Chiller ve soğutucu-nem alıcı serpantin çiftinin modellenmesi
Modelling of chiller and air cooling-dehumidifying coils
Yazar: ALİ KILIÇARSLAN
Danışman: PROF.DR. AHMET ARISOY
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü
Derece: Yüksek Lisans, 1994
Konu: Enerji = Energy ; Makine Mühendisliği = Mechanical Engineering
Dizin: Isı geçişi = Heat transfer ; Matematiksel modelleme = Mathematical modelling ; Serpantin = Serpentine

Özet:
ÖZET Bu çalışmada, ehiller ve soğutucu-nem alıcı serpantin çiftinin matematiksel modellenmesi yapılmış ve modeli oluşturan matematiksel denklemler, yapılan bilgisayar programı yardımıyla çözülmüştür. Modeli oluşturan denklemler hava, su ve refrigerant akışkanlarıyla ilgili denklemlerle beraber tablo ve diyagram kullanmadan bilgisayar yardımıyla çözülmüştür. Modelde etkili olan parametrelerden suyun soğutucu-nem alıcı serpantine giriş sıcaklığının, serpantinin soğutma kapasitesinin ve havanın hacimsel debisinin modeldeki diğer parametrelere etkisi incelenmiş olup ayrıca sisteme verilmesi gereken optimum su sıcaklığı bulunmaya çalışılmıştır. Grafikler incelendiğinde tasarlanan su sıcaklığının, soğutma kapasitesinin ve havanın hacimsel debisinin sağlanması için sistemin diğer parametrelerinin hangi oranda değişmesi gerektiği kolayca görülmektedir. Bu açıdan model uygulamada pratiklik sağlamaktadır.

Abstract:
SUMMARY MODELLING OF CHILLER AND AIR COOLING-DEHUMIDIFYING COILS This study is based on the air-conditioning and refrigerating system which consists of three cycles. The first cycle is composed of cooling and dehumidifying coil and refrigerated space. In the first cycle, the air circulates continuously and atmospheric air isn't used. The air of which entering conditions, such as dry-bulb temperature and humidity ratio are lowered is distributed to the refrigerated space. The second one includes air cooling and dehumidifying coil and evaporator part of the chiller. The chilled water is circulated throughout this cycle continously and any feedwater isn't used. The purpose of using chilled water is to lower dry-bulb temperature and humidify ratio of the air. The last one is refrigeration cycle, in other words, it is chiller itself. The refrigerant is circulated through this cycle. Freori-type refrigerant is used, such as Freon-22. As the refrigerant flows through the evaporator, it removes the heat from water, and water is chilled. Three different fluids are used in the cycles mentioned above. These are air, water and refrigerant The model consists of a chiller and an air cooiing-dehumidfying coils. Because dehumidfying coil is the most important of the model, purpose of using it and the work that it does, will be explained. Air cooling-dehumidifying coils are plate-fin-tube heat exchangers that are used to cool and dehumidify air in large building air-conditining systems. In a counterflow arrangement, the air enters the coil on the left side of the coil, and flows through passages formed by closely spaced wavy fin surfaces. Cold water flows in a counterflow arrangement, cooling and dehumidifying the warm air. A dehumidifying coil normally removes both moisture and sensible heat from entering air. In most air-conditioning processes, the air to be cooled is a mixture of water vapor and dry gases. Both lose sensible heat during contact with the first part of the cooling coil, which functions as a dry cooling coil. Moisture is removed only in the part of the cil that it is below the dew point of the entering air. As the leaving dry-bulb temperature is lowered below the entering dew-point temperature, the difference between the leaving dry-bulb temperature and the leaving dew point for a given coil, airflow, and the entering air condition is lessened. Generally, depending upon the entering conditions and flow rates. only part of the dehumidifying coil may be wet. A detailed analysis would involve determining the point in the coil at which the surface temperature equals the dew point of the entering air. A simpler approach is to assume that the coil is either completely wet or dry. Either assumed condition will tend to underpredict the actual heat transfer. With the assumption of a completely wet coil, the air is humidified during the portion of the coil in which the dew point the air is less than the surface temperature. The latent heat transfer to the air associated with VIthis "artificial" mass transfer reduces the overall calculated cooling capacity as compared with the actual situation. Since both the the completely dry and wet analyses underpredict the heat transfer, a simple approach is to utilize the results of the analysis that gives the largest heat transfer. In this study it is assumed that coil is both dry and wet. When the coil starts to remove moisture, the cooling surfaces carry both the sensible and latent load. As the air approaches saturation, each degree of sensible cooling is nearly matched by a corresponding degrree of dew point decrease. In cotrast, the latent heat removal per degree of dew point change varies considerably. The driving forces for the total heat Q transferred from air to chilled water are (1) an air enthalpy difference ( hh-hy) between airstream and surface interface and (2) a temperature difference ( th-y across the surface metal and into the chilled water. The coil model used in this study is a one one dimensional model with the thermodynamics relations for air and chilled water and heat transfer relations for a counterflow heat exchanger. The coil model is a steady-state analysis for a single-phase cooling fluid flowing through tubes. The specified air inlet conditions are airflowrate, air face velocity, dry-bulb temperature, and humidity ratio. The inlet conditions for a single- phase fluid, such as chilled water temperature, and mass flowrate must be given. The required heat transfer coefficients must also be given. The coil model calculates air, water, and surface temperatures through coil, the heat transfer for dry and wet surface, and reqired heat transfer areas. The chiller model of the system includes a motor compressor, kondenser, evaporator, and an expansion valve. Refrigerant vapor is compressed from a low-temperature state to a high pressure state in the compressor. The hot vapor then passes into the condenser where it condenses on heat exchanger tubes, heating the water that flows through an expansion valve, dropping to a low-presure with a mixture of liquid and vapor, two-phase refrigerant. This cold refrigerant then passes into the evaporator where it boils as it comes in contact with heat exchanger tubes, cooling the water that flows through the tubes. The following additional information can be given about the chiller model of the system. The refrigerant enters the compressor at the inlet port of the compressor and is assumed to be saturated vapor. Both the enthalpy and pressure rise as the refrigerant passes through the compressor to a superheated state at the exit port of the compressor. The approach to modelling the performance of the compressor is to use performance curves based on the manufacturer's data. The equation presented by Stoecker is used in order to calculate the power required by the compressor which uses R-22 as a refrigerant. In the condenser, the refrigerant is a constant pressure state and is assumed to be a saturated liquid at the exit of the condenser. During passing of refrigerant through the expansion valve, the enthalphy of the refrigerant is not be changed. In the evaporator refrigerant, refrigerant is a low pressure state and boils at the outside of the horizontal tubes and rises out the top. VIISome important assumptions are made in order to simplifiy the model. First, the refrigerant is assumed to undergo pure phase change so that the evaporator temparature, tr is assumed to be constant the evaporator. In reality this is reasonable insofar as the small amount of superheat carries away a proportionately small percentage of the total evaporator heat and thereby does not significantly raise the mean evaparator temperature above \ second, it is assumed that the heat exchangers were modeled as simple counterflow equipment. Third, it is assumed that leaving air is saturated state. Fourth, temperaturer of water on top of surface the temperature of saturated air on top of the water film, and the temperature of surface have been assumed to be same. Fifth, the log-mean temperature difference at the evaporator has been assumed as 5 °C. The entering velocity of the air to the air cooling- dehumidifying coil has been approximately taken as 3 m/s and the required heat transfer coefficients has been selected according to the this velocity. Last, lewis number is assumed to be 1 Most air-cooling coils consist of tubes with fins attached to the outside of the tubes to increase the area on the air side where the convection coefficients is generally much lower than the refrigerant or water side. Refrigerant or water flows inside the tubes, and air flows over the outside of the tubes and the fins. Cooling agent, such as chilled wter carries away the heat, this wter is chilled by an evaporator in the machine room. By using the mathematical equations of the coil and chiller together with axuliary equations, the required power to drive compressor and the fan power to distribute the air through air-distribution system can be calculated. In order to calculate the required power for compressor, the condenser temperature of the chiller must be given as an input. Pressure loss of the system through the air ducts, and static efficiency of the fan must be given in order to calculate the fan power. One of the main objectives of this model is to compare the system parameters with the manufacturer's catalog data. When the outlet temperatures of both air and water are compared with manufacturer performance data, the agreement between the model results and data is abaut in 10%. The second objective is to investigate the effects of effective parameters of the model on the other system parameters. After setting the model mathematical equations together with axuliary equations derived from related subjects, three effective system paremeters have been selected. These are outlet chiller water temperature, total heat load of the cooling and dehumidifying coil or refrigerating capacity of the chiller, and air volume flowrate. First it is investigated how system parameteres change when the chiller outlet Water temperature is changed in definite intervals. After the mathematical equations are solved by using the computer program, the different graphs are sketched from the results of the computer program. It is seen from the related graphs that chiller outlet water temperature increases, the leaving dry-bulb VIIItemperature of the air from the coil, the required heat transfer for wet surface, and the power required to drive the compressor decrease. The chiller inlet water, evaporator and surface temperatures, and the required heat transfer for dry surface increase. The required fan power to distribute air through the ducts and the surface temperature at the begining of the wet surface don't change. During increasing chiller outlet water temperature, the important effective parameters which are the dry-bulb temperature, humidity ratio of the entering air, andrefrigerating capacity of the coil are held constant The optimum chiller outlet water temperature at which the total of the required energy for fan and compressor is minimum has also been investigated. It has been found that the optimum temperature is around 8.5 °C. Second, it has been investigated how the system parameters are affected when the refrigerating capacity of the coil is increased regularly.The graphs show that the leaving water temperature from the coil, both dry and wet heat transfer.and the required heat transfer areas for dry and wet surface increase. Wet surface heat transfer and required heat transfer area increase more than dry surface heat transfer and required heat transfer area respectively. The leaving dry-bulb temperature of the air from the coil, and surface temperature at the end of the surface decrease. The increase in the leaving humidity ratio of the air is negligible. Finally, the effect of increasing airflowrate on the system parameters has been investigated when the other effective system parameters, such as chiller outlet water temperature, entering air dry-bulb temperature and humidity ratio, and refrigerating capacity of the coil are held constant. According to the results derived from graphs, the required fan power, leaving air dry-bulb tempererature, the dry surface heat transfer and the required heat transfer area for dry surface increase. The surface temperature at the beginning of the wet surface, water outlet temperature from the coil, and evaporator don't change. From the economical point of view, some important results are obtained. For aspecified cooling load, Q, the selection of the heat exchangers (specifying the heat transfer coefficients) and the pump to circulate water and water flowrate involves a trade-off between capital costs (pump, heat transfer surface) and operating costs (pumping power and chiller costs due to variations in the evaporator temperature. Given a specific set of heat exchangers, increasing the water flowrate is thought to produce decrasing temperature drops through the heat exchange network. This turns out to be valid only if the water circulating loop is transfering heat between two constant- temperature fluids. Howewer in the case of evaporator-to-air heat exchange, there is only one flowrate that minimizes the temperature drops through heat exchange for a given set of heat exchanger conductances (without consideration of capital costs). The power consumption of the chiller is sensetive to the consending water temperature, which is, in turn, affected by both the condenser water and tower air flow rates. Increasing either of these flows reduces the chiller power requirement but at the expense of an increase IXin the pump or fan power consumption. At any given load, chilled water setpoint, and wet bulb temperature, there exists an optimum operating point. By using the model which is composed of chiller and cooling dehumidifying coil, the effect of system parameters along the coil and through the chiller can be predicted with the accuracy of 10%. The model can be improved by adding the specified subprograms, and making assumptions. For example, coil properties such as number of coils and friction through the ducts and pipes in the system can be calculated. The selection procedure of the system parts can also done by using subprograms and related auxilary mathematical equations.