Heat Transfer Concepts Quiz
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Questions and Answers

Heat transfer always occurs from a cooler body to a hotter body.

False (B)

The rate of heat transfer is dependent on the temperature difference between two bodies.

True (A)

In conduction, molecular energy is exchanged between neighboring molecules directly.

True (A)

The Fourier equation of heat conduction includes parameters such as area and temperature gradient.

<p>True (A)</p> Signup and view all the answers

Convection is one of the modes of heat transfer that does not involve molecular movement.

<p>False (B)</p> Signup and view all the answers

To increase the rate of heat transfer, reducing the temperature difference is recommended.

<p>False (B)</p> Signup and view all the answers

Resistance to heat flow always facilitates heat transfer in food processing applications.

<p>False (B)</p> Signup and view all the answers

Heat transfer can occur through conduction, radiation, and convection.

<p>True (A)</p> Signup and view all the answers

A surface with high emissivity emits less heat through radiation than a surface with low emissivity.

<p>False (B)</p> Signup and view all the answers

Dull black surfaces have an emissivity approximately equal to 0.9.

<p>False (B)</p> Signup and view all the answers

Polished metal surfaces usually have an emissivity that is about or below 0.05.

<p>True (A)</p> Signup and view all the answers

The net heat transferred from one surface to another depends only on their temperatures.

<p>False (B)</p> Signup and view all the answers

Rough unpolished metal surfaces have emissivities that vary from 0.7 to 0.25.

<p>True (A)</p> Signup and view all the answers

The equation for net heat exchange for a small body surrounded by uniform temperature is given by q = Aεσ(T1^4 - T2^4).

<p>True (A)</p> Signup and view all the answers

For two parallel surfaces, the total energy absorbed is not dependent on their emissivities.

<p>False (B)</p> Signup and view all the answers

The radiation heat-transfer coefficient hr plays no role in calculating radiant heat transfer.

<p>False (B)</p> Signup and view all the answers

Thermal conductivity is a material property that describes the ability of a material to resist heat flow.

<p>False (B)</p> Signup and view all the answers

Metals like copper and aluminum have low thermal conductivity, making them poor conductors of heat.

<p>False (B)</p> Signup and view all the answers

Ice has a higher thermal conductivity than water.

<p>True (A)</p> Signup and view all the answers

A low thermal conductivity means that a material transfers heat efficiently.

<p>False (B)</p> Signup and view all the answers

The thermal conductivity of air is approximately 0.024 J m-1 s-1 °C-1 at 0°C.

<p>True (A)</p> Signup and view all the answers

Insulating materials have thermal conductivities in the range of approximately 0.03-0.06 J m-1 s-1 °C-1.

<p>True (A)</p> Signup and view all the answers

The heat conductance of a material decreases as the thickness increases.

<p>True (A)</p> Signup and view all the answers

Thermal diffusivity is defined as the product of thermal conductivity and density.

<p>False (B)</p> Signup and view all the answers

Heat will flow from the cooler face of a slab to the hotter face.

<p>False (B)</p> Signup and view all the answers

The thermal conductivity of cork is 0.042 J m-1 s-1 °C-1 in the temperature range specified.

<p>True (A)</p> Signup and view all the answers

Heat conductance is measured in units of J m-2 s-1 °C-1.

<p>True (A)</p> Signup and view all the answers

In series heat conduction, the same temperature difference occurs across each layer of material.

<p>False (B)</p> Signup and view all the answers

The equation q = AΔT1k1/x1 applies only when the areas of the layers are different.

<p>False (B)</p> Signup and view all the answers

The same quantity of heat must pass through each layer in steady-state conditions.

<p>True (A)</p> Signup and view all the answers

The values of conductance for each layer can vary significantly regardless of the materials used.

<p>False (B)</p> Signup and view all the answers

For a thickness x of material with thermal conductivity k, the conductance is calculated using the formula k/x.

<p>True (A)</p> Signup and view all the answers

The rate of heat transfer through a concrete wall can be calculated using its thermal conductivity and temperature difference.

<p>True (A)</p> Signup and view all the answers

Natural convection is caused by external forces acting on the fluid.

<p>False (B)</p> Signup and view all the answers

The thermal conductivity of steel is higher than that of concrete.

<p>True (A)</p> Signup and view all the answers

The rate of heat transfer for convection can be described using Newton's Law of Cooling as Q=hA(Ts−T∞).

<p>True (A)</p> Signup and view all the answers

In heat exchangers, heat energy is only transferred in one direction.

<p>False (B)</p> Signup and view all the answers

In forced convection, the movement of fluid is caused solely by density changes within the fluid.

<p>False (B)</p> Signup and view all the answers

The surface heat transfer coefficient can be calculated using the rate of heat transfer and the temperature difference.

<p>True (A)</p> Signup and view all the answers

The log mean temperature difference, denoted as ΔTm, is calculated as ΔTm = (ΔT1 - ΔT2) / ln(ΔT1/ΔT2).

<p>True (A)</p> Signup and view all the answers

The specific heat of milk is 2500 J kg-1 °C-1.

<p>False (B)</p> Signup and view all the answers

Heat transfer in a jacketed pan occurs only through conduction.

<p>False (B)</p> Signup and view all the answers

A jacketed pan uses steam condensing in the vessel jacket as a common source of heat.

<p>True (A)</p> Signup and view all the answers

The internal surface of the oven is cooler than the air entering the oven.

<p>True (A)</p> Signup and view all the answers

The buoyant effect causes cooler, denser fluid to rise in natural convection.

<p>False (B)</p> Signup and view all the answers

An overall heat transfer coefficient of 900 J m-2 s-1 °C-1 indicates a low efficiency in heat transfer.

<p>False (B)</p> Signup and view all the answers

To cool a fluid effectively, its final temperature must always be below the temperature of the surrounding fluid.

<p>True (A)</p> Signup and view all the answers

A heat exchanger can only transfer heat between solids.

<p>False (B)</p> Signup and view all the answers

Flashcards

Heat Transfer

The process where heat spontaneously moves from a hotter body to a cooler one.

Temperature Difference

The difference in temperature between the heat source and the receiver, which drives heat transfer.

Rate of Heat Transfer

The amount of heat energy transferred per unit of time, impacted by temperature difference and resistance.

Resistance to Heat Flow

The opposition a medium offers to the flow of heat, slowing down the transfer.

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Modes of Heat Transfer

The ways heat can travel: conduction, convection, and radiation.

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Conduction

Heat transfer through direct contact, where molecules pass energy between them.

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Fourier's Equation

A formula describing heat conduction: dQ/dt = kA dT/dx.

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Thermal Conductivity (k)

A material's ability to conduct heat, high k means good conductor.

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Thermal Conductivity

A material's ability to conduct heat, measured by how much heat passes through a unit area per unit time with a temperature difference.

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High vs. Low Thermal Conductivity

High thermal conductivity means heat transfers efficiently through the material, while low thermal conductivity means it resists heat flow.

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Thermal Conductivity of Metals

Metals generally have high thermal conductivity (50-400 J m-1 s-1 °C-1) due to their free electrons allowing for efficient heat transfer.

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Thermal Conductivity of Foodstuffs

Foodstuffs have thermal conductivities around 0.6 - 0.7 J m-1 s-1 °C-1 due to their high water content.

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Thermal Conductivity of Ice

Ice has a higher thermal conductivity than water (2.3 J m-1 s-1 °C-1), influencing the heat transfer in frozen foods.

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Thermal Conductivity of Non-Metallic Materials

Dense, non-metallic materials typically have conductivities of 0.5-2 J m-1 s-1 °C-1.

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Thermal Conductivity of Insulating Materials

Insulating materials like foamed plastics and cork have low conductivities (0.03-0.06 J m-1 s-1 °C-1) due to trapped air.

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Thermal Diffusivity

A material property that measures the rate at which heat spreads through a material, calculated by dividing thermal conductivity by the product of density and specific heat capacity.

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Heat Transfer Rate

The amount of heat energy transferred per unit time. It is measured in Watts (W).

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Heat transfer through a wall

Quantity of heat energy moving through a wall, depending on the wall's thickness, thermal conductivity, and temperature difference across the wall.

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Natural Convection

Convection driven by density differences due to temperature changes. Warmer, less dense fluid rises, while cooler, denser fluid sinks.

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Forced Convection

Convection driven by a mechanical force, like a fan or pump. The fluid movement is not solely due to density differences.

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Heat Transfer Coefficient

A measure of how efficiently heat is transferred between a surface and a fluid. Higher value means more efficient heat transfer.

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Simple Heat Conduction Equation

A fundamental equation used to calculate the rate of heat transfer through a uniform material, considering temperature difference and thermal conductivity.

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Heat Conductance

A property that describes how efficiently heat is transferred per unit area and thickness of a material under a unit temperature difference.

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Heat Conductances in Series

Situation where heat flows through multiple layers of different materials, each with its own heat conductance. The total conductance is determined by the combination of individual conductances.

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How does heat flow in series layers?

In steady-state conditions, the same quantity of heat flows through each layer per unit time, even though they may have different materials and properties.

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Impact of Different Conductances in Series

The total temperature difference across the layered system is equal to the sum of temperature differences across each individual layer.

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Formula for Combined Conductance

The reciprocal of the total conductance in series is equal to the sum of reciprocals of individual conductances.

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Why is understanding heat conductance crucial?

It allows predicting heat transfer through composite structures like walls, enabling efficient design of insulation and energy-saving solutions.

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Emissivity

A measure of how well a surface emits radiant heat. A high emissivity means the surface radiates more heat, while a low emissivity means it radiates less and reflects more.

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Emissivity of Different Surfaces

Emissivity varies depending on the surface material. Dull black surfaces have high emissivity (close to 1), while polished metals have low emissivity (close to 0).

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Radiant Heat Transfer Between Two Surfaces

The heat exchanged between two surfaces depends on their temperatures, their emissivities, and their geometric arrangement. The higher the temperature difference, the more heat is transferred.

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Net Heat Transfer Equation

The equation for net heat transfer between two parallel surfaces is: q = ACσ(T1⁴ - T2⁴), where C is a constant related to emissivities, σ is the Stefan-Boltzmann constant, T1 and T2 are the absolute temperatures of the surfaces.

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Radiation to a Small Body from Surroundings

When a small body is surrounded by surfaces at a uniform temperature, the net heat exchange is: q = Aεσ(T1⁴ - T2⁴), where ε is the body's emissivity, T1 is the body's temperature, and T2 is the surrounding temperature.

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Radiation Heat-Transfer Coefficient (hr)

The radiation heat-transfer coefficient relates the heat transfer rate to the temperature difference between the body and its surroundings: q = hrA(T1 - T2).

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Comparing HeatTransfer Methods

The equation q = hrA(T1 - T2) is similar to the heat transfer equations for convection and conduction, allowing comparison between different methods.

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Importance of Radiation in Food Processing

Radiation is important in food processing as it contributes to heat transfer in various operations, such as drying, roasting, and baking.

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Newton's Law of Cooling

Describes the rate of heat transfer through convection, stating that the heat transfer is proportional to the temperature difference between the surface and the surrounding fluid.

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Convective Heat Transfer Coefficient (h)

A measure of how well heat is transferred between a surface and a fluid. Higher h indicates better heat transfer.

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Log Mean Temperature Difference (LMTD)

A way to calculate the average temperature difference between two fluids in a heat exchanger, considering changing temperatures throughout the system.

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Heat Exchanger

A device designed to efficiently transfer heat energy between two fluids. It uses a surface that both fluids interact with, allowing heat flow.

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Jacketed Pan

A cooking vessel with a double wall, where the space between the walls is used to heat the liquid inside.

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Overall Heat Transfer Coefficient (U)

A measure of the combined resistance to the flow of heat through all the components of a system. It indicates the efficiency of heat transfer.

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Latent Heat of Condensation (λ)

The amount of heat energy released when a substance changes state from a gas to a liquid. This heat is released without a temperature change.

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Steam Requirement

The amount of steam needed to provide a specific amount of heat energy to a system.

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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.

    • 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)

  • 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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4510 Lec 7 - Heat Transfer PDF

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.

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