Abstract
The distribution of chilled water flow rate in terminal unit is a major factor used to evaluate the performance of central air conditioning unit. In this work, a theoretical chilled water distribution in the terminal units has been studied to predict the optimum heat performance of terminal unit. The central Air-conditioning unit model consists of cooling/ heating coil (three units), chilled water source (chiller), three-way and two-way valve with bypass, piping network, and pump. The term of optimization in terminal unit ingredient has two categories, the first is the uniform of the water flow rate representing in statically permanents standard deviation (minimum value) and the second category is the maximum heat transfer rate from all terminal units. The hydraulic and energy equations governing the performance of unit solved with the aid of FORTRAN code with considering the following parameters: total water flow rate, chilled water supply temperature, and variable valve opening. It was found that the optimum solution of three-way valve case at 8°C water supply temperature, 0.12 kg/s total water flow rate and valve opening order (valve 1: 100%, valve 2: 100% and valve 3: 75%) with total heat rate (987.92 Watt) and standard deviation (1.181E-3). Also, for the two-way valve case the results showed that the optimum condition at 8°C water supply temperature, 0.12 kg/s total water flow rate and valve opening order (valve 1: 75%, valve 2: 75% and valve 3: 50%) with total heat rate and standard deviation (717Watt) and (5.69E-4) respectively.
Highlights
Cooling is essential for all types of buildings in Iraq; Most of the air- central conditioning (A/C) systems used in commercial and institutional buildings are of large capacity
Figure (3) shows that the total heat rate of terminal units at different total water flow rates (0.06, 0.08, 0.1 and 0.12 kg/s) and various water supply temperatures were (8, 10, 12, 14 and 16 °C) at 100% valves opening:, Note that the higher total heat rate occurred at minimum water supply temperature (8°C) and at maximum total water flow rate (0.12 kg/s), due to the highest difference in temperature between the water and the air when the temperature of the air is constant, While the increase of the heat rate is slight because the flow type in most cases is laminar and the value of heat transfer coefficient (h) is almost constant
These figures indicate that the total water flow rate and water supply temperature has a significant impact on the total heat rate at any valve opening for any coil
Summary
Cooling is essential for all types of buildings in Iraq; Most of the air- central conditioning (A/C) systems used in commercial and institutional buildings are of large capacity These systems use chillers for cooling production and a network of chilled water piping for cooling distribution to the individual Air-Handling Units (AHUs) or Fan Coil Units (FCUs). The energy for heat distribution system and the total energy consumption in district heating and cooling system plant increases in considerable degrees [2] but the high chilled water temperature, low supply air temperature, and high outside air intake ratio may results in low cooling chilled water return temperature and the bypass bridge is not necessary and can be removed in the consumer chilled water system with the 2-way valve configuration. The chilled water distribution system stability is the basic engineering knowledge for air conditioning engineer; Proper sizing of control valves, layouts, and self-balancing, flow balancing and control the speed of the pump is an important fundamental factor of chilled water
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