HUMIDIFICATION AND DRYING Flashcards
Involves removal of moisture, usually water from a solid by allowing a composite material to come in contact with a drying medium that supplies the heat needed by moisture to evaporate.
Drying
Moisture remove is the unbound/free moisture, capable of generating partial pressure equivalent to vapor pressure time of drying.
Drying Rate Period
Moisture remove is bound moisture, water evaporates after capillary action/diffusion to the solids’ surface time of drying.
Falling Rate Period
Drying medium conditions subjecting the solids are changing.
Variable Drying Conditions
- tsolid and residual moisture = constant
- wet bulb temperature of air is constant
Adiabatic Dryers
Rf is assumed linear and passing through origin, Rf independent of ΔH
Drying is not Adiabatic
x > xc
Drying Rate Period
x < xc
Falling Rate Period
qL and qD = 0
Adiabatic Dryers
Involves the transfer of material between a pure liquid phase and a fixed gas phase that is nearly insoluble in the liquid
Humidification
- These operations are somewhat similar but simpler than absorption and stripping.
Humidification
- The liquid contains only one component, there are no concentration
gradients and no resistance to mass transfer in the liquid phase.
Humidification
the gaseous form of the component that is also present
as liquid (liquid that can become gas)
- Vapor
component present only in gaseous form (does not
condense)
Gas
- Sometimes also called as Absolute Humidity
- This refers to the ratio of the mass of vapor per unit mass of vapor-free gas. This may be determined from the partial pressure exerted by the vapor (A) in the gas (B).
HUMIDITY
- This refers to the maximum amount of vapor that a gas can contain which is in equilibrium with the liquid at a given temperature. The partial pressure of the vapor is in effect equal to the vapor pressure.
Saturation Humidity, HS.
- This refers to the ratio of the partial pressure to the vapor pressure at a specified temperature of the gas-vapor mixture.
Relative Humidity, RH or HR
- This refers to the ratio of the partial pressure to the vapor pressure at
a specified temperature of the gas-vapor mixture.
Percentage Saturation or Percentage Humidity, HA
This is the volume occupied by one pound of dry gas plus whatever vapor it contains at a given pressure and temperature.
Humid Volume, VH
The specific volume if the gas is saturated with the vapor
Saturated volume, Vs
This is the heat needed to raise the temperature of one pound of dry gas and the vapor it contains by 1 degree F
Humid Heat, Cs
This refers to the total enthalpy content of 1 lb gas and its vapor at a given temperature
Enthalpy of Mixture, Hy
- This refers to the steady state temperature attained by a gas-vapor mixture when saturated with vapor by spraying under adiabatic conditions.
Adiabatic Saturation Temperature, Ts
This refers to the steady state temperature attained by a small mass of liquid immersed in a large body of gas.
Wet Bulb Temperature, Tw
- This is the steady temperature attained when a vapor-gas mixture becomes saturated when cooled at constant humidity, that is pA= pA
Dew Point, Td
Adiabatic Humidification
In order for humidification to be considered adiabatic, the following conditions must be met:
1. Tx1 =Tx2 =Tx =constant
2. TW1=TW2= Tx= constant
That is, the temperature of the water is constant and equal to the wet-bulb temperature of the gas which is also constant. This, of course, is a simplification which may not be achieved in reality.
the air undergoes a non-adiabatic humidification process. Thus, the integrated design equations used are similar to that of dehumidification except that the limits are reversed.
Water Cooling
Due to the approximate nature of the mass and heat transfer equations generated for water cooling and dehumidification, it is necessary to use a more accurate working design equation. If the system is limited to air-water, the design may be based on the enthalpy driving force as given by the ________
Merkel’s Equation.
Presents Physical and thermal properties of moist air in a graphical form
Psychrometric Chart
Actual temperature of air-vapor mixture
Dry Bulb Temperature
lowest temperature at which air can be cooled by the evaporation of water into the air at a constant pressure
Wet Bulb Temperature
Temperature at which H = Hs
Dew point
