Lecture 12: Thermal Properties, Moisture Diffusivity Chpt 8

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Lecture 12 Thermal Properties, Moisture
DiffusivityChpt 8

• Test Monday 3/2, 230 when youre finished
• Lecture Wed 3/4 and Friday 3/6 230 320
  123AH
• See schedule for topics
• Lab reports due next Wednesday 3/4

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Lecture 12 Thermal Properties, Moisture
DiffusivityChpt 8

• Processing and Storage of Ag Products
• Heating
• Cooling
• Combination of heating and cooling
• Grain dried for storage
• Noodles dried
• Fruits/Vegetables rapidly cooled
• Vegetables are blanched, maybe cooked and canned
• Powders such as spices and milk dehydrated
• All include heat transfer and are dictated by
  thermal properties of material
• Generally diffusion of water in or out is involved

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Lecture 12 Thermal Properties, Moisture
DiffusivityChpt 8

• Heat is transferred by
• Conduction temperature gradient exists within a
  bodyheat transfer within the body
• Convection Heat transfer from one body to
  another by virtue that one body is moving
  relative to the other
• Radiation transfer of heat from one body to
  another that are separated in space in a
  vacuum. (blackbody heat transfer)
• Well consider
• Conduction w/in the product
• Convection transfer by forced convection from
  product to moving fluid
• Moisture moves similar to heat by conduction
• Moisture diffusivity
• Volume change due to moisture content change

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Lecture 12 Thermal Properties, Moisture
DiffusivityChpt 8

• Terms
• Specific heat
• Thermal conductivity
• Thermal diffusivity
• Thermal expansion coefficient
• Surface heat transfer coefficient
• Sensible and Latent heat
• Enthalpy

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Lecture 12 Thermal Properties, Moisture
DiffusivityChpt 8

• Specific heat Amount of heat required to raise
  the temp. of one unit of mass one degree.
• Cp specific heat at constant pressure
• Cp 4.18 kJ/kg-K 1.00 BTU/lb-R1.00 cal/g-K for
  water (unfrozen)
• oils and fats ½ H2O See Table 8.1 pg.
  219
• grains, powders ¼ - 1/3 H2O
• ice ½ H2O
• Good list
• http//www.engineeringtoolbox.com/specific-heat-ca
  pacity-food-d_295.html
• Q quantity of heat required to change
  temperature of a mass
• Q Mcp(T2-T1)
• M mass or weight

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Lecture 12 Thermal Properties, Moisture
DiffusivityChpt 8

• For liquid H2O
• Cp 0.837 3.348 M above freezing
• For solid H2O
• Cp 0.837 1.256 M below freezing

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Lecture 12 Thermal Properties, Moisture
DiffusivityChpt 8

• Thermal Conductivity
• measure of ability to transmit heat
• dQ/dt -kA (dT/dx)
• K coefficient of thermal conductivity
• W/mK, Btu/h ftF,
• 1 Btu/h ft F 1.731 W/m K
• Greater the water content, the greater the
  thermal conductivity
• Tables 8.2 and 8.3

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Lecture 12 Thermal Properties, Moisture
DiffusivityChpt 8

• If we dont know t-conductivity, approximate
  using...
• K VwKw VsKs
• K KwXw Ks(1-Xw) where X decimal fraction
• so K f(all the constituent volumes)
• Example 8.1 pg 224

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Lecture 12 Thermal Properties, Moisture
DiffusivityChpt 8

• Thermal Diffusivity, a, (m2/sec or ft2/sec)
• Materials ability to conduct heat relative to
  its ability to store heat
• a k/(?cp)
• Estimate the thermal diffusivity of a peach at 22
  C.

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Lecture 12 Thermal Properties, Moisture
DiffusivityChpt 8

• Surface Heat Transfer Coefficient, h
• Placed in a flowing stream of liquid or gas, the
  solids T will change until it eventually reaches
  equilibrium with the fluid
• Q/T hA(T2 T1)
• h is determined experimentally
• Look for research that matches your needs.
  (bottom of pg 227)

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Lecture 12 Thermal Properties, Moisture
DiffusivityChpt 8

• Sensible heat Temperature that can be sensed by
  touch or measured with a thermometer.
  Temperature change due to heat transfer into or
  out of product
• Latent heat transfer of heat energy with no
  accompanying change in temperature. Happens
  during a phase change...solid to liquid...liquid
  to gas...solid to gas

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Lecture 12 Thermal Properties, Moisture
DiffusivityChpt 8

• 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)

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Lecture 12 Thermal Properties, Moisture
DiffusivityChpt 8

• 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

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Lecture 12 Thermal Properties, Moisture
DiffusivityChpt 8

• Enthalpy, h, (kJ/kg or BTU/lb)
• Heat content of a material.
• Combines latent heat and sensible heat changes
• ?Q 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.
  ?h M cp(T2 T1) MXw L

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Lecture 12 Thermal Properties, Moisture
DiffusivityChpt 8

• 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.

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Lecture 12 Thermal Properties, Moisture
DiffusivityChpt 8

• Problem 1 Determine the amount of heat removed
  from 2 kg of sour cherries when cooled from 28C
  to -7C. Assume MC of 92.3 and at -7C, 27 wont
  be frozen.
• Problem 2 Estimate the thermal diffusivity of
  cheddar cheese at 22C.