* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download View a sample here
Survey
Document related concepts
Solar water heating wikipedia , lookup
Refrigeration wikipedia , lookup
Thermal comfort wikipedia , lookup
Hypothermia wikipedia , lookup
Radiator (engine cooling) wikipedia , lookup
Thermal conduction wikipedia , lookup
Insulated glazing wikipedia , lookup
Thermoregulation wikipedia , lookup
Solar air conditioning wikipedia , lookup
Intercooler wikipedia , lookup
Vapor-compression refrigeration wikipedia , lookup
Hyperthermia wikipedia , lookup
Atmospheric convection wikipedia , lookup
R-value (insulation) wikipedia , lookup
Transcript
2 L E S S O N Heat Transfer (Part 2) P M THE ROLE OF INSULATION IN ENERGY CONSERVATION T E L A S he previous Lesson defined the basic refrigeration cycle as the process of removing heat from one area, object, or material, and releasing it somewhere else. This process consumes one or more forms of energy. Continued population growth, necessary antipollution measures, ecology protection requirements, and fuel shortages all have made the conservation of energy—especially electrical energy—a matter of high priority. The phrase “energy conservation,” when it is used in the context of refrigeration, refers primarily to a reduction in electrical energy consumption. One way to cut down on energy consumption is to reduce the amount of heat to be transferred by the refrigeration cycle. Doing so means that equipment “ON” cycles will be shorter, and that smaller, more energy-efficient equipment can be used. Insulation plays an important role in reducing the heat load on a refrigeration application, regardless of whether the heat transfer occurs via radiation, conduction, or convection. TYPES OF INSULATION Insulation is used to retard (slow down) the flow of heat from a warm space to a cooler one. Recall that reflective types of insulation block the flow of radiant 21 R A C heat. Insulation materials that restrict the flow of heat by conduction are the subject of this Lesson. Materials vary in their ability to conduct heat. Most metals, for example, are good conductors of heat—in fact, they conduct heat so well and so rapidly that there is little difference between the temperature of the “hot” side and that of the “cold” side. Other materials are very poor conductors of heat. Because they conduct heat very slowly, these materials make good insulators. Some well-known materials that have been used as insulation over the years include cork, mineral wool, wood, paper, corn or cane stalks, and cotton. Today many insulation materials are synthetic products. Some of the more common modern insulators include fiberglass, polyurethane foam, phenolic foam, polystyrene, flexible elastomeric foam, and cellular glass foam. E L “Dead” air—that is, air that is not in motion—is a very good insulator. It has a theoretical K-value of 0.17. Why theoretical? Because it is very difficult to keep air absolutely still. Convection currents and radiation of heat across the air space decrease its insulating value. But the more a single space of open air can be separated into many smaller spaces, the more the convection currents are broken up—and the greater the insulating value. Convection currents in the tiny air cells of insulation materials are very small. These tiny, enclosed air cells are what give certain materials the ability to act as good insulators—that is, to impede the flow of heat by conduction. P M A S Insulation may take various forms: ª Fibrous insulation is composed of small-diameter fibers that separate the available air space into tiny areas. Silica, rock wool, slag wool, glass fiber, and aluminum silica fibers are used. ª Cellular insulation is composed of small, individual cells separated from each other. These closed-cell materials, as they are called, include glass, foamed elastomerics, and foamed plastics (such as polystyrene and polyurethane). ª Granular insulation is composed of small nodules that contain voids or hollow spaces. Vermiculite is an example of a granular insulation. These materials are manufactured in a variety of forms suitable for specific functions or applications. Some are rigid boards, blocks, or sheets. Some are made in preformed shapes so that they can be used as curved segments or for covering pipes. Flexible sheets, flexible blankets, and cements are also forms that are widely used. When a fibrous or granular material is pressed or formed 22 L E S S O N 2 into boards or sheets to give it more strength and rigidity, a binder of some kind must be used to make the loose material stick together. However, as little binder as possible is used, since the binder itself is often not a very good insulator, and tends to reduce the insulating value of the board. So-called “foamed-in-place” insulation is used for filling cavities that are hard to get at, or difficult to insulate by other methods. TEMPERATURES WITHIN INSULATION Do not assume that the high temperature on the hot side of a sheet of insulation suddenly becomes a low temperature on the cold side. Remember that insulation is not a positive barrier to heat flow—it merely retards the rate of heat transfer. The rate of heat flow through the insulation is reduced gradually. As a result, the temperature within the insulation also drops gradually, from the temperature of the warmer outer surface to that of the cooler inner surface. E L Figure 2-1 below shows the gradual change of temperature—the temperature gradient—within the insulation itself. These temperatures shift position if either the outer, warm-side temperature or the inner, cool-side temperature changes. For example, if the outer temperature drops from 90 to 70°F, but the inner temperature remains at 30°F, then the mean temperature (the halfway point) within the insulation moves toward the warm side. If the inner temperature drops but the outer temperature stays the same, the mean temperature within the insulation moves toward the cool side of the insulation. P M A S Cool side of insulation Warm side of insulation Direction of heat flow 30 40 50 60 70 80 Cool side of insulation 90 40 Temperature, °F 50 60 Temperature, °F FIGURE 2-1. Sample temperature gradients within insulation 23 Warm side of insulation Direction of heat flow 70 R A C MOISTURE IN INSULATION Water is an excellent conductor of heat. It has a K-value of about 4.1, assuming that there are no convection currents in the water. (Convection currents in water make it a much better conductor.) Thus, if water gets into insulation, the insulating value of the insulation is greatly reduced. On average, for every 1% increase (by volume) in the moisture content of an insulation material, its thermal conductivity (rate of heat transfer) increases by 7.5%. It is, therefore, absolutely essential for insulation to be dry when it is installed, and perfectly sealed so that it stays dry. The first step to preventing moisture from getting into insulation is to determine the source of the moisture. Water can get into insulation either as liquid or as water vapor in the air, which can condense on or in the insulation. Water getting into insulation as a liquid is a comparatively rare (and usually accidental) occurrence. It may result from washing or spillage, but for the most part does not represent a major problem. A far more challenging problem is to keep the water vapor in the air out of the insulation—because this is how most water gets into insulation. E L P M You have heard it said that air “contains” moisture. This is not strictly true. What people refer to as “air” in everyday speech is really a mixture of dry air (itself a combination of gases in relatively fixed amounts) and water vapor. Each exists independently of the other. Both are at the same temperature, but each has its own pressure. The pressure of the dry air alone is very large compared to that of the water vapor. References to atmospheric pressure or barometric pressure include both the pressure of the air itself and the pressure of the water vapor in it. Standard atmospheric pressure at sea level is 29.92 inches of mercury (in. Hg). A S The amount of water vapor in the air varies, depending on the temperature, relative humidity, and atmospheric pressure. The amount of water vapor that air can hold depends primarily on temperature. Warm air can hold more water vapor than the same volume of cold air. The separate and combined pressures of air and water vapor also vary with changes in weather conditions. Table 2-1 shows some of the properties of various mixtures of air and water vapor. As you can see, warm air has a much greater vapor pressure than cold air, even when the two are at the same relative humidity. If the total pressure inside a refrigerator, where the temperature is 40°F, is 29.92 in. Hg, then the total pressure outside the refrigerator must also be 29.92 in. Hg, even though the temperature of the air in the room may be 90°F. However, the partial pressures will differ dramatically. The vapor pressure inside the refrigerator (at 40°F db and 80% RH) is 0.2 in. Hg. The vapor pressure outside the refrigerator (at 90°F db 24 L E S S O N Dry-bulb temperature, °F Wet-bulb temperature, °F Relative humidity, % Dew point temperature, °F Vapor pressure, in. Hg 90 86 85 85 1.207 13.4 90 80 65 77 0.924 10.0 80 76 85 75 0.878 9.47 80 71 65 67 0.672 7.32 70 67 85 65 0.629 6.85 70 62 65 58 0.480 5.41 40 38 85 36 0.212 2.46 40 36 65 30 0.161 1.94 25 24 85 22 0.112 1.35 25 22 0 Grains* of moisture per ft3 65 16 0.085 1.03 0.5 85 –3 0.032 0.41 0.0245 0.31 0.0187 0.27 0.0142 0.18 0 –1.0 65 –8 –10 –10.3 85 –12 –15 –15.2 85 –18 E L P M *There are 7,000 grains in 1 lb of water. TABLE 2-1. Properties of typical mixtures of air and water vapor (at standard barometric pressure of 29.92 in. Hg) A S 53⁄4 and 80% RH) is 1.14 in. Hg, or times as great. The total pressure is the same inside and outside the refrigerator. But according to Dalton’s Law of Partial Pressures, the dry air and the water vapor act independently of each other. The pressure of each reacts to changes in volume and temperature in its own way. The higher vapor pressure outside the refrigerator can actually push the moisture from outside through the insulation unless a tight barrier is placed outside the insulation to stop it. This vapor barrier (or vapor seal, as it is sometimes called), must be made of some material that moisture cannot penetrate, such as metal foil or polyethylene. The edges must be tightly sealed where the barrier overlaps at the sides, top, and bottom of the refrigerator. The vapor barrier must be continuous—that is, there must be no breaks of any sort. Even a pinhole can defeat the purpose of the vapor barrier. In a refrigerator, cold storage room, or air conditioned house, the warm side normally is the outside of the insulation. But this is not always the case. An insulated house in winter is warmer inside. The same may be true of a refrigerator operating in a very cold room (colder than the inside of the refrigerator), or a refrigerated truck or refrigerator car in very cold weather. 25 2