Mass Transfer - Humidification Problems

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  1. The following data were obtained during a test run of a packed cooling tower of 0.4 m dia and 1 m packed height, operating at atmospheric pressure. Calculate the humidity of the exit air by means of an enthalpy balance. Average temperature of entering and leaving air is 38oC and 39oC respectively. Average temperature of water entering and leaving is 46oC and 35oC respectively.
  2. Rate of entering air = 13.6 m3/min

    Rate of entering water = 1000 kg/hr

    Humidity of entering air = 0.0175 kg water vapor / kg dry air

    Latent heat of evaporation of water = 589 kcal/kg

    Specific heat of air = 0.245 kcal/kgoC

    Specific heat of water vapor = 0.45 kcal/kgoC

  3. A wet solid material is dried from 0.7 kg water/kg dry solid to 0.08 kg water/kg dry solid in a continuous counter-current drier from which the product flows out at the rate of 500 kg/hr. The inlet air to the drier is at 54oC with an initial humidity of 0.015 kg water/kg of dry air and the exit air is at 32.2oC with 80% saturated humidity. Calculate the inlet air rate in m3/hr and the heat supplied by the preheater if the atmospheric temperature is 24oC.
  4. Data:

    Saturated humidity at 32.2oC = 0.025 kg water/kg dry air

    Specific volume of dry air at 54oC = 0.925 m3/kg

    Saturated volume = 1.09 m3/kg

    Humid heat 0.015 humidity = 0.243 cal/gmoC

    Humid heat at 32.2oC and at 80% saturated humidity = 0.250 cal/gmoC

  5. For a drying process, air is required at 38oC with a dew point of 21oC. To make this air at 15oC and 40 percent relative humidity is first heated and passed through an adiabatic humidifier until the required conditions are obtained. Estimate the temperature to which air is to be heated first. Vapor pressure of water: 12.8 mm Hg at 15oC and 18.7 mm Hg at 21oC. Latent heat of vaporization at 38oC = 576 cal/gm. Specific heat of dry air = 0.24 and that of water vapor = 0.48
  6. A continuous counter-current drier is to be used to dry 10000 kg per hour of wet solid containing 5% water (wet basis) to a water content of 0.1% (wet basis). Ambient air at 27oC and a humidity of 0.0075 will be heated to 150oC and the heated air is passed through the drier. The air leaving the drier is at 70oC with a percentage humidity of 10 percent. Calculate the air required and the heat supplied in the preheater.
  7. Saturation humidity at 70oC = 0.299

    Humid heat of inlet air = 0.243 cal/gmoloK

  8. Conditioned air at 24oC and 40% saturation is to be supplied to a laboratory room of size 4 X 20 X 6 meters, with no facility for recirculation. The air-conditioner takes outside air with 90% relative humidity at 38oC which it refrigerates and separates out the condensed water and then reheats it in a heat exchanger using condensing steam at 1 atm pressure. Determine the following:
      1. The volume of outside air at entry conditions, and the temperature to which it must be cooled.
      2. The tonnes refrigeration required and the kg of saturated steam required per hour.

  9. A mixture of oxygen and acetone vapor at a total pressure of 1050 mm Hg at 25oC has a percentage saturation of 75%. Calculate:
      1. The molal humidity
      2. Absolute humidity
      3. Relative humidity
      4. Volume percent acetone
      5. Molal humid volume and
      6. Molal humid heat.

    The saturation vapor pressure of acetone at 25oC is 290 mm Hg and the specific heats of oxygen and acetone vapor are 0.25 and 0.35 kcal/kgoC respectively.

  10. The air supply to a rotary drier has a dry bulb at 32.5oC and a wet bulb of 25oC. the air is heated by steam coils to 95oC before it enters the drier. In passing through the drier, the air cools along adiabatic cooling line and finally leaves the drier at 90% saturation.
      1. Estimate the temperature of exit air
      2. Calculate the diameter of the drier if it is to handle 10 tones per hour of a salt with 10% moisture on wet basis. Air mass velocity is given as 5000 kg per hour per sq.ft. and the salt is to be dried to 2% moisture content.

  11. A mechanical draft cooling tower is to be designed to cool 75000 kg/hr of water from 45oC using 62500 kg of dry air per hour. 24oC is suggested as the design air wet bulb temperature. Calculate the number of transfer units and hence the height of the packed section if the height of a transfer unit for the condition stated above is 4 m. It may be assumed that the liquid phase resistance to heat transfer is negligible. Temperature t oC - saturated enthalpy in kcal/kg dry air (H) data is as follows:
  12. t oC












  13. A horizontal spray chamber with recirculated water is to be used for the adiabatic humidification and cooling of air. The active part of the chamber is 1.5 m long and a cross section of 6 m with the nozzle arrangements provided and when operated with water circulation rate recommended by the nozzle manufacturer, the coefficient of heat transfer is expected to be hGa = 1600 kcal/ An amount of 200 cc/min of air at 65oC, Y' = 0.017 kg water / kg dry air, is to be blown through the spray. Determine
      1. What exit temperature and humidity can be expected from the air
      2. Express the performance in terms of kya, NtOG, and HG and stage efficiency

    Note: For air water system CS (humid heat) = 0.24 + 0.45 Y'

  14. The air supply to a drier has a dry bulb temperature of 21oC and wet bulb temperature of 15oC. It is heated to 90oC by heating coils and introduced into the drier. In the drier it cools along adiabatic cooling line and leaves the drier fully saturated.
      1. What is the dew point of the initial air?
      2. What is its humidity?
      3. How much water will be evaporated per 100 cubic meters of entering air?
      4. How much heat is needed to heat 100 cubic meters to 90oC?
      5. At what temperature does the air leave the drier?

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Last Modified on: 04-Feb-2022

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