vapour absorption refrigeration systems based on ......raoult’s law predicts a vapour pressure of...
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Vapour Absorption Refrigeration Systems Based On Ammonia- Water Pair
Nagendra M
CBM Engineer, Hindusthan Zink .Ltd
The specific objectives of this lesson are to: 1. Introduce ammonia-water based vapour absorption refrigeration systems. 2. Discuss the properties of ammonia-water mixtures and introduce pressure temperature- concentration (p-T-ξ) and enthalpy-temperature-concentration (h-T- ξ) charts. 3. Analyze some basic steady flow processes using ammonia-water mixtures such as adiabatic and non-adiabatic mixing, throttling of solution streams and the concept of rectification. 1.1. Introduction
In vapour absorption refrigeration systems based on ammonia-water pair, ammonia is the refrigerant and water is the absorbent. These systems are more adaptable than systems based on water-lithium bromide as they can be utilized for both below zero (refrigeration) too over 0 0C (cooling) applications. On the other hand, these systems are more unpredictable in outline and operation because of the littler boiling point temperature difference between the refrigerant and absorbent (around 1330C). Because of the littler boiling point temperature difference the vapour produced in the generator consists of both ammonia and additionally water. If water is allowed to circulate with ammonia in the refrigerant circuit, then:
i. Heat transfer in condenser and evaporator becomes non-isothermal ii. Evaporator temperature increases iii. Evaporation will not be complete iv. Water may get accumulated in the evaporator leading to malfunctioning of the plant iv. Circulation ratio increases
Since all the above effects are detrimental to the performance of the system, it is necessary to minimize the concentration of water vapour in ammonia at the inlet to the condenser. This requires additional components, namely a rectification column and a dephlegmator between generator and absorber, which increases the design complexity and cost and also reduces the system COP compared to water-lithium bromide system. 1.2. Properties of ammonia-water solutions 1.2.1. Composition
Similar to water-lithium bromide solutions, the composition of ammonia-water solution is also expressed either in mass fraction (ξ) or mole fraction (x). However, for ammonia-water solutions, the mass and mole fractions are defined in terms of ammonia. For example the mass fraction ξ is defined as the ratio of mass of ammonia to the total mass of solution, i.e.,
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where mA and mW are the mass of ammonia and water in solution, respectively. Similarly, the mole fraction of ammonia-water solution is defined as:
where n A and nW are the number of moles of ammonia and water in solution, respectively. The number of moles of ammonia and water can easily be obtained from their respective masses in solution and molecular weights, thus;
where MA (= 17.0 kg/kmol) and MW (= 18.0 kg/kmol) are the molecular weights of ammonia and water respectively. 1.2.2. Vapour pressure of ammonia-water solutions
Liquid ammonia and water are totally miscible in all proportions, henceforth can frame solutions of all concentrations from 0 to 1, at normal temperatures. The impact of ammonia in water is to bring down the vapour weight of water; comparatively the impact of water in ammonia is to bring down ammonia's vapour weight. Along these lines the aggregate weight over ammonia water solutions is comprised of incomplete weight of ammonia and fractional weight of water vapour, and is always in between the saturation pressures of unadulterated ammonia and water. If Raoult’s law is applied to ammonia-water mixtures, then the total pressure at any temperature, Ptotal is given by:
Ptotal = xPA + (1− x)PW
where x is the liquid phase mole fraction of ammonia, PA and PW are the saturation pressures of pure ammonia and pure water at that temperature. However, similar to water-lithium bromide solutions, ammonia-water solutions also deviate from ideal solution behaviour predicted by Raoult’s law in a negative manner, i.e., at a given temperature of the solution the actual vapour pressure will be less than that predicted by Raoult’s law (activity coefficient is much smaller than 1.0). For example, at a mass fraction of 0.4 and temperature of 400C, Raoult’s law predicts a vapour pressure of 6.47 bar, whereas the measured vapour pressure is 3.029 bar. The vapour pressure data of ammonia-water solutions is also available in the form of Dühring and other P-T-ξ plots. 1.2.3. Composition of ammonia-water vapour
Since the vapour above ammonia-water fluid consists of both ammonia and water vapour, it is crucial to recognize the composition in fluid phase and composition in vapour phase. The superscripts L and V will be utilized to recognize fluid and vapour phase
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when the coble and dewe point and
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Nand wateconcentrasaturatedvapour wliquid anwill be ξvapour wshown in
O
(ammoniexample,the conceof indivi(mtotal) an
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Fig.1.4:
Now since ther will be cation of su
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n the figure.
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The above equation is called as the mixing rule or lever rule for the binary mixtures such as ammonia and water. It implies that the fraction of liquid and vapour in the two-phase mixture is inversely proportional to the distance between the mixture condition 2 and the saturated liquid and vapour states 2 L and 2 V, respectively. 1.2.5. Enthalpy of ammonia-water mixtures Liquid phase: The enthalpy of ammonia-water solution in liquid phase, hL is calculated in a manner similar to that of water-lithium bromide solutions, i.e., by the equation:
Where ξL is the liquid phase mass fraction of ammonia, hAL and h WL are liquid phase enthalpies of pure ammonia and water respectively. Δh mix is the heat of mixing, which is negative (exothermic) similar to water-lithium bromide mixtures. Using the above equation one can calculate the specific enthalpy of ammonia water solutions at any concentration and temperature provided the heat of mixing is known from measurements.
Thus enthalpy charts for solution are plotted as a field of isotherms against mass fraction by taking suitable reference values for enthalpy of ammonia and water. Since pressure does not have a significant effect on liquid enthalpy (except at critical point), normally pressure lines are not shown on typical solution enthalpy charts. Also enthalpy of sub cooled liquid is generally assumed to be equal to the saturated enthalpy at that temperature without loss of much accuracy. Vapour phase:
Evaluation of enthalpy of a mixture of vapours of ammonia and water is more
complicated compared to liquid phase enthalpy. This is due to the dependence of vapour enthalpy on both temperature and pressure. However, to simplify the problem, it is generally assumed that ammonia and water vapour mix without any heat of mixing. Then the enthalpy of the vapour mixture, hV is given by:
Where ξ.V is the vapour phase mass fraction of ammonia and the specific enthalpies of ammonia vapour and water vapour respectively at the temperature of the mixture. However, since vapour enthalpies depend on temperature as well as pressure, one has to evaluate the vapour enthalpy at suitable pressure, which is not equal to the total pressure. An approximate, but practically useful method is to evaluate the vapour enthalpies of ammonia and water at pressures, PA and PW given by
where y is the vapour phase mole fraction of ammonia and Ptotal is the total pressure. It should be noted that PA and PW are equal to the partial pressures of ammonia and water only if they behave as ideal gases. However since ammonia and water vapour may not approach the ideal gas behaviour at all temperatures and pressures, in general PA and PW are not equal to the
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partial pressures. Using these method enthalpies of ammonia-water mixtures in vapour phase have been obtained as functions of temperature and mass fraction. 1.2.6. The complete enthalpy-composition diagram for ammonia-water mixtures:
Normally, charts of enthalpy-temperature-mass fraction are available which give both liquid phase as well as vapour enthalpy of mixtures. Figure 1.5 shows one such chart. Figure 1.6 shows the enthalpy-synthesis diagram at a constant weight P. In the figure point a represents to the state of saturated liquid mixture at a temperature T with a liquid phase mass fraction of ξL. The liquid phase enthalpy corresponding to this condition is given by h L. The organization and enthalpy of vapour mixture in harmony with the liquid mixture at temperature T and weight P are obtained by drawing a vertical line from an up to the auxiliary line and then drawing a horizontal line to one side from the convergence of the vertical line with the auxiliary line.
The crossing point of this horizontal line with the dew point line a' gives the vapour
phase mass fraction ξV and the vapour phase enthalpy h V as demonstrated in the figure. The isotherm T in the two-phase district is obtained by joining focuses on and an' as indicated in the figure. Point b in the figure lies in the two-phase district. The particular enthalpy of this point h b is given by:
where ψb is the quality or dryness fraction of the two-phase mixture at b. Since points a, a’ and b are co-linear, the dryness fraction ψb is given by:
In actual enthalpy-composition diagrams the isotherms are not shown in two-phase region as a different set of them exist for each pressure. It is important to note that it is not possible to fix the state of the mixture (sub cooled, saturated, two-phase or superheated) just from temperature and mass fraction alone, though one can calculate enthalpy of the mixture from temperature and mass fraction. This is due to the reason that at a given mass fraction and temperature, depending upon the pressure the point can be sub cooled or saturated or superheated. For example, a liquid mixture with a mass fraction of 0.4 and temperature of 80oC has an enthalpy of 210 kJ/kg, and it will be in sub cooled condition if the pressure is 4.29 bar and saturated if the pressure is 8.75 bar.
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F
Determ
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Fig.1.6: Enth
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the convepoint b' horizontathrough xcan be obpassing t
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F 1.3 Ba a)Adiabare mixedas:
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iabatic mixinomposition dhas occurred
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released written a
From the
Thus wit(Q/m3) bwould lie c) Throtakes plashows thare conse
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H
diagram flashing, pressure Taking pthe vapou
d) Heatarrangem(HX A) imixture t
Hence the inlas shown inthe exit coP2 then the
point 2 as in ur fraction a
ting and cment whereinin such a wathen flows i
Fig.1.10: M
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Fig.1.11:
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and saturstate 5 inmixture i(state 6) takes pla
Fi Now mas
rated vapourn another heis then fed toand saturatce at a const
g.1.12: Hea
ss and energ
r (state 4) areat exchangeo another aded vapour (tant pressure
ating and co
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re separated.er B (HX B
diabatic sepa(state 7) aree and is a ste
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to se id ss
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Similar equations can be obtained for heat exchanger B and separator B. The entire process is also shown on enthalpy-composition diagram in Fig.1.12.
It may be noticed that from the above arrangement comprising of heating, cooling and separation, one finally obtains a vapour at state 7 that is rich in ammonia. That is the combination of heat exchangers with separators is equivalent to the methodology of rectification. Heat exchanger A plays the part of generator, while heat exchanger B plays the part of dephlegmator. To enhance the procedure of rectification in actual vapour absorption refrigeration frameworks, a rectifying column is presented between the generator and dephlegmator. In the rectifying column, the vapour from the separator an interacts with the saturated liquid originating from separator B.
Therefore, there will be heat and mass transfer between the vapour and liquid and
finally the vapour turns out at a much higher concentration of ammonia. The practical ammonia-water based vapour absorption refrigeration framework incorporating rectifying column.
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