Steam for Indirect Heating
Saturated steam is the steam which is completely dry and has same temperature as the water from which it is formed. When we start boiling the water, its temperature goes on increasing till it reaches the boiling point at that pressure.
If further heat is added to the water, it converts to steam after absorbing the latent heat of evaporation. This steam, which exists at same temperature as that of the water is called as saturated steam.
If saturated steam does not contain even slightest amount of water droplets, it is called as dry saturated steam.
As we all know, at a particular pressure, saturated steam always exists at the boiling point of water at that pressure. Saturated steam exists roughly at 100 Deg. C. at atmospheric pressure. As we go on increasing the pressure, the boiling point of water also goes up and hence, the temperature of saturated also goes up.
What will happen to the temperature of saturated steam if we go on reducing the pressure instead of increasing it? Logical answer is, the temperature at which saturated steam will be formed will also go down. This steam, which is generated at sub-atmospheric pressure (vacuum conditions) is called as vacuum steam. Vacuum steam has temperature below 100 Deg. C.
Let us take an example where we have to heat a fluid which is at 30 Degrees to 80 Degrees. The task can be accomplished by using hot water or vacuum steam of suitable temperature. Using vacuum steam is an expensive affair as vacuum pump will be required to create vacuum.
On the other hand, in case of heating with water, the temperature of water will go on decreasing as it will be losing its sensible heat. Hence, a larger heat transfer area is required. In case of vacuum steam, there is transfer of latent steam with high heat transfer coefficients and will require lower heat transfer area. As vacuum steam heats the fluid by losing its latent heat, the heating will take place at same temperature.
Steam for Power Generation
If saturated steam is further heated, its temperature goes on increasing beyond the boiling point. Such steam, with temperature higher than the boiling point at that pressure is called as superheated steam.
It is always recommended to use saturated steam for process application and superheated steam for power generation purpose.
Steam for Injection (Direct Heating)
(also referred to as clean steam, WFI steam, high purity steam, sanitary clean steam, pharmaceutical clean steam, GMP clean steam and pyrogen free steam) is similar to clean steam; however, the resulting condensate must meet the standards of USP grade water for injection (WFI) and contain no bacteria or pyrogens. Pure steam must be produced by a pure steam generator; which is classed as sanitary in design, or from a multiple effect still.