Why superheated steam in steam turbine

In the sections that follow, we will discuss the types of steam used in these applications. Saturated dry steam results when water is heated to the boiling point sensible heating and then vaporized with additional heat latent heating.

If this steam is then further heated above the saturation point, it becomes superheated steam sensible heating. 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 of these are:. Having said this, it is necessary to be mindful of the following when heating with saturated steam: Heating efficiency may be diminished if steam other than dry steam is used for process heating.

Contrary to common perception, virtually all of the steam generated from a boiler is not dry saturated steam, but wet steam, which contains some non-vaporized water molecules. Radiant heat loss causes some of the steam to condense. The generated wet steam thus becomes even more wet, and condensate also forms, which must be removed by installing steam traps at appropriate locations. Heavy condensate that falls out of the steam flow can be removed through drip leg steam traps.

However, the entrained wet steam will reduce heating efficiency, and should be removed through point-of-use or distribution separation stations Steam that incurs pressure losses due to piping friction, etc. This is the most common form of steam actually experienced by most plants. When steam is generated using a boiler, it usually contains wetness from non-vaporized water molecules that are carried over into the distributed steam.

As the water approaches the saturation state and begins to vaporize, some water, usually in the form of mist or droplets, is entrained in the rising steam and distributed downstream. This is one of the key reasons why separation is used to dis-entrain condensate from distributed steam.

Superheated steam is created by further heating wet or saturated steam beyond the saturated steam point. This yields steam that has a higher temperature and lower density than saturated steam at the same pressure.

To maintain the dryness of the steam for steam-driven equipment, whose performance is impaired by the presence of condensate To improve thermal efficiency and work capability, e.

It is advantageous to both supply and discharge the steam while in the superheated state because condensate will not be generated inside steam-driven equipment during normal operation, minimizing the risk of damage from erosion or carbonic acid corrosion.

In addition, as the theoretical thermal efficiency of the turbine is calculated from the value of the enthalpy at the turbine inlet and outlet, increasing the degree of superheating as well as the pressure raises the enthalpy at the turbine inlet side, and is thereby effective at improving thermal efficiency.

For these reasons and others, saturated steam is preferred over superheated steam as the heating medium in exchangers and other heat transfer equipment. On the other hand, when viewed as a heat source for direct heating as a high temperature gas, it has an advantage over hot air in that it can be used as a heat source for heating under oxygen-free conditions. Research is also being carried out on the use of superheated steam in food processing applications such as cooking and drying.

Supercritical water is water in a state that exceeds its critical point: At the critical point, the latent heat of steam is zero, and its specific volume is exactly the same whether considered liquid or gaseous. In other words, water that is at a higher pressure and temperature than the critical point is in an indistinguishable state that is neither liquid nor gas.

Supercritical water is used to drive turbines in power plants which demand higher efficiency. If the saturated steam produced in a boiler is exposed to a surface with a higher temperature, its temperature will increase above the evaporating temperature.

The steam is then described as superheated by the number of temperature degrees through which it has been heated above saturation temperature.

Superheat cannot be imparted to the steam whilst it is still in the presence of water, as any additional heat simply evaporates more water. The saturated steam must be passed through an additional heat exchanger.

This may be a second heat exchange stage in the boiler, or a separate superheater unit. The primary heating medium may be either the hot flue gas from the boiler, or may be separately fired. Superheated steam has its applications in, for example, turbines where the steam is directed by nozzles onto a rotor.

This causes the rotor to turn. The energy to make this happen can only have come from the steam, so logically the steam has less energy after it has gone through the turbine rotor. If the steam was at saturation temperature, this loss of energy would cause some of the steam to condense. Turbines have a number of stages; the exhaust steam from the first rotor will be directed to a second rotor on the same shaft.

This means that saturated steam would get wetter and wetter as it went through the successive stages. Not only would this promote waterhammer, but the water particles would cause severe erosion within the turbine. Another very important reason for using superheated steam in turbines is to improve thermal efficiency.

The thermodynamic efficiency of a heat engine such as a turbine, may be determined using one of two theories:. Note: The values used for the temperature and energy content in the following examples are from steam tables. The exhaust is at 0. For the theoretical Rankine Cycle, Figure 2. The superheated steam tables display the properties of steam at various pressures in much the same way as the saturated steam tables. However, with superheated steam there is no direct relationship between temperature and pressure.

Therefore at a particular pressure it may be possible for superheated steam to exist at a wide range of temperatures. In general, saturated steam tables give gauge pressure, superheated steam tables give absolute pressure.

This may sound a useful increase in energy, but in fact it will actually make life more difficult for the engineer who wants to use steam for heating purposes. However, unlike the specific heat capacity of water, the specific heat capacity for superheated steam varies considerably with pressure and temperature and cannot be taken as a constant.

The value of 2. There is no direct relationship between temperature, pressure and the specific heat capacity of superheated steam. There is, however, a general trend towards an increase in specific heat capacity with increasing pressure at low degrees of superheat, but this is not always the case.

Can superheated steam be used in process heat exchangers and other heating processes? Although not the ideal medium for transferring heat, superheated steam is sometimes used for process heating in many steam plants around the world, especially in the HPIs Hydrocarbon Processing Industries which produce oils and petrochemicals. This is more likely to be because superheated steam is already available on site for power generation, being the preferred energy source for turbines, rather than because it has any advantage over saturated steam for heating purposes.

To be clear on this point, in most cases, saturated steam should be used for heat transfer processes, even if it means desuperheating the steam to do so. HPIs often desuperheat steam to within about ten degrees of superheat. This small degree of superheat is removed readily in the first part of the heating surface. Greater amounts of superheat are more difficult, and often uneconomic to deal with and for heating purposes are best avoided.

There are quite a few reasons why superheated steam is not as suitable for process heating as saturated steam:. Superheated steam has to cool to saturation temperature before it can condense to release its latent heat enthalpy of evaporation. The amount of heat given up by the superheated steam as it cools to saturation temperature is relatively small in comparison to its enthalpy of evaporation.

If the steam has only a few degrees of superheat, this small amount of heat is quickly given up before it condenses. However, if the steam has a large degree of superheat, it may take a relatively long time to cool, during which time the steam is releasing very little energy.

Unlike saturated steam, the temperature of superheated steam is not uniform. Superheated steam has to cool to give up heat, whilst saturated steam changes phase.

This means that temperature gradients over the heat transfer surface may occur with superheated steam. In a heat exchanger, use of superheated steam can lead to the formation of a dry wall boiling zone, close to the tube sheet.

This dry wall area can quickly become scaled or fouled, and the resulting high temperature of the tube wall may cause tube failure. This clearly shows that in heat transfer applications, steam with a large degree of superheat is of little use because it:.

So, superheated steam is not as effective as saturated steam for heat transfer applications. This may seem strange, considering that the rate of heat transfer across a heating surface is directly proportional to the temperature difference across it. If superheated steam has a higher temperature than saturated steam at the same pressure, surely superheated steam should be able to impart more heat?

This will now be looked at in more detail. It is true that the temperature difference will have an effect on the rate of heat transfer across the heat transfer surface, as clearly shown Equation 2.

Equation 2. In general, saturated steam tables give gauge pressure, superheated steam tables give absolute pressure. Blog Details Home Blog Details. Superheated Steam. Advantages of using superheated steam to drive turbines: To maintain the dryness of the steam for steam-driven equipment, whose performance is impaired by the presence of condensate To improve thermal efficiency and work capability, e.

Property Disadvantage Low heat transfer coefficient Reduced productivity Larger heat transfer surface area needed Variable steam temperature even at constant pressure Superheated steam needs to maintain a high velocity, otherwise the temperature will drop as heat is lost from the system Sensible heat used to transfer heat Temperature drops can have a negative impact on product quality Temperature may be extremely high Stronger materials of construction may be needed, requiring higher initial equipment outlay For these reasons and others, saturated steam is preferred over superheated steam as the heating medium in exchangers and other heat transfer equipment.

What is the temperature of superheated steam? How is superheated steam calculated? How do you calculate degrees of superheat?