ERT 206/4
THERMODYNAMICS
MISS WAN KHAIRUNNISA WAN RAMLI
• Introduce the concepts of refrigerators and heat
pumps and the measure of their performance.
• Analyze the ideal vapor-compression refrigeration
cycle.
• Analyze the actual vapor-compression refrigeration
cycle.
• Review the factors involved in selecting the right
refrigerant for an application.
• Discuss the operation of refrigeration and heat
pump systems.
• Evaluate the performance of innovative vapor
compression refrigeration systems.
• Analyze gas refrigeration systems.
• Introduce the concepts of absorption-refrigeration
systems.
OBJECTIVE:
MAINTAIN REFRIGERATED SPACE AT
TL BY REMOVING QL
OBJECTIVE:
MAINTAIN HEATED SPACE AT TH BY
ABSORBING QH
COOLING CAPACITY OF REFRIGERATORS
Rate of heat removal from refrigerated space
Eliminated the impracticalities of Reversed Carnot:
a. Refrigerant is vaporized completely before
compression
b. Turbine is replaced by throttling device
Consists of 4 main processes
The ideal vapor-compression refrigeration cycle involves
an irreversible (throttling) process to make it a more realistic
model for the actual systems, thus it is not internally
reversible cycle.
Superheated vapor
3-4’
Isentropic
turbine
3-4
Thtrottling
Δh = 0
1-2
Isentropic
compression
Schematic and T-s diagram for the ideal
vapor-compressionrefrigeration cycle.
All 4 components in vapor-compression
refrigeration cycle are steady flow devices.
Steady-flow energy balance on a unitmass basis:
The P-h diagram of an ideal vaporcompression refrigeration cycle.
In P-h diagram,
Process 2-3 & 4-1, constant
Pressure
Process 3-4, Thtrottling
Δh = 0
Condenser & evaporator do not involve
any work, compressor can be approxinated
as adiabatic.
COPs for refrigerators & heat pumps on
vapor-compression refrigeration cycle:
Where
A refrigerator uses R-134a as the working fluid and operates on an ideal vaporcompression refrigeration cycle between 0.14 & 0.8 Mpa. If the mass flow rate of
the refrigerant is 0.05 kg/s, determine
(a) the rate of heat removal from the refrigerated space and the power input to the
compressor, [7.1845 kW, 1.81145 kW]
(b) the rate of heat rejection to the environment [8.996 kW]
(c) COP of the refrigerator [3.9662]
SOLUTION: Rate of refrigeration, power input, rate of heat rejection, COPR
Assumption: Steady operating conditions, PE & KE negligible
Analysis:
1.Draw the T-s diagram of the system
2.Determine the enthalpies at all 4 states (1-2 Δs=0, 3-4 Δh=0)
3.Solve all
Actual vapor-compression refrigeration cycle differs from the ideal one owing
mostly to the irreversibilities that occur in various components  fluid friction and
heat transfer to or from the surroundings.
Schematic and T-s diagram for the actual vaporcompression refrigeration cycle.
1-2
Non-isentropic
compression,
Superheated vapor at
1, friction in
compressor, Δs≠0, Win
increase
2-4-5
Sub cooled liquid at
condenser exit, P drops
PARAMETERS
TO CONSIDER
1) Temperatures of the 2 media which the
refrigerant exchanges heat
2) Toxicity, flammability, chemical stability
3) Availability at low cost
For heat pumps: Tmin & Pmin may be
considerably higher
HYDROCARBON
CO2, AIR, H2O
AMMONIA
Industrial & heavy sectors
ADV low cost, high COP, low E
cost, high heat transfer, no effect on
ozone layer
DISADV toxicity
CFCs
Low cost & versatile
R-11: large capacity water chillers
R-12: domestic refrigerators & freezers
R-22: NH3 competitors
R-502: R-115 & R-22 blends,
commercial refrigerators (supermarket)
Fully haloganated CFCs damage
ozone layer
Developed R-134a chlorine-free
The most common energy source for heat
pumps is atmospheric air (air-to- air systems).
Water-source systems usually use well water
and ground-source (geothermal) heat pumps
use earth as the energy source. They typically
have higher COPs but are more complex and
more expensive to install.
Both the capacity and the efficiency of a heat
pump fall significantly at low temperatures.
Therefore, most air-source heat pumps require
a supplementary heating system such as
electric resistance heaters or a gas furnace.
Heat pumps are most competitive in areas
that have a large cooling load during the
cooling season and a relatively small heating
load during the heating season. In these areas,
the heat pump can meet the entire cooling and
heating needs of residential or commercial
buildings.
A heat pump can be used to heat a
house in winter and to cool it in
summer by adding a reversed valve
Gas Refrigeration Cycle The Reversed Brayton cycle can be used for
refrigeration.
Internally reversible,
executed in ideal gas
refrigeration cycle
T-s diagram
Area under 4-1 = QL
Area enclosed by
12341 = Win
Heat transfer not
isothermal, low COP
2 desirable
characteristics: simpler,
lighter component &
incorporated with
Simple gas refrigeration cycle
regeneration
where
An ideal gas refrigeration cycle using air as the working medium is to maintain a
refrigerated space at -18°C while rejecting heat to the surrounding medium at
27°C. The pressure ratio of the compressor is 4, determine
(a) the maximum & minimum T in the cycle [106°C, -71°C]
(b) the coefficient of performance [2.05]
(c) The rate of refrigeration for a mass flow rate of 0.05 kg/s [2.67 kW]
SOLUTION: Tmax & Tmin, COPR, Rate of refrigeration
Assumption: Steady operating conditions, air as ideal gases with variable
specific heats, PE & KE negligible
Analysis:
1.Draw the T-s diagram of the system
2.Determine the enthalpies at all 4 states (1-2 and 3-4isentropic for ideal gases)
3.Solve all
Absorption refrigeration is economical when there is a source of inexpensive thermal energy at a
temperature of 100 to 200°C.
EXAMPLES: Geothermal energy, solar energy, and waste heat from cogeneration or process steam
plants, and even natural gas when it is at a relatively low price.
Absorption refrigeration systems (ARS) involve the absorption of a refrigerant by a transport
medium. The most widely used system is the ammonia–water system, where ammonia (NH3)
serves as the refrigerant and water (H2O) as the transport medium.
Major advantage: A liquid is compressed instead of a vapor and as a result the work input is very
small and often neglected in the cycle analysis.
ARS are often classified as heat-driven systems & primarily used in large commercial and industrial
installations.
ARS are much more expensive than the vapor-compression refrigeration systems since it is more
complex and occupy more space, much less efficient & require much larger cooling towers to reject
the waste heat, and they are more difficult to service since they are less common.
Therefore, ARS should be considered only when the unit cost of thermal energy is low and is
projected to remain low relative to electricity.
PRINCIPLE OF ARS
The system is much like the vapor-compression cycle, except for the compressor which has
been replaced by a complex absorption mechanism
Absorption unit  absorber, pump, generator, rectifier, regenerator, valve
PURPOSE increased the Pressure of NH3
MECHANISM:
1. NH3 vapor leaves evaporator
enters the absorber, dissolves &
reacts with H2O (exo rxn, heat
released)
2. The NH3+H2O solution pumped
to generator, vaporize some
solution by heat transfer to the
solution
3. The vapor passes through
rectifier to separate the water
4. High P NH3 vpor enter
condenser & the rest of the cycle
5. The hot NH3+H2O solution
passes through the regenerator,
transfer some heat to rich
solution from the pump
6. The solution is throttled to the
absorber P.
The COP of ARS is defined as
The MAX COP is determined by assuming that the
entire cycle is totally reversible.
Heat from the source (Qgen) transferred to Carnot
HE & Woutput of this HE is supplied to Carnot
Refrigerator
Determining the maximum COP of
an absorption refrigeration system.
The COP of actual absorption
refrigeration systems is usually less
than 1.
A refrigerator uses R-134a as the working fluid and operates on an ideal vaporcompression refrigeration cycle between 0.12 and 0.7 Mpa. The mass flow rate of
the refrigerant is 0.05 kg/s. Show the cycle on a T-s diagram with respect to the
saturation lines. Determine
(a) The rate of heat removal from the refrigerated space & the power input to the
compressor
(b) The rate of heat rejection to the environment
(c) The coefficient of performance