Yapılarda geçici rejimde ısı transferi ve soğutma yükü hesabı

Abstract

Bu çalışmada çatı ve duvar gibi yapı elemanlarındaki sıcaklık dağılımı ve ısı iletkenliği incelenmiştir. Bu inceleme için bir bilgisayar programı geliştirilerek, Türkiye' de kullanılan bazı çatı ve duvarlar için çözümler şekillendirilmiştir. Bu çalışmada ayrıca soğutma yükü hesabı için iki yöntem incelenerek, bu iki yöntem örnek bir ortama uygulanmıştır. Soğutma yükü hesabında kullanılan güneş ışınımı miktarı ve güneş-hava sıcaklığı değerleri 11 ilimiz için geliştirilen bir bilgisayar programı aracılığı ile tablo aştırılmıştir. Binalarda, ısıl konforu sağlaması ve uzun vadede sağlıyacağı ekonomik kazanç nedeni ile ytong duvar ve izolasyonlu tuğla duvar kullanılmalıdır. To calculate a space cooling load, detailed building geometry and weather data are reqired. These are: 1. Characteristics of the building. Building metarials, component size, external surface colors, and shape are usually determined from building plans and specifications. 2. Building location, orientation, and external shading. Plans and specifications should contain this information. 3. Approprate weather data and select outer design conditions. Weather data may be obtained from local weather stations or from Directory of the Research and Data Processinq Center of the State Meteorological Office. Outer design conditions for eleven cities are given in Tables from 4.1 to 4.11, section 3. 4. Indoor design conditions, such as indoor dry-bulb temperature, indoor wet -bulb temperature, and ventilation rate. 5. A proposed schedule of lighting, occupants, internal equipment, appliances, and processes that would contribute to the internal thermal load. 6. The time of day and month to the cooling load calculation. Frequently, several different times on a given day are required. The particular day and month are often dictated by peak solar conditions, June 21 has been chosen for this study. In this study, the CLTD CCooloing Load Temperature Difference) and TFM (Transfer Function Method) methods proposed by ASHRAE C American Society of Heating, Refrigerating and Air -Conditioning Engineer^} are reviewed. Transfer Function Method has been developed by G.P. Mi thai as and Stephenson and uses room thermal response factors, to describe the transient thermal behaviour of the building space in question. The procedure can be summarrized as follows: 1. Heat gains are calculated for walls and roofs by using the transfer functions, 2. Heat gains are calculated for floors, ceiling and interior sections by using the transfer functions, 3. All of the heat gain components are converted to cooling loads by using room transfer functions.

Energy produced in an enclosed " space or heat transfered to it is defined as the heat gain of that space. Heat gain has sensible and latent. The components of heat gains for an enclosed space can be listed as follows: 1. Solar radiation through windows, 2. Conduction and convection through walls and roofs or ceilings, 3. Convection and radiation from furniture, 4. Air-conditioning and infiltration air, 5. Latent heat produced within the space. Cooling load on the other hand is defined as the amount of heat that must be removed from the space to keep it at the desired temperature and humudity. The calculation of cooling load is more complicated than the calculation of heating loads. The reason for this is because the cooling loads are calculated for the steady state conditions. The reason why the cooling load should be calculated for transient conditions is mainly due to the presence of solar radiation. Solar radiation shows a great variation over a day. The heat gain and the cooling load at any instant of time have different values. Due to solar radiation and heat conduction through windows and walls heat is stored in furniture. After same time the temperature of the furniture rise above that of the surrounding air and thus heat is transferred to room air. This causes a time delay between the heat gain and the cooling load. The same observation can be made for the outer and inner wall surfaces. The calculation of the maximum value the cooling load is important because it is used to determine the capacities of the system components. In this work, the longest day of the year, June 21, has been chosen as the day of maximum cooling load. - IX - The results obtained thus are expressed in terms of cofficients b, d, and c for calculation of heat gains and v and w for calculation of cooling loads as described in the equations below: S b CT An e, n=0 I d q n t.A I Q = ) C v q+v q.. ~,. t. L o t ı t-A 2 t-2A + v q. _A +...) 1=1 -wQ A-w Q A_w Q,. i t-A 2 t-zA 3 t-3A In the Transfer Function Method, indoor temperature the inner and outher convection heat transfer coefficients were assigned constant values and sol -air temperatures were used for outside temperatures. The use of Transfer Function Method requires extensive calculations, therefore computers are resorted to. For hand calculations Cooloing Load Temperature Different method is more suitable. Cooloing Load Temperature Difference method depends on the use of extensive tables. Cooloing Load Temperature Difference and Cooling Load Factor are given in these tables as function of various geographic, climatic and building parameters. In Cooloing Load Temperature Difference method, one dimensional heat transfer through walls and roofs are calculated by the Transfer Function Method. The values found are converted to cooling load transfer functions for three different having light, medium and heavy thermal For all of these calculations sol -air used and the indoor temperature is 25. 5 °C. by using room types of rooms characteristics. temperatures are assumed to be The results are divided by overall heat transfer coefficent, U, to obtain Cooloing Load Temperature Difference for the wall or roof in question. Cooloing Load Temperature Difference values are given in Table 2-1 and Table 2-3. - XI - The effects of lighting, electrical appliances and people on the cooling load are taken into account by Cooling Load Factor. The Cooling Load Factor values are given in Table 2-7, Table 2-8, Table 2-9 and Table 2-13. One example calculation are given in appendix A illustrating the calculational procedures of Cooling Load Temperature Difference and Transfer Function Method methods. In section 3 of this study, solar radiation arriving on surfaces of different orientations under clear sky conditions on June 21 were calculated for eleven cities. A computer program was written for this purpose. The results obtained were used to determine the sol -air temperatures for these surfaces. Characteristics of building materials used in Türkiye are also given in this section in tabular form. In section 4 of this study a computer program for transient heat transfer analysis of walls and roofs was developed. This program considers one dimensional heat conduction and can analyze multi-layer structures. This program was applied to four typical wall constructions and one roof construction. The transient temperature distributions obtained are presented in graphical form. The heat flux incident upon a wall is not passed entirely to the interior of the building. Some of this heat is absorbed in the wall and is then released to the interior at a later time. Therefore there is a time lag between the maximum heat fluxes on the inner and outher surfaces of the wall. The larger is the time lag, better it is for thermal comfort. Because if the heat transfer from the inner wall is minimum when the outside temperature is maximum, it will be easier to control the inner conditions. The examination of the results for the wall constructions considered yields the following: For the terrace roof, 12 percent of the heat flux incident on the outside surface is transferred to the interior. Rest of the heat is first stored in the roof elements and then transferred back to the outside air when the outside temperature is lower. In the terrace roof the time lag is apporroximately lO hours and the heat transferred to the interior varies between 6. 1 W/m and 2. 1 W/m2. - XII For the ytong wall, 17 percent of the heat flux incident on the outside surface is transferred to the interior. The time lag for the ytong wall is between 7 and 9 hours, and the heat transfer to the interior 2 2 varies between 5. 9 W/m and 0. 9 W/m. Minimum total daily heat transfer to the interior is realized with the ytong wall. For the concrete wall, 48 percent of the heat flux incident on the outside surface is transferred to the interior. The time lag for the concrete wall is between 5 and 7 hours, and the heat transfer to the interior 2 2 varies between 37. 9 W/m and 7. 7 W/m. Minimum total daily heat transfer to the interior is realized with the concrete wall. Concrete wall also has the shortest time lag between all of the wall costructions considered and is therefore the worst from the viewpoint of thermal comfort. For the brick wall with sandwich insulation, 13 percent of the heat flux incident on the outside surface is transferred to the interior. The time lag for the brick wall with sandwich insulation is between 8 and 11 hours, and the heat transfer to the interior varies 2 2 between 6 W/m and 3 W/m. The brick wall with sandwich insulation has the longest time lag between all of the wall constructions considered and is therefore the best from the viewpoint of the thermal comfort. For the brick wall, 41 percent of the heat flux incident on the outside surface is transferred to the interior. The time lag for the brick wall is between 6 and 8 hours, and the heat transfer to the interior varies between 26. 1 W/m2 and S. 8 W/m2. In conclusion, it can be said that the ytong wall and the brick wall with sandwich insulation are the best wall types for summer comfort conditions and that the extra investment made for insulation is compensated with the savings from the cooling load.