High Specific Heat of Water

3 Key excerpts on "High Specific Heat of Water"

• 

  eBook - ePub

  Physical Properties of Foods and Food Processing Systems

  • M J Lewis(Author)
  • 1990(Publication Date)
  • Woodhead Publishing

    (Publisher)

  8

  Sensible and latent heat changes

  8.1 INTRODUCTION

  The properties to be discussed in this chapter include specific heat, latent heat and specific enthalpy.

  These properties play an important role in heat transfer problems when heating or cooling foods. It is necessary to know the specific heat to determine the quantity of energy that needs to be added or removed. This will give an indication of the energy costs involved and in a continuous process will have an influence on the size of the equipment.

  Latent heat values, which are associated with phase changes, play an important role in freezing, crystallization, evaporation and dehydration processes.

  8.2 SPECIFIC HEAT

  The specific heat of a material is a measure of the amount of energy required to raise unit mass by unit temperature rise. As mentioned in Chapter 7 , specific heat is temperature dependent. However, for the purpose of many engineering calculations, these variations are small and an average specific heat value is used for the temperature range considered.

  The units of specific heat are kilojoules per kilogram per kelvin (kJ kg−1  K−1 ), kilocalories per kilogram (kcal kg−1  K−1 ) or British thermal units per pound per degree Fahrenheit (Btu lb−1 degF−1 ). From the definitions of the different thermal units, the specific heat of water in the respective units is

  1 .0

  . kcal kg −1

  K −1

  or 4

  .18 kJ kg

  −1

  K −1

  or 1 Btu lb

  −1

  degF −1

  In a batch heating or cooling process, the amount of heat (energy) Q required or removed is given by

  Q = mass × average specific heat × temperature change

  = MC Δ T

  J or kcal or Btu

  In a continuous process, the rate of heat transfer is given by

  Q / t = mass flow rate × specific heat × temperature range

  The units of Q /t are joules per second (J s−1 ), i.e. watts (W), or British thermal units per hour (Btu h−1 ). This is often termed the heating or cooling duty of the heat exchanger. If it is felt that this is not a sufficiently accurate procedure, the total energy requirement can be obtained by graphical integration. Specific heat is plotted against temperature; the total heat required to raise unit mass from T 1 to T 2 is given by ∫ 

  cp dT

  or the area under the curve (Fig. 8.1 ) This should, in most cases, be not too far removed from the value obtained by selecting a specific heat value at the average temperature (T 1  + T 2 )/2. If the relationship between the specific heat and temperature is known in terms of temperature, then the integral ∫ 

  cp

  dT can be evaluated directly, (see section 8.4

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  eBook - ePub

  Physical Properties of Tissues

  A Comprehensive Reference Book

  • Francis Duck(Author)
  • 2013(Publication Date)
  • Academic Press

    (Publisher)

  −1 .

  2.2 Specific heat capacity and latent heat

  2.2.1 Terminology and definitions

  The specific heat capacity, C, of a substance is the quantity of heat Q required to raise the temperature of a unit mass of the substance one degree.

  (2.21)

  where M is the mass and ΔT the change in temperature. The SI unit of specific heat capacity is joule per kilogram kelvin (J kg−1 K−1 ), with a common unit being J g−1 K−1 .

  The freezing of tissue is not characterised by the discontinuity in enthalpy which exists for many pure substances. The first–order transition from ice to water shows this discontinuity and the heat released is termed the latent heat of fusion. However, since the fluid water in tissue does not all freeze at the same temperature the concept of latent heat is not relevant, and therefore latent heats of fusion for tissues cannot be given. It has also been suggested that the concept of specific heat is also inappropriate for frozen tissue since there is no way to separate the heats associated with freezing from those associatd with change in temperature. For this reason enthalpy or total heat content is often reported for food industry calculations (Riedel, 1957b ; Fleming, 1969 ), referred to an arbitrary zero reference temperature, commonly −40°C. In the tables included here only values of specific heat for tissues are given, accepting the argument outlined above. For tabulations and diagrams of enthalpy values, reference may be made to food science texts (Dickerson, 1968 ; Rha, 1975 ).

  2.2.2 Values of specific heat of tissue

  Values of specific heats of various tissues are given in Table 2.11

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  eBook - ePub

  An Introduction to Thermogeology

  Ground Source Heating and Cooling

  • David Banks(Author)
  • 2012(Publication Date)
  • Wiley

    (Publisher)

  −3 , in some cases. Many sediments comprise solid (mineral), water and air phases: the composite volumetric heat capacity can be calculated as the volume-weighted arithmetic mean of the various components.

  Volumetric heat capacity varies somewhat with temperature, partly (but not wholly) due to changes in density of the material.

  Figures 3.1

  and 9.5 show how this affects the thermal properties of water at varying temperature (similar temperature-dependent variations also apply to solutions of antifreezes). For this reason, physicists tend to prefer using the term ρS C is their equations, rather than S VC . In this book, however, we are usually considering ground source heating/cooling systems that employ very modest temperature changes and I have decided to retain the term S VC for simplicity.

  Figure 3.1  The temperature dependence of the thermal properties of water. In this diagram, S C and S VC are the specific heat capacity and volumetric specific heat capacity, while λ is the thermal conductivity.

  Values on the graph are derived or calculated from data provided by Eskilson et al . (2000).

  We should remember that heat may also be stored or released from a substance due, not just to change in temperature (Box 3.2), but to change in phase. This stored heat is called latent heat (Box 3.3).

  BOX 3.2 Sensible Heat

  Sensible heat is the opposite of latent heat (Box 3.3). It is heat which, when added to a solid, liquid or gas, results in an increase in temperature. The relationship between heat and temperature is encapsulated in the concept of specific heat capacity. For water at around 20°C, 4.2 kJ of heat results in a 1°C temperature change per litre. In other words, this is sensible heat

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