Heat Exchanger Flashcards by Reindel Mallari

Heat exchangers

Used for exchange of heat between the two fluids that are at different temperatures

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Tubular Heat Exchangers

simplest heat exchanger in which the hot and cold fluids move in the same or opposite directions.

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Parallel-Flow

Hot and cold fluids enter at the same end, flow in the same direction, and leave at the same end.

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Counterflow

Fluids enter at opposite ends, flow in opposite directions, and leave at opposite ends.

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Cross Flow

Fluids move perpendicular to each other

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Tubesheet

Plates where tubes are fitted

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Baffle pitch

AKA Baffle Spacing
*Distance between baffles

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Baffle Cut

*Baffle Pitch / Shell Diameter
*Typically 20 percent

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Purpose of baffles

*increase the convection coefficient of the shell-side fluid by inducing turbulence
*physically support tubes

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Fouling vs Scaling

Fouling
*due to fluid impurities, rust and reactions between fluid and wall
Scaling
*type of fouling
*deposits of film

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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.

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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

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Finned tubular heat exchanger

*Give larger area per tube
*Cross flow
*Unmixed - fins inhibit motion transverse to the main flow direction

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Unfinned tubular heat exchanger

*Mixed - fins inhibit motion transverse to the main flow direction

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Factors affecting Heat Exchangers

Configuration (concurrent/ countercurrent)
Temperature Difference
Number of passes
Feed Fluid Properties
*Pressure drop
*Fouling and corrosion

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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

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TEMA Heat Exchanger

*Tubular Exchanger Manufacturers Association
*Three-Letter Designations
1st: Front-end head type
2nd: Shell types
3rd: Rear-end head types

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Economizers

*Type of HE
*Uses heat of recycled / recovered flue gases to heat fluids

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Boiler

Closed vessel where water is heated to generate superheated steam

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Fired Heaters

*AKA Furnace
*Uses direct heat transfer from combustion of fuels to fluids

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Why Saturated Steam is better than Superheated Steam?

*Safer for equipment
*Higher heat transfer coefficient (higher gas T = lower heat transfer coefficient)

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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