Advance Machine Design (ME-702 (D)) - Unit 1 Short Notes

UNIT 1: ADVANCED POWER AND MANUFACTURING SYSTEMS

I. POWER PLANT ENGINEERING SYSTEMS

A. Energy Sources and Conversion Fundamentals

Primary vs Secondary Energy Sources:

• Primary: Obtained directly from nature in raw form (e.g., coal, crude oil, natural gas, uranium, solar radiation, wind, hydro).

• Secondary: Derived from primary sources after conversion (e.g., electricity, gasoline, diesel, hydrogen, processed fuel oils).

• Interconvertibility: Energy can be converted from one form to another (e.g., chemical → thermal → mechanical → electrical in a thermal plant). Efficiency losses occur in each conversion stage due to the Second Law of Thermodynamics.

Direct Energy Conversion Methods:

1. Magnetohydrodynamic (MHD) Converter:

   • Working Principle: Ionized combustion gas (plasma) is passed through a magnetic field. The movement of charged particles (ions/electrons) across the field induces an electromotive force (EMF) perpendicular to both gas flow and magnetic field, generating direct current.

   • Schematic:

     DiagramCANVAS: Show a channel with electrodes on opposite walls, a magnet creating a perpendicular field, and hot ionized gas flowing through. Arrows indicate gas flow, magnetic field lines, and current direction.

   • Limitations: Requires very high temperatures (~2500°C) for sufficient ionization; material challenges for electrodes and channel walls; low electrical efficiency in open-cycle systems; high initial cost.

   [!TIP] MHD is a topping cycle that can increase overall plant efficiency when used with a steam bottoming cycle.

2. Fuel Cells:

   • Definition: An electrochemical device that converts chemical energy of a fuel (anode) and an oxidant (cathode) directly into electricity and heat, without combustion.

   • Types: Based on electrolyte: Phosphoric Acid Fuel Cell (PAFC), Molten Carbonate Fuel Cell (MCFC), Solid Oxide Fuel Cell (SOFC), Proton Exchange Membrane Fuel Cell (PEMFC).

   • Working Principle (General): Fuel (e.g., H₂) oxidized at anode → electrons flow through external circuit → oxidant (e.g., O₂) reduced at cathode. Electrolyte permits ion migration.

   • Thermodynamics: Maximum reversible work (and open-circuit voltage) given by Nernst Equation:

$$E = E^0 - \frac{RT}{nF} \ln Q$$

    where $$\displaystyle E^0 $$ is standard potential, $R$ gas constant, $T$ temperature, $n$ electrons transferred, $F$ Faraday's constant, $Q$ reaction quotient.

> [!TIP] Fuel cells are not heat engines; their efficiency is not limited by Carnot cycle and can be >60%.

B. Renewable Energy Systems

Hybrid Energy Systems: Combine two or more renewable sources (e.g., solar-wind, solar-biomass) with/without storage (batteries) to overcome intermittency and improve reliability.

• Feasible Options in India: Solar-Wind hybrid (complementary seasonal patterns in some regions), Solar-Biomass (biomass provides base load, solar daytime peak), Small Hydro-Solar in hilly/riverine areas.

C. Fossil Fuel Power Plants

1. Steam Power Plants (Coal-based)

• Coal Handling System Elements:

  1. Wagon Tippler: Unloads coal from rail wagons.

  2. Crushers & Screens: Reduce coal size & remove impurities.

  3. Conveyors: Transport coal to storage/boiler.

  4. Stacker-cum-Reclaimer: For stacking in yard & reclaiming.

  5. Magnetic Separator: Removes ferrous impurities.

  6. Weighing System: Measures coal quantity.

  DiagramSEARCH: thermal power plant coal handling system layout

• Fuel Burning Systems:

  • Overfeed: Coal fed above the grate. Suitable for low-volatile coals (anthracite). Primary air from below, secondary air above. Better control.

  • Underfeed: Coal fed below the grate. Suitable for high-volatile coals (bituminous). Coal moves upward as it burns. More compact.

• Fluidized Bed Combustion (FBC) System:

  • Working: Air blown at high velocity through a bed of sand/limestone + coal particles. At critical velocity, bed behaves like a fluid. Combustion occurs at 800-900°C.

  • Advantages: Low NOx formation (low temp), in-situ SO₂ capture with limestone, fuel flexibility (low-grade coal, biomass), high heat transfer.

  DiagramCANVAS: Sketch of a bubbling fluidized bed boiler showing air distributor, bed material, coal feed, and heat exchanger tubes.

• Boiler Trends: Larger capacity (≥660 MW), higher steam parameters (supercritical, ultra-supercritical: >22 MPa, >600°C), once-through boilers, low-NOx burners, digital control systems.

• Feed Water Treatment: Multi-stage: Clarification & Filtration (remove suspended solids) → Ion Exchange/Demineralization (remove dissolved salts) → Degasification (remove dissolved O₂, CO₂) → Chemical Conditioning (pH control, oxygen scavengers like hydrazine).

• Plant Heat Balance (Example): Energy input (fuel LHV) = Energy output (electricity) + Losses (stack, radiation, blowdown, auxiliary consumption). Typical efficiency: 38-42% (subcritical), 45%+ (supercritical).

• Cooling Towers: Natural Draft (hyperbolic, large, low pumping power) vs Mechanical Draft (induced/forced draft, compact, higher pumping power). Function: Cool circulating water via evaporative cooling.

2. Gas Turbine Power Plants

• Simple Open Cycle: Air compressed → fuel added & combusted → hot gases expand in turbine → produce work. Net work = Turbine work - Compressor work. Efficiency low (~30%) due to high exhaust heat loss.

• Reheating: Expand gases in high-pressure turbine → Reheat in combustion chamber → expand in low-pressure turbine.

  • Improves Efficiency: Increases average temperature of heat addition (T_max) without increasing compressor work or T_max in first stage. Reduces moisture content at final turbine stage. Efficiency increase ~2-4%.

D. Nuclear Power Plants

• Nuclear Fission Phenomenon: Heavy nucleus (U-235, Pu-239) absorbs a neutron → becomes unstable → splits into two lighter fission fragments + 2-3 fast neutrons + ~200 MeV energy. Chain reaction sustained if multiplication factor (k_eff) = 1.

• Reactor Moderators: Slow down fast fission neutrons to thermal energies where fission cross-section of U-235 is high.

  • Light Water (H₂O): Cheap, good moderator & coolant. Absorbs some neutrons (requires enriched uranium).

  • Heavy Water (D₂O): Excellent moderator, low neutron absorption (allows natural uranium). Expensive.

  • Graphite: Solid, good moderator, low absorption. Used with CO₂ gas coolant (AGR). Requires high purity.

• Pressurized Heavy Water Reactor (PHWR - e.g., CANDU):

  • Working: Natural UO₂ fuel in Zr-alloy pressure tubes. Heavy water coolant (under high pressure) flows through pressure tubes. Heavy water moderator (at lower pressure) in calandria surrounding tubes. On-power refueling.
  DiagramSEARCH: PHWR reactor core schematic calandria pressure tube

• Advanced Gas Cooled Reactor (AGR): Second-gen UK design. Graphite moderator, CO₂ coolant, enriched uranium fuel (2.5-3.5% U-235).

• Thermal vs Fast Breeder Reactors:

  | Feature | Thermal Reactor (PHWR, PWR, BWR) | Fast Breeder Reactor (FBR) | | :--- | :--- | :--- | | Neutron Energy | Thermal (slow, ~0.025 eV) | Fast (MeV range) | | Moderator | Yes (H₂O, D₂O, Graphite) | No | | Coolant | Water, Heavy Water, CO₂ | Liquid Sodium (Na) or Lead | | Fuel | U-235 (enriched/natural) | Pu-239 + U-238 (blanket) | | Breeding | No (consumes more fissile) | Yes (produces more fissile Pu-239 from U-238) | | Fuel Cycle | Once-through or limited recycle | Closed fuel cycle |

• Reliability Features: Redundant safety systems, diverse shutdown systems, containment building, emergency core cooling, passive safety features (natural circulation), rigorous quality assurance.

• Principles of Reactor Control: Control rods (B₄C, Ag-In-Cd) inserted/withdrawn to absorb neutrons. Chemical shim (soluble boron in PWR). Moderator temperature coefficient, fuel temperature coefficient (Doppler effect) provide inherent negative feedback.

E. Hydropower Plants

• Hydraulic Turbines:

  • Impulse (Pelton): High head (300-2000 m), low flow. No draft tube. High specific speed low.

  • Reaction:

    • Francis: Medium head (30-300 m), medium flow. Spiral casing, draft tube. Most common.

    • Kaplan: Low head (2-30 m), high flow. Adjustable blades. High specific speed.

  • Selection Factors: Net Head (H), Design Discharge (Q), Specific Speed (n_s). $$\displaystyle n_s = n \sqrt{P} / H^{5/4} $$ (where n=rpm, P=power).

• Site Selection Criteria: High annual rainfall & dependable flow, narrow deep gorge (reduce civil cost), good geology (rock foundation), proximity to load center, accessibility, sedimentation data, environmental & social impact.

• Micro vs Pico Hydro:

  | Feature | Micro Hydro | Pico Hydro | | :--- | :--- | :--- | | Capacity | 100 kW - 10 MW | < 100 kW (often < 50 kW) | | Head | Medium to high | Low to medium | | Application | Mini-grid, village power | Single community/home, very remote | | Civil Works | Significant | Minimal, run-of-river | | Grid Connection | Possible | Usually isolated |

• Spillways: Structures to safely pass flood flows. Types: Ogee (overflow, common), Side Channel, Shaft (Morning Glory), Chute.

• Flow & Power Duration Curves:

  • Flow Duration Curve (FDC): Discharge (Q) sorted descending vs % time exceeded. Indicates dependable flow (flow exceeded 90-95% time).

  • Power Duration Curve (PDC): Power output (P ∝ Q·H) sorted descending vs % time. Used for firm power calculation and plant sizing.

F. Power Plant Economics and Operation

• Key Performance Indices:

  1. Maximum Demand (MD): Peak load on the station in a given period (MW).

  2. Load Factor (LF):

$$\text{Load Factor} = \frac{\text{Average Load}}{\text{Maximum Demand}} = \frac{\text{Energy Produced in period}}{(\text{MD} \times \text{Period hours})}$$

. Measures utilization.

3.  **Diversity Factor (DF):** 

$$\text{Diversity Factor} = \frac{\sum \text{Individual Peak Loads}}{\text{Station Peak Load}}$$

. >1 indicates load diversity.

4.  **Plant Factor (PF) / Capacity Factor:** 

$$\text{Plant Factor} = \frac{\text{Actual Energy Produced}}{\text{Maximum Possible Energy (if run at MD continuously)}}$$

. Measures capacity utilization.

• Calculation Example (From Past Paper):

  Given: Peak loads of 4 regions = 10, 5, 8, 7 MW. DF = 1.5, Annual LF = 0.6.

  (i) Station MD = Σ Individual Peaks / DF = (10+5+8+7) / 1.5 = 30 / 1.5 = 20 MW.

  (ii) Annual Energy = MD × LF × Hours/year = 20 × 0.6 × 8760 = 105,120 MWh = 105.12 GWh.

• Tariff Structures:

  • Flat Rate: Fixed charge per kWh.

  • Block Rate: Different rates for different consumption blocks (slab system).

  • Two-Part Tariff: Fixed Charge (based on MD/capacity) + Variable Charge (based on kWh). Most common for industries.

  • Power Factor Tariff: Incentive/penalty based on PF (common for industries).

• Cost Analysis Methods:

  • Sinking Fund Method: Annual payment $A$ such that future salvage value (S) is accumulated. $$\displaystyle A = S \cdot \frac{i}{(1+i)^n - 1} $$.

  • Straight Line Method: Equal annual depreciation. $$\displaystyle A = \frac{\text{Initial Cost} - \text{Salvage Value}}{n} $$.

  Example: Cost=90,000 Rs, Salvage=5,000 Rs, n=15 yrs, i=6%.

  • Sinking Fund: $$\displaystyle A = (90000-5000) \times \frac{0.06}{(1.06)^{15}-1} \approx 85000 \times 0.043 = 3655 $$ Rs/yr.

  • Straight Line: $$\displaystyle A = (90000-5000)/15 = 85000/15 \approx 5666.67 $$ Rs/yr.

G. Comparative Analysis of Power Plants

II. ADVANCED MANUFACTURING PROCESSES

A. Non-Traditional Machining Processes

• Ultrasonic Machining (USM):

  • Mechanism: High-frequency (15-25 kHz) low-amplitude vibrations of tool (sonotrode) in an abrasive slurry (water + SiC/Al₂O₃). Micro-chipping due to impact of abrasive grains. Brittle fracture dominant.

  • Applications: Machining brittle, hard, non-conductive materials (ceramics, glass, carbides, quartz). Drilling, slotting, profiling.

• Electrical Discharge Machining (EDM):

  • Working Principle: Thermal erosion. Spark discharge between tool electrode (cathode) and workpiece (anode) submerged in dielectric fluid (kerosene, deionized water). Each spark vaporizes/ melts tiny material. Dielectric flushes debris.

  • Wire EDM (WEDM): Uses continuously fed thin brass/copper wire as electrode. Stratified Wire: Wire with coated layer (e.g., brass over copper core). Core provides strength, coating provides better discharge characteristics and wear resistance.

  • Errors in EDM:

    1. Tool Wear: Electrode erodes (especially in poor flushing).

    2. Overcut / Corner Wear: Gap between tool and workpiece; tool deflection at corners.

    3. Recast Layer: Resolidified molten metal on workpiece surface (hard, brittle, tensile stress).

    4. Thermal Cracks: Due to rapid heating/cooling, especially in hard materials.

    5. Spark Gap Variation: Affects dimensional accuracy.

• Laser Beam Machining (LBM):

  • Effect of Focusing: Focused beam (using lens) achieves high power density (W/cm²). Spot size ↓ → intensity ↑ → material removal rate (MRR) ↑, kerf width ↓, HAZ ↓. Defocused beam causes burning, poor finish, lower MRR.

• Electrochemical Machining (ECM):

  • Machining Rate Mechanism: Anodic Dissolution. Controlled by Faraday's Law:

$$W = \frac{I \cdot t \cdot M}{n \cdot F}$$

where W=mass removed, I=current, t=time, M=atomic mass, n=valency, F=96500 C/mol.

*   **Electrochemical Honing (ECM-Honing):** Combines ECM with abrasive honing. Removes recast layer, improves surface finish (Ra ~0.2-0.4 µm).

*   **Etch Factor (K):** Ratio of **actual depth of cut (h)** to **undercut (u)**. 

$$K = \frac{h}{u}$$

. Higher K (>1) indicates better shape control (less undercut). Depends on current density, electrolyte flow, electrode design.

B. Rapid Prototyping (RP)

• Definition & Significance: Additive manufacturing process to fabricate physical models directly from CAD data, layer-by-layer. Significance: Reduces product development time, enables design verification, functional testing, tooling (molds), complex geometries impossible with subtractive methods.

• Starting Materials (Three Types):

  1. Photopolymers: UV-curable resins (SLA, DLP).

  2. Thermoplastics: Filaments (FDM), powders (SLS).

  3. Composites/Metals: Powders (Metal SLS, DMLS), sheets (LOM), inks (PolyJet).

• Principles & Working of 3D Printing (FDM - most common):

  1. CAD model sliced into thin layers.

  2. Thermoplastic filament fed into heated nozzle.

  3. Nozzle moves in X-Y, extrudes molten material to form layer.

  4. Platform lowers (or nozzle raises), process repeats.

  5. Support material printed where needed, later removed.

• Application Issues in RP:

  • Material Limitations: Limited mechanical properties, anisotropy, UV degradation (photopolymers).

  • Accuracy & Surface Finish: Stair-stepping effect, layer lines. Post-processing often needed.

  • Build Size & Orientation: Part size constrained by machine volume. Orientation affects strength, surface, support requirement, build time.

  • Support Structures: Necessary for overhangs, difficult to remove, leaves marks.

  • Cost: High for machines/materials, economical only for complex/low-volume parts.

  • Intellectual Property & Standards: Lack of standards, file format issues (STL), IP concerns with digital files.

C. Micro-Fabrication

• Basic Types of Microsystem Devices (MEMS):

  • Sensors: Accelerometers, pressure sensors, gyroscopes.

  • Actuators: Micromotors, microvalves, micropumps.

  • Optical Devices: Micromirrors, optical switches.

  • Fluidic Devices: Microchannels, mixers, reactors (Lab-on-a-chip).

  • Biomedical: Implants, drug delivery systems, DNA chips.

• LIGA Process: German acronym (Lithographie, Galvanoformung, Abformung).

  1. Lithography: X-ray lithography using synchrotron radiation to create high-aspect-ratio resist structures.

  2. Electroforming (Galvanoformung): Electroplating (Ni) to create metal mold insert.

  3. Injection Molding (Abformung): Use metal insert to mass-produce plastic/metal microstructures.

  • Key Feature: Produces very high aspect ratio (height:width >100:1) microstructures with vertical sidewalls.

• Industrial Applications of Micro-Fabrication:

  • Automotive: Pressure sensors (tire, engine), accelerometers (airbag), inkjet printheads.

  • Consumer Electronics: MEMS microphones, projectors (DLP), hard disk drive heads.

  • Medical: Implantable sensors, drug delivery pumps, surgical tools, diagnostic chips.

  • Industrial: Inkjet printing, micro-optics, fluidics for chemical analysis, RF switches.

  • Aerospace: Inertial navigation systems (IMU), flow sensors.

UNIT 1 EXAM FOCUS: Past papers heavily test definitions & principles (MHD, Fuel Cells, USM/EDM/LBM/ECM mechanisms, RP significance), comparisons (Solar vs Wind, Micro vs Pico, Thermal vs FBR, Power Plants), sketch-based explanations (Coal handling, FBC, PHWR, Spillways), economic calculations (Load/Diversity factor, Sinking fund), and specific process details (Stratified wire, Etch factor, LIGA steps). Always include key formulas (Nernst, Faraday, Load Factor) and be ready for short notes on overfeed/underfeed, cooling towers, reactor control, flow duration curves, application issues in RP.