A Book on Refrigeration

A Book on Provision

Cooling

Plants on board ships.

By C. Rajesh Mahadevan

Preface

With great pleasure and immense joy I dedicate this book to my

dear friends and colleagues, marine engineers appearing for MEO

Class IV Part B.

The main objective of this book is to make the subject clear and

easy to understand. I have referred many books and sources on the

net to fulfill this objective and have almost covered all the terms

and process under this topic. I have also shared some of my self-

experience encountered on board my ship with provision plant.

I hope this book will be very useful to all, who appearing for Class

IV exams and will help and guide each and everyone in their

journey to success.

I would like to thank my parents, friends, colleagues and my elders

for helping me directly or indirectly for this work of mine.

I would specially like to thank Mr.Rethinam Stalin and

Mr.Munusami Surendar without whose continuous

encouragement, guidance and support this book, would not have

been possible.

- The author

Contents

1. Introduction.

2. Definitions & Accessories.

3. Properties of Refrigerants.

4. Vapour Compression Cycle.

5. Thermostatic Expansion Valve.

6. Back Pressure Valve.

7. Trouble Shooting.

8. Frequently Asked Questions.

9. Conclusion.

A Book On Provision Cooling Plants Onboard ships.

Page 1

Chapter I

Introduction

The Refrigeration is a process in which work is done to remove heat from one location to another. In this the room temperature is brought well

below to its atmospheric temperature. The first thing comes into all our mind

when talking about refrigeration is our domestic Refrigerator or Fridge. But in

this book we are going to deal with our Provision cooling plant.

Refrigeration is used widely onboard ships for various purposes

such as for Cooling Provision good, Cooling Perishable goods as cargo in cargo

holds, in refrigerated containers in container ships, in control air dryer for

removing moisture from air, and also in bulk CO2 system onboard ro-ro ships.

The purpose of refrigeration for provision good is that it will slow

down the bacterial process and keep the food items in a good condition for

longer time. Onboard ships there will be a minimum of two or three refrigerated

rooms namely Meat room, Vegetable room & Potato room. In all the three

rooms the temperature maintained are different and are as follows -24, 8 and 4

respectively. A single compressor and condensers serves these purposes.

The refrigerant used widely onboard ships now-a-days are R 134-a

and R 417-a being eco-friendly. R 22 is not used now due to its effect of

harming the environment.

The main components of refrigeration system

The refrigeration system mainly consists of four components,

they are, 1. Compressor

2. Condenser

3. Expansion Valve

4. Evaporator

The other components in the system namely the receiver, dryer,

filter, oily mixture separator, solenoid valves, back pressure valves etc., we will

study the working and function of all these components in the upcoming

chapters in a detailed manner.


Page 2

Chapter II

Definitions & Accessories

In this Chapter we will see the various terms and components associated with refrigeration process, definitions and working.

Refrigeration ton

The refrigeration ton is the unit used to indicate the heat removal

capacity of the system.

Boiling point

The boiling point is the temperature at which a liquid boils at a fixed

pressure, especially under standard atmospheric conditions.

The boiling point of a substance is the temperature at which the vapor

pressure of the liquid equals the environmental pressure. It is also called

Saturation temperature.

Specific heat of liquid

The specific heat of liquid is the amount of heat per unit mass required

to raise the temperature by one degree Celsius.

Critical temperature

The critical temperature is the temperature above which a gas cannot

be liquefied, regardless of the pressure applied.

Latent heat

The latent heat is the energy required to change the phase of a

substance.

Latent heat of Vaporization

The latent heat of vaporization is the change of phase of any matter

from liquid to gas.


Page 3

Heat transfer co-efficient

The heat transfer co-efficient is used in calculating the heat transfer,

typically by convection or phase change between a fluid and a solid.

Compressor

The compressor is a machine or pump in which the pressure and

temperature of gas or air is increased by compression.

The purpose of compressor in refrigeration unit is to raise the

pressure and move it along the system and to increase the temperature of gas

well above its critical temperature.

Unloader

The unloader is provided on the compressor to prevent the

compressor from starting on load, due to which the motor will draw high

current or damage of motor. It also serves a purpose that till the desired lube

oil pressure is achieved the compressor will not come on load. The unloader

unit will keep the suction valve open till the lube oil pressure is achieved.

Condenser

A condenser is a device or unit used to condense a substance from

its gaseous to its liquid state, typically by cooling it. They are typically heat

exchangers with various types and sizes.

It is used in refrigeration system

to get rid of heat from the interior of the unit

to the outside air or water. By doing so, the

latent heat is given up by the refrigerant,

and will transfer to the condenser coolant.

On ships the widely used condenser for

refrigeration plant is shell and tube type and

the cooling media being fresh water. i.e.,

Low temperature water. The fresh water is

obtained from a branch connection from the

Low temperature (LT) fresh water system

through a separate LT pump in modern

ships with central cooling system. A purge


Page 4

cock is mounted on the condenser on the refrigerant side to remove air or other

gases which are non-condensable or lighter than the vapour refrigerant. Cross-

sectional view of shell & tube condenser is shown in the fig. 2.1 above.

Oily mixture separator

The oily mixture separator is a device in the discharge side of the

compressor to remove the oil particles in the gas. There are possibilities of the

crankcase oil being carried to the piston crown as in air compressor due to

faulty piston rings and worn cylinder. The oily mixture separator thus removes

oil from the system and improves its efficiency.

Working principle of oily mixture separator

(1) Baffle type

The gas enters the inlet of the oil separator, this mixture flows

through a filter and a baffle device, making the oil particles aggregate and drop

to the bottom of the oil separator. The refrigerant gas will go through the outlet,

and enter the condenser under oil-less status when the filter separates the

remaining oil particles from the gas. The oil of the refrigerant aggregates at the

bottom of the oil separator, and a needle valve operated by a floating ball is

opened to let oil return to the compressor. Since the pressure of the oil

separator is higher than the pressure of the crankcase, the oil can quickly

return to the compressor. When the oil level drops, the needle valve is closed to

prevent the refrigerant gas from returning to the compressor. As shown in fig.

2.2 below.

Fig. 2.2 Baffle type Fig. 2.3 Helical type


Page 5

(2) Helical type

Helical separators depend on the interior shell design to force the gas/oil mixture along a spiral path. This creates a centrifugal force which forces the oil droplets to the outside of the spiral where they collect and drain to the float valve below. A supposed drawback to this type of unit is that centrifugal force only works effectively on relatively large oil droplets. Fig. 2.3.

Receiver

The function of a Receiver is to store the liquid refrigerant which is

supplied to evaporator and is also capable of storing the entire refrigerant in

system when the system is under maintenance. It acts as a reservoir on the

high side of a system with hold refrigerant. They have a dip tube to the outlet

side which ensures that a full column of liquid refrigerant feeds the liquid line.

They store refrigerant which may be needed at another time by an evaporator

with the Expansion valve when the evaporator load increases and more flow is

required.

Filter Drier

The system must go through a drying process, in order to remove the

moisture and water inside. The purpose of a drier is to trap any trace of

moisture of impurities. The removal of moisture from the system is necessary

because of the ice formation or ice plug appearance in the expansion valve and

it will also speed up rusty of the metal parts. The presence of impurities like

small foreign particles and also metal dusts can easily jam the solenoid and

expansion valve. The filter drier will play an important role in preventing ice

plug and dirty blockage appearance.

Accumulator

The accumulator prevents liquid

or any unevaporated refrigerant from

reaching the compressor suction which

may not have boiled off in the evaporator

thus preventing liquid slugging. It uses an

inverted trap to do so. At the same time it

can also trap the oil entertained in the

system. It consists of a small orifice which

is located on the underside of the inverted

trap which separates the oil. A Venturi


Page 6

Effect occurs and any trapped oil is sucked through the orifice and carried

away by the suction vapors to be returned to the compressor sump where it

belongs. As in Fig. 2.4.

Sight glass

It is a window through which the condition of the system can be

known, as follows 1) the quantity of refrigerant, 2) the condition of filter drier,

3) the level of refrigerant and 4) moisture ingress in the system. We will study

this briefly in Chapter 7 Troubleshooting.

Line Solenoid Valve

The Solenoid valve is provided in the liquid line before evaporator, if the

temperature of the refrigerated space falls below its desired value the valve will

close and vice versa. It is required only in the system in which there is more

than one evaporator.

Main Solenoid valve

The Main solenoid valve is provided in some large marine refrigeration

system in addition to the line solenoid valve. It is situated after the receiver and

its purpose is to shut off in case the compressor stops due to any reason other

than its safety cut outs. By doing so, it stops the refrigerant flooding in the

evaporator, thus preventing liquid refrigerant from coming in the suction.

Thermostatic expansion valve

The thermostatic expansion valve regulates the flow of refrigerant

through the evaporator, thus maintaining the desired room temperature. It

plays a major role in the system by doing so. It is also called TXV or metering

device. We will see about this in a detailed manner in Chapter V.

Back pressure valve

The back pressure valve or evaporator pressure regulating valve is

located in the outlet line of the evaporator. This valve will be used only in the

system where only one compressor is used for several refrigerated room. It will

be available only in the room, which is to be maintained at high humidity. Back

pressure valve will be detailed in Chapter VI.


Page 7

Super heating

The Super heating is a process which takes place in the evaporator. The

refrigerant is boiler off and converted into vapour at its boiling point in the eva-

porator. Any further increase in temperature above the saturation temperature

or boiling point is termed as superheating of the refrigerant.

It is measured as the difference between the refrigerant boiling

temperature in the evaporator and the temperature at the outlet of the

evaporator.

The refrigerant is little superheated to ensure no liquid is entering the

suction of compressor. But at the same time more superheating of refrigerant

should not be done because the cooling medium in the condenser will not be

able to extract the heat completely, which will affect the system performance.

Sub cooling

The sub cooling of refrigerant is a process which takes place in the

condenser of the system. In the condenser, the vapour refrigerant rejects its

latent heat to the cooling medium and changes its state to liquid at its

condensing temperature. Any further cooling below this condensing

temperature is termed as sub cooling of refrigerant.

It is measured as the difference between the condensing temperature of

refrigerant in the condenser and the temperature at the outlet of the

condenser.

The condenser in the system is so designed to cool the refrigerant little

below its condensing temperature, to make sure that the refrigerant is fully

converted into liquid and all the latent heat is given up.

Charging of refrigerant

The charging of refrigerant is the term used for adding the refrigerant

gas in the system. Before charging the gas one should purge the system with

refrigerant to ensure all the air in system has been removed.

The refrigerant is charged from a bottle of refrigerant to the system

through a refrigerant valve or a charging valve.

The system should never be overcharged. At least 5% of space in the

receiver should be left for expansion. The system is normally charged with 85%

of the receiver.


Page 8

Chapter - III

Properties of Refrigerants

The properties of refrigerant play a major role in the selection of refrigerant for the system. The properties that a refrigerant should have are,

1. The boiling point should be low.

The boiling point should be low to ensure that the refrigerant is

boiled off and becomes vapour in the outlet of the evaporator and the

suction line of the compressor.

2. The condensing pressure should be low.

The condensing pressure is the pressure corresponding to the

condensing temperature of the refrigerant. The condensing pressure

should be low, so that the refrigerant vapour in the inlet of the condenser

should be able give away its latent heat and condense into liquid.

3. Critical temperature should be high.

The critical temperature is the temperature above which a gas

cannot be liquefied, regardless of the pressure applied.

The liquid refrigerant is converted into vapour in the evaporator

and comes to the suction of the compressor. Now the compressor

further compress it and raises its pressure and hence temperature. If

the critical temperature is low, the temperature corresponding to the

discharge pressure of the compressor may rise above the critical

temperature and the refrigerant can never be liquefied thereafter. To

ensure that the discharge pressure of compressor does not rise above

the critical temperature, the refrigerant having critical temperature high

is preferred.

4. The latent heat of vaporization should be high. The latent heat of vaporization is the change of phase of any matter from liquid to gas.


Page 9

The latent heat of vaporization should be high so that, the evaporator absorbs maximum heat from the refrigerated room for boiling the refrigerant and changing its form to vapour, thus reducing the room temperature to its desired valve quickly.

5. Specific heat of liquid should be low.

The specific heat of liquid is the amount of heat per unit mass

required to raise the temperature by one degree Celsius. It should be low, so

that amount of heat per unit mass required to raise the temperature of

refrigerant is also low.

The amount of heat absorbed by the refrigerant in the evaporator

should be capable of raising the refrigerant temperature to its boiling point and

change its state.

6. It should be non-toxic.

The refrigerant should be non-toxic, so that it does not harm the

personnels in case of any leakage in the system or while charging.

7. It should not be flammable and explosive.

The refrigerant should be non-flammable and non-explosive, to

prevent fire hazards onboard.

If it is flammable and explosive it will act as fuel in fire, the

availability of air and heat in the engine room is more. And the required

components of fire triangle for fire to break out will be formed.

8. It should have satisfactory heat transfer co-efficient.

The heat transfer co-efficient is used in calculating the heat

transfer, typically by convection or phase change between a fluid and a

solid.

It must have a satisfactory heat transfer co-efficient for proper

phase change from liquid to vapour or vice versa.


Page

10

Chapter IV

Vapour Compression Cycle

A Book on Refrigeration-Index.pdfA Book on Refrigeration.pdf

Documents

A Book on Refrigeration