PROCESSING TECHNOLOGY 1

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PROCESSING TECHNOLOGY 1

• Power and Refrigeration Cycles

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Introduction
Refrigeration systems are wide-spread within the
brewing and food industries where cooling
utilities within different temperature regions
is a common requirement. Refrigeration is an
expensive utility and as such a greater
understanding of its thermodynamic detail is
important in ensuring that refrigeration duties
are employed and generated cost effectively
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Power Generation
Commonly done in a closed cycle system
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Power Generation
Step 1?2 Evaporation of water into steam
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Power Generation
Step 2?3 Expansion of steam through a turbine,
where shaft work is available from the turbine
for useful work
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Power Generation
Step 3?4 Condensation of turbine exhaust steam.
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Power Generation
Step 4?1 Pumping of condensate to boiler
pressure to begin the cycle again.
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Carnot Cycle
Simplest steam power cycle and represents an
ideal heat engine cycle of maximum thermal
efficiency. It consists of essentially the same
four steps as previously, i.e. heating,
expansion, condensation and compression
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Carnot Cycle
These steps have certain idealised characteristics
that limit the usefulness of the Carnot Cycle
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Carnot Cycle
Step 1?2 Isothermal and isobaric (constant
pressure) evaporation of water into steam from a
saturated liquid condition to a saturated vapour
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Carnot Cycle
Step 2?3 An isentropic (constant entropy)
expansion of steam through a turbine, where
shaft work is available from the turbine for
useful work
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Carnot Cycle
Step 3?4 Isothermal and isobaric condensation of
turbine exhaust steam
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Carnot Cycle
Step 4?1 An isentropic compression of condensate
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Carnot Cycle T/S Diagram
T
2
1
T
H
T
c
3
4
S
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Carnot Cycle
In practice it is extremely difficult to ensure
that the expansion of the steam through the
turbine and the recompression of the condensate
to the boiler (steps 1 ? 2 and 4 ? 1) are
isentropic.
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Carnot Cycle
Carnot cycle is thermodynamically very efficient
and as has the highest thermal efficiency
(termed the Carnot efficiency hc) of any power
cycle. It is useful to use as a benchmark
against which other practical cycles can be
assessed
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Carnot Cycle
The Carnot efficiency hc is given by, hc 1
(Tcondenser/Tevaporator) Tcondenser condenser
temperature (K) Tevaporator evaporator
temperature (K).
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Rankine Cycle
Rankine cycle is the basis for most modern power
generation plant
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Rankine Cycle
Step 1?2 Heating of condensate to boiling point,
followed by vaporization at constant temperature
and then super heating (i.e sensible heating of
dry saturated steam vapour).
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Rankine Cycle
Step 2?3 Isentropic (constant entropy) expansion
of superheated steam through a turbine, with
partial condensation
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Rankine Cycle
Step 3?4 Condensation at constant temperature
and pressure
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Rankine Cycle
Step 4?1 Pumping of condensate to boiler.
Ideally isentropic
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Rankine Cycle-P/H Diagram
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P/H and T/S Diagrams
Step 1?2 Pressurized water is preheated,
vaporized and superheated. Enthalpy increases
with constant pressure.
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P/H and T/S Diagrams
Step 2?3 Hot, pressurized vapour is expanded
through a turbine. Ideally this would be
isentropic. There is a decrease in both
pressure and enthalpy.
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P/H and T/S Diagrams
Step 3?4 Vapour is condensed. Enthalpy
decreases at constant T and P
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Rankine Cycle
It is a closed cycle, the same fluid is recycled
continuously. Fluid passes through a series of
thermodynamic states and returns to the same
state, i.e. the net enthalpy change of the cycle
is 0.
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Steady Flow Energy Equation (SFEE) for Rankin
Cycle
Use energy equation for an open system (each
individual process operates as an open system
even though overall system is closed
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Steady Flow Energy Equation (SFEE)
DH DEk DEp Q - W DH is the change in
enthalpy, DEk is the change in kinetic
energy, DEp is the change in potential energy, Q
is the net heat flow, W is the net work done.
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Steady Flow Energy Equation
DEk and DEp are negligible compared with the
enthalpy changes, equation reduces to DH Q
W (1) Net enthalpy change is 0, this
becomes Q W (2) Or net heat input net
work output
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Steady Flow Energy Equation
For each stage Stage 1?2 ve input of heat,
Q12 Stage 2?3 ve output of work, W23 Stage
3?4 -ve output of heat, Q34 Stage 4?1 -ve input
of work, W41
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Steady Flow Energy Equation
The overall efficiency of a power generation
cycle represents the efficiency with which heat
is converted into useful work. Net work W23
W41
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Steady Flow Energy Equation
W41 has a -ve sign (work done to the system), so
net work is less than W23. In practice, the work
done in pumping the condensate to the boiler
(W41) is usually much smaller than the work
obtained (W23), and can be ignored if an
approximate value is required.
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Steady Flow Energy Equation
Efficiency (h) is then given by h work
out/heat input W23 / Q12
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Steady Flow Energy Equation
Applying the reduced form of the SFEE to each
process within the cycle DH Q W (1)
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Steady Flow Energy Equation
Boiler W0, ?Q DH G(h2
h1) Turbine Q0, ?W - DH -G(h3 h2)
G(h2 h3)
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Steady Flow Energy Equation
Condenser W0, ?Q DH G(h3 h4) Feed
Pump Q0, ?W -DH -G(h1 h4) G(h4
h1)
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Refrigeration
Pub cold cabinets Pub beer coolers Air
conditioning units Ice beer production Site cold
utilities production - chilled water -
ethylene glycol - ammonia refrigeration
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Refrigeration
Refrigeration processes fall into two
categories, Vapour compression Vapour
absorption
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Vapour Compression Systems
Fundamental principle is The saturation
temperature of a fluid is proportional to the
pressure under which the fluid is kept. The
temperature at which a fluid will boil or
condense is higher if a high pressure is exerted
upon it, and vice versa.
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Vapour Compression Systems
KEYPOINT High pressure ? high condensation and
boiling temperatures. Low pressure ? low
condensation and boiling temperatures.
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Vapour Compression Systems
If we can control the pressure under which a
fluid is kept, we can absorb and reject heat from
the fluid at a controllable temperature
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Vapour Compression Systems
If we create a pressure higher than atmospheric
pressure, then the fluid will condense at a
temperature above ambient temperature, so we can
reject heat from the fluid into the atmosphere,
(thereby heating the surrounding atmosphere)
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Vapour Compression Systems
Conversely if we create a lower pressure than
atmospheric, then the fluid will boil at a
temperature lower than ambient temperature, so we
can absorb heat from the atmosphere, thereby
cooling the surrounding atmosphere
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Vapour Compression Plant
The basic vapour-compression refrigeration cycle
is essentially the reverse of the power
generation cycle. An important difference is
that a refrigerator transfers heat against a
temperature gradient i.e from cold to hot, and
may be regarded as a heat pump.
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Standard Refrigeration Cycle
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Vapour Compression Plant
Compression (1-2) Refrigerant vapour is
compressed (ideally isentropic). Pressure and
temperature rise.
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Vapour Compression Plant
Condensation (2-3) The vapour is condensed,
releasing heat which is lost to the atmosphere.
with sub
-cooling
2
Condensation
3
P
1
4
Evaporation
Specific enthalpy
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Vapour Compression Plant
Throttling (3-4) The condensate passes through a
throttling valve. An adiabatic process (constant
enthalpy) where the pressure drops irreversibly
and the temperature falls.
with sub
-cooling
2
Condensation
3
P
1
4
Evaporation
Specific enthalpy
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Vapour Compression Plant
Evaporation (4-1) The vapour absorbs heat energy
and produces the desired cooling effect within
the evaporator
with sub
-cooling
2
Condensation
3
P
1
4
Evaporation
Specific enthalpy
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Vapour Compression Plant
The vapour-compression process is a closed
steady-flow system, we can apply the steady flow
energy equation (SFEE) to it, DH DEk DEp
Q - W or ignoring DEk and DEp DH Q - W
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Steady Flow Energy Equation
Compressor Ignoring any heat loss/gain from the
compressor, Q0, W DH G(h2 h1) W
compression work (W) G mass flowrate of
refrigerant (kg/s) h2 specific enthalpy at
compressor outlet (kJ/kg) h1 specific enthalpy
at compressor inlet (kJ/kg)
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Steady Flow Energy Equation
Condenser No work is done within the condenser,
hence W0. QC G(h2 h3) QC condenser
(hot) load (W) G mass flowrate of refrigerant
(kg/s) h2 specific enthalpy at condenser inlet
(compressor outlet) (kJ/kg) h3 specific
enthalpy at condenser outlet (throttle inlet)
(kJ/kg)
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Steady Flow Energy Equation
Throttle No heat or work is done here, so Q W
0. The process is adiabatic, so, h3 h4 h3
specific enthalpy at throttle inlet (condenser
outlet) (kJ/kg) h4 specific enthalpy at
throttle outlet (evaporator inlet) (kJ/kg)
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Steady Flow Energy Equation
Evaporator No work is done, so W0. QE G(h1
h4) QE evaporator (cold) load (W) G mass
flowrate of refrigerant (kg/s) h1 specific
enthalpy at evaporator outlet (compressor inlet)
(kJ/kg) h4 specific enthalpy at evaporator
inlet (throttle outlet) (kJ/kg)
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Refrigerator or Heat-Pump?
Technically refrigeration systems can be
considered as both cooling and heating
systems. The evaporator removes heat (a cooling
system) whilst the condenser is supplied heat (a
heat pump)
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System Performance
Defined in terms of the coefficient of
performance (COP), which is defined differently
whether the system operates as a refrigeration or
heat-pump system. The COP quantifies the amount
of heat transferred per unit compressor work. A
more effective system has a higher COP.
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Refrigeration - COPR
Removal of heat within the evaporator,
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Heat-Pump COPHP
Supply of heat within the condenser,
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Relationship Between COPR and COPHP
?H2,3 ?H4,1 ?H1,2
?H Q - W
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Relationship Between COPR and COPHP
?H2,3 Q2,3
?H4,1 Q4,1
?H1,2 W1,2
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Relationship Between COPR and COPHP
?H2,3 ?H4,1 ?H1,2 Q2,3 Q4,1 W1,2 Q2,3/
W1,2 Q4,1 /W1,2 1 COPHP COPR 1
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The refrigerant fluid should
Have a critical temperature above atmospheric
temperature, but not necessarily much higher
(compare with power generation, where critical
temperature must exceed boiler temperature).
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The refrigerant fluid should
Have a vapour pressure higher than 1 atm at
operating temperatures - this prevents ingress of
air in case of leaks. Water vapour should not
enter the system as icing of the throttling valve
can occur.
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The refrigerant fluid should
Possess suitable freezing, boiling
characteristics so that changes from liquid to
vapour and back can be accomplished under cycle
conditions.
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The refrigerant fluid should
Be non-toxic in case of leaks.