Saturated Steam vs Superheated Steam in Heat Exchangers Superheated steam is sometimes used for process heating in many steam plants around the world, however it is not the best choice. In refieneries and petrochemical plants, superheated steam is already available on site for power generation, being the preferred energy source for steam turbines (STs), rather than because it has any advantage over saturated steam for heating purposes. In most cases, saturated steam should be used for heat transfer processes, even if it means desuperheating the steam to do so (to within about few degrees of superheat). This small degree of superheat (within 10 degrees) 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 because of the following:
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 (Sensible heat) is relatively small in comparison to its enthalpy of evaporation (Latent heat). 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 Superheated steam requires larger heat transfer areas. From the heat transfer equation: Q= U * A * delta T
If superheated steam has a higher temperature than saturated steam at the same pressure, surely superheated steam should be able to transfer more heat? The answer to this is ‘no’. 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 in the above equation, however the 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 shell and tube heat exchanger, ‘U’ values might be as low as 100 W/m2°C for superheated steam but about 500 W/m2°C for saturated steam can be expected.
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 heat exchanger operating at the same pressure to deliver the same heat flowrate. So, superheated steam is not as effective as saturated steam for heat transfer applications. If there is no choice but to use superheated steam, the superheated steam for heat transfer purposes should not hold more than about 10°C of superheat. It is relatively easy and practical to design a heat exchanger 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 heat exchanger will be used purely to reduce the temperature of superheated steam to its saturation point. The rest of the heat exchanger will then be able to take advantage of the higher heat transfer ability of the saturated steam. The effect is that the overall ‘U’ value may not be much less than if saturated steam were supplied to the coil. From practical experience (as per SpiraxSarco), if the extra heating area needed for superheated steam is 1% per 2°C of superheat, the heat exchanger will be large enough. This seems to work up to 10°C of superheat. It is not recommended that superheated steam above 10°C of superheat be used for heating purposes due to the probable disproportionate and uneconomic size of the heating surface, the propensity for fouling by dirt, and the possibility of product spoilage by the high and uneven superheat temperatures. Summary:
Disadvantages of using superheated steam for heating:
