Industrial Facility Design and Maintenance

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This section covers plant design, location factors, location theory and models, industrial building design and construction, the types of plant layout, the classes of layout problems, evaluation of layouts and systematic layout planning.

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Plant Design and Plant Location • Plant design covers everything needed to set up a production facility: product and process selection, capacity planning, location, layout, material-handling system, building and services (power, water, compressed air, drainage), storage, safety and environmental provisions, and the organisation to run it. • Location factors: nearness to raw material (for weight-losing processes) or to the market (for weight-gaining, perishable or bulky products); availability and cost of labour and skills; transport facilities (road, rail, port, airport); power, water and fuel; land cost and availability of room for expansion; climate and topography; waste disposal and environmental regulations; government policy — taxes, incentives, industrial estates, special economic zones; banking, communication and supporting industries; community attitude, housing, schools and health facilities; safety and security; and political stability.

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In Nepal, location is also shaped by the highway corridors and border points, the industrial districts/estates, electricity availability and land-use rules.

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Location theory / model Idea Weber's least-cost theory The plant locates where the total of transport cost (raw material + finished goods), labour cost and agglomeration effects is minimum; the material index (weight of localised material ÷ weight of product) decides whether it is material- or market-oriented Von Thünen / Hoover Location rent and transport-cost structures (terminal + line-haul costs, stepped freight rates) Factor rating (weighted score) method List factors, assign weights, score each site, choose the highest weighted score — handles qualitative factors Centre-of-gravity / load-distance model x* = Σdixi/Σdi, y* = Σdiyi/Σdi — minimises weighted transport distance for a single facility Break-even (cost-volume) analysis Plot total cost (fixed + variable × volume) for each site and pick the cheapest for the expected volume Transportation / LP models Allocate demand to plants and warehouses at least cost; capacitated plant location models add fixed costs Brown-Gibson model Combines critical (must-have), objective (cost) and subjective (judgement) factors into one location measure Others p-median and set-covering models, simulation, AHP for multi-criteria decisions Industrial Building Design and Construction • Single-storey buildings (the usual choice for manufacturing): easy material flow on one level, heavy machines on a strong floor, natural lighting and ventilation through roof monitors, easy expansion, cheaper per m² — but need more land.

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Multi-storey buildings suit light industries on costly land (garments, electronics), allow gravity flow downwards, but need lifts and have floor-load limits. • Design features: bay and span dimensions with column spacing to suit the layout; floor loading capacity and surface (oil- and abrasion-resistant); headroom for cranes; roof types — north-light (saw-tooth) roof for uniform daylight without glare, trusses, portal frames, shell roofs; pre-engineered steel buildings for speed; doors, aisles and gangways for handling equipment; natural and artificial lighting and ventilation; noise, dust and fume control; utilities distributed by overhead or trench routes; fire safety (exits, hydrants, compartments) and earthquake-resistant design under the Nepal National Building Code; amenities (canteen, toilets, first aid, parking) and room for future expansion.

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Types of Plant Layout Layout Features, merits and limitations Process (functional) layout Similar machines grouped (lathe section, milling section); suits job shops and low-volume, high-variety work; flexible, high machine utilisation, fewer machines needed, specialised supervision — but long travel, high WIP and throughput time, complex scheduling and more material handling Product (line) layout Machines arranged in the sequence of operations; suits mass production; short travel, low WIP, simple control, less skilled labour — but inflexible, breakdown of one machine stops the line, duplicate machines and high investment; needs line balancing Fixed-position layout The product stays put and men, machines and material come to it — ships, aircraft, buildings, large turbines; very flexible, no product movement — but low equipment utilisation and complex coordination Cellular (group technology) layout Machines grouped into cells that make a family of similar parts; combines the flexibility of the process layout with the flow of the product layout — shorter throughput time, less WIP, teamwork and ownership; needs part-family analysis and multi-skilled operators Combination (hybrid) layout Mix of the above — common in real factories (e.g., process layout for machining plus a product line for assembly) • Objectives of a good layout: minimum material handling and travel, smooth flow without back-tracking or congestion, effective use of space (including cubic space), flexibility for change, worker safety, comfort and convenience, minimum work in process, easy supervision and maintenance, and provision for expansion. • Classes of layout problems:

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(1) designing an entirely new plant;

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(2) expansion of an existing facility;

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(3) rearrangement of an existing layout (new products, new equipment, congestion or safety problems); and (4) minor changes/relocation of individual machines or work centres.

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Systematic Layout Planning (SLP) • Muther's SLP is the standard structured procedure.

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Inputs are Product, Quantity, Routing, Supporting services and Time (P-Q-R-S-T). • Steps:

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(1) flow of materials analysis — from-to chart, process charts, P-Q analysis;

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(2) activity relationship chart — closeness ratings A (absolutely necessary), E (especially important), I (important), O (ordinary), U (unimportant) and X (undesirable) with coded reasons;

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(3) relationship diagram combining flow and relationships;

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(4) determine space requirements and compare with space available;

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(5) draw the space relationship diagram;

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(6) apply modifying considerations (handling methods, services, building, safety) and practical limitations (cost, regulations, site shape);

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(7) develop layout alternatives; and (8) evaluate and select the best.

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Evaluation of Plant Layouts • Quantitative measures: total material-handling cost = Σ (flowij × distanceij × cost per unit distance), total travel distance or load-distance score, throughput/manufacturing lead time, work-in-process, space utilisation (m² per unit output, cubic utilisation), investment and operating cost, machine and labour utilisation, and simulation results for congestion and queues. • Qualitative measures: checklists, weighted factor comparison and ranking of alternatives on flexibility, safety, working conditions, supervision, expansion capability and appearance. • Computerised layout tools:

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CRAFT (improvement — exchanges departments to reduce handling cost), ALDEP and CORELAP (construction — build a layout from closeness ratings), PLANET, BLOCPLAN and modern simulation/3-D layout software.

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This section covers the fundamentals of safety, accidents, their causes and prevention, engineering methods of hazard identification and reduction, the physical environment, codes and regulations for workers' safety and health, and the rules of national and international organisations on hygiene and safety.

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Fundamentals and Accident Causation • Hazard = a source with the potential to cause harm; risk = likelihood × severity of that harm; incident/near miss = an unplanned event that could have caused harm; accident = an unplanned event that causes injury, illness or damage.

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Injuries are classified as fatal, permanent total or partial disability, and temporary disability (lost-time injury) or first-aid cases. • Heinrich's domino theory: injury is the last of five falling dominoes — social environment/ancestry → fault of the person → unsafe act or unsafe condition → accident → injury; removing the middle domino (the unsafe act/condition) prevents the injury.

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Heinrich's ratio: for every 1 major injury there are 29 minor injuries and 300 no-injury incidents — so near misses must be reported and acted on.

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Later models: multiple causation, energy-release theory, and Reason's Swiss cheese model of latent organisational failures. • Causes: unsafe acts (≈ 88%) — operating without authority, removing guards, wrong speed, unsafe loading/placing, horseplay, not using PPE, working on live or moving equipment; unsafe conditions (≈ 10%) — unguarded machines, defective tools, poor housekeeping, inadequate lighting or ventilation, noise, congested layout; and unavoidable/'acts of God' (≈ 2%).

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Underlying causes are usually management failures: poor training, supervision, maintenance, design or procedures. • Costs of accidents: direct (medical treatment, compensation, insurance) and indirect/hidden (lost production and time, damage to plant and material, replacement and retraining, investigation, legal penalties, low morale, loss of reputation) — the indirect costs are usually several times the direct ones (the 'iceberg').

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Hazard Identification and Risk Reduction • Identification techniques: safety inspections and audits, job safety analysis (JSA), checklists, what-if analysis, HAZOP (hazard and operability study using guide words on process deviations), HAZID, FMEA/FMECA, fault tree analysis (top-down with AND/OR gates) and event tree analysis, incident and near-miss reporting, and workplace monitoring of noise, dust and chemicals. • Risk assessment = identify hazards → estimate likelihood and severity → rank on a risk matrix → apply controls → re-assess residual risk → record and review. • Hierarchy of controls (most to least effective): elimination of the hazard → substitution with something less hazardous → engineering controls (machine guarding, interlocks, enclosure, local exhaust ventilation, isolation, ergonomic redesign, fail-safe devices) → administrative controls (safe procedures, permits to work, training, signage, job rotation, maintenance schedules) → personal protective equipment (PPE) — the last line of defence. • Machine safety: fixed, interlocked, adjustable and self-adjusting guards, two-hand controls, light curtains and safety mats, emergency stops, lock-out/tag-out before maintenance, guarding of transmission parts, safe access and platforms. • Permit-to-work systems for hot work, confined-space entry, work at height, excavation and electrical work; emergency preparedness — fire fighting (classes A, B, C, D and F fires with matching extinguishers), alarms, escape routes and drills, first aid, spill control and MSDS/GHS labelling of chemicals.

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Physical Environment and Occupational Health • Noise: limit commonly taken as 85 dB(A) for 8 hours (3 dB exchange rate — halve the time for each 3 dB rise); control at source, by enclosure, absorption, distance and finally hearing protection; effects — noise-induced hearing loss, stress, poor communication. • Illumination: sufficient and glare-free light for the task (rough work ≈ 100–200 lux, bench and office work ≈ 300–750 lux, fine inspection ≈ 1 000–2 000 lux). • Thermal environment: heat stress in furnaces, boiler houses and foundries — controlled by shielding, ventilation, cool rest areas, water and work-rest cycles; cold stress in cold stores. • Airborne contaminants: dust (silica, cement, wood), fumes (welding, lead), mists, gases and vapours (solvents, ammonia, CO) — controlled by substitution, enclosure, local exhaust ventilation, wet methods and respirators; exposure judged against threshold limit values (TLV/PEL). • Vibration (hand-arm and whole-body), radiation (welding arc, X-ray/NDT, UV), biological hazards and ergonomic hazards (Chapter 7.6). • Occupational diseases: silicosis and asbestosis, occupational asthma, noise-induced hearing loss, dermatitis, lead and solvent poisoning, carpal tunnel syndrome and back injuries; controlled by pre-employment and periodic medical examination, exposure monitoring, hygiene facilities and health education. • Safety performance measures: frequency rate = (number of lost-time injuries × 1 000 000) ÷ man-hours worked; severity rate = (days lost × 1 000 000) ÷ man-hours worked; incidence rate per 1 000 workers; accident cost; and leading indicators (near misses reported, training, audits, PM compliance) which are better predictors than the lagging injury rates.

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Codes, Regulations and Organisations • Nepal: the Labour Act, 2074 (2017) and Labour Rules, 2075 place the primary duty of a safe workplace on the employer — safety and health policy, safety committee in larger enterprises, PPE free of cost, machine guarding, hazard information and training, accident reporting to the Labour Office, medical examination and welfare facilities; workers must follow procedures and use PPE.

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Contribution Based Social Security Act 2074 (accident, medical and disability benefits through the SSF), Environment Protection Act 2076, Nepal National Building Code, electricity and explosives rules, Public Health Service Act and the Occupational Safety and Health directives issued under the Labour Act.

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(Verify the current provisions — thresholds such as committee size and reporting timelines are amended.) • International: the ILO — convention C155 (Occupational Safety and Health, 1981), C187 (Promotional Framework, 2006), C161 (occupational health services) and the ILO-OSH 2001 guidelines; a safe and healthy working environment is now an ILO fundamental principle.

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ISO 45001:2018 is the international management-system standard for OSH (replacing OHSAS 18001), built on the same high-level structure and PDCA cycle as ISO 9001/14001.

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OSHA (USA) standards, NFPA fire codes, NEBOSH/IOSH training qualifications, WHO guidance on occupational health, and the UN GHS for chemical classification and labelling. • Safety management system elements: policy and leadership commitment, organisation and responsibilities, hazard identification and risk assessment, operational control, training and communication, emergency preparedness, measurement and audit, incident investigation, management review and continual improvement — with worker participation throughout.

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This section covers the concepts of maintenance management, types of maintenance, maintenance practices in Nepal, maintenance of mechanical and electrical systems, lubricants and lubrication systems, condition monitoring techniques, maintenance facilities, and maintenance organisation and quality circles.

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Maintenance Management • Maintenance keeps equipment in, or restores it to, a condition in which it can perform its required function safely and economically.

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Objectives: maximise availability and reliability of plant, extend equipment life, ensure safety and environmental compliance, maintain product quality and minimise the total cost of maintenance plus downtime. • Functions of maintenance management: equipment register and criticality ranking (ABC/VED), planning (job methods, spares, tools, manpower), scheduling (with production), execution through work orders/job cards, recording history, spares and inventory control, budgeting and cost control, performance measurement and improvement; usually supported by a CMMS (computerised maintenance management system). • Economics: as preventive maintenance increases, PM cost rises but breakdown cost falls — the optimum is the minimum of the total-cost curve; equipment is replaced when the sum of operating, maintenance and downtime costs exceeds the cost of a new machine (replacement analysis, Chapter 10.2). • Performance measures: availability = MTBF/(MTBF + MTTR), MTBF, MTTR, OEE = availability × performance × quality, breakdown frequency and downtime hours, PM schedule compliance, maintenance cost as a percentage of replacement asset value (typically ≈ 2–5%), backlog and spare-parts service level.

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Type of maintenance Description Breakdown (corrective/reactive) Repair after failure — acceptable only for non-critical, cheap, easily replaced items; otherwise causes downtime, secondary damage and safety risk Preventive (planned/scheduled) Inspection, cleaning, lubrication, adjustment and replacement at fixed time or usage intervals to prevent failure; includes running and shutdown maintenance Predictive (condition-based) Monitoring the actual condition (vibration, temperature, oil, current) and acting before failure — maintenance only when needed Proactive / design-out Eliminating the root cause by redesign, better material, improved installation or operating practice Opportunistic and shutdown maintenance Work done while the machine is stopped for another reason, or during a planned plant shutdown/annual overhaul TPM Total productive maintenance — operator-led autonomous maintenance, planned maintenance, focused improvement, quality and safety pillars, measured by OEE, aiming at zero breakdowns and defects RCM Reliability-centred maintenance — choose the maintenance strategy for each item from its failure modes and their consequences (safety, environment, operations, cost) • Maintenance practices in Nepal: much of industry still relies on breakdown maintenance; common constraints are shortage of trained technicians, dependence on imported spares with long lead times, limited use of CMMS and condition monitoring, weak documentation of equipment history, and a tendency to defer preventive work under production pressure.

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Larger plants (cement, breweries, hydropower, aviation, hospitals) and public utilities practise planned maintenance and outsource specialised work through annual maintenance contracts (lifts, HVAC, DG sets, transformers, instruments).

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Hydropower plants schedule overhauls in the dry season when generation is low.

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Improvement usually comes from criticality-based PM schedules, spare-parts planning, operator training and TPM/5S programmes.

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Maintenance of Mechanical and Electrical Systems • Mechanical: shaft alignment (laser or dial gauge) and balancing of rotors; bearing lubrication, installation and replacement; belt and chain tensioning and alignment; gearbox oil level and change; mechanical seals and gland packing; fasteners and foundation bolts; pumps and compressors (clearances, valves, filters); hydraulic and pneumatic systems (fluid condition, filters, leaks); boilers and pressure vessels (statutory inspection, safety-valve testing, water treatment); structural inspection, corrosion protection and painting; and repair by welding, machining or replacement. • Electrical: insulation resistance (megger) testing and polarisation index for motors and cables; earth resistance measurement; motor checks — current balance, winding temperature, vibration, bearing greasing, terminal tightness; transformer maintenance — oil breakdown voltage and dissolved gas analysis, silica gel breather, Buchholz and temperature alarms, bushing cleaning; switchgear — contact resistance, breaker timing, relay testing and coordination; capacitor banks; battery systems; thermography of joints and panels to find hot spots; instrument calibration; and strict isolation and lock-out/tag-out with earthing before work.

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Lubricants and Lubrication Systems • Functions of a lubricant: reduce friction and wear, carry away heat, remove wear debris and contaminants, protect against corrosion, seal (e.g., piston rings) and damp shock and noise. • Types: liquid — mineral oils and synthetics (PAO, ester, PAG — better at extreme temperatures and longer life), vegetable/biodegradable oils; semi-solid greases — oil + thickener (lithium, calcium, complex soaps) graded by NLGI consistency numbers 000 to 6 (No.

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2 is the common general-purpose grade); solid lubricants — graphite, molybdenum disulphide, PTFE for high temperature or vacuum. • Properties: viscosity (the most important — graded by ISO VG for industrial oils and SAE for engine and gear oils) and viscosity index (resistance of viscosity to temperature change), flash and fire point, pour point, oxidation stability, demulsibility, total acid number, and additives — anti-wear (ZDDP), extreme-pressure (EP), detergent-dispersant, anti-oxidant, anti-foam, VI improver and rust inhibitors. • Lubrication regimes (Stribeck curve): boundary (surfaces touch — low speed, start/stop; additives protect), mixed, hydrodynamic (a full film generated by relative motion — the ideal for journal bearings) and elastohydrodynamic (rolling contacts, gears); hydrostatic lubrication uses external pressure. • Lubrication systems: manual (oil can, grease gun), drip feed, wick and ring/splash systems, forced (pressure) circulating systems with pump, filter and cooler, oil mist systems, and centralised automatic single-line or dual-line grease systems with metering valves and single-point lubricators. • Good practice: the right lubricant, in the right quantity, at the right place, at the right time, with the right method; avoid over-greasing (it overheats bearings) and cross-contamination; keep drums sealed and clean; filter new oil; monitor through oil analysis (viscosity, TAN, water, particle count, spectrometric wear metals, ferrography) and change on condition rather than by the calendar.

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Condition Monitoring Techniques Technique What it detects Vibration analysis The most widely used for rotating machines: overall velocity (mm/s rms, judged by ISO 10816/20816 zones) and FFT spectra — unbalance (1× rpm), misalignment (2×), looseness (harmonics), bearing defect frequencies, gear mesh frequencies, resonance Infrared thermography Hot spots in electrical joints, panels, motors, bearings, steam traps, refractory and insulation Oil/lubricant analysis Wear metals, contamination, water, oxidation, viscosity change — indicates internal wear before failure Ultrasonic detection Air/steam/vacuum leaks, electrical arcing and corona, early bearing and valve faults Motor current signature analysis Broken rotor bars, eccentricity and load problems from the motor current spectrum Performance and process monitoring Efficiency, pressure drop, temperature rise, output — a fall indicates fouling or wear NDT and others Ultrasonic thickness and crack detection, dye penetrant, magnetic particle, radiography, acoustic emission, corrosion coupons, endoscopy/borescope • The P-F curve shows the interval between the point where a potential failure becomes detectable (P) and the point of functional failure (F); the monitoring interval must be shorter than the P-F interval.

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Condition monitoring converts breakdowns into planned work and reduces both downtime and unnecessary preventive work.

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Maintenance Facilities, Organisation and Quality Circles • Facilities: a maintenance workshop (machine tools, welding, fitting), tool room and special tools, spare-parts store with codification and min-max levels, lifting equipment, calibration and instrument facilities, standby equipment and a documentation system (manuals, drawings, history cards, CMMS). • Organisation: centralised (one maintenance department serving the whole plant — better utilisation of specialists and equipment, uniform standards, but slower response in large plants), decentralised (crews attached to areas — fast response and familiarity, but duplication), or hybrid (area crews with a central workshop and planning cell).

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Typical structure: maintenance manager → planning and scheduling → trade sections (mechanical, electrical, instrumentation, civil, utilities) → stores; with clear work-order flow, priorities and a permit system.

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Staffing is calculated from the planned workload, and skills are maintained by training. • Quality circle: a small group of 6–12 volunteers from the same work area who meet regularly (often weekly, in work time) with a leader and a trained facilitator, under a steering committee, to identify, analyse and solve their own work-related problems using the seven QC tools, and to present solutions to management.

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Originating in Japan (Ishikawa), they are widely used in maintenance and shop-floor improvement — benefits include employee involvement, morale, skill development and continuous improvement; success needs management support, training, voluntary participation and recognition.

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This section covers the concepts of reliability, failure of systems and failure modes, measures of reliability, the reliability function, hazard rate and MTBF and their interrelations, reliability data analysis and performance parameters, calculation of failure rate, the Weibull distribution, system reliability modelling, simulation and reliability prediction.

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Concepts and Failure Modes • Reliability R(t) is the probability that an item performs its required function under stated conditions for a stated period of time.

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Related terms: maintainability (probability of restoring within a given time), availability (fraction of time the item is fit for use) and dependability (the combination). • Failure = termination of the ability to perform the required function.

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Classification: by degree — partial or complete; by rate — sudden (catastrophic) or gradual (degradation, drift, wear); by cause — design, manufacturing, material, assembly, misuse/overload, maintenance error or wear-out; intermittent failures.

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Failure modes (the way it fails — fracture, wear, leak, short circuit, open circuit, seizure, drift) are studied with FMEA/FMECA and fault trees. • Bathtub curve of hazard rate vs time:

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(1) infant mortality — decreasing failure rate from manufacturing and assembly defects, reduced by burn-in and better quality control;

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(2) useful life — approximately constant failure rate, failures are random (exponential law applies);

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(3) wear-out — increasing failure rate, controlled by preventive replacement and overhaul.

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Measures and Their Interrelations • If f(t) is the failure density and F(t) the cumulative failure (unreliability) function:

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R(t) = 1 − F(t); f(t) = dF/dt; and the hazard (instantaneous failure) rate λ(t) = f(t)/R(t) — the conditional rate of failure of items that have survived to t. • General relation:

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R(t) = exp[−∫λ(t)dt].

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For a constant hazard rate λ (useful life):

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R(t) = e−λt, F(t) = 1 − e−λt, and MTBF = 1/λ — so at t = MTBF, R = e−1 = 0.368 (36.8%). • MTTF (mean time to failure) is used for non-repairable items and MTBF (mean time between failures) for repairable ones;

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MTTR is the mean time to repair, and availability = MTBF/(MTBF + MTTR). • Calculation of failure rate from field or test data: λ = (number of failures) ÷ (total operating hours of all items); failure rates are often quoted in failures per 106 hours or FIT (failures per 109 hours).

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Data may be censored (items still running or withdrawn), which is handled by counting their running time in the denominator. • Reliability data analysis: collect time-to-failure data → plot the empirical hazard/cumulative distribution (probability plotting with median ranks) → fit a distribution (exponential, Weibull, normal, lognormal) → estimate parameters (graphically, by least squares or maximum likelihood) → compute confidence limits; accelerated life testing (higher stress, temperature or cycling) shortens test time; reliability growth during development is tracked with the Duane model.

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Weibull Distribution • The Weibull distribution is the most useful life model because its shape parameter adapts to all three regions of the bathtub curve:

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R(t) = exp[−(t/η)β] and hazard rate λ(t) = (β/η)(t/η)β−1. • β (shape parameter): β < 1 → decreasing hazard = infant mortality; β = 1 → constant hazard = random failures (the exponential distribution, η = MTBF); β > 1 → increasing hazard = wear-out (β ≈ 3.4 approximates the normal distribution). η (scale/characteristic life) is the time by which 63.2% of the population has failed; a three-parameter version adds a location (failure-free) parameter γ. • Weibull analysis is done by plotting on Weibull probability paper (or software): the slope gives β and hence the failure mechanism, which guides the maintenance strategy — preventive replacement only helps when β > 1.

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System Reliability Modelling Configuration Reliability Series (all components must work) Rs = R1R2…Rn — always less than the weakest component; for constant rates λs = Σλi Parallel (active redundancy) Rp = 1 − (1 − R1)(1 − R2)… — reliability increases with each redundant unit k-out-of-n System works if at least k of n units work (binomial summation) — e.g., 2 out of 3 voting systems Standby (passive) redundancy A spare is switched in on failure; with perfect switching and constant λ, R(t) = e−λt(1 + λt) for one standby — better than active redundancy but depends on the switch Series-parallel networks Reduce the reliability block diagram step by step; complex networks use decomposition, path/cut-set methods, fault tree analysis (AND gate = parallel, OR gate = series) or Markov models for repairable systems • Design for reliability: simplification and fewer parts, derating (operating below rated stress), redundancy for critical functions, robust/Taguchi design, proven components and standardisation, protection against environment, FMECA and fault trees during design, burn-in to remove infant mortality, and maintainability features (accessibility, modularity, diagnostics).

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Common-cause failures (same power supply, same environment, same maintenance error) can defeat redundancy and must be designed against. • Reliability prediction: parts-count and part-stress methods using standard failure-rate handbooks (e.g., MIL-HDBK-217), similarity analysis with existing equipment, test data, and Monte Carlo simulation for complex systems and maintenance/spares policies (Chapter 7.4).

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Predictions guide spares provisioning (often with the Poisson distribution), warranty costing and maintenance intervals. • Performance parameters summarised: reliability R(t), failure rate λ, MTBF/MTTF, MTTR, availability (inherent, achieved and operational), maintainability, failure-free operating period and confidence levels.

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This section covers the concepts of air refrigeration, the vapour compression system and cryogenics, refrigerants and their types, methods of food preservation, and the control components used in refrigeration systems — the automatic and thermostatic expansion valves and superheat setting.

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Basic Concepts • Refrigeration removes heat from a space or substance and rejects it to the surroundings, maintaining a temperature below ambient.

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Refrigerating effect (RE) is the heat removed in the evaporator per unit mass or per unit time;

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1 tonne of refrigeration (TR) = 3.5 kW = 3 024 kcal/h. • COP = refrigerating effect ÷ work input; for a heat pump COPHP = COPref + 1.

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The maximum (Carnot) value is COP = T2/(T1 − T2) with absolute temperatures; relative COP = actual ÷ theoretical.

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Practical vapour-compression plants achieve COP ≈ 2–5. • COP improves with a higher evaporator temperature and a lower condenser temperature — hence clean condensers, adequate air/water flow, correct charge, and not setting the evaporator colder than the process needs.

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Air Refrigeration (Bell-Coleman / Reverse Brayton Cycle) • Air is compressed, cooled in a heat exchanger, expanded in an expander (producing work and a large temperature drop) and then passed through the refrigerated space.

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Open and closed (dense-air) systems exist. • Merits: air is free, non-toxic, non-flammable and leak-tolerant, and the equipment is light — hence its use in aircraft air conditioning, where compressed bleed air is already available.

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Demerits: very low COP, large air volumes, bulky equipment and moisture/frost problems; it is not used for ordinary industrial refrigeration.

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Vapour Compression Refrigeration • Four processes round the cycle: compression (low-pressure vapour to high pressure and temperature), condensation (heat rejected, vapour → liquid), expansion (throttling to low pressure — an irreversible, constant-enthalpy process) and evaporation (liquid absorbs the refrigerating load and becomes vapour).

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The cycle is analysed on the pressure-enthalpy (p-h) chart:

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RE = h1 − h4, work = h2 − h1, COP = (h1 − h4)/(h2 − h1). • Superheating at the evaporator exit protects the compressor from liquid slugging (and may slightly improve COP); sub-cooling of the liquid before the expansion device increases the refrigerating effect. • Components: compressors — reciprocating, rotary, scroll, screw and centrifugal (large chillers); condensers — air-cooled, water-cooled (shell-and-tube with a cooling tower) and evaporative; evaporators — direct expansion (DX) coils, flooded and plate types; expansion devices — capillary tube (small sealed systems), automatic, thermostatic or electronic expansion valves; plus receiver, drier-filter, sight glass, accumulator, oil separator and service valves. • Multi-stage and cascade systems are used for very low temperatures; the vapour absorption system (Chapter 2.4) replaces the compressor with a generator-absorber circuit driven by heat.

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Cryogenics • Cryogenics deals with temperatures below about −150 °C (123 K).

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Liquefaction is achieved by the Linde-Hampson (Joule-Thomson throttling with regenerative heat exchange) and Claude (throttling plus an expansion engine) processes and by multi-stage cascade cycles. • Typical boiling points: liquid nitrogen −196 °C, liquid oxygen −183 °C, liquid natural gas ≈ −162 °C, liquid hydrogen −253 °C, liquid helium −269 °C.

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Storage in vacuum-insulated (Dewar) vessels. • Applications: medical (MRI superconducting magnets, cryosurgery, preservation of blood, tissue and semen), food (cryogenic freezing and cryo-grinding of spices), industry (shrink fitting, cryogenic treatment of tools, deflashing of rubber), air separation into O2, N2 and Ar, LNG transport, rocket propellants and superconductivity research.

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Refrigerants • Desirable properties: high latent heat, suitable evaporating and condensing pressures (above atmospheric in the evaporator, moderate in the condenser), non-toxic, non-flammable, non-corrosive, chemically stable, good oil miscibility, easy leak detection, cheap and available, and environmentally acceptable — zero ozone-depletion potential (ODP) and low global-warming potential (GWP). • Types:

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CFCs (R-11, R-12 — banned under the Montreal Protocol), HCFCs (R-22 — being phased out), HFCs (R-134a, R-404A, R-410A — no ODP but high GWP, being phased down under the Kigali Amendment), HFOs (R-1234yf/ze — very low GWP) and natural refrigerants — ammonia R-717 (excellent thermodynamically and cheap, used in large cold stores and ice plants, but toxic and irritant), carbon dioxide R-744, hydrocarbons R-290 propane and R-600a isobutane (very low GWP but flammable).

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Secondary refrigerants — chilled water, brines (NaCl, CaCl2) and glycols — carry cooling to the load. • Designation:

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R-number from the chemical formula; ammonia is R-717, water R-718, CO2 R-744; safety groups A1 (non-toxic, non-flammable) to B3.

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Methods of Food Preservation • Spoilage is caused by micro-organisms, enzymes, oxidation and pests; preservation slows or stops these actions. • Low-temperature methods: chilling/cold storage (≈ 0–4 °C — milk, vegetables, fruits, meat for short periods); freezing (storage at −18 °C or below; quick freezing gives small ice crystals and better texture than slow freezing; blast, plate, fluidised-bed, immersion and cryogenic freezers); controlled and modified atmosphere storage (lower O2, higher CO2 — apples, potatoes). • Other methods: drying and dehydration (sun drying, spray, drum, tray and freeze drying/lyophilisation), thermal processing (pasteurisation, sterilisation, canning, retorting), chemical preservatives and antioxidants, salting, sugaring, smoking, pickling and fermentation, irradiation, vacuum and modified-atmosphere packaging, and hurdle technology combining several methods. • In Nepal, cold stores for potato, apple and vegetables, dairy chilling centres and cold chains for vaccines and meat are important applications — the main constraints are power reliability, capital cost and transport.

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Control Components and Superheat Setting Control Function Capillary tube Fixed restriction used in small sealed systems (domestic refrigerators, window AC); cheap, no moving parts, but cannot adjust to load changes and needs an exact charge Automatic expansion valve (AEV, constant-pressure valve) Maintains a constant evaporator pressure (hence constant temperature) by balancing evaporator pressure against a spring and diaphragm; flow decreases as the load increases — suitable only for nearly constant loads; risk of starving or flooding under varying load Thermostatic expansion valve (TEV/TXV) Maintains a constant degree of superheat at the evaporator outlet: a sensing bulb on the suction line applies its pressure to the diaphragm (opening force), opposed by evaporator pressure (through the internal or external equaliser) and the superheat spring (closing forces).

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It matches refrigerant flow to the load, keeps the evaporator fully used and protects the compressor from liquid — the standard valve for commercial and industrial DX systems Electronic expansion valve (EEV) Stepper-motor valve controlled from temperature and pressure sensors — precise superheat control over a wide range, used with inverter systems Float valves High- and low-side float valves for flooded evaporators and large ammonia plants Other controls Thermostat, high- and low-pressure cut-outs, oil-pressure switch, solenoid valve with pump-down, defrost timer/thermostat, crankcase heater, water-regulating valve • Superheat setting: superheat = temperature of the suction gas at the bulb − saturation temperature corresponding to the evaporator pressure.

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Typical setting ≈ 4–8 K (higher for some systems).

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Turning the TEV adjusting screw clockwise increases spring force → more superheat, less flow; anticlockwise reduces superheat and increases flow.

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Too low a superheat risks liquid floodback and compressor damage; too high a superheat starves the evaporator, lowers capacity and overheats the compressor.

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Adjustments are made in small steps, allowing the system to stabilise, with the bulb properly clamped and insulated on a horizontal suction line.

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This section covers the concepts of the psychrometric chart, the factors influencing thermal comfort, air quality and contaminants, indoor air quality and its effects on health, controlling indoor air quality, and the determination of heating and cooling loads.

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Psychrometry and the Psychrometric Chart Property Meaning Dry-bulb temperature (DBT) Temperature measured by an ordinary thermometer Wet-bulb temperature (WBT) Temperature of a thermometer with a wetted wick in moving air — indicates the evaporative cooling potential Dew-point temperature (DPT) Temperature at which the air becomes saturated on cooling at constant pressure; condensation starts Specific/absolute humidity (humidity ratio ω) kg of water vapour per kg of dry air Relative humidity (RH) Ratio of the actual vapour pressure to the saturation vapour pressure at the same DBT (%) Enthalpy h Total heat of the air-vapour mixture per kg of dry air (kJ/kg) Specific volume, degree of saturation m³ per kg of dry air; ratio of actual to saturated humidity ratio • The psychrometric chart plots these properties (DBT on the x-axis, humidity ratio on the y-axis, with WBT, RH, enthalpy and specific-volume lines) so that any air-conditioning process can be drawn as a line on the chart. • Processes: sensible heating and cooling (horizontal lines — ω constant), humidification and dehumidification (vertical), cooling with dehumidification (the usual summer process — air is cooled below its dew point on the coil, water condenses; the line runs towards the apparatus dew point (ADP)), evaporative cooling (along a constant WBT line), heating with humidification (winter), and adiabatic mixing of two air streams (the state lies on the straight line joining them, divided in proportion to the masses). • Bypass factor (BPF) of a coil = fraction of air passing through unaffected = (tout − tADP)/(tin − tADP); sensible heat factor SHF = sensible heat ÷ total (sensible + latent) heat — it sets the slope of the room condition line.

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Total load = sensible + latent; sensible heat = 1.2 × V̇(m³/s) × Δt kW (approximately), latent heat = 3 000 × V̇ × Δω kW.

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Thermal Comfort • Comfort depends on six factors — four environmental and two personal: air temperature, mean radiant temperature, relative humidity and air velocity, plus clothing insulation (clo) and metabolic activity (met). • Typical comfort conditions:

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DBT ≈ 23–26 °C in summer and 20–24 °C in winter, RH ≈ 40–60%, air velocity ≈ 0.15–0.25 m/s (draughts above this are uncomfortable), vertical temperature difference and radiant asymmetry kept small. • Comfort indices: effective temperature, PMV (predicted mean vote) and PPD (predicted percentage dissatisfied) of Fanger, used in ISO 7730;

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ASHRAE Standard 55 defines the comfort zone (and an adaptive model for naturally ventilated buildings, relevant to Nepal's climate zones).

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Even in ideal conditions about 5% of occupants remain dissatisfied. • The body loses heat by convection, radiation, conduction and evaporation; high humidity hinders evaporative loss, making warm conditions uncomfortable, while very low humidity causes dryness and static electricity.

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Air Quality, Contaminants and Health • Indoor contaminants: particulate matter (PM10, PM2.5) and fibres; combustion products — CO, NO2, SO2 and smoke (biomass cooking, generators, vehicles);

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CO2 from occupants (not toxic at normal levels but the standard indicator of ventilation adequacy);

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VOCs and formaldehyde from paints, adhesives, furniture and cleaning agents; ozone from printers/copiers; radon from soil; bioaerosols — mould, bacteria, dust mites, pollen; tobacco smoke; and process fumes and dusts in industrial buildings. • Health effects: eye, nose and throat irritation, headache and fatigue, allergies and asthma, respiratory infections, sick building syndrome (symptoms that disappear on leaving the building) and building-related illness (e.g., legionellosis from cooling towers); long-term exposure to PM2.5 and combustion products causes chronic respiratory and cardiovascular disease — a serious issue with indoor biomass cooking in rural Nepal and with outdoor PM in the Kathmandu valley. • Controlling indoor air quality:

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(1) source control — low-emission materials, clean cooking fuels and improved stoves, banning smoking, enclosing and locally exhausting processes, maintaining combustion equipment;

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(2) ventilation — adequate outdoor air per person and per unit floor area (ASHRAE 62.1), natural or mechanical, with demand-controlled ventilation using CO2 sensors (target below ≈ 1 000 ppm) and correct location of intakes away from exhausts;

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(3) filtration and air cleaning — filters rated by MERV/HEPA classes, electrostatic precipitators, activated carbon for gases and UV for microbes;

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(4) humidity control (30–60% RH) to limit mould and dust mites;

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(5) maintenance — regular cleaning of coils, drain pans, ducts and cooling towers, filter replacement, and commissioning and monitoring of the system.

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Heating and Cooling Load Determination • Design conditions are first fixed: outdoor design DBT/WBT for the location (summer and winter percentiles) and the indoor condition (e.g., 24–25 °C and 50% RH for comfort; process requirements may differ). • Cooling (summer) load components: • External — conduction through walls, roof, floor and glass (Q = U·A·ΔT, using CLTD or the heat-balance/radiant-time-series method to allow for thermal storage), solar radiation through glass (Q = A × SHGF × SC × CLF — often the largest single item), and infiltration of outside air through cracks and door openings (sensible and latent). • Internal — occupants (sensible + latent heat, ≈ 75 W sensible and 55 W latent per person for office work), lighting (watts × use and ballast factors), equipment and appliances, and process heat. • Ventilation air — the outdoor air deliberately introduced (sensible and latent), which is a coil load rather than a room load. • The room sensible and latent loads give the RSHF; adding ventilation gives the grand total load and the GSHF/ESHF, from which the supply air quantity, coil ADP and equipment capacity are selected.

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Diversity and safety factors are applied, and the peak is found for the critical hour/month, not by adding all maxima. • Heating (winter) load: transmission losses through the building envelope plus infiltration and ventilation losses, usually computed at steady state for the design outdoor temperature with no credit for solar or internal gains (a conservative approach); seasonal energy is estimated by the degree-day method. • Air quantity:

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V̇ = room sensible heat ÷ (1.2 × Δt) (m³/s with Δt in K), with supply air typically 10–14 K below the room temperature; ducts, fans, diffusers and the refrigeration plant are then sized, and the equipment selected with attention to part-load performance and energy efficiency (EER/COP, ISO/ASHRAE ratings).