Drying calculation

For drying calculations, it is convenient to express Eq. (12-25) in terms of the decrease in moisture content rather than in the quantity of water evaporated. For evaporation from a tray of wet material, if no change in volume during diying is assumed, Eq. (12-25) becomes... 

It therefore appears that other hydrocarbons than medrane take part, but to a lesser extent. The other species which were considered in this calculation were SiCl, SiH4, CH3CI, SiHs, H, Si, Sia, Cl and CI2, all of which are gaseous at this temperature. It is difficult to predict before dris calculation what will be the major species, and attempts to limit die number of compounds which are included is to some extent subjective, but this can be conected by using the graphical method as a preliminaty. 

Drew. T. B. 477, 497, 499, 564, 565 Drift factor 578, 580 Drops, mass transfer 651 Dropwise condensation 476 DRTlNA, P. 307.312 Drying calculations, example 749 Duckworth, R. A. 209. 228 Duct, non-circular 86... 

Figure 3. Dust content (dry), calculated vi. observed. Key ------------, theoretical slope...

Comparing the approaches of physicists (R. Hooke) and mathematicians (himself), Newton wrote that Mathematicians that find out, settle do all the business must content themselves with being nothing but dry calculators drudges another that does nothing but pretend grasp at all things must carry away all the invention as well of those that were to follow him as of those that went before. ... 

Fig. 8 Variation of glass transition temperature and product temperature for moxalactam di-sodium during secondary drying. (Calculated from data in Refs. " °()...

Drying by Steam and Hot Water.—In dryers of this type, the heat is transferred through metallic surfaces, and where liquids are to be dried, calculations may be based on the equations given in the previous chapter on Heat Transmission. But where moisture is to be removed from solids, we have to resort again to experiments and tests in order to determine the capacity of a certain... 

Scale-up of Batch Dryers We can use the same equations as before but base drying time on an experimental value rather than one obtained from an unhindered drying calculation. 

As with many drying calculations, the most reliable design method is to perform experimental tests and to scale up. By measuring performance on a single tray with similar layer depth, air velocity, and temperature, the SDR (specific drying rate) concept can be applied to give the total area and number of trays required for the full-scale dryer. 

In fluid phases diffusion is governed by concentration differences expressed in mole fractions. In a wet solid, however, the term mole fraction may have little meaning, and for ease in drying calculations the moisture content is nearly always expressed in mass of water per unit mass of bone-dry solid. This practice is followed throughout the present chapter,... 

It is X, rather than X, that is of interest in drying calculations. 

The value of equilibrium moisture content, for many materials, depends on the direction in which equilibrium is approached. A different value is reached when a wet material loses moisture by desorption, as in drying, from that obtained when a dry material gains it by adsorption. For drying calculations the desorption values are preferred. In the general case, the equilibrum moisture content reached by losing moisture is higher than that reached by adsorbing it. 

In drying calculation, it is more convenient to use the mean values of heat capacity over a finite temperature step ... 

Soininen M., 1986, A perspectively transformed psychromet-ric chart and its applications to drying calculations. Drying Technol, 4(2), 295-305. 

One usually has the initial air conditions specified in terms of the relative humidity and temperature. Drying calculations are carried out in terms of the humidity ratio. Atypical use of psychrometric equations may follow the following sequences use T to calculate P, use RH and P to calculate P, and use P and P to calculate the humidity ratio H. For more equations relating other properties of moist air, see American Society of Agricultural Engineers (ASAE) standards [25]. 

A mathematical model with which the dryer and process parameters can be studied is of extreme utility to the industry. A reliable model often will prevent or minimize costly mistakes in prototype development. The model also can be utilized for the control of process, specifically in adaptive and feed-forward control strategies. With the proliferation of low cost and powerful computers, the simulation models are useful. In this chapter, the emphasis is on the presentation of formulas and equations rather than the conventional charts and graphs in drying calculations. 

Drying calculations are based on air and material properties and conditions. The air properties were discussed in Section 24.3. The material properties will be reviewed in this section. These properties are divided into two groups thermal properties and physical properties. Physical properties are important in the selection of the right drying system for the food material. 

Free software available on the Internet for humidity and psychrometric calculations is of limited value for practical drying calculations such as dryer flowsheet calculations since drying problems are much more complicated than simple psychrometric calculations. 

Evaporation Losses of Water in Irrigation Ditch. Water at 25°C is flowing in a covered irrigation ditch below ground. Every 100 ft there is a vent line 1.0 in. inside diameter and 1.0 ft long to the outside atmosphere at 25°C. There are 10 vents in the 1000-ft ditch. The outside air can be assumed to be dry. Calculate the total evaporation loss of water in lb yd. Assume that the partial pressure of water vapor at-the surface of the water is the vapor pressure, 23.76 mm Hg at 25°C. Use the diffusivity from Table 6.2-1. 

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