Wet-bulb depression

The wet-bulb depression is the difference between the dry-bulb and wet-bulb temperatures. The temperature is depressed by evaporative cooling of the wet bulb. The greater the difference between the amount of water in the air and the saturation water capacity the more rapid the evaporation and thus the greater the temperature depression. The wet-bulb depression allows one to determine the absolute humidity of dry air by means of the appropriate psychrometric chart. 

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Typical operating data for cocurrent rotary dryers are given in Table 12-18. (Note that the driving force AT must be based on wet-bulb depression and not on material temperatures. Use of material temperatures, particrrlariy when the dry solids are superheated after drying, will yield conservative results.)... 

Psychrometric coefficient The coefficient in the equation for the determination of the water vapor partial pressure from the wet bulb depression. 

The quantity Tq - t y is known as the wet-bulb depression. kH is the redefined mass transfer coefficient defined by... 

For example, evaporation takes place more rapidly in air with a lower relative humidity (giving a greater wet-bulb depression), but if the air is moist (high RH), evaporation is slow and the wet-bulb depression is small. At 100% RH, there is no wet-bulb depression, and thus, wet bulb equals dry bulb, which equals dew point. 

Corrected for 30% relative humidity of ambient air. Figures in brackets allow for full wet bulb depression. 

The gas velocity in the conveying duct must be sufficient to convey the largest particle. This may be calculated accurately by methocfs given in Sec. 12. For estimating purposes, a velocity of 25 m/s, calcu-fated at the exit-air temperature, is frequently employed. If mainly surface moisture is present, the temperature driving force for drying will approach the log mean of the inlet- and exit-gas wet-bulb depressions. (The exit solids temperature will approach the exit-gas dry-bulb temperature.)... 

The accurate determination of the humidity of air is carried out gravimetrically. The water vapor present in a known volume of air is chemically absorbed with a suitable reagent and weighed. In less laborious methods, the humidity is derived from the dew point or the wet-bulb depression of a water-vapor mixture. 

The derivation of the humidity from the wet-bulb depression requires a preliminary study of the transfer of mass and heat at a boundary between air and water. The difference between the air temperature and the wet-bulb temperature is the wet-bulb depression. If these temperatures are denoted by Ta and T b, the rate of heat transfer, Q, is given by Eq. (14),... 

Both the heat and mass transfer coefficients are functions of air velocity. However, at air speeds greater than about 15 ft/s (4.5 m/s), the ratio h kgis approximately constant. The wet-bulb depression is directly proportional to the difference between the humidity at the surface and the humidity in the bulk of the air. In the wet- and dry-bulb hygrometer, the wet-bulb depression is measured by two thermometers, one of which is fitted with a fabric sleeve wetted with water. These thermometers are mounted side by side and shielded from radiation, an effect neglected in the derivation above. Air is drawn over the thermometers by means of a small fan. The derivation of the humidity from the wet-bulb depression and a psychrometric chart are discussed later. 

Relative humidity is difficult to measure reliably and instead it is determined indirectly from the wet- and dry-bulb thermometers of a hygrometer. The wet-bulb thermometer is kept moist with a fabric sleeve whose other end is in a reservoir of clean water. As air passes over the wet sleeve water is evaporated and cools the wet-bulb thermometer the drier the air the greater the cooling effect. The dry-bulb measures the air temperature there is no cooling effect on the dry-bulb thermometer. The difference between the dry-bulb and wet-bulb temperatures, the wet-bulb depression (AT), and the dry-bulb temperature are the parameters used to... 

Table 8.2. The equilibrium moisture content of wood depends on the kiln condition, i.e. the wet-bulb depression and the dry-bulb temperature. The equibbrium moisture contents of individnal species can deviate 3% from these valnes (Brurmer-Hilderbrand, 1987).

Consider the surface temperature of lumber during the course of a kiln schedule in which the dry-bulb and wet-bulb temperatures, and so the wet-bulb depression, are held constant. The initial rate of evaporation is independent of the air temperature (dry-bulb temperature) but is proportional to the wet-bulb depression. This is because evaporation is sustained by the rate of heat transfer, and the rate of heat transfer from the warm air to the moist wood surface is proportional to the temperature difference between the air and the wood surface which is at the wet-bulb temperature. Thus, provided die wet-bulb depression is the same, say AT = 5°C, the rate of evaporation from a wet timber surface is essentially the same whether the kiln air temperature is 40, 70 or 100°C. 

During the schedule automatic controls (or manual operator) intermittently adjust the dry and wet-bulb temperatures so that the kiln eonditions correspond to the next stage of the schedule and so on until the sehedule is complete. The wet-bulb depression must be controlled accurately as it has a disproportionate effeet on the severity of the schedule, compared to any drift in both dry and wet-bulb temperatures (Table 8.2). If the wet-bulb depression is too great, the rate of drying will be too fast, and the lumber may suffer degrade. 

If lumber were taken out of the kiln immediately there would be a risk that the hot wood will heat the cool air around the stack, making the air warmer and much drier. The warm dry air would then lead to further drying and checking at the surface of the boards - eool saturated air at 20°C has a relative humidity of only 12% if heated to 60°C and the moisture content of the wood in equilibrium with that air would be only 2%. For the better grades of timber the heat is turned off and the load cooled under a constant wet-bulb depression of about 5°C until the temperature is within 15-20°C of that outside. Only then can the stacks be removed safely from the kiln. 

Traditional schedules result in a drying rate that decreases with time, only partly countered by increases in the dry-bulb temperature and the wet-bulb depression as the schedule proceeds. Modem automatic process control means that the kiln schedule can be adjusted continuously so ensuring a more constant rate of heat transfer and evaporation. Also this avoids any shock that an abrapt change in the schedule imposes on the timber. 

Some time ago Kamke and Vanek (1994) compared the performance of a number of within-the-timber drying models, representing mainly diffusion-like and multiple-transport mechanism approaches, for predicting average moisture contents and moisture-content profiles. Four data sets were used, with three sets representing idealised problems. The fourth data set was the experimental results of drying 40 mm boards of Norway spruce, Picea abies, from initial moisture contents of 29-66% at a dry-bulb temperature of 60°C, wet-bulb depressions of 8-25°C, and an air velocity of 6 m s. The required inputs for the models, including physical properties... 

AT = average temperature difference, taken as logarithmic mean of wet-bulb depressions at inlet and outlet of dryer... 

WET-BULB DEPRESSION - The difference between the dry-bulb temperature and the wet bulb temperature. 

This seems to take no account of variations in atmospheric pre.ssure, but the suggested tolerance of 10% seems not unreasonable given that if the dry bulb temperature is 20 C and the wet bulb depression is 4.2°C at 1013 hPa, the r.h.% will be 65%. but if the wet bulb depression changes to 3.2°C, at the same pressure, the r.h. % will be 72.9%. i.e,. a change of 7.9%. This of course assumes that wet bulb temperatures can be measured to an accuracy of O.TC. It seems clear therefore that the traditional standard atmosphere of 20 C 2 C and 65% r.h. 2% r.h, assumes that it is possible in the laboratory to maintain a constant barometric pressure, a dry bulb temperature that docs not vary by more than 2°C and that the wet bulb depression is constant at 4.2 C. Is this feasible ... 

Miller et al. [30], Friedman and Marshall [9], and Seaman and Mitchell [47] have done considerable amount of research for the evaluation of U a. The volumetric coefficient U a is the product of the heat transfer coefficient based on the effective area of contact between the gas and the solids, and the ratio a of this area to the volume of the dryer. When a considerable amount of surface moisture is removed from the solids and their temperature is unknown, a good approximation of (At)m is the logarithmic mean between the wet-bulb depressions of the drying air at the inlet and outlet of the dryer [35]. 

Ai, is the log mean of the drying gas wet-bulb depressions at the inlet and outlet of the dryer... 

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