3BSE013230R1 ABBTU810V1 紧凑型模块

　　相对湿度是最常用的湿度测量方法。它是存在的水蒸气量与在该温度下物理可能的量之间的比率，以百分比表示。使用相对湿度的主要缺点是它严重依赖于温度。空气保持水蒸气的能力取决于温度。例如，如果温度为 18°C，相对湿度为 96%，则空气温度仅增加 2°C，相对湿度就会降低到 85%。由于其熟悉性，相对湿度常用于冷却塔操作，但它不提供蒸发冷却效果停止的温度。但是，传感设备的相对湿度和温度都应用作计算湿球温度的输入。

　　测量湿球温度

　　传统上，湿球温度是通过用湿布（或湿袜子）覆盖并暴露在气流中的温度计来测量的。吊带干湿计是具有两个温度计的设备，一个温度计测量环境温度（干球温度），另一个测量湿球温度。湿球温度计配有湿灯芯，然后旋转以产生气流和蒸发冷却。这些低精度测量仪器需要手动操作，不能提供工业规模冷却塔控制系统所需的精度和可靠性。

　　今天，湿球温度是根据可靠的仪器测量的相对湿度和温度来计算的。这些计算可以编程到控制系统中，以消除手动计算错误。

　　结果的实践

　　Wilcox 建议冷却塔所有者采用四种实践来操作冷却塔的设计。

　　首先，设计系统的机械工程师应控制系统将被预编程，以根据相对湿度和温度的直接测量值自动计算湿球温度。这些公式可以在网上和白皮书中找到，包括来自维萨拉的。

　　其次，系统设计人员应准确、不易出现传感器漂移并保持校准的高质量传感器。“运营商并不总是了解依赖低成本、低质量传感器的重大财务后果，”Wilcox 说。“如果传感器产生的读数不准确，表明蒸发冷却可能发生但它不能发生，这些廉价、容易出错的传感器可能会浪费数十万美元的能源使用和过度使用造成的设备损坏。”

　　第三，在调试过程中，操作者所有者应确认传感器和控制系统程序符合规范。，塔运营商需要按照制造商的建议对传感器进行日常维护，以确保长期的准确性和可靠性。

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　　Relative humidity is the most commonly used humidity measurement. It is the ratio, expressed as a percentage, between the amount of water vapor present and the maximum amount that is physically possible at that temperature. The major drawback of using relative humidity is that it is heavily dependent on temperature. The capacity of air to hold water vapor is dependent on temperature. For example, if the temperature is 18°C and the relative humidity is 96%, an increase in the air temperature of only 2°C reduces the relative humidity to 85%. Due to its familiarity, relative humidity is often used in cooling tower operations, but it does not provide the temperature at which evaporative cooling effects stop. However, relative humidity and temperature from sensing devices should both be used as inputs to calculate wet-bulb temperature.

　　Measuring Wet-bulb Temperature

　　Traditionally, wet-bulb temperature was measured by a thermometer covered with wet cloth (or wet sock) and exposed to air flow. A sling psychrometer is a device with two thermometers, one thermometer measures ambient temperature (dry-bulb temperature) and the other measures wet-bulb temperature. The wet-bulb thermometer is fitted with a wet wick and then whirled around to generate air flow and evaporative cooling. These low-precision measurement instruments require manual operation and do not provide the accuracy and reliability required for industrial-scale cooling tower control systems.

　　Today, wet-bulb temperature is calculated based on measurements of relative humidity and temperature by accurate and reliable instruments. These calculations can be programmed into the control system to eliminate manual calculation errors.

　　Best Practices for Best Results

　　Wilcox recommends tower owners to adopt four best practices for operating the cooling tower as it was designed.

　　First, the mechanical engineer designing the system should specify that the control system will be preprogrammed to automatically calculate wet-bulb temperature based on direct measurements of relative humidity and temperature. The formulas can be found online and in white papers, including from Vaisala.

　　Second, the system designer should specify high-quality sensors that are accurate, not prone to sensor drift, and stay in calibration. “Operators do not always understand the significant financial consequence on relying on low-cost, low-quality sensors,” Wilcox says. “These inexpensive, errorprone sensors can cost hundreds of thousands of dollars in wasted energy use and equipment damage from overuse if the sensor produces inaccurate readings that indicate evaporative cooling can occur when it can’t.”

　　Third, during commissioning, the operator owner should confirm that the sensors and the control system program are in compliance with specification. Finally, tower operators need to perform routine maintenance on the sensor as recommended by the manufacturer for accuracy and reliability over the long run.