蒸汽性质 | Properties of Vapours

物质形态 | Phases of Pure Substances

Substances normally exist in different phases: solid, liquid and gas.

Liquid state:

A liquid that is about to vaporize (将要蒸发的液体) is called a saturated liquid
A saturated liquid–vapour mixture since the liquid and vapour phases coexist in equilibrium
A vapour that is about to condense (将要凝结的蒸汽) is called a saturated vapour
A vapour that is not about to condense (不将要凝结的蒸汽) is called a superheated vapour

Compressed liquid (state 1): At certain low temperature, liquid is not about to vaporize
Saturated liquid (state 2): A liquid that is about to vaporize
Saturated liquid (State 3): vapour mixture
Saturated vapour (state 4): A vapour that is about to condense
Superheated vapour (state 5): A vapour that is not about to condense

Saturation temperature: At a given pressure, the temperature at which a substance changes phase T_sat

Saturation pressure: At a given temperature, the pressure at which a substance changes phase p_sat

蒸汽相关物态 | Vapour related phrases

Gas: a state of matter (solid, liquid, gas, plasma)
Vapour: substance in gas state, it can be condensed to liquid
Steam: water vapour
Moisture: presence of a liquid (often water) in trace amount

潜热 | Latent heat

Latent heat: The amount of energy absorbed or released during a phase-change process

在温度保持不变的条件下，物质在从某一个相转变为另一个相的相变过程中所吸入或放出的热量。

恒压下相变过程的 T-v 图 | T-v diagram of constant-pressure phase-change processes

临界点 (Critical point) - the point at which the saturated liquid and saturated vapour states are identical.

三相点 | Triple point

At triple-point pressure and temperature, a substance exists in three phases in equilibrium.

升华 | Sublimation

物体由固体无需融化直接转变为气体 | Solids evaporates directly without melting first.

液体与蒸汽状态的焓 | Enthalpy of saturated liquid and saturated vapour states

f: for saturated liquid

g: for saturated vapour

fg: for difference between the saturated vapour and saturated liquid values of the same property

v_f: specific volume of saturated liquid

v_g: specific volume of saturated vapour

v_fg: difference between v_g and v_f that is, v_fg = v_g – v_f

h_fg: enthalpy of vaporization (or latent heat of vaporization), represents the amount of energy needed to vaporize a unit mass of saturated liquid at a given temperature or pressure.

超临界流体 | Supercritical fluid

any substance at a temperature and pressure above its critical point, where distinct liquid and gas phases do not exist.
can effuse(喷出) through solids like a gas, and dissolve(溶解) materials like a liquid.

饱和气液混合物 | Saturated Liquid–Vapour Mixture

x 为蒸气在气液混合物中的比例

Define quality x as the ratio of the mass of vapour to the total mass of the mixture.

同样地，对于内能、焓和熵，也有：

湿度 | Humidity

Absolute humidity | specific humidity | humidity ratio:

单位质量干燥空气中存在水蒸气的质量。

where R_a = 0.287 kJ/kgK, R_v = 0.462 kJ/kgK

饱和状态下的绝对湿度为当前温度下所能达到的最大湿度。

Relative humidity:

空气中实际水蒸气的质量与饱和状态下所能容纳的水蒸气质量之比。

where p_g is p saturated at T.

The amount of moisture air can hold is proportional to the saturation pressure, which increases with temperature. Air can hold more moisture at higher temperatures.

蒸汽压强 | Vapour pressure

p_atm = p_a + p_v

pv = ϕp_{sat @T}

where p_{sat @T} is the saturation pressure of water at the specified temperature.

水暴露于空气的标准相平衡 | Phase equilibrium criterion for water exposed to air

当环境湿度达到 100%，空气饱和，于是此时不存在从液态到气态的相变，并且这两种物态达到平衡。

When the environment relative humidity is 100% , the air is saturated, there is no transformation from the liquid phase to the vapour phase, and the two phases are in phase equilibrium.

对于暴露于空气的液态水而言，标准相平衡则为：空气中的蒸汽压强与水温下水的饱和压强相等。

For liquid water that is open to the atmosphere, the criterion for phase equilibrium is: the vapour pressure in the air must be equal to the saturation pressure of water at the water temperature.

p_v = p_{sat @T}