Basic Knowledge of Steam
Steam Related Terminology
The use of saturation state steam tables is essential in the steam engineering field (hereinafter referred to as "Steam Engineering"). In This chapter, we will first discuss the terms which are used in saturation state steam tables and some other related terms.
This is the force which is applied perpendicular to a surface.
Since Pascal is such a small unit, MPa or kPa is more commonly used in the field of steam engineering. (In this document, this shall be expressed hereafter as “MPa”) kgf/cm2 In addition, the unit kgf/cm2 is often used, and the relationship to Mpa is 1MPa ＝10.197 (kgf/cm2）
There are two types of pressure: absolute pressure, where the reference (0MPa) is set at a complete vacuum, and gauge pressure, where the reference is set at atmospheric pressure. The relationship between absolute pressure and gauge pressure is shown in the below formula.
（Gauge pressure）＝（Absolute pressure）－（Atmospheric pressure）
gauge pressure is 0MPa and absolute pressure is 0.10133MPa. The difference between these pressures is about 0.1MPa. In order to distinguish them, it is usual to add ‘a’ for absolute pressure and ‘g’ for gauge pressure after the unit symbol.
Specific volume, Specific weight
1kg The volume per 1kg of steam is called specific volume, and this is expressed as m3/kg. The value of the specific volume of steam is basically determined by pressure and temperature. When the pressure and temperature values change, the specific volume of steam also changes. The rate of change in the steam is much larger than the rate of change in liquid.
Specific mass (or density) is the reciprocal of specific volume, and the unit is kg/ m3.
The temperature rises when water is heated, but when it gets to a certain temperature, all added heat is used for evaporation of water and the same temperature of steam is produced. This temperature is called saturated temperature. The saturated temperature depends on the absolute pressure of water. The higher (lower) the pressure, the higher (lower) the saturated temperature.
Specific enthalpy is often used in thermodynamics and it describes energy quantity per mass unit (1kg) of steam or water. In the steam engineering field, in particular, the term usually means internal heat quantity. These units are described in the steam pressure table.
➀. Specific enthalpy of water (h’)
It refers to the amount of heat required to heat 1 kg of water from 0°C to the current temperature, and is a synonym for sensible heat.
Enthalpy is set to 0 (zero) for the temperature 0 C. Thus, it describes relative heat quantity which is easy to understand sensuously.
At atmospheric pressure, water boils at 100 C, but 419J of heat power is required to increase the temperature of 1kg water from 0 C to 100 C. The specific heat of water 4.19kJ/kgC is calculated from this value.
2. Specific enthalpy of vaporization（r)
This is the heat power which is necessary for 1kg of boiling water to change into steam. The temperature when mixing water and steam is not changed, and all energy is used for changing it from water into steam. This is synonymous with heat of vaporization and heat of evaporation.
3. Specific enthalpy of saturated steam（h”）
This is the total energy possessed by saturated steam and expresses the sum of specific enthalpy of the water and specific enthalpy of vaporization as shown in the following formula.
The heat for changing the -temperature of a substance is called sensible heat. When a substance absorbs sensible heat, the temperature increases and when the substance releases sensible heat, the temperature drops. In most cases, it refers to the heat power that water (liquid) has in the steam engineering field.
The heat that changes the phase of a substance is called latent heat. The temperature doesn’t change by absorbing and releasing latent heat. Latent heat is also known as heat of fusion, evaporation heat (heat of evaporation), heat of liquefaction, and heat of solidification. In the steam engineering field, latent heat often refers to enthalpy of evaporation.
The temperature increase for given heat power applied to a mass unit of a substance depends on the substance. The heat power which is necessary to raise the temperature of 1kg of a certain substance by 1C is called specific heat of the substance, and is displayed as kJ/kgC. Specific heat has two values. One is for constant volume of substances (specific heat at constant volume) and another is for constant pressure (specific heat at constant pressure). Generally, the difference between them only becomes relevant if the substance is gas. Thus, the specific heat of water which is liquid just represents 4.19 kJ/kg℃ as already mentioned in "➀. Specific enthalpy of water (h’) "