Performance
Rejection of the heat load produced at the chilling equipment is the primary goal of a cooling tower system. This heat rejection can be accomplished with an optimized system that minimizes the total compressor power requirements of the chiller and the tower loads such as the fans and condenser pumps. Several criteria must be determined before the designer can complete a thorough cooling tower analysis, including selection of tower range, water-to-air ratio, approach, fill type and configuration, and water distribution system. Table 4.2.6 lists some of the common design criteria and normally accepted ranges for cooling towers.
Most common HVAC applications requiring a cooling tower will use an “off the shelf” unit from a cooling tower manufacturer. Manufacturer representatives are usually well informed about their products and their proper application. After the project design process has produced the information called for in Table 4.2.6, it is time to contact one or more cooling tower representatives and seek their input on correct tower selection.
Control Scheme with Chillers
Most cooling towers are subject to large changes in load and ambient wet-bulb temperature during normal operations. For a typical cooling tower, the tower fan energy consumption is approximately 10% of the electric power used by the chiller compressor. The condenser pumps are about 2–5% of the compressor power. Controlling the capacity of a tower to supply adequately cooled water to the condenser while minimizing energy use is a desirable operational scheme. Probably the most common control scheme employed for towers serving an HVAC load is to maintain a fixed leaving water temperature, usually 27°C (80°F). Fan cycling is a common method to achieve this cooling tower control strategy and is applicable to multiunit and multicell tower installations. However, this control method does not minimize total energy consumed by the chiller/cooling tower system components.
Lowering the condensing water temperature increases a chiller’s efficiency. As long as the evaporator temperature is constant, a reduced condenser temperature will yield a lower pressure difference between the evaporator and condenser and reduce the load on the compressor. However, it is important to recognize that the efficiency improvements initially gained through lower condenser temperatures are limited. Improved chiller efficiency may be offset by increased tower fan and pumping costs. Maintaining a constant approach at some minimum temperature is
上一页 [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] ... 下一页 >>