In any milk plant, milk needs to be transported from one part of the plant to another, and this transportation is done in piping systems. Fluids comprise gases, vapours and liquids. Gases and vapours may be transported under increased or reduced pressure. Liquids are transported almost exclusively under pressure higher than that prevailing in the ambient, i.e. atmospheric. Depending on the pressure of the medium transported in the pipe the occurence of leaks will demonstrate itself either by the fluid coming out the pipe (fluids under increased pressure) or by the ambient air being sucked into the pipe (fluids under reduced pressure). In the majority of instances in the dairy industry the pressure inside pipe systems is higher than that in the ambient although there are exceptions such as milk evaporators (vapour ducts) and spray driers (air in the driers, in the cyclones and in the interconnecting components). Milk and liquid products are also transported within the milk plant in piping systems under increased pressure. However, the milk pipes need to meet special requirements not applicable to other fluid pipes in the milk plant and therefore they are discussed together with milk processing equipment in Chapter 4.
Pressures under which fluids are transported in piping systems in a milk plant are classified by industrial standards as low and medium. To the highest belong steam pressure which in milk powder factories may exceed 2.0 MPa (atmospheric pressure equals about 0.1 MPa) and compressed ammonia in refrigeration plants reaching about 1.2 MPa in single-stage compressors. Compressed air pressure for pneumatic devices may reach about 0.8 MPa, but air used in all air conditioning systems may be transported in air ducts either under slightly increased, or slightly reduced pressure as compared to the ambient. The pressure in water pipes usually does not exceed 0.3 to 0.5 MPa.
Depending on the type of fluid transported in the pipe and on its pressure, the design engineer selects the suitable type of piping and fittings. Pipelines may be flexible (plastic, rubber) or rigid (usually metallic). Various types of fitting are used to interconnect pipes or to connect pipes to machines. Obviously only flexible pipes can be connected to moving components. Worm drive clips and snap-on connector unions are in common use to secure flexible hose to metal pieces and adaptors. Screwed joints, flage joints and welded joints are most commonly used on mild steel piping, but in the case where galvanized pipes (zinc coated) are used - usually for water lines no welding should be applied since it removes the zinc coat and exposes the fluid and the steel to corrosive interactions. Soldered joints are used when lengths of thinwalled copper piping have to be connected and this is very often met in internal connections of automatic controls. Contrary to a frequent attitude, piping systems require the meticulous attention of the engineering staff of the milk plant. This attention should be given not only to the valves, traps, reducers, filters and other more complicated parts of a piping system, but also to the pipelines themselves. They need their share of scheduled inspection, care and maintenance.
The trouble most often met in piping systems is the deterioration of the flow of the fluid. Preventive maintenance should be aimed at preventing this from happening. This deterioration is caused by scale accumulation, by condensation inside the system or by leaks. The term “scaling” means depositing solids inside the pipe by which the free way for the passing fluid may be reduced and sometimes completely blocked. This occurs particularly often in water pipes transporting untreated water at elevated temperatures: if the water hardness caused by bicarbonates is high, the scale accumulation is rapid and the deposits are hard like stone. Condensation can occur in vapour (mainly steam) lines and gas (mainly air) piping systems. It always happens when the temperature of the fluid inside the pipe drops below a given value. Leaks are caused by corrosion or by distress cracks, which in turn may be the result of liquid hammer, faulty drainage or faulty pipe installation, particularly in anchoring, supporting and in designing the expansion couplings.
Securing high performance of a piping system begins with the proper layout in which all piping elements are well selected and properly assembled into a system. First of all allowance must be made for the fact that all dimensions of the pipeline change with a change of the temperature, increasing when heated and decreasing when cooled. As a result of this expansion the whole system moves and all hangers and supports have to be designed in such a way that they either move together with the pipe (roll or slide) or that they can swing without exposing any stress either on the pipe or on the part of the supporting anchoring structure. Expansion pipe bends are not often used in a milk plant since the “spring effect” of an expansion bend is usually achieved through the frequent change of the direction of the line. However, expansion joints are often in use on steam lines. In vertical runs that the expansion of the pipe may cause the even distribution of the weight of the cold pipe on all rigid hangers will change in such a way that the entire load will be shifted to the bottom hanger. Should, during preventive maintenance inspection, distress cracks on walls and footings near the pipe anchors be detected, the fault diagnosis will indicate improper layout with regard to thermal expansion.
Liquid hammer occurs when a moving column of liquid is rapidly stopped. It exposes the whole piping system to mechanical stresses which may cause serious damage and result in leaks. Slow moving valves do not cause hammering, but pneumatically operated spring loaded valves may. Air chambers are one of the common methods by which the hammer shocks are relieved. Condensate from the steam lines needs to be removed in order to avoid water hammer, but keeping traps in operating condition may not be sufficient when pipes are sagging and creating pockets where condensate can collect. The whole system should be adequately pitched. Condensate will tend to collect above closed valves in vertical lines and in the back of globe valves in horizontal lines. Draining the condensate from all such spots prior to admitting steam to cold pipes is one of the ways of preventing water hammer in steam pipes. Condensation in air pipes leads to rapid corrosion, but first of all it is dangerious because of the introduction of water in a part of equipment in which dry air only is wanted. For instance, air distribution valves for automatics are usually activated by solenoids; when air is discharged from the valve outlet together with water, short circuits in the solenoid wiring systems are unavoidable. Air drying is a must in most milk plants. It is done directly after compression and usually achieved by condensation of the water through cooling the air in a pipe heat exchanger.
Prevention from scaling depends much on whether the general technical standard of operations in the entire plant meets the requirements. A water treatment plant, filters and oil separators are the common components of the equipment, permitting fluids to flow smoothly in the pipes without scale formation. They all need proper care and maintenance.
The most difficult task in maintaining piping systems at expected levels of performance is the prevention of corrosion. Corrosion is unavoidable, but it can be slowed down by proper operation procedures and proper maintenance. Well-maintained mild steel piping systems may last 12 to 15 years or even longer, but instances of a complete breakdown in 3 to 4 years after commissioning are know. Corrosion can be caused by many factors but they are all of electro-chemical nature. Corrosion is generally caused by atmospheric oxygen dissolved in aqueous liquids, or by dissolved salts such as brines and detergent solution and finally some gases like carbon dioxide and sulphur hydroxide or diozide dissolved in water. The internal corrosion in the pipes can be reduced by strictly observing all rules in handling the fluids prior to their entering the piping system. Such rules must be introduced after a thorough analysis of the fluids, by drawing conclusions from the analytical results and by applying appropriate measures. Most typical in this respect is water treatment, not only for the steam raising plant, but also for general use. If aggressive solutions are transported in the pipes, corrosion can be slowed down by adding to the liquid certain selected chemicals such as sodium silicates. Scale formation and corrosion can be often reduced, particularly in water pipes, by adding up to 20 ppm sodium hexametaphosphate to the water. Basically corrosion is slower in liquids with pH values exceeding 7.0, or in other words in alkaline solutions, as compared to the acidic ones.
External corrosion on pipes is always increased on sweating pipes if the pipe is colder than the ambient dew point at which the air moisture condenses on the cold surface. Air and other gases dissolve in the condensate, and cause corrosion. A practical solution to the problem is to prevent the pipe from sweating. Watertight covering applied directly to the pipe (asphaltic coats, thermal insultation, spiral wrapping of strong fabrics) is the simplest remedy.
The layout of the piping and the selection of all components should depend on the parameters of the transported fluids and on the wanted capacities. The fluid velocity in a pipe is limited and depends on the type of the fluid: for water usually 2 m/s, for steam up to 50 m/s at pressures reaching 2.0 MPa but only 35 m/s for reduced pressures ranging between 0.2 to 0.6 MPa. The liquid velocity in milk pipes does not usually exceed 1 m/s, except during cleaning.
There are several technical rules guiding the installation, operation, care and maintenance of valves, and other components of a piping system. They are a part of the art of proper installation but, particularly concerning care and maintenance, they depend on the design of the component for which a good manufacturer always issues appropriate manuals. Globe-type valves cause higher pressure drops than gate valves, but they are useful for throttling service, and are easier to maintain tight and easier to repair. It is advisable to install whenever possible globe-type valves in such a way that the pressure is above the disc - this prevents vibrations. Automatic drainage needs to be provided in all places in which condensate could accumulate, particularly above valves in vertical lines, and above the discs on stop-check valves. All accessible points of friction of the piping system components, especially valves, should be frequently lubricated (steam threads, yoke sleeves, etc). Any leak discovered in the pipe system should be stopped without delay: stuffing boxes tightened, misalignments corrected, packings replaced, supports repaired or changed if they do not support their share of the load. Any repair on an insulated pipe may adversely affect the insulation. The insulation must be repaired immediately not only to prevent excessive heat exchange with the ambient, but also to prevent rapid outer corrosion.
Fluids move in a pipeline when there is a pressure difference between the ends of the line. This difference, if not caused by thermal convection, is usually created by pumps when the moving medium is a liquid and by compressors and fans when the moving medium is a gas. In many respects compressors and fans can be considered as gas pumps and there are several similarities in their mechanical design, installation and care. Therefore they could be considered as one group of machines supplying pipelines with fluids. In a milk plant, apart from milk, the most common liquids transported through pipelines are water and water solutions and air is the most common gas. (Gases used in refrigeration circuits are dealt with in section 9 below; milk pumps are dealt with in Chapter 4). Service pumps in a milk plant are nowadays almost exclusively of the centrifugal type and so are ventilating and exhaust fans used for building ventilation, for supplying air to burners in steam boilers, for air supply in spray driers, can washers, etc. Air compressors used in milk plants are commonly of the small and medium size and are built as single or multi-cylinder piston pumps. Air tanks are standard parts of the machine. In smaller machines the motor and the compressor are usually mounted on the air tank.
Centrifugal pumps. There are several types of centrifugal pumps used in a milk plant of which the simplest are those with motor and pump rotor built on a single shaft. Their care and maintenance is similar to milk pumps which are dealt with in another section below. In all other types of centrifugal pumps, the motor and the pump are joined by a coupling, flexible or rigid. The majority of centrifugal pumps are supplied by the manufacturers with bed-plates which need either to be grouted on a pump foundation or to be fixed on other types of rigid structures.
Misalignment is one of the most common sources of pump troubles. Proper alignment should be secured during pump installation and checked frequently throughout the pump's life. It should be noticed that neither a flexible coupling nor delivery of the pump on the manufacturer's bed-plate can be considered as guarantees for proper alignment. Therefore the pump must be checked for alignment - angular and paralleled - and necessary corrections must be done several times during installation: on the bed-plate prior to fixing this to its foundation, after fixing but prior to securing piping and finally after the pipes have been attached to the pump.
Absence of air, particularly in suction piping, is the next requisite for satisfactory pump performance. The capacity of a centrifugal pump is reduced by almost half when only 4 percent of free air is present in the pumped liquid. It leads in extreme cases to cavitation and to damage of the pump. The suction side of the pump is the most sensitive to entrained air and proper attention must be given to the following rules guiding the correct installation and operation of a pump:
Proper care of glands is most important during the operation of centrifugal pumps. The main function of a gland on centrifugal pumps is to hold in place a soft packing preventing the liquid from leaking or air from entering the pump casing. Glands may serve also as a device for removing heat from the shaft - they are then called quench glands. They have a hollow portion adjacent to the shaft, through which cooling liquids pass and lower the temperature of the shaft, thus protecting bearings mounted on the shaft. Quench glands must fit the shaft close to the outboard ends. The clearance between shaft and gland has to be kept at the correct tolerance, otherwise the pumped liquid will leak along the shaft. Glands may be treated as replaceable spare parts as a whole, but the necessary clearance may also be re-established by replaceable bushings or auxiliary packings. A normal packed-type stuffing box requires lubrication and cooling. Usually 40 to 60 drops per minute of the cooling/lubricating liquid (which most often is water) flowing out of the stuffing box are considered sufficient for the safe operation of the pump. The maintenance procedures for pumps must be guided - as they are with any other machine - by manufacturers' manuals, and experience, but a few indications may serve as a useful illustration of the work involved:
Most of the fans used in a milk plant are of much larger dimensions than the pumps and in addition their housings, ducts and blades are built out of relatively thin metal sheets. Because of this they are much more exposed to damage during shipment and installation. They require thorough inspection upon delivery, careful storage prior to installation and extra care in handling, particularly when lifted with a hoist. The installation of larger size fans means reassembling in the milk plant since they are usually disassembled by the manufacturer prior to shipment. The assembly has to be done with proper care being given to match the markings of the manufacturer. Exceptional importance should be attached to the correct direction of rotation. Many of the principles of a correct installation, care and maintenance of centrifugal pumps apply also to fans, particularly concerning alignment, foundations, lubrication of bearings, etc. However, there is a number of installation problems typical for fans only - one of them concerns vibration. Vibration-isolating bases are used in fan installations to reduce the transmission of sound and vibration from a fan to other areas of the plant. The isolating elements consist of resilient material and steel springs which deflect under the weight of the vibrating element and absorb structure-borne sounds and vibration. The isolating elements should support the rigid base on which the motor and the fan are mounted. They should be adequate in number and properly spaced. Flexible connections should be provided between air ducts and the fan. Large fans are usually belt driven and the efficiency of the drive depends on friction adjustable by the V-belt tension. Correct tension is attained when the belts just do not slip when operating at full speed and load. The initial run of a large-scale fan must start under reduced load: outlet dampers and variable inlet vanes should be partially - not completely - closed. Vibration can be measured by instruments indicating displacement: well-running fans will show displacement ranging from 0.05 mm for fans with low (600 rpm) rotation speed to 0.02 mm for those with high (1900 rpm) speed. The maintenance inspection procedures and inspection frequencies for fans are similar to those for centrifugal pumps: in addition isolation of the vibration bases and inspection of V-belts need to be included in the preventive maintenance programmes. Fans also need more devices to protect personnel from contact with rotating elements: protective screens, belt guards and coupling guards.
In many instances air compressors are assembled by the manufacturer as a unit together with the air tank. Since the highest permitted pressure in the tank is limited to a value declared by the manufacturer and in most countries it is also subject to inspection by legal authorities, the tank is provided with a safety valve and the compressor's operations are automatically controlled by an on-off regulator. The compressor is driven by an electric motor through a V-belt drive. Air filters are installed on the suction side of the compressor. A pressure unloader prevents the machine from starting with air pressure in the compressor's head. The crank case contains oil necessary for lubricating the main drive of the pistons. In many plants the compressed air should contain a very low level of moisture which requires an air cooling and water condensing device mounted between the compressor and the air tank. Moisture-free air is very difficult to obtain and for most purposes the reduction of the compressed air humidity serves the purpose. Even with air cooling installed, some water may condense in the air tank and therefore a drain valve at the bottom of the tank is necessary - preferably automatically operated. Oil separators are mounted on the compressed air pipe, but oil-free air used for milk agitation in large tanks is obtainable only from special type compressors in which the cylinder lubrication is achieved by application of self-lubricating rings. The inspection and maintenance of air compressors include:-
The electric motor is practically the exclusive prime-mover for powering all machines in a milk plant. The transmission of the power from the electric motor to the main shaft of the machine is performed by means of a drive. The most commonly used types of drives are gears, belts and chains. The acting part of the driven machine is sometimes mounted directly on the shaft of the electric motor, as in some of the most common types of milk pumps and in some designs of spraying discs in spray driers. Power can also be transmitted from a motor to a main shaft directly by means of couplings. Gears are most commonly used for driving dairy process equipment, belts - almost exclusively V-belts - are widely applied on auxiliary machinery like ammonia compressors, air compressors and fans, whereas chain drives are met in bottle and crate washers and in crate and package transportation equipment.
Those in most common use are plain spur, helical spur, bevel and worm and worm wheel gears. Plain spur gears transmit power between parallel shafts only. They are becoming less used in milk plant equipment, since helical gears are able to perform similar duties, are quieter in operation and have a higher transmitting capacity. Bevel gears transmit power between shafts meeting at an angle. They are called mitre gears when the angle is 90°. Worm and worm wheel gears provide a high reduction in speed; they operate very quietly and are commonly used as dairy machinery drives. They usually run in an oil bath enclosed in a gear box. Normally the drive can be transmitted only from the worm to the worm wheel.
As in any rotating part of a machine, correct alignment is the most crucial requisite of proper performance of a gear drive. It is of paramount importance that the gears mesh correctly. Excessive wear will occur very soon when the shafts of the gears are misaligned, or when the gears are too tight or too loose in mesh. In worm and worm wheel gears there is a heavy load on the worm shaft and adequate thrust bearings are incorporated in the design. They must be checked regularly for wear as should all gearbox bearings.
As with all other critical mechanical parts ball and roller bearings should be handled with greatest care to avoid mechanical abuse and corrosion damage. They should be constantly protected from all forms of dirt or foreign matter that might dent or wear the highly polished surfaces of the balls, rollers and races. Dirt causes 90 percent of early bearing failure. The services required on gears - including lubrication - are always described in manufacturers' manuals but local operating conditions may indicate additional sources of trouble and spots to which particular care needs to be given. Experience is a valuable source of information on how to maintain a gear at its optimal performance level.
These occupy an important place in the service section of a milk plant. The driving power of the motor can be transmitted by the belts when drive wheels (pulleys) are located on the shafts. V-belt pulleys are held in position by keys or by tapered locking bushes. Before installing belts the tension adjustment should be slackened completely, the driving surfaces of the pulley should be cleaned and the alignment of the pulley checked. Prior to installing a set of V-belts, the belts should be checked that they are a matched set. Standard V-belts are marked with a letter indicating their width across the tip and a number indicating their length. Widths are marked with letters A to E, lengths are stamped with a number which indicates the precisely measured length. A nominal pitch length will be marked, for example, (50), other figures (49, 51) will indicate the deviation from the nominal length. A matched set of V-belts is a set with the same stamped markings. Mismatched V-belts put on a drive will have a short life since the shorter belt will carry all the load whereas the longer ones will remain idle. Belts stretch on the drive, never shorten. Replacing V-belts one at a time (when wear is detected on one only) is a risky operation since the new one - being shorter - is likely to carry most of the load. For this reason it is always better to install a complete new matched set. The used belts can often be put on less demanding services in machines requiring a smaller number of V-belts.
Belts must be tensioned correctly to transfer the drive and prevent unnecessary wear. As they stretch in use, their tension must be regularly checked and adjusted. Testing the V-belt installation on tension is done when it is stopped or when running. The following are suggestions on how to do a simplified testing during routine maintenance inspection. When stopped, a correctly tensioned V-belt should, if pressed firmly with the thumb near the mid-point (half way from centre to centre), depress 3/4 of its own thickness for each one metre centre to centre distance. With belts running at full speed the sag on the slack side of the drive should be checked. Correctly tensioned belts will show a sag equal to the depression shown during stop testing, i.e. about 3/4 of its thickness for each one metre centre to centre distance. It should be noted that the adjustment of the tension of new belts needs checking and rechecking several times during the first 48 hours of operation since new V-belts stretch slightly and settle into the pulley grooves before reaching their working lengths.
These are subject to wear even when properly selected for installation, properly installed and adequately lubricated. They wear and stretch unevenly during use and checking chain wear means inspecting the complete length and all sprockets. Worn chains or sprockets will cause the chain to jump and possibly come off the sprockets which may mean damage to the machine. Proper alignment of sprockets minimizes chain wear but does not affect chain stretching which stresses the need for checking chain tension frequently and adjusting regularly. Chains need to be adjusted at the tightness tension as they stretch unevenly. The correct selection of the chain drive is given in manufacturers' manuals. Worn sprockets will also adversely affect the performance of a chain driven even with new chains. They should be replaced to ensure proper chain fit on the sprockets. In some cases the life of a worn sprocket may be extended by reversing it on the shaft to bring a new set of working tooth surfaces into use. Properly lubricated chains will not show discolouring at the joints and the connecting link pins will be brightly polished with a very high lustre. Frequent clearing of the chains and sprockets can greatly contribute to extending the life of a chain drive.
Any refrigeration system forms two circuits. In the primary circuit the refrigerant in the form of low-pressure vapour is compressed in a compressor and the compressed gas is liquified by cooling with air or water in a condenser, from where it passes to the liquid receiver. From the receiver the liquid passes under high pressure to a regulator or expansion valve where the pressure is reduced at its entry into the evaporator coils. In the coils the liquid evaporates by taking heat from the secondary circuit medium in which the coils are immersed. The refrigerant vapours return to the compressor.
The secondary circuit can be air in a cold store and brine or water in a container. In most modern milk plants there are usually two secondary circuits in which two sets of evaporator coils are installed: one for evaporation in the air of the cold stores and the second for evaporation in water used for cooling milk or liquid milk products in plate heat exchangers or in jacketed vats. In ice-cream plants, refrigerant evaporation takes place directly in a processing machine (freezer) but then the circuit is usually fed by a separate refrigeration system. There are numerous types of refrigerants, those in most common use are ammonia (R 717), R 12, R 22 and R 502 - the last three known under the trade names “Freon” or“Arcton”. They are supplied in cylinders marked with identification colours: R 717 in black cylinders with red and yellow bands, R 12 in grey cylinders with white markings, R 22 in grey cylinders with green markings and R 502 in grey cylinders with orchid markings.
Of the four refrigerants listed above ammonia (R 717) is extremely toxic and must be handled under strict controlled conditions. The remaining three are not toxic but they are gases heavier than air and any leak will tend to accumulate at ground level. In poorly ventilated rooms it may cause health hazards such as unconsciousness and suffocation. Before cylinders are returned for recharging they should be completely emptied by venting in an open space.
Refrigeration has become in the last few decades a very specialized subject in the engineering field. In many milk plants refrigeration specialists are employed to look after this essential part of a dairy enterprise. Their main responsibility is to maintain the performance of the refrigeration plant at its optimum level and to make it meet the respective requirements.
In many milk plants, even in those not manufacturing ice cream, the installed electric power of the refrigeration plant represents one third of the total electric power installed and often even more. The electric power consumption of the refrigeration equipment may come up to half of the total plant's consumption. Therefore the operation of the refrigeration equipment may greatly affect the total energy costs of the plant. A continuous performance checking of the refrigeration system is a requisite of the plant's good overall functioning.
The characteristics of the functioning of a refrigeration plant make it necessary to adjust pressures, flows, temperatures, etc. more frequently than in any other part of the milk plant. False readings caused by wear of the gauges and indicators lead to false adjustments. It is of paramount importance to make sure that all of them are checked for accuracy and kept in perfect mechanical condition. Thermostatic expansion valves require checking every month. If they open too wide, through wear or dirt, flooding may cause a frost back to the compressor. Gas leak to the bulbs may cause reduction of the flow to the evaporator and subsequent reduction of the overall capacity of the evaporator.
Cartridges in float controls need changing every three months. The float control valve is responsible for the refrigerant level in the evaporator. When closed too much, it reduces the capacity of the evaporator. If it is open too much the compressor may receive liquid on the suction side and get damaged.
Back -pressure regulating valves need checking every month. They are responsible for keeping the temperature of the gas within limits in the evaporator. If the temperature falls too low, frost accumulation on the blower units in the cold stores may occur leading to clogging of the units. Solenoid valves are used to stop the flow of either gas or liquid at different points in the system. Their functioning needs occasional checking. The trouble with the solenoid valves occurring most often is the burning of the coil: it should be warm when energized. Scale traps with screens ahead of all controls are important. They should be cleaned every six months. Only stainless steel screens should be allowed in ammonia circuits.
Thermostats, which automatically control the system, require checking on proper functioning by comparing the set and the true temperatures. Faulty thermostats can seldom be repaired and they usually have to be replaced with new ones. Relief valves need to be checked frequently on re-seating in order to avoid excessive loss of refrigerant.
Items requiring the major attention of maintenance staff in a refrigeration plant may be listed as follows:
Purging non-condensable gases, including air. The presence of these gases is checked by comparing the temperature of the vapour with its corresponding pressure, preferably near the expansion valve. Excessive pressure indicates the need for purging. The installation should be shut down during the check.
Draining oil traps on the refrigerant discharge side as frequently as indicated by experience.
Purging oil out of evaporators and receivers. Oil traps are not always sufficiently effective to prevent oil from the compressor travelling through the condenser pipes, the receiver and the evaporator coils. Oil reaching the evaporator may solidify and obstruct the flow of the refrigerant; in addition even the thinnest oil film on the heat exchange surface reduces the energy transfer.
Evaporative condensers are the most common in use in modern refrigeration systems in milk plants. They are usually located outdoors and there is a tendency for dirt accumulation and for growth of algae and fungi on the surfaces and in water basins. The unit needs cleaning with the frequency depending on local conditions under which the condenser operates. Fungicides and algicides are often added to the water basins which also help to keep the outer surfaces of the cooling pipes free of slime deposits.
Evaporator coils in cold stores need to be kept free of frost. Defrosting may be achieved by several methods of which passing hot compressed gas from the compressor's discharge side is the most common. Depending on the defrosting arrangements the frost accumulation needs frequent checking in order to keep heat exchange on the evaporator coils as effective as possible. Evaporator coils in brine basins seldom accumulate frost or ice, but in the ice-bank system the equipment is designed to accumulate ice on the evaporator coils. The heat transfer is reduced when ice grows on the pipes but up to a given thickness this reduction is allowed for in the design. The ice-water requirements of the plant vary in the course of the day and are usually nil during the night hours. This makes the thickness of the ice layer on the pipes grow. The plant should be shut down when the thickness of the ice layer reaches the highest calculated value. In many plants automatic switch-off devices are used; their efficiency needs frequent checking in order to operate the refrigeration plant at the lowest possible cost.
Compressor stuffing boxes need repacking at regular intervals. The condition of the compressor requires checking at least once a year. Refrigeration compressors are usually supplied with very comprehensive manuals, maintenance instructions and spare parts lists. They should be carefully studied and all instructions followed precisely. Refrigeration equipment - particularly that operating with ammonia as refrigerant - is long-lasting if well maintained, but any negligence in care and maintenance may lead to drastic accidents and to health hazards to the staff.
The check list on maintenance inspection schedule cards is particularly long for refrigeration equipment. This is also a long list of items for recording in the refrigeration log books. Their contents and mode of preparation are very often suggested by the manufacturers in their manuals. Additional assistance from a reputed refrigeration engineer may be of great help in establishing a preventive maintenance system for the refrigeration plant and will pay for its cost in the long run.
There is a great diversity of steam boilers utilized in milk plants. They differ in size from about 100 kg/h steam capacity to tens of metric tons per hour. They differ in the level of pressure of the produced steam, they use different fuels (coal, gas, oil, sometimes electricity) and they may be fully hand operated or fully automatic. The thermal efficiency of the boiler plant may be as low as 40 percent in small coal-fired plants and as high as above 90 percent in large-scale oil or gas-fired boilers. Depending on the type of processing equipment installed and on the layout of the milk plant, the condensate recovery may be nil but it may also exceed 80 percent of the total steam output.
Because of this diversity, it is not possible to draw up a universal standard maintenance programme for a steam raising installation. The operation of steam boilers is in most countries subject to official control of legal bodies which also set steam boiler regulations and standards. The diversity of boiler plants is taken into account in these standards and they are considered in the regulations accordingly. They are the first source of guidelines concerning care and maintenance of the plant. The second is the manufacturer's manual. The third could be the standard handbooks' advice on fuel burning, heat transfer in boiler plants, water treatment methods and requirements, feed water handling, etc. Experience of the plant in boiler operations is of course the last but not least important source of indications concerning plant care and maintenance.
Only few indications of general nature can be given as a list of first steps to be taken when establishing a preventive maintenance system in the steam boiler section. Periodical checking of the thermal efficiency of the plant will give the most crucial information on the overall performance of the boiler. The thermal efficiency may be considered as the ratio of energy obtained in steam to the energy supplied in the fuel and feed water. Estimating the efficiency requires data on the quantity of produced steam and on its parameters, the parameters of the feed water and the quantity and the caloric value of the fuel used. In large-scale installations instruments for recording most of the values needed for making the heat balance and for calculating efficiency are included. It is particularly easy to calculate the efficiency in boilers utilizing liquid fuels or gas since their calorific values once determined remain relatively constant for the given type of fuel. The situation is more complicated with coal which is difficult to weigh and for which the calorific value may change considerably from one supply to another. Besides, coal is normally stored in the open and is subjected to rain and snow which obviously change its moisture content and subsequently the calorific value. Milk plants seldom possess laboratory facilities for fuel analysis. Such analyses need to be done by specialized laboratories. The problems become even more difficult in small-scale plants in which the team quantity cannot be measured directly and indirect calculations may not give sufficiently accurate results. A common practice is to hire the services of a specialized engineering company or of an individual specialist in this field. The efficiency check should be repeated every year.
A useful indication - but indication only - on the overall performance of the boiler plant can be obtained by checking the composition and the temperature of the chimney gases. This can be done by the milk plant staff by means of relatively simple gas analysers and suitable thermometers. Excessive quantities of oxygen or carbon dioxide will indicate incorrect air supply; high temperatures may indicate scale accumulation on the heating surfaces.
Scale accumulation is one of the most commonly met reasons for poor boiler performance. It may reduce considerably the steam output and in extreme instances lead to damage of the boiler and be a health hazard to personnel. Scaling on the inner boiler surfaces is caused by improper quality of water which contains calcium, magnesium, iron and other salts. The quantity of these salts is expressed as water hardness. Hard water deposits scale very quickly reducing the heat transfer from the burning gases to boiling water. Feed water needs treatment prior to entering the boiler and the degree by which the original hardness must be reduced depends on the type of the boiler, although the ideal water for any boiler should be completely free of compounds causing scale deposits. There are many water treatment plants for boiler feed water. The most common in use are continuous ion-exchange water softeners. They are cheap in price and reliable in operations, but the resins need periodical regeneration and general care as instructed in the manuals. Gases dissolved in water such as oxygen and carbon dioxide should also be removed from feed water: suitable equipment is usually included as a component of the feed water treatment plants. It is a very good practice for the laboratory of the milk plant to analyse the feed water quality every day and inform the plant engineer on results. The analyses are very simple and can be carried out in a few minutes. They give essential information on boiler performance evaluation and for maintenance operations.
Feed water temperature should be as high as feasible in working installations. The amount of further heating with steam depends on the quantity of condensate collected from the processing sections of the plant and eventually from central heating and air conditioning installations. A very considerable energy saving can be made by collecting the maximum of available condensate: unfortunately this possibility is often neglected. All steam boilers require periodical internal inspection. Daily blowdown and periodic descaling are essential. The relevant instructions given in the manufacturers' manual must be followed strictly.
Steam distribution is a system which begins in the boilerhouse and continues through the entire plant. Saturated steam is most suitable for heating processes. Superheated steam is generally used for energizing steam prime movers such as turbines in electric power stations. However, saturated steam becomes superheated in the process of pressure reduction. For economic reasons the pressure in the boiler should be kept at the highest permissible level which in most plants not manufacturing milk powder is about 0.5 MPa to 1.0 Mpa. On the other hand the pressure permitted in most of the dairy machines does not exceed 0.2 MPa to 0.3 MPa, sometimes even lower. Steam pressure reducing valves are therefore installed at several points of the plant to ensure that steam reaches the machine at required pressure levels. The laws of thermo-dynamics must be followed when planning a pressure reduction and steam distribution system. One of the essential principles is to reduce the pressure step-wise when the ratio of high pressure to low pressure required after the reducing valve exceeds the value 1:83. In order to avoid supplying processing equipment with superheated steam (which might happen when the reduction valve is relatively close to the equipment receiving steam) the final length of the team supplying pipe should be left uninsulated. This system allows for cooling the superheated steam and saturating it prior to entering the equipment.
