REFRIGERATION & AIR CONDITIONING

CHAPTER 1:AIR REFRIGERATION SYSTEM

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M.S.E., Baripada- 757107 � PREPARED BY : ER. D DASH

LECTURER IN MECHANICAL ENGG DEPT.

INTRODUCTION:

• The mechanism used for lowering or producing low temp. in a body or a space, whose temp. is already below the temp. of its surrounding, is called the refrigeration system.
• Here the heat is being generally pumped from low level to the higher one & is rejected at high temp.

INDRODUCTION:

• 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 temp. of a body below the general temp. of its surroundings.

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AIR REFRIGERATION SYSTEM:

• The air refrigerator is one of the oldest type of refrigeration system.
• In this refrigerator air is used as the refrigerant.
• Air refrigeration system mainly works on two cycles.
• Reversed carnot cycle
• Reversed brayton cycle or belcoleman cycle or joule cycle

REVERSED CARNOT CYCLE

• Reverse Carnot Cycle is a reversible cycle, it is used as an example of a refrigeration cycle operating between a constant temperature heat source and sink. On this basis, various refrigeration cycles are being compared.
• Similarly, a refrigeration working on reverse Carnot cycle, has a maximum COP. It is not possible to make the refrigerating machines working on the principle of reversed Carnot cycle.

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REVERSED CARNOT CYCLE:

Schematic & TS diagram of reverse carnot cycle

PV & TS DIAGRAM OF REVESED CARNOT CYCLE:

Process of Reverse Carnot Cycle

• Four processes of the cycle are as follows.
• Process 1-2: Isentropic Compression.
• Process 2-3: Isothermal Compression.
• Process 3-4: Isentropic Expansion.
• Process 4-1: Isothermal Expansion.

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WORK DONE & COP:

• Work Done Work done during the cycle per kg of air

work done = Heat rejected – Heat absorbed = q_r – q_a

= T₂(S₂-S₃) – T₁(S₂-S₃)

= (T₂ – T₁)(S₂ – S₃)

• C.O.P Coefficient of performance of the system working on the Reverse Carnot cycle.

C.O.P = Heat absorbed/ work done

= q_a/q_a – q_r

= T₁( S₁ – S₃ )/ ( T₂ – T₁ ) (S₁ – S₃ )

= T₁/( T₂ – T₁ )

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BELCOLEMAN CYCLE:

DIFERENT PROCESSES:�

• There are four processes in Belcoleman cycle
• 1-2:const pressure heat addition
• 2-3:const. entropy(adiabetic) compression
• 3-4:const pressure heat rejection
• 4-1:const entropy(adiabetic) expansion

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� � ���Analysis of Bell-Coleman Cycle: �

• Work done per kg of air for the isentropic compression process 2-3 is given by,

W_c =C_P (T₃ – T₂ ) .

• Work developed per kg of air for the isentropic expansion process 4-1 is given by, W_E =C_P (T₄ – T₁).
• Net work required

Wnet = (WC - WE ) = Cp (T3 - T2 ) - Cp (T4 – T1 )…………….(I)

• Net refrigerating effect per kg of air is given by,

Rnet = Cp (T2 – T1)……………………….(II)

COP Continued…

• COP= R_net /W_net = Cp (T2 – T1 )/( Cp (T3 - T2 ) - Cp (T4 – T1 ))
• COP=(T2 – T1 )/((T3 - T2 ) -(T4 – T1 ))
• For isentropic condition COP= T2 / (T3 - T2 )

ADVANTAGES & DISADVANTAGES:

• Advantages:
• Air is a cheaper refrigerant and available easily compared to other refrigerants.
• There is no danger of fire or toxic effects due to leakage.
• The total weight of the system per ton of refrigerating capacity is less.
• Disadvantages:
• The quantity of air required per ton refrigerating capacity is far greater than other systems.
• The COP is low and hence maintenance cost is high.
• The danger of frosting at the expander valves is more as the air taken into the system always contains moisture.

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