Absorpsiyonlu soğutma sisteminin simülasyonu

Abstract

Bu çalışmada, enerji kaynağı olarak her türlü ısı enerjisini kullanan absorpsiyonlu soğutma sisteminin modeli elde edildi ve simülasyonu yapıldı. Önce soğutma sistemi elemanlarından olan absorber, çok detaylı bir şekilde incelendi. Absorber, film akışlı, dik borulu olarak düşünüldü. Akışın dik bir boru dışından olduğu ve boru cidar sıcaklığının, boru boyunca, lineer olarak değiştiği kabul edilerek, ısı geçiş, kütle geçiş ve sınır şart bağıntıları yazıldı. Ortaya çıkan simültane kısmi türevli diferansiyel denklemlerin çözümü için, sonlu fark denklemleri yazılarak bilgisayar yardımıyla, akış kalınlığı ve boru boyunca sıcaklık ve derişiklik dağılımları elde edildi. Elde edilen sıcaklık ve derişiklik dağılımlarından faydalanılarak, ısı taşınım ve kütle taşınım katsayıları bulundu. Soğutma sisteminin diğer elemanlarının herbiri, kovan-boru tipli ısı değiştiricisi olarak düşünüldü ve bilgisayar modelleri elde edildi. Absorpsiyonlu soğutma sistemi üzerinde daha önce yapılan ça lışmalarda, ısı değiştiricisi hesaplarında, parametre olarak toplam ısı geçiş katsayısı ile ısı geçiş alanının çarpımı (KxA) alınmıştır. Bu ça lışmada ise, soğutma sisteminin her bir elemanı için, akış şekline ve akışkan tipine bağlı olarak, boru içindeki ve dışındaki ısı taşınım katsayıları hesaplandı, bu değerler yardımıyla toplam ısı geçiş katsayısı bulundu. Herbir eleman için ısı değiştiricisi boyutlandırılması yapılarak, soğutma sistemi elemanlarının boyutları bulundu. Soğutma sistemi üzerinde parametrik çalışma yapılarak, sistem parametrelerinin soğutma tesir katsayısına (STK) ve sistem elemanlarının boyutları üzerine tesirleri incelendi. Elde edilen neticeler, grafikler ve tablolar halinde gösteri lerek yorumları yapıldı ve daha önceden yapılan teorik ve deneysel çalışmalarla karşılaştırıldı.

Absorption air conditioning units are among the most versalite water chillers available as standart commercial units. They may be energized by steam or hot water over a very wide range of input temperature and this range is made still wider if the energizing temperature is optimized according to existing loads and cooling tower water temperature. The effective range of energizing temperature for typical commercial units runs from 115 C down to 45 C as cooling tower water temperature ranges from 30 C down to 10 C. Since the building load diminishes as the outdoor ambient and cooling tower water temperature drop, there is seldom a problem of meeting the needed capacity through this whole range of conditions. The energy sources for which absorption equipment was initially designed was usually to make use of process heat or turbine exhausts instead of disposing of such heat in condensers, thus wasting it. A large portion of such heat is available at about 110-115°C, which became the design point for most commercial absorption equipment. But as the need for greater energy conservation and better utilization emerged, it soon became apparent that greater attention to low-tempera ture energization was needed. As solar cooling has come under consideration, using the heat from solar collectors to energize the absorption equipment, the benefits of lower temperature energization have become more important since collectors themselves can collect much more heat at the lower and intermediate temperatures than if all the heat is collected and stored at the highest temperature needed. In fact it is doubtful that any solar cooling system using conventional flat plate solar collectors can be competitive without having an arrangement to utilize low temperature heat over as great a portion of the season as possible. With the development of working absorption systems, there has been a growing need for reliable and effective system simulations. Several computer models have been described in the literature, dealing with a variety of applications, cycles and working substances [l-13]. The results of some of these models have been compared with experimen tal data and found to be in good agreement. The simulation, codes have proved to be a very valuable tool for research and development and for improvements in design to be implemented in manufacturing. The simulation studies are quiet superficial.Heat and mass transfer processes of the cooling system in absorber, which is extre mely effective on coefficent of performance (COP) is not taken into IX consideration. Also mathematical models for the other components of the system are taken to be simple. For these components each being considered as heat exchangers, product of overall heat transfer coef ficient and total heat transfer surface (KA) is taken as a parameter and calculation of heat transfer coefficient is excluded. Absorption of gases and vapors in liquids are encountered in numerous applications in the chemical technology. These processes normally involve simultaneous heat and mass transfer in the gas-liquid system. The heat of absorption gives rise to temperature gradients leading to the transfer of heat; the temperature influences the vapor pressure-concentration equilibrium between the two phases which in turn affects the exchange of mass. The combined heat and mass transfer process does not lend it self easily to mathematical analysis. Many studies of absorption prob lems described in the literature have considered the heat and mass transfer separately, neglecting the coupling between them. Fortunately, in many real cases the heat interaction is small and the process may be considered isothermal. In some processes, however, the effect of heat transfer is important and cannot be neglected. A typical example is when the absorbate is a vapor with high heat of absorption, such as water. Furthermore, there is growing interest in processes where mass transfer is initiated specifically to produce a temperature change. One such example, from which the present study originated, is in absorp tion heat pumps for heating or cooling. There the heat transfer accom panying the mass transfer is of primary importance. Only recently some work have been published on combined heat and mass transfer in failling films [14-19 ]. This thesis presents an attempt to improve upon the models described earlier an eliminate some of their limitations. The model, for a falling film of absorbent solu tion in laminar flow, aims at the calculation of the heat and mass transfer coefficients for typical wall conditions and finding their dependence on the systems parameters. In the study principals of operation of an absorption cooling system is explained first. Then various cooling machine models, manu factured by a variety of companies, are introduced. Some information about the working fluid pairs used in cooling systems is provided and calculation of LiBr-Water properties by computer is explained. Also some information is given about the relations used in programming. In chapter II cooling system is analyzed. COP of cooling system based on Carnot Cycle is determined first. Later the equation giving COP of the system, is obtained theoretically. Absorption of a gas or vapor into a laminar falling film on the surface of a vertical tube, which is one of the major parts of this thesis, is studied in Chapter III. Necessary boundary conditions are provided to solve simultaneaus parabolic partial-differantial Equation which are obtained by diffusion and energy equations characterizing absorption process. In formulating this model, the following assumptions have been made: (1) The liquid solution is Newtonian and its physical properties are costant and independent of temperature and concentration. (2) The mass of vapor absorbed per unit time is small compared to the mass flow rate of the liquid. Therefore, it is assumed that the latter is costant, and so are the film thickness and avarage flow velocity. (3) There is no heat transfer in the vapor phase. (4) There are no natural convection efects in the film due to temperature or concentration differences (This assumption is in fact a corollary of the first one). (5) Diffusion thermal effects are negligible. (6) Vapor pressure equilibrium exists between the vapor and liquid at the interface. To solve these partial differential equations Crank-Nichelson Method, (an implicit numerical solution method) was used. These equa tions were expressed in finite difference form in cylindirical coordi nates. Solutions for temperature and concentration variations were obtained by applying these equations in discritized flow field. These solutions were used in determining heat transfer coefficient from the bulk of fluid to the wall and mass transfer coefficient from the inter face to the bulk of the liquid. Stability analysis which is an impor tant film absorber problem was performed. Each component of the cooling system was modelled as shell and tube type and single-pass cross-flow heat exchangers. Overall heat- transfer coefficients were calculated and calculation for determination of dimensions, by using logarithmic mean temperature difference, (LMTD) were performed for each component. Individally modelled components were combined to complete the unique system simulation. There are some constraints for cooling system operation. These relations were dictated bay physico-chemical processes and heat transfer. Cyristallization problem arises at two different points of the system. One is the point at which rich solution, which comes from generator, leaves heat exchanger. The other is the weak solution leaving absorber. For these two points restrictive parameter for the system operation is cyristallization temperature. For each component which were considered as heat exchangers, some constraints were determined depending on heat transfer direction. It presents the algoritm for the solution procedure to find which operating conditions (output) enable the system to operate safely. The computer simulation is based on three practical assumptions (1) The temperature in the condenser and in the absorber are both the same. XI (2) The minimum temperature in the absorber is above the corres ponding crystallization temperature. (3) The pressure drops between the generator and condenser, and between the evaporator and absorber are both 0.3 kPa. The main input data are temperatures and heat load. Through computer calculations the other variables are found. The computer programme was written in FORTRAN 77. The main program calls the individual unit subroutines, which have been specified by the user, thus compofdes^ the complete system subroutines referring to calculation for each unit can be called in any order and the connection between different subroutines is made automatically by the program. In its present form, the program contains unit subroutines for the following components: an absorber, a generator, a recuperator (liquid- to-liquid heat exchanger), a condenser and an evaporator. In addition, there are a lot of FUNCTION subroutines. Each of them calcu lated a specific thermodynamic property at different points in the system. The program, simulating the cooling system, was run for various parametres and some results were obtained. One of these results is velocity, temperature and concentration distributions along the film thickness and length for a falling film over a vertical tube surface. The validity of the present approach was verified by comparing the results with some other theoretical and experimental one. Also Nusselt and Sherwood numbers were calculated by using previously obtained tem perature and concentration distributions. Film temperature at the end of the tube is the same of solution temperature while leaving absorber. Therefore this temperature is taken as absorber temperature in the iterations. Nusselt and Sherwood numbers used to determine heat trans fer coefficient (h) and mass transfer coefficient (h ) respectively. Thus necessary properties were obtained to determine absorber dimensions. Overall heat-transfer coefficient was determined for each com ponent by using heat transfer coefficients for inner and outer heat transfer surfaces of the tube. This made it possible to determine the dimensions of each component corresponding to specific input parameters by using computer program. The developed computer program is a general one so to allow utilization of different input parameters in determi ning the dimensions. Various parameters were tried. Calculations with variable parameters led us to evaluate COP variation with these parameters. COP shows a parabolic increase as the generator inlet water temperature increases but this increase slows down considerably after a certain temperature. There are constraints for upper and lower values of generator inlet temperature. Both values shift up with increasing condenser temperature. It was observed that the some tendency in condenser temperature results with a decrease in XII COP. Upper and lower values of condenser temperature were restricted too. This restriction is put by cyristallization and decay of concentration differences. Upper and lower limits changed depending on generator inlet water temperature. COP increased with evaporator temperature, but it was seen to be unchanged with evaporater cooling capacity. Condenser temperature was found to be effective on some dimen sions. An increase in this temperature increases absorber and heat exchanger cooling surfaces, while the dimensions of the other compo nents remains unchanged. Increasing generator tempearture, on the other hand, increased absorber and heat exchanger dimensions consi derably but condenser dimensions slightly. It didn't effect dimen sions of the others. When evaporater temperature increased so did the boiler dimensions while absorber and heat exchanger dimensions dec reased. Results were compared with those of similar studies and con firmed well. Consequently, in this study the performance of a detailed simulation of a cooling system is given and the effects of absorber on the system is outlined.