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Civil Law > Heat Exchanger > Flashcards

Heat Exchanger Flashcards

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1
Q

Heat exchangers

A

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

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2
Q

Tubular Heat Exchangers

A

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

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3
Q

Parallel-Flow

A

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

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4
Q

Counterflow

A

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

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5
Q

Cross Flow

A

Fluids move perpendicular to each other

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6
Q

Tubesheet

A

Plates where tubes are fitted

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7
Q

Baffle pitch

A

AKA Baffle Spacing
*Distance between baffles

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8
Q

Baffle Cut

A

*Baffle Pitch / Shell Diameter
*Typically 20 percent

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9
Q

Purpose of baffles

A

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

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10
Q

Fouling vs Scaling

A

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

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11
Q

NTU

A

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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12
Q

Series vs Parallel Heat Exchanger

A

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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13
Q

Finned tubular heat exchanger

A

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

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14
Q

Unfinned tubular heat exchanger

A

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

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15
Q

Factors affecting Heat Exchangers

A
  1. Configuration (concurrent/ countercurrent)
  2. Temperature Difference
  3. Number of passes
  4. Feed Fluid Properties
    *Pressure drop
    *Fouling and corrosion
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16
Q

WHEN TO USE?
*Double Pipe Heat Exchanger
*Which fluids to Shell-side or Tube-side?

A

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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17
Q

TEMA Heat Exchanger

A

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

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18
Q

Economizers

A

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

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19
Q

Boiler

A

Closed vessel where water is heated to generate superheated steam

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20
Q

Fired Heaters

A

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

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21
Q

Why Saturated Steam is better than Superheated Steam?

A

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

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22
Q

USES, PROS & CONS:
Plate Heat Exchanger

A

*USES
+Viscous
+Corrosive
+Slurry
-For boiling or condensing

*PROS
-Cheaper
-Less fouling

*CONS
-High pressure drop
-Low pressure and temperature capacity

23
Q

PROS & CONS:
Shell and Tube Exchangers

A

*PROS:
-Large heat transfer area
-High pressure
-Easily cleaned
-Well established design and fabrication

*CONS:
-More space than plate HE of similar capacity

24
Q

TUBESHEET LAYOUT:
*Triangular pitch
*Square pitch

A

*Triangular pitch
-For CLEAN fluids
-HIGHER heat transfer area and pressure drop

*Square pitch
-For FOULING fluids
-LESS heat transfer area and pressure drop

25
Q

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.

A

none of these.

26
Q

Economy of a multiple effect evaporator is not influenced by the ___

A

boiling point elevations

27
Q

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

A

reduce the capacity

28
Q

In a heat exchanger, floating head is provided to ___

A

relieve stresses caused by thermal expansion.

29
Q

Baffles are provided in heat exchangers to increase the ___

A

*enhance turbulence
*heat transfer co-efficient

30
Q

Terminal point temperature differences between fluids in case of a heat exchanger is termed as ____

A

approach

31
Q

For a cold viscous feed, backward feed gives __________ than forward feed.

A

a higher capacity

32
Q

Effects of Fouling

A

*Fouling has high thermal resistance
*Reduced cross-sectional area and velocity
*Increased pressure drop

33
Q

When to use Air Fin or Shell and Tube Heat Exchanger?

A

*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

34
Q

Floating Head

A

Allow for tubes to expand due to temperature differences

35
Q

Compact Heat Exchangers

A

+Higher area density (m2 area/ m3 volume)
- Fouling problems

*Plate and Frame
*Plate Fin
*Spiral

36
Q

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.

A

Swenson-Walker crystalliser is a batch crystalliser.

37
Q

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

A

Series and parallel set of shell and tube

38
Q

Vent pipes are provided in a condenser to ___

remove non-condensable gases.

purge the condenser.

facilitate easy cleaning of tubes.

none of these.

A

remove non-condensable gases.

39
Q

Tube pitch is the __________ of tube diameters and the clearances.
sum
difference
ratio
none of these

A

sum

40
Q

Economizer:
*For Heating
*For Cooling

A

*Heating
-use flue gas to reduce power consumption

*Cooling
-use outside air to reduce power consumption

41
Q

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.

A

there is no risk of contamination.

42
Q

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

A

abundance & high heat capacity

43
Q

Differentiate:
*Fixed Tube Sheet HE
*Floating Heat HE
*U-Tube HE

A

*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

44
Q

*Fixed Tubesheet HE

A

+Simple, cheapest construction
+No limit in tube passes
+Easy replacement of parts and cleaning
-Above 95C, expansion happens

45
Q

*Floating Head HE

A

=High temperature differential
=Dirty/clean fluids
+Prevents negative effects of thermal expansion
+Easy cleaning
+Can be used for fouling fluids
-Expensive
-Prone to leaking

46
Q

*U-Tube HE

A

=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

47
Q

Effectiveness NTU method

A

-Evaluates performance of heat exchangers
-LMTD not required
-Used when not all approach temperatures are given

48
Q

Types of Baffle Layout

A

*Disc and doughnuts
*Orifice
*Segmental cut (20% - 25%)

49
Q

Types of Reboiler and Vaporizers:

A

*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

50
Q

*Forced Circulation

A

= Uses pump
+ Viscous and fouling
+ Low vacuum and low vaporization rates
- Pumping is required (pumping costs)

51
Q

*Kettle-type

A

=Boiling of pool of liquid with no circulation
+ High vacuum and vaporization rates
- Least economical
- High residence time

52
Q

*Thermosiphon

A

= Natural circulation
+ Most economical type
- Column base must be elevated

53
Q

Ideal Ft value

A

> 0.85 is feasible

54
Q

*Spiral Heat Exchanger

A

+ High heat transfer area requirement
+Low pressure drop
-Small capacity / flow rates
-No mechanical cleaning for tubes

Civil Law (31 decks)

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