As long as there is air, oxygen and other dissolved gases in feedwater, there will be pressure-loss-induced condensate, whether the steam is saturated or superheated. The feedwater will always contain air. It can also contain bicarbonates and carbonate alkalinities, which oxidize to carbon dioxide at elevated temperatures. The oxygen and carbon dioxide together combine to produce acidic carbonization that has a corrosive effect. Deaeration and pretreatment of the feedwater does not remove all the air, some of them are carried over to the steam. This is a malignant fact associated with steam generation.
In gas mixtures or multiphase flows, each gas assumes a part of the total volume or pressure. This is referred to as the law of partial pressures. The partial pressure of each constituent gas depends upon its proportion of the total pressure of the mixture. This is true regardless of whether the gas is ideal or Van der Waal.
Consider a 100 psi line consisting of 80% quality steam and say 20% air, the total steam pressure would be 80 psi and the total air pressure 20 psi. Consequently, the line would have 80 psi steam in a 100 psi line at a temperature of 312 degrees instead of 327 degrees at 100 psi, a difference of 15 degrees between the two pressures. In addition, there will a change in BTU’s. Though the 80 pounds steam has more enthalpy of latent heat per pound than the 100 pounds steam, it has less by volume. In other words the air has displaced a portion of the enthalpy needed, by displacing a portion of the steam. An air film 0.04” thick has the same resistance to heat transfer as water 1” thick, or iron 4.3” thick or copper 43“ thick. As a film it acts as an insulator, in solution with steam it chokes the steam of its heating potential, moisturizes the steam and corroding pipe and equipment. The effect is more pronounced in superheated steam line for one additional reason, air flowing at a velocity of 10,000 fpm will induce pressure drop in the line (velocity and pressure are inversely proportional, one increases at the expense of the other), as a consequence of the decreased pressure the system will seek an equilibrium temperature to sustain the pressure – temperature relationship within the superheat regime, causing a drop in temperature, hence loss in latent heat.
Condensate will accumulate in a superheated steam line if (1) it is not properly trapped (2) if drip legs are not properly located at reasonable intervals and (3) if the pipeline is not properly pitched. Since it will be easier for the condensate to flow with the steam, the line should be sloped down the direction of steam flow. If the flow of condensate was against the flow of steam we would have the following scenario: steam moving with a velocity of say 6000 to 10000 fpm, the condensate would tend to eddy into pools trying to overcome the friction of the steam on its flow to the low point upstream (in other words the steam – water surface shear force will tend to overcome the force of gravity causing the water to flow downhill so that water builds up and plugs the pipe). As condensate builds up in this manner the velocity of the steam will cause it to be picked up and introduced into the steam as a mist, essentially humidifying the steam and lowering its heat content.
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