1. Psychrometrics – Lecture 3
Assignment:
Read Chapter 8 in Stroshine Book
Dr. C. L. Jones
Biosystems and Ag. Engineering

2. Thermal Properties and Moisture Diffusivity
Assignment:
Read Chapter 8 in Stroshine Book
Dr. C. L. Jones
Biosystems and Ag. Engineering

3. Thermal Properties, Moisture Diffusivity
Heat is transferred by conduction, convection or radiation
Heat may be conducted within the product
Heat transfer by forced convection between the product and a moving liquid surrounding the product
Dr. C. L. Jones
Biosystems and Ag. Engineering

4. Thermal Properties, Moisture Diffusivity
Basic thermal properties
Specific heat
Thermal conductivity
Thermal diffusivity
Thermal expansion coefficient
Surface heat transfer coefficient
Heat may be conducted within the product
Dr. C. L. Jones
Biosystems and Ag. Engineering

5. Thermal Properties, Moisture Diffusivity
Specific heat, cp , (kJ/kg-K, BTU/lb-F, cal/g-K)
Amount of heat (joules, BTU’s or calories) needed to increase the temperature of a unit of mass by one degree
Q = amount of heat added to mass to raise temp. from T2 to T1
Q = Mcp(T2-T1)
Dr. C. L. Jones
Biosystems and Ag. Engineering

6. Thermal Properties, Moisture Diffusivity
Table 8.1 gives cp for food and ag products
Materials containing mostly water have a cp close to water (4.18 kJ/kg-K, 1 BTU/lb-F, 1 cal/g-K)
Oils and fats are approx. ½ that of water
Ice is ½ of water…frozen foods take less heat to increase T than after it is thawed!!!
Cp = M above freezing
Cp = M below freezing
Equation 8.4 pg 220 takes into account the composition of the solids
Dr. C. L. Jones
Biosystems and Ag. Engineering

7. Thermal Properties, Moisture Diffusivity
Thermal Conductivity, k, (W/m-K, BTU/h ft-K)
Measure of material’s ability to transmit heat,…. quantity of heat Q that will flow across a unit cross sectional area A per unit of time, t
dQ/dt = -kA (dT/dx)
Table 8.2, pg. 221
The greater the proportion of water, the higher the thermal conductivity.
Porosity and fiber orientation make a difference, too.
See wood in Table 8.3 for fiber direction differences.
Dr. C. L. Jones
Biosystems and Ag. Engineering

8. Thermal Properties, Moisture Diffusivity
Thermal Diffusivity, α, (m2/sec)
Material’s ability to conduct heat relative to its ability to store heat
Estimate the thermal diffusivity of a peach at 22 C.
Dr. C. L. Jones
Biosystems and Ag. Engineering

9. Thermal Properties, Moisture Diffusivity
Thermal Diffusivity, α, (m2/sec)
Material’s ability to conduct heat relative to its ability to store heat
Estimate the thermal diffusivity of a peach at 22 C.
Dr. C. L. Jones
Biosystems and Ag. Engineering