REFRIGERATION AND AIR CONDITIONING

Lecture slides by

​

MR. C K SETHI

ASST. PROFESSOR

GIET, BBSR

REFRIGERATION AND AIR CONDITIONING

Refrigeration

• It is defined as the process of providing and maintaining a temperature well below that of surrounding atmosphere.
• In other words refrigeration is the process of cooling substance.

Refrigerators and heat pumps

• If the main purpose of the machine is to cool some object, the machine is named as refrigerator.
• If the main purpose of machine is to heat a medium warmer than the surroundings, the machine is termed as heat pump.

Refrigerator and Heat pump

Heat Pump

Warm Space

Cold Space

Refrig erator

Work Input

Q_R

Work Input

Q_R

Terminologies of Refrigeration

Refrigerating Effect (N): It is defined as the quantity of heat extracted from a cold body or space to be cooled in a given time.

N= Heat extracted from the cold space

Time taken

Specific Heat of water and ice : It is the quantity of heat required to raise or lower the temperature of one kg of water (or ice), through one kelvin or (1⁰

c) in one second.

Specific heat of water, C_pw = 4.19 kJ/kg K Specific heat of ice, C_pice = 2.1 kJ/kg K.

Terminologies of Refrigeration

Capacity of a Refrigeration Unit :

• Capacity of a refrigerating machines are expressed by their cooling capacity.
• The standard unit used for expressing the capacity of

refrigerating machine is ton of refrigeration.

• One ton of refrigeration is defined as, “the quantity of heat abstracted (refrigerating effect) to freeze one ton of water into one ton of ice in a duration of 24 hours at 0^o c”.

Heat extracted from at 0^o c = latent heat of ice

Latent heat of ice = 336 kJ/kg

i.e., 336 kJ of heat should be extracted from one kg of water at 0^o C to convert it into ice.

Terminologies of Refrigeration

One ton of refrigeration

One ton of refrigeration

= 336x1000 kJ/24 hrs.

= 336x1000 kJ/min

24x60

= 233.333 kJ/min

= 3.8889 kJ/sec

Terminologies of Refrigeration

Co efficient of Performance: It is defined as the ratio of heat extracted in a given time (refrigerating effect) to the work input.

Co efficient of performance = Heat extracted in evaporator

Co efficient of performance =

Work Input Refrigerating Effect

Co efficient of performance =

Work Input

N W

The COP is always greater than 1 and known as theoretical coefficient of performance.

Refrigerants

Refrigerant: Any substance that absorbs heat through expansion and vaporisation process and loses heat due to condensation is a refrigeration process is called refrigerant.

Some examples of refrigerants are,

• Air
• Ammonia (NH₃)
• Carbon dioxide (CO₂)
• Sulphur dioxide (SO₂)
• Freon – 12
• Methyl Chloride
• Methylene chloride.

Classification of Refrigerants

Refrigerants are classified as,

(a) Primary Refrigerants: It is a working medium which is used for cooling the substance by absorption of latent heat.

E.G Ammonia (NH₃), Carbon dioxide (CO₂), Sulphur dioxide (SO₂), Freon 12, etc.,

(b) Secondary Refrigerants: Secondary refrigerant is a substance already cooled by primary refrigerant and then employed for cooling purposes.

E.g Ice, solid carbon dioxide.

These refrigerants cool the substance by absorption of their sensible heat.

Types of Refrigerators

• Ice Refrigerators : Ice is kept in the cabinet of refrigerators and this acts as the refrigerating means.
• Air Refrigerators : Air is used as working agent in these types of refrigerators.

E.g., Bell Coleman Cycle.

• Vapour Refrigerators: The working agents employed in this type of refrigerators are ammonia, CO_2, SO_2, freons etc.,

Applications of Refrigeration

• In chemical industries, for separating and liquefying

the gases.

• In manufacturing and storing ice.
• For the preservation of perishable food items in cold storages.
• For cooling water.
• For controlling humidity of air manufacture and heat treatment of steels.
• For chilling the oil to remove wax in oil refineries.
• For the preservation of tablets and medicines in pharmaceutical industries.
• For the preservation of blood tissues etc.,
• For comfort air conditioning the hospitals, theatres, etc.,

Properties of Refrigeration

• A good refrigerant should have high latent heat of

vapourisation.

• It should have low boiling and low freezing point.
• It should be non toxic and should non corrosiveness
• It should be non flammable and non explosive.
• It should have high thermal conductivity
• It should be easy to handle
• It should have low specific volume of vapour.
• It should have high co efficient of performance

Vapour Compression Refrigeration System

S – entropy means transformation, increases with increase in temperature and decreases with decrease in temperature

ᵹ Q = T ds

Vapour Compression Refrigeration System - Construction

• This system consists of a compressor, condenser, a receiver tank, an expansion valve and an evaporator.
• Compressor : Reciprocating compressors generally used. For very big plants centrifugal

compressors directly coupled with high speed rotating engines (gas turbine) are used.

Vapour Compression Refrigeration System - Construction

Compressor: For very big plants Centrifugal compressors directly coupled with high speed rotating engines

(gas turbine) are used

Vapour Compression Refrigeration System - Construction

• Condenser : It is a coil of tubes made of copper.
• Receiver tank: It is the reservoir of liquid refrigerant.
• Expansion Valve: This is a throttle valve. High pressure refrigerant is made to flow at a controlled rate through this valve.
• Evaporator : It is the actual cooler and kept in the space to be cooled. The evaporator is a coil of tubes made of copper

Vapour Compression Refrigeration System -

Working

Working :

1. The low pressure refrigerant vapour coming out of the

evaporator flows into the compressor.

2. The compressor is driven by a prime mover.
3. In the compressor the refrigerant vapour is compressed.
4. The high pressure refrigerant vapour from the compressor is then passed through the condenser.
5. The refrigerant gives out the heat it had taken in the evaporator (N)

Vapour Compression Refrigeration System

- Working

Vapour Compression Refrigeration System

- Working

Working :

11. Most of the refrigerant is vapourised only in the evaporator, at a low pressure.
12. In the evaporator the liquid refrigerant absorbs its latent heat of vapourisation from the material which is to be cooled.
13. Thus the refrigerating effect (N) is obtained.
14. Then the low pressure refrigerant enters the compressor and the cycle is repeated.

Vapour Absorption Refrigeration system

In this system compression process of vapour compression cycle is eliminated. Instead of that the following three processes are carried out.

1. Absorbing ammonia vapour into water.
2. Pumping this solution to a high pressure cycle
3. Producing ammonia vapours from ammonia solution by heating.

Vapour Absorption Refrigeration system

Vapour Absorption Refrigeration system - Construction

Construction:

• The vapour absorption system consists of a condenser, an expansion valve and an evaporator.
• They perform the same as they do in vapour compression method.
• In addition to these, this system has an absorber, a heat exchanger, an analyser and a rectifier.

Vapour Absorption Refrigeration system – Working

Working:

1. Dry ammonia vapor at low pressure passes in to the absorber from the evaporator.
2. In the absorber the dry ammonia vapor is dissolved in cold water and strong solution of ammonia is formed.
3. Heat evolved during the absorption of ammonia is removed by circulating cold water through the coils kept in the absorber.
4. The highly concentrated ammonia (known as Aqua Ammonia) is then pumped by a pump to generator through a heat exchanger.

Vapour Absorption Refrigeration system - Construction

Working:

5. In the heat exchanger the strong ammonia solution is heated by the hot weak solution returning from the generator to the absorber.
6. In the generator the warm solution is further heated by steam coils, gas or electricity and the ammonia vapour is driven out of solution.
7. The boiling point of ammonia is less than that of water.
8. Hence the vapours leaving the generator are mainly of ammonia.

Vapour Absorption Refrigeration system - Construction

Working:

9. The weak ammonia solution is left in the generator is called weak aqua.
10. This weak solution is returned to the absorber through the heat exchanger.
11. Ammonia vapours leaving the generator may contain some water vapour.
12. If this water vapour is allowed to the condenser and expansion valve, it may freeze resulting in chocked flow.
13. Analyser and rectifiers are incorporated in the system before condenser.

Vapour Absorption Refrigeration system - Construction

Working:

14. The ammonia vapour from the generator passes through a series of trays in the analyser and ammonia is separated from water vapour.
15. The separated water vapour returned to generator.
16. Then the ammonia vapour passes through a rectifier.
17. The rectifier resembles a condenser and water vapour still present in ammonia vapour condenses and the condensate is returned to analyser.
18. The virtually pure ammonia vapour then passes through the condenser.

Vapour Absorption Refrigeration system - Construction

Working:

19. The latent heat of ammonia vapour is rejected to the cooling water circulated through the condenser and the ammonia vapour is condensed to liquid ammonia.
20. The high pressure liquid ammonia is throttled by an expansion valve or throttle valve.
21. This reduces the high temperature of the liquid ammonia to a low value and liquid ammonia partly evaporates.
22. Then this is led to the evaporator.
23. In the evaporator the liquid fully vaporizes.

Vapour Absorption Refrigeration system - Construction

Working:

24. The latent heat of evaporation is obtained from the brine or other body which is being cooled.
25. The low pressure ammonia vapour leaving the evaporator again enters the absorber and the cycle is completed.
26. This cycle is repeated again to provide the refrigerating effect.

Applications of refrigeration system

• Preservation of food items like vegetables, milk and eggs.
• Preservation of medicines.
• Preservation of blood, tissues, etc.,
• Preservation and cooling of cool drinks.
• Preservation of chemicals (Chemical industries)
• Cooling of water.
• Industrial and comfort airconditioning.
• Processing of dairy products.

Comparison between Vapour compression & Vapour Absorption refrigeration systems

Layout of Domestic Refrigerator

Layout of Domestic refrigerator

Domestic refrigerator……

• House hold refrigerators use vapor compression cycle
• Less energy: (90 W to 600 W)
• Due to small and high efficiency motors and compressors,
• better insulation materials,
• large coil surface area,
• better door seals
• Designed to maintain:
• Freezer section -18 °C
• Refrigeration section at 3°C

Domestic refrigerator……

• Insulation materials:
• Fiber glass, k= 0.032 W/m °C
• Urethane foam, k= 0.019 W/m °C
• Wall thickness for foam
• For freezer section reduced from 90 to 48 mm
• For refrigeration section reduced from 70 to 40 mm
• Works better up to the environment of 43°C
• Ice maker (2 to 3 kg/day)
• Vapor absorption is more expensive and less efficient

Domestic refrigerator……

• Energy consumption can be minimized for practicing good measures
• Open the refrigeration doors fewest times possible
• Cool the hot foods to room temperature
• Clean the condenser coil behind the refrigerator
• Check the door gaskets for air leaks
• Avoid unnecessary low temperature settings
• Avoid excessive ice build up
• Use the power saver switch
• Do not block the air flow passages to and from the

condenser coil.

AIR CONDITIONING

AIR CONDITIONING:

​

Air Conditioning is the process of conditioning the air according to the human comfort, irrespective of external conditions.

AIR CONDITIONING

Applications of Air Conditioning

• Used in offices, hotels, buses, cars.,etc
• Used in industries having tool room machines.
• Used in textile industries to control moisture.
• Used in printing press.
• Used in Food industries, Chemical plants.

Air conditioning systems are classified as

CLASSIFICATION OF AIR CONDITIONING

1. According to the purpose
1. Comfort Air conditioning.
2. Industrial Air conditioning.
2. According to Season of the year
• Summer Air conditioning.
• Winter Air conditioning.
• Year round Air conditioning.

AIR CONDITIONING

Types of Air conditioners

1. Room Air conditioners
2. Winter Air conditioners
3. Central Air conditioners Functions of Air conditioners
4. Cleaning air.
5. Controlling the temp of air.
6. Controlling the moisture content.
7. Circulating the air.

TERMINOLOGIES

1. Dry air: The atmospheric air which no water vapour is called dry air.
2. Psychrometry: Psychrometry is the study of the properties of atmospheric air.
3. Temperature: The degree of hotness (or) Coldness is called the temperature.
4. Moisture: Moisture is the water vapour present in the air.

TERMINOLOGIES

Humidity: mass of water vapor present in 1kg of dry air

Absolute humidity: mass of water vapor present in 1cu.m of dry air

5. Relative humidity: Relative humidity is the ratio of actual mass of water vapour in a given volume to the mass of water vapour actually can withhold by the same volume.
6. Dry bulb temperature: The temperature of air measured by the ordinary thermometer is called dry bulb temperature:

TERMINOLOGIES

7. Wet bulb Temperature: The temperature of air measured by the thermometer when it is covered by the wet cloth is known as wet bulb Temperature.
8. Dew point Temperature: The temperature at which the water vapour starts condensing is called dew point Temperature
9. Wet bulb depression: (DBT- WBT) indicates relative humidity
10. Dew point depression: (DBT- DPT)

Window Type Air Conditioner

Window Type Air Conditioner

Window Type Air Conditioner

Window Type Air Conditioner - Working

• The low pressure vapour refigerant from the evaporator is sucked by compressor through the open inlet valve.
• The compressor compresses the vapour

refrigerant.

• The high pressure and high temperature vapour refrigerant then flows to the condenser through the open outlet valve.
• In the condenser, the outside atmospheric temperature in summer being around 42^o C, air is circulated by fan.
• After condensation, the high pressure liquid refrigerant formed passes through an expansion valve which reduces its pressure

Window Type Air Conditioner - Working

• The low pressure refrigerant then enters the evaporator and evaporates, thus absorbing latent heat of vapourisation from the room air.
• The equipment which is used for evaporating the refrigerant is called evaporator.
• After evaporation, the refrigerant becomes vapour.
• The low pressure vapour is again passed to the compressor. Thus the cycle is repeated.
• A partition separates high temperature side of condenser, compressor and low temperature side of evaporator

Window Type Air Conditioner -

Working

ps

• The quantity of air circulated can be

controlled by the dampers.

• The moisture in the air passing over the evaporator coil is dehumidified and dri into the trays.
• The unit automatically stops when the required temperature is reached in the room. This is accomplished by the thermostat and control panel.
• Generally, the refrigerant monochloro difluro methane (CHCLF₂) is used in air conditioner. It is called Freon 22.

Merits and Demerits of Window type air

conditioner

Merits :

• A separate temperature control is provided in each

room.

• Ducts are not required for distribution.
• Cost is less.
• Skilled technician is required for installation.

Demerits:

• It makes noise.
• Large hole is made in the external wall or a large opening to be created in the window panel. This leads to insecurity to inmates.
• Air quantity cannot be varied.

Split Type Air Conditioner - Construction

Split Type Air Conditioner - Layout

Split Type Air Conditioner - Layout

Split Type Air Conditioner - Layout

• In split air type air conditioner noise making components like compressor and condenser are mounted outside or away from room.
• Split type air conditioning system has two main components.
1. Outdoor Unit (ii) Indoor unit.

​

• The outdoor unit consists of compressor and condenser.
• The indoor unit consists of power cables, refrigerant tube and an evaporator mounted inside the room.

Split Type Air Conditioner - Working

• Compressor is used to compress the refrigerant.
• The refrigerant moves between the evaporator and condenser through the circuit of tubing and fins in the coils.
• The evaporator and condenser are usually made of coil of copper tubes and surrounded by aluminium fins.
• The liquid refrigerant coming from the condenser

evaporates in the indoor evaporator coil.

• During this process the heat is removed from the

indoor unit air and thus, the room is cooled.

• Air return grid takes in the indoor air.
• Water is dehumidified out of air is drained through the

drain pipe.

Split Type Air Conditioner -Working

• The hot refrigerant vapour is passed to the compressor and then to the condenser where it becomes liquid.
• Thus the cycle is repeated.
• A thermostat is used to keep the room at a constant, comfortable temperature avoiding the frequent turning on off.

Merits and Demerits of Split type air

conditioner

Merits :

• It is compact
• It is energy and money saving.
• Duct is not used.
• Easier to install.
• It is noiseless, because rotary air compressor used is, kept outside.
• It is more efficient and powerful.
• It has the flexibility for zoning.

Merits and Demerits of Split type air

conditioner

DeMerits :

• Initial cost is higher than window air conditioner
• Skilled technician is required for installation.
• Each zone or room requires thermostat to control the air cooling.

Applications of air conditioning

• Used in houses, hospitals, offices, computer centres, theatres, departmental stores etc.,
• Air-conditioning of transport media such as buses, cars trains, aeroplanes and ships.
• Wide application in food processing, printing, chemical, pharmaceutical and machine tool, etc.,

Vapor-Compression Refrigeration Cycle

control volumes involving these components:

►Evaporator

►Compressor

►Condenser

►Expansion valve

►Most common refrigeration cycle in use today

►There are four principal

All energy transfers by work and heat are taken as positive in the directions of the arrows on the schematic and energy balances are written accordingly.

Two-phase

liquid-vapor mixture

The Vapor-Compression Refrigeration Cycle

work input.

Process 2-3: vapor refrigerant

condenses to liquid through heat

transfer to the cooler surroundings. Process 3-4: liquid refrigerant expands to the evaporator pressure.

►The processes of this cycle are

Process 4-1: two-phase liquid-vapor mixture of refrigerant is evaporated through heat transfer from the refrigerated space.

Process 1-2: vapor refrigerant is compressed to a relatively high temperature and pressure requiring

Two-phase liquid-vapor mixture

The Vapor-Compression Refrigeration Cycle

►Engineering model:

►Each component is analyzed as a control volume at steady state.

►Dry compression is presumed: the refrigerant is a vapor.

►The compressor operates adiabatically.

►The refrigerant expanding through the valve undergoes a throttling process.

►Kinetic and potential energy changes are ignored.

The Vapor-Compression Refrigeration Cycle

Evaporator

(Eq. 10.3)

►Applying mass and energy rate balances

1 4

in

_m&

^Q& = h − h

►The term ^Q& is referred to as the

in

refrigeration capacity, expressed in kW in the SI unit system or Btu/h in the English unit system.

►A common alternate unit is the ton of refrigeration which equals 200 Btu/min or about 211 kJ/min.

The Vapor-Compression Refrigeration Cycle

Compressor

Assuming adiabatic compression

Condenser

Expansion valve

Assuming a throttling process

^c = h₂ − h₁

_m&

_W&

h₄ = h₃

(Eq. 10.5)

(Eq. 10.6)

(Eq. 10.4)

►Applying mass and energy rate balances

out

= h₂ − h₃

_m&

_Q&

�The Vapor-Compression Refrigeration Cycle

►

Coefficient of Performance (COP)

Carnot Coefficient of Performance

This equation represents the maximum theoretical coefficient of performance of any refrigeration cycle operating between cold and hot regions at T_C and T_H, respectively.

Example 11-1

8

Refrigerant-134a is the working fluid in an ideal compression refrigeration cycle. The refrigerant leaves the evaporator at -20^oC and has a condenser pressure of 0.9 MPa. The mass flow rate is 3 kg/min. Find COP_R and COP_{R, Carnot} for the same T_max and T_min , and the tons of refrigeration.

​

Using the Refrigerant-134a Tables, we have

1

1

3

3

2s

2s 1

State1

State 2

Compressor exit

kJ kg

kJ kg

kJ

kJ

State 3

⎫

⎫_⎧

⎪_⎧

_⎪⎪h₁ = 238.41

h = 278.23

Compressor inlet _⎪⎪

⎪⎪

P_2s = P₂ = 900 kPa

⎬⎨

⎬⎨

T = −20^o C

⎪⎪_T

^⎪⎪s = 0.9456

1

= 43.79^o C

_⎪⎩ 2s

kg ⋅ K

s = s = 0.9456

x = 1.0

⎪ _⎩⎪

⎭

^{kg ⋅ K} ⎪_⎭

P = 900 kPa

3

4

4 1

3

4

3

x = 0.0

o

State 4

Throttle exit

kJ

kg

kJ

^⎫⎧h = 101.61 ^kJ

⎫

_⎧x = 0.358

⎪

Condenser exit ^⎪⎪

⎪⎪

⎪⎪

⎬⎨

⎬⎨

s₄ = 0.4053

T = T = −20 C

⎪⎪

^⎪⎪s = 0.3738

kg ⋅ K

⎪ ⎪

_⎪⎩

kg ⋅ K

h = h

⎩

⎭

⎭

₌ m^&(h − h ) ₌ h − h

9

2

1

L 1 4 1 4

R

net ,in 2 1

kg

COP =

(238.41−101.61) ^kJ

₌ kg

(278.23 − 238.41) ^kJ

_Q&

_W&

m^&(h − h ) h − h

min

min

1Ton

^kg 211 ^kJ

= 3 ^kg (238.41−101.61) ^kJ

= 1.94Ton

= 3.44

The tons of refrigeration, often called the cooling load or refrigeration effect, are

Q^& = m^&(h − h )

L 1 4

R, Carnot

COP

=

T_L T_H − T_L

(−20 + 273) K

=

(43.79 − (−20))K

= 3.97

power to the compressor, in horsepower, is required for each ton of cooling.

10

Another measure of the effectiveness of the refrigeration cycle is how much input

The unit conversion is 4.715 hp per ton of cooling.

4.715

net , in

L

R

COP

Ton

=

₌ 4.715 hp

3.44 Ton

= 1.37 ^hp

_W&

_Q&

Features of Actual Vapor-Compression Cycle

​

►Heat transfers between refrigerant and cold and warm regions are not reversible.

►Refrigerant temperature in evaporator is less than T_C.

►Refrigerant temperature in condenser is greater than T_H.

►Irreversible heat transfers have negative effect on performance.

Features of Actual Vapor-Compression Cycle

• refrigerant passing through the evaporator is reduced relative to the temperature of the cold region, T_C.
• ►temperature of the refrigerant passing

through the condenser is increased relative to the temperature of the warm region, T_H.

^Trefrigerant ^↓

^Trefrigerant ^↑

Features of Actual Vapor-Compression Cycle

​

►An increase in specific entropy accompanies an adiabatic irreversible compression process. The work input for compression process 1-2 is greater than for the counterpart isentropic compression process 1-2s.

►Since process 4-1, and thus the refrigeration capacity,

is the same for cycles

1-2-3-4-1and 1-2s-3-4-1, cycle

1-2-3-4-1 has the lower COP.

Isentropic Compressor Efficiency

►The isentropic compressor efficiency is the ratio of the minimum theoretical work input to the actual work input, each per unit of mass flowing:

(Eq. 6.48)

work required in an actual compression from compressor inlet state to exit pressure

work required in an isentropic compression from compressor inlet state to the exit pressure

Actual Vapor-Compression Cycle

Example: The table provides steady-state operating data for a vapor-compression refrigeration cycle using R-134a as the working fluid. For a refrigerant mass flow rate of 0.08 kg/s, determine the

1. compressor power, in kW,
2. refrigeration capacity, in tons,
3. coefficient of performance,
4. isentropic compressor efficiency.

Actual Vapor-Compression Cycle

(a) The compressor power is

W^& = m^&(h − h )

c 2 1

⎜

⎝ ⎠

⎟

s kg 1 kJ/s

c

W^& = ^⎛0.08 ^{kg ⎞} 280.15 − 241.35) ^{kJ 1 kW} = _{3.1 kW}

(

(b) The refrigeration capacity is

Q^& = m^&(h − h )

in 1 4

⎠

⎜

⎝

1 ton

kg 211 kJ/min min

_⎟(241.35 − 91.49) ^kJ

s

in

Q^& = ^⎛0.08 ^{kg ⎞}

^{60 s =} 3.41 tons

Actual Vapor-Compression Cycle

(c) The coefficient of performance is

(h₂ − h₁ )

C.O.P = ^{(h1 − h4 )}

(280.15 − 241.35)kJ/kg

_{C.O.P =} (241.35 − 91.49)kJ/kg ₌

3.86

Actual Vapor-Compression Cycle

c

c

(h − h )

s ₌ 2s 1

W^& / m^{& (h}₂ ^{− h}₁⁾

(W^& / m^&)

η_c =

_{ηc =} (272.39 − 241.35)kJ/kg _{= 0.8 = 80%}

(280.15 − 241.35)kJ/kg

(d) The isentropic compressor

efficiency is

p-h Diagram

►The pressure-enthalpy (p-h) diagram is a thermodynamic property diagram commonly used in the refrigeration field.

Other Refrigeration Cycles

20

Cascade refrigeration systems

​

Very low temperatures can be achieved by operating two or more vapor-compression systems in series, called cascading. The COP of a refrigeration system also increases as a result of cascading.

21

Multistage compression refrigeration systems

Multipurpose refrigeration systems

22

A refrigerator with a single compressor can provide refrigeration at several

temperatures by throttling the refrigerant in stages.

Liquefaction of gases

23

Another way of improving the performance of a vapor-compression refrigeration system is by using multi•stage compression with regenerative cooling. The vapor- compression refrigeration cycle can also be used to liquefy gases after some modifications.

Selecting Refrigerants

►Refrigerant selection is based on several factors:

►Performance: provides adequate cooling capacity cost-effectively.

►Safety: avoids hazards (i.e., toxicity).

►Environmental impact: minimizes harm to stratospheric ozone layer and reduces negative impact to global climate change.

Refrigerant Types and Characteristics

Global Warming Potential (GWP) is a simplified index that estimates the potential future influence on global warming associated with different gases when released to the atmosphere.

Refrigerant Types and Characteristics

►Chlorofluorocarbons (CFCs) and Hydrochlorofluorocarbons (HCFCs) are early synthetic refrigerants each containing chlorine. Because of the adverse effect of chlorine on Earth’s stratospheric ozone layer, use of these refrigerants is regulated by international agreement.

►Hydrofluorocarbons (HFCs) and HFC blends are chlorine-free refrigerants. Blends combine two or more HFCs. While these chlorine-free refrigerants do not contribute to ozone depletion, with the exception of R-1234yf, they have high GWP levels.

►Natural refrigerants are nonsynthetic, naturally occurring substances which serve as refrigerants. These include carbon dioxide, ammonia, and hydrocarbons. These refrigerants feature low GWP values; still, concerns have been raised over the toxicity of NH₃ and the safety of the hydrocarbons.

VAPOUR COMPRESSION CYCLE

COP for Ideal Vapor Absorption Refrigeration System

Practical Vapor – Absorption Refrigeration System

•

Practical vapor – absorption refrigeration system

Generator: The strong solution of ammonia refrigerant and water absorbent are heated by the external source of heat such as steam or hot water. It can also be heated by other sources like natural gas, electric heater, waste exhaust heat etc.

Practical vapor absorption refrigeration system

Generator - Due to heating the refrigerant ammonia gets vaporized and it leaves the generator. However, since water has strong affinity for ammonia and its vaporization point is quite low some water particles also get carried away with ammonia refrigerant, so it is important to pass this refrigerant through analyzer.

Vapour Absorption Refrigeration Systems

Based On Water-Lithium Bromide Pair

VARS based on H₂O – LiBr Pair

• Vapour absorption refrigeration systems using water-lithium bromide pair are extensively used in large capacity air conditioning systems.

​

• In these systems water is used as refrigerant and a solution of

lithium bromide in water is used as absorbent.

​

• Since water is used as refrigerant, using these systems it is not possible to provide refrigeration at sub-zero temperatures. Hence it is used only in applications requiring refrigeration at temperatures above 0^oC.

VARS based on H₂O – LiBr Pair

• Hence these systems are used for air conditioning applications. The analysis of this system is relatively easy as the vapour generated in the generator is almost pure refrigerant (water), unlike ammonia-water systems where both ammonia and water vapour are generated in the generator.

Sample Problem in Simple VARS

9. The operating temperatures of a single stage vapour absorption refrigeration system are: generator: 90^oC; condenser and absorber: 40^oC; evaporator: 0^oC. The system has a refrigeration capacity of 100 kW and the heat input to the system is 160 kW. The solution pump work is negligible.

• a) Find the COP of the system and the total heat rejection rate

from the system.

• b) An inventor claims that by improving the design of all the components of the system he could reduce the heat input to the system to 80 kW while keeping the refrigeration capacity and operating temperatures same as before. Examine the validity of the claim.

Sample Problem in Simple VARS

• Solution: