Wet wood chips are dried in a continuous rotary dryer that operates at atmospher

Question

Wet wood chips are dried in a continuous rotary dryer that operates at atmospheric pressure. The chips enter at 19 degree C with a water content of 40 wt% and must leave with a moisture content of less than 15%. Hot air is fed to the dryer at a rate of 11.6 m^3(STP)/kg wet chips. To monitor the performance of the dryer by sampling the exiting chips and determining their moisture content directly would be a cumbersome procedure and almost impossible to automate. Instead, wet- and dry-bulb thermometers are mounted in both the inlet and outlet air lines, and the moisture content of the exiting chips is determined by a material balance. After the unit goes on-stream, the inlet dry-bulb temperature is found to be 100 degree C, and the wet-bulb temperature is low enough so that the moisture content of the air may be neglected. The dry-bulb temperature of the exiting air is found to be 38 degree C, and the wet-bulb temperature is 29 degree C. Use the psychrometric chart to calculate the absolute humidity (kg H_2O/kg dry air) and specific enthalpy (kJ/kg dry air) of the outlet air stream. Then calculate the mass of water in the exiting air per kilogram of wet chips fed, assuming dry air has a molecular weight of 29.0. Calculate the moisture content of the emerging chips and determine whether the design specification of less than 15% H_2O has been achieved. Suppose a sample of the moist chips leaving the dryer is taken, the moisture content is measured, and the result is significantly different from the value calculated in Part (b). First briefly state how the moisture content was probably measured (keep it simple); then list possible reasons for the difference between the two values; and finally state in which value you would have more confidence and explain why. If the unit is operating adiabatically and the heat capacity of the dry chips is 1.70 kJ/(kg middot degree C), what is the exit temperature of the chips? (In estimating the specific enthalpy of the entering air, recall that the reference temperature for dry air used in constructing the psychrometric chart of Figure 8.4-1 is 0 degree C.)

Explanation / Answer

c.

Various methods descried by the American Society for measuring the moisture content of wood. These include oven-dry methods.

In the solid wood products industry, The moisture content (MC) is often defined on a dry basis.

MC = Weight of water* 100/Weight of dry wood

To be able to measure moisture content precisely, Following our parameters are significant which are possible reasons for the difference in the values.
- the representativeness of the sample,
- the reliability of the instrument used for measuring,
- the compliance of the operational mode by the person who measures.

- Cost involved

There are also improved psychrometer to solve practical issues arising in the relative humidity measurement of challenging drying environments for manufacturing in agricultural and agri-food industries.

So conclusion is we need to have a confidence in the data obtained and it comes out with experience.

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