Isı ve kütle geçişinin birarada gerçekleştiği sistemler

Abstract

Nemli hava su ikilisinin kullanıldığı birçok iklim lend ir me sisteminde ısı ve kütle geçişi birarada gerçek leşir. Neni i havadan ısı ve ne m alınması, suyun ısısı nın havaya verilmesi ve bu sırada havanın nemindeki ar tış, havanın nemlendirmesi bu konunun uygulamada karşı laşılan şekilleridir. Bu işlemlerde su ve nemli hava doğrudan yada dolaylı temas halindedir. Nemli hava ile su arasında ısı ve kütle geçişi olan sistemlerin hesabında psikrometrik tablo veya analitik ifadelerden yararlanılır. Bu nedenle, psikrometrik ta nım ve bağıntılar Bölüm 2 ile verilmiştir. Bu çalışma nın amaçlarından biri de hesaplamalar için bilgisayar programları geliştirmek olduğundan tablolar yerine ana litik ifadeler tercih edilmiştir. Doğrudan ısı ve kütle geçişinin sözkonusu olduğu sistemlerden uygulamada en çok karşılaşılanı soğutma ku lesi olduğundan, ters akışlı soğutma kulesinin hesabı ile bunun için geliştirilen bilgisayar programı Bölüm 3 de anlatılmıştır. Film tipi dolgu tipinin kullanımı ile, literatürde çarpma tipi (splash type) ve "rashing" denen dolgulara oranla daha yüksek kütle geçiş katsayısı (hrfftv) alındığı, bunun da daha küçük hacimli kuleler yapma olanağını verdiği görülmüştür. Nemli havanın soğutulmasında çok kullanılan kanatlı borulu ısı değiştiriciler, dolaylı ısı ve kütle geçişi nin incelenmesinde ele alınmıştır, Bölüm 4. Bu cihaz larda ekonomik olarak hava soğutmak için zorunluluk olmadıkça çok fazla sıra kullanılmaması gereği, sıra sayısının artışı ile sıra başına alınan ısıl gücün hızla düştüğü tesbit edilmiştir. Hava hızındaki artışın, ıs lak yüzey katsayısının (haw) artmasına, kanat veriminde (0W) düşmesine neden olduğu; toplam ısı geçiş katsayısı (UOM) bu parametrelere bağlı olduğundan, parabolik artış gösterdiği, sıra başına alınan ısıl gücün (q/Nr) de aynı eğilimde olduğu parametrik çalışmada tespit edilmiştir. Nemli hava ile su arasında ısı ve kütle geçişinin olduğu sistemlerin karakteristik eğri denklemleri ince lenirse aynı formda oldukları görülebilir. ayrı ayrı karakteristik eğri ifadelerinin belirli katsayılarla çarpılarak elde edilebileceği ortak bir karakteristik eğri ifadesi yazılabileceği düşünülmüştür. Bölüm 5.

Most of devices used in air conditioning applica tions are based on heat and mass transfers between bo ist air and water. Those transfer processes »ay be divided in two groups as direct contact processes and indirect contact processes. Cooling tower, air washer and spray dehumidifier nay be expressed as examples of direct con tact heat and mass transfer between the moist air and water. At the cooling and dehumidifying coils, indirect or induced heat and mass transfers occurs. In order to obtain thermodynamic properties of moist air, psychrometric tables and equations are used. Since one of the aims of this study is to improve com puter algorithms for calculations of the devices, analytical equations are preferred. In order to review psychrometric definitions of basic concepts and equa tions, Chapter 2 has been prepared. Direct contact heat and mass transfers between the moist air and water are realized by means of cooling of the water by atmospheric air, humidifying of air by in jection of water and rejection of heat and humid from the air by using chilled water in practical applica tions. Water cooling towers may be more important in these devices because of that is widely used in the practice. For this reason, water cooling towers are ex amined in Chapter 3. In a typical counter flow cooling tower, the warm water is admitted in the upper part of the tower and falls downward in counter flow to the air (Fig. 3.1). The fill retards the rate of water fall and increases the water surfaces to the air. Eliminator plates at the top of the tower minimize drift or carry-over of liquid water in the exhaust air. In order to build-up the calculation method of the cooling tower, application of first law of thermodynamic VI 11 on the differential voluıe eleaent, proceeding heat and ¦ass transfer and Lewis equations are adequate. Table 3.1 and equation 3.8 is used to calculate Lewis nuiber. When psychroaetric condition line of the aoist air cal culations completed, by using equation 3.11, the tower volute nay be obtained. An exaaple problea has been solved in Section 3.3 for better understanding of cal culation procedure. fi coaputer prograa has been developed for the coun ter flow cooling tower calculations. By using this coa puter prograa, paraaeters that influence to the average aass transfer coefficient (hrf0v) are examined. Results of this exaaination shows that cooling towers with film type fill are capable of showing average aass transfer coefficients between lOOOO kg/h-a* and 15000 kg/h-a*. Wet bulb teaperature of ataospheric air is impor tant in operating of water cooling towers. In practice, the water outlet teaperatures approach to the wet bulb teaperature of the air about 4°C maximally. Difference between the water inlet and outlet teaperatures, dif ference between the water outlet teaperatures and wet bulb teaperature of the air (approach), ratio of aass flow rate of the water to the air (aM/a«) are the aain factors that influences average aass transfer coeffi cient (hdflv) for the towers. Extended surface heat exchangers are widely used in cooling and dehuaidifying of the aoist air. When cool ing the aoist air, it is aore common that dehuaidif ica- tion of the air also occurs. With dehuaidif icat ion, air side surface is wetted by liquid water or frost. Besides sensible heat transfer, there is a transfer of heat because of condensation. For absorbing heat froa the air, two types of fluids are used inside the tubes. One type of fluids reaains constant teaperatures while absorbing heat and phase change occurs. For exaaple, Freon gases are this type fluids. Other type fluid's teaperatures change while absorbing heat and no phase change occurs. Well known exaaple for this type fluids is chilled water. Generally, copper tubes and aluainua fins are eaployed. If the air side surface teaperature is lower than dew point teaperature of the air, thin water fila covers the pipes and fins air side surfaces. Generally, face velocity of the air is about 1.5 to 3 a/s. IX Heat and ıass transfer fro» the moist air to the surface is examined and equations of the phenomenon is extracted in Section 4.1. Process line equation on the psychrometric chart for cooling and dehumidif y ing of aoist air by a cold surface is presented as a result of examinations in that section. fit the next section, wetted fin efficiency is studied and similarities of the equations for wetted and dry fins are shown. In order to est inat e air side heat transfer coeffi cient for dry surface, graphs prepared by Kays are used. To obtain heat transfer coefficient for wetted surface, equations given by Myers may be used (Section 4.3.1). The equations (4.22), (4.23) is used to calculate wetted surface air side heat transfer coefficient by using heat transfer coefficient calculated from graphs presented by Kays for far dry surfaces. Water film thickness at the air side surface is needed to predict. But it is not critical. Myers advice 0.1 mm. for the water film thickness. Wet surface coefficient (hOM) can be calcu lated by equation (4.14). Equations (4.24) and (4.25) are expressed to calcu late heat transfer coefficient inside the tubes in case chilled water is used as a refrigerant. For the sane purpose, equation (4.26) is given in case R-12 or R-22 is used as a refrigerant. After the calculations of the heat transfer coeffi cients and wetted fin efficiency, overall heat transfer coefficient (Uow) may be calculated by using equation (4.37). To perform those calculations mentioned above for extended surfaces, estimations for the pipe and the water film temperatures are needed. After the calcula tion of overall heat transfer coefficient, these estima tions must be checked. For this purposes, equations are given in Section 4.4. Mean enthalpy difference and using of logarithmic mean air enthalpy difference is discussed in Section 4.5. In order to best understanding of calculation pro cedures for wet surface, finned tube heat exchangers, an application example is solved completely in Section 4.7. fi computer program has been developed for calcula tions of wet surface, finned tube heat exchangers for cooling moist air. The algorithm of the computer program has been described in Section 4.8. By using the computer progran, parametric studies has been done. Firstly, effects of number of rows on the heat transfer rate has been examined. In this study, application example results obtained during the calculations of operational line of the moist air are used. According to the study results, adding a row to the heat exchanger, increases the heat transfer rate but decreases heat transfer rate per row (q/Nr). The heat exchangers have sore than 10 rows, would not been given an incredible additional heat transfer rates. Another study has been performed in order to obtain the effects of face velocity of the noist air, on the heat transfer contributors. Face velocity range is 0.5 to 4 m/s. Increase in face velocity of the air, results increase in wet surface heat transfer coefficient (heo), increase in wet surface coefficient (how) and the devia tions is nearly linear, but efficiency of wet fin decreases like a hyperbola (Fig. 4.24). As a result, while the outside-surface resistance decreases, fin resistance increases, so that overall heat transfer coefficient for wet surface, finned tube heat exchanger increases like a parabola. Deviation of the ratio of heat transfer rates to nuibers of rows is similar to Uov< deviation (Fig. 4.25). The equations of the operation lines on the psychrometric chart for devices that heat and mass transfer phenomenons occurs simultaneously between the moist air and water are obtained. Those equations are similar. Whether it is a direct contact process or not, since the mechanism of the phenomena is same, similarities of the operational line equations might not be surprised. Same similarities can be obtained, if calculation procedures examined at the computer program algorithms. So far, it may be thought that only one equation can be used for all the devices that take place heat and mass transfer between the moist air and water processes (5.1). To specify such a operation line of the moist air on psychrometric chart for a device a coefficient vector may be used (Table 5.1). In this study, separate computer program algorithms has been developed for counter flow cooling tower and wet surface, finned tube heat exchanger used for cooling XI of the moist air. In spite of separate algorithms, many procedures of the computer programs are same especially procedures that performs calculations of psychrometric properties of the moist air. Furthermore, a lot of pro cedures are very similar and may be improved so that be comes general procedures. Those works does not need a lot of time and efforts. Because computer programs has been developed so that such needs are thought to be necessary. Developed computer programs may be useful both in practical appl icat icat ions, and academic studies.