Marine vol 1 part 4

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.. Onecannot have sub-

MARINE ENGINEERING PRACTICE

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

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: 0 90007611 X

Printed by Hobbs the Printers in the UK

and Cargo Spaces

9 Preparation of Cargo Spaces, Loading and Stowage 67

AIR CONDITIONING

Grateful acknowledgement is made to the following companies for viding material for illustrations:

pro-Container Ship Product Division, Sterling Hydraulics Ltd.; Fig 36 Crane Packing Ltd.; Fig 10.

The Tilley Lamp Co Ltd.: Fig 25.

York Division of Borg-Warner Ltd.; Fig 15.

eau-de-Evaporation of perspiration produces a similar, :hough less nounced effect.

pro-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

3

4 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 inthe 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 amatter of providing a suitably low pressure in order to make someliquid boil and take up latent heat at the temperature desired Differentliquids, known as refrigerants. are used according to the temperaturerequired and the type of installation available for providing low pressures

FIG. I-Fundamental similarity bet wren refrigeratioll evaporator

and Economic boiler.

To illustrate the above points, Fig 1 compares sections through abrine "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 inrefrigeration Another parallel between the boiler and an evaporator may

be drawn If a boiler is "forced", Le attempts are made by increasingthe fuel supply to exceed its design rating, then boiling becomes sovigorous 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 5Returning 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 remainderfilled with its vapour, no air being present If there is no heat beinggiven 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 gaugewill read the pressure corresponding to the saturated vapour pressure ofthe liquid at this temperature

Pressure gauges used in refrigerating systems are often calibratedboth in units of pressure and in degrees of temperature The temperaturegiven on the gauge opposite any pressure reading is the temperature atwhich the saturated vapour of the refrigerant (for which the gauge iscalibrated) exerts this pressure In other words, the temperature scale onthe gauge shown in the figure could be inscribed at each pressure bymarking the corresponding reading of thermometers A, Band C Althoughone commonly reads a refrigerant pressure gauge as so many "degrees",its sensing element responds only to pressure not to temperature In anevaporator (or condenser) the "degrees" read from the gauge is thetemperature at that part of the heat exchange surface where the liquid

6 MARINE ENGINEERING PRACfICE

is evaporating (or condensing), i.e where saturated vapour and liquidco-exist

If then heat is applied at A, but not elsewhere, the gas surrounding

A increases in temperature, but the pressure is unaffected and meter B reads exactly as before (Hot gas, being less dense than coolergas, remains at the top of the tube.) The gas surrounding A is known as

thermo-superheated and the superheat temperature is the difference between thereading at B and that at A

Alternatively, if the bottom of the tube is immersed in colder roundings, the liquid at point e is cooled without affecting the tempera-ture reading or the pressure at B (The colder liquid being denser stays at thebottom of the vessel.) Liquid at point e is said to be sub-cooled. Onecannot have sub-cooled vapour or superheated liquid - if point A iscooled by removing heat at the top end, it immediately causes vapour

sur-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, warmliquid 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 3 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 tion is required is connected to a spray pipe in the top of the insulated body

refrigera-of the vehicle The release refrigera-of nitrogen through a valve is controlled by athermostat At atmospheric presure nitrogen boils, at -195°C (- 320°F),and the nitrogen issuing from the spray pipe is very nearly at this temp-erature To maintain a vehicle at, say, -20ce only intermittent squirts

of nitrogen are required This method of refrigeration is a "throwaway"

PRINCIPLES OF REFRIGERATION 7

system as no attempt is made to recover the nitrogen after it has porated The system is suitable for journeys measured in days ratherthan weeks, unless the nitrogen vessel can be replenished en route Gasesother than nitrogen can be used but nitrogen is usually employed because

eva-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 compressionrefrigeration cycle As the refrigerants used are too expensive to beallowed to blow to waste, after the refrigerant has done its cooling job

by evaporating in some form of evaporator, the gas is collected forreliquefaction This is accomplished by using a compressor to suck gasfrom the evaporator at low pressure and to deliver it as hot compressedgas to a condenser The work done on the gas by the compressor raisesits temperature above that of the atmosphere (or sea water) so that eitherair or water at normal atmospheric temperature can be used as thecooling 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 fromthe 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 andthe correct functioning of the expansion valve is necessary to maintainthe appropriate amounts of refrigerant in the high and low pressuresides For high efficiency the amounts of refrigerant must be correct sothat, as shown in Fig 4, there is enough refrigerant in the condenserfor 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 ofrefrigerant in the system is correct, and its distribution between the lowand high pressure sides being correctly maintained by the expansionvalve

Figure 4 illustrates an evaporator being used to cool br-ine, but therefrigerant cycle is just the same if the evaporator is designed to coolair directly, i.e by blowing air over the surface of the evaporator ratherthan by circulating brine

Typical temperature differences for correct operation of marine plantsare:

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 importantfor 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 toohigh and cause the compressor to overheat

2) The volume of vapour that has to be pumped around the circuit for a given cooling effect must be low;

3) The working pressure on the high pressure side must be low enough to keep down the mechanical strength required in the compressor, pipework and condenser;

4) The working pressure on the low pressure side must not be too low as pressures below atmospheric result in air being drawn into a system through any minute leaks This air carries water vapour into the system which freezes out and causes chokes; 5) The refrigerant must be chemically inert and non-corrosive to any material used in the structure of the plant;

6) The refrigerant must be non-toxic, non-explosive, and inflammable;

non-7) Its solubility in oil and the miscibility of the liquid refrigerant and oil must be such that compressor lubrication can be satis- factory;

R) It should be low in cost and readily available throughout the world.

Not all of the above desirable properties are found in one refrigerant suitable for all types of plant Depending on the size of plant and the temperatures required one or other of the desirable properties becomes paramount and points to the use of a particular refrigerant.

9

The numbered refrigerants listed above such as Refrigerant 12 (or R12) are also known by various trade names, e.g Freon 12 etc As will

be seen from the chemical formulae those quoted are halogenated carbons i.e compounds of carbon and hydrogen in which some of the hydrogen has been replaced by chlorine or fluorine The code of numbers

hydro-is internationally agreed to mean the following:

R followed by a two digit number denotes a refrigerant derived from methane (CHI) the first digit being the number of hydro- gen atoms -'-) the second digit being the number of fluorine atoms:

R followed by a 3 digit number (the first number being ")") denotes a refrigerant derived from ethane (C"H,) the second digit being the number of hydrogen atoms +1 the third digit being the number of fluorine atoms:

REFRIGERANTS 11

R followed by a 3 digit number, the first number being "5" denotes a mixture made from other halogenated hydrocarbons (It is perhaps a surprising quirk of chemistry that these refrigerants, whose vital features are their chemical inertness and safety, are formed from a combination of halogen gases, themselves dangerous and very reactive, with hydrocarbons which are also explosive and inflammable.) For convenience for the rest of this chapter the word Freon will be used

as this is more readily understood than the correct "halogenated carbons "

2.2.2 Ammonia

Ammonia has never been a popular refrigerant for marine use owing

to its irritant and toxic properties in the event of a leak However, it does have cost advantages for large installations operating at low temperatures, e.g fish factory vessels Any engineer posted to an ammonia plant is advised to make his first job a check that all pressure relief devices (bursting disc or pressure relief valves) are correctly installed and piped

to atmosphere so that in the event of their relieving an excess pressure there will be no escape of gas into the machinery space.

2.2.3 Refrigerant 12

Historically, Refrigerant 12 was the first of the halogenated carbon refrigerants to become widely available at reasonable costs It ousted carbon dioxide in the marine field as it permitted the use of lower pressure systems and simpler compressors Its behaviour with oil facilitates lubrication With the use of lower temperatures it has the disadvantage that evaporator pressures fall below atmospheric.

12 MARINE ENGINEERING PRACTICE

2.2.5 Refrigerant 11

Table I shows that this is a very low pressure refrigerant, and largevolumes of it have to be circulated for a given duty These propertiesmake it particularly suitable for large air conditioning installations, whereonly modest temperature drops are required and centrifugal compressorscan be used Its high coefficient of performance gives a significant saving

in horse power for large installations

2.2.6 Refrigerant 502

Refrigerant 502 was introduced commercially in the early 1960'sand is particularly suitable for so-called "hermetic" compressors i.e.compressors in which the reciprocating compressor and its motor arecontained within a gastight shell This construction eliminates rotatingshafts penetrating the compressor casing and reduces the risk of leaks.However, it does mean that the refrigerant vapour is drawn over theelectrical windings of the motor, and in this respect Refrigerant 502 hasthe advantage that it attacks the electrical insulating materials far lessthan other refrigerants Also, for a given refrigerating capacity, a largerweight of vapour passes through the motor and the temperature riseduring compression is less Both of these factors help to reduce tempera-tures within the hermetic shell

3 LAWS OF HEAT TRANSFER

The basic laws of heat transfer can be a great help in operating arefrigeration plant sucessfully in adjusting it for most efficient workingand in diagnosis of faults that may arise

Application of scientific method and studying the plant operation

in terms of the basic laws of heat transfer will often resolve difficultiesmore effectively than searching through manufacturers instruction books

In this respect refrigeration engineering differs from some other branches

of marine engineering in that temperatures in refrigeration installationsalways react slowly to varying conditions and there is always time to

THINK before taking action

Heat is transmitted from a warmer to a colder body by threemethods:

1) rariiation;

2) conduction;

3) convection

3.1 RADIATION

The transfer of heat from a warm surface to a cooler surface across

an intervening space Heat is radiated from surface to surface with theintervening material playing no part - radiation transfer can occuracross a vacuum

At the temperatures prevailing in refrigeration plants, radiation dom plays any significant part The one instance where this form ofheat transfer is important is solar radiation Direct sunlight can raisetemperatures to 60°C (140°F.)

sel-3.2 CONDUCTION

The transfer of heat within a material from a place at high ture to one at a lower temperature without there being any bodily move-ment of material The rate at which heat is conducted through differentmaterials subjected to the same temperature gradients varies enormously.Metals are the best conductors of heat, most vegetable substances arepoor conductors and gases are the worst conductors

tempera-13

Some consideration of the practical application of the above equation

H small, e.g if insulating the walls of a refrigerated space one chooses

a material with a lowK value and makes its thickness /, large

LAWS OF HEAT TRANSFER 15 Equation (1) can also be used as a guide to investigate, say, the performance of a condenser that is not up to design standard It could

be that some tubes are blocked, i.e part of the area A is not available.

It could also be that a film of oil has built up on the refrigerant side

of the metal, or a film of solid deposit on the water side of the metal, both of which increase the effective thickness, I, and reduce the average

value of K.

3.3 CONVECTION

Natural convection is the transfer of heat from one part of a fluid to another by a bodily movement of the warm fluid caused by buoyancy effects - warm fluid is less dense and therefore rises.

Forced convection, where the bodily movements of the fluid are increased by the use of pumps or fans, transfers heat more rapidly than natural convection The term forced convection is also applied to heat transfer from a solid surface to a fluid circulated over it and the heat transfer is ,given by

C is roughly proportional to the velocity of the fluid over the surface Thus, to improve the performance of a heat exchanger using forced convection one has the choice of either increasing the velocity, or of increasing the temperature difference (t 1 -t 2).

In many heat exchangers the heat is transferred from a fluid to a metal surface by forced convection, through the metal by conductance, and then to another fluid by forced convection To diagnose faulty perfor- mance all three steps in the heat transfer should be considered in the light of the equations (1) and (2).

4 MAIN COMPONENTS OF REFRIGERATION SYSTEMS

4.1 COMPRESSORS

4.1.1 CO2 Compressors

The type of CO2 compressor finally evolved before this refrigerantwas superseded consisted of twin horizontal cylinders, driven by variablespeed d.c electric motor It is the only type of CO2 compressor stilIlikely to be met at sea today

Cylinders are constructed from blocks of forged steel, to withstandthe high pressures involved, and the crankcase is open to the atmosphere.The one peculiarity of this type of machine, and which needs regularinspection, is the gland on the piston rod which prevents escape ofrefrigerant

MAIN COMPONENTS OF RHRIGERATION SYSTEMS 17 This gland is shown in Fig 7 together with the pump provided for its lubrication If the piston rod becomes worn or scored or if the gland packing is too loose excessive oil consumption and loss of refrige- rant occur Oil is reclaimed both that which escapes out of the gland

to atmosphere at the machine and from oil separators on the discharge side of the compressor This oil is filtered and re-used Maintaining rods and glands in working order is good practice since this minimizes the time spent in hand pumping and reclaiming oil.

Glands cannot be repacked by first fuIly assembling them and then tightening up Each soft metal ring must be compressed in turn as it is inserted using bushes as necessary and using a I m (3ft) tube over the spanner for the rings first inserted before the lantern bush Rings after the lantern bush should be slightly less tight.

A good habit for a watch keeping refrigerating engineer to form is

to check the temperature of the compressor rods soon after taking over

a watch Particular care is needed with a newly repacked gland which requires adjusting as it beds down and must be slackened back if over- heating commences.

Table lIT shows the discharge temperatures which have been found suitable with these machines.

4.1.2 Modern Reciprocating Compressors

The most common compressor at sea today is the reciprocating pressor for use with Freon Designs cover a power range of I to 200kW

com-(I to 300hp) the small machines being vertical in line and larger ones with cylinder in V or W formation A typical modern compressor is shown in Figs 8 and 9 Most manufacturers use an iron casting for the

18 MARINE ENGINEERING PRACTICE

crankcase and main carcase of the machine, although one manufactureremploys an all-welding technique which eliminates the use of castings.The general design is now well proven and generally trouble free.Such developments as there have been in recent years have been directed

FIG. 8-Vee block six cylinder compressor viewed from back end cover.

at increasing rotational speeds to give smaller and cheaper machines Withthis increase in rotational speeds piston strokes have fallen to maintainreasonable piston speeds Simplified valves of various reed types havebeen introduced - again with lower costs in mind

Apart from hermetic compressors which are referred to below, oneend of the crankshaft has to project through the crankcase in order toreceive the motor drive, either direct coupled or belt The sealing of thisshaft presents a greater problem than the usual "shaft seal" encountered

on other mechanical items because:

a) crankcases are subject to suction gas pressure which causes apressure difference across the seal;

MAIN COMPONENTS OF REFRIGERATION SYSTEMS 19

b) the refrigerants are what are known as "searching liquids", i.e.ones which will escape through minute clearances This is asso-ciated with their being very good cleansing agents - theirchemical formulations are of similar type to the familiar cleanser,carbon tetrachloride;

c) a very high standard of tightness is required to prevent eitherloss of refrigerant or air entering the refrigeration system

FIG. 9-Vee block four cylinder compressor viewed from drive end.

A typical gland used is the rotating seal and is illustrated in Fig 10

The essential components are a carbon ring, stationary and sealed to the

gland cover, with a flat surface against which is pressed the rotating face

MAIN COMPONENTS OF REFRIGERATION SYSTEMS 21

of a cast iron ring secured to the shaft Oil lubrication is maintainedbetween the rubbing faces

Either oil or water cooling is applied to the gland on larger machines

In earlier machines sight glasses were common in the cooling liquid line.but these are nowadays often dispensed with The gland of this type

of compressor should be examined daily - the sight glass if fitted, beingexamined and the gland cover checked for overheating

Scrupulous cleanliness is essential when fitting spare glands

With multicylinder compressors it is possible for one valve to bedefective or even completely broken without any very obvious differencesbeing evident in gauge readings or output of the machine Routine checks

of valve cover temperatures, by touch (cautiously as temperatures may

be uncomfortably high) are advised as local overheating in way of adefective valve can usually be detected in this way

With the introduction of a.c to ships electrical systems in place ofd.c compressors are usually arranged for single speed operation Toallow capacity reduction cylinder unloading gear which allows suctionvalves to be held open has been introduced (This is a "swings and round-abouts" situation where the simplicity of the electrical installation for thewhole ship has been achieved at the price of complications to themechanical system of the refrigeration machinery) Although cylinderunloading gear is designed for continuous operation, its use causes arise in temperature of the heads of the non-working cylinders and upsetsthe balance of the machine Accordingly on ships with a number ofcompressors it is advisable to use as many compressors as possible fullyloaded, and only to use the unloading gear to adjust total output on onecompressor

Mechanical linkages of unloading gear have been known to "stick"and correct operation cannot always be taken for granted On strippingcompressors for overhaul or survey it is easy to lose the correct sequence

of unloading cylinders and care should be taken to number or markparts and leads to ensure their correct replacement

With direct coupled compressors various proprietory couplings areused for most of which accurate alignment of motor and compressorshafts is needed

Correct alignment should be checked after any overhaul Periodicalchecks on the tightness of holding down bolts should be made as anyslackening off can lead to vibration and coupling troubles

It is usual for compressors to contain safety cut-outs to guardagainst excessive high pressure on the high pressure side of the system,excessive low pressure on the low pressure side or failure of oil pressure.They should be checked for correct functioning at least at the start ofeach voyage

A "semi-hermetic" compressor is illustrated in Fig 11 The pressor end of the unit is basically the same as a V or W compressor

com-22 MARINE ENGINEERING PRACTICE

driven by a separate motor To avoid the use of a gland seal, an a.c induction motor is arranged within the shell, all gastight with the com- pressor casing With this arrangement the mechanical end of the unit can

be stripped for overhaul in the usual manner.

A weakness of the design is that if a motor burns out, contaminated oil and refrigerant can be circulated round the system before the machine stops To guard against this, it is usual to provide motor protection in the form of a temperature sensor mounted in the motor windings.

Oil pressure cutouts are essential and should be checked periodically Semi-hermetic compressors for use at sea are seldom of more than 22kW (30hp) and a complete unit is carried as spare In the event of a unit failing, special large filters - known as burn-out filters - are fitted

on the compressor suction and kept in place until contamination from the burn-out is removed After a burn-out all oil must be removed and replaced as far as practicable, and it may be necessary to break open pipe-work for cleaning.

For small power compressors, fractional horsepower up to about 2-3kW (3hp), the most reliable unit available is the fully hermetic unit -

a typical design being illustrated in Fig 12 These units are assembled

in specially clean workshops with very small tolerances for working

24 MARINE ENGINEERING PRACTICE

clearances If such units break down at sea there is seldom any point inopening the shell; a complete new unit should be fitted Manufacturersusually guarantee this type of unit for 5 years Burn-out proceduresapply as with semi-hermetic compressors Both hermetic and semi-hermetic units are very reliable in themselves, but in view of the burn-outpossibility, which does not present a major problem with other types ofcompressor, particular attention should be paid to the rest of the refrige-ration circuit components to prevent moisture entering the system or oilbeing lost Either possibility may precipitate motor failure

Figures 13 and 14 show the rotors of a screw compressor, theessentials being a male rotor with 4 lobes which meshes with a femalerotor with 6 lobes It is difficult to explain the workings of the compressor

from a drawing - even transparent plastic models fail to fully clarify

it To follow the gas path through the compressor one may start at theinlet port; as a passage between the lobes of the female rotor pass thisport a "gulp" of gas is drawn in As the rotor continues to turn, a lobe

of the male rotor progressively fills up the space which is available

MAIN COMPONENTS OF REFRIGERATION SYSTEMS 25

for gas between the female lobes, the gas is forced forwards axially andcompressed in the ever diminising space available to it, until it escapes fromthe outlet port

To obtain efficient compression and pumping, leakage of gas betweenthe lips of the lobes and the casing must be minimized This is achieved

by keeping clearances small and by injecting oil to ensure continuity of

FIG. 14-Screw compressor.

the oil film The oil also serves as a coolant to remove some of the heat

of compression, thus reducing the operating temperature The necessityfor small clearances is one of the factors that so far has restricted themanufacture of small compressors Allowing for manufacturing tolerances,one cannot just scale down the diameter of the rotors and casing whilststill maintaining the same ratio of clearance to diameter

With deliberate oil injection, the oil passes out with the compressedgas in far larger quantities than with reciprocating compressors, whereany oil carried out is only the accidental seepage that passes the pistonscraper rings Oil separators on compressor discharge thus have to befar larger in capacity than for other types of compressor The capability

of the screw compressor to pump out oil also means it can pump outliquid refrigerant should this be drawn in with the suction gas Whilst it

is not suggested that this should be allowed to happen, if it does so due

to some part of the refrigerant circuit malfunctioning, the results are not

as damaging as they can be with reciprocating compressors

Rotors are designed with different length/diameter ratios to vary thecompression ratio according to the refrigerant to be used, and to the

26 MARINE ENGINEERING PRACfICE

temperature difference required Although designed in this way foroptimum efficiency at one compression ratio the compressor is versatileand can work over a range of temperature differences

Compressors are invariably driven by single speed a.c motors and, toallow the capacity to be varied sliding sleeve valves are used which havethe effect of bringing the outlet port back along the axial length of therotor towards the inlet port The control is progressive down to about 10per cent of full output and is not confined to steps as occurs with unload-ing cylinders of reciprocating compressors With some types of evapora-tors and screw compressors there is a tendency for oil to accummulate

in the evaporator when running for long periods at v~ry low capacity.This oil can be recovered by a brief period of operation at full capacity.The screw compressor is not as fully developed as reciprocating com-pressors Early screws used gearing to link the rotors This was dispensedwith as it was found that with large quantities of oil injected it wassatisfactory to drive only the male rotor and to let the female rotor idleround Present developments towards smaller and faster rotating screwsinvolve the reintroduction of gearing and the elimination of oil injection.Oil pumps have usually been driven from the screw shaft or provided

as independent unit with their own electric motors Finality as to which

is the better arrangement does not appear to have been reached

However, at its present stage of development the screw compressor isfulfilling the hopes that it would prove to be a more reliable unit thanthe reciprocating compressor and one needing less routine maintenance.(A reciprocating compressor with reed valves can be regarded as a form

of fatigue testing machine for the material of the reeds The springs ofplate valves also have a limited life.)

In the early days of screw compressors at sea there were suggestionsthat opening up the compressor for routine examination was not desirable

on board and th3.t a "survey by replacement" system was neces~ary Infact, no more skill is needed to strip a screw compressor than a recipro-cating type This is, however, seldom necessary and the classificationsocieties will consider up to 25000 h running before a full survey forscrews compared to 10 000 h for reciprocating

4.1.4 Centrifugal Compressors

A typical centrifugal compressor is shown in Fig ]5

As mentioned previously for marine use centrifugal compressors arefound only on air conditioning duties, as they are not flexible enough forthe range of operating conditions necessary for cargo operations How-ever some adjustment of capacity is needed even for air conditioningduties and one of the best ways of reducing capacity is by the use ofadjustable inlet guide valves Other methods include use of a dampervalve in the suction pipe speed variation, or "hot gas bypass" which

MAIN COMPONENTS OF REFRIGERATION SYSTEMS 27

involves passing a proportion of the discharge gas from the compressor directly to the evaporator, bypassing the condenser.

1 Pinion or high speed gear 8 Loll' speed pump

2 Hig" speed pump 9 Impeller

3 Tlifllst bca;"i;;g 10. Vanes

4 Loll' specd sliaft 11. Pre-rotation Vall£' linkage

5 High speed shaft 12 Shaft seal

6 Loll' spccd gear 13 York flex coupling

7 Main bearings

FIG ]5-Centrifugal compressur.

If the ca:1uc;:y control is a correctly engineered automatic system,

it will always keep the comrressor within a satisfactory range. If capacity control is manuai then care must be takcn not to set the control at too great r: dcviat't):1 from the design optimum If this is not done there is a possibility of sla~Jjn:; th::: bbdes of the compressor with cons2quent surging

or vibration.

28 MARINE ENGINEERING PRACTICE

Efficiency of multi-stage compressors is improved if the liquidrefrigerant is expanded in several stages, the "flash" gas from each stagebeing returned to the appropriate stage of the compressor The entrainment of liquid refrigerant in the suction gas can be disastrous, the mech-anical effects being similar to water entering a steam turbi.ne To guardagainst this, flooded evaporators for use with centrifugal compressorsincorporate spray eliminator plates within the evaporator shell

Correct alignment is most important for turbine drive compressors;expansion and contraction of steam pipes have on occasion pulled unitsout of line

4.2 CONDENSERS

For the smallest refrigeration plants, up to 3-4 kW (5 hp), condensersare air cooled Above this size of plant the space required for an aircooled condenser becomes so large compared to that required for a watercooled condenser of the same duty, that water cooling is usually used.Air cooled condensers consist of finned tubes, usually with a fan toprovide forced draught through the bank Two operations are necessary

to maintain efficient working Firstly, fluff and dirt must be brushed offthe surface every few months Secondly, suitable access of fresh air to theunit must be available This elementary precaution is often forgotten, orthe provision made for it is of such a meagre kind that excessive tem-pertures build up

For carbon dioxide systems the condensers consist of tubes insidewhich the refrigerant condenses, the tubes being nested within some form

of shell circulated with sea water One type is shown in Figs 16 and 17

Concentric coils of aluminium brass are housed in copper shells, and therequired number of shells built up with a common cast nickel iron waterbox Gas enters at one end of the outer coil and as it condenses it pro-gresses through coils of smaller bore, so that the liquid velocity within

30 MARINE ENGINEERING PRACTICE

the final coils is increased to improve heat transfer In spite of the cated design and superior materials employed, these condensers have arather high tendency for developing leaks If a leak is suspected it can betraced by stopping the water circulation, removing a top connexion tothe header and siowly draining out the water Bubbling of gas throughthe water can be heard until the level drops below the failed coil The

sophisti-A. Water outlet J. Tube support baffle (stainless steel)

B. End cover (cast iron neoprene K. Vellt connexion

coated, or gun meta/) L. Air cock

C. Joint (lleoprene) M. Drain cock

D. Tube plates (stainless steel clad N Elld cover

COI't iron or brass or gllll meta/) O. Joint

E. Air purges Q. Tubes (aluminium brass or

cupro-F. Branch for safety disc nickel)

FIG. I't',,-Shell and tube condenser (4-pass).

failed coil can thus be isolated and the remainder of the unit brought backinto service

For Freon systems, with their lower pressures, the usual form ofcondenser is a shell and tube, with sea water circulating through the tubes

MAIN COMPONENTS OF REFRIGERATION SYSTEMS 31

and Idrigerant condensing in the shell A typical condenser, with details

of materials embodied is shown in Fig 18

External fins to improve heat transfer are sometimes used on theoutside of the tubes, often formed by rolling "threads" on to the outside

of the tube

In spite of the materials used, corrosion and erosion can occur onthe water side of condensers Corrosion plugs of iron arc often fitted inthe end water space The intended function of these plugs is that they act

as source of iron ions in the water and reduce the attack of the sea water

on the non-ferrous materials in the condenser However, in themselvesthe corrosion plugs do not provide sufficient concentrations of iron if thesea water piping is either internally galvanized or non-ferrous, when it isgood practise to include a "sacrificial" length of steel pipe just before thewater inlet to the condenser

To avoid erosion water velocities through the tubes should be kept

below 2·5 m/s On large plants it is usual to provide pressure gauges to

enable the pressure difference between the inlet and outlet sea waterconnexions to be maintained between stated limits The upper limit is toprevent erosion through excessive water velocity, the lower limit to pre-vent the lodging of foreign bodies which may enter from the sea

On small plants, such as those for domestic chambers, where a smallcondenser may be supplied from a sea water pump serving other pur-poses, it is often necessary to incorporate an orifice plate to limit thewater flow through the condenser Whatever the size, the tubes of con-densers should be brushed through every few months This is to removeany weed growth or sea shells Apart from the detrimental effect of dirtytubes on heat transfer, there is a real risk of local erosion if a shell orother foreign body almost blocks a tube so that there is an abnormallyhigh water velocity through the very restricted passage around theobstruction

Whn.n replacing end covers it is essential that the sealing gaskets (Cand 0 of Fig 18) are correctly located in way of the dividing plates, wherethese abutt onto the tube plate Any leakage at this gasket allows highwater velocities where the leak is by-passing the tubes and short circuitingfrom one pass to the next The result is erosion of the dividing plate andl

or the tube plate

To guard against fouling of condensers on the water side "chIoros"

or sodium hypochlorite solution is injected into the sea water The tion system should be used whenever the ship is in port, and the con-centration at the sea outlet maintained at about 2 to 3 parts per million

32 MARINE ENGINEERING PRACfICE

4.3 EVAPORATORS

An evaporator is a heat exchanger in which liquid refrigerant isturned into gas, performing the desired cooling operation for which theplant is designed There are many types of evaporators, and they aresometimes given different names For example they may be caI.led "aircoolers" if the heat transfer is direct from refrigerant to air, "waterchillers" if the refrigerant is cooling water, "brine coolers" if the refriger-ant is cooling brine or "inner fin chillers" if the tubes in which therefrigerant evaporates are in some way finned to increase heat transfer

to a liquid outside the tubes

As evaporators are subject to low pressures and do not suffer fromthe corrosive action of sea water as do condensers, they are generally atrouble free item Sources of trouble are most likely to be externalatmospheric corrosion, particularly if parts are insulated and the insula-tion is allowed to remain in a damp condition

The most elementary air cooler evaporator is the nest of pipessecured to the wall of the room to be cooled Except in small low tem-perature rooms, this arrangement is little used owing to the inconveniencecaused when the ice has to be periodically thawed from the pipes

The more popular "air cooler" is the battery of pipes, with externalfins, provided with fan circulated air All-copper subsequently tinned, orall-steel subsequently galvanized are suitable materials For air condition-ing requirements a cooler commonly used ashore and constructed ofcopper pipes with pressed on aluminium fins, is sometimes used; but thecombination of dissimilar metals in a permanently damp, salt-laden atmos-phere is a perfect environment for electrolytic corrosion

The CO~ evaporators for brine cooling were usually multi-coils of steelpipe encased in steel tanks through which brine circulated The weaklink in this system is the connexion between the coil ends and header sothat it must be kept clean and well greased or painted to prevent corrosion.Orifice plates arranged to equalize flow of refrigerant through differentcoils must not be lost when headers are opened up for inspection or repair.This type of evaporator accummulates oil in the coils during use and aftercompletion of discharge of cargo, the evaporator should be circulatedwith warm brine to make any congealed oil more fluid, and the compres-sor run to suck the oil out of the evaporator into the oil separators fittedafter the evaporator

The shell and tube Freon evaporator with brine passing through thetubes and which has the shell flooded with refrigerant has already beenreferred to (see Fig I) The maintenance of a correct working level ofrefrigerant in the shell is necessary to prevent either liquid carryover orexcessive suoerheat Devices for controllin~ this level are dealt with inSection 4.5 In spite of these automatic devices, it is often necessary withthis type of evaporator to vary the charge of the refrigerant in the plantdepending on the duty At high duties less charge is needed and some

MAIN COMPONENTS OF REFRIGERATION SYSTEMS 33 Freon may have to be withdrawn to the reservoir provided for this pur- pose The behaviour of oil return from this type of evaporator is also affected by the duty; for example, with Freon 22 there is a tendency for oil to collect after prolonged running on light duties This oil can be retrieved by a brief run at a higher duty.

A typical Freon evaporator with the refrigerant inside the tubes is shown in Fig 19 Compared to the refrigerant-in-shell evaporator, the inner fin evaporator offers a more compact design, in some ways simplifies the oil return system and permits the use of simpler expansion valves Materials used are steel for the shell, with tubes of steel or aluminium brass with aluminium fins inserted inside the tubes.

During construction the aluminium fins are expanded to give good thermal contact with the tubes With steel tubes the inserts have been known to move and work out of the tubes, probably as a result of dif- ferential rates of contraction at low temperature between aluminium and steel In this respect aluminium-brass tubes have a coefficient of expan- sion closer to that of aluminium than does steel.

The secret of successful design in this type of evaporator is the means used to ensure uniform distribution of Freon liquid to the tubes,

so that no tubes are sta'rved of liquid Plugs with graded orifice holes may

be used for this purpose but care should be taken that their correct tion is maintained if the evaporator is opened up.

loca-It is usual to provide two or more independent banks of tubes, so that only a reduced number of tubes is used when the associated com- pressor is running unloaded The separation of these banks is maintained

by dividing plates in the end covers As with condenser end covers, one must ensure that gaskets are correctly fitted to seal these dividing plates when a cover is replaced.

4.4 FREON LIQUID/FREON GAS HEAT EXCHANGERS

As adjuncts to evaporators, tubular heat exchanges are often fitted

in which all the liquid from the condenser passes in contra flow to all the gas from the evaporator This has two advantages:

I) the liquid is sub-cooled and this reduces the "flash" gas formed

at the expansion valve, easing the working conditions of this valve;

2) it increases the superheat in the gas preventing any liquid vertantly passed from the evaporator reaching the compressor.

inad-On small plants this heat exchanger is not fitted as a unit, but the liquid pipe is clipped to the suction pipe and insulation applied overall,

so that heat exchange is effected between the pipes.

Heat exchangers are often of an all-welded steel construction and as Freon is non-corrosive, their operation seldom causes trouble However,

on the rare occasions when an internal leak develops, its diagnosis can

34 MARINE ENGINEERING PRACfICE

MAIN COMPONENTS OF REFRIGERATION SYSTEMS 35

be difficult Once a leak is located, the unit can be cut out of the systemaltogether in order to keep the plant running-but with reduced efficiency.4.5 REGULATORS OR EXPANSION VALVES

The regulator or expansion valve is fitted to ensure that the correctvolume of liquid refrigerant flows from the high pressure side of thesystem through to the low pressure evaporator side In passing throughthe valve there is a sudden drop in pressure and temperature, and a pro-portion of the liquid "flashes" into gas The importance of the regulatorcannot be over emphasized Most components of a refrigeration plant willcontinue to work, perhaps with reduced efficiency, even if their mechanicalstate is poor The effects of a faulty regulator are cumulative and cansoon cause complete failure of the equipment

Expansion valves have small orifices in order to effect the desiredpressure reduction They are therefore prone to choking from any dirt

in the system and so are always protected by fine filters, which should becleaned if any blockage is suspected As the expansion valve is the firstpoint in the refrigerant circuit at which the temperature falls, and if theplant is operating at temperatures below O°C, then any moisture in thecircuit will freeze out and the ice may choke the expansion valve

A further situation which can cause erratic performance of an sion valve occurs if it is made to perform outside what may be termedits "normal" operating range Any automatic control valve metering aflow of liquid has to be sized to suit that particular flow and pressuredrop in order to obtain optimum performance However, a refrigerationplant has to operate in different climates and to produce various ranges

expan-of cold temperatures-calling for a wide variation in refrigerant flowthrough the valve To provide this flexibility of operation, some plants areprovided with more than one expansion valve Sometimes two valves areused in parallel or a larger valve is brought into use for large flows Manyplants are fitted with a recirculation system linked to the condenser cool-ing water When operating in cold climates the condenser temperatureand pressure can then be kept closer to their values for operation inwarm sea waters This is detrimental to the power consumption by theplant as the compressor is having to pump up against an unnecessarilyhigh pressure However, this wasteful procedure is adopted as a way ofstabilizing the pressure upstream of the expansion valve and keeps thevalve operating nearer to its optimum design pressure drop (For hightemperature cargoes, say bananas, being carried into Northern waters inwinter, the sea water temperature is below the carrying temperature andsome recirculation or restriction of condenser cooling water is then needed

to ensure that even the condenser pressure remains greater than the orator pressure.)

evap-All CO2 regulators are manually operated valves with fine pitchthreads Sometimes the valves are spring loaded, the spring tension being

36 MARINE ENGINEERING PRACTICE

manually adjusted so that once set, the valve maintains a more or lesssteady pressure difference between the condenser and the evaporator.For Freon systems the simplest regulator is that found on domesticcabinets, and consists of a length of capillary tubing No adjustments arepossible but at the design stage the length and bore of the tubing are cal-culated to give the pressure drop required for normal working conjitions

For larger Freon systems the most common regulator is the

thermo-static expansion valve, the meaning of the word "thermostatic" beingthat the valve is designed to maintain a constant amount of superheattemperature at the evaporator outlet The principles of its operation can

be deduced from the diagrammatic valve shown in Fig 20 Above the

valve diaphragm is a closed system with a temperature sensing bulb ped to the evaporator outlet pipe to ensure good thermal contact Assumethat this closed system is charged with the same refrigerant as is used inthe plant Then the pressure generated above the diaphragm is thesaturation pressure corresponding to the temperature of the bulb An

strap-"equalizing" pressure pipe is taken from the evaporator outlet to theunderside of the diaphragm If the evaporator is operating with no super-heat, the pressures above and below the diaphragm are equal The dia-phragm is also spring loaded so that the equilibrium position of the dia-phragm is obtained with a higher pressure on top of the diaphragm, i.e.when the temperature sensing bulb is at a higher temperature than that

MAIN COMPONENTS OF REFRIGERATION SYSTEMS 37corresponding to the evaporator outlet saturation gas pressure The springsetting controls the amount of superheat; a typical superheat value is6'6°C (l2°F).

The mode of operation of the valve is as follows If the superheatstarts to rise, then the needle valve opens up slightly to admit more liquidand reduce the superheat, and vice versa A given change in superheataffects approximately the same amount of valve movement regardless ofthe actual operating temperature, or size of the evaporator For this reasonthe size of the valve has to be matched to the size of the evaporator andits duty and good valve performance occurs over a limited range ofpressure differences For small evaporators it may not be necessary to usethe "equalizing" pipe connexion shown in Fig 20, if instead the lowerside of the diaphragm is subjected to refrigerant pressure immediately onthe downstream side of the expansion valve (This arrangement ignoresthe pressure difference between evaporator inlet and outlet and is notsuitable for large evaporators.)

The charged system of the thermostatic valve does not have to becharged with the same refrigerant used in the plant, although it often is.Other fluids, liquid or gas, can be used Suitable compensation for dif-ferent temperature/pressure properties is achieved by spring adjustment

In some valves bellows are used in place of a diaphragm

Once a plant is correctly set up, the spring of the thermostatic valverarely needs adjustment If a thermostatic valve appears to be malfunc-

tioning, one should first look for dirt (or ice) in the valve, and secondly,the plant should be checked for leaks to ensure that the refrigerant charge

is correct Only after carrying out these checks should any valve

38 MARINE ENGINEERING PRACl'ICE