CH2020, Project Report,

Department of Chemical Engineering, IIT Madras 1

DRYING

Sam David C P, CH10B056

Safeer Rahman, CH10B052

Shivani Patel, CH10B101

Neha P., CH10B044

Rajesh, CH10B069

Tarun Guntuka, CH10B082

Department of Chemical Engineering,

Indian Institute of Technology Madras

Chennai 600036

Assigned paper: 1. Authors: H.S.F. Awadalla , A.F. El-Dib , M.A. Mohamad , M.

Reuss ,H.M.S. Hussein

2. Title: Mathematical modeling and experimental verification of

wood drying process

3. Journal: Energy conservation and management

4. Volume: 45

5. Page range: 197-207

6. Year: 2004

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Department of Chemical Engineering, IIT Madras 2

1. Applications / Motivations of the paper

Industrial/practical applications cited in the paper

1. To make a number of products in building construction

2. Applied in furniture industry

Motivations of the study, given in the paper

1. The possibility of producing a high quality product at low cost in a solar powered dryer, or

optimizing drying schedules to reduce drying time and increase product quality.

2. To develop a computer simulation model for a solar timber dryer and a solar dehumidification

dryer.

3. To develop an optimized solar kiln plus heating system

2. Related topics from class discussion / assignments

1. Energy rate balance.

The energy rate balance (kW) of a drying air segment adjacent to the wood segment (0)

throughout the wood board is calculated.

2. Momentum rate balance.

The mass rate balance (kg/s) of wood segment (0) throughout the wood board is calculated.

3. Diffusion from a slab

This is a case of one dimensional diffusion from a slab of finite dimensions (thickness is

smaller than its width) at steady state.

3. Definitions of keywords / phrases used in paper

Dryer: Device used to remove moisture from the wood.

Ambient temperature: The temperature surrounding the object.

EMC: Equilibrium moisture content.

Pr (Prandtl number) : the ratio of momentum diffusivity to thermal diffusivity.

Convection: It is the concerted, collective movement of ensembles of molecules within fluids.

Conduction: It is the transfer of heat between substances that are in direct contact with each

other.

Humidity ratio: It is the ratio between actual mass of water vapour present in moist air to the

mass of dry air.

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Department of Chemical Engineering, IIT Madras 3

4. Governing equations used in the paper

Problem specification

1. The wood stack inside the drying chamber is divided into (m) columns in the air flow direction.

2. For each column, each wood board is divided into (n) segments from its surface to its center,

while the drying air volume between wood boards in each column is divided into two segments.

3. The changes of temperature and moisture content of wood segments in each column are in one

dimension because the thickness of the wood segments is small compared to its width.

4. The change of the drying air temperature between wood boards in the wood stack is one

dimensional in the flow direction, while it is constant in each column.

5. Because of the heterogeneous structure of wood, average values of the physical properties of

wood, such as sorption isotherms and diffusion coefficient, are independent of the position in the

structure.

6. The density of the drying air in each column is constant.

7. As the thickness of wood segments is small compared to their width, the heat and mass transfer

between drying air and the sides of wood segments can be neglected.

8. Spruce boards having dimensions of 0.5 m* 0.1 m*0.025 m were used.

9. The drying air velocity was kept constant at 3 m/s during the drying process.

Balance equations, initial and boundary conditions

Balance Equations:

ENERGY BALANCE EQUATION:

cp : specific heat, kJ/kgK

ρ : density, kg/m3

V : volume, m3

v : air velocity, m/s

Q evap (kW) : evaporation heat transfer rate

Qconv (kW) : convection heat transfer rate

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Using the energy balance equation, the following equation can be derived for the wood segment (0):

MASS RATE BALANCE:

Where,

y : humidity ratio of air, kgwv/kga;dr

ṁ : mass flow rate per unit surface area, kg/m2 s

Initial and boundary conditions:

1. The outlet temperature of the drying air from each column is equal to the temperature of the

drying air in this column at the previous time step.

2. The initial temperature of the wood segments is constant at ambient temperature at the start of

simulation.

3. Initial moisture content was 0.35 kgwa/kgw;dr.

Correlations, overall trasfer coefficient

The mass transfer coefficient (hD) (m/s) can be calculated from the convection heat transfer

coefficient (h) (kW/m2 K).

Le : Lewis number, dimensionless p : atmospheric pressure/partial pressure, kPa

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5. Most important result graph from the paper

Comparison between transient previous experimental and present theoretical results of wood average moisture content.

6. Discussion based on the above figure

1. At steady state and transient conditions, the previous experimental results and their

corresponding computational ones from the present model have the same trend.

2. The transient experimental and theoretical results of wood average moisture content have similar

trends for drying features, where the curves exhibit hard drying at the day, followed by

humidification of wood at night, when the EMC of wood is greater than the moisture content of

wood.

3. The deviation of the present theoretical results of wood average moisture content from the

previous experimental ones ranges from) 7% to +13%. This deviation may be attributed to the

condensation of moisture at the wood surface at night.

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Department of Chemical Engineering, IIT Madras 6

7. Conclusions from the paper

1. Computational results from the present analysis show considerable agreement with the previous

experimental and theoretical results at steady state and transient condition;

2. The present simulation model proved to be an effective tool for the design of a solar timber dryer

and the prediction of its moisture content behavior.

8. References

1. Krischer O, Kr€oll AJ. Trocknungstechnik. Berlin, Germany: Springer-Verlag; 1992.

2. Koponen H. Drying 87. Berlin, Germany: Springer-Verlag; 1987.

3. [5] Duffie NA, Close DJ. Solar Energy 1978;20:405–11.