The excerpt is taken from my learning experience at Lotte Chemical, detailed report can be accessed here: Lotte By Syed Ahsan Imam
Steam Tables has been in service for ages, from calculations to sizing and to estimating certain process requirements, it is an essential tool of an engineer. Although, there is nothing that much cumbersome to understand the Steam Tables but still, there are many observations and certain understandings present in the tables. Here, I try to discuss some of them that sound handy and critical to know.
Saturated Steam Table is mainly classified as: Temperature based or Pressure based, both tables are similar in nature but sorted differently. The most common and important thing to remember is whether there is a Pressure Table or Temperature Table based Steam Table, only one saturation condition corresponds to the other parameter either it is temperature or pressure. Obviously, all the experiments are based on pure component bases which means there should not be any impurity present in water otherwise the process condition may vary. Superheated Steam Table is mainly presented as a single table. Because of having additional amount of sensible heat, the temperature varies on a single pressure condition. There are two most common terminology used in industry: Dry Steam and Wet Steam, the first one is predominantly known as Saturated Steam, the word dry is associated because it has high quality which means less water content, the latter one is when the steam gives up major portion of the latent heat and condensate droplet appears. When steam is used in driving turbine, it is called Propulsion or Drive Fluid, if used is any non-moving equipment like steam ejector* it is called as Motive Fluid. When used in stacks/chimneys or in burners, it is termed as Steam Atomizers because it causes to decrease the amount of soot as well as plume (as explained in previous section) and increases the surface area for effective combustion.
Saturated Steam or Superheated Steam: which one to choose and when?
This is probably one of the most important thing to consider whenever there is a process requirement related to steam use. Although, this might be debatable but a simple thumb rule suggests that, for heat transfer needs like in heat exchangers, evaporators etc., saturated steam is preferably used while for momentum and energy transfer applications like in steam turbines etc., superheated steam is preferred to use. In this regard, following important points were observed:
o Superheated steam has relatively low heat transfer coefficient as compared with saturated steam, so it requires large heat transfer area in order to transfer heat efficiently.
o Variable steam temperature at constant pressure as explained above is the main cause of disregarding the use of superheated steam. Because of having
additional sensible heat content, it must be maintained at high velocity otherwise the temperature fluctuates and drops significantly.
o Usually, latent heat is preferred to transfer because of its high value and consistency but in case of superheated steam, sensible heat is mainly used to do this which cause problems in temperature controlling.
o Obviously, if the temperature is very high as in the case of superheated steam, it might cause severe problems related to the material of construction specially in the case of heat exchangers.
o Temperature can control pressure or vice versa in the case of saturated steam which ensures rapid and uniform heat transfer.