TER208 Thermodynamics II
Instructor: Assoc. Prof. Elif Begüm Elçioğlu
TA: Res. Asst. Fırat Sezgin
Schedule: Wednesday 10-13
Check https://mergen.anadolu.edu.tr/login/canvas regularly to be notified when
a course material or announcement is posted.
E-mail: ebelcioglu@eskisehir.edu.tr
Most of the course content are taken from Moran, Shapiro, Boettner, Bailey, 2014, John Wiley & Sons., Inc.
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Let’s first remember the formulations & analyses we discussed in the previous weeks.
✓ Exergy
✓ Exergy (rate) balance for a closed system
✓ Exergy (rate) balance for an open system
✓ Flow exergy
✓ Exergetic (2nd law) efficiency
✓ Rankine cycle (ideal & w/ irreversibilities)
✓ Isentropic turbine & pump efficiencies
✓ Rankine cycle w/ superheat & reheat
✓ Regenerative Rankine cycle w/ open fwh
✓ Regenerative Rankine cycle w/ closed fwh
✓ Regenerative Rankine cycle w/ multiple fwh’s
✓ Refrigeration and Heat Pump Cycles
✓ Carnot Refrigeration cycle
✓ Ideal and non-ideal vapor compression systems
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Refrigeration and Heat Pump Cycles (cont’d)
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Refrigeration and Heat Pump Cycles
• The purpose of a refrigeration system is to maintain a cold region at a temperature below the temperature of its
surroundings. This is commonly achieved using the vapor refrigeration systems.
Carnot Refrigeration Cycle
(This cycle is obtained by reversing the Carnot vapor power cycle.)
→Carnot refrigeration cycle operating between a region
@ TC and another region at a higher temperature, TH.
The cycle is executed by a refrigerant circulating steadily
through a series of components.
All processes are internally reversible.
Since HTs b/w the refrigerant and each region occur
with no temperature differences, there are no external
irreversibilities.
The energy transfers are positive in the directions
indicated by the arrows.
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Carnot Refrigeration Cycle – details (cont’d)
Since the Carnot vapor refrigeration cycle is made up of internally reversible processes,
areas on the T–s diagram can be interpreted as ……. .
Area 1–a–b–4–1 is the heat added to the refrigerant from the cold region per unit mass
of refrigerant flowing.
Area 2–a–b–3–2 is the heat rejected from the refrigerant to the warm region per unit
mass of refrigerant flowing.
• The enclosed area 1–2–3–4–1 is the net HT from the refrigerant.
• The net HT from the refrigerant equals the net work done on the refrigerant.
• The net work is the difference between the compressor work input and the turbine work output.
The COP of any refrigeration cycle is the ratio of the refrigeration effect to the net work input required to achieve that
effect. For the Carnot vapor refrigeration cycle, the coefficient of performance is:
This is the the max. theoretical COP of any refrigeration
cycle operating between regions at TC and TH.
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Analyzing Vapor-Compression Refrigeration Systems
Components of a vapor
compression refrigeration system
Evaluating Principal Work and Heat Transfers
Let us consider the steady-state operation of the vapor-compression system on the left.
Shown on the figure are the principal work and HT’s, which are
positive in the directions of the arrows. KE and PE changes are neglected.
We begin with the evaporator, where the desired refrigeration effect is achieved.
As the refrigerant passes through the evaporator, HT from the refrigerated space
results in the vaporization of the refrigerant. For a c.v. enclosing the refrigerant side of
the evaporator, the mass and energy rate balances reduce to give the rate of HT per
unit mass of refrigerant flowing as:
Refrigeration capacity
mass flow rate of the refrigerant
The refrigerant leaving the evaporator is compressed to a relatively high p. and T. by the compressor. Assuming no HT to or
from the compressor, the mass and energy rate balances give:
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Analyzing Vapor-Compression Refrigeration Systems
Evaluating Principal Work and Heat Transfers (cont’d)
Next, the refrigerant passes through the condenser, where the refrigerant condenses and there is HT from the
refrigerant to the cooler surroundings. For a c.v. enclosing the refrigerant side of the condenser, the rate of HT
from the refrigerant per unit mass of refrigerant flowing is:
Finally, the refrigerant at state 3 enters the expansion valve and expands to the evaporator p. This process is
usually modeled as a throttling process for which h3=h4.
The refrigerant p. decreases in the irreversible adiabatic expansion, and there is an accompanying increase in
specific entropy. The refrigerant exits the valve at state 4 as a two-phase liquid–vapor mixture.
In the vapor-compression system, the net power input is equal to the compressor power, since the expansion
valve involves no power input or output. The COP of the vapor compression refrigeration system is:
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Ideal Vapor-Compression Systems
If irreversibilities within the evaporator and condenser are
ignored, there are no frictional pressure drops, and the
refrigerant flows at constant pressure through the two
hex’s.
If compression occurs without irreversibilities, and stray HT
to the surroundings is also ignored, the compression
process is isentropic.
With these considerations, the vapor-compression
refrigeration cycle labeled 1–2s–3–4–1 on the T–s diagram
results.
The cycle consists of the following series of processes:
Process 1–2s: Isentropic compression of the refrigerant from
state 1 to the condenser pressure at state 2s.
Process 2s–3: HT from the refrigerant as it flows at constant p.
through the condenser. The refrigerant exits as a liquid at state
3.
Process 3–4: Throttling process from state 3 to a two-phase
liquid–vapor mixture at 4.
Process 4–1: HT to the refrigerant as it flows at constant p.
through the evaporator to complete the cycle.
All processes of the cycle are internally reversible except for the
throttling process.
Despite the inclusion of this irreversible process, the cycle is
commonly referred to as the ideal vapor-compression cycle.
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Example-1: Refrigerant 134a is the working fluid in an ideal vapor-compression refrigeration cycle that communicates
thermally with a cold region at 0oC and a warm region at 26oC. Sat. vapor enters the compressor at 0oC and sat. liquid leaves
the condenser at 26oC. The mass flow rate of the refrigerant is 0.08 kg/s.
Determine: (a) the compressor power, in kW, (b) the refrigeration capacity, (c) the COP, (d) the COP of a Carnot refrigeration
cycle operating between warm and cold regions at 26 and 0oC, respectively.
Engineering Model:
1. Each component of the cycle is analyzed as a c.v. at steady state.
2. Except for the expansion through the valve, which is a throttling process, all processes of the refrigerant
are internally reversible.
3. The compressor and expansion valve operate adiabatically.
4. KE and PE effects are negligible.
5. Sat. vapor enters the compressor, and sat. liquid leaves the condenser.
Let’s begin by fixing each of the principal states.
At the inlet to the compressor, the R134a is a sat. vapor (assumption
5) at 0oC, Table A-10, h1 = 247.23 kJ/kg, s1 = 0.9190 kJ/kg.K.
The pressure at state 2s is the saturation pressure corresponding to
26oC, or p2 = 6.853 bar. State 2s is fixed by p2 and the fact that the
specific entropy is constant for the adiabatic, internally reversible
compression process.
The R134a at state 2s is a superheated vapor with h2s = 264.7 kJ/kg.
State 3 is sat. liquid at 26oC, so h3 = 85.75 kJ/kg.
The expansion through the valve is a throttling process (assumption 2), so h4 = h3.
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Solution to Example-2:
(a) The compressor work input is:
(b) The refrigeration capacity is the HT rate to the refrigerant passing through the evaporator, which is given by:
In the SI unit system, the capacity is normally expressed in kW. In the English unit system, the
refrigeration capacity may be expressed in Btu/h. Another commonly used unit for the refrigeration
capacity is the ton of refrigeration, which is equal to 200 Btu/min or about 211 kJ/min.
(c) The COP is:
(d) For a Carnot vapor refrigeration cycle operating at TH = 299 K and TC = 273 K, the COP is:
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Example-1: Modify the example on
the right to allow for temperature
differences b/w the refrigerant and
the warm and cold regions as follows:
Sat. vapor enters the compressor @
210oC. Sat. liquid leaves the
condenser @ 9 bar.
Determine for this modified vaporcompression refrigeration cycle:
(a) the compressor power, in kW,
(b) The refrigeration capacity, in tons,
(c) the COP.
Engineering Model:
1. Each component of the cycle is analyzed as a c.v. at steady state.
2. Except for the process through the expansion valve, which is a throttling process, all
processes of the refrigerant are internally reversible.
3. The compressor and expansion valve operate adiabatically.
4. KE and PE effects are negligible.
5. Sat. vapor enters the compressor, and sat. liquid exits the condenser.
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Soln. to Example-1:
Let’s begin by fixing each of the principal states located on the accompanying T–s diagram.
The inlet to the compressor: R134a is a sat. vapor at 210oC.
From Table A-10, h1 = 241.35 kJ/kg and s1 = 0.9253 kJ/kg K.
The superheated vapor at state 2s is fixed by p2 = 9 bar and the specific entropy is constant
for the adiabatic, internally reversible compression process.
Interpolating in Table A-12 gives h2s = 272.39 kJ/kg.
State 3 is a sat. liquid at 9 bar, so h3 = 99.56 kJ/kg. The expansion through the valve is a
throttling process; thus, h4 …… h3.
(a) The compressor power input is:
(b) The refrigeration capacity is:
(c) The coefficient of performance:
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Your turn!
Study Question:
Reconsider the previous example, but this time include in the analysis that the compressor has an isentropic
efficiency of 80%. Also, let the temperature of the liquid leaving the condenser be 30oC.
Determine for the modified cycle:
(a) the compressor power, in kW,
(b) the refrigeration capacity, in tons,
(c) The coefficient of performance, and
(d) the rates of exergy destruction within the compressor and expansion valve, in kW, for T0 = 299 K (26oC).
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Your turn! – Part 1 – Numerical Example
NQ1- A heat engine operates on the Carnot cycle. It produces 50 kW of power
while operating between temperature limits of 800°C and 100oC. Determine
the engine efficiency and the amount of heat added.
You have 10 mins
to solve this
question.
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Your turn! – Part 2 – Conceptual Examples
CQ1-When heating a solution, a scientist detects a temperature increase in the solution during a period of time.
Which of the following statements accurately characterizes the solution during this period?
a. The solution is at boiling point.
b. The solution is undergoing a phase change.
c. The velocity of molecules in the solution is increasing.
d. The solutions temperature increase is proportional to its ΔHvaporization.
You have 9 mins
to solve these
questions.
CQ2- In a system undergoing adiabatic compression, what are the values of internal energy and heat if work done on
the system is 500 J?
a. Internal energy is 0 J and heat is 500 J.
b. Internal energy is -500J and heat is 0 J.
c. Internal energy is 0 J and heat is -500 J.
d. Internal energy is 500 J and heat is 0 J.
CQ3- Which of the following scenarios violates the first law of thermodynamics, “the conservation of energy’’?
a. A spring that extends and retracts forever, alternating between potential and kinetic energy.
b. An isolated electrochemical cell that indefinitely generates an electric current.
c. An efficient wind turbine that converts all of its energy from mechanical movement into electrical potential
energy.
d. A machine that converts heat energy into work energy.
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Your turn! – Part 2 – Conceptual Examples (cont’d)
CQ4- Work is a
a) point function
b) path function
c) depends on the state
d) none of the mentioned
You have 5 mins
to solve these
questions.
CQ5- For a constant pressure process, work done is:
a) zero
b) p*(V2-V1)
c) p1*V1*ln(V2/V1)
d) none of the mentioned
CQ6- For a constant volume process, work done is
a) zero
b) p*(V2-V1)
c) p1*V1*ln(V2/V1)
d) none of the mentioned
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Selecting Refrigerants
• Refrigerant selection for refrigeration and air-conditioning applications is generally based on:
1- Performance,
2- Safety,
3- Environmental impact.
• Performance refers to providing the required cooling or heating capacity reliably and cost-effectively.
• Safety refers to avoiding hazards (e.g., toxicity, flammability).
• Environmental impact refers to using refrigerants that do not harm the stratospheric ozone layer or
contribute significantly to global climate change.
• The temperatures of the refrigerant in the evaporator and condenser of vapor-compression cycles are governed by the
temperatures of the cold and warm regions, respectively, with which the system interacts thermally. This determines
the operating pressures in the evaporator and condenser.
• Consequently, refrigerant selection is based partly on the suitability of its p-T relationship in the range of the particular
application.
• It is generally desirable to avoid excessively low pressures in the evaporator and excessively high pressures in the
condenser.
• Other considerations in refrigerant selection include chemical stability, corrosiveness, and cost.
• The type of compressor also affects the choice of refrigerant. Centrifugal compressors are best suited for low evaporator
pressures and refrigerants with large specific volumes at low pressure. Reciprocating compressors perform better over
large pressure ranges and are better able to handle low specific volume refrigerants.
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Refrigerant Types and Characteristics
• Prior to the 1930s, accidents were prevalent among those who worked closely with refrigerants due to the toxicity and
flammability of most refrigerants at the time.
• Because of such hazards, two classes of synthetic refrigerants were developed, each containing chlorine and possessing
highly stable molecular structures: CFCs (chlorofluorocarbons) and HCFCs (hydrochlorofluorocarbons). These refrigerants
were widely known as “freons,” the common trade name.
• In the early 1930s, CFC production began with R-11,
R-12, R-113, and R-114. In 1936, the first HCFC
refrigerant, R-22, was introduced. Over the next
several decades, nearly all of the synthetic
refrigerants used in the United States were either
CFCs or HCFCs, with R-12 being most commonly used.
• To keep order with so many new refrigerants having
complicated names, the “R” numbering system was
established in 1956 by DuPont and persists today as
the industry standard.
This table lists information including
refrigerant number, chemical
composition, and global warming
potential for selected refrigerants.
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Global Warming:
• Global warming refers to an increase in global average temperature due to a combination of
natural phenomena and human industrial, agricultural, and lifestyle activities.
• The Global Warming Potential (GWP) is a simplified index that aims to estimate the potential
future influence on global warming of different gases when released to the atmosphere.
• The GWP of a gas refers to how much that gas contributes to global warming in comparison to the
same amount of CO2.
Image from: https://www.wwf.org.au/what-we-do/climate/impacts-of-global-warming#gs.x0grzi
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Environmental Considerations
• After decades of use, compelling scientific data indicating that release of chlorine containing refrigerants into the
atmosphere is harmful became widely recognized.
• Concerns focused on released refrigerants depleting the stratospheric ozone layer and contributing to global climate
change. Because of the molecular stability of the CFC and HCFC molecules, their adverse effects are long-lasting.
• In 1987, an international agreement was adopted to ban production of certain chlorine-containing refrigerants.
• In response, a new class of chlorine-free refrigerants was developed: the HFCs (hydrofluorocarbons). One of these,
R-134a, has been used for over 20 years as the primary replacement for R-12.
• Although R-134a and other HFC refrigerants do not contribute to atmospheric ozone depletion, they do contribute to
global climate change. Owing to a relatively high Global Warming Potential of about 1430 for R-134a, we may soon
see reductions in its use in the United States despite widespread deployment in refrigeration and air-conditioning
systems, including automotive air conditioning. Carbon dioxide (R-744) and R-1234yf are possible replacements for
R-134a in automotive systems.
• Another refrigerant that has been used extensively in air-conditioning and refrigeration systems for decades, R-22, is
being phased out under a 1995 amendment to the international agreement on refrigerants because of its chlorine
content. Effective in 2010, R-22 cannot be installed in new systems. However, recovered and recycled R-22 can be
used to service existing systems until supplies are no longer available.
• As R-22 is phased out, replacement refrigerants are being introduced, including R-410A and R-407C, both HFC blends.
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Reading: Natural Refrigerants
• Nonsynthetic, naturally occurring substances also can be used as refrigerants. Called natural refrigerants, they
include carbon dioxide, ammonia, and hydrocarbons. Natural refrigerants typically have low Global Warming
Potentials.
• Ammonia (R-717), which was widely used in the early development of vapor compression refrigeration,
continues to serve today as a refrigerant for large systems used by the food industry and in other industrial
applications. In the past two decades, ammonia has been increasingly used because of the R-12 phaseout and is
receiving even greater interest today due to the R-22 phaseout.
• Hydrocarbons, such as propane (R-290), are used worldwide in various refrigeration and air-conditioning
applications including commercial and household appliances.
• In the United States, safety concerns limit propane use to niche markets like industrial process refrigeration. Other
hydrocarbons—methane (R-50) and butane (R-600)—are also under consideration for use as refrigerants.
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Heat Pump Systems
• The objective of a heat pump is to maintain the temperature within a dwelling or other building above the
temperature of the surroundings or to provide a HT for certain industrial processes that occur at elevated
temperatures.
→ Remember what the objective of a refrigeration cycle is?
The purpose of a refrigeration system is to maintain a cold region at a temperature below the
temperature of its surroundings.
• Heat pump systems have many features in common with the refrigeration systems considered thus far and
may be of the vapor-compression or absorption type.
• Vapor-compression heat pumps are well suited for space heating applications and are commonly used for
this purpose. Absorption heat pumps have been developed for industrial applications and are also
increasingly being used for space heating.
• To introduce some aspects of heat pump operation, we will start with Carnot heat pump cycle.
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Carnot Heat Pump Cycle - Cycle analysis
By changing our viewpoint, we can regard the below cycle (which we studied as the Carnot vapor refrigeration cycle) as a
heat pump. The objective of the cycle now, however, is to deliver the HT (𝑸ሶ 𝒐𝒖𝒕 ) to the warm region, which is the space to
be heated.
At steady state, the rate at which energy is supplied to the
warm region by HT is the sum of the energy supplied to
the working fluid from the cold region, Qሶ in , and the net
rate of work input to the cycle, Wሶ net . That is:
The COP of any heat pump cycle is defined as the ratio
of the heating effect to the net work required to achieve
that effect. For the Carnot heat pump cycle:
the maximum theoretical coefficient of
performance for any heat pump cycle
operating between two regions @ TC and TH.
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Vapor-Compression Heat Pumps
• Actual heat pump systems depart significantly from the Carnot cycle model. Most systems in common use today are of the vaporcompression type.
• The method of analysis of vapor-compression heat pumps is the same as that of vapor-compression refrigeration cycles considered
previously. Also, the previous discussions concerning the departure of actual systems from ideality apply for vapor-compression heat
pump systems as for vapor-compression refrigeration cycles.
•
•
As illustrated herein, a typical vaporcompression heat pump for space heating
has the same basic components as the
vapor-compression refrigeration system:
compressor, condenser, expansion valve, and
evaporator.
The COP of a simple vapor-compression heat pump is:
can never be
less than unity
The objective of the system is different,
however. In a heat pump system, Qሶ in comes
from the surroundings, and Qሶ out is directed
to the dwelling as the desired effect. A net
work input is required to accomplish this
effect.
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Vapor-Compression Heat Pumps
Air-source heat pump
• In the most common type of vapor-compression heat pump for space heating, the evaporator communicates
thermally with the outside air. Such air-source heat pumps also can be used to provide cooling in the summer with
the use of a reversing valve, as illustrated below.
The solid lines show the flow path of the refrigerant in the
heating mode. To use the same components as an air conditioner,
the valve is actuated, and the refrigerant follows the path indicated by
the dashed line. In the cooling mode, the outside hex becomes the
condenser, and the inside hex becomes the evaporator.
Although heat pumps can be more costly to install and operate than
other direct heating systems, they can be competitive when the
potential for dual use is considered.
Example of an air-to-air reversing heat pump.
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Example-2: R134a is the working fluid in an electric-powered, air-source heat pump that maintains the inside temperature of
a building at 22oC for a week when the average outside temperature is 5oC. Sat. vapor enters the compressor at -8oC and exits
at 50oC, 10 bar. Sat. liquid exits the condenser at 10 bar. The refrigerant mass flow rate is 0.2 kg/s for steady-state operation.
Determine:
(a) the compressor power, in kW,
(b) the isentropic compressor efficiency,
(c) the HT rate provided to the building, in kW,
(d) the coefficient of performance.
Engineering Model:
1. Each component of the cycle is analyzed as a c.v. at
steady state.
2. There are no Δp’s through the evaporator and
condenser.
3. The compressor operates adiabatically. The expansion
through the valve is a throttling process.
4. KE and PE effects are negligible.
5. Sat. vapor enters the compressor, and sat. liquid exits
the condenser.
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Soln. to Example-2: Let’s begin by fixing the principal states located on the schematic and T–s diagram.
State 1 is sat. vapor at 28oC; thus h1 and s1 are obtained directly from Table A-10.
State 2 is superheated vapor; knowing T2 and p2, h2 is obtained from Table A-12.
State 3 is sat. liquid at 10 bar and h3 is obtained from Table A-11.
Finally, expansion through the valve is a throttling process; therefore, h4 … h3.
(a) The compressor power is:
(b) The isentropic compressor efficiency is:
where h2s is the specific entropy at state 2s. State 2s
is fixed using p2 and s2s … s1.
Interpolating in Table A-12, h2s = 274.18 kJ/kg.
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Soln. to Example-2 (cont’d):
(c) The HT rate provided to the building is:
(d) The heat pump COP is:
Is this value meaningful?
Next week we will continue with gas refrigeration systems and Brayton cycle.
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