shmt+4
TRANSCRIPT
SIMULTANEOUS HEAT AND
MASS TRANSFER
(SHMT)
7th Semester,
B.Sc. Chemical Engineering
Session 2008
Delivered by:
Mr. Usman Ali
Department of Chemical Engineering
University of Engineering & Technology, Lahore
Humidification Operations
Unit Operation.
Simultaneous heat and mass transfer
Transfer between pure liquid phase and a fixed gas that is insoluble in liquid.
Air-water contacting: o Water Cooling
o Humidification
o Dehumidification
o Gas Cooling
The degree of moisture has a strong effect on
o heating, cooling, and comfort
o insulation, roofing, stability and deformation of building materials
o sound absorption, odor levels, ventilation
o industry and agriculture
Humidification Operations
Dry Air and Water Vapor
Nitrogen 78.084 28.0134 Oxygen 20.448 31.9988 Argon 0.934 39.9430 Carbon Dioxide 0.031 44.0100
Dry Air Component % by vol MW
Effective MW 28.9645
Water Vapor 18.0153
Humidity
Saturated gas
Saturation Humidity
Relative humidity
Percentage humidity
Humid heat
Humid volume
Dew point / Saturation temperature
Total Enthalpy
Dry-bulb Temperature
Wet-bulb Temperature
Adiabatic Saturator
Example
In a vessel at 101.3 kN/m2 and 300 K, the percentage relative humidity of the water vapour in the air is 25. If the partial pressure of water vapour when air is saturated with vapour at 300 K is 3.6 kN/m2, calculate:
(a) the partial pressure of the water vapour in the vessel;
(b) the specific volumes of the air and water vapour;
(c) the humidity of the air; and
(d) the percentage humidity.
In a process in which it is used as a solvent, benzene is evaporated into dry nitrogen. At 297 K and 101.3 kN/m2, the resulting mixture has a percentage relative humidity of 60. It is required to recover 80 per cent of the benzene present by cooling to 283 K and compressing to a suitable pressure. What should this pressure be? The vapour pressure of benzene is 12.2 kN/m2 at 297 K and 6.0 kN/m2 at 283 K.
Example
Humidity charts / Psychrometric charts
Abscissa – temperature
Ordinate – humidity
Percentage humidity lines
Adiabatic cooling lines
Specific volume of dry air & saturated volume lines
Humid heat vs humidity
Point on chart
At saturation line
Above
Below
Use of humidity charts
Any pressure
Any system other than air-water
Vapor pressure & Latent heat of vaporization
Specific heats of dry gas & vapor
Molecular weights of gas & vapor
Humidity charts other than air-water
Wet-bulb temperature
• Steady-state , non-equilibrium temperature reached by small amount of liquid immersed under adiabatic conditions in a continuous stream of gas.
• It is very close to the adiabatic saturation temperature for the air-water system, but not for most other vapor-gas systems
How to measure ?
• Adiabatic conditions
• Covered by wick
• Must be saturated with pure liquid
Heat of vaporization
+
sensible heat of vapors
=
sensible heat flowing from gas to liq.
Precautions
1. The wick must be completely
wet.
2. Velocity of gas should be large.
3. Make up water should be at wet-
bulb temperature.
WET DRY
Cooling Towers
Introduction:
Dry cooling
Evaporative cooling
Compression cooling
Evaporative Cooling
Advantages Reduced ground area (higher thermal power/m²)
More efficient heat exchange
Less electrical consumption
Limits set by wet bulb temperature
Disadvantages
Substantial water consumption
Water treatment may be necessary
Cooling in Industry
A cooling tower is an equipment used to reduce the temperature of a water stream by extracting heat from water and emitting it to the atmosphere.
Principle : Evaporation
Cooling Towers
Cooling Towers
Cooling towers are evaporative coolers used for cooling water or other working medium to near the ambient wet-bulb temperature of the air.
http://www.industry-animated.org/coolingtwr.swf
Important Factors for Operation
Dry bulb and wet bulb temperature of air
Temperature of warm water
Efficiency of contact between air and water
Uniformity of distribution of phases
Air pressure drop
Desired temperature of cooled water
Types of Cooling Towers
Induced Draft
Cooling Towers
Natural Draft Mechanical Draft
Forced Draft
Counter Current
Cross Flow
Spray Ponds
Atmospheric Cooling Tower
Rectangular chamber with louvers
Cheap and inefficient
Performance depends on direction and velocity of wind
Natural draft Cooling Tower
Natural draft Cooling Tower
Large reinforced concrete shell of hyperbolic shape
At bottom , small part is filled with high void packing.
Factors for Natural Draft:
i. Rise in T and H of air in column reduces its
density
ii. Wind velocity at tower bottom
Natural draft Cooling Tower
Hyperbolic Shape:
More packing in bigger area as in bottom
Entering air gets smoothly directed towards centre and creating a strong draft
Greater strength ant stability
Mechanical draft towers have large fans to force
or draw air through circulated water.
Types :
Forced draft
Induced draft
Mechanical Draft Cooling Tower
Air blown through tower by centrifugal fan at air inlet
Advantages:
Suited for high air resistance
Velocity head is converted in press. Head on entering tower.
Fans are relatively quiet and less vibration
Disadvantages:
Recirculation due to high air-entry
Air flow through fill may not be uniform.
Low air-exit velocities
Forced Draft Cooling Tower
Forced Draft Cooling Tower
Counter-flow
Cross-flow
Advantage:
Less recirculation than forced draft towers
Disadvantage:
Fans and motor drive mechanism require weather-proof.
Induced draft Cooling Tower
Counter-flow Cooling Tower
Dry air contacts coldest water at bottom and Humid air contacts warm water at top
Creating maximum average driving force for both heat and mass transfer
More horse power of fan as there is restricted area for air flow at bottom
Counter-flow Cooling Tower
Cross-Flow Cooling Tower
Less horse power of fan as compared to counter flow.
Growth of algae on fill is more
Cross-Flow Cooling Tower
Components of a cooling tower
Shell, Frame and casing: support exterior enclosures
Tower Fill: facilitate heat transfer by maximizing water / air contact
o Splash fill
o Film fill
Cold Water Basin: receives water at bottom of tower
Drift Eliminators: capture droplets in air stream
Air Inlet: entry point of air
Louvers: equalize air flow into the fill and retain water within tower
Water Distributor(Nozzles): spray water to wet the fill
Fans
Mechanical Support
Performance of cooling towers
Range Difference in temperature
between the inlet hot water and the outlet cold water.
Approach Difference in temperature
between the outlet cold water and the wet bulb temperature of the entering air.
Effectiveness. This is the ratio between the range and the ideal range
(in percentage)
Evaporation loss.
This is the water quantity evaporated for cooling duty.
Theoretically the evaporation quantity works out to 1.8 m3 for every 1,000,000 kCal heat rejected.
= Range / (Range + Approach)
Blow-out Water droplets blown out of the cooling tower by wind, generally at the air
inlet openings. Water may also be lost, in the absence of wind, through
splashing or misting.
Devices such as wind screens, louvers, splash deflectors and water diverters
are used to limit these losses.
Blow-down
The portion of the circulating water flow that is removed in order to maintain
the amount of dissolved solids and other impurities at an acceptable level.
Plume
The stream of saturated exhaust air leaving the cooling tower.
The plume is visible when water vapor it contains condenses in contact with
cooler ambient air, like the saturated air in one's breath fogs on a cold day.
Problems Related To Cooling Water
Interference and Recirculation
Others Problems
1. Scale
2. Fouling
3. Microbiological growth
4. Corrosion
Problems Related To Cooling Water