Why superheated steam

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.

Research on supercritical water is being performed with an emphasis on its use as a fluid that has the properties of both a liquid and a gas, and in particular on its suitability as a solvent for chemical reactions. This is water in its most recognizable state. In water's liquid form, hydrogen bonding pulls water molecules together.

As a result, unsaturated water has a relatively compact, dense, and stable structure. The auxiliary boiler components including safety devices, water treatment systems, feedwater pumps, and control valves must be properly sized and selected to meet the more stringent requirements of the operation and control of the superheat boiler system.

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What are the Advantages of Superheated Steam Boilers? What are the Disadvantages of Superheated Steam Boilers? Superheated Steam Boiler Considerations There are special considerations and designs that must be accounted for in a system intended to use superheated steam.

Once the water is heated to boiling point, it is vaporized and turned into saturated steam. When saturated steam is heated above boiling point, dry steam is created and all traces of moisture are erased. This is called superheated steam. Superheated steam has a lower density, so lowering the temperature does not revert it back to its original liquid state.

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. For any single application, the heat transfer area might be fixed. These figures are typical; actual figures will vary due to other design and operational considerations.

Although the temperature of superheated steam is always higher than saturated steam at the same pressure, its ability to transfer heat is therefore much lower. The overall effect is that superheated steam is much less effective at transferring heat than saturated steam at the same pressure. Not only is superheated steam less effective at transferring heat, it is very difficult to quantify using Equation 2. Predicting the size of heat transfer surfaces utilising superheated steam is difficult and complex.

In practice, the basic data needed to perform such calculations is either not known or empirically obtained, putting their reliability and accuracy in doubt. Clearly, as superheated steam is less effective at transferring heat than saturated steam, then any heating area using superheated steam would have to be larger than a saturated steam coil operating at the same pressure to deliver the same heat flowrate.

If there is no choice but to use superheated steam, it is not possible to maintain steam in its superheated state throughout the heating coil or heat exchanger, since as it gives up some of its heat content to the secondary fluid, it cools towards saturation temperature. The amount of heat above saturation is quite small compared with the large amount available as condensation occurs. If this is so, it is relatively easy and practical to design a heat exchanger or a coil with a heating surface area based upon saturated steam at the same pressure, by adding on a certain amount of surface area to allow for the superheat.

Using this guideline, the first part of a coil will be used purely to reduce the temperature of superheated steam to its saturation point. The rest of the coil will then be able to take advantage of the higher heat transfer ability of the saturated steam. Fouling is caused by deposits building up on the heat transfer surface adding a resistance to heat flow. Many process liquids can deposit sludge or scale on heating surfaces, and will do so at a faster rate at higher temperatures.

Further, superheated steam is a dry gas. Heat flowing from the steam to the metal wall must pass through the static films adhering to the wall, which resist heat flow. By contrast, the condensation of saturated steam causes the movement of steam towards the wall, and the release of large quantities of latent heat right at the condensing surface.

The combination of these factors means that the overall heat transfer rates are much lower where superheated steam is present, even though the temperature difference between the steam and the secondary fluid is higher.

Estimate the area of primary steam coil required. Arithmetic mean temperature differences are used to keep this calculation simple; in practice, logarithmic mean temperatures would be used for greater accuracy. Please refer to Module 2. All the above applies when steam is flowing through a relatively narrow passage, such as the tubes in a shell and tube heat exchanger or the plates in a plate heat exchanger.

In some applications, perhaps a drying cylinder in a paper machine, superheated steam is admitted to a greater volume, when its velocity plummets to very small values.

Here, the steam near the wall of the cylinder quickly drops in temperature to near saturation and condensation begins. The heat flow through the wall is then the same as if the cylinder were supplied with saturated steam. There are instances where the presence of superheat can actually reduce the performance of a process, where steam is being used as a process material.