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PROCESS DESIGN > Heat Exchanger Design > Flashcards

Heat Exchanger Design Flashcards

1
Q

What is a heat exchanger?

A

A device or piece of hardware which promotes the transfer of heat between 2 or more entities at different temperatures.

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2
Q

What are the 3 types of flow configuration?

A
  1. Parallel (co-current) flow
  2. Counter-current flow
  3. Cross-flow
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3
Q

Describe a parallel (co-current) flow heat exchanger

A

The 2 inlet ports are positioned at the same end of the heat exchanger, where stream-to-stream temperature difference is the greatest. Fluids flow in the same direction.

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4
Q

Describe a counter-current flow heat exchanger

A

The 2 inlet ports are positioned at opposite ends of the heat exchanger, where stream-to-stream temperature difference is more evenly distributed along the heat exchanger. Fluids flow in opposite direction.

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5
Q

What is a cross-flow heat exchanger used for?

A

Gas heating & cooling.

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6
Q

What is meant by ‘mixed’ & ‘unmixed’ in a cross-flow heat exchanger?

A

Mixed - stream fluid can move freely in the exchanger as it exchanges heat.

Unmixed - stream fluid is confined in separate channels in the exchanger so that it cannot mix with itself during the heat-transfer process.

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7
Q

What are the 4 types of heat exchanger when considering their construction? Give an example for each.

A
  1. Tubular - shell & tube
  2. Plate - gasketed
  3. High corrosion-resistant material - graphite
  4. Special - rotary regenerator
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8
Q

What are the purpose of baffles in a shell & tube heat exchanger?

A

Increase heat exchange area.

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9
Q

What is the purpose of a floating head on a shell & tube heat exchanger?

A

Allows expansion in exchanger, less hazardous with temperature increase & decrease.

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10
Q

What type of shell & tube exchanger is a kettle reboiler? Draw & label 7 things.

A

U-tube - see diagram in notes

  1. Heating medium inlet
  2. Heating medium outlet
  3. Liquid from distillation column
  4. Vapour to distillation column
  5. Liquid product out
  6. Weir plate
  7. Entrainment plate
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11
Q

Where should the exiting vapour of the kettle reboiler enter the distillation column?

A

Below the first plate in the column.

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12
Q

What are the tubes called in a kettle reboiler?

A

Hairpin tubes.

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13
Q

Describe a double pipe heat exchanger. Give an example.

A

Used for high pressure fluids, small area, bulky & expensive. eg. hairpin heat exchanger.

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14
Q

Describe a spiral tube heat exchanger.

A

Used for clean fluids, higher area than shell & tube.

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15
Q

Describe a plate heat exchanger.

A

Area < 500m2, compact, modest temperature & pressure. Can be sealed by gasket, fully welded or fusion bonded.

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16
Q

Describe an extended surface (fin) heat exchanger.

A

Fins extend surface for increased heat transfer. Plate fin gas-gas, tube fin liquid-air.

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17
Q

Give 5 examples of heating fluids. State the temperature range for each.

A
  1. Steam 110-260C
  2. Hot oil to 350 C
  3. Dowtherm to 450 C
  4. Molten Salt to 660 C
  5. Hot gases to 1200 C
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18
Q

Give 5 examples of cooling fluids. State the temperature range for each.

A
  1. Cooling water 30 C
  2. Chilled water 10 C
  3. Brine -20 C
  4. Ammonia -40 C
  5. Propylene -50 C
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19
Q

How should fluid allocation be determined with respect to corrosion? Why?

A

The more corrosive fluid allocated to the tube-side as easier to clean & reduces cost of expensive alloys.

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20
Q

How should fluid allocation be determined with respect to fouling?

A

Most fouling fluid allocated to the tube-side as easier to clean & better control over tube velocity.

21
Q

How should fluid allocation be determined with respect to fluid temperature?

A

Higher temperature fluid allocated to tube-side as reduces need for alloys (high temperature) or need for lagging (moderate temperature).

22
Q

How should fluid allocation be determined with respect to operating pressures?

A

Higher pressure stream on tube-side as high pressure tubes cheaper than high pressure shell.

23
Q

How should fluid allocation be determined with respect to pressure drop?

A

Same pressure drop - higher heat transfer coefficient = tube-side
Different pressure drop - lowest pressure drop = tube-side

24
Q

How should fluid allocation be determined with respect to viscosity?

A 
Turbulent flow (Re = 200) - more viscous fluid (higher heat transfer coefficient) on shell-side.
Non-turbulent flow - more viscous fluid on tube-side (heat transfer coefficient more easily predicted).
25
Q

How should fluid allocation be determined with respect to stream flow-rate?

A

Lowest flow-rate allocated to the shell-side

26
Q

What coefficient represents the inlets & outlets of the hot & cold fluids in a heat exchanger?

A 
T1 = hot fluid inlet (tube)
T2 = hot fluid outlet (tube)
t1 = cold fluid inlet (shell)
t2 = cold fluid outlet (shell)
27
Q

How do you calculate heat duty?

A

Q = m x Cp x T

28
Q

How do you generally calculate the LMTD for a shell & tube heat exchanger?

A

LMTD = [(TOP) - (BOTTOM)]/[ln(TOP)/(BOTTOM)]

29
Q

How do you calculate the overall heat transfer coefficient?

A

1/Uo = 1/ho + 1/hi (do/di) + Rw + Rs

Uo = W/m2 K
ho/hi = outside/inside film coefficient
Rw = tube wall resistance 
Rs = total scale resistance (fouling)
30
Q

How do you calculate the heat transfer area?

A 
Q = U x A x LMTD
A = Q / (U x LMTD)
31
Q

How do you calculate temperature difference if one fluid doesn’t change temperature in a heat exchanger?

A

Calculate average temperature difference (T1-T2)/2

32
Q

How do you calculate heat duty from enthalpy of vapourisation?

A

Q = m x H

33
Q

Why can’t a horizontal shell & tube exchanger be used when condensing vapour to liquid?

A

Liquid will not be able to escape, could either use shell & tube vertically or kettle reboiler.

34
Q

What is the problem calculating the temperature difference in multi-pass exchanger? How do you calculate it?

A

Some parts are parallel (co-current) & some parts are counter-current. We calculate a modified temperature difference (Tm) using LMTD for counter-current & correction factor.
Tm = F x LMTD

35
Q

How do you calculate the ‘R’ & ‘S’ coefficients to obtain the correction factor ‘F’ for a multi-pass exchanger?

A 
R = (T1-T2)/(t2-t1)
S = (t2-t1)/(T1-t1)
*read F from graph*
T=shell
t=tube
36
Q

How do you calculate the heat transfer area of a multi-pass exchanger?

A

Area (multi-pass) = Area (counter-current) / F

37
Q

What is the typical range for tube velocity for liquids & gases?

A 
Liquids = 1 - 3 m/s
Gases = 10 - 30 m/s
38
Q

What is the typical range for pressure drop?

A
  1. 2 - 0.6 bar

0. 1 bar for vapourisation

39
Q

When iterating a overall heat transfer coefficient, what range of the trial value can it be accepted?

A

< 5%

40
Q

What does TEMA stand for? What does it tell us & what is the most common combination?

A

Tubular Exchanger Manufacturer’s Association. It gives the minimum shell thickness. Most common = BEM.

41
Q

What is the benefit of having multiple passes? How many passes are possible?

A

It increases the length of the flow path therefore varies tube-side velocity. 1-16 passes possible.

42
Q

What is the standard internal thickness & tube length?

A

Internal thickness = 2 inch (50.8mm)

Length = 16 ft (4.88m)

43
Q

What are the 3 types of tube patterns? Give examples where each is used?

A
  1. Triangular - high heat transfer/higher PD
  2. Square - fouling fluids
  3. Rotated square - fouling fluids/high heat transfer/higher PD
44
Q

How do you calculate the pitch (Pt)?

A

Pt = 1.25 x tube outer D

45
Q

Give 3 types of shell & tube heat exchanger featured in TEMA

A

Fixed tubesheet - tubes welded to shell
U-tube - tubes free in shell
Floating head - 1 tube fixed, 1 free in shell

46
Q

What is the ‘j (h)’ factor used for?

A

Used to estimate heat transfer coefficients for tubes using Reynold’s number.

47
Q

What are the 2 main sources of pressure loss on the tube-side of a shell & tube heat exchanger?

A
  1. Friction loss

2. Losses due to sudden contraction & expansion & flow reversals

48
Q

Name 2 CO2 removal fluids

A

MEA (monoethanolamine)

NaOH

49
Q

On what condition can a system not be parallel flow (co-current)?

A

t2 > T2

PROCESS DESIGN (6 decks)

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