R&AC Lecture 16 - Lesson 16 Vapour Absorption...

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Lesson 16 Vapour Absorption Refrigeration Systems Based On Ammonia- Water Pair Version 1 ME, IIT Kharagpur 1
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The specific objectives of this lesson are to: 1. Introduce ammonia-water based vapour absorption refrigeration systems ( Section 16.1 ) 2. Discuss the properties of ammonia-water mixtures and introduce pressure- temperature-concentration (p-T- ξ ) and enthalpy-temperature-concentration (h-T- ξ ) charts ( Section 16.2 ) 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 ( Section 16.3 ) At the end of the lecture, the student should be able to: 1. Differentiate between water-lithium bromide and ammonia-water systems vis-à- vis their properties 2. Explain the concepts of bubble point and dew point temperatures 3. Obtain thermodynamic properties of ammonia-water mixtures using p-T- ξ and h- T- ξ charts 4. Analyze important steady flow processes involving binary mixtures 16.1. Introduction In vapour absorption refrigeration systems based on ammonia-water pair, ammonia is the refrigerant and water is the absorbent. These systems are more versatile than systems based on water-lithium bromide as they can be used for both sub-zero (refrigeration) as well above 0 o C (air conditioning) applications. However, these systems are more complex in design and operation due to the smaller boiling point temperature difference between the refrigerant and absorbent (about 133 o C). Due to the smaller boiling point temperature difference the vapour generated in the generator consists of both ammonia as well as 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. Version 1 ME, IIT Kharagpur 2
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16.2. Properties of ammonia-water solutions 16.2.1. 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., W A A m m m + = ξ (16.1) where m A and m W are the mass of ammonia and water in solution, respectively.
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R&AC Lecture 16 - Lesson 16 Vapour Absorption...

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