Topics in Processing Dr. C. L. Jones Biosystems and Ag. Engineering

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

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

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

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

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 = 0.837 + 3.348 M above freezing Cp = 0.837 + 1.256 M below freezing Equation 8.4 pg 220 takes into account the composition of the solids Dr. C. L. Jones Biosystems and Ag. Engineering

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

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. α = k/(ρcp) Dr. C. L. Jones Biosystems and Ag. Engineering

Thermal Properties, Moisture Diffusivity Surface Heat Transfer Coefficient, h, (W/m2K, BTU/h ft2 F) What a flowing stream of liquid or gas surrounds a solid, the solid will eventually change to the temperature of the fluid. h relates the rate of heat transfer across the surface boundary to the total surface area and the difference between the surface T and the fluid T. Depends on fluid velocity, solid’s surface characteristics, size and shape of solid, fluid properties Dr. C. L. Jones Biosystems and Ag. Engineering

Thermal Properties, Moisture Diffusivity Latent Heat, L, (kJ/kg or BTU/lb) Heat that is exchanged during a change in phase Dominated by the moisture content of foods Requires more energy to freeze foods than to cool foods (90kJ removed to lower 1 kg of water from room T to 0C and 4x that amount to freeze food) 420 kJ to raise T of water from 0C to 100C, 5x that to evaporate 1 kg of water. Heat of vaporization is about 7x greater than heat of fusion (freezing) Therefore, evaporation of water is energy intensive (concentrating juices, dehydrating foods…) Dr. C. L. Jones Biosystems and Ag. Engineering

Thermal Properties, Moisture Diffusivity Latent Heat, L, (kJ/kg or BTU/lb) Determine L experimentally when possible. When data is not available (no tables, etc) use…. L = 335 Xw where Xw is weight fraction of water Many fruits, vegetables, dairy products, meats and nuts are given in ASHRAE Handbook of Fundamentals Dr. C. L. Jones Biosystems and Ag. Engineering

Thermal Properties, Moisture Diffusivity Enthalpy, L, (kJ/kg or BTU/lb) Heat content of a material. Combines latent heat and sensible heat changes ΔQ = M(h2-h1)…amount of heat to raise a product from T1 to T2 ASHRAE Handbook of Fundamentals When data is not available use eqtn. 8.15 pg 230. Dr. C. L. Jones Biosystems and Ag. Engineering

Thermal Properties, Moisture Diffusivity Example 8.3: Calculate the amount of heat which must be removed from 1 kg of raspberries when their temperature is reduced from 25C to -5C. Assume that the specific heat of raspberries above freezing is 3.7 kJ/kgC and their specific heat below freezing is 1.86 kJ/kgC. The moisture content of the raspberries is 81% and the ASHRAE tables for freezing of fruits and vegs. Indicate that at -5C, 27% will not yet be frozen. Dr. C. L. Jones Biosystems and Ag. Engineering

Thermal Properties, Moisture Diffusivity Sample questions for this section. Problem 1: do 8.1 in your book (see example of a solution on next slide Problem 2: Determine the amount of heat removed from 2 kg of blueberries when cooled from 28C to -7C. Assume MC of 83% and at -7C, 27% won’t be frozen. Dr. C. L. Jones Biosystems and Ag. Engineering

Thermal Properties, Moisture Diffusivity Sample solutions for problem #1 A thawed turkey (whole) is placed in a turkey fryer. The turkey is at 3 C while the oil is at 400 C. The turkey is at 3 C while the oil is at 400 C. Assuming the oil temperature is maintained at 400 C, the temp. at the surface of the turkey (boundary condition) will remain at 400C throughout the frying process. This assumes that the heat transfer coefficient across the oil-turkey fry boundary does not limit the heat transfer process. The thermal conductivity of the turkey can be estimated from the perpendicular model since the meat is fibrous and the thermal conductivity will be directionally dependent. The turkey can be modeled as a finite cylinder. Dr. C. L. Jones Biosystems and Ag. Engineering