lecture 6,7,8 introduction to...
TRANSCRIPT
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Introduction to refrigeration
Md Ferdous AlamLecturer, MEE, SUST
Lecture 6,7,8
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Refrigeration is the removal of heat from a material
or space, so that it’s temperature is lower than that of
it’s surroundings.
What is saturation temperature?
“Saturation Temperature” – can be defined as the
temperature of a liquid, vapor, or a solid, where if
any heat is added or removed, a change of state takes
place.
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“Latent Heat”- The heat required to change a liquid to a gas (or
the heat that must be removed from a gas to condense it to a
liquid), without any change in temperature.
• Heat is a form of energy that is transferred from
one object to another object.
• Heat Is a form of energy transferred by a
difference in temperature.
• Heat transfer can occur, when there is a
temperature difference between two or more
objects. Heat will only flow from a warm
object to a colder object.
• The heat transfer is greatest, when there is a
large temperature difference between two
objects.
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Do you know the laws of thermodynamics?
First law of thermodynamics – Energy can neither be
created nor destroyed. It can only change forms. In any
process, the total energy of the universe remains the
same. For a thermodynamic cycle the net heat supplied
to the system equals the net work done by the system.
Second law of thermodynamics – The entropy of an
isolated system not in equilibrium will tend to increase
over time, approaching a maximum value at equilibrium.
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A perpetual motion machine is a hypothetical
machine that can do work indefinitely without an
energy source. This kind of machine is impossible,
as it would violate the first or second law of
thermodynamics.
Never build this machines!!!!!!!!!!!
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Fig: Heat Engine
Brainstorm:
Is refrigerator a Heat
pump?
Is refrigerator a heat
engine?
Do you know the
difference between
refrigerator and heat
pump?
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What is enthalpy?Enthalpy is defined to be the sum of the
internal energy E plus the product of the
pressure p and volume V. Using the
symbol H for the enthalpy:
H = E + p * V
What is entropyEntropy is a measure of the uncertainty of the
energy state of the system.
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The reversed Carnot cycle is the most efficient
refrigeration cycle operating between TL and TH.
evaporator
compressor
condenser
turbine
4 1
23
4 1
23
QL
QH
T
s
Replace turbine by an expansion valve in case of vapor compression cycle
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Two important types of refrigeration:
Vapor compression refrigeration
Vapor absorption refrigeration
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Schematic and T-s diagram for the ideal vapor-compression refrigeration cycle.
H.P. side
L.P. side
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P-h diagram of refrigeration
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The Vapor Compression Refrigeration Cycle:
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There are four main components in a refrigeration
system:
The Compressor
The Condensing Coil
The Expansion Device
The Evaporator
Two different pressures exist in the refrigeration cycle. The
evaporator or low pressure, in the "low side" and the
condenser, or high pressure, in the "high side".
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The concept of an efficiency being greater
than 100% makes people uneasy!!!!!!!!
Therefore, the conversion efficiency for a
refrigerator is called the Coefficient of
Performance (COP).
CCOPinE
cycle
Q
W
energy sought
energy that costs
may be >1 100%E E
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R
1COP
/ 1
in inth
cycle out in out in
Q Q
W Q Q Q Q
For the Refrigeration cycle …
Working fluid = Refrigerant = R134a
Melting point of R134a is -103.3 degree Celsius
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Refrigeration Effect and Capacity:
Refrigeration Effect:
Refrigeration
Capacity:
Compressor Work
done:
1 4inq h h
WE = m (h2 - h1)
Qin = m (h1 - h4)
COP = (h1 - h4)
(h2 - h1)
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Refrigeration capacity is often expressed in tons of
refrigeration. Definition …
1 ton of refrigeration is the steady state heat
transfer rate required to melt 1 ton of ice at 0°C in
24 hours.
1 ton = 3.516 kW
Try the calculation yourself!!!!!!!!!!!!
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Practice math problems from
reference books and class
lectures………