Introduction to refrigeration

1. AIR REFRIGERATION

2. INTRODUCTION:
Air cycle refrigeration systems uses air as the working
fluid.
The gas does not undergo any phase change during the
cycle, consequently, all the internal heat transfer processes are
sensible heat transfer processes.
Gas cycle refrigeration systems find applications in air
craft cabin cooling and also in the liquefaction of various gases.

3. Analysis of air refrigeration cycle is made with the following
assumptions:
i. The working fluid is a fixed mass of air that behaves as
an ideal gas
ii. The cycle is assumed to be a closed loop cycle with
all inlet and exhaust processes of open loop cycles
being replaced by heat transfer processes to or from
the environment
iii. All the processes within the cycle are reversible, i.e.,
the cycle is internally reversible
iv. The specific heat of air remains constant throughout
the cycle

4. REVERSED CARNOT CYCLE
P-V diagram
T-S diagram

5. Reversed Carnot cycle is an ideal refrigeration cycle for constant
temperature external heat source and heat sinks. Figure shows
the schematic of a reversed Carnot refrigeration system using a
gas as the working fluid along with P-v and T-s diagrams. 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

6. ò dq = (q 4-1 + q 2-3 ) = ò dw = (w 2-3 - w 4-1 ) = - w net
The heat transferred during isothermal processes 2-3 and 4-1 are
given by:
q 2-3 = ∮ 𝑇𝑇. 𝑑𝑑𝑠𝑠 = Th (s3 - s2 )
2
q 4-1 = ∮ 𝑇𝑇. 𝑑𝑑𝑑𝑑 = Tl (s1 - s4 )
s1 = s2 and s3 = s4, hence s2 - s3 = s1 - s4
Applying first law of thermodynamics to the closed cycle,
∮ 𝑑𝑑𝑞𝑞 = (q 4-1 + q 2-3 ) = ∮ 𝑑𝑑𝑑𝑑 = (w2-3 - w4-1 ) = - wnet
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:
COP =
𝑄𝑄4−1
𝑊𝑊𝑛𝑛𝑛𝑛𝑛𝑛
=
𝑇𝑇𝑙𝑙
𝑇𝑇ℎ − 𝑇𝑇𝑙𝑙

7. Ideal reversed Brayton cycle or Bell-Coleman cycle

8. Ideal reversed Brayton cycle or Bell-Coleman cycle
Bell-Coleman cycle consists of following four processes:
Process 1-2: Reversible, adiabatic compression in a compressor
Process 2-3: Reversible, isobaric heat rejection in a heat exchanger
Process 3-4: Reversible, adiabatic expansion in a turbine
Process 4-1: Reversible, isobaric heat absorption in a heat exchanger

9. WORKING OF BELLCOLEMAN CYCLE

10. Process 1-2:
Gas at low pressure is compressed isentropically from state 1 to
state 2. Applying steady flow energy equation by neglecting
changes in kinetic and potential energy, we can write:
W1-2 = m(h2-h1) = mcp(T2-T1)
S2 = S1
And T2=T1 (P2/P1) =T1 rp
Where rp =(P2 /P1 ) = pressure ratio
Process 2-3:
Hot and high pressure gas flows through a heat exchanger and
rejects heat sensibly and isobarically to a heat sink. The enthalpy
and temperature of the gas drop during the process due to heat
exchange, no work transfer takes place and the entropy of the gas
decreases. Again applying steady flow energy equation and second
T ds equation.
(γ-1)/ γ (γ-1)/ γ

11. Process 3-4: High pressure gas from the heat exchanger flows
through a turbine, undergoes isentropic expansion and delivers net
work output. The temperature of the gas drops during the process
from T3 to T4. From steady flow energy equation:
Process 4-1: Cold and low pressure gas from turbine flows through the low temperature heat
exchanger and extracts heat sensibly and isobarically from a heat source, providing a useful
refrigeration effect. The enthalpy and temperature of the gas rise during the process due to
heat exchange, no work transfer takes place and the entropy of the gas increases. Again
applying steady flow energy equation and second T ds equation:
Q2-3=m(h2–h3) = mcp(T2–T3)
s2-s3 = cpIn(T2/T3)
P2=P3
W3-4=m(h3-h4)=mcp(T3-T4)
s3=s4
And T3=T4(P3/P4) =T4rp
Where rp =(P3/P4)=pressure ratio
(γ-1)/ γ
(γ-1)/ γ

12. From the above equations, it can be easily shown that:
using the relation between temperatures and pressures, the COP can also be written as:
Applying 1st law of thermodynamics to the entire cycle
(T2/T1)=(T3/T4)
Q4-1 = m(h1-h4) = m cp(T1-T2)
s4-s1=cp ln(T4/T1)
P4=P1
∮𝛿𝛿q = (q4-1 - q2-3)= ∮𝛿𝛿(w3-4 - w1-2) = -wnet
COP of the reversed Brayton cycle is given by:
q4-1 (T1-T4)
Wnet
(T2-T1)-(T3-T4)
COP = =
(T1-T4) T4 (T1-T4)
(T2-T1)-(T3-T4) T3-T4
(T1-T4)(rp - 1)
(γ-1)/ γ
(γ-1)/ γ -1
COP = = = =(rp - 1)

13. • Advantages
• Refrigerant is easily available and cheaper
• No danger of fire or toxic effects due to leakage
• Weight of system / TR is less
• Disadvantages / Limitations
• Quantity of air required per TR is far greater (sensible heat)
• Large volume to be handled and bulky compressor
• Low COP and hence higher running cost (4KW vs 1KW/TR)
• Possibility of frost formation due to moisture in air
• Air Refrigeration is to be used only when others are not.
• The only system used in Aircraft refrigeration

14. The air refrigeration system can be divided in two types:
(i) open system, (ii) closed system.
In the open system the refrigerator is replaced by the
actual space to be cooled with the air expanded to atmospheric
pressure, circulated through the cold room and then compressed to
the cooler or condenser pressure. The pressure in this system is
limited to atmospheric pressure in the refrigerator.
In closed (or dense air) system the air refrigerant is
contained within the piping or components parts of the system at
all times and refrigerator with usually pressures above atmospheric
pressure.
closed system
open system

15. Advantages of closed system over open system:
i) In a closed system the suction to compressor may be at high
pressure. The sizes of expander and compressor can be kept
within reasonable limits by using dense air;
(ii) In open air system, the air picks up moisture from the
products kept in the refrigerated chamber; the moisture may
freeze during expansion and is likely to choke the valves whereas
it does not happen in closed system.
(iii) In open system, the expansion of the refrigerant can be
carried only upto atmospheric pressure prevailing in the cold
chamber but for a closed system there is no such restriction.