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Psychrometrics



S.Seifullin Kazakh Agro-Technical University

Technical faculty

Department of Agricultural and post-harvesting Machines

LABORATORY WORK

) on subject) on subjecton subject «Adjustment and linkage of Agricultural machines»

Theme: Grain drying

Astana 2012

Considered and approved at the meeting of Methodological Council of S.Seifullin Kazakh Agro-Technical University Protocol №___ «___»_______________2012 «APPROVED» Chairman of Methodological Council of S.Seifullin Kazakh Agro-Technical University Protocol №___ _____________A.M.Abdyrov «___»_______________2012  

 

Authors: Yeskhozhin К. – Candidat of Techical Sciense, Associate Professor of the Department of Agricultural and post-harvesting Machines.

 

 

Educational-methodical __________ is designed for training on discipline "Fundamentals of tractors" for students on specialty 5B080600 - "Agricultural techniques and technology"

 

Educational-methodical instructions are made according to the working curriculum of specialty.

Methodical instructions are intended for students of specialties 5В080600 – « Agricultural techniques and technology» and includes laboratory occupation tasks and independent work, educational literature lists and control questions for self-examination.

 

Reviewers: Doctor of Technical Science, Professor S.O. Nukeshev;

 

Reviewed and recommended at a meeting of the Department of Agricultural and post-harvesting Machines.

Protocol № ___, of "__" ___________ 2012.

 

Reviewed and recommended at a meeting of the methodical commission of Technical Faculty.

Protocol № ___, of "__" ___________ 2012.

Grain drying is a process of simultaneous heat and moisture transfer. To adequately understand the operation of drying, the reader has to be acquainted with the basics of psychrometrics, equilibrium moisture content, airflow, and drying rate. These are the topics briefly reviewed in this subsection.

 

Psychrometrics

Air is the medium in which grain is dried. The major physical properties of air that affect the drying rate of grains are the relative humidity or humidity ratio, the dry-bulb temperature, the specific volume, and the enthalpy.

The relative humidity (RH) of air is the ratio of the vapor pressure of the water molecules in the air to the saturated vapor pressure at the same temperature. The relative humidity usually is expressed as a percentage. A second term expressing the moisture content of the air is the humidity ratio (W), the mass of water vapor per unit mass of dry air (kg/kg).

Figure 1.9. Psychrometric chart in 0–50C temperature range and

at a barometric pressure of 101.325 kPa.

 

The dry-bulb temperature (T) of air is the temperature measured with an ordinary thermometer. If the term temperature is used without a prefix, dry-bulb temperature is implied. Another temperature commonly used in grain drying is the wet-bulb temperature (it is measured by covering the bulb of a thermometer with a wet wick). Knowledge of the dry-bulb and wet-bulb temperatures allows rapid determination of the relative humidity of the air on a psychrometric chart (see Fig. 1.9). The temperatures of air are expressed in degrees Centigrade (C).

The specific volume (v) of moist air is the volume per unit mass of dry air and is expressed in cubic meters per kilogram of dry air. The power required by the fan on a drying system is affected by the specific volume of the drying air.

Figure 1.10. Psychrometric chart in 10–120C temperature range and

at a barometric pressure of 101.325 kPa.

 

The enthalpy (h) of moist air is the energy content per unit mass of dry air above a certain reference temperature (usually 0C). It is denoted in kilojoules per kilogram of dry air. Determination of the burner size in a dryer requires knowledge of the enthalpy values of the air before and after heating.

Equations and computer packages have been developed for the calculation of the psychrometric properties of air. Before the evolution of computer technology, the so-called psychrometric charts for moist air were developed to facilitate the determination of the psychrometric properties. Figures 1.9 and 1.10 show psychrometric charts in the 1 to 50C and 10 to 120C temperature ranges, respectively.

The horizontal axis on the psychrometric chart represents the dry-bulb temperature; the humidity ratio serves as the vertical axis. The curved lines on the chart represent constant relative humidity values. Figure 1.11 illustrates how the various psychrometric values of moist air can be determined once two properties (e.g., dry-bulb temperature and RH) are known.

Figure 1.11. Illustration of the use of the psychrometric chart

 

The process of grain drying is represented on a skeletal psychrometric chart in Fig. 1.12. As the air passes through the heater, its dry-bulb temperature and enthalpy increase. The relative humidity and humidity ratio of the air increase as it passes through the grain, while the dry-bulb temperature decreases and the enthalpy remains almost constant.

Equilibrium Moisture Content.The equilibrium moisture content (EMC) of a grain species is the moisture content to which the grain will dry after it has been exposed to the drying air for an infinite period of time. The EMC of a grain sample is a function of the air temperature and RH, and of the grain species and (to some degree) of the sample history. Table 1.14 gives the EMC values of some grains at 25C.

Plotting the EMC values of a grain species at a specific temperature versus the RH of the surrounding air results in a sigmoid curve. This is illustrated for three grains in Fig. 1.13.

Figure 1.12. Representation on the psychrometric chart of the process of adiabatic drying.

 

Figure 1.13. Equilibrium moisture-content curves of hard wheat, maize, and soybeans at 16.7C.

 

The relationship between the EMC and the air conditions can be represented by the following empirical EMC equation:

where a, b, and c are product constants, T is the temperature (C), RH is the relative humidity (decimal), and Meq is the equilibrium moisture constant (decimal dry basis). Table 1.15 contains the product constants in Eq. (1.1) for selected grains.

 

Equilibrium moisture content of selected grains at 25C (percentage wet basis)

Table 1.14.

  20% Relative Humidity 40% Relative Humidity 60% Relative Humidity
Maize 7.1 10.0 12.4
Rough rice 6.5 9.4 12.2
Soybeans 5.3 6.9 9.7
Wheat (hard red) 7.2 9.9 12.1

 

Airflow.As grain kernels lose moisture during the drying process, the evaporated water is carried by a flow of air from the grain bed in the form of water vapor. To properly design and operate a grain-drying system, the principles of air movement must be under­stood, especially as it relates to static pressure, fan characteristics, and system operating condition.

 




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