Dairy Science & Technology · Foundations · also Engineering Drawing, Workshop Practice, Basic Electrical Engineering, Engineering Mechanics
Drawing, workshop, mechanics, materials and electricity as used in a milk plant. Go to the agricultural engineering track (engineering-drawing-cad, workshop-technology, basic-electrical-electronics) for depth.
In the exams: ICAR AIEEA-PG group 17 Unit III (mechanics, strength of materials, Hooke's law, materials, machine tools, electrical engineering, AC fundamentals)
A milk plant is a factory of pumps, pipes, tanks, heat exchangers, separators, homogenisers, fillers and the motors, drives and electrical systems that run them. A dairy technologist need not be a mechanical or electrical engineer, but must be able to read the drawings of the plant, specify materials that will not corrode or taint the product, understand why a shaft broke or a motor tripped, talk with welders and fitters about the finish inside a pipe, and work safely around electricity in a wet environment. These are the engineering fundamentals that dairy degrees teach in the first year, before the specialised subjects on fluid flow, heat transfer, refrigeration and plant design.
This manual covers ten topics: engineering drawing and how to read plant drawings such as piping and instrumentation diagrams; workshop practice in fitting, welding and sheet-metal work; stainless steels and the other materials of dairy equipment; corrosion and its control; statics, friction and the basic strength of materials; power transmission by belts, chains, gears and couplings; direct and alternating current circuits and three-phase supply; electric motors and their starters; electrical safety and earthing; and lubrication and maintenance. Each topic is taught at the depth a dairy technologist needs, with plant examples. Readers who need more should go to the agricultural engineering track, whose subjects on engineering drawing and CAD, workshop technology and basic electrical and electronics engineering treat the general theory fully.
Postgraduate entrance examinations in dairy technology and dairy engineering include a unit on mechanics, strength of materials, Hooke's law, engineering materials, machine tools and electrical engineering, including alternating current fundamentals. Questions are typically short calculations of stress, strain, belt speed ratio, gear ratio, motor synchronous speed or three-phase power, and recall of stainless steel grades, corrosion forms, projection methods and starter types.
Engineering drawing is the formal language by which designers describe an object so that it can be made, installed and maintained. Its conventions are set by international standards, chiefly the ISO 128 series for general principles and ISO 5456 for projection methods, with national standards that follow them closely.
Orthographic projection shows an object by separate two-dimensional views, usually the front view (elevation), top view (plan) and side view, each looking straight at one face. Two systems arrange these views. In first-angle projection, used in most of Europe, Asia and Africa, the plan is drawn below the front elevation and the view from the left is placed on the right. In third-angle projection, standard in North America, the plan is drawn above the elevation and the view from the left is placed on the left. A symbol in the title block shows which is used.
Pictorial drawings show the object in three dimensions. Isometric drawing draws the three principal axes at 120° to one another, with the receding axes at 30° to the horizontal, and true lengths along the axes; it is the basis of isometric piping drawings.
Line types carry meaning: continuous thick lines for visible outlines, dashed lines for hidden edges, thin chain lines for centre lines and axes of symmetry, and thin continuous lines for dimensions and hatching. A sectional view imagines the object cut by a plane, with the cut surfaces hatched with thin lines at 45°, and is used to show the inside of valves, pumps and heat-exchanger plates.
Dimensions give sizes and positions, in millimetres on most engineering drawings worldwide, with inches still used in some industries in North America. Each feature should be dimensioned once, from a stated datum, so that the drawing is neither incomplete nor contradictory. Diameters are marked with the symbol ⌀ and radii with R.
Drawings are made to scale: full size 1:1, reductions such as 1:2, 1:5, 1:10, 1:50 or 1:100 for equipment and building plans, and enlargements such as 2:1 for small parts. A dimension written on the drawing always gives the real size, whatever the scale, so measurements should never be scaled off a print.
No part can be made to an exact size, so drawings state tolerances, the permitted variation. A shaft of 50 mm with a tolerance of ±0.02 mm may be anywhere from 49.98 to 50.02 mm. Fits between mating parts, such as a bearing on a shaft, are specified in the ISO system of limits and fits by letters and grades, for example H7/g6 for a close running fit; a clearance fit always leaves a gap, an interference fit always needs force or heat to assemble, and a transition fit may do either. Surface finish is specified by the arithmetic mean roughness, Ra, in micrometres. For dairy product-contact surfaces, hygienic-design guidelines commonly call for Ra of 0.8 µm or smoother, because rougher surfaces harbour bacteria and are harder to clean.
A dairy plant is described by several kinds of drawing. A block diagram shows the main process steps as boxes joined by arrows. A process flow diagram (PFD) shows the major equipment, the main process lines and key operating conditions, such as flow rates, temperatures and pressures, often with a mass balance. A piping and instrumentation diagram (P&ID) is the detailed working document: every vessel, pump, heat exchanger, valve and pipe, with line sizes, materials and every instrument and control loop. Layout drawings show where equipment sits in the building, in plan and elevation, and isometric piping drawings show each pipe run in three dimensions with its dimensions, fittings, slopes and welds for fabrication.
P&IDs use standard symbols, from ISO 10628 and the ISA-5.1 standard of the International Society of Automation. Instruments are shown as circles with a tag of letters and numbers. The first letter gives the measured variable: T for temperature, P for pressure, F for flow, L for level, A for analysis (such as conductivity or pH). The following letters give the function: I for indicator, T for transmitter, C for controller, V for valve, S for switch, A for alarm. A tag TIC-101 is therefore a temperature-indicating controller in loop 101, and FT-205 a flow transmitter. Lines inside or across the circle show whether the instrument is in the field, on a control panel or in the control system.
Reading a P&ID of a pasteuriser, a technologist can follow the milk from the balance tank through the regenerative, heating and holding sections, find the temperature sensor and the flow-diversion valve after the holding tube, and check that the diversion line returns to the balance tank. The same drawing is used for hazard and operability studies, for validating cleaning circuits and for maintenance planning.
Fitting is the hand and bench work of shaping and assembling metal parts. It begins with measurement and marking out. A steel rule reads to about 0.5 mm. A vernier calliper reads to 0.02 or 0.05 mm, depending on its vernier scale; its least count is the difference between one main-scale division and one vernier-scale division. An outside micrometer reads to 0.01 mm, using a screw of 0.5 mm pitch and a thimble with 50 divisions. Dial indicators measure small displacements and are used to check the run-out of a shaft or the alignment of couplings. Feeler gauges measure gaps. Marking out uses a surface plate, a scriber, a try square, dividers and a centre punch.
The basic operations are sawing, filing, drilling, tapping and reaming. A hacksaw blade is chosen by its teeth per unit length: coarse blades for soft or thick material, fine blades for thin sheet and tubing, so that at least three teeth are always in contact. Files are classified by length, shape and cut, from rough to smooth. Drilling makes round holes; the cutting speed in metres per minute is v = πDN/1000, where D is the drill diameter in millimetres and N the spindle speed in revolutions per minute, and harder materials such as stainless steel need lower speeds, sharp drills and cutting fluid. Taps cut internal threads, starting with a taper tap; dies cut external threads. Reaming finishes a drilled hole to an accurate size and smooth finish. Machine tools, such as the lathe, milling machine, shaper and grinder, extend these operations.
Welding joins metals by melting them together, usually with added filler metal. In shielded metal arc welding, an electric arc between a flux-coated electrode and the work melts both, and the flux forms a gas shield and a slag. In gas metal arc welding (MIG or MAG), a continuously fed wire electrode is shielded by gas. In gas tungsten arc welding (TIG), an arc from a non-consumable tungsten electrode melts the joint under a shield of inert gas, usually argon, with or without filler rod. Oxy-acetylene gas welding uses a flame whose character is set by the gas ratio: neutral for most steels, oxidising for brasses, and carburising (reducing) for some special work. Brazing and soldering join parts with a filler of lower melting point without melting the parent metal.
Dairy pipework and vessels are made of stainless steel and welded almost entirely by TIG, often by automatic orbital welding machines that rotate the torch around a tube. A hygienic weld must be fully penetrated, smooth and flush with the bore, without cracks, pits, crevices or misalignment, because any defect traps product and bacteria and resists cleaning. The inside of the joint must be purged with argon during welding; otherwise the hot metal oxidises into a rough, dark, crumbly surface known as sugaring, which corrodes and is impossible to clean. Heat tint, the coloured oxide film left beside a weld, reduces corrosion resistance and is removed by pickling and passivation.
Weld quality is checked by visual inspection, including borescopes inside tubes, by dye-penetrant testing for surface cracks, and by radiography or ultrasonic testing for internal defects in pressure equipment. Welders are qualified by procedure and performance tests under international or national standards.
Much dairy equipment, from tank cladding and hoppers to guards, trays and ducts, is made from sheet metal. The work involves marking out the flat pattern, cutting, forming and joining. Development is the drawing of the flat shape that will form the finished three-dimensional part: parallel-line development for prisms and cylinders, radial-line development for cones and pyramids, and triangulation for transition pieces such as square-to-round ducts and hopper outlets. When sheet is bent, the outer surface stretches and the inner compresses, so the flat length must include a bend allowance calculated along the neutral axis, which lies inside the sheet at a fraction of the thickness from the inner surface.
Cutting uses hand snips, guillotine shears, nibblers, plasma and laser cutters. Forming uses press brakes for bends, rolls for cylinders and cones, and spinning or pressing for dished ends. Joints include folded seams, rivets, spot welds and continuous welds; for product-contact parts only continuous, smooth welds are acceptable, because folded seams and rivets create crevices.
Sheet thickness is specified in millimetres in most of the world, and by gauge numbers in some industries, where a larger gauge number means a thinner sheet. Stainless sheet surface finishes are designated by mill and polished finish codes; a bright-annealed or finely polished finish is used for product contact. Hot work inside a running food plant needs a permit-to-work.
Stainless steels are iron alloys containing at least about 10.5% chromium. The chromium reacts with oxygen to form an extremely thin, invisible, self-repairing film of chromium oxide, the passive layer, which protects the metal beneath. If the surface is scratched, the film re-forms in air or oxygenated water. This passivity, together with strength, ease of fabrication, smoothness and resistance to cleaning chemicals, makes stainless steel the standard material for every product-contact surface in a modern dairy: tanks, pipes, valves, pumps, heat-exchanger plates, separators and fillers.
Stainless steels are grouped by their crystal structure. Austenitic stainless steels, the main dairy group, contain chromium and nickel; they are non-magnetic in the annealed state, tough, easily welded and formed. Type 304 (EN 1.4301), containing about 18% chromium and 8% nickel, is the general-purpose grade, often called 18/8. Type 316 (EN 1.4401) adds about 2 to 3% molybdenum, which greatly improves resistance to pitting by chlorides; it is preferred for contact with brines, whey, salted products, acid products and chlorinated water. The low-carbon grades 304L (1.4307) and 316L (1.4404), with carbon no more than 0.03%, resist the loss of corrosion resistance near welds, described in the next chapter, and are the usual choice for welded construction.
Ferritic stainless steels, such as type 430, contain chromium without nickel; they are magnetic, cheaper and resistant to stress corrosion cracking, but less tough and harder to weld, and are used for non-contact cladding and some appliances. Martensitic grades, such as 420, can be hardened and are used for knives and cutting blades. Duplex stainless steels, with mixed austenitic and ferritic structure, such as grade 2205, are stronger and resist chloride stress corrosion cracking and pitting, and are used where chloride exposure is severe.
Several metals once common in dairies are now avoided or excluded from product contact. Copper and its alloys, brass and bronze, dissolve slightly in milk; even traces of copper catalyse the oxidation of milk fat, giving oxidised, cardboard or metallic flavours, and destroy vitamin C. Copper is therefore not used for product contact in modern dairies, though copper vats are kept for some traditional cheeses in a few regions, where the copper is part of the tradition and its effects are controlled. Iron and plain carbon steel rust and also catalyse oxidation. Galvanised (zinc-coated) steel is attacked by acid and alkaline cleaners and is not used for product contact. Aluminium was once used for milk cans; it is light and conducts heat well but is attacked by alkaline cleaners and some sanitisers, so it has largely been replaced by stainless steel and plastic. Lead and cadmium are prohibited in food-contact materials.
Titanium, which resists chlorides extremely well, is used for some heat-exchanger plates in brine or seawater service.
Food-contact materials are regulated in most jurisdictions by framework rules that require that materials do not transfer constituents to food in quantities that could endanger health or change the food unacceptably, supported by specific rules for plastics, rubbers and other groups. Equipment makers declare compliance, and hygienic-design organisations publish material guidance for dairy equipment.
Gaskets, seals, O-rings and diaphragms are made of elastomers, chosen for compatibility with the product, the temperature and the cleaning chemicals. EPDM (ethylene propylene diene monomer) resists hot water, steam and alkaline and acid cleaners well but swells in fats and oils, so it suits milk and cleaning circuits but not high-fat products such as cream or butter oil. NBR (nitrile rubber) resists fats and oils but ages in steam, ozone and strong oxidisers. FKM (fluoroelastomer) resists fats, oils and high temperatures and most chemicals, though not all, and is expensive. Silicone rubber tolerates a wide temperature range but has low mechanical strength. PTFE (polytetrafluoroethylene) is chemically inert and is used for seals, valve seats and linings, though it creeps under load. Elastomers wear, harden and crack with time and repeated heating, so they are replaced on a schedule; a worn gasket is a common source of contamination.
Plastics used in dairies include polypropylene and high-density polyethylene for crates, tanks and some pipes, polycarbonate for transparent covers and some moulds, and polyamides for gears and guides. Their limits are temperature, scratching and absorption of flavour or colour.
Surface finish matters as much as material. Hygienic design guidance from organisations such as the European Hygienic Engineering and Design Group (EHEDG) and 3-A Sanitary Standards calls for smooth, non-porous, crevice-free product-contact surfaces, commonly with roughness Ra no more than 0.8 µm, achieved by cold rolling, mechanical polishing or electropolishing. Electropolishing removes a thin surface layer electrochemically, smoothing microscopic peaks and enriching the surface in chromium, which improves both cleanability and corrosion resistance.
Corrosion is the deterioration of a material by reaction with its environment. For metals in water it is an electrochemical process with four elements: an anode, where metal atoms lose electrons and pass into solution as ions; a cathode, where those electrons are consumed, typically by reducing dissolved oxygen or hydrogen ions; an electrolyte, the water or product that carries ions between them; and a metallic path that carries electrons. Remove any one, for example by keeping the surface dry, and corrosion stops.
Metals differ in their tendency to corrode, shown by their position in the galvanic series in a given environment such as seawater. When two different metals are in electrical contact in an electrolyte, the more active (anodic) one corrodes faster and the more noble (cathodic) one is protected. This galvanic corrosion is why carbon-steel bolts on stainless-steel frames rust quickly in a wet dairy, and why a small anode coupled to a large cathode is the worst arrangement. Passive stainless steel lies near the noble end of the series; if its passive film breaks down locally, it behaves as an active metal at that spot.
Stainless steel does not corrode uniformly in normal dairy service. Its failures are almost always localised attacks on the passive film, and they are often traced to chlorides, high temperature, crevices, deposits, welding or poor cleaning practice.
Pitting is the commonest attack on stainless steel in dairies. Chloride ions, from water, salt, brine, whey, cleaning chemicals or hypochlorite sanitisers, break the passive film at small points, and the pits grow deep and narrow while the surface around them looks unharmed; a pit can perforate a tank wall or plate. Resistance to pitting is ranked by the pitting resistance equivalent number, PREN = %Cr + 3.3 × %Mo + 16 × %N. Type 304 has a PREN of about 18 to 20, type 316 about 23 to 26, and duplex 2205 about 34 to 36, which is why molybdenum-bearing grades are chosen for chloride service.
Crevice corrosion is similar but occurs in narrow gaps where the solution stagnates, such as under gaskets, between plates, under deposits and at incompletely penetrated welds; oxygen is used up inside, the chemistry becomes acidic and chloride-rich, and the film fails. Stress corrosion cracking occurs when a susceptible material is under tensile stress in a corrosive environment; austenitic grades such as 304 and 316 can crack in chloride solutions above about 60 °C, as on the outside of insulated hot tanks where chlorides from insulation or spillage concentrate. Intergranular corrosion follows sensitisation: when ordinary-carbon grades are held between about 450 and 850 °C, as in the heat-affected zone of a weld, chromium carbides precipitate at grain boundaries and the adjacent metal is depleted of chromium. Low-carbon L grades and stabilised grades prevent this. Galvanic corrosion, erosion–corrosion at high velocities, and microbiologically influenced corrosion under biofilms complete the list. Plate heat exchangers, with their many gasket crevices, thin plates and hot chlorinated water, show most of these forms.
Corrosion control begins with design and material selection. Grades are chosen for the worst conditions the equipment will meet, including cleaning: 316L where chlorides, salt, acid products or high temperatures are expected, duplex or titanium for severe chloride service. Equipment is designed to drain completely, avoid crevices and dead legs, use continuous smooth welds, and keep dissimilar metals apart or insulated from each other.
Fabrication must preserve passivity. Welds are made with proper back-purging; heat tint and embedded iron from carbon-steel tools are removed by pickling and passivation. Passivation treats stainless steel with an oxidising acid, traditionally nitric acid and now often citric acid, to dissolve free iron from the surface and allow a uniform chromium-rich film to form. Tools, wire brushes and grinding discs used on stainless steel must not have been used on carbon steel.
Operation is the third line of defence. Chloride content of process and cleaning water is controlled; hypochlorite sanitisers are used at the lowest effective concentration and temperature, for the shortest time, and rinsed off promptly, and are never left in equipment overnight; peracetic acid is often preferred for sanitising stainless steel. Acid and alkaline cleaners are used at specified concentrations and temperatures. Deposits are not allowed to build up. Heat-exchanger plates and gaskets are inspected periodically, with dye-penetrant checks for cracks and pinholes, because a perforated plate lets raw milk or cooling water leak into pasteurised milk. For external structures, coatings, galvanising and cathodic protection are used, and a brown surface discoloration called rouge in high-purity hot-water and steam systems is monitored and removed by derouging.
Statics is the study of bodies at rest under forces. A force is a vector, with magnitude and direction, measured in newtons (N); a mass of 1 kg weighs 9.81 N under standard gravity. Forces acting at a point are combined into a single resultant by the parallelogram or triangle law, or by resolving each into horizontal and vertical components and adding them. The moment of a force about a point is the force multiplied by the perpendicular distance from the point to its line of action, M = F × d, in newton-metres. A couple is a pair of equal, opposite, parallel forces; it produces pure turning, as in turning a valve handwheel.
A body is in equilibrium when the sum of all forces in each direction is zero and the sum of moments about any point is zero. These conditions find unknown forces from a free-body diagram, a sketch of the body isolated from its surroundings with every force acting on it. For three forces in equilibrium meeting at a point, Lami's theorem states that each force is proportional to the sine of the angle between the other two. Beam problems use the same conditions: a simply supported beam carrying a load W at its centre has reactions of W/2 at each support. In a dairy, these calculations size the supports for a pipe bridge, the legs of a storage tank and the lifting points for a separator bowl.
Friction is the force that resists sliding between surfaces in contact. The laws of dry friction state that the limiting friction force is proportional to the normal force between the surfaces, F = μN, where μ is the coefficient of friction, and is nearly independent of the apparent area of contact. The coefficient of static friction, which must be overcome to start motion, is larger than the coefficient of kinetic friction during sliding. The angle of friction φ satisfies tan φ = μ, and a block on an inclined plane begins to slide when the angle of the plane exceeds φ, the angle of repose.
For a rope or flat belt wrapped around a drum or pulley, the ratio of tensions on the tight and slack sides is T₁/T₂ = e^(μθ), where θ is the angle of wrap in radians. This exponential law explains why a few turns of rope around a bollard can hold a large load, and it sets the power a belt drive can transmit. For V-belts the effective coefficient is increased by the wedge action of the belt in its groove.
Friction is useful in brakes, clutches, belts and conveyor drives, and harmful in bearings, gears and slides, where it wastes energy and causes wear and heat. Lubrication, treated in the last chapter, reduces it. Rolling resistance is much smaller than sliding friction, hence ball and roller bearings.
When a bar is loaded in tension or compression, the stress is the force per unit cross-sectional area, σ = F/A, measured in pascals or, more conveniently, in megapascals (1 MPa = 1 N/mm²). The strain is the change in length divided by the original length, ε = ΔL/L, and has no units. Hooke's law states that, within the elastic limit, stress is proportional to strain: σ = Eε, where E is the modulus of elasticity or Young's modulus, about 200 GPa for steels including stainless steel. Shear stress, τ = F/A for a force parallel to the area, is related to shear strain by the modulus of rigidity G. Lateral contraction under tension is described by Poisson's ratio, about 0.3 for steels.
A tensile test of a ductile metal gives the stress–strain curve: a straight elastic region up to the proportional limit, a yield point at which permanent deformation begins, strain hardening up to the ultimate tensile strength, and then necking and fracture. Ductility is measured by the percentage elongation at fracture. Working stresses are kept well below failure by a factor of safety, the ratio of the failure stress (yield or ultimate) to the allowable working stress.
Several simple formulas recur in dairy engineering. A thin cylinder under internal pressure p, of diameter d and wall thickness t, carries a hoop stress σ_h = pd/(2t) and a longitudinal stress σ_l = pd/(4t), so the hoop stress is twice the longitudinal and cylinders split lengthwise; this governs the wall thickness of pipes, tanks and pressure vessels. A bar heated while prevented from expanding develops thermal stress σ = EαΔT, where α is the coefficient of thermal expansion, which is why long hot pipelines need expansion loops or bellows. Bending stress in a beam is σ = My/I, and shear stress in a twisted shaft is τ = Tr/J.
Power transmission carries mechanical power from a motor to a machine, often changing speed and torque on the way. The power transmitted by a rotating shaft is P = Tω = 2πNT/60, where T is the torque in newton-metres, ω the angular velocity in radians per second and N the speed in revolutions per minute. For a given power, torque is inversely proportional to speed: a gearbox that reduces speed tenfold increases torque nearly tenfold, less the losses. A 7.5 kW motor running at 1,450 rpm delivers a torque of about 7,500 × 60/(2π × 1,450) ≈ 49 N·m.
Shafts are sized so that the shear stress from torque, combined with bending stress from belt pulls and gear forces, stays below the allowable value. Power is transferred from shaft to hub by keys in keyways, splines or shrink fits. Shafts run in bearings: rolling-element bearings (ball bearings for light and medium radial loads at high speed, roller bearings for heavier loads, tapered roller and angular-contact bearings for combined radial and axial loads) and plain bearings or bushes, where a shaft runs in a sleeve with a lubricating film. Bearings in dairy machines must be sealed against washdown water and cleaning chemicals, and those near product zones use food-grade lubricants.
The efficiency of a transmission is the output power divided by the input power. Losses appear as heat in belts, gears and bearings. A chain of components multiplies efficiencies: a motor of 92% efficiency driving a gearbox of 95% gives 87% overall.
Belt drives transmit power between shafts that may be far apart, absorb shock, run quietly and need no lubrication. Flat belts run on crowned pulleys; V-belts wedge into grooved pulleys, giving greater grip in less space, and several are often run side by side; timing or synchronous belts have teeth that mesh with toothed pulleys and transmit power without slip, keeping shafts in step. Belts slip slightly and also creep, as the belt stretches more on the tight side than on the slack side, so the speed ratio is not exact for flat and V-belts.
Ignoring slip, the speed ratio of a belt drive is N₁/N₂ = D₂/D₁: the smaller pulley turns faster. A motor at 1,450 rpm with a 150 mm pulley driving a 300 mm pulley turns it at 725 rpm. The belt speed is v = πDN/60. The power transmitted is P = (T₁ − T₂)v, where T₁ and T₂ are the tight- and slack-side tensions; the maximum tension is limited by the belt's strength and the ratio by the friction law of the previous chapter. Belts must be correctly tensioned and pulleys aligned; under-tensioned belts slip, squeal and wear; over-tensioned belts overload bearings.
Chain drives use roller chains running on toothed sprockets. They give a positive drive without slip, carry heavy loads at moderate speeds and are compact, but need lubrication and protection from wear, and are noisier than belts. In dairies, chains drive conveyors, crate stackers and some fillers, often in stainless steel or plastic for washdown areas. All belt and chain drives must be fully guarded.
Gears transmit power positively by meshing teeth. Spur gears have straight teeth parallel to the axis and connect parallel shafts; they are simple but noisy at high speed. Helical gears have teeth at an angle, run more smoothly and quietly and carry more load, but produce an axial thrust that bearings must take. Bevel gears connect intersecting shafts, usually at right angles. A worm and worm wheel connect non-intersecting perpendicular shafts and give a large speed reduction in one stage, often 20:1 to 60:1 or more; at high ratios they can be self-locking, so the wheel cannot drive the worm, but their efficiency is lower than that of other gears. A rack and pinion converts rotation into straight-line motion.
The speed ratio of a gear pair is N₁/N₂ = T₂/T₁, where T is the number of teeth. In a compound gear train the overall ratio is the product of the ratios of each pair. A motor at 1,440 rpm driving a 20-tooth pinion meshed with an 80-tooth gear turns the gear at 360 rpm.
Couplings join two shafts end to end. Rigid couplings, such as flange and sleeve couplings, need precise alignment. Flexible couplings, such as jaw couplings with an elastomer insert, tyre couplings and gear couplings, accommodate small angular and parallel misalignment and damp shock. Universal joints connect shafts at an angle. Fluid couplings give a smooth start for heavy loads. Misalignment is the commonest cause of coupling, bearing and seal failure in pumps, and is checked with dial indicators or laser alignment tools.
Mechanical variable-speed pulleys and gearboxes have largely given way to variable-frequency drives, which change motor speed electronically and are described under motors.
Electric current is the flow of charge, measured in amperes (A). Potential difference, or voltage, measured in volts (V), drives it, and resistance, measured in ohms (Ω), opposes it. Ohm's law states V = IR. The power dissipated in a resistance is P = VI = I²R = V²/R, in watts, and the energy used is power multiplied by time, billed in kilowatt-hours (1 kWh = 3.6 MJ).
Resistors in series carry the same current and their resistances add: R = R₁ + R₂ + …. Resistors in parallel share the same voltage and their conductances add: 1/R = 1/R₁ + 1/R₂ + …; two equal resistors in parallel give half the resistance of one. Kirchhoff's current law states that the currents entering a junction equal those leaving it; his voltage law states that the voltages around any closed loop sum to zero. Together they solve any network.
The resistance of a conductor is R = ρL/A, where ρ is the resistivity of the material, L the length and A the cross-sectional area; copper and aluminium are used for cables because of their low resistivity. Resistance rises with temperature in metals. Undersized cables have high resistance, overheat and cause a voltage drop at the far end, which makes motors run hot and weak. Direct current is used in control circuits, often at 24 V, in batteries, in electronics, and in some drives and electroplating; direct-current motors were once common for variable speed but have largely been replaced by alternating-current motors with electronic drives.
Most power is distributed as alternating current, whose voltage and current reverse direction many times per second, following a sine wave. The frequency is 50 Hz in most of Europe, Africa, Asia and Oceania and 60 Hz in most of the Americas and some other countries. The root-mean-square (RMS) value is the steady value that would deliver the same power in a resistor; for a sine wave it is the peak value divided by √2, so a 230 V RMS supply peaks at about 325 V. Nominal voltages, single-phase and three-phase, differ by country and must be checked when equipment is imported.
In a pure resistance, current and voltage are in phase. An inductance, such as a motor winding, opposes changes in current; its reactance is X_L = 2πfL, and current lags voltage by 90°. A capacitance opposes changes in voltage; its reactance is X_C = 1/(2πfC), and current leads voltage by 90°. The combined opposition of resistance and reactance is the impedance, Z = √(R² + X²), and I = V/Z.
When current and voltage are out of phase, not all of the apparent power, S = VI in volt-amperes, does useful work. The real power, P = VI cos φ in watts, is the useful part; the reactive power, Q = VI sin φ in volt-amperes reactive, flows back and forth. The power factor, cos φ, is the ratio of real to apparent power. Induction motors, especially when lightly loaded, have lagging power factors, often 0.7 to 0.85. A low power factor means higher current for the same useful power, larger cables and transformers, and, under many tariffs, penalty charges; capacitor banks are installed to correct it.
Industrial plants are supplied with three-phase alternating current: three voltages of equal magnitude, each displaced by 120° from the others. Three-phase supply transmits more power for a given amount of conductor than single-phase, and it produces the rotating magnetic field that makes induction motors self-starting and smooth-running. Single-phase supply for lighting and small loads is taken between one phase and neutral.
Three-phase loads and windings are connected in star (wye) or delta. In a star connection, the ends of the three windings are joined at a neutral point; the line voltage between any two lines is √3 times the phase voltage across each winding, and the line current equals the phase current. A supply of 400 V between lines gives 400/√3 ≈ 230 V from each line to neutral, a common arrangement in many countries; other systems, such as 480/277 V and 208/120 V, are used elsewhere. In a delta connection, the windings form a closed triangle; the line voltage equals the phase voltage, and the line current is √3 times the phase current.
The power of a balanced three-phase load is P = √3 V_L I_L cos φ, whichever the connection. For example, a motor drawing 15 A per line at 400 V with a power factor of 0.85 takes √3 × 400 × 15 × 0.85 ≈ 8.8 kW of electrical power. If its efficiency is 90%, its shaft output is about 7.9 kW. A plant's electrical system includes the incoming supply and transformer, the main switchboard, motor control centres, distribution boards, cables and protective devices, and the electrical drawings and single-line diagram that describe them.
The three-phase squirrel-cage induction motor drives almost every pump, agitator, fan, compressor and conveyor in a dairy. Its stator carries three-phase windings that produce a magnetic field rotating at synchronous speed, Ns = 120f/P rpm, where f is the supply frequency and P the number of poles. A four-pole motor runs at a synchronous speed of 1,500 rpm on 50 Hz and 1,800 rpm on 60 Hz; a two-pole motor at 3,000 or 3,600 rpm. The rotor is a set of conducting bars joined by end rings, the squirrel cage, in which the rotating field induces currents that produce torque.
The rotor must turn slightly slower than the field, or no current would be induced. The difference is the slip, s = (Ns − N)/Ns, typically 2 to 5% at full load: a four-pole 50 Hz motor runs at about 1,440 to 1,470 rpm. The motor's nameplate gives its rated power output, voltage, current, frequency, speed, power factor, efficiency, insulation class, enclosure protection and duty. Rated power is the mechanical output at the shaft, not the electrical input.
Motors are classified for efficiency in international classes, IE1 to IE4 and above, under IEC 60034-30-1, and many countries set minimum efficiency classes for new motors. Because motors consume most of a dairy's electricity, choosing efficient motors and not oversizing them saves energy throughout their life. Enclosures are rated for protection against solids and water by IP codes under IEC 60529: the first digit for solids and the second for water, so IP55 is protected against dust and water jets, and IP66, IP67 and IP69 are used for washdown and high-pressure, high-temperature cleaning areas.
At the moment of starting, an induction motor draws a current several times its full-load current, commonly five to eight times, for a few seconds. Small motors can be started direct-on-line (DOL), connected straight to the full supply through a contactor; this is simple and gives full starting torque, but the current surge causes voltage dips on weak supplies and mechanical shock on the driven machine.
Larger motors use reduced-voltage starting. A star–delta starter connects the windings in star for starting and switches them to delta when the motor nears full speed. In star each winding receives 1/√3 of the line voltage, so the starting current and starting torque are both reduced to one third of their direct-on-line values; this suits pumps and fans that need little torque at start, but not heavily loaded conveyors or positive-displacement machines. Autotransformer starters reduce the voltage by a chosen tap. Soft starters use thyristors to ramp the voltage up smoothly, limiting current and mechanical shock, and are common on centrifugal pumps, where they also reduce water hammer when stopping.
Variable-frequency drives (VFDs), also called inverters, rectify the supply to direct current and then produce an alternating output of variable frequency and voltage, so that motor speed can be set anywhere over a wide range and changed smoothly. They give soft starting, speed control for pumps, fans, agitators and conveyors, and large energy savings: by the affinity laws, the power taken by a centrifugal pump or fan varies with the cube of its speed, so running at 80% speed needs only about half the power. VFDs replace throttling valves and dampers, but they generate electrical harmonics and need correct installation and screened motor cables.
A motor circuit includes isolation, protection, switching and control. An isolator or switch-disconnector allows safe isolation for maintenance. Fuses or circuit breakers protect the cable against short circuits. A contactor, an electrically operated switch controlled by push-buttons or the plant's control system, starts and stops the motor. An overload relay, thermal or electronic, trips the contactor if the motor draws more than its set current for too long, protecting the windings from overheating. Motor protection relays may also detect phase loss, phase imbalance and earth faults; running a three-phase motor on two phases, called single-phasing, quickly burns it out. In the classic start–stop control circuit, the start button energises the contactor coil and an auxiliary contact holds it in, while the stop button and overload contact are in series so that either breaks the circuit. Motor control centres house the starters of many motors in one assembly.
Common motor faults include overheating from overload, blocked ventilation or high ambient temperature; bearing failure from poor lubrication, misalignment or water ingress; and insulation breakdown from moisture, which is checked with an insulation-resistance tester before restarting a motor that has been flooded or washed down.
Other motors have particular uses. Single-phase induction motors need an auxiliary winding, often with a capacitor, to start, and are used for small loads. Servo and stepper motors give precise position control in fillers and packaging machines.
Electricity injures by current flowing through the body, and the danger depends on the size of the current, its path, its duration and its frequency. Approximate thresholds for alternating current at mains frequency through the trunk are: about 1 mA, perception; about 10 mA, the let-go threshold above which muscles may clamp the hand onto the conductor; and from about 30 to 50 mA upward for a second or so, a serious risk of ventricular fibrillation, the disordered heart rhythm that kills in electrocution. Higher currents cause burns and tissue damage. The current is set by the voltage and the body's resistance, which falls sharply when the skin is wet. A dairy, with water on floors, wet hands, metal equipment and conductive milk, is therefore a high-risk electrical environment.
Contact may be direct, with a live conductor, or indirect, with exposed metalwork, such as a motor frame or tank, that has become live through an insulation fault. Protection against direct contact relies on insulation, enclosures and barriers, and keeping live parts out of reach. Protection against indirect contact relies on earthing and automatic disconnection, supplemented by residual current devices. Arc flash, the explosive release of energy in a short circuit, is a separate hazard for those working on switchgear.
If someone is receiving a shock, the supply must be switched off or the person separated from it with a dry non-conducting object before anyone touches them; then cardiopulmonary resuscitation and, where available, an automated external defibrillator are used, and medical help is called.
Earthing (called grounding in North America) connects the exposed metal parts of equipment to the general mass of earth through protective conductors. If an insulation fault makes a motor frame live, a large fault current flows through the protective conductor and operates the fuse or circuit breaker quickly, disconnecting the supply before anyone is harmed. The earth path must have a low enough impedance for this to happen within the times set by wiring rules. Supply systems are described by codes: in TN systems the supply neutral is earthed and exposed parts are connected to it by protective conductors (TN-S with separate neutral and protective conductors throughout, TN-C-S with them combined in part of the supply); in TT systems exposed parts are connected to a local earth electrode, and residual current devices are essential.
Equipotential bonding connects all exposed and extraneous metal parts that can be touched together, such as tanks, pipes, platforms, steel structures and water pipes, so that no dangerous voltage can exist between them during a fault. It is especially important in wet areas.
A residual current device (RCD), also called a ground-fault circuit interrupter, compares the current in the live and neutral conductors and trips when they differ by more than a set amount, meaning current is leaking to earth, possibly through a person. Devices of 30 mA rated residual current are widely required for socket outlets and wet locations under international wiring rules, and some countries use 5 mA devices for personal protection. Other protective measures include double-insulated tools, which need no earth, and safety extra-low voltage for hand lamps and equipment used inside tanks.
Most electrical accidents in plants happen during maintenance, cleaning or fault-finding, not normal operation. The central control is isolation before work: lockout–tagout. The equipment is switched off, isolated at a point that can be locked, locked with each worker's personal padlock and tagged, and then proved dead with a tested voltage detector before work begins. Stored energy, such as charged capacitors in drives, compressed air, steam, springs and gravity, must also be released or blocked. Work on or near live conductors is avoided and, where unavoidable, is done only by competent persons under a permit-to-work with suitable insulated tools and protective equipment.
Washdown is a particular hazard in dairies. High-pressure hoses must not be directed at motors, junction boxes or control panels unless they carry suitable IP ratings, and damaged cables, glands and plugs must be repaired at once. Portable electrical equipment is inspected and tested at regular intervals. Electrical work is done to the national wiring regulations, most of which follow the IEC 60364 series or, in North America, national electrical codes.
Some dairy areas carry explosion risks. Fine milk powder, lactose and whey powder form explosive dust clouds in spray dryers, fluid beds, silos, conveying systems and bagging rooms, and ammonia refrigeration systems can release flammable concentrations. Such areas are classified into hazardous zones under international schemes, and electrical equipment there must be certified for the zone. Static electricity from powder flow and plastic pipes must be dissipated by earthing and bonding of all conductive parts.
Lubrication separates moving surfaces with a film that reduces friction and wear, carries away heat, protects against corrosion, seals out contaminants and flushes away debris. The film can work in three regimes, shown by the Stribeck curve of friction against a parameter combining viscosity, speed and load. In boundary lubrication, at low speed or high load, the surfaces touch at their asperities and friction depends on thin adsorbed layers and additives. In hydrodynamic lubrication, at adequate speed and viscosity, a full fluid film forms and the surfaces never touch, giving the lowest friction and wear. Mixed lubrication lies between. Elastohydrodynamic lubrication, in which very high pressures briefly stiffen the oil, protects rolling bearings and gear teeth.
Viscosity is the key property of a lubricating oil. Industrial oils are graded by the ISO viscosity grade (ISO VG), which is the kinematic viscosity in square millimetres per second (centistokes) at 40 °C; ISO VG 68, for example, has a viscosity of about 68 mm²/s. Viscosity falls as temperature rises, and the viscosity index measures how little it changes; a high index is desirable. Oils are mineral or synthetic, such as polyalphaolefins, esters and polyalkylene glycols, which last longer at high temperature. Additives include antioxidants, anti-wear and extreme-pressure agents, rust inhibitors and anti-foam agents. Greases are oils thickened with soaps or other thickeners, graded for consistency by NLGI numbers, and are used where oil would leak away, as in most rolling bearings. Solid lubricants, such as graphite, molybdenum disulfide and PTFE, work at high temperature or where liquids cannot be used.
In food plants a lubricant may accidentally contact the product through drips, leaks or mists. Lubricants are therefore classified by their permitted use. Under the widely used registration scheme that originated with the United States Department of Agriculture and is now run by an independent certifier, H1 lubricants are acceptable where incidental food contact is possible, made only from listed ingredients and restricted in the amount that may contact food; H2 lubricants are for use where there is no possibility of food contact; and H3 lubricants are soluble oils used to clean and prevent rust on hooks and similar equipment. The international standard ISO 21469 sets hygiene requirements for the formulation, manufacture and use of lubricants with incidental product contact. Many dairies use H1 lubricants throughout production areas so that the wrong grade cannot be applied by mistake.
Good lubrication practice means the right lubricant, in the right amount, at the right place and the right interval. Over-greasing a bearing is as harmful as under-greasing: excess grease churns, overheats and pushes past seals. Mixing incompatible greases can soften them. Lubrication points, products and intervals are recorded on a lubrication schedule, with colour-coded containers and grease guns dedicated to each product. In washdown areas, cleaning chemicals and hot water wash lubricant out of bearings and chains, so sealed-for-life bearings, stainless or plastic chains and water-resistant greases are used, and lubrication follows cleaning. Lubricant leaks, from gearboxes on agitators above open vats for example, are a foreign-material and allergen hazard and are covered in food-safety plans.
Maintenance keeps equipment able to perform its function safely, hygienically and economically. Breakdown or reactive maintenance repairs equipment after it fails; it is acceptable only for cheap, non-critical items whose failure causes little harm. Preventive maintenance replaces or services parts at fixed intervals of time or running hours, such as changing pump seals, homogeniser valves and heat-exchanger gaskets on a schedule; it reduces unexpected failures but can replace parts that still have life left. Predictive or condition-based maintenance monitors the condition of equipment and intervenes when the data show deterioration: vibration analysis detects bearing wear, imbalance and misalignment; infrared thermography finds hot electrical connections and failing bearings; oil analysis shows wear metals and contamination; ultrasonic detection finds compressed-air and steam leaks. Reliability-centred maintenance chooses among these strategies for each failure mode according to its consequences.
Total productive maintenance (TPM) involves operators in basic care: cleaning, inspection, lubrication and tightening, called autonomous maintenance, so that small problems are seen early. Its measure, overall equipment effectiveness (OEE), is the product of availability, performance and quality rates. Reliability is described by the mean time between failures (MTBF) and maintainability by the mean time to repair (MTTR); inherent availability is MTBF/(MTBF + MTTR). A filler with an MTBF of 95 h and an MTTR of 5 h has an availability of 95%.
In a dairy, maintenance is also a food-safety activity. Work in product areas is planned with production and quality staff, tools and spare parts are counted in and out, swarf and debris are removed, and equipment is cleaned and released by quality staff before production restarts. A computerised maintenance management system records work orders, histories, spare parts and costs.
Learn to read drawings early: during in-plant training, take the P&ID of the pasteuriser or CIP system and trace every line and instrument on the real plant. Practise the standard calculations until they are quick: stress and strain, hoop stress, belt and gear ratios, shaft power and torque, three-phase power and motor synchronous speed and slip. In the workshop, try TIG welding on stainless steel and look at a weld made with and without back-purging; you will never again accept a sugared weld in a product line.
A production or maintenance engineer five years into the job specifies replacement materials and gaskets, checks welding quality on new pipework, investigates corrosion of heat-exchanger plates and tanks, plans preventive and condition-based maintenance, selects motors, drives and starters, and enforces lockout–tagout. Typical decisions: whether a plate heat exchanger leak needs immediate shutdown, whether a pump failure came from misalignment or cavitation, whether to fit a variable-frequency drive, and which lubricant is acceptable above an open vat. Good records of each failure make the next decision faster and better.
An engineering manager or plant head with 25 years' experience sets standards for the whole site: material specifications (304L versus 316L versus duplex) for new projects, welding procedure and inspection requirements for contractors, electrical design and hazardous-area classification for powder plants, maintenance strategy and spare-parts policy, and energy efficiency through motor and drive choices. These decisions shape capital cost, downtime, product safety and worker safety for decades, and must be defended in audits, insurance surveys and incident investigations long after the people who made them have moved on.
A professor or research engineer uses these fundamentals to study and teach hygienic design, corrosion of stainless steels under dairy cleaning regimes, the mechanics of process equipment, and energy-efficient drives and motors. Research on surface finish and cleanability, on stress corrosion in hot chloride service, and on condition monitoring with sensors and data analysis builds on this foundation, and so does contribution to hygienic-design guidelines and national standards for dairy equipment. Teaching these fundamentals with real plant failures as case studies prepares graduates far better than abstract problems alone.
This is a condensed subject. Engineering drawing practice (sketching, CAD), machine-tool operation, thermodynamics of welding, metallurgy of heat treatment, beam deflection, columns, detailed gear design, electronics, transformers and power distribution design are only named; the agricultural engineering track covers them. Nominal supply voltages, wiring rules, RCD requirements, lockout procedures and lubricant registration schemes differ by country and are described in general terms only; national rules must be followed. Pressure-vessel design codes are left to advanced-dairy-engineering, and hygienic design to cip-sanitation-hygienic-design.
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