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5 Cards in this Set
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When heat is applied to a body or substance, their internal energies will increase and an increase in temperature or a change of state will occur.
Sensible Heat When heat is supplied or taken away and causes an immediate change in temperature without changing the state, the heat is known as sensible heat. Latent Heat When heat is supplied or taken away and causes a change in state without a change in temperature, then the heat is known as latent heat. Latent Heat of Fusion The latent heat of fusion is the heat required to change a unit mass of solid to a unit mass of liquid to a unit mass of vapour or gas of the same temperature and pressure. For example, at at atmospheric conditions , the latent heat of fusion of ice is 335 kJ/kg. That is ,it requires 335 kj of heat to change every kg of ice at 0°C into water at 0°C. |
Latent Heat of Evaporation
The latent heat of evaporation is the heat required to change a unit mass of liquid to a unit mass of vapour or gas at the same temperature and pressure. For example, at atmospheric conditions, the latent heat of evaporation of water is 2257 RJ/kg. That is, it requires 2257 kJ of heat to change every kg of water at 100°C into steam at 100°C. Note: Latent heat of fusion and evaporation are found by experiment. The latent heat of evaporation varies with pressure; it decreases in value as the pressure rises. |
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Saturation Temperature
The temperature at which the change from water to steam takes place is called the boiling point or saturation temperature; this temperature will depend on the pressure on the surface of the water. Saturation Temperatures Absolute Pressure Boiling Point or Saturation Temperature 101.3kPa 100°C 200 KPa 120.23 C 300 Kpa 133.55 C 400 kPa 143.63 C Calculations concerning steam are complex due to variations in temperature and pressure. The relationships are seen in Table 1. All values of the properties of steam have been determined accurately and are listed in available and easy to use steam tables. Similar tables are also available for most liquids and gases, such as: refrigerants and industrial gases. |
Saturated Steam
As mentioned before, the change of state from water to steam takes place at the saturation temperature (or boiling point) corresponding to the pressure acting on the water surface. Water at the saturation temperature corresponding to a particular pressure is called saturated water because it is saturated with sensible heat. The steam produced is called saturated steam. If this steam does not contain any particles of water in suspension, it is called dry saturated steam. It is called wet steam if the steam has particles of water suspended in it. Thus, saturated steam is just at the temperature of the change of state from liquid to vapour and if any heat is removed from it, it will immediately begin to condense. |
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Superheated Steam
Saturated steam may be perfectly dry; that it may contain no water particles in suspension as it leaves the boiler. But as it begins to lose heat and fall in temperature, there will be a certain amount of condensation Joss in the steam line. To avoid or minimize these losses, the saturated steam can be heated to a higher temperature than that corresponding to its pressure by passing it through rows of tubes placed in the combustion chamber of the boiler before entering the main steam header. These tubes are known as a superheater. This heating of the steam to a higher temperature than its saturation temperature is called superheating; the steam produced is called superheated steam. This steam will not condense until the temperature has dropped to the saturation temperature meaning that the superheated steam can be transmitted long distances to turbines or other equipment without encountering excessive condensation losses. Buildings using steam for heating, almost always use saturated steam. Superheated steam is used mainly for steam turbines or very high temperature processes. |
CHANGE OF STATE
Certain properties of steam the volume per kilogram, the sensible and latent heat per kg and the saturation temperature corresponding to certain absolute pressures, are known as the thermodynamic properties of steam . these values and many others must be known to solve problems involving the use of steam. All these properties of steam have been obtained by careful experiments and the values are tabulated in the steam tables. Referring to the Handbook of Formulae and Physical Constants, Table 1 of the SI Steam Tables for Reference in Power Engineering lists the Properties of Saturated Steam Pressure Table for various saturation pressures. For discussion here, consider the columns as being numbered from left to right (note: they are not actually numbered in the steam table). |
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Columns 1, 2, 3 and 4 These columns list various pressures and their corresponding temperatures, plus the specific volumes of saturated liquids and vapours. Column 1 lists the absolute pressure (p) in kPa . It should be remembered that the absolute pressure is obtained using the following formula:
Absolute pressure = Gauge pressure +101.3 kPa Column 2 lists the corresponding saturation temperature (t) in °C for each absolute pressure. in columns 1 and 2, the student will see that the temperature of saturated steam increases as the pressure increases. That is, as Table 2 illustrates, the boiling point of water increases as the pressure increases. |
Column 1 = Absolute pressure Absolute pressure = Gauge pressure + 101.3 Kpa Column 2 = Lists the corresponding saturation temperature (t)(Boiling Point) in C for each absolute pressure Column 3 = Lists the specific volume (vf) of the saturated liquid in cm^3/gram at each saturation temperature and pressure Column 4 = Lists the specific volume (vg) of saturated vapour in cm^3/ gram saturated steam for each pressure(1 gram of steam at the temperature .) |
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Column 5,6,7 = No concern of 4th class Column 8,9,10 = these columns of the steam tables indicate specific enthalpy . Enthalpy is a measure of heat added to a unit mass of substance . Column 8 = Lists the liquid enthalpy (hf) in KJ/KG of saturated water for each pressure.( This is sensible heat ) Column 9 = Lists the enthalpy of evaporation (hfg) in Kj/Kg for each pressure.(Latent Heat of evaporation) Column 10 = In the list of enthalpy (hg) in KJ/Kg for saturated stream for each pressure.(No water particles present) It should be noted hg = hf +hfg Column 11,12,13 = Not for 4th class |
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