Causes of filtrate cloudiness.
Torn or oversized filter cloth.
Very fine particles due to crystal breakage.
High viscosity mother liquor.
Too high feed rate or pressure.
Incomplete formation of initial cake layer.
140. Why solids buildup happens in housing?
Poor discharge from basket.
Sticky or compressible solids.
Ineffective scraper mechanism.
Inadequate cleaning between batches.
Can lead to imbalance, contamination, and vibration.
141. What are ATEX zones?
Classification for areas where explosive atmospheres may occur.
Zone 0: Continuous presence of explosive vapors.
Zone 1: Likely during normal operation.
Zone 2: Present only during abnormal conditions.
Determines proper centrifuge design and certification.
142. What causes overloaded motor?
Excessive solids load due to high feed rate.
Sticky or compact cake causing resistance.
Bearing failure or poor lubrication.
Mechanical binding or scraper issues.
Must be fixed immediately to avoid motor burnout.
143. Why backpressure affects filtration?
High backpressure reduces filtrate flow.
Causes wet cake, longer cycle time.
May force fine particles through cloth.
Must maintain free discharge path for filtrate.
144. What is hydro-extraction?
Removal of water or solvent from solids by applying centrifugal force.
Synonym for spin drying.
Achieves low moisture before thermal drying.
145. Difference between screen bowl and solid bowl centrifuges.
Screen Bowl:
Has perforated screen followed by solid bowl section.
Performs filtration + sedimentation.
Ideal for coal, minerals, and salts.
Solid Bowl:
No perforations; relies purely on sedimentation.
Used for fine solids or liquid–liquid separation.
146. What is a tricanter centrifuge?
Three-phase centrifuge separating:
1. Solid phase
2. Heavy liquid (e.g., water)
3. Light liquid (e.g., oil)
Used in oily wastewater, fermentation broth, edible oil industry.
147. When to use three-phase separation?
When feed contains solids + two immiscible liquids.
When liquid density difference is significant.
Required in extraction, oil recovery, biotech, solvent-water separation.
148. What is batch cycle time?
Total time for one operating cycle:
Filling
Filtration
Washing
Spin drying
Discharge
Cleaning
Shorter cycle time → higher productivity but must not reduce quality.
149. Effect of RPM ramp rate.
Fast ramp → risk of imbalance, cake cracking, vibration.
Slow ramp → longer cycle time.
Must balance to protect equipment and ensure uniform cake formation.
150. What is cake densification?
Compaction of cake due to high centrifugal force.
Results in low moisture, but may reduce permeability.
Excessive densification → difficult washing and discharge.
Controlled by adjusting RPM, feed rate, and cake thickness.
151. What causes scroll torque increase in decanter centrifuge?
High solids loading in the bowl.
Sticky or compressible solids.
Low differential speed → solids accumulate.
High viscosity feed slurry.
Must be corrected to avoid torque overload trip.
152. Why solids settle faster in centrifuge than gravity?
Centrifuge generates centrifugal force thousands of times greater than gravity.
Shortens settling time drastically.
Enables separation of fine particles that cannot settle under gravity.
153. What is basket run-out?
Deviation of rotating basket from perfect circular motion.
Causes vibration, noise, and premature wear.
Checked using dial gauge and corrected during maintenance.
154. Why correct nozzle size is important in centrifuge feed?
Ensures uniform distribution of slurry.
Prevents jetting, which causes uneven cake.
Controls feed velocity → better separation and clarity.
Avoids erosion of basket wall.
155. What is wear strip in a centrifuge?
Replaceable component to protect basket or scroll from abrasion.
Installed in high-wear zones.
Extends equipment life and reduces maintenance cost.
156. Why solids accumulate at discharge end of decanter?
Incorrect differential speed.
Low scroll torque.
High viscosity or high solids concentration.
Shallow pond depth restricting solids movement.
157. What is purging in peeler centrifuge?
Cleaning operation before next batch.
Removes residual heel and solids from basket.
Ensures GMP compliance and avoids cross-contamination.
158. Importance of basket wall thickness.
Determines strength and durability at high RPM.
Thicker wall = higher safety margin but more weight.
Must meet ASME, GMP, and ATEX design standards.
Regular UT testing ensures no thinning due to corrosion.
159. Why housing design is critical in centrifuges?
Contains solvent vapors, vibration, and noise.
Protects operator in case of basket failure.
Maintains inert atmosphere for solvent handling.
Ensures CIP spray coverage.
160. What is scroll wear?
Erosion of scroll flights in decanter due to abrasive solids.
Leads to reduced conveying efficiency.
Requires periodic coating or replacement.
161. Why feed uniformity matters in centrifuges?
Ensures even cake formation.
Reduces vibration and imbalance.
Improves filtrate clarity.
Prevents sudden load fluctuations on motor.
162. What is overflow in decanter centrifuge?
Excess liquid flowing over weir plates.
Indicates improper weir setting, high feed, or insufficient separation.
May reduce clarity and stability.
163. How do you control noise in centrifuges?
Use acoustic enclosures.
Ensure proper balancing of rotating assembly.
Lubricate and maintain bearings.
Reduce vibration through shock absorbers.
164. What is dynamic load in centrifuge?
Load acting on components during rotation (centrifugal + mechanical).
Higher dynamic load → higher wear.
Depends on RPM, cake mass, and imbalance.
165. Why motor heating occurs during operation?
Overloading due to high solids or viscosity.
Poor ventilation or blocked cooling fins.
Misalignment increasing friction.
Electrical issues like voltage fluctuation.
166. What is polymer dosing in centrifugation?
Addition of flocculants/polymers in decanter feed.
Helps fines to agglomerate, improving sedimentation.
Increases clarity and dryness of cake.
Common in ETP/STP decanter operations.
167. Why is feed degassing required sometimes?
Removes air bubbles that cause:
Foaming
Poor separation
Imbalance at high speed
Degassing improves clarity and stability.
168. What is torque protection system?
Automatically stops centrifuge when torque exceeds safe limits.
Prevents damage to scroll, gearbox, and motor.
Essential for decanters handling high solids load.
169. Why sealing system important in centrifuges?
Prevents solvent vapor leakage.
Protects bearings from slurry ingress.
Maintains inert atmosphere during operation.
Ensures long equipment life.
170. What is a discharge chute?
Pathway for cake or solids to exit centrifuge.
Must be smooth, corrosion-resistant, and avoid blockages.
Ensures clean, fast discharge and GMP compliance.