Intro to Marine Engineering 2 2011 Part 7 potx

170 Refrigeration, air conditioning and ventilationinspection port tubjng Figure 9.5 Evaporator stores in which the fan and coil unit are one, and a larger version indirect expansion car

170 Refrigeration, air conditioning and ventilation

inspection port tubjng

Figure 9.5 Evaporator

stores in which the fan and coil unit are one, and a larger version indirect expansion cargo or air conditioning systems where the fan or fansmay be remote

A more elaborate design is used for secondary refrigerant coolingwhich takes the form of a shell and tube vessel Such a type is illustrated

in Figure 9.5 and employs direct expansion In this case the refrigerantpasses through the tubes and the secondary refrigerant is passed overthe tube bank The refrigerant is sprayed into the tubes so as to ensure

an even distribution through all the tubes Any oil present is not sprayedand drains away In this type of evaporator two features are employed toimprove heat transfer efficiency On the refrigerant side there is acentre tube with a spiral fin fitted around it (as illustrated) or the insertmay be in the form of an aluminium star which has a spiral twist on it.Also, baffles are arranged on the brine side to deflect the brine acrossthe tube bank

Refrigerant flow control valves

It is usual to have a solenoid valve in the liquid line prior to theexpansion valve or regulator This shuts or opens as determined by thethermostat in the space or the secondary refrigerant being cooled Itmay also be used to shut off various circuits in a cooler when themachine is operating on part-load conditions

The expansion valve/regulator is a more complex piece of equipmentwhich meters the flow of refrigerant from the high-pressure to thelow-pressure side of the system This may be of the thermostatic type, asshown in Figure 9.6 The bulb senses the temperature of the refrigerant

Figure 9.6 Therrnostatic expansion valve or regulator

at the outlet from the evaporator and opens or closes the valveaccordingly The design of the valve is critical and is related to thepressure difference between the delivery and expansion side There-fore, it is essential that the delivery pressure is maintained at or near themaximum design pressure Thus, if the vessel is operating in cold seawater temperatures it is necessary to re-circulate the cooling water tomaintain the correct delivery pressure from the condenser If this is notdone, the valve will 'hunt' and refrigerant liquid may be returned to thecompressor suction

Ancillary fittings

Delivery oil separators are essential for screw compressors, but for othersystems, depending on the design criteria and length of pipe run, theymay or may not be fitted

Refrigerant driers are essential with the Freon gases to remove waterfrom the system, otherwise freezing of the water can take place in theexpansion valve

A liquid receiver may be fitted for two reasons Firstly, to give asufficient reserve of refrigerant in the system to cater for variousoperating conditions (this is known as a back-up receiver) Secondly, forstorage of the refrigerant where it is required to pump over, i.e store,the charge for maintenance purposes In very small systems this pumpover can sometimes be achieved in the condenser

172 Refrigeration, air conditioning and ventilation

Cargo refrigeration

Refrigerated cargo vessels usually require a system which provides forvarious spaces to be cooled to different temperatures The arrange-ments adopted can be considered in three parts: the central primaryrefrigerating plant, the brine circulating system, and the air circulatingsystem for cooling the cargo in the hold

A central refrigerating plant is shown in Figure 9.7 The refrigerantflow through the chiller splits into four circuits, each with its ownexpansion valve The four circuits are used to control the amount ofevaporator surface, depending on the degree of condenser loading atthe time, thus giving greater system flexibility The large oil separator is

a feature of screw compressor plants and the circuit for oil return isshown in the illustration

Each primary refrigerant circuit has its own evaporator within thebrine chiller (as shown in Figure 9.7) which results in totallyindependent gas systems There will probably be three such systems on acargo or container ship installation Since they are totally independenteach system can be set to control the outlet brine at differenttemperatures Each brine temperature is identified by a colour and willhave its own circulating pump The cold brine is supplied to the cargospace air cooler and the flow of this brine is controlled by thetemperature of the air leaving the cooler

Figure 9.7 Central refrigerating plant

Refrigeration, air conditioning and ventilation 173The cooler in the cargo space is arranged for air circulation over it andthen through the cargo before returning An arrangement of fans andducting direct the air to the cooler and below the cargo (Figure 9.8) Thecargo is stacked on gratings which allow the passage of cooled air upthrough the cargo.

For small refrigerated cargo spaces or provision rooms a directexpansion primary refrigerant system may be used (Figure 9.9) The

Figure 9,8 Cargo space arrangements

Figure 9.9 Direct expansion system

174 Refrigeration, air conditioning and ventilation

twin circuit arrangement for each cooler (evaporator) provides flexibilityand duplication in the event of one system failing The back pressurevalve maintains a minimum constant pressure or temperature in theevaporator when working a space in high-temperature conditions toprevent under-cooling of the cargo If one space is operating at alow-temperature condition at the same time the back pressure valvewould be bypassed The liquid cooler illustrated in the diagram isnecessary where an abnormal high static head has to be overcomebetween the machinery and the coolers In this vessel the liquid issub-cooled to prevent it flashing off before reaching the thermostatkexpansion valve

Containers which require refrigeration present particular problems.Where only a few are carried or the ship has no built-in arrangement forrefrigerating containers, then clip-on or integral refrigeration plantswould be provided The clip-on or integral unit may be either air orwater cooled In the case of air cooled units adequate ventilation has to

be supplied if they are fitted below decks For water cooled units somesort of cooling water arrangement must be coupled up to each unit Also

an electrical supply is required for each type

Vessels designed for specific refrigerated container trades havebuilt-in ducting systems These can be in two forms: a horizontal fingerduct system in which up to 48 containers are fed from one coolersituated in the wings of the ship or, alternatively, a vertical duct system inwhich each stack of containers has its own duct and cooler This type ofsystem is employed for standard containers having two port holes in thewall opposite the loading doors Air is delivered into the bottom openingand, after passing through a plenum, rises through a floor grating overthe cargo and returns via another section of the plenum to the top port.The connection between the duct arid containers is made by couplingswhich are pneumatically controlled

Controlled atmosphere

Refrigerated cargo ships are making increased use of ControlledAtmosphere (CA), a technique which increases the storage life of fruitand vegetables Oxygen and carbon dioxide levels, and relative humidity,are independently controlled to within close tolerances within aparticular CA zone This slows down the ripening of fruit and vegetablesduring their transportation In a CA zone oxygen levels may be as low

as from 1 to 12 per cent, carbon dioxide from 0 to 25 per cent andrelative humidity is kept within 40 to 90 per cent

Refrigeration, air conditioning and ventilation 175

The chamber or zone must be airtight, and any leakage of gas isreplaced by injecting the required volume into the zone A low oxygenalarm and sampling points within the chamber protect the cargo fromsuffocation, which would occur if the oxygen level was less than 0.5 percent by volume

The CA chamber will not support human life and rules exist for theuse of CA, which ensure adequate safety precautions are taken prior toentry Locks and alarms are fitted to CA spaces, and if entry is requiredcomplete aeration must take place Ventilation outlets must be safely ledout into the atmosphere, well way from air conditioning units andaccommodation

Air in the system will also show as a high condenser pressure With thecondenser liquid outlet closed the refrigerant charge should be pumped

in and cooled Releasing the purge valve will vent off the air which willhave collected above the refrigerant

Under-charge will show as a low compressor pressure and largebubbles in the liquid line sightglass A leak test should then be carriedout over the system to determine the fault and enable its rectification Aleak detector lamp for Freon refrigerants may be of the methylatedspirit type, but more commonly uses Calor gas (butane/propane) TheFreon is drawn into the flame and the flame will change colour, goingfrom green to blue depending on the concentration of the gas

When charging the system with more gas the main liquid valve should

be closed and gas introduced before the regulating valve until the system

is correctly charged (It is possible to charge on the outlet side of theregulating valve and is quicker, but this requires a good amount ofexperience to prevent liquid carrying over and damaging thecompressor.)

Moisture in the system may change to ice and close up the regulatingvalve, resulting in a drop in pressure on the evaporator side and a rise inpressure on the condenser side The drier should be examined and thedrying chemicals will probably require replacing A correctly operatingregulating valve will have frosting on the outlet side but not on the inletside

176 Refrigeration, air conditioning and ventilation

Air conditioning

Ships travel the world and are therefore subject to various climaticconditions The crew of the ship must be provided with reasonableconditions in which to work regardless of the weather Temperaturealone is not a sufficient measure of conditions acceptable to the humanbody Relative humidity in conjunction with temperature more trulydetermines the environment for human comfort Relative humidity,expressed as a percentage, is the ratio of the water vapour pressure inthe air tested, to the saturated vapour pressure of air at the sametemperature The fact that less water can be absorbed as air is cooled andmore can be absorbed when it is heated is the major consideration in airconditioning system design Other factors are the nearness of heatsources, exposure to sunlight, sources of cold and the insulationprovided around the space

An air conditioning system aims to provide a comfortable workingenvironment regardless of outside conditions Satisfactory air treatmentmust involve a relatively 'closed' system where the air is circulated andreturned However, some air is 'consumed' by humans and somemachinery so there is a requirement for renewal Public rooms andaccommodation will operate with a reduced percentage of air renewalsince the conditioning cost of 100% renewal would be considerable.Galleys and sanitary spaces, for instance, must have 100% renewal, buthere the air quantities and treatment costs will be much smaller Systemsmay however be designed for 100% renewal of air although notnecessarily operated in this way Noise and vibration from equipmentused in the system should be kept to a minimum to avoid a differentkind of discomfort Three main types of marine air conditioning system

are in general use, the single duct, the twin duct and the single duct with reheat,

The single-duct system is widely used on cargo ships (Figure 9.10).Several central units are used to distribute conditioned air to a number

of cabins or spaces via a single pipe or duct In warm climates a mixture

of fresh and recirulated air is cooled and dehumidified (some water isremoved) during its passage over the refrigeration unit In cold climatesthe air mixture is warmed and humidified either by steam, hot water orelectric heating elements The temperature and humidity of the air iscontrolled automatically at the central unit Within the conditionedspace control is by variation of the volume flow of air

The twin-duct system provides increased flexibility and is mainly used

on passenger ships (Figure 9.11) A central unit is used with cooleddehumidified air provided through one duct The other duct is suppliedwith cooled air that has been reheated Each treated space is providedwith a supply from each duct which may be mixed as required at the

Refrigeration, air conditioning and ventilation 177

Figure 9.10 Single-duct system

Warmer air ducts

Fresh air

_£E

Cooler air ducts

1 2 3 4 "i

' ,fc12 Recircuiated air U13

11 Warmer air plenums

12 Cooler air plenums

13 Pre insulated spiro ducting

178 Refrigeration, air conditioning and ventilation

outlet terminal In cold climates the preheater will warm both supplies ofair, resulting in a warm and a hot supply to each space

The 'single duct with reheat' system is used for vessels operating inmainly cool climates The central unit will cool and dehumidify orpreheat and humidify the air as required by outside conditions Inaddition, before discharge into the treated space a local reheating unitwill heat the air if required, depending upon the room thermostatsetting

The refrigeration system used in the central unit is shown in Figure9.12 A direct-expansion system is shown using a reciprocating

Cooler Heater

D—#

Compressor

Figure 9.12 Direct-expansion refrigeration system for an air cooler

compressor, sea water cooled condenser and a thermostaticallycontrolled regulating valve The air to be cooled passes over theevaporator or cooler The cooling effect of the unit may need to bereduced if there is no great demand and the hot gas bypass systemprovides this facility

Maintenance of the above systems will involve the usual checks on therunning machinery and the cleaning of filters Air filters in the centralunits are usually washable but may be disposable The filters should beattended to as required, depending upon the location of the ship

Refrigeration, air conditioning and ventilation 179

Ventilation

Ventilation is the provision of a supply of fresh untreated air through aspace Natural ventilation occurs when changes in temperature or airdensity cause circulation in the space Mechanical or forced ventilationuses fans for a positive movement of large quantities of air

Natural ventilation is used for some small workshops and stores but isimpractical for working areas where machinery is present or a number

of people are employed

Forced ventilation may be used in cargo spaces where the movement

of air removes moisture or avoids condensation, removes odours orgases, etc

The machinery space presents another area which requires tion As a result of its large size and the fact that large volumes of air areconsumed a treatment plant would be extremely costly to run.Ventilation is therefore provided in sufficient quantities for machineryair consumption and also to effect cooling The usual distributionarrangement is shown in Figure 9.13

ventila-Fan room

Machinery

>space ' casing

Flat fan

Floor plates Tank top

Purifier extraction

Figure 9.13 Machinery space ventilation—diagrammatic

Several axial-flow fans provide air through ducting to the variousworking platforms The hot air rises in the centre and leaves throughlouvres or openings, usually in the funnel The machinery control room,

as a separate space, may well be arranged for air conditioning with anindividual unit which draws air through trunking from the outside andexhausts back to the atmosphere

The operations of mooring, cargo handling and anchor handling allinvolve controlled pulls or lifts using chain cables, wire or hemp ropes.The drive force and control arrangements adopted will influence theoperations Several methods are currently in use, and these will beexamined before considering the associated equipment.

Three forms of power are currently in use: steam, hydraulic and electric,

Each will be described in turn, together with its advantages anddisadvantages for particular duties or locations

Steam

With a steam powering and control system the steam pipelines are runalong the deck to the various machines Steam is admitted first to adirectional valve and then to the steam admission valve Double-actingsteam engines, usually with two cylinders, are used to drive themachinery Additional back pressure valves are used with mooringwinches to control tension when the machine is stalled or brought to astop by the load Arrangements must also be made, often associated withthe back pressure valve, to counteract the fluctuations in main steam linepressure as a result of other users of steam

The steam-powered system was widely used on tankers since itpresented no fire or explosion risk, but the lengths of deck pipeworkand the steam engines themselves presented considerable maintenancetasks which have generally resulted in their replacement by hydraulicallypowered equipment

180

Deck machinery and hull equipment 181

Hydraulic systems

The open-loop circuit takes oil from the tank and pumps it into thehydraulic motor A control valve is positioned in parallel with the motor.When it is open the motor is stationary; when it is throttled or closed themotor will operate The exhaust oil returns to the tank This method canprovide stepless control, i.e smooth changes in motor speed

The live-line circuit, on the contrary, maintains a high pressure fromwhich the control valve draws pressurised oil to the hydraulic motor (inseries with it), as and when required

In the closed-loop circuit the exhaust oil is returned direct to thepump suction Since the oil does not enter an open tank, the system isconsidered closed

Low-pressure systems use the open-loop circuit and are simple indesign as well as reliable The equipment is, however, large, inefficient

in operation and overheats after prolonged use

Medium-pressure systems are favoured for marine applications, usingeither the open or closed circuit Smaller installations are of theopen-loop type Where considerable amounts of hydraulic machineryare fitted the live-circuit, supplied by a centralised hydraulic powersystem, would be most economical

Electrical operation

Early installations used d.c supply with resistances in series to providespeed control (see Chapter 14) This inefficient power-wasting methodwas one possibility with d.c., but a better method was the use of WardLeonard control The high cost of all the equipment involved in WardLeonard control and its maintenance is, however, a considerabledisadvantage

Machines operated on an a.c supply require a means of speed controlwith either pole-changing or slip-ring motors being used Slip-ringmotors require low starting currents but waste power at less than fullspeed and require regular maintenance Pole-changing motors are ofsquirrel cage construction, providing for perhaps three different speeds.They require large starting currents, although maintenance is negligible(see Chapter 14)

Apart from the advantages and disadvantages for each of the driveand control methods, all electric drives have difficulty with heavycontinuous overloads Each system has its advocates and careful designand choice of associated equipment can provide a satisfactoryinstallation

S82 Deck machinery and hull equipment

Mooring equipment

Winches with various arrangements of barrels are the usual mooringequipment used on board ships A mooring winch is shown in Figure 10,1where the various parts can be identified The winch barrel or drum isused for hauling in or letting out the wires or ropes which will fasten theship to the shore The warp end is used when moving the ship usingropes or wires fastened to bollards ashore and wrapped around the warpend of the winch

The construction of a mooring winch will now be examined, againwith respect to Figure 10.1 The motor drive is passed through a spur

Driving

motor

Figure 10.1 Mooring winch

gear transmission, a clutch and thus to the drum and warp end Asubstantial frame supports the assembly and a band brake is used to holdthe drum when required The control arrangements for the drive motorpermit forward or reverse rotation together with a selection of speedsduring operation

Modern mooring winches are arranged as automatic self-tensioningunits The flow of the tides or changes in draught due to cargooperations may result in tensioning or slackening of mooring wires Toavoid constant attention to the mooring wires the automatic self-tensioning arrangement provides for paying out (releasing) or recover-ing wire when a pre-set tension is not present

Anchor handling equipment

The windlass is the usual anchor handling device where one machinemay be used to handle both anchors A more recent development,

184 Deck machinery and hull equipment

particularly on larger vessels, is the split windlass where one machine isused for each anchor

One unit of a split windlass is shown in Figure 10.2 The rotating unitsconsist of a cable lifter with shaped snugs to grip the anchor cable, amooring drum for paying out or letting go of mooring wires and a warpend for warping duties Each of these units may be separately engaged

or disengaged by means of a dog clutch, although the warp end is oftendriven in association with the mooring drum A spur gear assemblytransmits the motor drive to the shaft where the various dog clutchesenable the power take-off Separate band brakes are fitted to hold thecable lifter and the mooring drum when the power is switched off.The cable lifter unit, shown in Figure 10.2, is mounted so as to raiseand lower the cable from the spurling pipe, which is at the top andcentre of the chain or cable locker Details of the snugs used to grip thecable and of the band brake can be seen

Anchor capstans are used in some installations where the cable lifterrotates about a vertical axis Only the cable lifter unit is located on deck,the driving machinery being on the deck below A warping end or barrelmay be driven by the same unit and is positioned near the cable lifter

Cargo handling equipment

Cargo winches are used with the various derrick systems arranged forcargo handling The unit is rated according to the safe working load to

be lifted and usually has a double-speed provision when working at halfload

In the cargo winch, spur reduction gearing transfers the motor drive

to the barrel shaft A warp end may be Fitted for operating the derricktopping lift (the wire which adjusts the derrick height) Manuallyoperated band brakes may be fitted and the drive motor will have abrake arranged to fail-safe, i.e it will hold the load if power fails or themachine is stopped

A derrick rig, known as 'union purchase', is shown in Figure 10.3 Onederrick is positioned over the quayside and the other almost verticallyover the hold Topping wires fix the height of the derricks and stays tothe deck may be used to prevent fore and aft movement Cargo handlingwires run from two winches and join at the hook A combination ofmovements from the two winches enables lifting, transfer and lowering

of the cargo This is only one of several possible derrick arrangements orrigs Although being very popular for many years, it requiresconsiderable crew time to set up and considerable manpower foroperation