Let us decide upon the pressure at which the steam is to be generated. After fixing the pressure, we can know the saturation temperature of steam at this pressure from steam tables. This means, water has to be heated to this temperature before the steam will be produced. We can therefore, understand the heat content of formation steam in two stages, the warming stage and the evaporation stage.
i. Warming Stage
During the Warming Stage, heat is added to water so that its temperature is increased to the saturation temperature. At atmospheric pressure the saturation temperature of steam is 100 o C. Therefore we will have to heat water up to 100 o C for producing steam. For one kg. of water, the addition of heat will raise the temperature of water at the rate of 1 o C for every 4.19 kJ of heat added. The heat supplied during the warming stage is known as Sensible Heat because it can be detected by sense of touch and produces a rise in temperature to be seen on a thermometer. The amount of sensible heat required to raise the temperature of water from 0 to 100 o C would thus be equal to 419 kJ / kg. The sensible heat content increases if the saturation temperature is increased.
ii. Evaporation Stage
In the Evaporation Stage, further heat is applied to 1 kg of water at saturation temperature and pressure. During this stage heat is utilized in changing the state of water from liquid to Vapour state. Temperature remains constant as the evaporation is in progress. Because the heat added during this stage cannot be recorded by a rise in temperature on the thermometer, it is a hidden heat and is called as the Latent Heat of Vapourization. The latent heat of vapourization of water is 2257kJ / kg. Any further addition of heat at 100 o C would increase the temperature of vapours approximately at the rate of 1 o C for every 1.97 kJ of heat added per kg and will produce superheated steam.
Total heat content of steam would thus depend upon the type of steam. Dry saturated steam is obtained when all traces of liquid in steam is vapourized with just enough of heat to vapourize the liquid. Removal of heat from dry steam causes moisture to condense out and would result in a wet steam. If we measure from 0 o C, one kg of dry saturated steam has a total heat content of 419 + 2257 = 2676 kJ. On the other hand, the heat content of one kg. of 95% quality steam (5% moisture ) would be 419 + ( 2257 x 0.95) = 2563 kJ. Addition of 2257 x .05 = 113 kJ of heat would be required to change 95% quality steam to dry saturated steam.
Super heated steam contains more heat energy than dry saturated steam. Heat content of dry saturated steam and the superheated steam is usually indicated in a table form along with its saturation pressure and the corresponding temperature.These tables are known as steam tables and are used in estimating the steam requirement for a required milk processing operation.
For example, let us calculate the amount of steam required to pasteurize 500 kg. of milk from 20 o C to 72 o C. The specific heat of milk is 3.914 and that of water is 4.187 kJ/kg/ o C. We shall now require the information on type of steam and the available steam pressure. Let the steam available is of 90% quality, the steam pressure is 3 kg/cm 2 and the temperature of steam condensate leaving the vessel jacket is 50 o C. We shall now make the calculations in the following way:
We now know what steam is and what the basic principles of steam generation are. We have also understood the different types of steam and how to make calculations for the quantity of steam required for accomplishing a given processing operation.Now let us know about the equipment in which steam is generated, that is a steam boiler and understand its construction and functioning.
i. Warming Stage
During the Warming Stage, heat is added to water so that its temperature is increased to the saturation temperature. At atmospheric pressure the saturation temperature of steam is 100 o C. Therefore we will have to heat water up to 100 o C for producing steam. For one kg. of water, the addition of heat will raise the temperature of water at the rate of 1 o C for every 4.19 kJ of heat added. The heat supplied during the warming stage is known as Sensible Heat because it can be detected by sense of touch and produces a rise in temperature to be seen on a thermometer. The amount of sensible heat required to raise the temperature of water from 0 to 100 o C would thus be equal to 419 kJ / kg. The sensible heat content increases if the saturation temperature is increased.
ii. Evaporation Stage
In the Evaporation Stage, further heat is applied to 1 kg of water at saturation temperature and pressure. During this stage heat is utilized in changing the state of water from liquid to Vapour state. Temperature remains constant as the evaporation is in progress. Because the heat added during this stage cannot be recorded by a rise in temperature on the thermometer, it is a hidden heat and is called as the Latent Heat of Vapourization. The latent heat of vapourization of water is 2257kJ / kg. Any further addition of heat at 100 o C would increase the temperature of vapours approximately at the rate of 1 o C for every 1.97 kJ of heat added per kg and will produce superheated steam.
Total heat content of steam would thus depend upon the type of steam. Dry saturated steam is obtained when all traces of liquid in steam is vapourized with just enough of heat to vapourize the liquid. Removal of heat from dry steam causes moisture to condense out and would result in a wet steam. If we measure from 0 o C, one kg of dry saturated steam has a total heat content of 419 + 2257 = 2676 kJ. On the other hand, the heat content of one kg. of 95% quality steam (5% moisture ) would be 419 + ( 2257 x 0.95) = 2563 kJ. Addition of 2257 x .05 = 113 kJ of heat would be required to change 95% quality steam to dry saturated steam.
Super heated steam contains more heat energy than dry saturated steam. Heat content of dry saturated steam and the superheated steam is usually indicated in a table form along with its saturation pressure and the corresponding temperature.These tables are known as steam tables and are used in estimating the steam requirement for a required milk processing operation.
For example, let us calculate the amount of steam required to pasteurize 500 kg. of milk from 20 o C to 72 o C. The specific heat of milk is 3.914 and that of water is 4.187 kJ/kg/ o C. We shall now require the information on type of steam and the available steam pressure. Let the steam available is of 90% quality, the steam pressure is 3 kg/cm 2 and the temperature of steam condensate leaving the vessel jacket is 50 o C. We shall now make the calculations in the following way:
We now know what steam is and what the basic principles of steam generation are. We have also understood the different types of steam and how to make calculations for the quantity of steam required for accomplishing a given processing operation.Now let us know about the equipment in which steam is generated, that is a steam boiler and understand its construction and functioning.
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