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Millwright Level 3 > Heat Exchangers > Flashcards

Heat Exchangers Flashcards

1
Q

Why is understanding the heat exchanger process so important?

A

Many processes use dangerous fluids(toxic, flammable or termperature extreme)

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2
Q

Depending on The fluids in the exchanger, ________ and purging may be required and valves pumps or compressors will need to be_______

A

Venting

Locked out

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3
Q

What are the different types of matter used in a heat exchangers to cool the material?

A

Gas to gas

Liquid to gas

Liquid to liquid

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4
Q

Gas to gas heat exchange is typically used with what ________

A

Compressors

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5
Q

As compressors get into higher compression ratios or higher flow the heat exchange is ______to gas method

A

Liquid - glycol or water is common to cool compressed gas

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6
Q

Liquid to liquid is very common heat exchange method in the industry. What is the main difference?

A

Both sides of the tubes has liquid

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7
Q

What are the three means of heat transfer?

A

Conduction

Convection

And radiation

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8
Q

Which is the most important forms of heat transfer used in heat exchangers?

A

Conduction

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9
Q

Why is copper typically used in heat exchangers?

A

Has a high thermal conductivity rating

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10
Q

All metals are better thermal conductors than liquids, liquids are better conductors than______.

A

Gas

Conductivity of silver is 750 times that of water

Water conducts heat 25 times better than air

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11
Q

Convection occurs when?

A

Movement of a mass of fluid from a warm location to a cooler one.

Central heating is a good example of that

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12
Q

Convection currents are often increased through the use of auxiliary equipment such as? This is called ________\

A

Pumps or fans are used to move mass

This is called forced convection

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13
Q

Heat exchangers use both conduction and ________ methods of heat transfer.

A

Convection

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14
Q

The last type of heat transfer? Minor part in heat exchangers.

A

Radiation

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15
Q

What are the different flow path arrangements in heat exchangers?

A

Parallel flow

Counter flow

Cross flow

Mixed flow

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16
Q

Parallel flow

A

Fluid flow in same direction

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17
Q

Counter flow

A

Much more efficient than parallel flow.

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18
Q

Cross flow

A

More efficient that parallel but not as efficient as counter. Advantage is able to produce more turbulence, which makes for greater efficiency for given size exchanger.

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19
Q

Mixed flow

A

Uses baffles to cause flow to switch between parallel and counter flow

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20
Q

What are the different designs of heat exchanger?

A

Finned tube
Plate
Spiral
Cooling towers - natural draft
- mechanical draft
Double pipe
Shell and tube

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21
Q

Finned tube

A

Most common

Car radiator

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22
Q

Plate heat exchanger

A

A number of pressed plates clamped together on a frame with gaskets between the plates.

Spacing is between 2-5 mm

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23
Q

Advantages of plate heat exchangers

A
  • Low fouling
  • high heat transfer
  • little floor space required
  • little clearance needed
  • no insulation required
  • little vibration
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24
Q

Disadvantages of plate heat exchangers

A
  • limited by pressure and temperature
    2.07 kPa and 175degree C
  • not recommended for viscous (thick) fluids or fluids that change viscosity with heat
  • small clearance between plays limits the size of solids passing though
  • high pressure drops not suitable for air coolers and gas to gas services
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25
Q

Spiral heat exchanger

A

Very efficient do to both fluids having a true counter flow path. The single path for each side eliminates dead bypass areas which lessons fouling.

Continually curving passage creates high turbulence

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26
Q

What is the purpose of a cooling tower?

A

Recycle cooling water for large scale applications such as air conditions and power generation.

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27
Q

What is the general principle of a cooling tower?

A

Water to be cooled is pumped to the top of the tower where it is sprayed through nozzles to give a fine spray

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28
Q

Efficiency of a cooling tower depends on?

A
  • area of water (smaller droplets give larger surface area)
  • length of contact time between air and water
  • relative humidity
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29
Q

What are the two classes of cooling towers?

A

Natural draft and

Mechanical draft

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30
Q

Natural draft cooling towers depend on ________ operate effectively in _________ with fairly steady wind. Maintenance is fairly simple since limited moving parts

A

Wind conditions

Large open areas

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31
Q

Mechanical draft cooling towers use _______ to increase the air flow though the exchanger and are divided into two classes: forced draft and _______-

A

Fans

Induced draft

32
Q

Forced draft towers

A

Use fan to push air through the tower
- little vibration or noise
- air is fairly dry which reduces problems with erosion, corrosion and scale build up on the fans

33
Q

What recaptures most of the water that is being pulled out with the air flow in a mechanical draft tower

A

Drift eliminators

34
Q

What is the difference between induced forced draft cooling towers?

A

Induced tower has fan on top pulling air up through the water as it flows down through the packing.

35
Q

Venturi towers can either forced or ______

A

Induced

36
Q

Maintenance required on cooling towers consist of :

A
  • lubrication of fan motor every 3 months
  • inspect fan belt regularly
  • inspect bearings on floating shaft assembly
  • ensure float valve operates freely
  • inspect structural integrity yearly
  • clean and inspect fan blades
  • consult water treatment experts if excessive scaling or algae is present
37
Q

Double pipe exchanger

A

Compact and components and fittings are small in dimension, which makes natural choice for high pressure services

Does not provide large heat transfer surface areas and is commonly used for low heat duties with smaller surface area requirements

When larger heat exchange is needed multiple units can be hooked together

38
Q

Advantages of double pipe heat exchangers?

A
  • units can be added or removed as needed
  • modular Type construction simplifies maintenance
  • suitable for high pressure application
  • can be grouped together in parallel or in series for the required cooling
39
Q

Disadvantages of double pipe heat exchangers

A

Limited to low heat duties requiring smaller surface areas

40
Q

Most common type of heat exchanger?

A

Shel and tube

Capable of high temps and high pressure

41
Q

Heat exchangers are typically made of what materials

A

Non corrosive:
- copper
- brass
- stainless
- Carbon steels

42
Q

Why would a anorode be used in a heat exchanger?

A

Anarods are used as sacrificial replaceable parts that take the electrolytic actions instead of the heat exchanger itself

43
Q

Longer the heat exchanger tubes means _________ the number of tubes and holes necessary and a smaller shell diameter can be used. Disadvantage of longer tubes is _______ pressure drops

A

Lower

Higher

44
Q

The greater the surface area of the tubing the quicker the heat is _____. _______ is one of the more common tube materials due to its high thermal conductivity

A

Conducted away

Copper

45
Q

Welded tube joints are commonly used in _________ pressure or extreme temperature applications where considerable expansion must be handled

A

High

46
Q

Tube sheet layout

A

30 degree triangle pitch greatest heat transfer rate but greater pressure drop across the exchanger

47
Q

What two functions do baffles do?

A

Support tubes to prevent tube vibration and sagging

Cause turbulence to increase the heat transfer rate and prevent dead airspace areas

48
Q

What are the different designs of the baffles?

A

Segmental - most common

Disc and doughnut - smooth flow can cause sedimmate buildup of fluids are not clean

Impingement baffles - provided over the tubes to reduce corrosion by the entering fluid

Longditudinal baffles - split the shell Side flow into two or more passes allowing more contact time between fluids

Single pass tube side/single pass shell side -

49
Q

What are shell and tube exchangers?

A

Cylindrical outer shell surrounding a bundle of tubes that are parallel to the longitude axis of the shell.

50
Q

The principle types of shell and tube exchangers are fixed tube sheet, u-tube and _______

A

Floating head

51
Q

Fixed tubesheet

A

Tube sheets are welded to the shell at both ends. Tubes are straight and can almost completely fill the shell.

  • no gasket joints so it is ideal for high pressure and dangerous fluid
52
Q

What is the main disadvantage of a fixed tubesheet heat exchanger?

A

Tube bundle cannot be removed for cleaning. Best suited when fluids are clean

53
Q

Advantage of a u-tube heat exchanger?

A

Handles differential thermal expansion

  • u-tube bundle can easily be removed
  • vibration and erosion damage can be avoided by locating nozzles at a point beyond the u-bends
  • well suited for high pressure high temperature
54
Q

What are the different types of floating head shell tube exchangers?

A

Floating head shell

Floating head with split backing ring exchanger

Floating head, outside-packed, lantern ring exchanger

Double tube sheet exchanger

55
Q

Advantage and disadvantage of a floating head shell tube exchanger?

A

Can handle extremely high fluid temperature differential since the tubes can expand and contract without stress.

Disadvantage: large clearance requirements needed between the outermost tubes and the inside of the shell

56
Q

Advantages of the floating head and split backing ring exchanger

A

Widely used in oil and gas. Backing ring allows for greater number of tubes to fit inside the same diameter shell. Greatly increases the efficiency for a exchanger this size.

57
Q

What is the disadvantage of the outside packed lantern ring exchanger?

A

Can leak so hazardous toxic, lethal or explosive chemicals should not be used with it

58
Q

What is the use of a double tubesheet exchanger.

A

Applications. Where it is not desirable to have mixing of the tube side and shell side fluids

Only option if mixing of two fluids must be prevented.

59
Q

How do you decide which fluid goes through the tubes and which fluid goes through the shell of an exchanger?

A

No hard and fast rule but some things to consider are:
- dirtier fluid should go through tubes since they are easier to clean
- higher pressure fluid should go through tubes since they can collapse
- corrosive fluids should go through the tubes or tube and shells will corrode
- fluid with lower heat transfer coefficient usually travels on the outside of the tubes

60
Q

What are the major factors that determine shell and tube exchangers efficiency?

A

Tube length and shell diameter

61
Q

What factors affect heat exchangers efficiency

A

Fluid flow
- temperature differentials
- heating surface area
- contact time
- conductivity of the metal wall and nature of the fluids
- fluid films adjacent to the inside outside or metal wall surfaces
-Thickness of the heating surface
- cleanliness of the heating surface

62
Q

Increased velocity of fluids results in!

A

Turbulent motion and therefore a better rate of heat transfer is obtained

63
Q

Thick wall tubes require a high conductance metal to transfer the heat rapidly and efficiently what types of metals are typically used?

A

Copper
Copper alloys
Brass
Steel
Nickel aluminum
Low carbon steels

64
Q

What are some factors when trouble shooting heat exchangers?

A
  • performance criteria (flow rates, thermal design, operating pressures and temps)
  • proper pressure and temperatures
  • fouling characteristics, frequency and quality of cleaning
  • proper handling and storage
  • proper design, base supports and piping
  • materials, tools and workmanship
65
Q

Poor heat exchanger performance can be traced to one of the following:

A
  • dirty exchanger
  • failure to remove preservation materials after storage
  • operating conditions different than design conditions
  • corrosion from incorrect application of exchanger materials or conditions
  • air or gas build up
  • leaking tube sheets or gaskets
  • excessive clearance causing dead bypass areas
  • tube failure cause by erosion or vibration
  • incorrect piping connections
66
Q

Fouling results in?

A

Increase of resistance to both fluid flow and heat transfer

An increase in pressure drop through the exchanger or drop heat exchange efficiency usually indicates that cleaning is needed

67
Q

What are the 3 most common problems found in exchangers?

A

Leaking tube sheets, fouling, and vibration

68
Q

What should steam never be used to clean exchanger tubes and why?

A

Steam

Will overheat resulting in severe expansion strain and leaking tube sheet joints

69
Q

What is one way to clean the insides of the tubes?

A

Brushes can be simply pushed through or rotated on a flexible shaft with a rotary tool

70
Q

When removing tube bundles don’t lift ________

A

Never wrap slings around the tubes!
Never drag bundles

Support by end baffles

71
Q

How do you pull the bundle out from the shell?

A

Through bolts can be inserted through the tubes and bolted to a heavy bearing plate and pulled out

72
Q

When bolting tube sheets you need to do what?

A

Torque to specific torque slowly and follow the bolting pattern

73
Q

Where do most tube failure occurs

A

Where tube spans are long and unsupported.

I let baffle area is a prime candidate for failure because of the span length and high local velocity

74
Q

For the most accurate leak detections what would you use?

A

Hydro testing

1 1/2 times the working pressure

75
Q

What must be done before loosening head studs?

A

Check for leaks
Ensure has been purged if necessary
Vented
Drained

76
Q

Pressurizing a fixed tubesheet for a leak test with hot water could?

A

Result in enough expansion to seal the leak.

Always use cold water

Millwright Level 3 (15 decks)

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