28 CHAPTER 03 THEORETICAL CONCEPTS OF VAPOUR ABSORPTION CHILLERS & EXHAUST GAS This chapter describes the about the vapour absorption chillers. Further to that describe the types and characteristics of the vapour absorption chillers. Vapour absorption chiller is the most important part of the cogeneration system 3.1 Absorption Chillers Absorption chiller technologies are one of a group of technologies classified as heat pumps. Heat pumps may be either heat driven or work driven. Absorption technologies are heat driven, transferring heat from a low temperature to a high temperature using heat as the driving energy. Heat pumps operate on the principle of the absorption refrigeration cycle, which is similar to the vapor-compression cycle. Both the absorption refrigeration cycle and the vapor-compression cycle will be examined to draw analogies between the two. Vapor-compression refrigeration systems are the most common refrigeration systems in use today. The vapor-compression cycle is a work- driven cycle that is illustrated in Figure 3.1. In the vapor-compression cycle, work is input to compress the refrigerant to a high pressure and temperature at State 2. At State 2, the refrigerant condensation temperature is below the ambient temperature. As the high- pressure and high-temperature refrigerant vapor passes through the condenser, heat is rejected to the ambient air and the refrigerant vapor condenses to a liquid to achieve State 3. The high-pressure liquid at State 3 passes through an expansion valve. As the liquid passes through the expansion valve, the refrigerant experiences a reduction in both temperature and pressure to reach State 4. At State 4, the boiling temperature of the refrigerant is lower than that of the surroundings. The low- pressure liquid refrigerant passes through the evaporator, absorbing heat from the ambient environment when boiling occurs in the evaporator and creating a low- pressure refrigerant vapor at State 1. The low-pressure refrigerant vapor at State 1 enters the compressor completing the cycle.
21
Embed
CHAPTER 03 THEORETICAL CONCEPTS OF VAPOUR ABSORPTION ...
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
28
CHAPTER 03
THEORETICAL CONCEPTS OF VAPOUR ABSORPTION
CHILLERS & EXHAUST GAS
This chapter describes the about the vapour absorption chillers. Further to that
describe the types and characteristics of the vapour absorption chillers. Vapour
absorption chiller is the most important part of the cogeneration system
3.1 Absorption Chillers
Absorption chiller technologies are one of a group of technologies classified as heat
pumps. Heat pumps may be either heat driven or work driven. Absorption
technologies are heat driven, transferring heat from a low temperature to a high
temperature using heat as the driving energy. Heat pumps operate on the principle of
the absorption refrigeration cycle, which is similar to the vapor-compression cycle.
Both the absorption refrigeration cycle and the vapor-compression cycle will be
examined to draw analogies between the two.
Vapor-compression refrigeration systems are the most common refrigeration
systems in use today. The vapor-compression cycle is a work- driven cycle that is
illustrated in Figure 3.1. In the vapor-compression cycle, work is input to compress
the refrigerant to a high pressure and temperature at State 2. At State 2, the
refrigerant condensation temperature is below the ambient temperature. As the high-
pressure and high-temperature refrigerant vapor passes through the condenser, heat
is rejected to the ambient air and the refrigerant vapor condenses to a liquid to
achieve State 3. The high-pressure liquid at State 3 passes through an expansion
valve. As the liquid passes through the expansion valve, the refrigerant experiences
a reduction in both temperature and pressure to reach State 4. At State 4, the boiling
temperature of the refrigerant is lower than that of the surroundings. The low-
pressure liquid refrigerant passes through the evaporator, absorbing heat from the
ambient environment when boiling occurs in the evaporator and creating a low-
pressure refrigerant vapor at State 1. The low-pressure refrigerant vapor at State 1
enters the compressor completing the cycle.
29
Figure 3.1: Vapor-compression cycle schematic
Source: Mississippi state university, 2005
The absorption cycle has some features in common with the vapor compression
cycle. For example, the absorption cycle has a condenser, an evaporator, and an
expansion valve. However, the absorption cycle and the vapor-compression cycle
differ in two very important aspects. The absorption cycle uses a different
compression process and different refrigerants than the vapor-compression cycle.
The absorption cycle operates on the principle that some substances (absorbents)
have an affinity for other liquids or vapors and will absorb them under certain
conditions. Instead of compressing a vapor between the evaporator and condenser as
in Figure 3.1, the refrigerant of an absorption system is absorbed by an absorbent to
form a liquid solution. The liquid solution is then pumped to a higher pressure.
Because the average specific volume of a liquid is much smaller than that of the
refrigerant vapor, significantly less work is required to raise the pressure of the
refrigerant to the condenser pressure. This corresponds to less work input for an
absorption system as compared to a vapor- compression system.
30
Because the absorbent used in the absorption cycle forms a liquid solution, some
means must also be introduced to retrieve the refrigerant vapor from the liquid
solution before the refrigerant enters the condenser. This process involves heat
transfer from a relatively high-temperature source. Because the thermal energy input
into the system is much higher than the work input through the pump, absorption
chillers are considered to be heat driven.
The components used to achieve the pressure increase in an absorption chiller are
viewed as a “thermal compressor” and replace the compressor in the vapor-
compression cycle shown in Figure 3.1. The components of the absorption cycle are
shown schematically in Figure 3.2. The components of the thermal compressor are a
pump, an absorber, and a (heat) generator and are shown to the right of the dashed
Z-Z line. The components to the left of the dashed Z-Z line are the same as the ones
used in the vapor-compression system
Figure 3.2: Basic absorption cycle schematic
Source: Mississippi state university, 2005
The operation of the absorption cycle shown in Figure 3-2 is as follows: At State 1,
the low-pressure refrigerant vapor exits the evaporator and enters the absorber.
31
In the absorber, the refrigerant vapor is dissolved in an absorbent and rejects the heat
of condensation and the heat of mixing to form a liquid solution. The
refrigerant/absorbent solution is then pumped to the condenser pressure and passed
to the generator. In the generator, heat is added to the refrigerant/absorbent solution
to vaporize the refrigerant, removing the refrigerant from the solution. The liquid
absorbent has a higher boiling temperature than the refrigerant and, therefore, stays
in the liquid form. There are two streams exiting the generator. The refrigerant exits
to the condenser at a high temperature and pressure (State 2) while the absorbent
passes through an expansion valve, decreasing the pressure of the absorbent to the
evaporator pressure before entering the absorber again.
The remainder of the operation is much the same as the vapor compression cycle.
The high-temperature, high-pressure refrigerant vapor at State 2 enters the
condenser with a pressure such that the ambient temperature is higher than the
condensation temperature of the refrigerant.
The refrigerant vapor condenses as it passes through the condenser, rejecting heat to
the ambient environment to achieve State 3. At State 3, the high-pressure, low-
temperature liquid refrigerant enters the expansion valve where the refrigerant
experiences a decrease in pressure to the evaporator pressure. The low- pressure,
low-temperature liquid refrigerant that results at State 4 is at a pressure such that the
boiling temperature of the refrigerant is lower than the ambient temperature of the
environment. As the liquid refrigerant passes through the evaporator, the refrigerant
boils, absorbing heat from the ambient air. The refrigerant exits the evaporator as a
high-temperature, low-pressure vapor to complete the cycle.
3.2 Refrigerant-Absorbent Selection
Though all absorption chillers operate on the basic cycle presented in Figure 3.2,
each chiller design is dependent on the refrigerant-absorbent selection. Current
refrigerant/ absorber media for absorption chillers are either water/lithium bromide
or ammonia/water. Water/lithium bromide absorption chillers utilize water as the
refrigerant and lithium bromide as the absorbent.
32
Because water is used as the refrigerant, applications for the water/lithium bromide
absorption chillers are limited to refrigeration temperatures above 00C. This
combination of refrigerant and absorbent is advantageous in areas where toxicity is a
concern because lithium bromide is relatively non-volatile. Absorption machines
based on water/lithium bromide are typically configured as water chillers for air-
conditioning systems in large buildings. Water/lithium bromide chillers are available
in sizes ranging from 10 to 1500 tons. The coefficient of performance (COP) of
these machines typically falls in the range of 0.7 to 1.2 (Herold et al.1996).
3.2.1 Water lithium bromide vapour absorption system
Vapour absorption refrigeration systems using water-lithium bromide pair are
extensively used in large capacity air conditioning systems. In these systems water is
used as refrigerant and a solution of lithium bromide in water is used as absorbent.
Since water is used as refrigerant, using these systems it is not possible to provide
refrigeration at sub-zero temperatures.
Hence it is used only in applications requiring refrigeration at temperatures above
0oC. Hence these systems are used for air conditioning applications. The analysis of
this system is relatively easy as the vapour generated in the generator is almost pure
refrigerant (water), unlike ammonia-water systems where both ammonia and water
vapour are generated in the generator.
3.3. Properties of Water-Lithium Bromide Solutions
3.3.1. Composition
The composition of water-lithium bromide solutions can be expressed either in mass
fraction (ξ) or mole fraction (x). For water-lithium bromide solutions, the mass
mass of solution, i.e,
Where mL and mW are the mass of anhydrous lithium bromide and water in solution,
respectively.
33
The composition can also be expressed in terms of mole fraction of lithium bromide
as:
Where, nL and nW are the number of moles of anhydrous lithium bromide and water
in solution, respectively. The number moles of lithium bromide and water can easily
be obtained from their respective masses in solution and molecular weights, thus;
: and
Where ML (= 86.8 kg/kmol) and MW (= 18.0 kg/kmol) are the molecular weights of
anhydrous lithium bromide and water respectively.
3.3.2. Vapour pressure of water-lithium bromide solutions
Applying Raoult’s law, the vapour pressure of water-lithium bromide solution with
the vapour pressure exerted by lithium bromide being negligibly small is given by:
Where, PW is the saturation pressure of pure water at the same temperature as that of
the solution and x is the mole fraction of lithium bromide in solution. It is observed
that Raoult’s law is only approximately correct for very dilute solutions of water
lithium bromide ( 0). Strong aqueous solutions of water-lithium bromide
are found to deviate strongly from Raoult’s law in a negative manner. For example,
at 50 percent mass fraction of lithium bromide and 25oC, Raoult’s law predicts a
vapour pressure of 26.2 mbar, whereas actual measurements show that it is only 8.5
mbar. The ratio of actual vapour pressure to that predicted by Raoult’s law is known
as activity coefficient. For the above example, the activity coefficient is 0.324
34
The vapour pressure data of water-lithium bromide solutions can be very
conveniently represented in a Dühring plot. In a Dühring plot, the temperature of the
solution is plotted as abscissa on a linear scale, the saturation temperature of pure
water is plotted as ordinate on the right hand side (linear scale) and the pressure on a
logarithmic scale is plotted as ordinate on the left hand side. The plot shows the
pressure-temperature values for various constant concentration lines (isosters),
which are linear on Dühring plot. Figures 15.1 show the Dühring plot. The Dühring
plot can be used for finding the vapour pressure data and also for plotting the
operating cycle. Figure 15.2 shows the water-lithium bromide based absorption
refrigeration system on Dühring plot. Other types of charts showing vapour pressure
data for water-lithium bromide systems are also available in literature. Figure 15.3
shows another chart wherein the mass fraction of lithium bromide is plotted on
abscissa, while saturation temperature of pure water and vapour pressure are plotted
as ordinates. Also shown are lines of constant solution temperature on the chart.
Pressure-temperature composition data are also available in the form of empirical
equations
Fig.3.3: A Typical Duhring plot
Source: IIT Kharagpur, 2008
35
Fig.3.4: H2O- LiBr System with a solution heat exchanger on Duhring plot
Source: IIT Kharagpur, 2008
Fig.3.5: Pressure-temperature-concentration diagram for H2O-LiBr solution
Source: IIT Kharagpur, 2008
36
3.3.3. Enthalpy of water-lithium bromide solutions
Since strong water-lithium bromide solution deviates from ideal solution behavior, it
is observed that when water and anhydrous lithium bromide at same temperature are
mixed adiabatically, the temperature of the solution increases considerably. This
indicates that the mixing is an exothermic process with a negative heat of mixing.
Hence the specific enthalpy of the solution is given by:
Where hL and hW are the specific enthalpies of pure lithium bromide and water,
respectively at the same temperature. Figure 15.4 shows a chart giving the specific
enthalpy-temperature-mass fraction data for water-lithium bromide solutions. The
chart is drawn by taking reference enthalpy of 0 kJ/kg for liquid water at 0oC and
solid anhydrous lithium bromide salt at 25oC.
Fig.3.6: Enthalpy –temperature - concentration diagram for H2O-LiBr solution
Source: IIT Kharagpur, 2008
37
3.3.4. Enthalpy values for pure water (liquid and superheated vapour)
The enthalpy of pure water vapour and liquid at different temperatures and pressures
can be obtained from pure water property data. For all practical purposes, liquid
water enthalpy, hW, liquid at any temperature T can be obtained from the equation:
Where Tref is the reference temperature, 0oC
The water vapour generated in the generator of water-lithium bromide system is in
super heated condition as the generator temperature is much higher than the
saturation water temperature at that pressure. The enthalpy of superheated water
vapour, hW,sup at low pressures and temperature T can be obtained approximately by
the equation:
hW,sup = 2501 + 1.88 (T – T ref ) (3.8)
3.3.5. Crystallization
The pressure-temperature-mass fraction and enthalpy-temperature-mass fraction
charts (Figs. 3.5 and 3.6) show lines marked as crystallization in the lower right
section. The region to the right and below these crystallization lines indicates
solidification of LiBr salt. In the crystallization region a two-phase mixture (slush)
of water-lithium bromide solution and crystals of pure LiBr exist in equilibrium. The
water-lithium bromide system should operate away from the crystallization region as
the formation of solid crystals can block the pipes and valves. Crystallization can
occur when the hot solution rich in LiBr salt is cooled in the solution heat exchanger
to low temperatures. To avoid this condenser pressure reduction below a certain
value due to say, low cooling water temperature in the condenser should be avoided.
Hence in commercial systems, the condenser pressure is artificially maintained high
even though the temperature of the available heat sink is low. This actually reduces
the performance of the system, but is necessary for proper operation of the system. It
should be noted from the property charts that the entire water-lithium bromide
system operates under vacuum.
38
3.4. Commercial systems
Commercial water-lithium bromide systems can be:
1. Single stage or single-effect systems, and
2. Multi stage or multi-effect systems
Single stage systems operate under two pressures – one corresponding to the
condenser-generator (high pressure side) and the other corresponding to evaporator-
absorber. Single stage systems can be either:
1. Twin drum type, or
2. Single drum type
Since evaporator and absorber operate at the same pressure they can be housed in a
single vessel, similarly generator and condenser can be placed in another vessel as
these two components operate under a single pressure. Thus a twin drum system
consists of two vessels operating at high and low pressures. Figure 3.8 shows a
commercial, single stage, twin drum system.
Fig.3.7: A commercial, twin-drum type, water-lithium bromide system
Source: IIT Kharagpur, 2008
39
As shown in the figure, the cooling water (which acts as heat sink) flows first to
absorber, extracts heat from absorber and then flows to the condenser for condenser
heat extraction. This is known as series arrangement. This arrangement is
advantageous as the required cooling water flow rate will be small and also by
sending the cooling water first to the absorber, the condenser can be operated at a
higher pressure to prevent crystallization. It is also possible to have cooling water
flowing parallel to condenser and absorber; however, the cooling water requirement
in this case will be high.
A refrigerant pump circulates liquid water in evaporator and the water is sprayed
onto evaporator tubes for good heat and mass transfer. Heater tubes (steam or hot
water or hot oil) are immersed in the strong solution pool of generator for vapour
generation.
Pressure drops between evaporator and absorber and between generator and
condenser are minimized, large sized vapour lines are eliminated and air leakages
can also be reduced due to less number of joints.
Figure 3.9 shows a single stage system of single drum type in which all the four
components are housed in the same vessel. The vessel is divided into high and low
pressure sides by using a diaphragm.
Fig.3.8: A commercial, single-drum type, water-lithium bromide system
Source: IIT Kharagpur, 2008
40
In multi-effect systems a series of generators operating at progressively reducing
pressures are used. Heat is supplied to the highest stage generator operating at the
highest pressure. The enthalpy of the steam generated from this generator is used to
generate some more refrigerant vapour in the lower stage generator and so on.
Double-effect absorption systems use a second generator, condenser, and heat
exchanger that operate at higher temperature. A double-effect water/lithium bromide
absorption system is shown schematically in Figure 3.10. Refrigerant vapor is
recovered from the first-stage generator in the high- temperature condenser. The
refrigerant/absorbent in the second-stage generator is at a lower temperature than the
solution in the first-stage generator. The refrigerant vapor from the first stage
generator flows through the second- stage generator where a portion of the
refrigerant condenses back into liquid while the remainder remains in the vapor
phase.
Additional refrigerant is vaporized in the second-stage generator by the high
temperature and the heat of vaporization supplied by the refrigerant from the first-
stage generator. The refrigerant vapor from both generator stages flows to the
condenser while the absorbent solution flows back to the absorber.