The ensuring of the required parameters of the air, used in air-conditioning systems, is directly related to the change of the flow rate of each fan that is installed in these systems. This change can be achieved by using different methods of flow rate regulation. However, it is well-known that the selected method determines the energy efficiency of the given fan system. This work represents a comparative theoretical analysis, concerning the energy efficiency of three different methods of regulating a fan’s flow rate: by throttling, by using inlet guide vanes and by changing the voltage frequency - the motor’s speed of rotation has been changed with the help of a variable frequency drive (VFD). For the aims of this research, the change of the coefficient of efficiency for some of the key system elements – fan, motor and VFD, at different work regimes, have also been indicated. To fulfil the objectives of this research a given number of fans, having different specific speeds of rotations: nq = 3.31; 7.11 and 8.82 min−1, have been selected. As a criteria of effectiveness, concerning the different methods of flow rate regulation, the specific fan power PSFP - the invested energy in transporting a unit volume of fluid (gas), has been used: P S F P = p η t o t [ k W m 3 / s ] , where p is the total pressure of the fan; ηtot is the overall efficiency of the fan. As a result of this theoretical investigation some graphics representing the change of the specific energy consumption, when the three different methods of flow rate regulation are used and indicating the fan’s initial work regime and its specific speed of rotation, have been given.
It is well-known that one of the reasons for reducing the head of a centrifugal pump, when trimming its impeller, is the reduction of the slip factor. There are various methods used to determine it, but there is lack of information on whether they are applicable in case that a pump with a trimmed impeller operates. In the present work, a comparative experimental study concerning the applicability of some of the methods used to determine the slip factor, on two centrifugal pumps with different specific speed of rotation, is performed. Based on the experimental data, CFD models of the impellers of the two pumps have been validated and a numerical simulation study concerning the change of the slip factor during trimming, has also been performed. The impact of the rate of impeller trimming on the the slip factor variation has also been established. Based on the results found, concerning the performed numerical study, equations describing the relationship between the slip factor and the rate of trimming of the studied pumps are obtained.
This work represents results concerning different methods, used to determine the minor loss coefficient of a cone diffuser, installed at the system’s outlet. These results are compared with experimental data and numerical results, obtained after using ANSYS CFX and the data base of Pipe Flow Expert. For determining the minor loss coefficient of a cone diffuser, being installed at the system outlet and having a straight pipe section in front of it, some models in ANSYS CFX are established. These models are validated by using previously published experimental data. An empirical equation, based on the results found, enabling the accomplishment of analytical studies concerning the energy efficiency of fan systems, using an outlet diffuser, has been established.
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