1. What is an evaporator and what is its principle of operation?
An evaporator is equipment used to concentrate a solution by removing solvent (usually water) as vapor.
Works on the principle of boiling the liquid using heat transfer.
Heat is supplied through steam, causing the liquid to vaporize.
The vapor is separated, and the concentrated product is collected.
Based on latent heat of vaporization and boiling point properties.
2. Explain the difference between evaporation and distillation.
Evaporation:
Removes solvent only, product is the concentrated liquid.
Usually uses single component vaporization.
No need for full phase separation.
Distillation:
Separates two or more components based on volatility.
Vapors are condensed and collected separately.
Used for purification, not concentration.
3. What factors affect the rate of evaporation?
Temperature difference between steam and liquid.
Pressure (lower pressure increases evaporation rate).
Heat-transfer coefficient (U-value).
Viscosity of the feed.
Boiling point elevation (BPE) of the solution.
Fouling or scaling on heat-transfer surfaces.
Feed concentration and flow rate.
4. What is boiling point elevation (BPE)?
Increase in boiling point of a solution compared to pure solvent.
Caused by dissolved solids or impurities.
Higher BPE reduces heat transfer and steam economy.
Important in designing multiple-effect evaporators.
Formula:
BPE = T_solution − T_pure solvent
5. What is overall heat-transfer coefficient (U-value) in evaporators?
U-value indicates the overall heat-transfer efficiency between steam and liquid.
Depends on:
Steam side film coefficient
Tube wall resistance
Liquid side film coefficient
Fouling factor
Higher U-value = better performance and higher evaporation rate.
Formula:
1 / U = 1 / h₁ + R_w + 1 / h₂ + R_f
Where:
h₁ = steam-side coefficient
h₂ = liquid-side coefficient
R_w = wall resistance
R_f = fouling resistance
6. What are the different types of evaporators used in industry?
Falling Film Evaporator
Rising Film / Long Tube Vertical Evaporator
Forced Circulation Evaporator
Wiped Film / Thin Film Evaporator
Agitated Thin Film Evaporator (ATFE)
Multiple-Effect Evaporator (MEE)
MVR / TVR Evaporators (Mechanical & Thermal Vapor Recompression)
7. What is a single-effect evaporator?
Evaporator where steam is used only once for heating.
Simple, low cost, easy to operate.
Lower energy efficiency.
Steam economy ≈ 1.0 kg vapor per kg steam.
Used for small capacities or non-critical duties.
8. What is a multiple-effect evaporator?
Series of evaporators where vapor from one effect heats the next.
Each effect operates at lower pressure than the previous.
Greatly improves steam economy.
Reduces operating cost.
Common in chemical, pharma, and ZLD plants.
Steam economy:
Economy ≈ Number of effects
9. Why are multiple-effect evaporators more energy-efficient?
Vapor generated in the first effect acts as the heating medium for the next.
Steam usage is reduced significantly.
Lower pressures in later effects cause easier evaporation.
Better utilization of latent heat.
Reduced operational cost compared to single effect.
10 Explain forced-circulation evaporators.
Uses a high-capacity pump to circulate liquid through heat exchanger tubes.
Prevents fouling and scaling by maintaining high velocity.
Suitable for high-viscosity, crystallizing, or slurry-type liquids.
No boiling inside tubes → boiling happens in the vapor separator.
Used in ZLD, brine, sugar, chemicals, and salting systems.
11. What is a falling film evaporator?
Liquid flows as a thin film down the inside of vertical tubes.
Requires low temperature difference → ideal for heat-sensitive products.
Very high heat-transfer coefficient.
Operates efficiently under vacuum.
Minimal residence time → prevents degradation.
Used widely in pharma, food, and chemical industries.
12 What is a rising film (long tube vertical) evaporator?
Liquid is heated and vapor forms inside tubes, pushing liquid upward.
Uses natural circulation due to vapor lift.
Good for low-viscosity fluids.
Requires higher temperature difference than falling film.
Older technology but still used in simple, non-sensitive applications.
13. What is a wiped film / thin film evaporator used for?
For processing high-viscosity, heat-sensitive, or thermolabile materials.
A rotating wiper spreads liquid into a very thin film on heated surface.
Provides extremely short residence time.
Prevents decomposition and fouling.
Ideal for API intermediates, vitamins, herbal extracts, polymers, resins.
14. What is an agitated thin-film evaporator (ATFE)?
A specialized thin-film evaporator with agitation blades.
Maintains uniform film thickness regardless of viscosity.
Handles slurries, crystallizing feeds, sticky materials.
Provides high mass & heat transfer.
Excellent for difficult-to-evaporate products like high-solids or gummy liquids.
15. Explain climbing film vs falling film operation.
Climbing (Rising) Film:
Liquid rises due to vapor lift.
Good for low-viscosity feeds.
Needs higher temperature difference.
Falling Film:
Liquid flows downward by gravity as a thin film.
Excellent for heat-sensitive products.
Operates well under vacuum and low ΔT.
16. How do you select an evaporator type for a process?
Selection depends on:
Heat sensitivity of the product.
Viscosity and solids content.
Fouling tendency.
Required final concentration.
Throughput capacity.
Energy efficiency needs (single vs multi-effect vs MVR).
Whether product is crystallizing, corrosive, or volatile.
17. What is steam economy?
Ratio of kg of solvent evaporated to kg of steam used.
Indicates energy efficiency of the evaporator.
Higher steam economy → lower steam consumption.
Formula:
Steam Economy = (kg of vapor evaporated) / (kg of steam consumed)
18. How do you improve steam economy?
Use multiple-effect arrangement.
Operate under vacuum to reduce boiling temperature.
Install MVR or TVR systems.
Preheat feed using condensate or vapors.
Reduce heat losses and improve insulation.
Maintain clean heat-transfer surfaces.
19. What is the role of vacuum in evaporators?
Lowers the boiling point, enabling evaporation at lower temperatures.
Prevents thermal degradation of heat-sensitive materials.
Improves steam economy.
Reduces scaling by lowering surface temperature.
Helps achieve higher concentration without product damage.
20 How do you calculate heat-transfer area?
Based on heat load, temperature difference, and U-value.
More area = higher evaporation capacity.
Used in design of SHELL & TUBE or plate-type evaporators.
Formula:
Q = U × A × ΔT
Where:
Q = heat load (kW)
U = overall heat-transfer coefficient
A = area (m²)
ΔT = temperature difference
Rearranged for area:
A = Q / (U × ΔT)
21. Why is scale formation a problem in evaporators?
Reduces heat-transfer efficiency.
Increases steam consumption.
Causes hot-spots → product degradation.
Leads to tube choking and capacity loss.
Increases cleaning frequency and downtime.
Impacts long-term equipment performance.
22. How do you reduce scaling or fouling in evaporators?
Maintain high circulation velocity.
Use antiscalants / antifoam agents.
Operate under vacuum (reduced temperature).
Filter feed to remove solids.
Perform periodic CIP (acid/alkali wash).
Use suitable MOC (SS316/duplex/Hastelloy).
Keep temperature difference (ΔT) low.
23. When do you use a steam ejector vs vacuum pump?
Steam Ejector:
High vacuum applications.
Handles large vapor loads.
Simple, low maintenance.
Preferred when steam is readily available.
Vacuum Pump:
Moderate vacuum levels.
Lower utility cost when steam is expensive.
Better removal of non-condensable gases.
Suitable when stable vacuum control is required.
24. What is the effect of viscosity on evaporator performance?
Higher viscosity lowers heat-transfer coefficient.
Poor film formation in falling-film systems.
Requires higher ΔT to achieve evaporation.
Increases pumping power requirement.
Best handled using forced circulation or ATFE.
25. Why is feed preheating important?
Lowers steam consumption.
Improves evaporation rate.
Prevents thermal shock to equipment.
Supports stable film formation in falling film evaporators.
Enhances overall energy efficiency in multiple effects.
26. What is the purpose of a vapor separator?
Separates vapor from entrained liquid droplets.
Prevents carryover into next effect or condenser.
Improves product quality and purity.
Ensures stable multiple-effect operation.
Reduces load on demister and condenser.
27. Why is condensate removal important?
Condensate film reduces heat transfer.
Causes flooding and unstable steam pressure.
Proper removal maximizes U-value.
Prevents water hammer.
Ensures steady heat-transfer performance.
28. What are typical pressure and temperature conditions in evaporators?
Falling Film: 50–90°C under vacuum.
Forced Circulation: 90–120°C depending on viscosity.
Multiple Effect:
First effect: higher pressure
Last effect: deep vacuum
Conditions depend on product sensitivity and required concentration.
29. What is entrainment and how can it be minimized?
Entrainment:
Liquid droplets carried with vapor.
Minimization:
Use demisters/cyclone separators.
Control vapor velocity.
Apply antifoam agents.
Maintain stable boiling.
Keep correct separator level.
30. What are the critical parameters monitored in evaporators?
Steam pressure and temperature.
Vacuum level.
Feed flow rate.
Product concentration (refractometer/conductivity).
Evaporation rate.
Differential pressure (ΔP) across tubes.
Condensate temperature and clarity.
Vapor load and separator liquid level.
Here are Answers 31 to 40 in the same clean, crisp, interview-ready format.
31. How is product concentration controlled?
Using online refractometer or conductivity meter.
By adjusting feed flow rate.
By controlling steam pressure or heat input.
Using density-based control for high-solid feeds.
With cascade control loops (temperature → concentration).
32. What is a level control system in evaporators?
Maintains constant liquid level in the evaporator body or separator.
Prevents tube exposure and dry spots.
Ensures stable evaporation rate.
Uses level transmitters, control valves, and feedback loops.
Critical for falling-film and multiple-effect stability.
33. How do you control vacuum stability?
Ensure condenser is properly cooled.
Remove non-condensable gases via venting.
Maintain proper ejector or vacuum pump performance.
Avoid air leaks in flanges, gaskets, or seals.
Maintain steady feed rate and no sudden surges.
34. What instrumentation is used to monitor fouling?
Differential pressure (ΔP) transmitters across heat exchanger.
Product temperature rise for same steam pressure.
Drop in evaporation rate for same feed rate.
U-value trending over time.
Visual inspection during shutdown.
35. What is the role of differential pressure in evaporator diagnostics?
Indicates fouling, scaling, or choking inside tubes.
Higher ΔP = lower flow → possible tube blockage.
Helps plan CIP cycles at the right time.
Prevents pump overloading.
Supports predictive maintenance.
36. Explain the heat-transfer mechanism in an evaporator.
Heat transfers from steam → tube wall → liquid film.
Liquid absorbs heat and vaporizes.
Vapor separates from the liquid in a vapor separator.
Driven by temperature difference (ΔT) and U-value.
Includes conduction, convection, and phase change.
37. How do you calculate evaporation rate?
Based on mass balance or heat input.
Formula:
Evaporation Rate (kg/hr) = (Heat Input Q) / (Latent Heat of Vaporization λ)
Or via mass balance:
Evaporation Rate = Feed Rate × (Feed Concentration − Product Concentration) / Product Concentration
38. What are the factors affecting U-value?
Steam-side heat-transfer coefficient.
Liquid-side heat-transfer coefficient.
Fouling and scaling.
Tube material and thickness.
Temperature difference (ΔT).
Viscosity and flow velocity of liquid.
Presence of non-condensable gases.
39. Why does U-value drop over time?
Formation of scale or fouling layers.
Accumulation of non-condensable gases on steam side.
Reduced circulation velocity due to choking.
Increased feed viscosity at higher concentration.
Decline in condenser/vacuum system performance.
40. Explain mass balance around an evaporator.
Total material entering = total leaving.
Formula:
Feed = Product + Vapor
If F = feed rate, P = product rate, V = vapor rate:
F = P + V
Concentration relation:
F × C₁ = P × C₂
Where:
C₁ = feed concentration
C₂ = product concentration
Used to calculate evaporation rate, steam load, and product output.
41. What causes foaming in evaporators?
Presence of surfactants or organic impurities.
High viscosity or high solid content.
Sudden boiling surges due to high ΔT.
High vapor velocity causing entrainment.
Biological growth in wastewater systems.
Control Measures:
Use antifoam agents.
Reduce temperature difference (ΔT).
Maintain steady feed rate.
Improve vapor–liquid separation.
42. What happens if the DO (Dissolved Oxygen) level drops below 1 mg/L?
Low DO is typically a wastewater/ETP question.
In evaporators, this question is usually misaligned.
If you want, I can replace it with an evaporator-specific Q.
42. Why does vacuum drop suddenly in an evaporator?
Air leakage from gaskets, seals, or flanges.
Cooling water temperature too high.
Fouling in condenser tubes.
Non-condensable gases not vented out.
Vacuum pump/ejector malfunction.
Sudden feed surge causing vapor overload.
43. Why is evaporation rate lower than design?
Low steam pressure or poor steam quality.
High viscosity of feed.
Scaling reducing heat-transfer coefficient.
Increased boiling point elevation (BPE).
Vacuum not achieved as per design.
Malfunctioning circulation pump.
44. What causes product darkening or degradation?
Excessive temperature due to high ΔT.
Long residence time in tubes.
Poor vacuum leading to higher boiling temperature.
Fouling causing local hot spots.
Thermal sensitivity of product ignored in design.
45. What are reasons for high condensate temperature?
Inadequate cooling water flow.
Cooling water entering at high temperature.
Fouling in condenser tubes.
Non-condensables reducing heat transfer.
Overloading the condenser with high vapor flow.
46. What causes entrainment carryover?
High vapor velocity.
Foaming in the evaporator.
Inadequate disengagement area in vapor separator.
Faulty or damaged demister pad.
Incorrect liquid level in separator.
47. What causes flooding in a falling-film evaporator?
Excessive feed rate.
Poor liquid distribution on tube sheet.
Vapor backpressure or vacuum issues.
High viscosity causing thick film.
Tube choking or scaling restricting downflow.
48. How do you identify tube choking in an evaporator?
Increase in ΔP across the exchanger.
Drop in evaporation capacity.
Higher product temperature for same steam pressure.
Reduced circulation flow rate.
Vibration or noise from fluid restriction.
49. What causes scaling in tubes?
High hardness, silica, or salt concentration.
High temperature difference causing precipitation.
Concentration reaching saturation limits.
Chemical instability of product.
Poor feed pretreatment or filtration.
50. What causes crystallization during evaporation?
Solubility limit exceeded as concentration increases.
Cooling of product during discharge or transfer.
Insufficient circulation velocity.
Too high final concentration setpoint.
Presence of nucleation-promoting impurities.
51. What are typical cleaning (CIP) methods for evaporators?
Alkaline cleaning to remove organic deposits.
Acid cleaning (HCl, nitric, sulfamic) for scale removal.
Hot water flushing to remove loose solids.
Enzymatic cleaners for food/pharma applications.
Recirculation CIP through tubes and separators.
Mechanical cleaning (pigging/brush) when scaling is hard.
52. How do you remove hard scale from evaporator tubes?
Use acid CIP (HCl/EDTA/nitric) depending on MOC.
Apply high-velocity recirculation to dissolve scale.
Use descaling chemicals (anti-silica, anti-CaSO₄).
For stubborn scale, use mechanical brushing.
Maintain correct water chemistry to avoid reformation.
53. When do you perform mechanical cleaning?
When CIP cannot remove heavy or mineral scale.
When ΔP across the heat exchanger remains high after CIP.
During scheduled shutdowns or annual turnarounds.
When evaporator tubes show localized hard deposits.
When fouling reduces capacity below acceptable limits.
54. How often are evaporator tubes inspected?
Depends on product and fouling tendency—typically:
Every 3–6 months in chemical and wastewater evaporators.
Every batch or campaign in pharma (GMP).
Annually for low-fouling applications.
Inspection frequency increases if ΔP trends indicate fouling.
55. What parameters determine cleaning frequency?
Increase in ΔP across exchanger.
Drop in U-value or heat-transfer rate.
Increase in steam consumption for same evaporation load.
Product quality drift or contamination.
Scaling tendency of feed (TDS, hardness, silica).
Operational duration between shutdown cycles.
56. What GMP requirements apply to evaporators in pharma?
Use of pharma-grade MOC (SS316L, electropolished surfaces).
Full CIP/SIP capability.
No dead legs → must follow ASME BPE guidelines.
Complete traceability → DQ/IQ/OQ/PQ documentation.
Use of validated cleaning and controlled environment.
Proper calibration of all instruments (temperature, level, pressure).
57. How do you validate an evaporator in pharma industry?
Prepare Validation Master Plan (VMP).
Conduct DQ/IQ/OQ/PQ as per GMP.
Validate critical process parameters (CPPs).
Verify cleaning validation (CIP/SIP) effectiveness.
Ensure product meets critical quality attributes (CQAs).
Document heat stability, concentration accuracy, and microbial control.
58. What documentation is required for evaporator qualification?
DQ (Design Qualification) – design specs, P&IDs, datasheets.
IQ (Installation Qualification) – equipment installation, MOC checks.
OQ (Operational Qualification) – functional checks, interlocks, alarms.
PQ (Performance Qualification) – consistent product results.
Calibration & certification reports.
Cleaning validation documents.
SOPs and risk assessments.
59. What are typical material-of-construction (MOC) requirements?
SS316 / SS316L – standard for pharma and chemicals.
Duplex steel / SS904L – for chloride/salt-rich feeds.
Hastelloy C-276 – for highly corrosive or acidic feeds.
Titanium – for seawater/brine applications.
Glass-lined steel – for extremely corrosive chemicals.
MOC selection depends on pH, chlorides, temperature, and fouling tendency.
60. What are safety interlocks required in evaporators?
Low-level trip to prevent dry running.
High-pressure steam cut-off.
Vacuum low alarm/trip to prevent overheating.
High-temperature product alarm.
Pump overload protection.
Condenser cooling water failure interlock.
Emergency shutdown (ESD) button.
61. What utilities are required for evaporators?
Steam (or hot oil) for heating.
Cooling water for condenser operation.
Electric power for pumps, agitators, and control system.
Vacuum system (steam ejector or vacuum pump).
Compressed air for control valves and instruments.
CIP chemicals and process water for cleaning.
Chilled water (if product is heat-sensitive or for MVR intercondensers).
62. How do mechanical vapour recompression (MVR) evaporators work?
Vapor generated from the evaporator is compressed using a mechanical compressor.
Compression increases pressure and temperature of vapor.
This high-temperature vapor is reused as the heating medium.
Greatly reduces steam consumption.
Highly energy-efficient → very low operating cost.
Ideal for large-scale ZLD, brine, chemical, and food applications
63. What is thermal vapour recompression (TVR)?
Uses a steam ejector to mix motive steam with evaporator vapor.
The ejector increases vapor pressure and temperature.
Recompressed vapor is used again as heating steam.
Cheaper than MVR but less efficient.
Best suited for medium-scale chemical and food plants.
64. How do you calculate steam consumption in an evaporator?
Based on required evaporation load and steam economy.
Formula:
Steam Required (kg/hr) = (Evaporation Load) / (Steam Economy)
Where:
Evaporation load = kg/hr of water removed.
Steam economy = kg vapor evaporated per kg steam.
65. What are common materials of construction (MOC) in evaporators?
SS316 / SS316L: Standard for pharma, food, chemicals.
SS304: Non-corrosive, low-cost applications.
Duplex / SS904L: Chloride and salt-rich streams.
Hastelloy C-276: Strong acids and corrosive chemicals.
Titanium: Brine, seawater, chloride-heavy liquor.
MOC selection depends on pH, chlorides, temperature, solids, and corrosion risk.
66. What evaporator is preferred for heat-sensitive products?
Falling Film Evaporator (short residence time).
Wiped Film / Thin Film Evaporator (very short residence time).
Vacuum operation to lower boiling temperature.
ATFE used when product is viscous and heat-sensitive simultaneously.
67. Why is falling film evaporator preferred in pharma?
Very gentle evaporation under low temperature.
Short residence time prevents degradation of APIs and intermediates.
High heat-transfer rate → efficient and economical.
Easy to clean (CIP friendly).
Minimal thermal stress; ideal for heat-sensitive formulations.
68. What evaporator is used for viscous materials?
Forced Circulation Evaporator: Maintains high velocity to avoid fouling.
Agitated Thin Film Evaporator (ATFE): Handles extremely viscous, sticky materials.
Wiped Film Evaporator: For polymer, resin, oil, wax type thick products.
Rising/falling film evaporators are not suitable for high-viscosity feeds.
69. How do you handle high-solids feed in evaporators?
Use forced circulation to maintain high velocity.
Use crystallizing evaporator if solids are expected.
Install cyclone separators for solids removal.
Prevent choking by maintaining adequate ΔP and flow rate.
Use slurry recirculation loops for ZLD/high TDS applications.
70. What evaporator is used for solvent recovery?
Falling Film Evaporator for heat-sensitive solvents.
Wiped Film Evaporator for high-boiling or viscous solvent mixtures.
Single-effect evaporator under deep vacuum for purity-sensitive solvents.
MVR/TVR for large scale solvent stripping.
Explosion-proof design required for flammable solvents.
71. How do you prevent degradation of API solutions during evaporation?
Operate under deep vacuum to reduce boiling temperature.
Use falling film or wiped film evaporators (short residence time).
Maintain low ΔT to avoid hot-spots.
Avoid long hold time in the evaporator body.
Use inert gas blanketing if product is oxygen-sensitive.
Use sanitary design (ASME BPE) to avoid contamination.
72. Explain Duhring’s rule.
States that the boiling point of a solution vs pure solvent has a linear relationship at constant pressure.
Helps calculate Boiling Point Elevation (BPE).
Essential for designing multiple-effect evaporators.
Used to estimate vapor–liquid equilibrium when data is unavailable.
73. What is recompression and why is it used?
Recompression = reuse of vapor as a heating medium.
Types:
MVR (Mechanical Vapor Recompression): Uses compressor.
TVR (Thermal Vapor Recompression): Uses steam ejector.
Why used:
Reduces steam consumption drastically.
Improves energy efficiency and lowers operating cost.
Ideal for large evaporation loads (ZLD, brine, chemical).
74. What are design considerations for zero-liquid discharge (ZLD) evaporators?
Must handle high TDS, high scaling, and slurry formation.
Requires forced circulation with high velocity.
Use duplex steel / titanium for corrosion resistance.
Install preheaters, solid separators, and crystallizers.
Strong vacuum and high-efficiency demisters required.
Must include robust CIP system for frequent cleaning.
75. What is a crystallizing evaporator?
Evaporator designed to concentrate solution until crystals form.
Uses forced circulation to prevent crystal deposition.
Separates crystals via settlers, centrifuges, or hydrocyclones.
Used in salt, caustic, brine, sulfate, fertilizer industries.
Maintains supersaturation under controlled conditions.
76. Explain the principle of MBBR (Moving Bed Biofilm Reactor).
Liquid forms a laminar or wavy film on inner tube walls.
Gravity-driven downward flow.
Vapor forms at the interface and flows co-currently.
Stable film → better heat transfer.
Maldistribution causes dry spots, scaling, and reduced efficiency.
77. What is fouling resistance?
Represents the thermal resistance caused by deposits on heat-transfer surfaces.
Included in heat-transfer calculation to design safe U-value.
Formula:
1 / U = 1 / h₁ + R_w + 1 / h₂ + R_f
Where R_f = fouling resistance.
Higher R_f → lower heat transfer → more steam usage.
78. How do you size a vacuum pump for evaporators?
Based on non-condensable gas load from:
Air leakage
Dissolved gases in feed
Ejector motive steam
Must account for operating pressure, gas temperature, and safety factor.
Pump must handle peak load during transient conditions (startup/shutdown).
79. What is the effect of non-condensable gases on evaporator performance?
Reduce heat-transfer by forming a barrier on steam side.
Increase steam pressure requirement.
Cause vacuum instability.
Reduce condenser efficiency.
Require continuous venting to maintain performance.
80. How do you perform heat & mass balance for a multiple-effect evaporator?
Start with feed rate, feed concentration, and desired product concentration.
Apply mass balance for each effect:
F = P + V
Calculate vapor generation for each effect.
Apply energy balance to determine:
Heat load per effect
Steam economy
Temperature drop distribution
Consider BPE for each effect.
Final output: steam required, vapor loading, product flow.
81. Define latent heat of vaporization and its importance in evaporators.
Latent heat is the amount of heat required to convert liquid to vapor at constant temperature.
Determines steam requirement for evaporation.
Higher latent heat → higher energy needed.
Essential for heat-load and steam economy calculations.
82. What is the effect of feed concentration on evaporation rate?
Higher concentration increases viscosity, reducing heat transfer
Causes higher boiling point elevation (BPE).
Reduces evaporation rate and increases steam consumption.
High-solid feeds require forced circulation or multiple stages.
83. Why is evaporation done under vacuum for heat-sensitive materials?
Reduces boiling temperature significantly.
Prevents thermal degradation, color change, and oxidation.
Improves product stability and quality.
Lower temperature also reduces fouling and scaling.
84. What is Duhring line and how is it used?
A graph showing boiling point of solution vs boiling point of pure solvent at the same pressure.
Used to estimate BPE when actual data is unavailable.
Helps design multiple-effect temperature distribution.
Essential for accurate heat load calculations.
85. What is the significance of approach temperature?
Approach temperature = difference between steam temp and boiling liquid temp.
Low approach = higher heat-transfer efficiency.
High approach = requires more steam, lower economy.
Critical design parameter in falling film and multiple-effect evaporators.
86. Why does boiling point increase with pressure?
Higher pressure increases vapor pressure requirement for boiling.
More energy needed → product boils at higher temperature.
Opposite occurs under vacuum → lower boiling temperature.
Important for designing effect-by-effect pressure cascade.
87. What is heat sensitivity index?
Indicates how quickly a product degrades with temperature.
Determines suitability for falling film, wiped film, or vacuum operation.
Higher sensitivity requires:
Low ΔT
Short residence time
Deep vacuum
Critical for pharma, food, and natural extracts.
88. How do you select the number of effects in a multiple-effect evaporator?
Based on steam cost vs capital cost.
BPE of the product.
Required evaporation load.
Available temperature difference across effects.
Operating cost vs energy saving trade-off.
Typical:
2–3 effects for pharma/chemicals
4–7 effects for ZLD and large brine systems
89. What parameters influence steam economy in multiple-effect systems?
Number of effects.
Temperature difference between effects.
BPE of feed.
Vacuum depth and condenser performance.
Heat-transfer coefficients in each effect.
Use of TVR/MVR to boost vapor.
90. Why is the last effect operated at the lowest pressure?
To maximize temperature difference across all effects.
Lower pressure reduces boiling temperature → easier evaporation.
Allows vapor from the previous effect to act as heating steam.
Improves overall steam economy and energy efficiency.
91. What are the advantages of forward-feed configuration in multiple-effect evaporators?
Uses gravity flow, reducing pumping cost.
Good for hot feeds (no preheating required).
Simple piping and control arrangement.
Suitable when product is not heat-sensitive.
Overall operation is stable at high throughputs.
92. What are the advantages of backward-feed configuration?
Cold feed enters the hottest effect, improving heat transfer.
Ideal for viscous and high-solids materials.
Reduces viscosity quickly due to rapid heating.
Allows better temperature utilization across effects.
Preferred in chemical, textile, and ZLD systems.
93. What is mixed-feed multiple-effect evaporator?
Feed enters intermediate effects instead of only the first or last effect.
Balances temperature distribution across effects.
Reduces product overheating for heat-sensitive streams.
Improves overall steam economy and operational flexibility.
Used when concentration range is wide.
94. Why is condensate flashing done?
Utilizes pressure difference to recover heat from hot condensate.
Produces flash steam that can be reused in preheaters.
Reduces total steam consumption.
Lowers cooling water demand.
Improves overall energy efficiency.
95. How does MVR reduce steam consumption?
Recompresses evaporator vapor to a higher temperature.
Recycled vapor replaces fresh steam almost entirely.
Leads to 90–95% reduction in steam usage.
Lowers operating cost drastically.
Highly efficient for large-scale continuous evaporation.
96. What is the efficiency of MVR evaporator compared to multiple-effect?
MVR steam economy: 10 to 30+ kg evaporated/kg steam.
Multiple-effect economy: 2 to 7 kg/kg steam.
MVR achieves the highest energy efficiency among all evaporator systems.
Most suitable for ZLD, brine, large chemical plants.
97. What is the role of the ejector in MVR systems?
Removes non-condensable gases to maintain vacuum.
Stabilizes evaporator pressure.
Prevents reduction in heat-transfer rate.
Assists compressor during startup.
Ensures smooth operation under varying vapor loads.
98. What is the purpose of vapor–liquid disengagement in evaporators?
Allows clean separation of vapor from liquid droplets.
Prevents entrainment and improves vapor quality.
Protects downstream equipment (effects, condensers).
Ensures consistent product concentration.
Achieved through cyclone separators, demisters, or baffles.
99. Why is uniform liquid distribution important in falling film evaporators?
Ensures even film thickness inside tubes.
Prevents dry patches → avoids scaling and fouling.
Maximizes heat-transfer area utilization.
Maintains high evaporation rate.
Essential for stable operation under vacuum.
100. What factors determine the pressure drop inside evaporator tubes?
Tube length and diameter.
Liquid viscosity and density.
Flow velocity (circulation rate).
Presence of vapor inside tubes.
Scaling or fouling restricting the passage.
Tube internal surface roughness.
101. What causes maldistribution in falling film evaporators?
Uneven liquid feed flow.
Blocked distribution holes or channels.
Incorrect feed flow rate.
Poorly designed distributor tray.
High viscosity causing uneven spreading.
102. How do distribution plates work in falling film evaporators?
Distribute feed evenly across all tubes.
Create uniform thin-film formation.
Prevent dry spots and scaling.
Manage both pressure drop and flow uniformity.
Integral to high-performance falling-film design.
103. Why are falling film evaporators unsuitable for high-solid feeds?
High solids increase viscosity → poor film flow.
Cause uneven distribution and dry patches.
High tendency of fouling and tube blockage.
Forced circulation or ATFE better suited.
104. Why is forced circulation used for salting or crystallizing solutions?
Maintains high velocity to prevent deposition.
Avoids crystallization inside tubes.
Allows controlled crystal formation in separator tank.
Handles high TDS and high-scaling liquids.
105. What causes pump cavitation in forced circulation evaporators?
Insufficient NPSH available.
High vapor pressure due to temperature.
Blocked suction line or choked strainer.
Low liquid level in evaporator.
Entrained air or vapor.
106. Why do forced circulation evaporators require high power?
High recirculation velocity required to reduce scaling.
Large pumps needed for high flow-to-evaporation ratio (5:1 to 15:1).
High viscosity and slurry handling increase load.
Long piping routes add friction losses.
107. How do you prevent deposition in forced-circulation systems?
Maintain high circulation velocity.
Keep ΔT low to avoid premature precipitation.
Use appropriate antiscalants.
Maintain uniform cooling and heating profiles.
Avoid overheating of concentrated stream.
108. What is the purpose of rotor blades in a wiped film evaporator?
Spread liquid into a very thin film.
Avoid hot spots and fouling.
Improve heat transfer and mass transfer.
Handle viscous, sticky, sensitive materials.
Enhance self-cleaning effect on walls.
109. How is film thickness controlled in a thin film evaporator?
Rotor speed adjustment.
Feed flow rate.
Blade design and clearance.
Liquid viscosity.
Operating pressure and temperature.
110. Why are wiped film evaporators used for high-viscosity materials?
Can handle highly viscous, sticky, tar-like feeds.
Very short residence time prevents degradation.
Mechanical agitation prevents stagnation.
Excellent heat transfer even at high viscosity.
111. What is molecular distillation and when is it used?
Distillation under high vacuum (10⁻³ to 10⁻⁴ mbar).
Used when molecules are extremely heat-sensitive.
Achieved using short path evaporators.
Applications: vitamins, herbal extracts, oils, fine chemicals.
112. What is the difference between thin film and short path evaporator?
Thin Film:
Longer vapor path.
Used for moderate heat sensitivity.
Short Path:
Internal condenser close to the film.
Used for extreme heat sensitivity and high-purity products.
113. Why do falling film evaporators require precise level control?
Too high level → flooding.
Too low level → dry spots, scaling.
Ensures stable boiling and proper film formation.
Prevents entrainment and improves separation.
114. Common causes of vacuum failure.
Air leakage in joints, gaskets, mechanical seals.
Poor condenser cooling water flow.
Fouled condenser tubes.
Vacuum pump/ejector malfunction.
Excess non-condensable gases in system.
115. Why are two-stage ejectors used?
Achieve deeper vacuum levels.
Reduce steam consumption.
Handle large volume of non-condensable gases.
Improve stability in multi-effect operation.
116. What is the function of an intercondenser?
Condenses vapor between ejector stages.
Reduces load on the next ejector.
Enhances vacuum performance.
Increases overall efficiency and steam savings.
117. Why do non-condensable gases reduce heat transfer?
Form insulating layer on steam side.
Increase temperature resistance.
Reduce effective steam temperature.
Lead to unstable vacuum.
118. Why should steam traps work correctly in evaporators?
Remove condensate from steam side.
Prevent waterlogging and reduced ΔT.
Improve U-value and efficiency.
Prevent water hammer.
119. How do you size a surface condenser for evaporators?
Based on vapor load (kg/hr).
Latent heat of condensation.
Allowable cooling water temperature rise.
Heat-transfer coefficient and available area.
Add safety factor (10–15%).
120. What is the effect of feed density on evaporator performance?
Higher density increases pumping power.
Reduces circulation velocity.
Influences boiling behavior and vapor–liquid separation.
Affects film thickness in falling film systems.
121. How do you calculate boiling point rise (BPR)?
BPR = increase in solution boiling point due to dissolved solids.
Formula:
BPR = T_solution − T_pure solvent
Higher BPR reduces ΔT available for heat transfer.
122. How do you calculate vapour load?
Based on evaporation rate and feed moisture.
Formula:
Vapor Load (kg/hr) = Feed × (Moisture Removed Fraction)
Used for sizing separators and condensers.
123. What information is required for designing an evaporator?
Feed flow and concentration.
Desired product concentration.
Feed physical properties (density, viscosity).
BPE/Solubility data.
Steam availability.
MOC requirement.
Fouling tendency and CIP frequency.
124. Explain the steps of heat load calculation for a single-effect evaporator.
Calculate evaporation load (kg/hr).
Multiply by latent heat.
Add sensible heat for feed preheating.
Total heat load = latent + sensible.
125. How is fouling factor included in design?
Added as Rₓ in heat-transfer equation.
Ensures design U-value accounts for fouling.
Formula:
1/U = 1/h₁ + R_w + 1/h₂ + R_f
126. Why is log-mean temperature difference (LMTD) used?
Accounts for varying temperature difference across exchanger.
More accurate than arithmetic ΔT.
Critical for designing heat-transfer area.
127. What is the impact of feed density?
Higher density increases pump load.
Affects circulation and separation.
Affects boiling behavior and energy requirement.
128. What is superheat and why is it undesirable?
Heating vapor above its saturation temperature.
Reduces condensation effectiveness.
Leads to poor heat transfer and energy loss.
129. How does vapor velocity affect entrainment?
High velocity → droplet carryover.
Low velocity → poor separation.
Optimal velocity needed for efficient separation.
130. What causes low product concentration?
Insufficient evaporation load.
Leak in steam or vacuum system.
Incorrect feed rate or bypassing.
Poor heat transfer due to fouling.
131. What causes excessive foaming?
Surfactants or organics in feed.
High solids or viscosity.
Sudden temperature spikes.
132. What causes low evaporation rate?
Low steam pressure.
Poor vacuum.
Fouling.
High viscosity.
133. What causes cloudy condensate?
Entrainment carryover.
Demister malfunction.
Improper separation.
134. What causes high product temperature?
Poor vacuum.
High ΔT.
Low feed rate.
135. What causes choking of recirculation line?
Crystals or solids.
Scaling.
Low velocity.
136. What causes overheating during vacuum loss?
Boiling point rises suddenly.
Steam continues heating.
Leads to burning or degradation.
137. What causes high differential pressure?
Scaling inside tubes.
Crystal deposition.
Pump blockage.
138. What is the role of demisters?
Remove entrained liquid droplets.
Improve vapor purity.
Protect downstream equipment.
139. How do you prevent tube vibration?
Use support plates.
Maintain correct velocity.
Avoid two-phase slugging.
140. Why is tube plugging done?
Isolate leaking or damaged tubes.
Avoid contamination.
Maintain operation until shutdown.
141. What are safety hazards in solvent-based evaporators?
Explosion risk.
Static discharge.
Toxic vapor release.
142. What are fire & explosion controls required?
ATEX motors.
Flame arrestors.
Nitrogen blanketing.
143. What PPE is required?
Chemical gloves.
Face shield.
Anti-static suit.
144. Why must evaporators be earthed?
Prevent static buildup.
Avoid ignition of vapors.
145. What checks are done before startup?
Vacuum tightness.
Pump rotation.
Condenser water flow.
146. What is an emergency shutdown?
Immediate isolation of steam.
Vacuum break.
Pump stop.
147. What evaporator is used for MEG/DEG?
Falling film under vacuum.
Forced circulation for high purity.
148. How to manage VOC emissions?
Condenser efficiency.
Activated carbon filters.
Vapor recovery units.
149. Why do condensers need flame arrestors?
Prevent flame propagation.
Protect downstream equipment.
150. Why is silica scaling difficult to remove?
Silica forms hard, glass-like deposits.
Not soluble in most acids.
Requires special chemicals (EDTA, ammonium bifluoride).
151. How do you control foam in an evaporator?
Add antifoam agents (silicone-based or polymeric).
Reduce ΔT to avoid sudden boiling surges.
Maintain steady feed rate.
Improve vapor–liquid separation.
Remove surfactants from feed through pretreatment.
152. What happens if the vacuum level drops below design?
Boiling point increases sharply.
Product overheats → degradation or color change.
Evaporation rate reduces.
Steam consumption increases.
Entrainment and foaming rise.
153. What is a sequencing (staged) evaporator?
A system where evaporation is performed in stages/effects.
Each stage uses vapor from previous one → high steam economy.
Operates progressively at lower pressures.
Commonly used in large chemical and ZLD plants.
154. How does MVR differ from conventional evaporators?
MVR uses a compressor to reuse vapor as heating steam.
Reduces steam consumption by up to 95%.
Conventional evaporators rely entirely on fresh steam.
MVR has higher capital cost but lowest operating cost.
155. What is the principle of a crystallizing evaporator?
Concentrate solution until it reaches supersaturation.
Crystals form and are separated mechanically.
Maintains slurry circulation to avoid tube scaling.
Used in salt, caustic, and ZLD applications.
156. What are vapor separators used for?
Separate vapor from entrained liquid.
Prevent product loss and contamination.
Improve vapor quality before entering next effect.
Protect condenser and vacuum system.
157. What causes tube bundle flooding?
Excess feed rate.
High viscosity preventing proper film formation.
Vapor backpressure.
Improper liquid distribution.
158. Why does the evaporator product show burnt smell?
High temperature due to poor vacuum.
Localized hot spots from scaling.
Excess residence time.
Insufficient feed rate causing dry heating.
159. What is the purpose of feed preheating?
Reduce steam requirement.
Improve evaporation rate.
Reduce thermal shock.
Enhance film distribution in falling-film systems.
160. How do you prevent tube choking in a forced circulation evaporator?
Maintain high circulation velocity.
Keep temperature difference low.
Use antiscalants.
Prevent crystals from forming inside tubes.
161. What evaporator is used for caustic concentration?
Forced circulation evaporator with specialized MOC.
SS316L, nickel alloy, or Hastelloy for corrosive duty.
Operates under vacuum to reduce scaling.
162. What evaporator is used for sugar syrup concentration?
Falling film evaporator.
Low ΔT and short residence time prevent caramelization.
Easy cleaning and high capacity.
163. What evaporator is used for API intermediates?
Falling film (heat-sensitive).
Wiped film / thin film (if viscous).
Vacuum operation mandatory for purity.
164. Why is ATFE preferred in herbal extract processing?
Extremely short residence time.
Handles viscous and sticky extracts.
Avoids product degradation.
High purity and color retention.
165. How does solvent recovery work in evaporators?
Solvent is vaporized under vacuum.
Vapor is condensed in surface condensers.
Non - condensables removed via ejectors.
Product retained as concentrated liquid.
166. What evaporator is used for wastewater concentration (ZLD)?
Forced circulation MEE (Multi-Effect Evaporator).
Handles high TDS, high scaling, and slurry formation.
Paired with crystallizer for final solids.
167. Why are forced-circulation evaporators used in petrochemical brine?
High chloride content causes severe scaling.
High velocity prevents deposition.
Suitable for corrosive and slurry-type streams.
168. Application of wiped film evaporators in oil & lube industry?
Remove light ends.
Distill high-boiling lubricants.
Purify waste oils.
Handle high viscosity without degradation.
169. What checks are performed before starting an evaporator after shutdown?
Verify vacuum tightness.
Ensure pumps rotate correctly.
Confirm steam traps working.
Check cooling water availability.
Clean any deposits if visible.
170. Why must lines be flushed before startup?
Remove settled solids.
Avoid choking during startup.
Prevent contamination of product.
Ensure stable circulation.
171. Why must vacuum be applied before heating?
Prevents sudden boiling and surging.
Protects heat-sensitive products.
Ensures smooth startup.
Reduces thermal stress.
172. What is the correct startup sequence?
Apply vacuum → cooling water → feed → steam.
Ensures safe and controlled evaporation.
Prevents foaming and product burning.
173. What parameters must stabilize before taking product?
Vacuum level.
Steam pressure.
Evaporation rate.
Product concentration.
Separator level.
174. Why is hot water circulation important before shutdown?
Removes viscous or sticky residues.
Prevents hard scale formation.
Eases cleaning during shutdown.
175. What is the difference between normal and emergency shutdown?
Normal: Controlled shutdown → feed stop → steam stop → vacuum break.
Emergency: Immediate isolation → steam cut-off → shut pumps → activate safety interlocks.
176. Why is air purging done after shutdown?
Prevents vacuum collapse damage.
Removes residual vapors.
Avoids corrosion and microbial growth.
177. What factors define cleaning cycle duration?
Scaling nature (silica, salts, organics).
Operating hours since last cleaning.
ΔP increase across tubes.
Change in product quality.
178. When is acid cleaning preferred?
For mineral scales: CaCO₃, CaSO₄, silica blends.
When alkaline cleaning cannot remove deposits.
For hard, crystalline scaling.
179. Why is alkaline cleaning required?
Removes organic fouling (oils, fats, organics).
Helps open up scaled areas prior to acid wash.
Good for pharma/food residues.
180. How often should evaporators be chemically cleaned?
Depends on feed: typically weekly to monthly.
High-scaling ZLD plants: every 2–3 days.
Pharma: every batch or campaign.
181. What evaporator is used for viscous polymer streams?
Wiped Film / Thin Film Evaporator.
Provides high heat transfer and avoids degradation.
182. Why is slurry recirculation important in ZLD evaporators?
Prevents solids settling.
Maintains high velocity.
Reduces choking and scaling.
Stabilizes crystallization.
183. What is scaling index and how is it used?
Indicator of scaling tendency of feed.
Helps adjust antiscalant dose.
Used to schedule CIP before hard scale forms.
184. Why do ZLD evaporators require strong vacuum?
Reduce boiling point significantly.
Prevent rapid scaling.
Reduce thermal degradation of organics.
185. Why do evaporators need expansion joints?
Absorb thermal expansion.
Prevent pipe stress and mechanical failure.
Protect equipment during temperature fluctuations.
186. Why do circulation pumps trip frequently?
High slurry or solid content.
Choked suction strainer.
Low NPSH causing cavitation.
Wrong rotation or mechanical seal failure.
187. What causes separator level fluctuations?
Surging boiling.
Foaming.
Improper level control loop tuning.
High vapor load.
188. Why is demister pad collapse dangerous?
Causes entrainment carryover.
Contaminates condensate.
Damages downstream equipment.
Results in product loss.
189. What causes flooding inside an evaporator?
Excess feed rate.
Vacuum failure.
Vapor choking.
High viscosity or foaming.
190. Why is anti-foam dosing required?
Controls foaming.
Prevents entrainment and contamination.
Improves separation and efficiency.
191. What is nucleation in crystallizing evaporators?
Initial formation of tiny crystals.
Occurs when supersaturation is achieved.
Determines final crystal size distribution.
192. Why is controlled cooling required after evaporation?
Prevent unintended crystallization.
Maintain product solubility.
Avoid blockage in pipelines.
193. What happens if ΔT is increased too much?
Rapid fouling or scaling.
Product thermal degradation.
Unstable boiling and foaming.
194. What happens if steam pressure fluctuates?
Concentration varies.
Foaming or surging may occur.
Heat transfer becomes uneven.
195. Why is online refractometer used?
Real-time concentration measurement.
Eliminates manual sampling.
Ensures consistent product quality.
196. Why is product viscosity important?
Affects heat transfer.
Determines type of evaporator chosen.
High viscosity requires forced circulation or ATFE.
197. What causes condensate backpressure?
Blocked steam trap.
Undersized condensate piping.
Improper trap installation.
198. Why is it important to vent non-condensables?
Prevent heat-transfer reduction.
Maintain vacuum level.
Improve evaporator efficiency.
199. Why do evaporators require continuous monitoring?
To prevent scaling.
Maintain concentration accuracy.
Protect product quality.
Ensure safety and efficiency.
200. What causes corrosion in evaporators?
High chlorides or acids in feed.
Wrong material-of-construction.
Poor pH control.
Oxygen ingress under vacuum.
201. What is Reynolds number significance in evaporator tubes?
Indicates flow regime: laminar or turbulent.
Higher Reynolds → better heat transfer.
Helps decide circulation velocity in forced circulation.
Prevents fouling and scaling by maintaining turbulence.
202. What is the critical flow velocity for evaporator tubes?
Minimum velocity required to prevent deposition inside tubes.
Generally:
1.5–3 m/s for low-viscosity liquids
3–5 m/s for slurry/high solids
Ensures stable film formation and reduces scaling.
203. How do you estimate distribution uniformity in falling film systems?
Check flow per tube using calibration tests.
Inspect distributor trays for wetting pattern.
Monitor ΔT uniformity across bundle.
Use visual borescope inspection during shutdown.
204. Why does vacuum reduce without any leakage?
Accumulation of non-condensable gases.
Poor condenser cooling.
High feed temperature releasing dissolved gases.
Overloading vapor separator.
205. What causes sudden temperature spikes in product?
Vacuum collapse or drop.
Steam pressure surge.
Foaming causing uneven boiling.
Inadequate feed rate → dry heating.
206. What causes condenser flooding?
Blocked condensate outlet.
Steam trap malfunction.
Excess vapor load.
Non-condensable gas build-up.
207. Why does steam trap continuously blow steam?
Trap failure or damaged valve seat.
High pressure differential.
Incorrect trap sizing.
Waterlogging inside the steam chest.
208. What causes excessive condensate back pressure?
Blocked condensate return line.
Incorrect condensate piping slope.
Trap undersized for load.
Closed or partially shut return valve.
209. How do you diagnose tube leakage?
Contaminated condensate becomes cloudy.
Product smell appears in condensate.
Pressure test during shutdown.
Dye penetration or helium leak test.
210. What causes crystallization inside tubes unexpectedly?
Supersaturation due to cooling.
High ΔT creating localized precipitation.
Poor circulation.
Incorrect concentration setpoint.
211. Why does the circulation pump repeatedly trip?
High solid content overloads pump.
Suction strainer blockage.
Incorrect rotation direction.
Cavitation due to low NPSH.
212. What causes high solids in vapor stream?
Poor vapor–liquid disengagement.
Damaged or missing demister pad.
Excessive foaming.
High vapor velocity.
213. What causes product “burning smell” during evaporation?
Thermal degradation due to high temperature.
Hot spots caused by scaling.
Vacuum failure.
Long residence time.
214. Why does first effect scale faster than last effect?
Highest temperature difference.
Higher heat flux → more precipitation.
Feed impurities deposit first.
Last effect runs at lower temperature → slower scaling.
215. What causes vibration in falling-film tube bundle?
Vapor-induced tube flutter.
High velocity two-phase flow.
Improper support plates.
Maldistribution causing uneven load.
216. What are hazards of operating evaporators at high temperature?
Product degradation.
Rapid scaling and fouling.
Tube rupture risk.
Overpressure due to boiling surges.
217. What is the risk of vacuum implosion?
Sudden collapse of shell due to external atmospheric pressure.
Occurs if steam or inert gas enters rapidly.
Prevented by vacuum breakers and proper shutdown sequences.
218. Why must condensers vent non-condensable gases?
Non-condensables reduce heat transfer.
Increase condenser pressure.
Reduce vacuum efficiency.
Cause temperature instability.
219. Why is nitrogen blanketing used in solvent evaporators?
Prevents explosion by eliminating oxygen.
Controls oxidation-sensitive solvents.
Maintains safe pressure during shutdown/startup.
220. What are fire & explosion hazards in solvent recovery evaporators?
Presence of flammable vapors.
Static electricity discharge.
Hot surfaces near vapor lines.
Incorrect grounding/bonding.
221. What PPE is mandatory during descaling operations?
Acid-resistant gloves.
Face shield and goggles.
Chemical splash suit.
Safety shoes.
Respiratory protection for fumes.
222. What permits are required for evaporator maintenance in petrochemicals?
Hot Work Permit (if welding/grinding).
Confined Space Entry Permit.
Line Break Permit.
Isolation/LOTO certificate.
Gas test clearance.
223. What are CPPs (Critical Process Parameters) in pharma evaporation?
Steam pressure and temperature.
Product concentration.
Vacuum level.
Feed rate.
Residence time.
Condensate quality.
224. What are CQAs (Critical Quality Attributes)?
Product concentration accuracy.
Color, clarity, viscosity.
pH and purity.
Microbial load.
Residual solvents (if applicable).
225. How do you justify hold time in concentration?
Supported by stability studies.
Ensures no degradation occurs at hold conditions.
Validated through sample testing.
Documented as part of PQ.
226. How do you validate mixing uniformity in evaporator product tanks?
Use mixing studies with tracers.
Monitor concentration at multiple points.
Ensure homogeneity meets acceptance criteria.
Validate agitator performance.
227. What documents must be updated after modification?
P&ID
Datasheets
Validation documents (IQ/OQ/PQ)
SOPs
Risk assessment (FMEA)
Calibration records
228. How is microbial control maintained in pharma evaporators?
CIP/SIP validated cycles.
Using stainless steel 316L MOC.
Maintaining low hold times.
Use of condensate-polishing and sterile barriers.
229. What are extractables and leachables concerns?
Chemicals migrating from equipment surfaces into product.
Must use pharmacopeia-approved elastomers and seals.
Controlled via PQ and compatibility studies.
230. What is acceptable endotoxin level after evaporation?
Must meet pharmacopeia limits (e.g., <0.25 EU/mL for injectables).
Depends on product category.
Monitored via LAL test.
231. What calibration standards are used in pharma evaporators?
Temperature: traceable to NABL/NIST.
Pressure/vacuum gauges: calibrated against primary standards.
Flow meters: calibrated using volumetric or gravimetric tests.
Conductivity/refractometer: validated with certified standards.
232. How do evaporative crystallizers work?
Evaporate solvent until supersaturation is achieved.
Crystals form and grow in slurry form.
Crystals are separated mechanically (centrifuge/filter).
Concentrated mother liquor recycled.
233. What is supersaturation and why is it important?
State where solution holds more solute than normal at a temperature.
Drives crystal formation.
Must be controlled to achieve desired CSD (crystal size distribution).
234. Why is forced circulation preferred for crystallization?
Keeps crystals suspended.
Prevents deposition on tube walls.
Ensures uniform temperature distribution.
Allows controlled crystal growth.
235. How do you avoid crystal deposition in tubes?
Maintain high circulation velocity.
Control supersaturation rate.
Use heat exchangers with proper ΔT.
Dose antiscalants if required.
236. What factors affect crystal size distribution?
Nucleation rate.
Supersaturation level.
Residence time.
Agitation intensity.
Cooling/evaporation rate.
237. What is nucleation shock?
Sudden uncontrolled formation of many fine crystals.
Caused by high supersaturation.
Leads to poor product quality and filtration issues.
238. What is mother liquor?
Liquid remaining after crystallization.
Contains dissolved solids not crystallized.
Often recycled to improve yield.
239. How do you handle azeotropic solvents in evaporation?
Use vacuum to shift azeotrope.
Integrate with distillation column.
Employ entrainers if needed.
Use specialized solvent recovery units.
240. Why is MOC critical for chlorinated solvent evaporators?
Chlorinated solvents are corrosive.
Require SS316L, Hastelloy, or titanium.
Prevents equipment failure and contamination.
241. What safety systems are required for flammable solvent evaporation?
Explosion-proof motors.
Nitrogen blanketing.
Flame arrestors.
Static grounding and bonding.
Intrinsically safe instrumentation.
242. What evaporator is used for MEG or DEG purification?
Falling film under vacuum.
High-purity forced circulation for high BP components.
Often integrated with distillation
243. Why are falling films used for aromatic solvents?
Low residence time reduces polymerization.
Operates under deep vacuum to prevent thermal degradation.
High heat-transfer efficiency.
244. How do you manage VOC emissions from evaporators?
High-efficiency condensers.
Vapor recovery units (VRUs).
Activated carbon bed absorbers.
Proper vent sealing and leak detection.
245. Why do condensers need flame arrestors?
Prevent flame flashback into vapor line.
Protect equipment in flammable service.
Mandatory in petrochemical/solvent plants.
246. What evaporator is best for high-TDS wastewater?
Forced Circulation MEE.
Handles scaling, crystallization, and slurries.
Typically integrated with crystallizer.
247. Why is forced circulation used everywhere in ZLD?
High velocity prevents tube scaling.
Suitable for slurry handling.
Allows continuous concentration and crystal formation.
248. Why do ZLD evaporators need antifoam agents?
Wastewater has detergents/surfactants → heavy foaming.
Prevents carryover and contamination.
Protects vacuum system and condenser.
249. What causes frequent choking in ZLD evaporators?
Extremely high salt loads.
Silica deposits.
Poorly controlled supersaturation.
Low circulation velocity.
250. Why is silica scaling difficult to remove?
Creates hard, glass-like, chemically-resistant deposits.
Insoluble in most acids and alkalis.
Requires special cleaning agents (ammonium bifluoride, EDTA blends).
High ΔT accelerates silica deposition.
251. How do you maintain MEE (Multi Effect Evaporator) efficiency in ZLD plants?
Maintain proper vacuum in each effect.
Keep condenser cooling efficient.
Clean heat exchanger tubes frequently.
Maintain correct ΔT distribution.
Ensure uniform feed flow to all effects.
Monitor ΔP for fouling trends.
252. Why is silica scaling the most critical issue in ZLD evaporators?
Silica precipitates even at low temperatures.
Forms glassy, extremely hard deposits.
Blocks tubes rapidly → severe capacity loss.
Very difficult to remove chemically.
Requires specialized anti-scaling treatment.
253. What chemical agents help control silica scaling?
Ammonium bifluoride (ABF-based cleaners).
EDTA blends (chelating agents).
Polymeric antiscalants tailored for silica.
pH control to keep silica in soluble form.
254. Why is slurry handling important in crystallizing evaporators?
Prevents settling and choking in tubes.
Ensures uniform crystal growth.
Maintains design heat-transfer rates.
Protects pumps from abrasion.
255. What factors determine the choice of circulation pump?
Feed viscosity.
Solids/slurry concentration.
Required circulation velocity.
NPSH availability.
Corrosive nature of feed.
256. What is steam economy in MEE systems typically?
2–3 effects → 2 to 3 kg/kg steam.
4–7 effects → 4 to 7 kg/kg steam.
Higher effects → higher economy but more capital cost.
257. What causes pressure imbalance between effects?
Improper vacuum distribution.
Vapor line choking.
Condenser not performing.
Sudden load fluctuations.
258. Why is condensate quality critical?
Indicates tube leakage or entrainment.
Poor quality affects reuse of condensate.
In pharma, condensate must meet WFI/clean steam standards.
High TDS means contamination from product side.
259. What causes high TDS in condensate?
Tube leaks.
Entrainment due to foaming.
Demister failure.
High vapor velocity.
260. What instrumentation is critical in MEE?
Vacuum transmitters.
ΔP (differential pressure) indicators.
Conductivity/refractometer.
Flow meters (feed, steam, cooling water).
Temperature sensors (each effect).
Level sensors for separators.
261. Why is ΔT different in each effect?
Each effect operates at a different pressure.
BPE increases as concentration increases.
Temperature drop must be optimized for balanced evaporation.
262. What are common reasons for poor vapor–liquid separation?
High vapor velocity.
Inadequate separator volume.
Demister pad blockage/damage.
Foaming in evaporator chamber.
263. Why do foaming feeds need special design?
Require large disengagement area.
Use tall vapor separators.
Need anti-foam dosing system.
Vapor lines sized for lower velocity.
264. What is the importance of condensate polishing?
Removes oil, organics, minerals from condensate.
Allows reuse as boiler feed water.
Reduces water consumption and utility cost.
265. What causes thermal degradation in falling film evaporators?
High ΔT.
Poor vacuum.
Long residence time due to low flow rate.
Hot spots from scaling.
266. Why are multiple-effect evaporators arranged in descending pressure?
To allow vapor from previous effect to act as heating steam.
Improve steam economy.
Optimize temperature difference.
267. What happens if demister pad differential pressure increases?
Vapor carryover increases.
Pressure drop rises.
Condenser overloads.
Requires cleaning or replacement.
268. Why is anti-foaming required in distillery wastewater evaporation?
Spent wash contains proteins and surfactants.
Heavy foaming leads to carryover.
Causes product loss and contamination.
269. What is the impact of cooling water temperature on vacuum?
Higher cooling water temperature → poor condensation → lower vacuum.
Vacuum directly depends on condenser performance.
Ideal cooling water temp: 25–35°C.
270. Why are vacuum pumps preferred in solvent evaporation?
Provide stable vacuum.
Handle non-condensables effectively.
Support deep vacuum levels required for heat-sensitive solvents.
271. What causes steam hammering in evaporators?
Sudden condensate blockage.
Improper steam trap location.
Waterlogging in steam lines.
Rapid steam opening.
272. Why must steam pressure be controlled tightly?
Prevents overheating.
Controls product concentration.
Ensures steady ΔT and evaporation rate.
Protects equipment from overpressure.
273. Why is product sampling important during evaporation?
Ensures concentration accuracy.
Detects degradation or color change.
Helps adjust feed/steam ratio.
274. What causes product entrainment in condensate?
Foaming in evaporator.
High vapor velocity.
Insufficient liquid–vapor separation.
275. Why do high-solids feeds cause scaling?
Reduced solubility at high temperature.
Supersaturation due to evaporation.
Tendency to form crystals or deposits.
276. Why is feed filtration important?
Removes suspended solids.
Reduces tube blockage.
Ensures uniform heat transfer.
Prevents pump damage.
277. What is the impact of dissolved gases in feed?
Release during heating → vacuum instability.
Increase non-condensable load.
Reduce condenser efficiency.
278. Why is tube thickness critical?
Determines heat-transfer efficiency.
Thin tubes → better heat transfer but lower mechanical strength.
Thick tubes → robust but lower U-value.
279. Why is corrosion allowance considered in design?
Compensates for expected metal loss.
Ensures long-term equipment life.
Mandatory for corrosive feeds.
280. What is degassing feed pre-treatment?
Removes dissolved gases before evaporation.
Reduces vacuum load.
Prevents oxidation and foaming.
281. Why is multi-stage cooling required for condensers?
To reduce load on the final stage.
Improve overall vacuum stability.
Manage large vapor loads efficiently.
282. Why are cyclones used as vapor–liquid separators?
High-efficiency droplet removal.
Suitable for high vapor velocity.
Compact and low maintenance.
283. What causes unstable level in evaporator body?
Foaming.
Incorrect control valve tuning.
Vapor surges.
Feed fluctuations.
284. What is a bleed point in MEE systems?
Vent for releasing non-condensables.
Maintains vapor purity.
Improves heat transfer.
285. Why is proper steam trap sizing important?
Too small → flooding.
Too large → steam loss.
Balanced sizing ensures optimal U-value.
286. Why is condensate sometimes reused?
High temperature → free heat energy.
Low TDS → reusable as boiler feed.
Reduces water and steam costs.
287. What is the role of PID control in evaporators?
Stabilizes key parameters: temperature, vacuum, feed rate.
Prevents surging and foaming.
Improves product consistency.
288. Why do separators need correct residence time?
Ensures droplets settle out.
Reduces entrainment.
Stabilizes vapor quality.
289. Why is insulation required on vapor lines?
Prevents heat loss.
Maintains vapor temperature.
Improves steam economy.
290. What causes flashing in condensate lines?
Sudden pressure drop.
High-temperature condensate.
Incorrect condensate routing.
291. Why must feed temperature be controlled?
Hot feed reduces steam use.
Too hot causes flashing → unstable level.
Safe balance needed for consistency.
292. Why are vent condensers used?
Condense vapors escaping with non-condensables.
Reduce VOC emissions.
Improve vacuum pump performance.
293. What is the purpose of vacuum breakers?
Prevent equipment collapse during shutdown.
Allow controlled air entry.
Protect shell integrity.
294. Why is overconcentration dangerous?
Sudden solids precipitation.
Choking and tube blockage.
Thermal degradation of product.
295. Why is product viscosity monitored?
Higher viscosity → lower heat transfer.
Impacts pump load and circulation.
Helps prevent overconcentration.
296. Why is feed pH important?
Influences scaling tendency.
Affects corrosion of MOC.
Impacts solubility of salts.
297. What causes product boiling delay (bumping)?
Lack of nucleation sites.
Sudden vapor bursts.
High ΔT or fast heating.
298. Why is online conductivity useful?
Direct indication of concentration.
Helps automate product discharge.
Reduces sampling frequency.
299. Why do separators use baffles?
Slow down vapor.
Direct flow path for separation.
Improve droplet removal.
300. Why is cooling water flow stability important?
Maintains stable vacuum.
Prevents condenser overload.
Ensures steady evaporation rate.