Actual drying

The actual drying process takes place in two stages. 

In the first stage water evaporates uniformly from the inside of the fruit to the outside through the capillaries. The temperature at the surface of the fruit and the drying rate remain to some extent constant. In this phase, which lasts until the capillaries are broken, the temperature of the surface is significantly lower than the air temperature. 

If the temperature in this phase is made too high, entrained constituents block the capillaries, the surface becomes incrusted and the drying rate is greatly reduced. 

In the second stage water vapour has to pass through the dry capillaries horn the inside of the huit to the outside. The drying rate diminishes, while at the same time the surface temperature of the huit rises. 

In the first phase there is still a difference in temperature between the air and the substance to be dried. The longer the drying process lasts, the warmer the huit gets. The temperature in the second phase of drying is reduced by up to 10°C in order to avoid browning and loss of flavour. 

---

Figure 10-24 shows a schematic of an actual dry low emission NO combustor used by ALSTOM in their large turbines. With the flame temperature being much closer to the lean limit than in a conventional combustion system, some action has to be taken when the engine load is reduced to prevent flame out. If no action were taken flame-out would occur since the mixture strength would become too lean to burn. 

Spray drying is one of the most cost effective and widely used methods of flavor encapsulation. While the process of spray drying is fairly straight forward, there are many variables relative to both sample preparation and actual drying conditions. 

Vacuum Flash Drying. In this most recent technique, drying takes place m a vacuum vessel similar to an atmospheric tower but smaller. The soap is similarly heated before flashing but under less pressure, so that boiling (actually drying of the soap) occurs in the heat exchangers. Since there is boiling in the heaters, the moisture of the dried soap depends primarily upon soap flow rate, soap pressure, and steam pressure to the heater and to a minor extent on the absolute pressure in the vacuum chamber The final temperature of the soap depends entirely upon the absolute pressure m the vacuum chamber. 

Surface cracks. Most solid substances have very numerous small cracks in their surfaces.6 The first evidence for this comes from a comparison of the actual strength of crystals with that deduced from theoretical considerations. In the case of the ionic lattice of sodium chloride the theoretical strength calculated from consideration of the electrostatic forces between the ions is of the order 200 kg. per sq. mm. actually dry crystals of rock salt can be broken at 0 4 kg. per sq. mm. If strained in air the deformation of rock salt is very small, before it breaks. It has long been known, to those who work in salt mines, that rock salt can be bent... 

Equation (12-46) may be solved by trial and error or gr hically to estimate the true values of ff, and t and, hence, the actual drying rate. The values of X and h. depend on the value of but can generally be considered constant over the range of temperatures usually encountered in air drying. 

The third heat transfer mechanism is radiation, examples of which are infrared and radio frequency dryers. Radiant heaters are often used as predryers, removing much of the moisture from wet fabric prior to entering the actual drying process (Fig. 2.14). Use of predryers minimises finish migration and increases dryer productivity since less water must be removed in the dryer. A more detailed description of drying methods and machines is given by Miles. ... 

Combination microwave/vacuum technology does not suffer the scale-up inefficiencies of vacuum drying alone. One study (39). shows that despite a 40-fold increase in batch size, the actual drying time remained similar in the laboratory and scale-up vessels. 

Again, these methods take no account of the actual drying kinetics of the particle, which are included in the next section. 

In previous work drying curves were fitted to four drying models and the goodness of fit of each model (Correlation Coefficient and Standard Error) was evaluated [7], The main aim of this work is to find out a model for drying time and to predict the required time for drying samples to desired moisture content. In the second part the forecast time is compared with the theoretical approach. The predicted values by the theoretical model are compared with experimental data taken under actual drying conditions to demonstrate the efficiency of the predictive model. 

To constant weight at <35-105°C (actual drying temperature dependent on analytical requirements). 

Remarkably these consecutive processes can be detected from the determination of the crystal structures of 7, 8 and 9. The activity of the processes seems to be directly proportional to the rate of loss of crystal water molecules, i.e. the actual drying conditions. 

The actual drying (sublimation) time 4 is thus seen to depend on the following quantities ... 

Selection Solvent recovery dryer. Nominal diameter, 1 ft 6 in. length, 7 ft 0 in. Actual drying time Assuming 20 per cent of the dryer volume is occupied by the solids, the retention time will be 1.74 hr. 

The total crop of grapes produced in California varies from year to year according to the vagueries of climate. Further, the percentage of the raisin grapes that are actually dried into raisins varies with the world-wide market for raisins. In years when this market is depressed, sizable quantities of California raisin grape varieties will be used for fresh consumption and for wine production. For example, in 1979, for which preliminary figures are now available, (1 ) of the total crop of 4,493,000... 

These relationships can be used to assess spray rate requirements for aqueous coating systems. For example, 72 g/min water sprayed into a process at 300 fE/min process air, lO C dew point, and 30°C exhaust temperature would be equivalent to 40% of the available drying capacity (0.60 g/min capacity x 0.4 x 300 = 72 g/min). A total spray rate of 90 g/min would apply if the coat suspension consists of only 80% water (0.72/0.8 = 90). If the coater were a two gun configuration, each gun would be set to deliver 45 g/min spray rate. Depending on the critical parameters of the process, a spray rate range can be set to allow spray adjustment to actual drying conditions across the applicable... 

---