MARINE ENGINEERING Volume PRACTICE Part REFRIGERA_TING MACHINERY AND AIR CONDITIONING PLANT by J R STOTT, C.Eng., F.LMar.E THE INSTITUTE OF MARINE ENGINEERS Published by The Institute of Marine Engineers 80 Coleman Street London EC2R 5BJ Copyright © 1974 The Institute of Marine Engineers A Charity Registered in England and Wales Reg No 212992 Reprinted 1980 Reprinted 1981 Reprinted 1990 Reprinted 1997 Reprinted 1999 Reprinted 2000 Reprinted 2001 All rights reserved No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form of by any means, electronic, mechanical, photocopying, recording or otherwise, without the prior permission of the publisher Enquiries should be addressed to The Institute of Marine Engineers ISBN: 90007611 X Printed by Hobbs the Printers in the UK CONTENTS REFRIGERATION Page Principles of Refrigeration Refrigerants Laws of Heat Transfer 13 16 Main Components of Refrigerating Systems Ancillary Equipment 41 Operation of Primary Refrigerant Systems 44 Cooling Arrangements in Provision Rooms and Cargo Spaces 53 Instrumentation 64 Preparation of Cargo Spaces, Loading and Stowage 67 10 Cargo Carrying Procedures 71 11 Insulation Maintenance 75 12 Planned Maintenance 77 AIR CONDITIONING 13 Principles of Air Conditioning 81 14 Air Conditioning Systems 85 15 Ancillary Components 91 16 General Operation of the Installation 94 17 Routine Maintenance 96 ACKNOWLEDGEMENTS Grateful acknowledgement is made to the following companies for providing material for illustrations: Container Ship Product Division, Sterling Hydraulics Ltd.; Fig 36 Crane Packing Ltd.; Fig 10 Danfoss; Fig 25 Hall-Thermotank International Ltd.: Figs 8,9, 16, 17, 18, 19, 22, 24, 28, 29, 30, 44 and 45 Stal Refrigeration AB.; Figs 23 and 31 Temperature Ltd: Fig 43 The Tilley Lamp Co Ltd.: Fig 25 York Division of Borg-Warner Ltd.; Fig 15 PRINCIPLES OF REFRIGERATION 1.1 FUNDAMENTAL PRINCIPLES When a liquid evaporates a cooling effect is produced Common examples of this effect are well known For example, a few drops of volatile liquid, e.g ether or eau-decologne poured on to the hand gives a cold sensation, as it evaporates rapidly, taking heat out of the skin Evaporation of perspiration produces a similar, :hough less pronounced effect Milk bottles can be kept cool by wrapping them in a damp cloth; cooling is more effective if the bottles are placed in a draught to accclerate the rate of evaporation Refrigeration on board ship is always based on evaporating a liquid but under more controlled conditions than in the above examples To elaborate on the principles of evaporation, the distinction must be made between sensible heat and latent heat If heat is applied to a liquid in an open vessel well below its boiling point, the effect is to raise the temperature and the heat taken up by the liquid is known as sensible heat As more heat is applied, the rate of evaporation from the surface of the liquid increases and the temperature rises until the boiling point of the liquid is reached All heat supplied from this point onwards has no effect on the temperature of the liquid Instead, all heat goes to turn the liquid into vapour The heat absorbed by the liquid in turning into vapour is known as latent (or hidden) heat When the vapour is condensed back again to liquid, the same amount of latent heat is released Unfortunately, there are no obvious everyday examples of condensation of vapour where heat is being given up However, marine engineers will be aware that a steam condenser soon ceases to function if there is no continuous flow of cold water to take away the heat given up when the steam condenses A second natural law basic to refrigeration is that the boiling point of any liquid varies with pressure The temperature 100°C is the boiling point of water at normal atmospheric pressure If the pressure of a steam boiler is increased by weighting or screwing down a safety valve to prevent the escape of steam at atmospheric pressure, then the temperature steadily rises in direct proportion to the rise in pressure If one now imagines MARINE ENGINEERING PRACTICE this process being reversed, i.e the pressure being reduced to atmospheric in stages and a vacuum pump then used to further lower the pressure in the boiler below atmospheric, then the same law continues to apply: boiling occurs at lower and lower temperatures as the pressure is reduced To achieve useful refrigeration at a particular desired temperature is a matter of providing a suitably low pressure in order to make some liquid boil and take up latent heat at the temperature desired Different liquids, known as refrigerants are used according to the temperature required and the type of installation available for providing low pressures FIG I-Fundamental similarity bet wren refrigeratioll and Economic boiler evaporator To illustrate the above points, Fig compares sections through a brine "evaporator" in which Freon 22 is boiling, and an Economic boiler Such a relatively advanced item of refrigeration equipment is introduced at this stage in order to emphasize the importance of evaporation in refrigeration Another parallel between the boiler and an evaporator may be drawn If a boiler is "forced", Le attempts are made by increasing the fuel supply to exceed its design rating, then boiling becomes so vigorous with so much frothing at the surface that water is carried over in liquid form with the steam Likewise, if an evaporator is "forced" too much, liquid refrigerant froths over with the gas PRINCIPLES OF REFRIGERATION Returning to basic principles two further terms must be understood, viz sub-cooling and super-heating Consider a closed vessel in the form of a thin vertical tube as shown in Fig 2, and equipped with thermometers at A, Band C The vessel is partly filled with a liquid and the remainder filled with its vapour, no air being present If there is no heat being given to or taken away from this vessel by its surroundings, all three FIG 2-Vessel with pressure gauge and three thermometers thermometers will indicate the same temperature and the pressure gauge will read the pressure corresponding to the saturated vapour pressure of the liquid at this temperature Pressure gauges used in refrigerating systems are often calibrated both in units of pressure and in degrees of temperature The temperature given on the gauge opposite any pressure reading is the temperature at which the saturated vapour of the refrigerant (for which the gauge is calibrated) exerts this pressure In other words, the temperature scale on the gauge shown in the figure could be inscribed at each pressure by marking the corresponding reading of thermometers A, Band C Although one commonly reads a refrigerant pressure gauge as so many "degrees", its sensing element responds only to pressure not to temperature In an evaporator (or condenser) the "degrees" read from the gauge is the temperature at that part of the heat exchange surface where the liquid MARINE ENGINEERING PRACfICE is evaporating (or condensing), i.e where saturated vapour and liquid co-exist If then heat is applied at A, but not elsewhere, the gas surrounding A increases in temperature, but the pressure is unaffected and thermometer B reads exactly as before (Hot gas, being less dense than cooler gas, remains at the top of the tube.) The gas surrounding A is known as superheated and the superheat temperature is the difference between the reading at B and that at A Alternatively, if the bottom of the tube is immersed in colder surroundings, the liquid at point e is cooled without affecting the temperature reading or the pressure at B (The colder liquid being denser stays at the bottom of the vessel.) Liquid at point e is said to be sub-cooled One cannot have sub-cooled vapour or superheated liquid - if point A is cooled by removing heat at the top end, it immediately causes vapour to condense and the liquid falls, thus reducing both B, and eventually e, to the same lower temperature as A If heat is applied at e, warm liquid rises to increase the temperature at B, causing more evaporation at the surface and eventually increasing the reading of A 1.2 LIQUID NITROGEN REFRIGERATION Figure illustrates an elementary form of refrigeration that is applied to road vehicles and containers FIG 3-Liql/id nitrogcl! cooling fo, ,'clriclcs or comainers A pressure vessel which is filled with liquid nitrogen before refrigeration is required is connected to a spray pipe in the top of the insulated body of the vehicle The release of nitrogen through a valve is controlled by a thermostat At atmospheric presure nitrogen boils, at -195°C (- 320°F), and the nitrogen issuing from the spray pipe is very nearly at this temperature To maintain a vehicle at, say, -20ce only intermittent squirts of nitrogen are required This method of refrigeration is a "throwaway" PRINCIPLES OF REFRIGERATION system as no attempt is made to recover the nitrogen after it has evaporated The system is suitable for journeys measured in days rather than weeks, unless the nitrogen vessel can be replenished en route Gases other than nitrogen can be used but nitrogen is usually employed because of its price and its inertness, and hence its safety 1.3 THE VAPOUR COMPRESSION REFRIGERATION CYCLE Most marine refrigeration plants make use of the vapour compression refrigeration cycle As the refrigerants used are too expensive to be allowed to blow to waste, after the refrigerant has done its cooling job by evaporating in some form of evaporator, the gas is collected for reliquefaction This is accomplished by using a compressor to suck gas from the evaporator at low pressure and to deliver it as hot compressed gas to a condenser The work done on the gas by the compressor raises its temperature above that of the atmosphere (or sea water) so that either air or water at normal atmospheric temperature can be used as the cooling medium in the condenser To complete the circuit the liquid from the condenser passes through a regulator, or expansion valve, which controls the flow of liquid to the FIG 4-Diagrammatic illustration of the refrigaatioll cycle, etc evaporator (see Fig 4) The correct functioning of the expansion valve is of paramount importance The part of the circuit downstream from the expansion valve to the suction valve of the compressor is called the g MARINE ENGINbERING PRACTICE low pressure side of the system, and that from the compressor delivery valve to the upstream side of the expansion valve is the high pressure side Compressors are usually of the continuous-running, fixed-speed type and the correct functioning of the expansion valve is necessary to maintain the appropriate amounts of refrigerant in the high and low pressure sides For high efficiency the amounts of refrigerant must be correct so that, as shown in Fig 4, there is enough refrigerant in the condenser for the liquid refrigerant to be sub-cooled, and only enough refrigerant in the evaporator to ensure that there is some superheating of the gas This correct working of the cycle is obtained when the total charge of refrigerant in the system is correct, and its distribution between the low and high pressure sides being correctly maintained by the expansion valve Figure illustrates an evaporator being used to cool br-ine, but the refrigerant cycle is just the same if the evaporator is designed to cool air directly, i.e by blowing air over the surface of the evaporator rather than by circulating brine Typical temperature differences for correct operation of marine plants are: Condenser gauge above sea water goC (l5°F); Liquid sub-cooled by 6°C (lO°F); Superheat 3°C (5°F) (for carbon dioxide plants); 14°C (25°F) (for R12 and R22 plants) Apart from efficiency considerations, correct superheat is important for the mechanical well being of the compressor If there is no superheat, liquid may be drawn into the compressor and cause damage to valves If there is too much superheat, then the discharge temperature will be too high and cause the compressor to overheat PRINCIPLES OF AIR CONDITIONING 83 humidity-thus 29°e (84°F) does not feel too warm if the relative humidity is down to 40 per cent, but 22°e (72°F) will feel too cold unless the relative humidity is above 40 per cent As well as providing conditions within the comfort zone, ship air conditioning plants must be operated at such a level that there is no excessive temperature difference inside and outside Passengers using sun decks are liable to sense "cold shock" on re-entering the accommodation if the air conditioning temperature is too low Ten years ago, a limit of (9°F) for the dry bulb temperature drop was common, but over the years, this figure has tended to increase Reverting to air velocity-which as stated above, should be unimportant in well designed systems-it is found that passenger complaints of draughts are as common as complaints of uncomfortable temperatures A degree of air movement past the body, which is unnoticed under heating conditions, and which may even be welcome under natural venti lation conditions, is felt as an unpleasant draught when the air stream is cooler than the surroundings There is a design compromise involved here To achieve a given degree of cooling either large amounts of slightly cooled air, or smaller amounts soe 84 MARINE ENGINEERING PRACTICE of colder air can be supplied If the air supply is too cold, then there are excessive temperature differencies between areas of the room close to the air inlets and elsewhere; this is avoided by using higher rates of air circulation, but hopefully not so high as to cause draughts The correct design of air circulation systems, and in particular the type and siting of air inlet distributors, is the key to successful air conditioning The usual rate of air circulation in each room lies between and 12 air changes per hour 14 AIR CONDITIONING SYSTEMS Air conditioning systems fall into two main classes: individual unit systems, in which each room contains its own small refrigeration plant and fan and air cooler; and central systems, where larger refrigeration machinery units are installed and their output distributed about the ship by a variety of means The many possibilities are illustrated diagrammatically in Fig 42 Self-contained units are noisier than central systems, require more maintenance and have been found to have a relatively short life (about years.) The single duct system only allows for adjustment of temperature in each room by the occupant manually controlling the air volume admitted It is thus less flexible than any of the other systems which allow individual temperature control, at least of sections of the ship if not of individual rooms With ducted systems, the modern tendency is to use "high velocity" in the air ducts with fans generating up to 2550 mbar (250 mm H20 g) pressure compared to "low velocity" systems with fans generating about 520 mbar (50 mm H20 g) This tendency helps installation as the size of ducts is reduced and prefabricated standard ducts can be used, but it incurs the heavier running costs of more powerful fans Air terminals lined with sound insulation material are necessary to reduce the noise passing into the room with high velocity systems 14.1 SELF CONTAINED UNITS A typical marine pattern self-contained unit is shown in Fig 43 Air circulation is usually effected by means of a centrifugal fan, for quiet running, and a direct expansion cooler served by a hermetic compressor Water cooled condensers are used As these contain small water passages, choking develops rapidly with direct seawater circulation and a better method is to circulate with fresh water, itself cooled in a sea water/fresh water heat exchanger Control is on/off by a thermostat sensing the temperature of air returning to the unit 85 14.2 CINTRAL AIR CONDITIONING UNITS Figure 44 shows a type of air cooler, and its constructional details as litted about 10 years ago and Fig 45 shows its modern counterpart The cooling coil of the central unit may be of the direct expansion, brine or chilled water cooled type When cooling is by direct expansion, a separate steam heater coil is fitted in the unit for winter heating With brine or water coolers, a central he'ater is used so that the same coil serves for summer or winter Thermo- 15 ANCILLARY ]5.1 COMPONENTS THERMOSTATS All types of thermostats are found in air conditioning systems, direct acting, pneumatic and electrical In themselves, they are all satisfactory instruments, but the results they achieve are dependent on the correct siting of their sensing elements Even the site for a direct acting thermostat to control one single berth cabin must be chosen with care-if it is masked behind curtains, or too far away from the air inlet control will be too sluggish The correct location for a thermostat to control a block of cabins is more difficult to find One can pick on a "typical" cabin-but if the occupant opens his porthole he can upset the whole block Another possibility is to site the thermostat in the alleyway of the block of cabins This position may be affected more by an open door or draught in the alleyway than by the temperature of the cabins Yet another possibility is to site the thermostat in the recirculation air trunk, carrying air back from the accommodation to the unit If the recirculation grille is close to an outside door, this position too can be affected by outside air temperature when the door is open, rather than by cabin temperature The large public room, say a lounge used for dancing, may be impossible to control satisfactorily by one thermostat If one thermostat is placed, sayan a pillar, in the dancing area it will sense a temperature higher than the average in the room and cause air to be delivered which will be too cold for the comfort of those sitting around the edge of the room Similarly, a thermostat sited at the edge of the room may leave too high a temperature in the central area The only satisfactory arrangement for such rooms is to have different controls for different parts of the room ]5.2 FII.Tf:RS Filtering the air before it passes over the cooling coils is necessary to prevent atmospheric dirt, particularly funnel smuts, entering the accommodation and also to prevent the cooling coils, which have closely pitched fins becoming choked on the air side The protection of the coils calls for filtering of recirculated air as well as fresh air as the circulating air picks up fluff from carpets and blankets 91 92 MARINE ENGINEERING PRACTICE Filters are either washable or disposable A washable type still in use consists of brass wool retained in galvanized steel frames These are cleaned in boiling water and detergent and dipped in an oil bath before re-use Modern washable types are of foamed plastic, again requiring washing in hot water and detergent, but suitable for re-use without oiling after drying Disposable types of filters are glass or mineral wool (either in mattress form or corrugated sheet form) in cardboard frames, which fit into metal racks on the unit There is little to choose between the different types in terms of efficiency of dirt collecting or in life between cleaning or renewal Present 93 ANCILLARY COMPONENTS economic considerations and reduced manning on ships favour the disposable type 15.3 AIR DIFFOSERS To deliver cooled air into cabins the simplest method is a "punkah louvre" or grille blowing air out from a bulkhead just below ceiling level More uniform temperatures and freedom from draughts are obtained from a diffuser fitting mounted centrally in the ceiling These diffusers, shown in Fig 46 are designed to deliver the air in a blanket over the ceiling, and if this is coupled with extract of exhaust air at floor level a good distribution of air is obtained They are made in circular and rectangular types, the larger versions of which are also used for public rooms The rectangular type, if sited close to a bulkhead, can cause discomfort to anyone sitting close to the bulkhead, as shown in Fig 47 Circular diffusers - FIG 47-Rectangular ceiling diffuser too close to bulkhead causes draughts are not so bad in this respect, but with both circular and rectangular types it is sometimes necessary to fit a blanking baffle over a section of the periphery to prevent draughts in a particular direction 15.4 THERMOMETERS On passenger ships it is not advisable to have either thermometers or thermostats with calibrated scales mounted in cabins They provide too ready a topic for conversation and perhaps unfavourable comparisons between one cabin and another 16 GENERAL OPERATION OF THE INSTALLATION For an installation with a central refrigerating plant, the operation of the compressors, condensers and associated controls is generally for cargo plants as previously described The first essential in operating the air cooling appliances throughout the ship is to have all thermostats correctly set and correctly functioning In extreme weather conditions, either hot or cold, control of the plant usually presents few difficulties The capacity of many installations is such that under tropical conditions nearly all control valves move to the fully open position; although automated control has been lost, internal conditions are by and large acceptable Control difficulties arise in intermediate weather conditions when there is a call for only a small amount of cooling The worst case is when part of the ship, say inboard cabins against the engine room, require cooling and other parts, say exposed upper cabins, require warming Only well zoned systems, or those with a good reheat installation can cope with this condition For this intermediate condition, thermostats must be correctly set by trial and error It is found that a uniform setting of say 21°e (71°F) throughout the ship is not satisfactory, but slight variations of a few degrees up or down are needed to suit particular regions of the ship Unfortunately, these variations in thermostat setting are not always the same for the cooling and heating condition and frequent resetting may be needed for a ship repeatedly passing from cold to warm weather The control problem is eased if the chilled brine (or water) of systems using chilled liquid circulation is held at about 13°e (S5°F) in the intermediate weather conditions and lowered progressively to about soe (41°F) as tropical weather conditions are approached If a thermostat is found to be unduly influenced by opening doors, or some other local effect, it may well be advantageous to experiment in resit ing it When air cooling is in use it is good practice to keep all portholes windows and doors shut On passenger ships, some public announcement requesting that this be done is worthwhile However, insistence on compliance with the request should not be too emphatic Certainly a port open which happens to be adjacent to a thermostat can throw temperatures out over all the region controIled by the thermostat but most ports are 94 GENERAL OPERATION OF THE INSTALLATION 95 not adjacent to thermostats and the individual passenger who wishes to open a port may be allowed to so It should not have a large effect on a well designed system Good staff communications have already been mentioned in connexion with cargo storage The communication channel which must be quick and definite on passenger ships is from bedroom stewards to the engineer in charge of air conditioning On no account should a bedroom steward be allowed to fob off a passenger complaint instead of reporting it In addition to attending promptly to any passenger complaints, a wise precaution for an engineer to take is to go through accommodation and public rooms periodically recording wet and dry bulb temperatures Keeping a log of these readings then serves to identify any malfunctioning of the installation as soon as it arises, and if all is in order it exists as evidence to refute any unjustified complaints The quantity of cooled air delivered by an air conditioning unit should balance the sum of the quantity of air recirculated to the unit and the quantity mechanically exhausted If this balance is not correct in a public room there will be a draught whenever the door is open (and through any gap in the door fitting when it is shut) An exception to this rule is the dining room which should be over supplied so that there is a flow of air from the dining room to the galley This flow must be affected by the galIey exhaust sucking the air out of the dining room-not by over pressurizing the dining room as this would spread food smells out of the room into the accommodation The correct balance between supply and exhaust fans should be checked periodically Even with filters fitted ducts can become partially blocked and fan performance can fall off to upset the balance On older ships it may be that the capacity of the cooling installation is not adequate to provide the temperature drop which is now required If this is the case thermostats should nevertheless be set to aim at keeping as steady a temperature as possible The practice of cooling down public rooms before they are occupied to have a little "cold in hand" can cause cold shock to people first entering the room, and com· plaints when the temperature subsequently rises A steady, albeit higher temperature is to be preferred On older ships, temperature maintenance can be made easier by increasing the ratio of recirculated to fresh air Most air conditioning units have dampers for adjusting this ratio and the effect of these can be extended after they have reached full travel by partially blocking fresh air inlets Care must be taken not to reduce the fresh air so that stuffiness or smells arise 17 ROUTINE MAINTENANCE 17.1 SELF CONTAINED UNITS Both air filters and filters in the condenser circulating water must be cleaned regularly These units sometimes develop annoying rattles which may be noticeable particularly when the compressor starts or stops Worn resilient mounts for compressors may be the cause, or pipes may require additional clipping 17.2 CENTRAL AIR CONDITIONING UNITS Cleaning or renewal of filters is necessary at about 3-monthly intervals, the time varying according to location on the ship Disposable filters can be vacuum cleaned so that in fact two or three "lives" are obtained before they need to be thrown away The use of a simple gauge to measure the pressure drop across filters is a good guide as to when replacement is necessary Clean filters are designed for about 30 mbar (3 mm H20 g) ROUTINE MAINTENANCE 97 pressure drop and need changing when this increases to 100-150 mbar (10-15 mm H20 g) A suitable pressure gauge for this purpose is the type shown in Fig 48, in which a small light weight ball rises vertically in a calibrated plastic tube These gauges can also be used for quick approxim:J.te measurements of air velocity If filters are choked and the unit is run without them for expediency, the resultant accumulation of dirt on the cooling coils will require more effort to remove it than would filter replacement in the first place In addition to normal mechanical attentions, such as lubrication of bearings, and adjustment of fan belts and cleaning of motors, careful greasing of linkages of automatic controls is necessary Automatic damper activators and brine valve activators are always subject to condensation and corrosion eventually renders them inoperative unless they are kept well greased Some air conditioning units, of the type shown in Fig 45, have internal unlined foam insulation which with age becomes friable This should be inspected and removed before it disintegrates and passes into the air ducts Cooled air ducts should be examined to see that the insulation vapour seal remains in good order If a plastic film vapour seal becomes damaged, or sealing tapes become removed, condensation forms within the film As well as making the insulation wet and ineffective, the condensation may become serious enough to cause drips and damp patches on ceilings ... Procedures 71 11 Insulation Maintenance 75 12 Planned Maintenance 77 AIR CONDITIONING 13 Principles of Air Conditioning 81 14 Air Conditioning Systems 85 15 Ancillary Components 91 16 General... Ltd.; Fig 10 Danfoss; Fig 25 Hall-Thermotank International Ltd.: Figs 8,9, 16 , 17 , 18 , 19 , 22, 24, 28, 29, 30, 44 and 45 Stal Refrigeration AB.; Figs 23 and 31 Temperature Ltd: Fig 43 The Tilley... of Marine Engineers 80 Coleman Street London EC2R 5BJ Copyright © 19 74 The Institute of Marine Engineers A Charity Registered in England and Wales Reg No 212 992 Reprinted 19 80 Reprinted 19 81 Reprinted