Theory

Dehydration involves the simultaneous application of heat and removal of moisture from foods. Factors that control the rates of heat and mass transfer are described elsewhere in the encyclopedia. Dehydration by heated air or heated surfaces is described in this article. Microwave, dielectric, radiant, and freeze-drying are described in other entries.

Psychrometrics

The capacity of air to remove moisture from a food depends on the temperature and the amount of water vapor already carried by the air. The content of water vapor in air is expressed as either absolute humidity—the mass of water vapor per unit mass of dry air (in kilograms per kilogram), termed moisture content in Fig. 1)—or relative humidity (RH) (in percent)—the ratio of the partial pressure of water vapor in the air to the pressure of saturated water vapor at the same temperature, multiplied by 100. Psychrometry is the study of the interrelationships of the temperature and humidity of air. These properties are most conveniently represented on a phychrometric chart (Fig. 1).

The temperature of the air, measured by a thermometer bulb, is termed the dry-bulb temperature. If the thermometer bulb is surrounded by a wet cloth, heat is removed by evaporation of the water from the cloth and the temperature falls. This lower temperature is called the wet-bulb temperature. The difference between the two temperatures is used to find the relative humidity of air on the psychrometric chart. An increase in air temperature, or reduction in RH, causes water to evaporate from a wet surface more rapidly and therefore produces a greater fall in temperature. The dew point is the temperature at which air becomes saturated with moisture (100% RH). Adiabatic cooling lines are the parallel straight lines sloping across the chart, which show how absolute humidity decreases as the air temperature increases.

Sample Problems 1. Using the psychrometric chart (Fig. 1), calculate the following.

1. The absolute humidity of air that has 50% RH and a dry-bulb temperature of 60°C.

2. The wet-bulb temperature under these conditions.

3. The RH of air having a wet-bulb temperature of 45°C and a dry-bulb temperature of 75°C.

4. The dew point of air cooled adiabatically from a dry-bulb temperature of 55°C and 30% RH.

5. The change in RH of air with a wet-bulb temperature of 39°C, heated from a dry-bulb temperature of 50°C to a dry-bulb temperature of 86°C.

6. The change in RH of air with a wet-bulb temperature of 35°C, cooled adiabatically from a dry-bulb temperature of 70°C to 40°C.

Solution to Sample Problems 1.

1. 0.068 kg/kg of dry air. Find the intersection of the 60°C and 50% RH lines and then follow the chart horizontally right to read the absolute humidity.

2. 47.5°C. From the intersection of the 60°C and 50% RH lines, extrapolate left parallel to the wet-bulb lines to read the wet-bulb temperature.

Psychrometric Chart 101 325 Kpa

Specific enthalpy (kJ/kg)

Figure 1. Psychometric chart (10-120°C based on barometric pressure of 101.325 kPa. Source: m

Specific enthalpy (kJ/kg)

Figure 1. Psychometric chart (10-120°C based on barometric pressure of 101.325 kPa. Source: m

Courtesy of Chartered Institution of Building Services Engineers. -<

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Responses

  • clara
    What are the application of psychrometric chart in food processing industry?
    4 months ago

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