Heat Exchanger Flashcards
Heat exchangers
Used for exchange of heat between the two fluids that are at different temperatures
Tubular Heat Exchangers
simplest heat exchanger in which the hot and cold fluids move in the same or opposite directions.
Parallel-Flow
Hot and cold fluids enter at the same end, flow in the same direction, and leave at the same end.
Counterflow
Fluids enter at opposite ends, flow in opposite directions, and leave at opposite ends.
Cross Flow
Fluids move perpendicular to each other
Tubesheet
Plates where tubes are fitted
Baffle pitch
AKA Baffle Spacing
*Distance between baffles
Baffle Cut
*Baffle Pitch / Shell Diameter
*Typically 20 percent
Purpose of baffles
*increase the convection coefficient of the shell-side fluid by inducing turbulence
*physically support tubes
Fouling vs Scaling
Fouling
*due to fluid impurities, rust and reactions between fluid and wall
Scaling
*type of fouling
*deposits of film
NTU
Number of Transfer Units
*measure of the physical size (heat transfer area) of the heat exchanger.
*used to calculate the rate of heat transfer in heat exchangers when insufficient data to calculate LMTD.
Series vs Parallel Heat Exchanger
Series
*When temperature difference is too large
*Temp difference needs to be reduced gradually
*Temperature change for multiple process streams
Parallel
*Heat transfer requirement is large (high flow rate)
*A single heat exchanger is enough to satisfy the heat transfer requirements
Finned tubular heat exchanger
*Give larger area per tube
*Cross flow
*Unmixed - fins inhibit motion transverse to the main flow direction
Unfinned tubular heat exchanger
*Mixed - fins inhibit motion transverse to the main flow direction
Factors affecting Heat Exchangers
- Configuration (concurrent/ countercurrent)
- Temperature Difference
- Number of passes
- Feed Fluid Properties
*Pressure drop
*Fouling and corrosion
WHEN TO USE?
*Double Pipe Heat Exchanger
*Which fluids to Shell-side or Tube-side?
Double Pipe Heat Exchanger
*Heat Transfer Area: 100 - 200 ft^2
Tube-side - easy and cheap cleaning and replacement
*High pressure
*Fouling
*Corrosive
*Non-condensing vapors
*Non-solidifying liquids
Shell-side
*Viscous
*Limited allowable pressure drop
*Condensing / Solidifying fluids
TEMA Heat Exchanger
*Tubular Exchanger Manufacturers Association
*Three-Letter Designations
1st: Front-end head type
2nd: Shell types
3rd: Rear-end head types
Economizers
*Type of HE
*Uses heat of recycled / recovered flue gases to heat fluids
Boiler
Closed vessel where water is heated to generate superheated steam
Fired Heaters
*AKA Furnace
*Uses direct heat transfer from combustion of fuels to fluids
Why Saturated Steam is better than Superheated Steam?
*Safer for equipment
*Higher heat transfer coefficient (higher gas T = lower heat transfer coefficient)
USES, PROS & CONS:
Plate Heat Exchanger
*USES
+Viscous
+Corrosive
+Slurry
-For boiling or condensing
*PROS
-Cheaper
-Less fouling
*CONS
-High pressure drop
-Low pressure and temperature capacity
PROS & CONS:
Shell and Tube Exchangers
*PROS:
-Large heat transfer area
-High pressure
-Easily cleaned
-Well established design and fabrication
*CONS:
-More space than plate HE of similar capacity
TUBESHEET LAYOUT:
*Triangular pitch
*Square pitch
*Triangular pitch
-For CLEAN fluids
-HIGHER heat transfer area and pressure drop
*Square pitch
-For FOULING fluids
-LESS heat transfer area and pressure drop
Shell side pressure drop in a shell and tube heat exchanger does not depend upon the ____
baffle spacing & shell diameter.
tube diameter & pitch.
viscosity, density & mass velocity of shell side fluid.
none of these.
none of these.
Economy of a multiple effect evaporator is not influenced by the ___
boiling point elevations
In a multiple effect evaporator, the effect of boiling point elevation is to ____
reduce the capacity
reduce the economy
increase the economy
none of these
reduce the capacity
In a heat exchanger, floating head is provided to ___
relieve stresses caused by thermal expansion.
Baffles are provided in heat exchangers to increase the ___
*enhance turbulence
*heat transfer co-efficient
Terminal point temperature differences between fluids in case of a heat exchanger is termed as ____
approach
For a cold viscous feed, backward feed gives __________ than forward feed.
a higher capacity
Effects of Fouling
*Fouling has high thermal resistance
*Reduced cross-sectional area and velocity
*Increased pressure drop
When to use Air Fin or Shell and Tube Heat Exchanger?
*Air Fin Heat Exchanger
-Small approach temperature
-Cooling water not available
-Lower HE cost is desired
*Shell and Tube Heat Exchanger
-High approach temperature
-Cooling water is available
-Smaller size, lower energy cost and more efficient heat transfer is desired
Floating Head
Allow for tubes to expand due to temperature differences
Compact Heat Exchangers
+Higher area density (m2 area/ m3 volume)
- Fouling problems
*Plate and Frame
*Plate Fin
*Spiral
Pick out the wrong statement.
Swenson-Walker crystalliser is a batch crystalliser.
Super saturation of the solution is the driving potential for a crystal growth.
The liquor left after the removal of crystals from a solution is called mother liquor.
The first stage of crystal formation is called nucleation.
Swenson-Walker crystalliser is a batch crystalliser.
Which type of heat exchanger is preferred for heavy heat loads ?
Double pipe
Plate fine
Series and parallel set of shell and tube
None of these
Series and parallel set of shell and tube
Vent pipes are provided in a condenser to ___
remove non-condensable gases.
purge the condenser.
facilitate easy cleaning of tubes.
none of these.
remove non-condensable gases.
Tube pitch is the __________ of tube diameters and the clearances.
sum
difference
ratio
none of these
sum
Economizer:
*For Heating
*For Cooling
*Heating
-use flue gas to reduce power consumption
*Cooling
-use outside air to reduce power consumption
Indirect contact heat exchangers are preferred over direct contact heat exchangers, because ____
heat transfer co-efficient are high.
there is no risk of contamination.
there is no mist formation.
cost of equipment is lower.
there is no risk of contamination.
Water is normally used as a coolant in the heat exchange equipments mainly because of its ___
abundance & high heat capacity
low density
low viscosity
high fluidity
abundance & high heat capacity
Differentiate:
*Fixed Tube Sheet HE
*Floating Heat HE
*U-Tube HE
*Fixed Tubesheet HE
+Simple, cheapest construction
+No limit in tube passes
+Easy replacement of parts and cleaning
-Above 95C, expansion happens
*Floating Head HE
=High temperature differential
=Dirty fluids
+Prevents negative effects of thermal expansion
+Easy cleaning
+Can be used for fouling fluids
-Expensive
-Prone to leaking
*U-Tube HE
=High temperature differential
=Clean fluids
+Cheap (only 1 tube sheet)
+Easy inspection
-For clean fluids only (hard to clean)
-Tube replacement is difficult
-Erosion in bends
*Fixed Tubesheet HE
+Simple, cheapest construction
+No limit in tube passes
+Easy replacement of parts and cleaning
-Above 95C, expansion happens
*Floating Head HE
=High temperature differential
=Dirty/clean fluids
+Prevents negative effects of thermal expansion
+Easy cleaning
+Can be used for fouling fluids
-Expensive
-Prone to leaking
*U-Tube HE
=High temperature differential
=Clean fluids
+Cheap (only 1 tube sheet)
+Easy inspection
-For clean fluids only (hard to clean)
-Tube replacement is difficult
-Erosion in bends
Effectiveness NTU method
-Evaluates performance of heat exchangers
-LMTD not required
-Used when not all approach temperatures are given
Types of Baffle Layout
*Disc and doughnuts
*Orifice
*Segmental cut (20% - 25%)
Types of Reboiler and Vaporizers:
*Forced Circulation
= Uses pump
+ Viscous and fouling
+ Low vacuum and low vaporization rates
- Pumping is required (pumping costs)
*Kettle-type
=Boiling of pool of liquid with no circulation
+ High vacuum and vaporization rates
- Least economical
- High residence time
*Thermosiphon
= Natural circulation
+ Most economical type
- Column base must be elevated
*Forced Circulation
= Uses pump
+ Viscous and fouling
+ Low vacuum and low vaporization rates
- Pumping is required (pumping costs)
*Kettle-type
=Boiling of pool of liquid with no circulation
+ High vacuum and vaporization rates
- Least economical
- High residence time
*Thermosiphon
= Natural circulation
+ Most economical type
- Column base must be elevated
Ideal Ft value
> 0.85 is feasible
*Spiral Heat Exchanger
+ High heat transfer area requirement
+Low pressure drop
-Small capacity / flow rates
-No mechanical cleaning for tubes
