Properties of Steam like Sensible Heat, Latent Heat, Total Heat
In Sugar process industry Equipment Design and drawing calculations, the properties of saturated steam are involved at every step. So we understand some definitions in steam properties:
Enthalpy:
Enthalpy can understand in the simple way by the following. (here not considered for use enthalpy in thermodynamics).
The terms formerly known as “heat of water” (sensible heat), “heat of evaporation” (latent heat) and “total heat of steam” are now known as the “enthalpy of (saturated) water”, “enthalpy of evaporation” and “enthalpy of saturated steam” respectively.
Similarly, the term total heat of superheated steam is now known as “enthalpy of superheated steam”. Enthalpy of evaporation is the difference between the enthalpy of dry saturated steam and enthalpy of (saturated) water, i.e,
Enthalpy of evaporation = Enthalpy of dry saturated steam – Enthalpy of boiling water
Generally, it is expressed in Kcal/kg or KJ/kg
Sensible Heat (Specific enthalpy of water):
Sensible heat is the energy required to change the temperature of a substance with no phase change.
When an object is heated, its temperature rises as heat is added. The increase in heat is called sensible heat. Similarly, when heat is removed from an object and its temperature falls, the heat removed is also called sensible heat. The heat that causes a change in temperature in an object without phase change is called sensible heat.
Latent heat (Enthalpy of Evaporation or vaporization)
An amount of heat required to convert its phase of one kilogram of water at a given temperature.
All pure substances in nature are able to change their state. Solids can become liquids (ice to water) and liquids can become gases (water to vapor) but changes such as these require the addition or removal of heat. The heat that causes these changes is called latent heat.
Sensible Heat Vs Latent heat
Latent and sensible heat are types of energy released or absorbed in the atmosphere. Latent heat is related to changes in phase between liquids, gases, and solids. Sensible heat is related to changes in temperature of a gas or object with no change in phase.
Total Heat or Specific enthalpy of the steam:
It is the total heat contained in 1 kg of steam. Total heat is the sum of the enthalpy of the various states, liquid (water) and gas (vapour) and also total heat can defined as the total amount of heat received by 1 kg of water from OoC at constant pressure to convert it to the desired form of steam.
The specific volume of steam:
Volume occupied in M3 by 1 kg of steam. The value in cubic metre per kg of dry saturated steam (M3/kg)
The density of the steam:
Specific mass of the steam in a volume of one M3. is the reciprocal of specific volume.
Dry Saturated Steam :
Dry saturated steam is the sum of the enthalpy of saturated water and enthalpy of evaporation and is defined as the quantity of heat required to raise the temperature of one kilogram of water from freezing point to the temperature of evaporation (corresponding to given pressure p)and then convert it into dry saturated steam at that temperature and pressure.
Wet Steam :
The steam in the steam space of a boiler generally contains water mixed with it in the form of a mist (fine water particles). Such a steam is termed as wet steam. The quality of steam as regards its dryness is termed as dryness fraction.
Super Saturated Steam
The steam having lesser temperature and greater density with respect to the steam table values for a particular saturation pressure is called super saturated steam.
If the water is entirely evaporated and further heat is then supplied, the first effect on the steam is to make it dry if it is not already dry. The temperature of steam will then begin to increase with a corresponding increase in volume. Steam in this condition, heated out of contact with water, is said to be superheated. Superheating is assumed to take place at constant pressure. The amount of superheating is measured by the rise in temperature of the steam above its saturation temperature.
This condition is obtained when it is cooled by its own expansion in a nozzle. but it is very unstable and the steam soon resumes the saturated condition.
Absolute pressure:
Absolute pressure = Gauge pressure + Atmospheric pressure
(Atmospheric pressure at 1.01325 bar or kg/cm2, i.e. normal atmospheric pressure on the sea level at 0°C).
The formulae of Regnault Given
Q = T
λ = 607 – 0.7 T
H = Q + λ = 607 + 0.3T
-
- Q = sensible heat supplied to unit weight of water to raise it from freezing point to to T oC expressed in Kcal/kg
- λ = latent heat of vaporization of water at ToC i.e. the quantity of heat to change unit weight of water into steam at the same temperature, in kcal/kg “
- H = total heat to be supplied to unit weight of water, starting at 0°C (32°F), to change it to steam at T° C, expressed in kcal/kg
Online Calculator of Steam Table for Saturated Steam
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Robert Evaporator Design and online calculation sheet with formulas.
Rillieux’s principles and pressure drops calculation for Multiple Effect Evaporator bodies.
Wetting rate concepts and formulas for evaporators calculations.
