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Psychrometrics Unit operation 2 Example 1. Air at 60 C dry bulb temperature and 27.5 C wet bulb temperature, and a humidity ratio of 0.01 kg water/kg dry air is mixed with water adiabatically and is cooled and humidified to a humidity ratio of 0.02 kg water/kg dry air. What is the final temperature of the conditioned air? Given Inlet: dry bulb temperature = 60C wet bulb temperature = 27.5 C Initial humidity ratio W 1 = 0.01 kg water/kg dry air Final humidity ratio W 2 =0.02 kg water/kg dry air Solution From Table A.4.2, latent heat of vaporization at 27.5 C = 2436.37 kJ/kg Example 2. Calculate the rate of thermal energy required to heat 10 m3 /s of outside air at 30 C dry bulb temperature and 80% relative humidity to a dry bulb temperature of 80 C. Solution 1. Using the psychrometric chart, we find at 30 C dry bulb temperature and 80% relative humidity, the enthalpy H1 = 85.2 kJ/kg dry air, humidity ratio W1 = 0.0215 kg water/kg dry air, and specific volume V1= 0.89 m3 /kg dry air. At the end of the heating process, the dry bulb temperature is 80 C with a humidity ratio of 0.0215 kg water/kg dry air. The remaining values are read from the chart as follows: enthalpy H2 =140 kJ/kg dry air; relative humidity φ2 =7%. 2. 3. The rate of heat required to accomplish the given process is 615.7 kW 4. In these calculations, it is assumed that during the heating process there is no gain of moisture. This will not be true if a directly fired gas or oil combustion system is used, since in such processes small amounts of water are produced as part of the combustion reaction. Example 3. In efforts to conserve energy, a food dryer is being modified to reuse part of the exhaust air along with ambient air. The exhaust air flow of 10 m3/s at 70 C and 30% relative humidity is mixed with 20 m3/s of ambient air at 30 C and 60% relative humidity. Using the psychrometric chart, determine the dry bulb temperature and humidity ratio of the mixed air. Solution 1. From the given data, locate the state points A and B, identifying the exit and ambient air as shown on the skeleton chart ( Fig. E9.3 ). 2. Join points A and B with a straight line. 3. The division of line AB is done according to the relative influence of the particular air mass. Since the mixed air contains 2 parts ambient air and 1 part exhaust air, line AB is divided in 1:2 proportion to locate point C. Thus, the shorter length of line AC corresponds to larger air mass. 4. The mixed air, represented by point C, will have a dry bulb temperature of 44 C and a humidity ratio of 0.032 kg water/kg dry air. Example 4. Heated air at 50 C and 10% relative humidity is used to dry rice in a bin dryer. The air exits the bin under saturated conditions. Determine the amount of water removed per kg of dry air. Solution 1. Locate point A on the psychrometric chart, as shown in Figure. Read humidity ratio 0.0078 kg water/kg dry air. 2. Follow the constant enthalpy line to the saturation curve, point B. 3. At point B, read the humidity ratio 0.019 kg water/kg dry air. 4. The amount of moisture removed from rice 0.019 – 0.0078 = 0.0112 kg water/kg dry air. EXAMPLE 5. Relative humidity, enthalpy and specific volume of air If the wet-bulb temperature in a particular room is measured and found to be 20°C in air whose dry-bulb temperature is 25°C (that is the wet-bulb depression is 5°C) estimate the relative humidity, the enthalpy and the specific volume of the air in the room. On the humidity chart follow down the wet-bulb line for a temperature of 20°C until it meets the dry-bulb temperature line for 25°C. Examining the location of this point of intersection with reference to the lines of constant relative humidity, it lies between 60% and 70% RH and about 4/10 of the way between them but nearer to the 60% line. Therefore the RH is estimated to be 64%. Similar examination of the enthalpy lines gives an estimated enthalpy of 57 kJ kg-1, and from the volume lines a specific volume of 0.862 m3 kg-1. Once the properties of the air have been determined other calculations can easily be made. EXAMPLE 6 . Relative humidity of heated air If the air in Example 5 is then to be heated to a dry-bulb temperature of 40°C, calculate the rate of heat supply needed for a flow of 1000 m3 h-1 of this hot air for a dryer, and the relative humidity of the heated air. On heating, the air condition moves, at constant absolute humidity as no water vapour is added or subtracted, to the condition at the higher (dry bulb) temperature of 40°C. At this condition, reading from the chart at 40°C and humidity 0.0125 kg kg-1, the enthalpy is 73 kJ kg-1, specific volume is 0.906 m3 kg-1 and RH 27%. Mass of 1000 m3 is 1000/0.906 = 1104kg, DH = (73 - 57) = 16 kJ kg-1. So rate of heating required = 1104 x 16 kJ h -1 = (1104 x 16)/3600 kJ s -1 = 5 kW If the air is used for drying, with the heat for evaporation being supplied by the hot air passing over a wet solid surface, the system behaves like the adiabatic saturation system. It is adiabatic because no heat is obtained from any source external to the air and the wet solid, and the latent heat of evaporation must be obtained by cooling the hot air. Looked at from the viewpoint of the solid, this is a drying process; from the viewpoint of the air it is humidification. EXAMPLE 7 . Water removed in air drying Air at 60°C and 8% RH is blown through a continuous dryer from which it emerges at a temperature of 35°C. Estimate the quantity of water removed per kg of air passing, and the volume of drying air required to remove 20 kg water per hour. Using the psychrometric chart, the inlet air condition shows the humidity of the drying air to be 0.01 kg kg-1 and its specific volume to be 0.96 m3 kg-1. Through the dryer, the condition of the air follows a constant wet- bulb line of about 27°C , so at 35°C its condition is a humidity of 0.0207kg kg-1. Water removed = (0.0207 - 0.010) = 0.0107 kg kg-1 of air. So each kg, i.e. 0.96 m3, of air passing will remove 0.0107kg water, Volume of air to remove 20 kg h-1 = (20/0.0107) x 0.96 = 1794 m3 h-1 If air is cooled, then initially its condition moves along a line of constant humidity, horizontally on a psychrometric chart, until it reaches the saturation curve at its dew point. Further cooling then proceeds down the saturation line to the final temperature, with water condensing to adjust the humidity as the saturation humidity cannot be exceeded. EXAMPLE 8. Relative humidity of air leaving a dryer The air emerging from a dryer, with an exit temperature of 45°C, passes over a surface which is gradually cooled. It is found that the first traces of moisture appear on this surface when it is at 40°C. Estimate the relative humidity of the air leaving the dryer. On the psychrometric chart, the saturation temperature is 40°C and proceeding at constant humidity from this, the 45°C line is intersected at a point indicating: relative humidity = 76% EXAMPLE 9. Reheating of air in a dryer A flow of 1800 m3 h-1 of air initially at a temperature of 18°C and 50% RH is to be used in an air dryer. It is heated to 140°C and passed over a set of trays in a shelf dryer, which it leaves at 60 % RH. It is then reheated to 140°C and passed over another set of trays which it leaves at 60 % RH again. Estimate the energy necessary to heat the air and the quantity of water removed per hour. In dryers, it is sometimes useful to reheat the air so as to reduce its relative humidity and thus to give it an additional capacity to evaporate more water from the material being dried. This process can easily be followed on a psychrometric chart. From the psychrometric chart the humidity of the initial air is 0.0062 kg kg-1, specific volume is 0.834 m3 kg-1, and enthalpy 35 kJ kg-1. Proceeding at constant humidity to a temperature of 140°C, the enthalpy is found to be 160 kJ kg-1. Proceeding along a wet-bulb line to an RH of 60% gives the corresponding temperature as 48°C and humidity as 0.045 kg kg-1. Reheating to 140°C keeps humidity constant (0.045 kg kg-1 )and enthalpy goes to 268 kJ kg-1. Thence along a wet-bulb line to 60 % RH gives humidity of 0.082 kg kg-1. Total energy supplied = DH in heating and reheating = 268 - 35 = 233 kJ kg-1 Total water removed = DY = 0.082 - 0.0062 = 0.0758 kg kg-1 1800 m3 of air per hour = 1800/0.834 = 2158 kg h-1 = 0.6 kg s-1 Energy taken in by air = 233 x 0.6 kJ s-1 = 140 kW Water removed in dryer = 0.6 x 0.0758 = 0.045 kg s-1 = 163kgh -1 Exit temperature of air (from chart) = 60°C. Consideration of psychrometric charts, and what has been said about them, will show that they can be used for calculations focused on the air, for the purposes of air conditioning as well as for drying. EXAMPLE 10. Air conditioning In a tropical country, it is desired to provide processing air conditions of 15°C and 80% RH. The ambient air is at 31.5°C and 90% RH. If the chosen method is to cool the air to condense out enough water to reduce the water content of the air sufficiently, then to reheat if necessary, determine the temperature to which the air should be cooled, the quantity of water removed and the amount of reheating necessary. The processing room has a volume of 1650 m3 and it is estimated to require six air changes per hour. Using the psychrometric chart : Initial humidity is 0.0266 kg kg-1. Final humidity is 0.0085 kg kg-1. Saturation temperature for this humidity is 13°C. Therefore the air should be cooled to 13°C At the saturation temperature of 13°C, the enthalpy is 33.5 kJ kg-1 At the final conditions, 15°C and 80 % RH, the enthalpy is 37 kJ kg-1 and the specific volume of air is 0.827 m3 kg-1. Assuming that the air changes are calculated at the conditions in the working space. Mass of air to be conditioned = (1650 x 6)/0.827 = 11,970 kg h-1 Water removed per kg of dry air DY = 0.0266 - 0.0085 = 0.018 kg kg-1 Mass of water removed per hour = 11,970 x 0.018 = 215 kg h-1 Reheat required DH = (37 - 33.5)= 3.5 kJ kg-1 Total reheat power required = 11,970 x 3.5 = 41,895 kJ h-1 = 11.6 kJ s-1 = 11.6 kW. Problem 1. Atmospheric air at 760 mm Hg is at 22 C dry bulb temperature and 20 C wet bulb temperature. Using the psychrometric chart, determine: a. Relative humidity. b. Humidity ratio. c. Dew-point temperature. d. Enthalpy of air per kg dry air. e. Volume of moist air/kg dry air. Problem 2. Moist air flowing at 2 kg/s and a dry bulb temperature of 46 C and wet bulb temperature of 20 C mixes with another stream of moist air fl owing at 3 kg/s at 25 C and relative humidity of 60%. Using a psychrometric chart, determine the (a) humidity ratio, (b) enthalpy, and (c) dry bulb temperature of the two streams mixed together. Problem 3. Air at a dry bulb temperature of 20C and relative humidity of 80% is to be heated and humidified to 40C and 40% relative humidity. The following options are available for this objective: (a) by passing air through a heated water-spray air washer; (b) by preheating sensibly, and then passing through a water spray washer with recirculated water until relative humidity rises to 95% and then again heating sensibly to the final required state. Determine for (a) and (b) the total heating required, the make-up water required in water-spray air washer, and the humidifying efficiency of the recirculated spray water. Problem. 4 Moist air at 35 C and 55% relative humidity is heated using a common furnace to 70C. From the psychrometric chart, determine how much heat is added per m3 initial moist air and what the final dew-point temperature is. Problem 5. A water-cooling tower is to be designed with a blower capacity of 75 m3/s. The moist air enters at 25C and wet bulb temperature of 20 C. The exit air leaves at 30 C and relative humidity of 80%. Determine the flow rate of water, in kg/s, that can be cooled if the cooled water is not recycled. The water enters the tower at 40C and leaves the tower at 25C. Problem 6. Air is at a dry bulb temperature of 20C and a wet bulb temperature of 15C. Determine the following properties from a psychrometric chart. a. Moisture content b. Relative humidity c. Enthalpy d. Dew point e. Specific volume Problem 7. Air at a dbt (dry bulb temprature) of 30 C and a relative humidity of 30% is conveyed through a heated dryer where it is heated to a dbt of 80 C. Then it is conveyed through a bed of granular pet food to dry it. The air exits the dryer at a dbt of 60 C. The exit air is again heated to 80 C and conveyed through another dryer containing another batch of pet food. The exit air from the second dryer leave at saturation. Clearly show the paths of air, starting from the ambient air to the saturated air exiting the second dryer on a copy of a psychrometric chart. Determine the amount of water removed in the first and second dryer per kg of dry air.

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posted: | 6/8/2014 |

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