Document 7OjXxm6Zxz2XnBQ7DxZ3OaN2a

12 CHAPTER 1 1965 Guide And Data .Book effectiveness of the absorber as a compressor.-The effects the--;, result of the attraction between the absorbent and refrigerant molecules, a subject which will be discussed in more detail in connection with the discussion of absorbent-refrigerant combinations. After a given quantity.,of absorbent has dissolved its quota of vapor, it must be returned to the generator to be reclaimed. Since the generator is at the high tide pressure, a' pump is required to make the transfer. This pump is not a compressor, because the absorber has already accomplished the compres sion; it is merely a circulating device, and the amount of energy required to'drive it is small in comparison to'that re-` quired to drive's vapor compressor.'' '' ''' ' ; . .The function of the generator,is to separate the refrigerant from the absorbent. This is accomplished by vaporizing the refrigerant so that the physical separation-of the refrigerant" vapor from the absorbent liquid is relatively easy..This process requires the addition of heat, most,of which is used to.vaporize ; the refrigerant. The absorbent in concentrated form flows back to the absorber through a flow control, and the refriger-' ant vapor flows through an unrestricted passage to' thecoh- ' denser. As indicated in Fig. 17, beat is put into the absorption cycle-* in two places, i.e., into the generator and into the evaporator. Energy must also be supplied for the solution pump; this is generally very small in comparison to the other heat quanta-.- ties, and the heat equivalent of the energy used for circulating the solution is simply added to the heat applied to the genera tor. Heat is dissipated;from both the condenser and-tire absorber. r. . An absorption cycle may be viewed as -the combination of a heat engine and a-refrigerating machine; the heat engine converts heat into work which, in turn, drives the refrigerating machine to produce a refrigerating effect. For an ideal, re versible cycle, consider the'ease in which a quantity of heat Qi is added to the generator at a temperature of Tg and a quantity of heat Qe is rejected at temperature T,, According to the Second Law, the work W produced can be calculated as follows: W (15) where W ~ work, Btu. Qr ~ heat added to geoerator, Btu.' T, = temperature of generator. Rantrine degrees. T, -- temperature of condenser, Rankine degrees. * For a reversible refrigeration cycle taking in a quantity of heat Qt through ah evaporator at temperature T, and reject- mg heat through the absorber at temperature T,, : , .; >--(^).. : where ' ... Q. *= heat added to evaporator, Btu. T, -- temperature of evaporator, Rankine degrees. T. = temperature of absorber, Rankine degrees. : Since the work quantity W is common. Equations.15 and 16 may be combined: ` <-. (17) The coefficient of performance (CP) of a refrigerating ma chine is defined as the ratio of refrigerating effect to heat in put; therefore Equation 17'can be rearranged to give Q, T,(T, -- 7V\ : 'q.TY, \t. - tJ. (18) fig. 18 .... Ideal Coefficients.of. Performance for 'Reversible Absorption Refrigeration Cydes-'-'* It is frequently stated that tire CP for an absorption cycle must always be less than one; this is not a theoretical limit but rather an artificial limit imposed by conventional design. A study of Equation 18 reveals that there is no finite limit. .The value of the factor TJTt must be less than unity for a refriger-. ation cycle, but the value of the factor {Tt Tt)/(Ta --'T',). will approach infinity as 71. approaches 7V. . -The statement concerning a limit of unity for .the CP is based on the concept that if .one pound of refrigerant -is evaporated from the generator, then only one pound'-of. re-i frigerant can be available in the evaporator. This artificial limit can be overcome by design, e.g., by using the multiple effect evaporator principle in the generator. In some cases, the condenser and absorber will have a com mon heat rink, and Te will equal Tm\ Equation 18 can then be written: (CP) T. (T, -- T,\ T, \T. - TJ (19) To show the relation between the rink temperature,-the generator temperature, and the ideal coefficient of perform ance, Equation 19 has been plotted in Fig.. 18 for generator temperatures of 200, 300, and 400 F. ., Nature of the Absorbent-Refrigerant Combi nation5415 By definition, the working fluids are liquids; solid materi-T' als which, with a suitable refrigerant, cam constitute a re frigeration system are called adsorbents, and the systems are called adsorption systems. The liquids, the absorbent, and the refrigerant must, to some degree, meet' all of a series of re-' Thermodynamics and Refrigeration Cycles 1 quiremente; the more significant requirements are discussed materials but, at this time, only two pairs have been found ju the following paragraphs. ' practically usefuL These are (I) the ammonia-water, combi nation and (2) the water-aqueous solution of a hygroscopic . salt combination, in which lithium bromide has been the only absorber_____ gSrS the refrigerant vapor. This affinity is>e r^Jt of mild X2.WI bonding, generally hydrogen bonding." While it is ftlLssrr that a strong affinity exist, it is also necessary that the two materials not be so strongly bonded that they cannot be read- really practical salt. A third combination makes use of dimethoxytetraethylene glycol*7 as the absorbent, and some material such as methylene chloride or Refrigerant 21 as the refrigerant.** f/te-k of stability and a low value for the CP Ov separated by the application of heat. The fact that hydrogen . is the yielding satisfactory bonding forces : why so many of the absorbent-refrigerant combinations' - ,,tili containing highly electro-negative atoms,: have been the main deterrents to the use of this system. The ammonia-water combination,' in. which ammonia serves as the refrigerant and water as the absorbent, is the such as oxygen and nitrogen. ' classical example in absorption refrigeration literature. -Both In the case of systems using aqueous solutions of hygroscopic- molecules meet the requirements for-hydrogen bonding ex salts for the absorption of water vapor, the bonding is generally.-, attributed to solvation. .m This affinity is indicated by a large negative deviation from:.'/ tremely well; both are highly stable,, ammonia has a high . latent heat of vaporization, and, in general, bothare,quite fiaoult's Law; the absorbent lowers the vapor pressure of the , .compatible with ordinary materials of construction. Even so, frigerant by a far greater amount than would be expected on -. the combination is far from perfect, the major difficulties. the K**;` of solution behavior. The marked evolution of heat '; when the liquid refrigerant is added to the liquid absorbent ia'also." ; being the volatility of the absorbent and the high condensing evidence of the affinity. pressure. There are other, disadvantages, such as the'toxicity 2. Solubility- It ts necessary that the refrigerant and absorbent'.' of ammonia in higher concentrations ami the fact that copper' be mutually soluble over the whole range of operating conditions.' and its alloys are not suitable for use in systems containing: If crystallization occurs, the solid particles can make it difficult or iTTPrt--hl1* for the fluids to circulate properly. Usually, with ammonia. These disadvantages have been successfully hari-- matenaJs of this nature, the problem is one of solvate formation,' -; died, with the result that millions of such refrigeration units and the solution simply goes solid. have been made and used. 3. Absorbent Volatility. Ideally, the absorbent should be non-,,> . The fact that the water (absorbent)-is volatile means that volatile so the vapor leaving the generator will be pure re--- frigerant If the absorbent is volatile to any appreciable extent ?, the vapor leaving the generator will contain appreciable a separation of refrigerant vapor from absorbent vapor is required 'T- . quantities of water vapor. If allowed to pass to the oondenser between the generator and toe condenser; this generally neoesa- - . and flow on to the evaporator, the water would raise the fates a rectification column which requires energy for operation / evaporator temperature and carry much unevaporated re and thus reduces the CP of the system. 4. Stability. In the ease of absorption systems^ almost absolute'.\ frigerant from the evaporator, thus reducing the CP. There- " stability is required because the fluids will be refluxed together1 - - fore, it is necessary to insert a rectifying or distilling column continuously in a hermetically sealed system which is expected to -.- between the generator and condenser. The distilling column function without trouble for many years. . 5. Corrosion. It ts obvious, because of the long life require- "'' mentis, that the fluids must not be corrosive to any detectable ex-^ tent to the materials used in constructing the equipment.^ requires that a part of the distillate be returned to the top of the column as reflux,liquid. This return of condensate to the generator constitutes a loss in refrigerating effect, but this 6. Pressure. Ideally, the working pressures should bo ashear to loss is much less than that which would result if the water- atmospheric as-posible, to-minimize equipment weight and to' minimigft leakage- into or out of the system. Furthermore, thfiTT pressure difference between the high side and the low-side:-: _ rich tor. refrigerant were allowed to pass directly to the evapora Fortunately, the relative volatilities of ammonia and should be tow, to facilitate circulation of the solution. water are different enough to make this a relatively easy sepa 7. Safety. Low toxicity and noninflammability requirements - - are generally included in building codes and are quite strict. 8. Viscosity. Low viscosity for both fluids b highly desirable, both to promote good heat transfer and to circumvent excessive energy requirements for fluid circulation.' ration. The use of water as a refrigerant is quite old. One such system was in use in 1845. A number of strong absorbents for water vapor 'are' well known, e.g., sulfuric acid, phosphoric 9. A refrigerant with a high latent heat of vaporization b'de sirable because it holds to a minimum the quantities of fluids to be circulated. , . - ,. acid, potasium hydroxide solutions and a series of hygroscopic salt solutions. However, the corrosiveness of many of these, particularly the acids, makes their use impractical, so the No combinations meeting all these requirements have been aqueous solution of the hydroscopic salts are the only ab found. However, a few absorbent-refrigerant puis fulfill most sorbents that have been found usefuL of them, and are practicaL They are discussed in detail in the The chlorides, bromides, and iodides of. lithium, mag nexteection. nesium, calcium, and zinc are typical of the most useful hy Workable Absorbent-Refrigerant Combinations groscopic salts. Of these, lithium bromide is almost exclusively used. It has the advantage over the salts of magnesium, Since hydrogen bonding** is the primarymp^hanitim- creat fAle.inm, and zinc that it is soluble in solutions haveng a pH ing the attraction between absorbents and refrigerants, the higher than 7, and the use of slightly baric solutions makes general requirements are that: (1) the molecules be wnall^ corrosion much easier to control. The' bromide of lithium is (2) at least one of the pair contain a highly electronegative also more desirable than the chloride or iodide, in that it has atom such as fluorine, nitrogen' or oxygen, and- (3) the other the best solubility-vapor pressure towering relationship. . molecule of the pair contain hydrogen. Molecules meeting The advantages of the water-hygroscopic salt systems are these general requirements include water, the lower alcohols, as follows: ammonia, amines, -ketones, and ethers. Actually, the- list of materials is quite lengthy. However, in most cases where Affinity requirements are met, failure to meet some other feqtiirement will eliminate a material from consideration. -For example, sulfuric arid would be an ideal absorbent for 1. Water, as a refrigerant, has a very high latent heat of vaporization. 2. The absorbent is nonvolatile.-. ... j .; 3. The system operates at low pressures. 4. The materials are nontoxic and nonflammable. -water vapor, but it-is too corrosive to the.materials of con There are certain disadvantages, as follows: struction. < -Much effort has gone into the search for suitable pairs of 1. Water as a refrigerant is limited to evaporator temperatures above 32 F, its freezing point. /