Question 1

Bubble-cap columns: the maximum allowable vapor velocity is primarily set by which design consideration?

Accepted Answer

Introduction / Context: Tray towers must strike a balance between vapor throughput and separation quality. In bubble-cap trays, vapor passes through risers and caps into the liquid, creating froth and interfacial contact. Excessive vapor rates cause liquid carry-over (entrainment) to the tray above, degrading efficiency and risking flooding. Hence, a physical phenomenon—not a single geometric parameter—sets the limit. Given Data / Assumptions: Conventional bubble-cap tray design with adequate downcomers. Steady operation; normal hydrocarbon or aqueous service. Goal: determine what ultimately caps allowable superficial vapor velocity. Concept / Approach: As vapor velocity increases, droplet formation intensifies and more liquid is carried upward. When entrainment exceeds acceptable levels, tray efficiency drops and flooding may begin in downcomers. While vapor density and column diameter influence velocities (via hydraulics and area), the governing constraint is the entrainment limit that ensures proper phase disengagement and mass transfer on each stage. Step-by-Step Solution: Recognize that tray capacity is controlled by froth and droplet behavior.Link vapor velocity to entrainment rate and downcomer back-up.Conclude that maximum allowable vapor velocity is set by entrainment considerations. Verification / Alternative check: Tray rating methods use capacity factors adjusted for system properties to limit velocities before entrainment/flooding; these are anchored in entrainment criteria. Why Other Options Are Wrong: Vapor density and column diameter affect velocity but do not alone define the limit; the limit arises from entrainment onset. “None of these” is incorrect because entrainment is the accepted governing factor. Common Pitfalls: Neglecting foaming or surface-tension effects that lower the entrainment threshold; oversizing based on nominal velocities without checking downcomer capacity. Final Answer: Entrainment considerations

Question 2

Tray hydraulics and efficiency: when the liquid path length on a tray exceeds about 5 ft, how does the overall column efficiency typically respond?

Accepted Answer

Introduction / Context: On crossflow trays, liquid travels laterally from the inlet downcomer to the outlet downcomer while contacting rising vapor. The path length affects residence time, lateral gradients, and backmixing. Designers must balance area utilization with mass-transfer effectiveness to achieve high stage efficiency without inducing maldistribution or stagnant zones. Given Data / Assumptions: Conventional single-pass crossflow trays. Liquid path length increased beyond ~5 ft. No drastic changes in foaming tendency or weeping/flooding regime. Concept / Approach: As path length increases, the liquid tends to exhibit a larger concentration and temperature gradient from inlet to outlet, and non-uniform froth quality can develop across the tray width. Excessive path length promotes backmixing, dead zones near the walls, and maldistribution, which reduce the approach to equilibrium and hence the effective tray (Murphree) efficiency. Therefore, very long paths are generally disfavored unless compensated by multiple passes or improved distribution. Step-by-Step Solution: Relate path length to hydrodynamics: more residence does not guarantee better contacting if gradients form.Recognize increasing maldistribution and backmixing with long travel distances.Conclude that overall efficiency typically decreases beyond about 5 ft unless the tray is reconfigured. Verification / Alternative check: Multi-pass tray designs shorten individual path lengths while maintaining capacity; field experience shows efficiency recovery compared with single long paths. Why Other Options Are Wrong: “Increases” and “Remains same” ignore well-documented maldistribution effects. “May increase or decrease … plate spacing” misattributes the dominant factor; spacing matters, but path-length-driven maldistribution is primary. Common Pitfalls: Assuming residence time alone controls efficiency; overlooking tray leveling and weir uniformity; pushing capacity at the expense of contacting quality. Final Answer: Decreases

Question 3

Shell-and-tube heat exchangers: identify the incorrect statement regarding baffles, spacing limits, and unsupported tube spans.

Accepted Answer

Introduction / Context: Baffles in shell-and-tube exchangers control shell-side flow pattern, enhance heat transfer, and limit tube vibration by reducing unsupported spans. Several conventions exist for minimum and maximum baffle spacing and for typical unsupported tube spans. Knowing which statements match common practice helps avoid design misconceptions. Given Data / Assumptions: Conventional TEMA-type exchanger, single or multiple shell passes. Standard materials such as carbon steel, stainless steel, and nonferrous alloys. Normal process services without extraordinary fouling or vibration issues. Concept / Approach: Segmental baffles (single or double) are by far the most commonly used in industry because they are simple, effective, and economical. Disk-and-doughnut configurations are used but are less common. Maximum baffle spacing is typically limited to about the shell inside diameter. Minimum baffle spacing is often the greater of roughly 50 mm or 0.2 times the shell diameter to preserve crossflow and cleaning access. Unsupported spans on the order of 75 times the tube diameter are common guide values, reduced for softer metals. Step-by-Step Solution: Check each statement versus practice.(a) 50 mm as a basic minimum aligns with common lower bounds (subject to shell size) → acceptable as a minimum reference.(b) Maximum spacing ≈ shell I.D. → consistent with practice.(c) Span guidance 75×OD, reduced for softer metals → consistent.(d) “Disk-and-doughnut most common” contradicts practice; single segmental is most common → incorrect. Verification / Alternative check: Design guides and vendor standards highlight single-segmental prevalence; disk-and-doughnut is niche for pressure drop or fouling reasons. Why Other Options Are Wrong: (a), (b), and (c) align with widely used rules of thumb; they are not the incorrect statements here. Common Pitfalls: Applying minimum spacing blindly without checking shell diameter; neglecting vibration checks for long spans; assuming nonstandard baffles are default. Final Answer: Disk-and-doughnut baffle is the most commonly used baffle.

Question 4

Bubble-cap tower design: for towers with diameter greater than 3 ft, what is the typical ratio of total riser area to the tower cross-sectional area?

Accepted Answer

Introduction / Context: On bubble-cap trays, risers convey vapor from below the tray into the liquid pool under the caps. The combined cross-sectional area of these risers determines the superficial vapor velocity through each riser and influences pressure drop, entrainment, and capacity. Designers select a modest fraction of the tower area for risers to maintain proper contacting without excessive restriction. Given Data / Assumptions: Tower diameter > 3 ft. Conventional bubble-cap geometry; adequate downcomers and weirs. Normal hydrocarbon/aqueous service; no extreme foaming. Concept / Approach: If the riser area is too small, vapor velocity through slots and caps is too high, leading to entrainment and pressure drop. If too large, active bubbling area is lost and tray efficiency may suffer. Industry practice typically targets a riser-area fraction on the order of 10–20% of column cross-section to balance these effects across a wide range of services. Step-by-Step Solution: Express riser fraction = total riser area / column area.Apply standard design window used in tray rating: approximately 0.1–0.2.Confirm that this maintains acceptable slot velocities and pressure drop. Verification / Alternative check: Vendor rating calculations consistently yield riser fractions in this band for middle-of-the-road designs; extremes are tuned only for special services. Why Other Options Are Wrong: 0.35–0.75 or 0.55–0.85: implausibly large, would severely reduce active bubbling area. 0.4–0.6: likewise unrealistically high for normal tray design. Common Pitfalls: Ignoring slot velocity limits; neglecting the accompanying need for sufficient downcomer area; assuming a single fraction suits highly foaming systems. Final Answer: 0.1–0.2

Question 5

Thermo-mechanical effects: what is the name of the stress produced in a metal bar when its temperature is changed but deformation is fully restrained?

Accepted Answer

Introduction / Context: Materials expand on heating and contract on cooling. When a component is free to expand, no significant stress arises from temperature change. However, if expansion or contraction is restrained (e.g., fixed ends), internal stresses develop. Recognizing and naming this stress is fundamental to thermal design of piping, vessels, and structural members. Given Data / Assumptions: Uniform temperature change across the member. Both ends or boundaries restrain deformation (no free expansion). Linear elastic response within the elastic limit. Concept / Approach: Thermal strain, if prevented, converts to stress according to Hooke’s law. Depending on whether the tendency is to expand (heating) or contract (cooling), the induced stress can be compressive or tensile in nature; collectively, such stress is termed thermal stress. Engineers quantify it using material properties and thermal expansion coefficients, and mitigate it with expansion joints or flexibility in piping. Step-by-Step Solution: Recognize that restrained thermal strain → internal stress.Note that the sign depends on heating (compression) or cooling (tension).Use the general term for this phenomenon: thermal stress. Verification / Alternative check: Thermal analysis calculations (e.g., σ = E * α * ΔT for full restraint) confirm the stress nature and magnitude. Why Other Options Are Wrong: Shear, tensile, compressive describe directions or signs; the encompassing term under thermal cause is “thermal stress.” Common Pitfalls: Ignoring partial restraint; neglecting temperature gradients that introduce bending; overlooking creep/relaxation at high temperatures. Final Answer: Thermal stress

Question 6

Bubble-cap trays: for a fixed cap geometry, which factors set the required number of caps per tray in an industrial column?

Accepted Answer

Introduction / Context: In bubble-cap trays, vapor rises through risers and discharges via cap slots into the liquid layer. The count of caps per tray governs total vapor-handling area, slot velocities, and contacting uniformity. Determining this count requires a holistic view of hydraulics and geometry rather than a single parameter. Given Data / Assumptions: Cap size and slot geometry are fixed (catalog item). Normal distillation/absorption service with crossflow trays. Goal: meet capacity, efficiency, and pressure drop constraints. Concept / Approach: Allowable slot velocity limits entrainment and pressure drop; plate spacing affects froth height and disengagement, constraining workable vapor rates; column diameter sets available plan area and hence the feasible layout (cap pitch, active area versus downcomer area). Therefore, the cap count emerges from all three factors acting together through tray rating procedures. Step-by-Step Solution: Estimate required vapor area from throughput and allowable slot velocity → sets total slot area and thus number of caps.Check plate spacing and froth behavior → ensures operability without entrainment or weeping.Lay out caps within the column diameter, preserving downcomer and active area requirements → yields final cap count. Verification / Alternative check: Vendor rating software iterates these constraints; sensitivity shows each factor materially impacts cap counts and spacing. Why Other Options Are Wrong: Choosing only one parameter ignores the coupled hydraulic and geometric constraints that define workable cap layouts. Common Pitfalls: Overcrowding caps and starving downcomers; ignoring fouling allowances; setting slot velocities too high for foamy systems. Final Answer: All (a), (b) and (c)

Question 7

Binary distillation methods: Smoker’s equation for estimating the number of equilibrium stages is specifically applied under which condition?

Accepted Answer

Introduction / Context: Several shortcut correlations estimate the number of ideal stages for binary distillation: Fenske’s method at total reflux, Underwood’s method for minimum reflux, Gilliland-type correlations for operating conditions, and specialized equations such as Smoker’s. These tools simplify early design when detailed stage-to-stage calculations are not yet warranted. Given Data / Assumptions: Binary system with relative volatility α not much larger than 1. Goal: obtain a realistic stage count without full rigorous simulation. Ordinary tray or packed columns with equilibrium stages as the modeling basis. Concept / Approach: When α approaches unity, the separation becomes difficult; traditional shortcuts can lose accuracy or require many stages. Smoker’s equation is tailored to such near-ideal, close-boiling systems and offers a way to approximate stage numbers accounting for the small driving force inherent to α ≈ 1. Step-by-Step Solution: Identify that α ≈ 1 signals a close-boiling, hard separation.Select Smoker’s equation as the appropriate shortcut correlation.Use product purities, feed composition, and α to estimate the required number of stages for preliminary sizing. Verification / Alternative check: Cross-check against rigorous simulation or McCabe–Thiele methods shows Smoker’s predictions are most useful when α is near 1; deviations grow as α increases. Why Other Options Are Wrong: Feed thermal condition (bubble/dew) affects operating lines but is not the defining trigger for Smoker’s equation. Limiting to stripping section only is unrelated. “Very small” stage counts do not describe close-boiling separations, which usually need more stages. Common Pitfalls: Applying the correlation outside its intended α range; ignoring non-idealities or azeotropy; skipping verification with rigorous methods before final design. Final Answer: Relative volatility is close to one (e.g., separation of close-boiling isomers).

Question 8

Corrosion allowance policy: in the design of pressure vessels and process equipment, when is an explicit corrosion allowance generally not provided?

Accepted Answer

Introduction / Context: Corrosion allowance (CA) is an extra thickness added to account for expected uniform metal loss during service. It is distinct from mechanical allowances such as mill tolerance or fabrication margins. The need for CA depends on corrosion resistance of the chosen material and the expected environment, not simply on initial wall thickness. Given Data / Assumptions: Standard pressure vessel codes followed; material selection aligned with process corrosion data. Question focuses on when CA is not typically required. High alloy steels (e.g., austenitic stainless, nickel alloys) are used in suitably compatible services. Concept / Approach: When highly corrosion-resistant alloys are correctly matched to the process environment—such that expected metal loss is negligible—designs commonly omit a specific CA and rely on the intrinsic resistance of the alloy. In contrast, merely increasing wall thickness (e.g., > 30 mm) does not remove the need for CA; corrosion would still proceed and could compromise life if not accounted for. Step-by-Step Solution: Identify that CA addresses expected corrosion rate, not structural strength.If the material is a high alloy with negligible uniform corrosion in service → CA may be omitted.Wall thickness alone does not justify omitting CA; carbon steels typically require CA in corrosive service. Verification / Alternative check: Owner standards often specify zero CA for stainless/nickel alloys in non-corrosive or passivating media, while mandating CA for carbon steels even at large thicknesses. Why Other Options Are Wrong: Plain carbon steel/cast iron: usually require a CA in corrosive environments. Wall thickness > 30 mm: thickness does not eliminate corrosion; life would still diminish. Both (b) and (c): includes an unjustified condition (b), hence incorrect. Common Pitfalls: Confusing CA with mill tolerance; assuming localized corrosion modes are covered by CA; neglecting cladding/lining strategies. Final Answer: Material of construction is high alloy steel.

Question 9

Industrial piping identification: in chemical plants, oxygen-carrying pipelines are painted with which standard line color for quick recognition and safety?

Accepted Answer

Introduction / Context: Color coding of industrial pipelines is a fundamental safety practice. It enables operators and maintenance teams to instantly recognize the service contained in a line, preventing accidental cross-connections and improving emergency response. Oxygen service, in particular, must be clearly distinguished because it strongly promotes combustion and requires strict cleanliness standards. Given Data / Assumptions: Context is general chemical-plant piping color standards used for service identification. We focus on line (pipe) color, not compressed-gas cylinder body/shoulder colors. The question tests recall of the standard color commonly assigned to oxygen lines. Concept / Approach: Standard industrial schemes (widely adopted plant codes) assign blue to oxygen pipelines. While local or national standards can vary in shade or in auxiliary banding, the base identification for oxygen in plant piping is blue, aiding rapid visual identification near manifolds, headers, and equipment tie-ins. Step-by-Step Solution: Identify the service: oxygen (combustion-supporting gas).Recall standard plant color code: oxygen lines → blue.Eliminate distractors: yellow (often fuel gas), red (fire water), black or green used for other utilities depending on site code.Select the color used for oxygen pipelines: blue. Verification / Alternative check: Typical plant standards, vendor P&ID legends, and pipe-marking guidelines consistently depict oxygen service with a blue base color and appropriate text arrows/markers. Why Other Options Are Wrong: Yellow: frequently used for flammable gases; not oxygen. Black/Green: often used for different utilities (e.g., waste lines, water) depending on scheme. Red: commonly fire water, not oxygen. Common Pitfalls: Confusing cylinder color codes with pipeline color codes; relying on memory from a different regional standard without checking the plant legend. Final Answer: Blue

Question 10

Shell-and-tube heat exchangers: which of the following tube lengths is normally NOT used in practice for standard bundles and maintenance access?

Accepted Answer

Introduction / Context: Standard tube lengths in shell-and-tube heat exchangers balance thermal performance, pressure drop, and maintainability. Very short tubes complicate tube-to-tubesheet fabrication, reduce area per pass dramatically, and hinder economical layout. Understanding typical lengths helps in quick “sanity checks” during specification and vendor bid reviews. Given Data / Assumptions: Conventional exchangers with removable or fixed bundles. Normal fabrication practices using standard tube stock lengths (often in ~2.4 m, 3 m, 4.5 m, 6 m ranges, etc.). Focus is on what is normally not used, not what is theoretically possible. Concept / Approach: Common practice favors tube lengths of a few meters (e.g., 2.5–6 m or more), which provide sufficient heat-transfer area without excessive shell length. Extremely short tubes (e.g., 0.5 m) cause disproportionate header space, poor economic utilization of shell hardware, and limited thermal effectiveness per unit cost. Step-by-Step Solution: List typical lengths available from tube mills and used by TEMA vendors (2.5 m, 3 m, 6 m, etc.).Assess practicality: very short tubes lead to costly headers versus area gained.Therefore identify 0.5 m as atypical in normal practice.Select the “not used” option accordingly. Verification / Alternative check: Vendor catalogs and TEMA-based datasheets show preferred standard lengths aligned with stocking and bundle assembly considerations. Why Other Options Are Wrong: 2.5 m, 3 m, 6 m, 10 m are all common in various duties and sizes. Common Pitfalls: Confusing small lab/pilot units (which could use short tubes) with industrial practice; overlooking maintenance access space for pulling bundles. Final Answer: 0.5 metre

Question 11

Rotary dryer design: the typical height of lifting flights is what fraction of the drum inside diameter d?

Accepted Answer

Introduction / Context: Lifting (or showering) flights inside rotary dryers pick up solids and cascade them through the gas stream, greatly enhancing heat and mass transfer. Flight geometry, especially flight height relative to drum diameter, affects curtain density, residence time, and risk of choking or poor showering. Given Data / Assumptions: Cocurrent or countercurrent rotary dryer handling granular solids. Typical industrial operating speeds and standard flight layouts. Focus on widely used rule-of-thumb proportions. Concept / Approach: Practical experience indicates flights around 8–12% of drum diameter produce a good solids curtain without excessive holdup or blockage. Too small flights yield thin curtains and poor contact; excessively large flights can cause avalanching, erratic showering, and higher drive loads. Step-by-Step Solution: Define design target: robust solids curtain → choose moderate flight height.Adopt empirical range used by vendors: 0.08–0.12 d.Reject values that are too small (0.02–0.05 d) or unrealistically large (≥0.2 d).Select the accepted range. Verification / Alternative check: Design handbooks and vendor application notes provide comparable ranges, with fine-tuning by test runs. Why Other Options Are Wrong: Very small flights underperform in mixing/contacting. Very large flights lead to instabilities, high torque, and poor showering quality. Common Pitfalls: Ignoring material stickiness or shrinkage; not adjusting flight angle along the dryer length; overlooking influence of gas velocity on curtain stability. Final Answer: 0.08 to 0.12 d

Question 12

Packed absorption columns: the onset of flooding typically corresponds to a pressure drop of about how many millimetres of water per metre of packing height?

Accepted Answer

Introduction / Context: Flooding in packed towers is a hydraulic limit where liquid holdup rises sharply and pressure drop increases rapidly, causing loss of mass-transfer efficiency and potential entrainment. Designers use pressure-drop criteria and capacity correlations to estimate safe operating envelopes and turndown. Given Data / Assumptions: Random packing operation under typical gas–liquid services. Pressure drop referenced as mm H2O per metre of packed height. Interest is in an indicative range at the onset of flooding. Concept / Approach: Empirical practice suggests that flooding onset corresponds to modest pressure-drop levels, often within several tens of mm H2O per metre. A commonly referenced bracket is roughly 50–75 mm H2O/m at the approach to flooding for many ring or saddle packings, though exact values depend on fluid properties, packing size, and distributor quality. Step-by-Step Solution: Recognize flooding as a sharp inflection in ΔP–throughput behavior.Recall typical onset range: around 50–75 mm H2O per metre.Exclude very high choices (hundreds to thousands) which represent severe maloperation.Select the realistic onset bracket. Verification / Alternative check: Packed-tower design literature and vendor curves align with similar onset ranges, adjusting for packing geometry and size. Why Other Options Are Wrong: 25–50 can be too conservative for many packings. 200–250 or higher represent deep flooding, not onset. Very small values (5–10) are well below typical limits. Common Pitfalls: Ignoring distributor maldistribution; extrapolating one packing’s data to another; operating too close to flooding without adequate control. Final Answer: 50–75

Question 13

Tray design guideline: the minimum clearance between the overflow weir and the last row of bubble caps on a distillation tray should be approximately how many centimetres?

Accepted Answer

Introduction / Context: On bubble-cap trays, proper hydraulic layout requires adequate clearance between the overflow weir and the last row of caps. This allows liquid to approach the weir uniformly, reduces localized high velocities, and avoids cap submergence anomalies that can impair mass transfer or induce weeping/entrainment. Given Data / Assumptions: Conventional circular tower with bubble-cap trays. Standard cap sizes and weir heights. Rule-of-thumb engineering practice for layout. Concept / Approach: Providing a few centimetres of open approach length improves flow distribution into the downcomer and reduces hydraulic interaction with the weir crest. A commonly cited minimum is about 7.5 cm between the weir and the last bubble-cap row, balancing active area use with hydraulic stability. Step-by-Step Solution: Identify the needed “quiet zone” near the weir.Apply the practice value ~7.5 cm for clearance.Reject too-small gaps (2.5 cm) and unnecessarily large gaps that waste active area.Select 7.5 cm as the recommended minimum. Verification / Alternative check: Tray layout guides and vendor standards specify comparable minimum clearances to ensure stable weir hydraulics. Why Other Options Are Wrong: 2.5 cm is usually insufficient; can cause hydraulic interference. 15–20 cm reduce active area more than necessary for typical trays. Common Pitfalls: Not scaling clearance with cap size; ignoring foaming systems that may require additional approach length; placing hardware (bolts/bars) that intrude into the approach zone. Final Answer: 7.5 cm

Question 14

Distillation operation: the working reflux ratio can vary between which practical limits when compared with the minimum reflux ratio?

Accepted Answer

Introduction / Context: The reflux ratio R is a key design and operating parameter in distillation. It trades off energy (reboiler and condenser duties) with capital (number of stages). Theoretical extremes define bounds for feasibility and optimization during column design and operation. Given Data / Assumptions: Binary or multicomponent distillation with total condenser. Minimum reflux ratio R_min is the smallest value at which separation is just possible with infinite stages. Infinite reflux corresponds to total reflux operation with zero product withdrawal. Concept / Approach: In practice, the operating (working) reflux must be at least the minimum (R ≥ R_min). As R approaches infinity, the number of theoretical stages needed approaches the minimum possible. Economic optimum typically lies between 1.1–1.5 times R_min for binaries, but the theoretical upper limit is unbounded (infinite) while the lower limit is R_min. Step-by-Step Solution: Recognize the lower bound: R cannot be less than R_min.Recognize the theoretical upper bound: R → ∞ at total reflux.Therefore, the variation is between minimum and infinity.Select the option reflecting these bounds. Verification / Alternative check: McCabe–Thiele and Ponchon–Savarit analyses show stage and reflux tradeoffs and confirm these bounds. Why Other Options Are Wrong: “Zero and infinity” ignores the minimum reflux constraint. “Zero and one” is unrelated to column feasibility. Common Pitfalls: Confusing minimum reflux with minimum stages; forgetting composition/spec constraints when choosing operating R. Final Answer: Minimum and infinity

Question 15

Noncircular ducts: for a square duct of side d, what is the equivalent (hydraulic) diameter to use for heat-transfer calculations?

Accepted Answer

Introduction / Context: Engineering correlations for pressure drop and convection in ducts often employ the hydraulic (equivalent) diameter to account for noncircular cross-sections. For turbulent flow and most design work, the same hydraulic diameter is used for both friction and heat-transfer correlations. Given Data / Assumptions: Square duct with side length d. Use of standard equivalent diameter definition Dh = 4A/P, where A is flow area and P is wetted perimeter. Application to heat-transfer calculations under typical conditions. Concept / Approach: Compute Dh using geometry: for a square, A = d^2 and P = 4d. Therefore Dh = 4A/P = 4 d^2 / (4 d) = d. This provides a simple, exact equivalence for the square section. Unless special laminar, developing-flow, or constant-heat-flux corrections are required, Dh = d is used directly in Nusselt, Reynolds, and friction factor correlations. Step-by-Step Solution: Write Dh = 4A/P.Insert A = d^2 and P = 4d.Evaluate Dh = 4 d^2 / (4 d) = d.Use Dh = d in heat-transfer correlations. Verification / Alternative check: Handbooks list Dh = characteristic dimension for common shapes; for square ducts Dh equals side length. Why Other Options Are Wrong: d/2 or d/√2: not consistent with 4A/P. 4d/π: corresponds to circular equivalence that does not apply here. 2d: overestimates the scale and would mispredict Re and Nu. Common Pitfalls: Mixing hydraulic diameter definitions for laminar fully developed specialized cases; forgetting to use wetted perimeter for partially filled ducts. Final Answer: d

Question 16

Surface condensers: why is cooling water routed on the tube side in a multi-pass arrangement during design?

Accepted Answer

Introduction / Context: Condenser designs aim for high overall heat-transfer coefficients while controlling pressure drop and accommodating water quality. Routing cooling water through the tubes allows multiple passes so velocity can be increased to enhance the tube-side film coefficient, often yielding a smaller, more economical exchanger. Given Data / Assumptions: Surface (shell-and-tube) condenser service. Tube-side fluid is cooling water; shell side contains condensing vapor. Pass partitioning is available to adjust water velocity. Concept / Approach: Tube-side convection coefficient generally increases with velocity. Using multiple passes shortens individual pass length and raises velocity for a given flowrate, boosting h_i and overall U. This improvement reduces the required area, making the exchanger more compact. The tube side also tolerates fouling/cleaning better and contains the higher-pressure utility safely. Step-by-Step Solution: Place water on tube side to allow pass partitions and controllable velocity.Increase velocity → increase film coefficient → higher U.Higher U → less area for the same duty → smaller condenser.Therefore, multi-pass tube-side routing makes the unit more compact. Verification / Alternative check: Design equations for convective coefficient (e.g., Dittus–Boelter-type) show h proportional to velocity^n, explaining the benefit of multi-pass arrangements. Why Other Options Are Wrong: Pressure drop is not reduced by adding passes; it usually increases for a given flow. “More thinner tubes” is not the primary reason; tube count is design-dependent. It cannot “avoid fouling entirely.” Common Pitfalls: Over-increasing passes causing excessive ΔP; ignoring erosion limits; neglecting water-side scaling when choosing velocity. Final Answer: It makes the condenser more compact by permitting higher water velocity and better heat transfer.

Question 17

Thin-walled cylindrical vessel under internal pressure: what is the expression for the longitudinal (axial) stress?

Accepted Answer

Introduction / Context: Design of thin-walled pressure vessels distinguishes between circumferential (hoop) stress and longitudinal (axial) stress. Correct formulas are critical for sizing wall thickness, selecting materials, and verifying code compliance. For thin cylinders (t ≪ D), membrane theory provides simple expressions used in preliminary design and quick checks. Given Data / Assumptions: Internal pressure p (uniform). Cylinder mean diameter D and wall thickness t, with t ≪ D. Closed ends carrying internal pressure load. Concept / Approach: The longitudinal stress arises from the pressure force on the end caps, balanced by the axial stress in the cylindrical wall across the cross-sectional metal area. Force balance yields σ_l = p D / (4 t), whereas the hoop stress from tangential force balance is σ_h = p D / (2 t). The longitudinal stress is thus half the hoop stress in a thin cylinder. Step-by-Step Solution: End force = pressure * end area = p * (π D^2 / 4).Metal resisting area (axial) = π D t.Set equilibrium: σ_l * (π D t) = p * (π D^2 / 4).Solve: σ_l = p D / (4 t). Verification / Alternative check: Compare with hoop stress formula σ_h = p D /(2 t); longitudinal is half of hoop for thin cylinders, matching classic results. Why Other Options Are Wrong: p D /(2 t) is the hoop stress, not longitudinal. 2 p D / t and p t / D do not follow membrane equilibrium for thin shells. p / 2 lacks dimensional consistency. Common Pitfalls: Using internal radius rather than mean diameter inconsistently; applying thin-wall formulas when t is not small relative to D. Final Answer: σ_l = p D / (4 t)

Question 18

Why are industrial pipes most commonly circular in cross-section for fluid transport and structural economy?

Accepted Answer

Introduction / Context: Cross-sectional shape influences both mechanical integrity and material economy in piping. A sound choice minimizes stress concentration, maximizes resistance to internal pressure, and reduces required wall area per unit flow capacity. Circular pipes dominate industrial practice for these reasons. Given Data / Assumptions: Thin-walled pipe approximation for pressure analysis. Comparison among alternative shapes at equal enclosed flow area or volume. Material cost roughly proportional to surface area and wall thickness. Concept / Approach: For a given internal pressure, a circular cylinder develops uniform hoop stress; other shapes develop bending and stress concentrations at corners. From geometry, a circle encloses the maximum area for a given perimeter, or equivalently, provides the minimum perimeter (surface area) for a given enclosed area. Thus circular pipes achieve both better structural efficiency and material economy. Step-by-Step Solution: Assess strength: circular sections distribute hoop stress uniformly.Assess economy: minimal perimeter → minimal wall material for a given bore.Combine benefits: strong and material-efficient → preferred for piping.Conclude both reasons apply. Verification / Alternative check: Thin-cylinder stress formulas and isoperimetric inequality underpin these conclusions mathematically and are standard in mechanical design texts. Why Other Options Are Wrong: (a) alone or (b) alone are incomplete—both advantages are relevant. “None of these” contradicts well-established design reasoning. “Easier to paint” is irrelevant to fundamental design criteria. Common Pitfalls: Ignoring fabrication/connection issues; misapplying to non-pressure structural members where other shapes may be preferred. Final Answer: Both (a) & (b)

Question 19

Continuous distillation economics: the typical “optimum” reflux ratio is selected as a multiple of the minimum reflux ratio. Over common industrial conditions, this optimum is usually in the range of __________ times the minimum value.

Accepted Answer

Introduction / Context: In continuous binary (or multicomponent) distillation, designers rarely operate exactly at minimum reflux (which would require an infinite number of stages) or at excessively high reflux (which would demand large condenser/reboiler duties). Instead, an economic optimum is selected as a modest multiple of the minimum reflux ratio to balance capital and energy costs. Given Data / Assumptions: Constant molal overflow approximation is acceptable for preliminary design. Minimum reflux ratio is obtained from total/partial condenser methods (Underwood, McCabe–Thiele pinch at the top). Equipment and utility costs have typical industrial weightings (no extreme energy pricing). Concept / Approach: The “optimum” reflux ratio minimizes the total annualized cost. As reflux increases, stage count decreases (smaller column height but larger utilities). As reflux decreases toward minimum, stage count and column height grow rapidly. The shallow minimum in total cost typically occurs near 1.1–1.5 times the minimum reflux ratio for many separations and relative volatilities. Step-by-Step Solution: Identify Rmin from design methods (pinch analysis/Underwood).Evaluate total cost vs. R: capital roughly decreases with R, utilities increase with R.Select the economic minimum, commonly at R ≈ 1.1–1.5 Rmin. Verification / Alternative check: Sensitivity studies on tray count versus reflux show diminishing returns beyond about 1.3–1.5 Rmin; the drop in stages is modest while energy use escalates. Why Other Options Are Wrong: 1.6 to 2 / 2.2 to 2.6 / 2.7 to 3: These are typically above the economic sweet spot and imply unnecessary energy costs for little reduction in stages under ordinary utility and capital cost ratios. Common Pitfalls: Confusing “optimum” with “minimum”; assuming a single universal value without checking utility pricing and tray/packing costs. Final Answer: 1.1 to 1.5

Question 20

Internally pressurised cylindrical vessels: comparing closures of the same material and thickness, which head shape can withstand the highest internal pressure before reaching the same membrane stress limits?

Accepted Answer

Introduction / Context: Pressure vessel ends (heads) convert internal pressure into membrane stresses. For a given material and thickness, geometry strongly influences the stress distribution. Identifying the most efficient head helps reduce weight or increase allowable design pressure. Given Data / Assumptions: All heads have identical nominal thickness and are made from the same material. Elastic membrane theory applies; local discontinuities at knuckles are neglected for comparison. Internal pressure loading only; no external loads or vacuum buckling. Concept / Approach: Hemispherical heads develop uniform membrane stresses and have the most favorable curvature in both principal directions. As a result, for the same thickness, the allowable internal pressure is highest. Ellipsoidal (2:1) heads are next best, followed by torispherical or conical. Flat plates are worst because they carry bending in addition to membrane stresses. Step-by-Step Solution: Rank shapes by curvature efficiency: hemispherical > ellipsoidal > conical > flat.For equal thickness, the head with the lowest stress at a given pressure can withstand the highest pressure.Select hemispherical as best performer. Verification / Alternative check: Design codes use larger allowable pressure or smaller required thickness for hemispherical heads than for ellipsoidal/torispherical heads under the same conditions. Why Other Options Are Wrong: Ellipsoidal (2:1): efficient but not as strong as hemispherical for the same thickness.Conical: stress concentrations at the knuckle; lower pressure capacity.Flat plate: carries bending, lowest pressure capacity for a given thickness. Common Pitfalls: Comparing unequal thicknesses; ignoring required knuckle radii and code joint efficiencies. Final Answer: hemispherical

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