State any five process where humidity control is necessary for an efficient operation
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Answer:
LOW ENERGY HUMIDITY MANAGEMENT STRATEGIES
The humidity control objectives described in the previous section imply that the most demanding humidity control criterion is the maintenance of relative humidity below 70% at low dry bulb temperatures. Within ASHRAE Standard 55-81, this requirement has been expressed in an unnecessarily restrictive form, namely the maintenance of the interior dewpoint temperature below 17°C for all dry bulb temperatures.
The recognition of the role of relative humidity as the controller of microorganisms, and the insensitivity of preferred comfort temperature to dewpoint suggests two energy efficient microorganism control strategies; high dry bulb comfort and controlled ventilation with cooling recovery. These two strategies are actually modern expressions of the strategies which were followed in the days before air conditioning.
The “high dry bulb comfort” strategy is simply the use of air motion, clothing and mean radiant temperature control to provide comfort at the highest possible dry bulb temperature. The maintenance of relative humidity below 70% in buildings with ceiling fans and mean radiant temperature control will be much less energy intensive than the maintenance of dewpoint below the ASHRAE limit of 17°C. The increased dry-bulb temperature will allow microorganism control at high dewpoint temperatures. In many climates the required relative humidity can be maintained by simple ventilation through air to air heat exchangers. This “high dry bulb comfort” strategy for microorganism control is a reexpression of the “light bulb in the closet” strategy of mold control. The purpose of the light bulb is simply to decrease the relative humidity in the cool stagnant closet air. The air motion induced by fans provides a secondary microorganism control function as well as a direct comfort effect. Air circulation helps to prevent pockets of high relative humidity which can promote local mold and mildew growth (e.g., in bathrooms, kitchens and closets). It is fortunate that the elements of an energy conserving human comfort strategy are also the elements of an energy conserving microorganism control strategy.
An example will illustrate a method of evaluation of the maximum allowable dewpoint. Fanger's (1970) analysis predicts that the preferred dry bulb temperatures for a lightly clothed sedentary person will be 27°C (81°F) at a relative humidity of 70%, an air speed of 0.5 m/s (100 fpm) and a mean radiant temperature of 27°C. The corresponding dewpoint temperature is 22°C (70°F).
Depression in the mean radiant temperature will allow comfort at even higher dry bulb temperatures. However, unless designed for deliberate condensation, the temperature of the surface which provides radiant cooling must not be below the interior dewpoint. For example, the ceiling temperature in a residence cooled by an open surface roof pond will begin the day at a temperature which is depressed below the ambient dry bulb by 70% of the wet bulb depression (Clark and colleagues, 1982). Even though this ceiling temperature is still above the ambient dewpoint, interior moisture sources can raise the interior dewpoint above the ambient dewpoint.
The most energy efficient method of removal of the moisture generated in the interior is ventilation. In American homes this moisture will total 5 to 12 liters/day (10 to 25 lbs/day) for a typical family of four people. A psychrometric chart and ambient dewpoint temperature data allow easy calculation of the effect of a given ventilation rate on the interior dewpoint and relative humidity.
In many places the ambient air dewpoint (a measure of the water vapor mass per pound of dry air) is low enough to provide dilution of interior water sources to 70% relative humidity at low ventilation rates. Table 1 lists 24 hour average dewpoint temperatures in selected U.S. cities for a typical July. In coastal cities, the diurnal variation from the daily average value is usually less than 1°C. However, in inland cities the diurnal variation is as much as 3°C above and below the daily average. In all locations the ambient dewpoint rises to a maximum near dawn and reaches a minimum in late afternoon when insolation driven convective mixing of the atmoshpere is maximized.
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