1. Saturated steam
Steam that has not been heat-treated is called saturated steam. It is a colorless, odorless, inflammable and non-corrosive gas. Saturated steam has the following characteristics.
(1) There is a one-to-one correspondence between the temperature and pressure of saturated steam, and there is only one independent variable between them.
(2) Saturated steam is easy to condense. If there is heat loss during the transmission process, liquid droplets or liquid mist will form in the steam, resulting in a decrease in temperature and pressure. Steam containing liquid droplets or liquid mist is called wet steam. Strictly speaking, saturated vapor is more or less a two-phase fluid containing liquid droplets or liquid mist, so different states cannot be described by the same gas state equation. The content of liquid droplets or liquid mist in saturated steam reflects the quality of steam, which is generally expressed by the parameter of dryness. The dryness of steam refers to the percentage of dry steam in a unit volume of saturated steam, represented by "x".
(3) It is difficult to accurately measure the flow of saturated steam, because the dryness of saturated steam is difficult to guarantee, and general flowmeters cannot accurately detect the flow of two-phase fluids, and fluctuations in steam pressure will cause changes in steam density, and additional errors will occur in the indications of flowmeters . Therefore, in steam measurement, we must try to keep the dryness of the steam at the measurement point to meet the requirements, and take compensation measures if necessary to achieve accurate measurement.
2. Superheated steam
Steam is a special medium, and generally speaking, steam refers to superheated steam. Superheated steam is a common power source, which is often used to drive a steam turbine to rotate, and then drive a generator or a centrifugal compressor to work. Superheated steam is obtained by heating saturated steam. It contains absolutely no liquid droplets or liquid mist, and belongs to the actual gas. The temperature and pressure parameters of superheated steam are two independent parameters, and its density should be determined by these two parameters.
After the superheated steam has been transported for a long distance, with the change of working conditions (such as temperature and pressure), especially when the degree of superheat is not high, it will enter saturation or supersaturation from the superheated state due to the decrease of heat loss temperature state, transforming into saturated steam or supersaturated steam with water droplets. When the saturated steam is decompressed suddenly and greatly, the liquid will also be saturated steam or supersaturated steam with water droplets when it expands adiabatically. The saturated steam is suddenly decompressed greatly, and the liquid will also be transformed into superheated steam when it expands adiabatically, thus forming a vapor-liquid two-phase flow medium.
