HEAT
Heat is one of the many forms of energy and is commonly generated from chemical sources. The heat of a body is its thermal or internal energy, and a change in this energy may show as a change of temperature or a change between the solid, liquid and gaseous states.
Matter may also have other forms of energy, potential or kinetic, depending on pressure, position and movement. Enthalpy is the sum of its internal energy and flow work and is given by:
H = u + Pv
In the process where there is steady flow, the factor Pv will not change appreciably and the difference in enthalpy will be the quantity of heat gained or lost.
Enthalpy may be expressed as a total above absolute zero, or any other base which is convenient. Tabulated enthalpies found in reference works are often shown above a base temperature of -40°C, since this is also -40° on the old Fahrenheit scale. In any calculation, this base condition should always be checked to avoid the errors which will arise if two different bases are used.
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-M4- |
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-H«- |
If a change of enthalpy can be sensed as a change of temperature, it is called sensible heat. This is expressed as specific heat capacity, i. e. the change in enthalpy per degree of temperature change, in kJ/(kg K). If there is no change of temperature but a change of state (solid to liquid, liquid to gas, or vice versa) it is called latent heat. This is expressed as kJ/kg but it varies with the boiling temperature, and so is usually qualified by this condition. The resulting total changes can be shown on a temperature-enthalpy diagram (Figure 1.3).
373.15 K
273.16 K
Enthalpy
Figure 1.3 Change of temperature (K) and state of water with enthalpy
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334.91 (80 — 0) |
The specific enthalpy of water at 80°C, taken from 0°C base, is 334.91 kJ/kg. What is the average specific heat capacity through the range 0-80°C?
4.186kJ/(kg K)
If the latent heat of boiling water at 1.013 bar is 2257 kJ/kg, the quantity of heat which must be added to 1 kg of water at 30°C in order to boil it is:
4.19(100 — 30) + 2257 = 2550.3kJ
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