Thermal Energy Storage Systems and Applications. Ibrahim Dincer

patterns. While these changes in barometric pressure are usually less than one‐half inch of mercury, they need to be taken into account when precise measurements are required.

      (b) Gauge Pressure

The gauge pressure is any pressure for which the base for measurement is atmospheric pressure expressed as kPa (gauge). Atmospheric pressure serves as a reference level for other types of pressure measurements, for example, gauge pressure. As shown in Figure 1.1, the gauge pressure is either positive or negative depending on its level above or below atmospheric level. At the level of atmospheric pressure, the gauge pressure becomes zero.

Figure 1.1 Illustration of pressures for measurement.

      (c) Absolute Pressure

      A different reference level is utilized to obtain a value for absolute pressure. The absolute pressure can be any pressure for which the base for measurement is a complete vacuum, and is expressed in kPa (absolute). Absolute pressure is composed of the sum of the gauge pressure (positive or negative) and the atmospheric pressure as follows:

      (1.4)

      For example, to obtain the absolute pressure, we simply add the values of atmospheric pressure to gauge pressure. The absolute pressure is the most common one used in thermodynamic calculations, despite the fact that the reading by most pressure gauges and indicators is the pressure difference between the absolute pressure and the atmospheric pressure existing in the gauge.

      (d) Vacuum

      A vacuum is a pressure lower than atmospheric pressure and occurs only in closed systems, except in outer space. It is also called negative gauge pressure. In fact, a vacuum is the pressure differential produced by evacuating air from the closed system. A vacuum is usually divided into four levels: (i) low vacuum representing pressures above 1 Torr absolute (a large number of mechanical pumps in industries are used for this purpose; flow is viscous), (ii) medium vacuum varying between 1 and 10⁻³ Torr absolute (most pumps serving this range are mechanical; fluid is in transition between viscous and molecular phases), (iii) high vacuum ranging between 10⁻³ and 10⁻⁶ Torr absolute (nonmechanical ejector or cryogenic pumps are used; flow is molecular or Newtonian), and (iv) very high vacuum representing absolute pressure below 10⁻⁶ Torr (primarily for laboratory applications and space simulation).

      It is important to note an additional term, the saturation pressure, which is the pressure of a liquid or vapor at saturation conditions.

      1.3.3 Temperature

      Temperature is an indication of the thermal energy stored in a substance. In other words, we can identify hotness and coldness with the concept of temperature. The temperature of a substance may be expressed in either relative or absolute units. The two most common temperature scales are Celsius (°C) and Fahrenheit (°F). The Celsius scale is used with the SI unit system and the Fahrenheit scale with the English system of units. There are two additional scales, the Kelvin scale (K) and the Rankine scale (R), which are absolute temperature scales and are often employed in thermodynamic applications.

The degree Kelvin is a unit of temperature measurement; zero kelvin (0 K) is absolute zero and is equal to −273.15°C. Increments of temperature in units of K and °C are equal. For instance, when the temperature of a product is decreased to −273.15°C (or 0 K), known as absolute zero, the substance contains no thermal energy and all molecular movement stops. Temperature can be measured in a large number of ways by devices. In general, the following devices are commonly used:

       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.