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ME 410 - EXPERIMENT NO . 3 Dr. Cemil Yamalı Heat Transfer Lab. MASS AND ENERGY BALANCES IN PSYCHROMETRIC PROCESSES Air Conditioning Control Temperature and Humidity Industrial Comfort ASHRAE Comfort conditions Winter: 20 - 23 oC 50% +/- 20 RH Summer: 24 - 27 oC BASICS The air in the atmosphere that normally contains some water vapor is called atmospheric air. Air that contains no water vapor is called dry air. Although the amount of water vapor in the air is small, it plays a major role in human comfort. The temperature of air in air conditioning applications ranges from about -10 to about 50°C. In this range, the dry air can be treated as an ideal gas with a constant specific heat. The enthalpy and enthalpy change of dry air T is the air temperature in °C Treat the water vapor in the air as an ideal gas. Water vapor in the air behaves as if it existed alone and obeys the ideal-gas relation Pv = RT hv(T, low P) = hg(T) The amount of water vapor in the air: humidity (also called humidity ratio) As more vapor or moisture is added, the specific humidity will keep increasing until the air can hold no more moisture. At this point, the air is said to be saturated with moisture, and it is called saturated air. Any moisture introduced into saturated air will condense. Comfort level depends on the amount of moisture the air holds relative to the maximum amount of moisture the air can hold at the same temperature The enthalpy of air is expressed in terms of the enthalpies of the dry air and the water vapor Dry-bulb temperature: The ordinary temperature of atmospheric air Dew-point temperature Tdp is defined as the temperature at which condensation begins if the air is cooled at constant pressure Tdp is the saturation temperature of water corresponding to the vapor pressure: Relative humidity and specific humidity are not easy to measure directly we must relate them to easily measurable quantities such as temperature and pressure. Another way of determining the absolute or relative humidity is related to an adiabatic saturation process Wet bulb temperature: Psychrometric charts: Air- water vapor mixture properties presented in the form of a chart at a given total pressure The dry-bulb temperatures are shown on the horizontal axis, and the specific humidity is shown on the vertical axis. Lines of constant enthalpy (in kJ/kg dry air) lie very nearly parallel to the lines of constant wet-bulb temperature. For saturated air, the dry-bulb, wet-bulb, and dewpoint temperatures are identical The psychrometric chart also serves as a valuable aid in visualizing the air conditioning processes HUMAN COMFORT AND AIR CONDITIONING The comfort of the human body depends primarily on three factors: the (dry-bulb) temperature, relative humidity, and air motion. The temperature of the environment is the most important index of comfort. Most people feel comfortable when the environment temperature is between 22 and 27°C The relative humidity also has a considerable effect on comfort since it affects the amount of heat a body can dissipate through evaporation Most people prefer a relative humidity of 40 to 60 percent. Air motion also plays an important role in human comfort. It removes the warm, moist air that builds up around the body and replaces it with fresh air. Therefore, air motion improves heat rejection by both convection and evaporation Most people feel comfortable at an airspeed of about 15 m/min. Very high-speed air motion causes discomfort. For example, an environment at 10°C with 48 km/h winds feels as cold as an environment at -7°C with 3 km/h winds as a result of the body-chilling effect of the air motion (the wind-chill factor). AIR CONDITIONING PROCESSES Simple Heating and Cooling Heating with Humidification Problems associated with the low relative humidity resulting from simple heating can be eliminated by humidifying the heated air If steam is introduced in the humidification section, this well result in humidification with additional heating. If humidi fication is accomplished by spraying water into the airstream instead, part of the latent heat of vaporization will come from the air, which will result in the cooling of the heated airstream Cooling with Dehumidification It may be necessary to remove some moisture from the air, i.e., to dehumidify it. This requires cooling the air below its dew-point temperature. Evaporative Cooling In desert (hot and dry) climates, one can avoid the high cost of cooling by using evaporative coolers Evaporative cooling is based on a simple principle: As water evaporates, the latent heat of vaporization is absorbed from the water body and the surrounding air. As a result, both the water and the air are cooled during the process Twb = constant h = constant Adiabatic Mixing of Airstreams Many air conditioning applications require the mixing of two airstreams. This is particularly true for large buildings, most production and process plants, and hospitals, which require that the conditioned air be mixed with a certain fraction of fresh outside air before it is routed into the living space REFRIGERATION THE IDEAL VAPOR-COMPRESSION REFRIGERATION CYCLE The impracticalities associated with the reversed Carnot cycle can be eliminated by vaporizing the refrigerant completely before it is compressed and by replacing the turbine with a throttling device. Isentropic compression in a compressor P = constant heat rejection in a condenser Throttling in an expansion device P = constant heat absorption in an evaporator ACTUAL VAPOR-COMPRESSION REFRIGERATION CYCLES Two common sources of irreversibilities are fluid friction (causes pressure drops) and heat transfer to or from the surroundings Instead, the system is designed so that the refrigerant is slightly superheated at the compressor inlet This slight overdesign ensures that the refrigerant is completely vaporized when it enters the compressor The line connecting the evaporator to the compressor is usually very long, thus the pressure drop caused by fluid friction and heat transfer from the surroundings to the refrigerant can be very significant. the line connecting the evaporator to the compressor is usually very long, thus the pressure drop caused by fluid friction and heat transfer from the surroundings to the refrigerant can be very significant. The refrigerant is subcooled somewhat before it enters the throttling valve REFRIGERANTS Cholorofluorocarbons, ammonia, hydrocarbons(propane, ethane, ehtylene, etc), carbon dioxide, air (in the air conditioning of aircraft R-11, R-12, R-22, and R-502 account for a large percent of the market The industrial and heavy commercial sectors were satisfied with ammonia, and still are, although ammonia is toxic. The advantages of ammonia over other refrigerants are its low cost, higher COPs (and thus lower energy cost), more favorable thermodynamic and transport properties and thus higher heat transfer coefficients (requires smaller and lower-cost heat exchangers), greater detectability in the event of a leak, and no effect on the ozone layer Refrigerants used in the light commercial and household sectors such as sulfur dioxide, ethyl chloride, and methyl chloride were highly toxic The versatility and low cost of CFCs R-11 is used primarily in large-capacity water chillers serving air conditioning systems in buildings. R-12 is used in domestic refrigerators and freezers. R-22 is used in window air conditioners, heat pumps, air conditioners of commercial buildings, and large industrial refrigeration systems, and offers strong competition to ammonia. R-502 (a blend of R-115 and R-22) is the dominant refrigerant used in commercial refrigeration systems such as those in supermarkets because it allows low temperatures at evaporators while operating at single-stage compression. The ozone crisis: CFCs allow more ultraviolet radiation into the earth's atmosphere while preventing the infrared radiation from escaping the earth and thus contributing to the greenhouse effect which causes global warming. Fully halogenated CFCs (such as R-11, R-12, and R-115) do the most damage to the ozone layer . The nonfully halogenated refrigerants such as R-22 have about 5 percent of the ozone-depleting capability of R-12. Chlorine-free R-134a is expected to replace R-12. The lowest pressure in a refrigeration cycle occurs in the evaporator, and this pressure should be above atmospheric pressure to prevent any air leakage into the refrigeration system The saturation pressure of the refrigerant at this temperature should be well below its critical pressure if the heat rejection rocess is to be approximately isothermal. If no single refrigerant can meet the temperature requirements, then two or more refrigeration cycles with different refrigerants can be used in series Cascade Refrigeration Systems In actual cascade refrigeration systems, the two cycles would overlap somewhat since a temperature difference between the two fluids is needed for any heat transfer to take place. Compressor work decreases and the amount of heat absorbed from the refrigerated space increases as a result of cascading. Therefore, cascading improves the COP of a refrigeration system Multistage Compression Refrigeration Systems When the fiuid used throughout the cascade refrigeration system is the same, the heat exchanger between the stages can be replaced by a mixing chamber (called a flash chamber), since it has better heat transfer characteristics. Such systems are called multistage compression refrigeration systems The compression process in this system resembles a two-stage compression with intercooling, and the compressor work decreases Multipurpose Refrigeration Systems with a Single Compressor Some applications require refrigeration at more than one temperature A practical and economical approach would be to route all the exit streams from the evaporators to a single compressor and let it handle the compression process for the entire system.