Saturated Steam
Introduction:
Saturated steam is produced when water is heated to the boiling point and then vaporized with additional heat. If this steam is then further heated above the saturation point, it becomes superheated steam. Saturated steam occurs when steam and water are in equilibrium. Dry steam is saturated steam that has been very slightly superhe
Pressure –Temperature Relationship of Water & Steam:
If water is heated beyond the boiling point, it vaporizes into steam, or water in the gaseous state. However, not all steam is the same. The properties of steam vary greatly depending on the pressure and temperature to which it is subject.
Saturated (dry) steam results when water is heated to the boiling point (sensible heating) then vaporized with additional heat (latent heating).
If this steam is then further heated above the saturation point, it becomes superheated steam (sensible heating).
Saturated Steam:
As indicated by the black line in the above graph, saturated steam occurs at temperatures and pressures where steam (gas) and water (liquid) can coexist. In other words, it occurs when the rate of water vaporization is equal to the rate of condensation.
Some Common Terminology:
Saturated Steam: is pure steam at the temperature that corresponds to the boiling temperature of water at the existing pressure.
Absolute and Gauge Pressures: Absolute pressure is pressure in pounds per square inch (psia) above a perfect vacuum. Gauge pressure is pressure in pounds per square inch above atmospheric pressure which is 14.7 pounds per square inch absolute. Gauge pressure (psig) plus 14.7 equals absolute pressure. Or, absolute pressure minus 14.7 equals gauge pressure.
Pressure/Temperature Relationship: For every pressure of pure saturated steam there is a corresponding temperature. Example: The temperature of 17 Kg/cm2g (250 psig) pure steam is always 206.1OC (406°F).
Heat of Saturated Liquid: This is the amount of heat required to raise the temperature of unit mass that is 1.0Kg (1.0lb) of water from 0OC (32°F) to the boiling point at the applied pressure. It is expressed in Kilo-Calorie (kCal) [British thermal units (Btu)]. This also known as Sensible Heat.
Latent Heat: Heat quantity responsible only for changing the phase of any material without changing its pressure & temperature is known as Latent Heat of that material for particular that phase change. Phase change of materials only can possible at saturation temp of applied pressure. Quantity of latent heat is always same for same material, at same pressure for particular phase change, qty also same for reverse phase change at same condition.
For a material at constant pressure-
Latent Heat of Vaporization = Latent Heat of Condensation
Latent Heat of Solidification = Latent Heat of Melting
Heat of Vaporization/evaporation of water:
This is the amount of heat required to change the state of water at its boiling temperature, into steam. It involves no change in the temperature of the Seam/Water mixture, and all the energy is used to change the state from liquid (water) to vapor (saturated steam).
The amount of heat (expressed in kCal / Btu) required to changing one Kg / Pound of boiling water to one Kg /Pound of steam at fixed applied pressure is Heat of Vaporization of water. This same amount of heat is released when one Kg / Pound of steam is condensed back into one Kg / Pound of water (Heat of Condensation of water). This heat quantity is different for every pressure / temperature combination, as shown in the steam table.
This, ‘Enthalpy of evaporation’ may be considered as the useful portion of heat in the steam for heating purposes, as it is that portion of the total heat in the steam that is extracted when the steam condenses back to water.
Total Heat of Steam:
This is the total energy in saturated steam, and is simply the sum of the enthalpy of water and the enthalpy of evaporation. The sum of the Heat of the Liquid (Sensible Heat) and Latent Heat of Vaporization in kCal / Btu. It is the total heat in steam above 0OC / 32°F.
hg = hf + hfg
Where:
hg = Total enthalpy of saturated steam in Kcal/kg (Total heat)
hf = Liquid enthalpy at saturation condition in Kcal/kg (Sensible heat)
hfg = Enthalpy of evaporation in Kcal/kg (Latent heat)
Example:
At atmospheric pressure (0 bar g), water boils at 100°C, and kCal 100 (actual 99 kCal) of energy are required to heat 1 kg of water from 0°C to its saturation temperature of 100°C. Therefore the enthalpy of water at 0 bar g and 100°C is 100 kCal/kg.
Another 540 (actual 539.8 kCal) kCal of energy are required to evaporate 1 kg of water at 100°C into 1 kg of steam at 100°C. Therefore at 0 bar g the specific enthalpy of evaporation is 540 kCal/kg.
Therefore:
Total enthalpy of steam hg = 100 + 540 = 640 kCal/kg. (Actual 638.8 kCal/kg).
At 7.0bar-g, the saturation temperature of water is 170°C. More heat energy is required to raise its temperature to saturation point at 7.0bar-g than would be needed if the water were at atmospheric pressure. The Steam table gives a value of 171 kCal to raise 1 kg of water from 0°C to its saturation temperature of 170°C at 7.0bar-g.
The heat energy (enthalpy of evaporation) needed by the water at 7 bar-g to change it into steam is 489.8 kCal/kg, actually less than the heat energy required at atmospheric pr. This is because the specific enthalpy of evaporation decreases as the steam pressure increases.
Steam in Industries :
The Boiler
Boiler is the heart of the process Plant and steam system. In case of Liquid or Gas Fired Boiler, fuel burns in the burner, whereas, in case of Solid Fuel Fired Boiler, fuel burns in furnace chamber and transfers heat to the boiler tubes.
The hot gases from the burner pass backwards and forwards multiple times through a series of tubes to gain the maximum transfer of heat through the tube surfaces to the surrounding boiler water.
Once the water reaches saturation temperature (the temperature at which it will boil at that pressure) bubbles of steam are produced, which rise to the water surface and burst. The steam is released into the space above, ready to enter the steam system. The stop or crown valve isolates the boiler and its steam pressure from the process or plant.
If steam is pressurized, it will occupy less space. Steam boilers are usually operated under pressure, so that more steam can be produced by a smaller boiler and transferred to the point of use, by means of smaller bore pipe work. When required, the steam pressure is reduced at the point of use.
As long as the amount of steam being produced in the boiler is the same amount as that leaving the boiler, the boiler will remain pressurized. The burner will operate to maintain the correct pressure. This also maintains the correct steam temperature, because the pressure and temperature of saturated steam are directly related.
The boiler has a number of fittings and controls to ensure that it operates safely, economically, efficiently and at a consistent pressure.
Boiler Feed Water (BFW)
Boiler Feed Water (BFW) is the water supply into the boiler. This is one of two main Raw Materials required to generate Steam.
BFW quality is very important. It must be optimally at a minimum temperature, usually around 80°C; to avoid thermal shock, also to reduce the amount of dissolved oxygen in water, as oxygenated water is corrosive, and to operate efficiently. It must also be of the correct quality to avoid damage to the boiler.
Ordinary, untreated potable water is not entirely suitable for boilers and can quickly cause them to foam and scale up. The boiler would become less efficient and the steam would become dirty and wet. The life of the boiler would also be reduced.
Therefore the water must be treated with chemicals before entering not only into Boiler but also in Feed Water tank to reduce the impurities it contains.
Both feed water treatment and heating take place in the Feed Water Tank, which is usually situated high above the boiler. The Feed Pump delivers BFW to the boiler when required. Feed water pumps are not ordinary pumps; these are Multi-stage centrifugal pumps.
Water Level control:
Water level inside the boiler has to be carefully controlled. If the water level drops too low and the boiler tubes are exposed, the boiler tubes could overheat and fail, causing an explosion. If the water level becomes too high, water could enter the steam system and upset the process. For this reason, automatic level controls are used.
To comply with legislation, level control systems also incorporate alarm functions which will operate to shut down the boiler and alert attention if there is a problem with the water level. A common method of level control is to use probes which sense the level of water in the boiler. At a certain level, a controller will send a signal to the feed pump which will operate to restore the water level, switching off when a predetermined level is reached. The probe will incorporate levels at which the pump is switched on and off, and at which low or high level alarms are activated. Alternative systems use floats.
It is a legal requirement in most countries to have two independent low level alarm systems.
Boiler Blow-down:
Chemical dosing of the boiler feed water will lead to the presence of suspended solids in the boiler. These will inevitably collect at the bottom of the boiler drum in the form of sludge, and are removed by a process known as bottom blow-down. This can be done manually – the boiler attendant will use a key to open a blow-down valve for a set period of time, usually twice a day.
Other impurities remain in the boiler water after treatment in the form of dissolved solids. Their concentration will increase as the boiler produces steam and consequently the boiler needs to be regularly purged of some of its contents to reduce the concentration. This is called control of total dissolved solids (TDS control). This process can be carried out by an automatic system which uses either a probe inside the boiler, or a small sensor chamber containing a sample of boiler water, to measure the TDS level in the boiler. Once the TDS level reaches a set point, a controller signals the blow-down valve to open for a set period of time. The lost water is replaced by feed water with a lower TDS concentration, consequently the overall boiler TDS is reduced.
Water Boiler Analysis:
pH (pH in boiler is about 11,....)
Electric conductivity (measuring the value of ions in water)
M-alkalinity (measuring the value of carbonat, bicarbonat and OH- in water)
Chloride
Hardness (prevent the scale)
Silica (calculate the blowdown volume)
Iron (prevent the corrossion)
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