The mechanism used for lowering or producing low temperature in a body or a space, whose temperature is already below the temperature of its surrounding, is called the refrigeration system.
The transfer of heat from a low-temperature region to a high-temperature one requires special devices called refrigerators.
Refrigerators are cyclic device, and the working fluids used in the refrigeration cycles are called refrigerants. Here the heat is being generally pumped from low level to the higher one & is rejected at high temp.
Another device that transfers heat from a low-temperature medium to a high-temperature one is the heat pump. Refrigerators and heat pumps are essentially the same devices; they differ in their objectives only.
The term refrigeration may be defined as the process of removing heat from a substance under controlled conditions. It also includes the process of reducing heat & maintaining the temperature of a body below the general temperature of its surroundings.
In other words the refrigeration means a continued extraction of heat from a body whose temp is already below the temperature of its surroundings. A refrigerator is a reversed heat engine or a heat pump which takes out heat from a cold body & delivers it to a hot body. The refrigerant is a heat carrying medium which during their cycle in a refrigeration system absorbs heat from a low temperature system & delivers it to a higher temperature system.
Its advantages in comparison with other types of refrigeration are the compact design of the system’s elements; high coefficient of performance (COP); reliability, safety, and flexibility in operation; relative simplicity of its maintenance; and reasonable price. This rejection of heat from low level to higher level of temperature can only be performed with the help of external work according to second law of thermodynamics.
Refrigeration Terms:
Refrigeration Terms:
Heat Exchanger -
A device for the
transfer of heat energy from the source to the conveying medium, with the
latter often is being air or water. Most common combinations are: Refrigerant
to air or Refrigerant to water, Water to air, Steam to air, Steam to
water.
Humidity -
The total amount of
moisture in air. Relative humidity (RH), is the amount of moisture in air,
relative to its total capability based upon its temperature (dew point).
Moisture will condense on surfaces which are below this dew point.
Absorbent -
Substance with
ability to take-up, or absorb another substance.
Absorber -
A solution or surface
that is capable of soaking up (taking in) another substance or energy form.
Absorption Chiller -
A chiller
that uses a brine solution and water to provide refrigeration without the aid
of a compressor.
Absorption Refrigerator -
Refrigerator which creates low temperatures by using the cooling effect formed
when a refrigerant is absorbed by chemical substance.
Heat -
Invisible energy (except
high intensity infra-red) caused by the motion of molecules within any
substance or matter. Will always travel from warm/hot to cold, via either or a
combination of conduction, convection or radiation. Materials which resist flow
or transfer of heat are called insulators, or insulation.
Temperature -
Degree of hotness
or coldness as measured by a thermometer; measurement of speed of motion of
molecules.
Accumulator -
Storage tank which
receives liquid refrigerant from evaporator and prevents it from flowing into
suction line.
Adiabatic Compression -
Compressing refrigerant gas without removing or adding heat.
Wet Bulb -
Device used in
measurement of relative humidity. Evaporation of moisture lowers temperature of
wet bulb compared to dry bulb temperature in same area.
Sub Cooling -
Process whereas
additional sensible heat (as opposed to latent heat) is removed from condensed
refrigerant liquid prior to the metering device. The proper method for charging
a system utilizing a TXV.
TON -
A unit of measurement used
for determining cooling capacity. One ton is the equivalent of 3024 kcal per
hour.
Adsorbent -
Substance which has
property to hold molecules of fluids without causing a chemical or physical
change.
Air -
Invisible, odorless, and
tasteless mixture of gases (consisting mostly of Nitrogen, Oxygen and Carbon
Dioxide) which form earth's atmosphere.
Calorimeter -
Device used to
measure quantities of heat or determine specific heats.
CFM -
A standard of airflow
measurement. Cubic feet per minute.
Compressor -
The heart or
"pump" within an air conditioning or heat pump system. The compressor
maintains adequate pressure to cause refrigerant to condense and flow in
sufficient quantities to meet the cooling requirements of the system.
Economizer - A mechanism that
removes flash gas from the evaporator.
Fan - A radial or axial flow
device used for moving or producing artificial currents of air.
Filter - Device for removing
small particles from a fluid.
Gas - Vapor phase or state of a
substance.
Thermostat - A temperature control
device. Typically mounted in conditioned space.
Absolute Humidity - Amount of
moisture in the air, indicated in kg water/kg dry air.
Absolute Pressure - Gauge
pressure plus atmospheric pressure.
Absolute Temperature -
Temperature measured from absolute zero.
Absolute Zero Temperature -
Temperature at which molecular motion ceases.
Insulation - Any material or
substance which has the ability to retard the flow or transfer of heat.
Latent Heat -
Heat energy
absorbed in process of changing form of substance (melting, vaporization,
fusion) without change in temperature or pressure. Also referred to as
"hidden" heat.
Manometer -
Instrument to
measuring pressure of gases and vapors. Gas pressure is balanced against column
of liquid such as mercury, in U-shaped tube.
Metering Device -
TXV, capillary
tube assembly, constant pressure expansion valve or bullet type piston orifice
designed to regulate flow of liquid refrigerant entering the evaporator.
Creates pressure drop to allow liquid refrigerant to boil and absorb latent
heat. Separates high side of system from low side.
Refrigerant -
A substance
produces a refrigerating or cooling (heat absorbing) effect while expanding or
vaporizing.
Refrigeration -
The moving of
heat from an undesirable location, to that of a location where its presence is
less undesirable.
Safety Control -
Device used to
electrically shut down a refrigerating unit when unsafe pressures and/or
temperatures exist.
Saturation Temperature -
The
temperature where a refrigerant exists in both liquid and vapor form relative
to its measured pressure.
Super Heat -
The temperature rise within an
evaporator/suction line assembly from the evaporator's saturation temperature.
TXV - Thermostatic Expansion
Valve-
A metering valve which acts as a superheat controller. Most are
mechanically operated, and utilize a remote sensing bulb attached to the outlet
of the evaporator assembly to regulate flow of sub-cooled liquid refrigerant at
the evaporator inlet.
Valve, Solenoid - Valve actuated
by magnetic action by means of an electrically energized coil.
Pascal's Law - A pressure imposed
upon a fluid is transmitted equally in all directions.
Psychrometer -
Either a sling type, or electronic. Instrument
used to determine wet bulb temperatures and relative humidity. Combining RH
with dry bulb temperature will yield total heat.
Range - Pressure or temperature
settings of a control; change within limits.
Water -Cooled Condenser -
Condensing unit which is cooled through use of water.
The total amount of heat being rejected to the outside body consists of two parts:-
- The heat extracted from the body to be cooled.
- The heat equivalent to the mechanical work required for extracting it.
- The objective of a refrigerator is to remove heat (QL) from the cold medium.
-The objective of a heat pump is to supply heat (QH) to a warm medium.
(COP) R Refrigeration = Desired Output/Required Input
= Cooling effect/Work input
= Q L /Wnet, in
(COP) H Heat pump = Desired Output/Required Input
= Heating effect/Work input
= Q H /Wnet, in
(COP) H Heat pump = (COP) L refrigeration + 1
A refrigerator is a reverse heat engine run in the reverse direction by means of external aid.
Every type of refrigeration system used for producing cold must have the following four basic units:-
• Low temperature thermal sink to which the heat is rejected for cooling the space.
• Means of extracting the heat energy from the sink, raising its level of temperature before delivering it to heat receiver.
• A receiver is a storage to which the heat is transferred from the high temperature, high pressure
refrigerant.
The cooling capacity of a refrigeration system – is the rate of heat removal from the refrigeration space – is often expressed in terms of ton of refrigeration.
The capacity of a refrigeration system that can freeze 1 ton of liquid water at 0 °C into ice at 0 °C in 24 h is said to be 1 ton.
Two modes of operations:
1. Ideal vapor-compression refrigeration cycle
2. Actual vapor-compression refrigeration cycle
Principles of Refrigeration
Gases give off heat when changed from gas to liquid. Liquids absorb heat when changed from liquid to gas. The refrigerant must be used continually. So, that all air conditioners use the same cycle of compression, condensation, expansion, and evaporation in a closed circuit. The same refrigerant is used to move the heat from one area, to cool this area, and to expel this heat in another area.
The refrigerant comes into the compressor as a low-pressure gas, it is compressed and then moves out of the compressor as a high-pressure gas. The gas then flows to the condenser. Here the gas condenses to a liquid, and gives off its heat to the outside air. The liquid then moves to the expansion valve under high pressure. This valve restricts the flow of the fluid, and lowers its pressure as it leaves the expansion valve. The low-pressure liquid then moves to the evaporator, where heat from the inside air is absorbed and changes it from a liquid to a gas. As a hot low-pressure gas, the refrigerant moves to the compressor where the entire cycle is repeated.
Ideal vapor-compression refrigeration cycle
The vapor-compression refrigeration cycle is the ideal model for refrigeration systems, air conditions and heat pumps.
It consists of four processes:
1-2 Isentropic compression in compressor.
2-3 Constant-pressure heat rejection in a condenser.
3-4 Throttling in an expansion devise.
4-1 Constant-pressure heat absorption in an evaporator.
Schematic and T-s diagram for the ideal vapor-compression refrigeration cycle.
The process in ideal vapor compression refrigeration cycle:
The refrigerant enters the compressor at state 1 as saturated vapor and is compressed isentropically to the condenser pressure. The temperature of the refrigerant increases during this isentropic compression process to well above the temperature of the surrounding medium. The refrigerant then enters the condenser as super heat vapor at state 2 and leaves as saturated liquid at state 3 as a result to the heat rejection to the surrounding. The saturated liquid at state 3 enters an expansion valve or capillary tube and leaves at evaporator pressure. The temperature of refrigerant drop below the temperature of refrigerated space during this stage. The refrigerant enters the evaporator at stage 4 as saturated mixture and it completely evaporate by absorbing the heat from the refrigerated space. The refrigerant leaves the evaporator as saturated vapor and reenters the compressor, completing the cycle.
The area under the curve for the process 4-1 represents the heat absorbed from the refrigeration space.
Area under the curve 2-3 represented the heat rejected to the surrounding.
COP improves by 2-4% for each compressor if the evaporating temperature is raised or the condensing temperature is lowered.
process curve 4’-4 in fig) and the net work input would decrease (by the amount of work output of the turbine). Replacing the expansion valve by the turbine is not practical, since the added benefits cannot justify the added cost and complexity
(q in – q out) + (W in – W out) = h e – h i
COP Refrigeration = qL/W net, in = (h1 – h4)/(h2 – h1)
COP Heat pump = qH/W net, in = (h2 – h3)/(h2 – h1)
An actual vapor-compression refrigeration cycle
An actual vapor-compression refrigeration cycle differs from the ideal one owing mostly to the irreversibility that occur in various components, mainly due to fluid friction (causes pressure drops) and heat transfer to or from the surroundings.
The COP decreases as a result of irreversibility.
DIFFERENCES
Non-isentropic compression
Superheated vapor at evaporator exit
Sub cooled liquid at condenser exit
Pressure drops in condenser and evaporator
Schematic and T-s diagram for the actual vapor-compression refrigeration cycle.
Heat Transfer Rates
One thing that we would like to optimize in the
refrigeration loop is the rate of heat transfer. Materials like copper and
aluminum are used because they have very good thermal conductivity. In other
words heat can travel through them easily. Increasing surface area is another
way to improve heat transfer. In small engines cooling fins formed into the
casting around the piston area. This is an example of increasing the surface
area in order to increase the heat transfer rate. The hot engine can more
easily reject the unwanted heat through the large surface area of the fins
exposed to the passing air. Refrigeration heat transfer devices such as air
cooled condensers and evaporators are often made out of copper pipes with
aluminum fins and further enhanced with fans to force air through the fins.
SELECTING THE RIGHT REFRIGERANT
Several refrigerants may be used in refrigeration systems such as chlorofluorocarbons (CFCs), ammonia, hydrocarbons (propane, ethane, ethylene, etc.), carbon dioxide, air (in the air-conditioning of aircraft), and even water (in applications above the freezing point).
• R-11, R-12, R-22, R-134a, and R-502 account for over 90 percent of the market.
• The industrial and heavy-commercial sectors use ammonia (it is toxic).
• R-11 is used in large-capacity water chillers serving A-C systems in buildings.
• R-134a, R-407C (replaced R-12, which damages ozone layer) is used in domestic refrigerators and freezers, as well as automotive air conditioners.
• R-22 is used in window air conditioners, heat pumps, air conditioners of commercial buildings, and large industrial refrigeration systems, and offers strong competition to ammonia.
• CFCs allow more ultraviolet radiation into the earth’s atmosphere by destroying the protective ozone layer and thus contributing to the greenhouse effect that causes global warming. Fully halogenated CFCs (such as R-11, R-12, and R-115) do the most damage to the ozone layer. Refrigerants that are friendly to the ozone layer have been developed.
• Two important parameters that need to be considered in the selection of a refrigerant are the temperatures of the two media (the refrigerated space and the environment) with which the refrigerant exchanges heat.
• Means of reducing the pressure & temp. of the refrigerant before it return to the sink.
• This is the most important system from the point of commercial & domestic utility & most practical form of refrigeration.
• The working fluid refrigerant used in this refrigeration system readily evaporates & condenses or changes alternatively between the vapour & liquid phases without leaving the refrigerating plant
• During evaporation it absorbs heat from the cold body or in condensing or cooling it rejects heat to the external hot body .
• The heat absorbed from cold body during evaporation is used as its latent heat for converting it from liquid to vapour.
• Thus a cooling effect is created in working fluid.
• This system of refrigeration thus act as latent heat pump since its pump its latent heat from the cold body or brine & rejects it or deliver it to the external hot body or the cooling medium.
• According to the law of thermodynamics , this can be done only on the expenditure of energy which is supplied to the system in the form of electrical energy driving the compressor.
• The vapour compression cycle is used in most of the modern refrigeration systems in large industrial plants.
• The vapour in this cycle is circulated through the various components of the system, where it undergoes a number of changes in its state or condition.
• Each cycle of operation consists of the four fundamental changes of state or processes:-
- Expansion
- Vaporisation
- Compression
- Condensation
The low pressure & temp. refrigerant from evaporator is drawn into the compressor through the inlet or suction valve , where it is compressed to a high pressure & temp.
The high pressure & temp vapour refrigerant is discharged into the condenser through the delivery or discharge valve.
Reciprocating compressor:
The compressors in which the vapour refrigerant is compressed by reciprocating motion of the piston are called reciprocating compressors. These compressors are used for refrigerant which have comparatively low volume per Kg and a large differential press. Such as NH¬3 (R-717), R-12, R-22 and CH3Cl (R-40). The reciprocating compressors are available in sizes as small as ½ KW which are used in small domestic refrigeration and up to about 150 KW for large capacity.
The two types of reciprocating compressor in general are: -
• Single acting vertical compressor.
• Double acting horizontal compressor.
The single acting compressors usually have their cylinder arranged vertically radially or in ‘V’ or ‘W’ form. The double acting compressors usually have their cylinder arranged horizontal.
Working: -
When the piston moves downwards, the refrigerant left in the clearance space expands. Thus, the volume of the cylinder increase and the pressure inside the cylinder decreases. When the pressure become slightly less than the valve gets opened and the vapour refrigerant flows into the cylinder. This flow continuous until the piston reaches the bottom of the stroke. At bottom of the stroke, the suction valve closes because of spring action. Now, when the piston moves upwards, the volume of the piston moves upwards, the volume of the cylinder decreases and the pressure inside the cylinder increases. When the pressure inside the cylinder becomes greater than that on the top of the discharge valve, the discharge valve gets opened & the vapour refrigerant is discharged into the condenser and the cycle is repeated.
Rotary compressor
In rotary compressor, the vapour refrigerant from the evaporator is compressed due to movement of blades. The rotary compressors are positive displacement type compressor. Since, the clearance in rotary compressors is negligible; therefore, they have high volume. These may be used for refrigerants like R-134a, R-22, R-407C and R-144 & NH3.
The two types of rotary compressors are: -
Single stationary blade type
Rotating blade type
Single stationary blade type
It consists of a stationary cylinder, a roller and a shaft. The shaft has an eccentric on which the roller is mounted. A blade is set into the slot of a cylinder in such a manner that it always maintains contacts with a sloter by means of a spring. The blade moves in and out of the slot to follow the rotor when it rotates. Since the blade separates the suction and discharge parts, therefore it is often called a sealing blade. When the shaft rotates, the roller also rotates the roller rotates so that it always touches the cylinder wall.
Rotating Blade type
It consists of a cylinder and a slotted rotor containing a number of blades. The centre of the rotor is eccentric with the centre of the cylinder. The blades are forced against the cylinder wall by the centrifugal action during the rotation of the motor.
The low pressure and temperature vapour refrigerant from the evaporator is drawn through the suction port. As the rotor turns, the suction vapour refrigerant entrapped between the two adjacent blades is compressed. The compressed refrigerant at high pressure and temp is discharged through the discharge port to the condenser.
Centrifugal Compressor
The centrifugal compressor increases the pressure of low pressure vapour refrigerant to a high pressure by centrifugal force. The centrifugal compressor is generally used for refrigerants that require large displacement and low condensing pressure, such as R-134a is and R-407C.
A single stage centrifugal compressor, in its simplest form, consists of an impeller to which a number of curved vanes are fitted symmetrically. The impeller rotates in an air volute casing with inlet and outlet points.
The impeller draws in low pressure vapour refrigerant from the evaporator. When the impeller rotates, it pushes the vapour refrigerant from the centre of the impeller to its periphery by centrifugal force. The high speed of the impeller leaves the vapour refrigerant at a high velocity at the vane tips of the impeller. The kinetic energy thus attained at the impeller outlet is converted into pressure energy when the high velocity vapour refrigerant passes over the diffuser. The diffuser is normally a vane less type as it permits more efficient part load operation which is quite and it further converts the kinetic energy into pressure energy before it leaves the refrigerant to the evaporator.
Condensers
In the condenser heat is removed from the refrigerant and it
is transferred to the ambient, which has a lower temperature. The heat rejected
is the heat absorbed during evaporation plus the heat added by compression.
Types
Water Cooled
Air Cooled
Evaporative
Purpose: Converts High Pressure/Temperature Gas to High Pressure/Temperature Liquid and Rejects Heat to the Air or Water
The condenser or the cooler consists of coils of pipe in which the high pressure & temp. vapour refrigerant is cooled & condensed. The refrigerant while passing through the condenser, rejects its latent heat to surrounding condensing medium which is normally air or water. Thus hot refrigerant vapour received from compressor is converted into liquid form in condenser. The condensed liquid refrigerant from the condenser is stored in a vessel, known as receiver, from where it is supplied to the expansion valve or refrigerant control valve.
Expansion valve
The function of this valve is to allow the liquid refrigerant under high pressure & temp. to pass at a controlled rate after reducing its pressure & temp.
Evaporators
A refrigerant in liquid state will take in / consume heat when evaporating. It is this change of state, which produces cooling effect in refrigeration. Remember that evaporation is a pressure dependent process. If the atmospheric pressure changes (higher altitude for example) we will have a different evaporation temperature. The component, in which the evaporation of the refrigerant takes place, by taking the heat from its surroundings, is called the evaporator.
some of liquid refrigerant evaporates as it passes through the expansion valve, but the greater portion is vaporised in the evaporator at the low pressure & temp.
An evaporator consists of coils of pipes in which the liquid vapour refrigerant at low pressure & temp. is evaporated & changed into vapour refrigerant at low pressure & temp.
During evaporation process, the liquid vapour refrigerant absorbs its latent heat of vaporization from the medium which is to be cooled.
Advantages
• Smaller size for a given refrigerating capacity
• Higher coeff. of performance
• Lower power requirements for a given capacity
• Less complexity in both design & operation
• It can be used over large of temp.
The controls are very essential for satisfactory and economical working of a refrigerant. The electrical connection diagram of a domestic refrigerator is shown in fig. The refrigerant is fitted with following controls.
Starting Relay: -
The starting relay is used to provide the necessary starting torque required to start the motor. It also disconnects the starting winding of the motor when the motor speed increases. When the compressor motor is to be started, the thermostat is in closed position. When the electric supply is given, an electric current passes through the running winding of the motor and the starting relay. Due to the flow of electric current through relay coil & due to electromagnetism, its armature is pulled thereby closing the starting winding contacts. The current through starting winding provides the starting torque and the motor starts. As, the motor speed increase, the running winding current decrease. The current in the starting relay is no longer able to hold the relay and it gets released thereby opening the starting winding contacts. Thus, the starting winding gets disconnected.
Overload protector : -
The basic function is to protect the compressor motor winding from damage due to excessive current, in the event of overloading or due to some fault in the electric circuit. It consists of a bimetallic strip. During the normal working of the compressor, the contacts are closed. Whenever there is any abnormal behavior, the bimetallic strip gets heated and bands, thereby opening the motor contacts, and de-energizing it. The overload protector is fitted on the body of the compressor and operates due to the combined action of heat produced when current passes through the bimetallic strip and a heater element, and heat transferred from the compressor body. It may be noted that the abnormal behavior of compressor may be due to low voltage, high voltage, high load, low suction pressure, high suction & discharge pressure.
Thermostat: -
A thermostat is used to control the temperature in the refrigeration. The bulb of the thermostat is clamped to the evaporator or Freezer. The thermostat bulb is charged with few drops of refrigerant. The thermostat can be set to maintain different temperature at a time. When the desired temperature is obtained, the bulb of the thermostat senses it; the liquid in it compresses and operates the bellows of the thermostat and open compressor motor contacts. The temperature at which motor stops is called cut-out temperature. When the temperature increases, the liquid in the bulb expands thereby closing the bellow contact of the compressor motor. The temperature, at which compressor motor starts, is called cut-in temperature. A thermostat is very crucial in operation of refrigerator as the running time of compressor is reduced considerably thereby cutting the operation cost as well as enhancing the compressor life due to non-continuous working.
Principles of absorption refrigeration
Both the mechanical vapor compression refrigeration cycle and the absorption refrigeration cycle accomplish the removal of heat through the evaporation of a refrigerant at a low pressure and the rejection of heat through the condensation of the refrigerant at a higher pressure.
The method of creating the pressure difference and circulating the refrigerant is the primary difference between the two cycles.
The mechanical vapor compression cycle employs a mechanical compressor to create the pressure difference necessary to circulate the refrigerant.
In the absorption system, another liquid, which is called absorbent, is used to circulate the refrigerant.
The main components of a basic absorption system are shown diagrammatically in Fig.
Comparing the Fig, it can be seen that the condenser, expansion valve and evaporator in these two cycles are the same, the mechanical compressor, however, is replaced by a thermal compressor which consists of absorber, solution pump, generator (or boiler) and liquid valve.
This group of components ‘sucks’ vapor from the evaporator, and delivers high pressure vapor to the condenser, just as the mechanical compressor does but the vapor is actually absorbed by a liquid absorbent .
Aqua ammonia and aqua lithium bromide solutions are commonly used in vapor absorption refrigeration systems. In an ammonia-water absorption system, ammonia is used as the refrigerant and water as the absorbent.
In lithium bromide-water absorption refrigeration systems, water is the refrigerant and lithium bromide is the absorbent.
This explains that the lithium bromide absorption system is strictly limited to evaporation temperatures above 0˚C; and the ammonia absorption system is mainly used for low temperatures below 0˚C.
Composition of mixtures
Calculation of absorption refrigerators requires some knowledge of the thermodynamics of solutions and of how their properties depend on the composition.
Composition of a mixture is expressed as the mass fraction ΞΎ of one of the components.
For example, in H2O–LiBr solution it contains mass mL of LiBr and mW of H2O, the mass fraction of LiBr is defined as:
Sigma = ML/ML + MW
Vapor pressure of LiBr-water solution
The vapor pressure of aqua lithium bromide solution is determined by its temperature and mass fraction. Their relationship is shown in Fig.
The abscissa is temperature in linear scale; the ordinate on the left-hand is vapor pressure in logarithmic scale; the ordinate on the right-hand is temperature in linear scale, shows the saturation temperature of pure water which has the same vapor pressure as a BrLi solution at the temperature given by the abscissa.
The line of pure water is also shown in the figure, which is corresponding to a solution of sigma=0 , all the points on the line of pure water have the same values of temperature both on the abscissa and on the ordinate on the right-hand.
Basic Lithium bromide-water absorption refrigeration system
The diagram shown in Fig. is a basic lithium-bromide vapor absorption refrigeration system.
A basic H2O–LiBr absorption refrigeration system consists of 8 main components.
Apart from the evaporator, the condenser and the expansion valve which are found in a mechanical powered vapor compression refrigerator, other five components, namely, a pump, and absorber, a generator, a heat exchanger and a valve fulfill the function of “thermal compressor”:
In the evaporator, the heat (Qe) from low temperature source (To) is transferred to the refrigerant which changes its state from liquid to vapor at low pressure from state 3 to state 4.
In the evaporator, the heat (Qe) from low temperature source (To) is transferred to the refrigerant which changes its state from liquid to vapor at low pressure from state 3 to state 4.
The vapor is then absorbed by the absorbent LiBr in the absorber at low pressure and temperature. During the absorption, the vapor changes into liquid state 5. The absorption is an exothermic process. The heat (Qa) released during the absorption process is rejected to the surroundings. The weak solution is delivered by the pump to the generator (G) at high pressure at the state 7. The solution in the generator is heated to boil off the refrigerant (water) with heat (Qb) from the high temperature source so that the solution is concentrated.
Solution in the generator becomes strong and leaves the generator at state 8. The high pressure and temperature refrigerant vapor at state 1 leaves the generator and flows to the condenser where the refrigerant is cooling down and condensed. The condensing heat (QC) is rejected to the surroundings. The liquid refrigerant then expands through the expansion valve-1 from state 2 to state 3.
This completes the refrigerant cycle. The strong solution leaving the generator flows back to the absorber after passing through the valve-2 and changing its state from state 9 to 10 to complete the solution circulation. It worth noting that the compression done by the pump in the absorption system only consumes a small amount of mechanical work (Wp).