Basic concepts of RAC

1. Basic Concepts
of
Refrigeration & Air Conditioning
Prepared by
Dr. Rakesh Kumar
Professor, Mechanical Engineering
MG Govt. Engg. College Kotla (Jeori)

2. Definition
• Refrigeration: withdrawal of heat from a
substance/space so that its temperature is
lower than that of the natural surroundings
• Air Conditioning: process of treating air to
control simultaneously its temperature,
humidity, cleanliness, and distribution to meet
the requirements of the conditioned space

3. Types of Refrigerations
• Natural Refrigeration
– Use of ice
– Evaporative cooling
• Artificial/Mechanical Refrigeration
– Vapour compression systems
• Domestic refrigeration
• Air Conditioning system
– Vapour absorption systems,
• Ammonia vapour refrigeration system
• Electrolux refrigeration
– Gas cycle systems
– Steam jet refrigeration system
– Thermo electric refrigeration system

4. Applications of Refrigeration
• Food processing, preservation and distribution
• Chemical and process industries
• Special Applications
• Comfort air-conditioning

5. Food Processing, Preservation and
Distribution
– Storage of Raw Fruits and Vegetables
– Fish
– Meat and poultry
– Dairy Products
– Beverages
– Candy
– Processing and distribution of frozen food

6. Chemical and Process Industries
– Separation of gases
– Condensation of Gases
– Dehumidification of Air
– Solidification of Solute
– Storage as liquid at low pressure
– Removal of Heat of Reaction
– Cooling for preservation
– Recovery of Solvents

7. Special Applications
– Cold Treatment of Metals
– Medical
– Ice Skating Rinks
– Construction
– Desalination of Water
– Ice Manufacture

8. Air Conditioning
– Industrial Air conditioning
• Laboratories
• Printing
• manufacturing of Precision parts
• Textile industries
• Pharmaceutical industries
• Photography material
• Computer labs
• Power plants
• Vehicular air conditioning
– Comfort air conditioning
• residences, offices, shopping centers, stores, large buildings,
theatres, auditorium etc.

9. Second Law of Thermodynamics
• It is common sense that heat will not flow spontaneously from a body at
lower temperature to a body at higher temperature. In order to transfer
heat from lower temperature to higher temperature continuously (that is,
to maintain the low temperature) a refrigeration system is needed which
requires work input from external source. This is one of the principles of
second law of thermodynamics, which is known as Clausius statement of
the second law.
Clausius’ statement of second law
• It is impossible to transfer heat in a cyclic process from low temperature to
high temperature without work from external source.
Kelvin-Planck statement of second law
• It is impossible to construct a device (engine) operating in a cycle that will
produce no effect other than extraction of heat from a single reservoir and
convert all of it into work.

10. Heat Engines, Refrigerators & Heat
Pumps
• Heat Engine: A device that operates in a thermodynamic cycle and
does a certain amount of net positive work through the transfer of
heat from a high temperature body to a low temperature body.
– Examples: IC Engines, steam power plants.
• Refrigerator: A device that operates in a thermodynamic cycle and
transfers a certain amount of heat from a body at a lower
temperature to a body at a higher temperature by consuming
certain amount of external work.
– Examples: Domestic refrigerators and room air conditioners
• Heat Pump: It is similar to a refrigerator but required output is the
heat rejected to the high temperature body.
• Examples: Blower

11. REFRIGERATOR AND HEAT PUMP
The transfer of heat from a low-temperature region to a
high-temperature one requires special devices called
refrigerators.
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.
𝐶𝑂𝑃𝑅 =
𝐷𝑒𝑠𝑖𝑟𝑒𝑑 𝑂𝑢𝑡𝑝𝑢𝑡
𝑅𝑒𝑞𝑢𝑖𝑟𝑒𝑑 𝐼𝑛𝑝𝑢𝑡
=
𝐶𝑜𝑜𝑙𝑖𝑛𝑔 𝐸𝑓𝑓𝑒𝑐𝑡
𝑊𝑜𝑟𝑘 𝐼𝑛𝑝𝑢𝑡
=
𝑄𝐿
𝑊𝑛𝑒𝑡,𝑖𝑛
𝐶𝑂𝑃𝐻𝑃 =
𝐷𝑒𝑠𝑖𝑟𝑒𝑑 𝑂𝑢𝑡𝑝𝑢𝑡
𝑅𝑒𝑞𝑢𝑖𝑟𝑒𝑑 𝐼𝑛𝑝𝑢𝑡
=
𝐻𝑒𝑎𝑡𝑖𝑛𝑔 𝐸𝑓𝑓𝑒𝑐𝑡
𝑊𝑜𝑟𝑘 𝐼𝑛𝑝𝑢𝑡
=
𝑄𝐻
𝑊𝑛𝑒𝑡,𝑖𝑛
for fixed values of QL and QH

12. Unit of Refrigeration
• Ton of Refrigeration(TR)
Quantity of heat required to be removed from one
ton of ice within 24 hours when initial condition of
water is zero degree centigrade
• 1 ton of ice (British)=2000 pounds= 909.1 kg
Latent heat of ice = 335 kJ/kg
1𝑇𝑅 =
909.1 ×335
24 ×3600
= 3.5
𝑘𝐽
𝑠
1𝑇𝑅 = 3.5 𝑘𝑊

13. Reversed Carnot cycle
• Reversed Carnot cycle is an ideal refrigeration
cycle for constant temperature external heat
source and heat sinks. Figure 9.1(a) shows the
schematic of a reversed Carnot refrigeration
system using a gas as the working fluid along
with the cycle diagram on T-s and P-v
coordinates. As shown, the cycle consists of
the following four processes:
• Process 1-2: Reversible, adiabatic
compression in a compressor
• Process 2-3: Reversible, isothermal heat
rejection in a compressor
• Process 3-4: Reversible, adiabatic expansion
in a turbine
• Process 4-1: Reversible, isothermal heat
absorption in a turbine

14. Reversed Carnot cycle
The heat transferred during isothermal processes 2-3
and 4-1 are given by:
Applying first law of thermodynamics to the closed
cycle,
• the work of isentropic expansion, w3-4 exactly
matches the work of isentropic compression w1-2.
the COP of the Carnot system is given by:
 
2
3
3
2
3
2 s
s
T
ds
T
q h 



 

 
4
1
1
1
4
1
4 s
s
T
ds
T
q 



 

3
2
4
1 s
s
s
s 


    net
w
w
w
w
q
q
q 





 
 


 1
4
3
2
3
2
1
4 

 










 
1
1
1
4
T
T
T
w
q
COP
h
net
CARNOT

15. Limitations of Carnot cycle
• Carnot cycle is an idealization and it suffers from several
practical limitations. One of the main difficulties with
Carnot cycle employing a gas is the difficulty of achieving
isothermal heat transfer during processes 2-3 and 4-1. For a
gas to have heat transfer isothermally, it is essential to carry
out work transfer from or to the system when heat is
transferred to the system (process 4-1) or from the system
(process 2-3). This is difficult to achieve in practice. In
addition, the volumetric refrigeration capacity of the
Carnot system is very small leading to large compressor
displacement, which gives rise to large frictional effects. All
actual processes are irreversible; hence completely
reversible cycles are idealizations only.