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Questions and Answers
Heat transfer always occurs from a cooler body to a hotter body.
Heat transfer always occurs from a cooler body to a hotter body.
False
The rate of heat transfer is dependent on the temperature difference between two bodies.
The rate of heat transfer is dependent on the temperature difference between two bodies.
True
In conduction, molecular energy is exchanged between neighboring molecules directly.
In conduction, molecular energy is exchanged between neighboring molecules directly.
True
The Fourier equation of heat conduction includes parameters such as area and temperature gradient.
The Fourier equation of heat conduction includes parameters such as area and temperature gradient.
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Convection is one of the modes of heat transfer that does not involve molecular movement.
Convection is one of the modes of heat transfer that does not involve molecular movement.
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To increase the rate of heat transfer, reducing the temperature difference is recommended.
To increase the rate of heat transfer, reducing the temperature difference is recommended.
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Resistance to heat flow always facilitates heat transfer in food processing applications.
Resistance to heat flow always facilitates heat transfer in food processing applications.
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Heat transfer can occur through conduction, radiation, and convection.
Heat transfer can occur through conduction, radiation, and convection.
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A surface with high emissivity emits less heat through radiation than a surface with low emissivity.
A surface with high emissivity emits less heat through radiation than a surface with low emissivity.
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Dull black surfaces have an emissivity approximately equal to 0.9.
Dull black surfaces have an emissivity approximately equal to 0.9.
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Polished metal surfaces usually have an emissivity that is about or below 0.05.
Polished metal surfaces usually have an emissivity that is about or below 0.05.
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The net heat transferred from one surface to another depends only on their temperatures.
The net heat transferred from one surface to another depends only on their temperatures.
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Rough unpolished metal surfaces have emissivities that vary from 0.7 to 0.25.
Rough unpolished metal surfaces have emissivities that vary from 0.7 to 0.25.
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The equation for net heat exchange for a small body surrounded by uniform temperature is given by q = Aεσ(T1^4 - T2^4).
The equation for net heat exchange for a small body surrounded by uniform temperature is given by q = Aεσ(T1^4 - T2^4).
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For two parallel surfaces, the total energy absorbed is not dependent on their emissivities.
For two parallel surfaces, the total energy absorbed is not dependent on their emissivities.
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The radiation heat-transfer coefficient hr plays no role in calculating radiant heat transfer.
The radiation heat-transfer coefficient hr plays no role in calculating radiant heat transfer.
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Thermal conductivity is a material property that describes the ability of a material to resist heat flow.
Thermal conductivity is a material property that describes the ability of a material to resist heat flow.
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Metals like copper and aluminum have low thermal conductivity, making them poor conductors of heat.
Metals like copper and aluminum have low thermal conductivity, making them poor conductors of heat.
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Ice has a higher thermal conductivity than water.
Ice has a higher thermal conductivity than water.
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A low thermal conductivity means that a material transfers heat efficiently.
A low thermal conductivity means that a material transfers heat efficiently.
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The thermal conductivity of air is approximately 0.024 J m-1 s-1 °C-1 at 0°C.
The thermal conductivity of air is approximately 0.024 J m-1 s-1 °C-1 at 0°C.
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Insulating materials have thermal conductivities in the range of approximately 0.03-0.06 J m-1 s-1 °C-1.
Insulating materials have thermal conductivities in the range of approximately 0.03-0.06 J m-1 s-1 °C-1.
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The heat conductance of a material decreases as the thickness increases.
The heat conductance of a material decreases as the thickness increases.
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Thermal diffusivity is defined as the product of thermal conductivity and density.
Thermal diffusivity is defined as the product of thermal conductivity and density.
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Heat will flow from the cooler face of a slab to the hotter face.
Heat will flow from the cooler face of a slab to the hotter face.
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The thermal conductivity of cork is 0.042 J m-1 s-1 °C-1 in the temperature range specified.
The thermal conductivity of cork is 0.042 J m-1 s-1 °C-1 in the temperature range specified.
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Heat conductance is measured in units of J m-2 s-1 °C-1.
Heat conductance is measured in units of J m-2 s-1 °C-1.
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In series heat conduction, the same temperature difference occurs across each layer of material.
In series heat conduction, the same temperature difference occurs across each layer of material.
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The equation q = AΔT1k1/x1 applies only when the areas of the layers are different.
The equation q = AΔT1k1/x1 applies only when the areas of the layers are different.
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The same quantity of heat must pass through each layer in steady-state conditions.
The same quantity of heat must pass through each layer in steady-state conditions.
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The values of conductance for each layer can vary significantly regardless of the materials used.
The values of conductance for each layer can vary significantly regardless of the materials used.
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For a thickness x of material with thermal conductivity k, the conductance is calculated using the formula k/x.
For a thickness x of material with thermal conductivity k, the conductance is calculated using the formula k/x.
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The rate of heat transfer through a concrete wall can be calculated using its thermal conductivity and temperature difference.
The rate of heat transfer through a concrete wall can be calculated using its thermal conductivity and temperature difference.
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Natural convection is caused by external forces acting on the fluid.
Natural convection is caused by external forces acting on the fluid.
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The thermal conductivity of steel is higher than that of concrete.
The thermal conductivity of steel is higher than that of concrete.
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The rate of heat transfer for convection can be described using Newton's Law of Cooling as Q=hA(Ts−T∞).
The rate of heat transfer for convection can be described using Newton's Law of Cooling as Q=hA(Ts−T∞).
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In heat exchangers, heat energy is only transferred in one direction.
In heat exchangers, heat energy is only transferred in one direction.
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In forced convection, the movement of fluid is caused solely by density changes within the fluid.
In forced convection, the movement of fluid is caused solely by density changes within the fluid.
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The surface heat transfer coefficient can be calculated using the rate of heat transfer and the temperature difference.
The surface heat transfer coefficient can be calculated using the rate of heat transfer and the temperature difference.
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The log mean temperature difference, denoted as ΔTm, is calculated as ΔTm = (ΔT1 - ΔT2) / ln(ΔT1/ΔT2).
The log mean temperature difference, denoted as ΔTm, is calculated as ΔTm = (ΔT1 - ΔT2) / ln(ΔT1/ΔT2).
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The specific heat of milk is 2500 J kg-1 °C-1.
The specific heat of milk is 2500 J kg-1 °C-1.
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Heat transfer in a jacketed pan occurs only through conduction.
Heat transfer in a jacketed pan occurs only through conduction.
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A jacketed pan uses steam condensing in the vessel jacket as a common source of heat.
A jacketed pan uses steam condensing in the vessel jacket as a common source of heat.
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The internal surface of the oven is cooler than the air entering the oven.
The internal surface of the oven is cooler than the air entering the oven.
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The buoyant effect causes cooler, denser fluid to rise in natural convection.
The buoyant effect causes cooler, denser fluid to rise in natural convection.
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An overall heat transfer coefficient of 900 J m-2 s-1 °C-1 indicates a low efficiency in heat transfer.
An overall heat transfer coefficient of 900 J m-2 s-1 °C-1 indicates a low efficiency in heat transfer.
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To cool a fluid effectively, its final temperature must always be below the temperature of the surrounding fluid.
To cool a fluid effectively, its final temperature must always be below the temperature of the surrounding fluid.
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A heat exchanger can only transfer heat between solids.
A heat exchanger can only transfer heat between solids.
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Study Notes
Central Luzon State University
- Located in Science City of Munoz, Nueva Ecija, Philippines
- Established in 1907
- Postal Code: 3120
Food Process Engineering (ABEN 4510)
- Instructor: Melba Domes Denson
- Department of Agricultural and Biosystems Engineering
- College of Engineering
Heat Transfer in Food Processing
- Cooking, baking, drying, sterilizing/pasteurizing, freezing, and other processes involve heat transfer.
- Heat transfer is a dynamic process where heat moves spontaneously from a warmer body to a cooler one.
- The rate of heat transfer depends on the difference in temperature; larger differences lead to faster transfer.
- Heat transfer through a medium experiences resistance.
Modes of Heat Transfer
-
Conduction: Heat transfer through direct contact; molecules with higher energy transfer energy to those with lower energy (e.g., heat transfer through refrigerated store walls).
- Fourier equation of heat conduction: dQ/dt = kA dT/dx (where dQ/dt is heat transfer rate, k is thermal conductivity, A is area, and dT/dx is temperature gradient)
- Thermal conductivity: a material property describing a material's ability to conduct heat. In many applications it's considered constant for a given material but changes slightly with temperature.
- Higher thermal conductivity = material transfers heat efficiently
- Lower thermal conductivity = material resists heat flow better (e.g., copper/aluminum vs wood/rubber)
- Example: Heat transferred through walls of refrigerators.
-
Radiation: Heat transfer through electromagnetic waves (e.g., food being heated under red-hot electric resistance heaters); depends on temperature and wavelength.
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Stefan-Boltzmann Law: q = εσAT⁴
- q = radiation heat transfer rate
- ε = emissivity of the body
- σ = Stefan-Boltzmann constant
- A = surface area
- T = absolute temperature (K)
- Emissivity is the measure of how well a body emits radiation (compared to a perfect blackbody which has ε = 1). Dull black surfaces (ε ≈ 1), most food surfaces ε ≈ .9, rough/unpolished metal (ε ≈ .7 - .25), polished metal (ε ≈ 0.05)
-
Stefan-Boltzmann Law: q = εσAT⁴
-
Convection: Heat transfer in fluids (liquids or gases) caused by molecular movement, density changes (e.g., density of fluid changes cause heat transfer in jacketed pans). - Natural Convection: Driven by density differences due to temperature, creates a continuous flow. - Forced Convection: External forces (like fans/pumps) move the fluid.
Convection Heat Transfer Equation
- For convection, Q=hA(Ts – T∞) - Q = heat transfer rate - h = convection heat transfer coefficient - A = heat transfer area - Ts = surface temperature - T∞ = surrounding fluid temperature
Heat Transfer Applications
-
Heat Exchangers: Equipment where one fluid transfers heat to another.
- Types: parallel/counterflow/crossflow
- q = UA∆Tm (where ∆Tm is log mean temperature difference)
- Thermal Processing: Controlled use of heat for various effects in food (e.g., sterilization, pasteurization).
- Refrigeration/Chilling/Freezing: Reduction of food temperature to desired levels; chilling (above freezing point), freezing (latent heat removal).
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Description
Test your understanding of heat transfer principles, including conduction, convection, and radiation. This quiz covers key concepts such as the Fourier equation, emissivity, and the factors affecting the rate of heat transfer in various applications.
