Document Dmz07EKy28ewez4Bgbbp6LQN

158____________ __ ________ CHAPTER 7 J .1946 Guide -In the case of finned tubes, values of hi may be "lower than those obtained by use of Equation 10 and .actual testResults are preferred if available. When saturated steam is condensed in . the tubes of coils, the film coefficient hr varies from 1000 to 2000, depending on freedom from air in the steam, and upon good drainage of the tubes. The coefficient is fairly' constant for a particular coil, giving values ofAi that are directly propor tional to qt. Performance of Coils and Refrigeration Compressor Practically all data published by various makers of. direct expansion .. cooling coils are based upon maintaining a predetermined refrigerant' temperature within the coils. While it is often possible to maintain a Fig. 13. Graphical Analysis of Coil-Compressor Performance definite refrigerant temperature within a given cooling coil, for the greater part it is either impossible or impractical. \This is due to' the fact that the capacity, of standard refrigeration compressors-is usually fixed and in matching a given cooling coil and a standard.compressor the capacityof the latter is often somewhat smaller or greater than that of the.former. Consequently, the refrigerant temperature resulting within.a cooling coil and correspondingly the capacity- of the coil-compressor combination are very often not what they were originally , calculated -to be. In order to determine the actual performance of a given coil-compressor combination under varying conditions of operation, a graphical solution, of. the balance point is highly desirable. A typical procedure for graphical analysis of a coil-compressor combination performance is shown in Fig. 13, which is constructed as follows: 1. The equipment capacity scale, total Btu per hour, is laid out along-the' vertical axis while the refrigerant suction temperature scale-is laid out-along the horizontal axis using rectangular coordinates. r : ...... . ... : Heat Transfer Surface Coils 159 /, 2. The performance curve of a given compressor with a definite condenser (com bination usually called a condensing unit) is. plotted as a function of suction temperature; corresponding to the saturation suction pressure at the compressor suction service valve for a given inlet water temperature and quantity supplied to the condenser. 3. The performance curve.of the given cooling coil is'next plotted as a function ofmean suction temperature within the coil, and the wet-bulb temperature of air entering the coil, for a given mass air velocity over the coil. 4. The refrigerant pressure drop between the center of the cooling coil and the com pressor suction service valve is computed and converted into the terms of temperature difference. This temperature difference is then fitted in horizontally between the performance curves of the cooling coil and the compressor, as shown, and the total capacity of the coil-compressor combination is read along the horizontal line upon which the above mentioned temperature-difference segment falls. COIL SELECTION In the selection of a coil it is necessary to consider several factors: - 1. The duty required--heating, cooling, dehumidifying. 2. Temperature of entering air--dry-bulb only if there is no dehumidification, dryand wet-bulb if moisture is to be removed. 3. Available' heating and 'cooling media. - 4. Space and dimensional limitations. 5. Air quantity limitations. 6. Allowable resistances in air circuit and through tubes. 7. Peculiarities of individual designs of coils. 8. Individual installation requirements, such, for example, as type of automatic con trol to be used. The duties required may be determined from information in Chapters 6, 8, 14 and 15. There may or may not be a choice of cooling and heating media, as well as temperatures available, depending upon whether the installation is new or is in combination with present sources of heating or cooling. Space limitations are dictated by the requirements of individual cases. The air quantity is influenced by a number of considerations. The air quantity through heating coils is often made the same as that necessary to handle the summer cooling load. The air handled may be fixed by the use of old ventilating' ducts as an air distribution system for new air conditioning apparatus, or may. be dictated by requirements of satisfac tory air distribution or, ventilation. The resistance through the air circuit influences the fan horsepower and speed. This resistance may be limited to allow the use of a given size of fan motor, or to keep the opera ting expense low, or it may be limited by the maximum fan peripheral velocity which requirements of quietness may permit. The friction, through the water or brine circuit may be dictated by the head available from a given size of pump and pump motor. As the fan and pump motor inputs represent a refrigerating load on cooling installations, it is eco- , nomical to keep them low. Proper performance of a surface heating or cooling coil depends upon correct choice of the original equipment and upon certain other factors. The usual coil ratings are based on a uniform face velocity of air. If the' air is brought in at odd angles or if the fan is-located so as to block part .of ' the air flow, the performance as given in the manufacturer's ratings cannot usually be: obtained. To obtain this performance it is necessary ' also that the air quantity be adjusted on the job to that used in deter-': mining the.coil selection, and must also be kept at this value. The most -.