Superheated Steam

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. The solution is to supply the turbine with superheated steam at the inlet, and use the energy in the superheated portion to drive the rotor until the temperature/pressure conditions are close to saturation; and then exhaust the steam.

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:The Carnot cycle, where the change in temperature of the steam between the inlet and outlet is compared to the inlet temperature.

The Rankine cycle, where the change in heat energy of the steam between the inlet and outlet is compared to the total energy taken from the steam
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 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:

Gives up little heat until it has cooled to saturation temperature.

Creates temperature gradients over the heat transfer surface as it cools to saturation temperature.

Provides lower rates of heat transfer whilst the steam is superheated.

Requires larger heat transfer areas.

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? The answer to this is ‘no’. This will now be looked at in more detail.

Q=U*A*DT

Where:
= Heat transferred per unit time (W)
U = Overall thermal transmittance (heat transfer coefficient) (W/m2°C)
A = Heat transfer area (m2)
DT = Temperature difference between primary and secondary fluid (°C) Equation also shows that heat transfer will depend on the overall heat transfer coefficient ‘U’, and the heat transfer area ‘A’.

For any single application, the heat transfer area might be fixed. However, the same cannot be said of the ‘U’ value; and this is the major difference between saturated and superheated steam. The overall ‘U’ value for superheated steam will vary throughout the process, but will always be much lower than that for saturated steam. It is difficult to predict ‘U’ values for superheated steam, as these will depend upon many factors, but generally, the higher the degree of superheat, the lower the ‘U’ value.

Typically, for a horizontal steam coil surrounded with water, ‘U’ values might be as low as 50 to 100 W/m2)°C for superheated steam but 1 200 W/m2)°C for saturated steam, as depicted in Figure 2.3.2.

For steam to oil applications, the ‘U’ values might be considerably less, perhaps as low as 20 W/m2)°C for superheated steam and 150 W/m2)°C for saturated steam.

In a shell and tube heat exchanger, 100 W/m2)°C for superheated steam and 500 W/m2)°C for saturated steam can be expected. 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. The next Section ‘Fouling’ gives more detail.

Not only is superheated steam less effective at transferring heat, it is very difficult to quantify using Equation 2.5.3, = U A DT, as the temperature of the steam will fall as it gives up its heat while passing along the heating surface.

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.

The steam should reach saturation relatively soon in the process; this allows the steam to condense to produce higher heat transfer rates and result in a higher overall ‘U’ value for the whole coil, .

To help to enable this, superheated steam used for heat transfer purposes should not hold more than about 10°C of superheat.