Thermal Energy Storage Systems and Applications. Ibrahim Dincer

Чтение книги онлайн.

       Thermometers: Thermometers contain a volume of fluid which expands when subjected to heat, thereby raising its temperature. In practice, thermometers work over a certain temperature range. For example, the common thermometer fluid, mercury, becomes solid at −38.8°C and its properties change dramatically at that condition.

       Resistance thermometers: A resistance thermometer (or detector), also known as a wire‐wound thermometer, has great accuracy for wide temperature ranges. The wire used has to have known, repeatable, electrical characteristics so that the relationship between the temperature and resistance value can be predicted precisely. The measured value of the resistance of the detector can then be used to determine the value of an unknown temperature. Among metallic conductors, pure metals exhibit the greatest change of resistance with temperature. For applications requiring higher accuracy, especially where the temperature measurement is between −200 and 800°C, the resistance thermometer comes into its own. The majority of such thermometers are made of platinum. In industries, in addition to platinum, nickel (−60 to 180°C) and copper (−30 to 220°C) are frequently used to manufacture resistance thermometers. Resistance thermometers can be provided with two, three, or four wire connections, and for higher accuracy at least three wires are required.

       Averaging thermometers: An averaging thermometer is designed to measure the average temperature of liquids stored in bulk. The sheath contains a number of elements with different lengths, all starting from the bottom of the sheath. The longest element that is fully immersed is connected to the measuring circuit to allow a true average temperature to be obtained. For this type of thermometer, several parameters are significant, namely, sheath material (stainless steel for the temperature range from −50 to 200°C or nylon for the temperature range from −50 to 90°C), sheath length (to suit the application), termination (flying leads or terminal box), element length, element calibration (to copper or platinum curves), and operating temperature ranges. In many applications, where a multielement thermometer is not required, such as in air ducts, cooling water and gas outlets, a single‐element thermometer stretched across the duct or pipe work can provide a true average temperature reading. Despite the working range from 0 to 100°C, the maximum temperature may reach 200°C. To maintain high accuracy, these units are normally supplied with three‐wire connection. However, up to 10 elements, made of platinum, nickel, or copper, can be mounted in the average bulb fittings, and fixed at any required position.

       Thermocouples: A thermocouple consists of two electrical conductors of different materials connected together at one end (the so‐called measuring junction). The two free ends are connected to a measuring instrument, for example, an indicator, a controller, or a signal conditioner, by a reference junction (the so‐called cold junction). The thermoelectric voltage appearing at the indicator depends on the materials with which the thermocouple wires are made and on the temperature difference between the measuring junction and the reference junction. For accurate measurements, the temperature of the reference junction must be kept constant. Modern instruments usually incorporate a cold junction reference circuit and are supplied ready for operation in a protective sheath, to prevent damage to the thermocouple by any mechanical or chemical means. Table 1.1 lists several types of thermocouples along with their maximum absolute temperature ranges. As can be seen in Table 1.1, a copper–constantan thermocouple has an accuracy of ±1°C, and is often employed for control systems in refrigeration and food processing applications. The iron–constantan thermocouple with its maximum temperature of 850°C is used in applications in the plastics industry. The chromel–alumel‐type thermocouples, with a maximum temperature of about 1100°C, are suitable for combustion applications in ovens and furnaces. In addition, it is possible to reach temperature of about 1600 or 1700°C using platinum rhodium–platinum thermocouples, which are particularly useful in steel manufacturing. It is worth noting that one advantage that the thermocouple has over most other temperature sensors is that it has a small thermal capacity, and thus a prompt response to temperature changes. Furthermore, its small thermal capacity rarely affects the temperature of the body under examination.

       Thermistors: These devices are made of semiconductors and act as thermal resistors with a high (usually negative) temperature coefficient. In use, thermistors are either self‐heated or externally heated. Self‐heated units employ the heating effect of the current flowing through them, to raise and control their temperature and thus their resistance. This operating mode is useful in such devices as voltage regulators, microwave power meters, gas analyzers, flow meters, and automatic volume and power level controls. Externally heated thermistors are well suited for precision temperature measurement, temperature control, and temperature compensation due to the large changes in resistance versus temperature. These are generally used for applications in the range −100 to 300°C. Despite early thermistors having tolerances of ±20 or ± 10%, modern precision thermistors are of a higher accuracy, for example, ±0.1°C (less than ±1%).

       Digital display thermometers: A wide range of digital display thermometers, for example, hand‐held battery‐powered displays and panel‐mounted mains or battery units, СКАЧАТЬ