REFRIGERATION MACHINE

A. Definition of Refrigeration Machine
Refrigeration is the production and maintenance of temperature degree of a material or chamber at a degree which is lower than surrounding temperature by material or chamber’s heat absorption. In other hand, refrigeration is a process of heat transfer from a material or chamber to other material or chamber.
According to definition explanation above, we can say that refrigeration machine is a device that applied refrigeration principle.

B. Principle of Refrigeration Machine
The principle of refrigeration machine is so simple i.e. absorbing heat of an insulated chamber and then carried it over surrounding.
The principle of refrigeration machine also can be explained broader i.e.:
1. Managing heat in order to create low temperature. The purpose is creating a lower temperature degree than surrounding temperature at a material or chamber, by getting heat absorption from a material or insulated chamber and then carried it over surrounding.
2. Employing the change of energy form. In heat absorption, refrigerant changes from liquid to vapor. Afterwards refrigerant vapor is compressed in order to increase its pressure and temperature and then refrigerant is condensed again as a liquid, so heat condensation which is absorbed before is released and gone with the coolant (air or water). At whole system, heat only flow from a hotter material to a colder material. The change of heat and other energy form (electricity energy, mechanic energy, etc) is involved in all process and cycle.
3. Employing engineering, thermodynamic and physic process. In the change of refrigerant form process, the change of pressure and temperature refrigerant is involved. In the flow of heat from a hotter material to a colder material, with refrigerant flow regulation in the system, low pressure side and high pressure side will be created. Temperature difference will be created if there is a pressure difference.
4. Employing thermal and physic characteristics of various type of material. The exchange of heat which is going on all cycle is employing thermodynamic characteristics of materials included refrigerant, material that has high thermal conductivity (ex. steel, aluminum, copper) and material that has low thermal conductivity (insulation material such as polyurethane).
5. Restraining and controlling process. All process and cycle of refrigeration system must be restrained and controlled in order to make heat transfer and pressure and temperature change run smoothly.

C. Vapor Compression Cycle
Processes that run in refrigeration system i.e.:
1. Evaporation. In this process, liquid refrigerant that exist in evaporator metal pipe boils and evaporate at constant temperature, although it has absorbed much heat from surrounding i.e. fluid and food in close insulated chamber. Heat absorption during boiling and evaporation that runs without refrigerant temperature increase, shows how refrigerant act as material or freezing and refrigeration agent effectively. Absorbed heat is called evaporation latent heat.
Component of refrigeration system where evaporation process runs, is called evaporator. Vapor which is formed leave evaporator in saturated state and because it absorbs heat that comes from outside suction pipe or heat exchanger device, so its state becomes superheated but the pressure doesn’t change when sucked into compressor.
2. Compression. Low pressure and temperature refrigeration superheated vapor that comes from evaporation process is compressed by compressor becomes high pressure and temperature vapor in order to condensed easily then, vapor becomes liquid again in condenser.
Compression process runs at high velocity (in short time) and the difference of temperature between refrigerant vapor and compressor wall is very small, so then heat flow between both of them can be neglected. Therefore, vapor compression process in compressor is assumed runs adiabatically, it means that there is no heat which is added or eliminated, or more specific called isentropically i.e. adiabatic without friction. As a consequence, gas energy increase accord with amount of heat which its value equal to work which is done to gas to compress it (compression heat). It means that vapor enthalpy and temperature proportional to energy or compression heat which is provided by compressor. Theoretically, required power to move compressor also can be determined directly from compression heat.
Vapor temperature which is discharged by compressor always bigger than saturation temperature that appropriate to vapor pressure. That vapor is in superheated state which its pressure and temperature some times over pressure and temperature inlet compressor.
3. Condensation. Condensation process is a rejection or heat transfer process from high pressure and temperature vapor refrigerant which is result of compression compressor to condensate outside condenser. Condensate is air, water or air – water combined. All latent heat which is absorbed by refrigerant during evaporation process is discharged to outside refrigeration system. Heat which is discharged per vapor refrigerant weight unit to be condensed is called condensation latent heat, whereas the numeric value is equal to evaporation latent heat numeric value.
In order to maintain the directness of refrigeration effect, so, refrigerant temperature must be maintained below refrigerated product or chamber temperature degree. In order to proportion, heat that discharged with condensate in condenser must be at the same velocity like heat that absorbed by system in evaporator, suction pipe and compressor.
Heat transfer rate through condenser is depend on temperature difference between refrigerant and condensate, because in every condenser, condensation surface area and conductance coefficient of condenser pipe wall has been already a constant factor when condenser made. Condensation temperature is equal to condensate temperature added temperature difference between condensed refrigerant and condensate. That condensation temperature is varied to heat transfer rate which is required by condenser. Then, liquid refrigerant that formed during condensation process at high pressure and temperature flows into refrigerant controller device (expansion valve).
4. Expansion. Expansion is an opportunity regulation process for liquid refrigerant to expansion in order to able to boils and evaporates in evaporator then. Liquid pressure is reduced to evaporator pressure, so then refrigerant saturation temperature below refrigerated chamber temperature. Refrigeration system needs refrigerant regulator in order to make compression and suction process in compressor can create and maintain a pressure difference which is required to change refrigerant state from liquid to vapor. This task is done by refrigerant controller device, expansion valve or other type.

D. Refrigeration Machine Component
Refrigeration machine has 3 types component i.e.:
1. Main Component
Main component of refrigeration machine consist of:
a. Compressor
The function of compressor is moving refrigeration system in order to maintain the difference of pressure between low pressure side and high pressure side of the system. In implementing this function, compressor:
i. Creating low pressure side. Compressor sucks vapor refrigerant from evaporator, creating low pressure that makes possible liquid refrigerant to boil and evaporate at low temperature in evaporator. By this evaporating, heat is absorbed from food or product which is cooled or frozen in evaporator.
ii. Creating high pressure side. Compressor compress vapor refrigerant which is absorbed from evaporator, increase that vapor temperature and pressure then flow it to condenser in order to makes possible to be condensed becomes liquid by air or cooling water in condenser, so heat in refrigerant gone with air or cooling water.
So compressor has function as a pump that creates condition which is required to heat transfer in evaporator and condenser.
Compressor has many types i.e.:
- Reciprocating compressor
- Rotary compressor
- Screw compressor
- Centrifugal compressor
b. Condenser
Condenser is a part of refrigeration machine that receives high pressure superheated vapor refrigerant from compressor then reject condensation heat by cooling high pressure superheated vapor refrigerant to its dew point by rejecting its sensible heat. Next latent heat rejecting caused that vapor condense becomes liquid.
Types of condenser:
- Finned static natural convection
- Finned force convection
- Air cooled condenser
o Base mounted air cooled condenser
o Remote air cooled condenser
- Water cooled condenser
o Shell and tube condenser
o Shell and coil water cooled condenser
o Tube and tube condenser
o Evaporative condenser
c. Expansion valve
Expansion valve is used to expansion adiabatically low pressure and temperature liquid refrigerant until reach low pressure and temperature state; so, implementing throttle process or constant enthalpy expansion process. Besides that, expansion valve regulate refrigerant intake according to cooling load which have to be served by evaporator.
So, expansion valve regulate in order that evaporator always run well until obtained maximum efficiency of refrigeration cycle.
Expansion valve has many types i.e.:
i. Thermostatic automatic expansion valve
ii. Manual expansion valve
iii. Constant pressure expansion valve
iv. Capillary tube
d. Evaporator
Evaporator is a heat exchanger that plays the most important a part in refrigeration cycle, i.e. cools product.
There are some evaporator types, according to the purpose of use, the type can be different. It is caused media that will be cooled can be gas, liquid or solid. So evaporator can be divided into some group, according to refrigerant state which in evaporator, i.e.: dry expansion type, semi-wet type, wet type, and liquid pump type.
i. Dry expansion type
In dry expansion type, liquid refrigerant which is expanded through expansion valve, when go into evaporator it has already in air water mixture state, so it leaves evaporator in dry vapor state.
Because a considerable part of evaporator filled by vapor refrigerant, so heat transfer which is occurred is not so much, if compared with state where evaporator is filled by liquid refrigerant. But, dry expansion type evaporator doesn’t need refrigerant in large amount. Besides that, the amount of lubrication oil that left in evaporator is very small.
The amount of refrigerant that go into evaporator can be regulated by expansion valve so that all refrigerant leave evaporator in saturated vapor state, and even in superheated state.
ii. Semi-wet type evaporator
Semi-wet type evaporator is an evaporator with refrigerant state is between dry expansion type evaporator and wet type evaporator. In this type, there is always liquid refrigerant in its evaporation pipe. Therefore, heat transfer rate in semi-wet type evaporator is higher than dry expansion type, but lower than wet type.
In dry expansion type, refrigerant enter from up section of coil; whereas in semi-wet type evaporator, refrigerant is entered from down section of evaporator coil.
iii. Wet type evaporator
In wet type evaporator, considerable part of evaporator is filled by refrigerant liquid. The evaporation process runs like at boiler. Refrigerant bubble which is formed due to heating will raise, collapse on liquid surface or apart from its surface. Then some of refrigerant flow into accumulator that separate vapor from liquid. So, only refrigerant which in vapor form that flow into compressor. Liquid refrigerant which is separated in accumulator will go back into evaporator, together with refrigerant (liquid) which come from condenser.
So, evaporator tube is filled by liquid refrigerant. Refrigerant liquid absorbs heat from fluid which will be cooled (brine), which flows in pipe. Refrigerant vapor which formed is collected at up section of evaporator before enter compressor.
Refrigerant liquid surface height that is in evaporator is regulated by float valve; usually a little bit higher than half of tube height. The amount of refrigerant that is sent to evaporator tube is adjusted with cooling load which have to be served.
2. Auxiliary Component
Auxiliary component of refrigeration machine consist of:
a. Oil Separator
Oil separator is a device for separating oil from refrigerant vapor after leaving compressor and returning it to compressor. A little oil that brought to refrigerant system won’t harm, but if much oil that brought to refrigerant system will disrupt condenser, refrigerant controller, evaporator and filter.
Lubricant oil which is separated will be collected at down section of oil separator. If that oil surface has already reached a certain height, lubricant oil will flow to compressor crank-case automatically, i.e. if it float reach a certain position.
Because refrigerant can mix with lubricant oil, so oil presence in refrigerant system can’t be avoided completely. Thereby, in some refrigerant machine, lubricant oil re-enter to compressor over and over after trough condenser and evaporator, without using lubricant oil separator.
Lubricant oil separator must be set near compressor and must be kept in order to untouched by low temperature from condenser and receiver, when refrigeration machine not run.
b. Receiver
Receiver is used to temporary receive refrigerant which is condensed in condenser, before enter expansion valve.
Receiver volume must be able to serve refrigerant need according to load change. Besides that, receiver also has function to receive refrigerant from refrigeration machine, when the machine repaired or stop running for a long time. In this case, condenser also can be used to collect refrigerant too.
In Freon refrigeration machine with low capacity, receiver is oftentimes unused, because water-cooled condenser which is used can be function as receiver. But, in Freon refrigeration machine with large capacity, usually receiver is used, because condenser can’t receive all refrigerant from the machine. It is caused by condenser volume which is too small, moreover if finned cooling pipe is used. So it is if air-cooled condenser is used, with same reason above, receiver is always needed. In this case, refrigerant that already condensed in water-cooled condenser flows to (down) receiver without any left in condenser.
Therefore, receiver must be set lower than condenser and refrigerant temperature in receiver must lower than refrigerant temperature in condenser. The flow of refrigerant from condenser to receiver occurs because of gravitation.
But, if coolant temperature in condenser is relative low, and machine chamber temperature where receiver set is relative high, sometimes refrigerant liquid that occurs in condenser can’t flow easily. In this case, up section of condenser must be connected to up section of receiver by pressure equalizer.
c. Filter Drier
Filter drier is a device that besides evict vapor, also use to strain dirt, metal and clips from liquid refrigerant before flow to controller valve of a refrigeration system. Filter drier usually put in high pressure side liquid line, but there is a refrigeration system that puts filter drier at suction line (low pressure) to compressor.
In order to operate efficiently, a system must be clean. Refrigerant that allow to circulate in the system is only pure refrigerant which clean and dry (without water vapor) and at part of system, clean and dry oil (without water vapor, and free acid too.
Too much water vapor in the system will makes water vapor freeze and form ice in capillary tube and cause clogging at refrigerant controller valve, instead it can happens at whole system that operate at low temperature. Water vapor can fasten corrosion process. Acid formation also happens if lubricant oil expansion in system runs.
d. Accumulator
In compression system, especially that use reciprocating compressor, refrigerant that flows from evaporator should be in vapor form before entering compressor. If the refrigerant is still in liquid form, the compressor could be damage.
In order to avoid the entry of liquid refrigerant into suction pipe in compressor, some ways and equipments have already devised i.e. (a) installing an accumulator in suction pipe (b) using heat exchanger to heat up suction pipe (c) using electric heater to heat up suction pipe, and (d) using hot gas by-pass valve to push up hot gas in suction pipe in order to be able to evaporate every liquid, etc.
e. Heat exchanger
The duty of the refrigerant system is transferring heat. In processes and during refrigeration cycle, heat transfer process is developed i.e. the process that gets and loses heat. This thing can be balanced by heat transfer, in order to create efficiency of refrigeration system.
This heat exchange, in refrigerant system, runs in some methods:
i. Heat transfer that occurs by doing contact between suction pipe (low pressure) and liquid (high pressure) by attaching each other.
ii. By using heat exchanger.
iii. Heat transfer process also occurs at condensation process in condenser.
3. Control component
In order to work efficiently and economically, refrigeration machine must stop running when not needed. Refrigeration machine is not designed to work all day or year. So, it must use refrigeration machine control device to regulate compressor or electric motor, in order to run only when needed. The capacity of the refrigeration machine is designed at maximum load, whereas maximum load condition only occurs some hours in a day and some weeks in a year.
The control device of the refrigeration machine has to be able to regulate refrigeration machine to maintain required temperature and relative humidity as accurate as possible. That control device can be used to regulate the work: heating, cooling, drying, humidifying, etc. Besides that, control device is designed to do a little work but it can make cooling system provides maximum capacity and gives safety to component, material and the man whose near it.
The control device of the refrigeration machine only works mechanically, electrically or mechanic – electric combination.
The control device of the refrigeration machine can be divided into three:
a. Refrigeration flow control
b. Electric control
c. Combination control
3.1. Refrigerant flow control
Refrigerant flow control can control pressure and volume of refrigerant that flows in refrigeration system.
This control device is an obstacle, such as a valve that puts at one of cooling system section.
Compressor compress low pressure refrigerant becomes high temperature. Refrigerant flows back from high pressure to low pressure through refrigerant flow control or expansion valve. That control device can control volume and pressure of refrigerant from high pressure side to low pressure side. High pressure liquid refrigerant is decreased to low pressure in exact amount, in order to evaporate easily in evaporator at maximum efficiency, but doesn’t make the compressor gets over load.
Refrigerant flow control runs according to:
a. Pressure change
b. Temperature change
c. Volume change
d. Combination of changing above.
Refrigerant flow control can run manually or automatically, because of pressure difference or outside effect. In the past, refrigeration system use refrigerant flow control that its work is manually regulated by needle valve. In the present, with experience, advance design and maintenance, more efficient, economic and automatic refrigerant flow control has already created.
Refrigerant flow control can be used to control fluid (liquid and gas). This control device has a lot of types and each type has its own function. Generally, this control device is put at liquid line near evaporator, but there is a control device which is put at suction line or discharge line.
Refrigerant flow control consists of:
a. Automatic valve
b. Thermostatic expansion valve
c. Low pressure float valve
d. High pressure float valve
e. Capillary tube
f. Evaporator pressure regulator
g. Suction pressure regulator
h. Discharge pressure regulator
i. Check valve
3.2. Electric control
Electric control device is a device which runs use electric power and able to regulate electric current. Electric current that flows to coil can makes magnet and generate mechanic motion, so that can open or close: electric contact, valve hole, spin rotor of motor, etc.
Electric control device can control electric current that flows to electric motor and other control device of refrigeration system. This control device also can changing compressor capacity, automatically melting down ice in evaporator, transferring refrigerant from one section to another section of refrigeration section, starting, stopping, regulating and protecting refrigeration system and its component.
Electric control device consists of: switch, solenoid, contactor, electric motor protection, signal lamp, transformer and many more.
3.3. Combination control
Control device is a device which can start or stop, regulate and protect refrigeration system.
Refrigeration control device such as expansion valve has very important function as main control device of refrigeration system. Whereas other control device has function for adjusting if load changing occurred, giving safety and increasing work efficiency of refrigeration system.
Combination control has many types. Mechanic control device can makes straight motion that can move electric switch open or close. That motion can stop or start compressor and other control device. Electric control switch can directly work if changing of temperature, pressure or humidity happened.
Electric energy usually used to transfer controlling result or changing of controlled condition to other section of the system and to change measurement result becomes work at actuating control device. Using electric power has many advantages because electric current can be transferred trough electric cable which is simple and easily installed.
Combination control consists of:
a. Operating or primary control
b. Actuating or secondary control
c. Limiting and safety control
3.3.1. Operating or primary control
Operating control runs as electric switch to start or stop refrigeration system directly or indirectly. This control device must be able to make quick and accurate change in the limit which is determined.
Operating control device must be sensitive in order to be able to change quickly in required condition, if change of temperature, pressure or humidity happened. Operating control receives that changing and change it to mechanic motion, to open or close electric switch contact spot.
Some operating control devices such as:
a. Temperature control or thermostat
b. Pressure control or pressurestat
c. Humidity control or humidistat
Pressure control is used to regulate pressure limit of refrigeration system. Temperature control and humidity control is used to regulate temperature and humidity in a chamber which being regulated.
3.3.2. Actuating or secondary control
Actuating control device regulate and/or protect refrigeration system due to condition in system or required by operating control to do like that.
Actuating control device can’t directly control and improve refrigeration system output. This control device has assignment to do what commanded by operating control to control some sections of refrigeration system.
Some actuating control devices such as:
a. Relay starting or contactor
b. Solenoid valve
c. Flow reverse valve
d. Water regulating valve
e. Evaporator pressure regulating valve
f. Suction pressure regulating valve
3.3.3. Limiting and safety control
Refrigeration system requires certain pressure or temperature limit to protect compressor and other component from damage due to run out of designed limit. This is limiting control device duty to open its contact and stop refrigeration system activity before determined limit is crossed over.
Electric control device can makes secure condition and automatic too. Mechanic control device automatically controls system to run and stop, regulate work of refrigeration system and signal condition. This control device controls liquid or air flow and connected to safety device.
Limiting and safety control can makes temperature and pressure of refrigeration system still in determined limit. If pressure and temperature of refrigeration system over safety limit, limiting and safety control will run, so electric motor can’t start and which is running will stop.
Some limiting and safety control devices such as:
a. High pressure circuit breaker
b. Low pressure circuit breaker
c. Oil pressure control switch
d. Flow switch
e. Pressure relief valve
f. Overload motor protector
g. Fuse
h. Fusible plug