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MEC203 > Chapter 11 > Flashcards

Chapter 11 Flashcards

1
Q

Types of heat exchanger

A

counter flow and parallel flow

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2
Q

what is assumed for ideal heat exchangers

A

complete heat transfer from one to another

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3
Q

Draw the graph of temperature change for hot and cold fluid for parallel and counter flow heat exchangers

A

see book

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4
Q

what does a compact heat exchanger have

A

large heat transfer surface area per unit volume

area density beta > 700

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5
Q

difference between cross flow and counter flow

A

cross flow fluids flowing perpendicular to each other

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6
Q

what does unmixed mean for heat exchangers

A

has cross flow steam plates, reduce separation, small turbulence, lower pressure gradient but less mixing and cooling

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7
Q

What is the most common type of heat exchanger

A

shell and tube

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8
Q

what does a shell and tube heat exchanger contain

A

large number of tubes, (several hundred) packed in a shell, axis parallel to shell

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9
Q

regenerative heat exchanger

A

involves the alternate passage of hot and cold fluid steams through the same flow area

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10
Q

how are shell and tube heat exchangers further classified

A

by number of shell and tube passes

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11
Q

Dynamic type regenerator

A

rotating drum and continuous flow of the hot and cold fluid through different portion of the drum so that any portion of the drum passes periodically through the hot stream, storing heat and then through the cold steam rejecting this stored heat

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12
Q

Condenser is

A

one of the fluids is cooled and condeses as it flows through the heat exchanger

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13
Q

boiler is

A

one of the fluid absorbs heat and vaporizes

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14
Q

plate heat exchangers consist of

A

series of plates with corrugated flat flow passages the hot and cold fluids flow in alternate passages and thus each fluid stream is surrounded by two hot fluid streams
very effective heat transfer

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15
Q

What type of analysis can be used on heat exchangers

A

assume 1d steady radial flow, no heat generation no radiation and use network analysis

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16
Q

in double pipe heat exchangers what is the total thermal resistance

A

Rtot = Ri + Rwall + Ro

convection inner flow, conduction through wall, convection second surface

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17
Q

Rwall in double pipe heat exchangers

A

lnDo/Di / 2pikL

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18
Q

what is often assumed when using network analysis on heat exchangers

A

thickness of the wall -> therefore resistance of wall -> 0

therefore total resistance = Ro + Ri

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19
Q

What is U

A

the overall heat transfer coefficient

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20
Q

In double pipe heat exchangers what does Q. =

A

DELTAT/R = UA*DELTAT

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21
Q

relationship between U and Rtot

A

1/UAs = R

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22
Q

relationhsip between U and heat transfer coefficient

A

when Rwall approx = 0 and Ai approx = Ao approx = As

1/U approx = 1/hi + 1/ho

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23
Q

What does 1/U approx = 1/hi + 1/ho mean

A

U is dominated by the smaller convection coefficient

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24
Q

what happens when a fluid and gas heat exchanger is used

A

U tends towards the h of the gas, such cases fins used on gas side to enhance product UA, and inc heat transfer

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25
Q

When tube is finned what happens to As

A

As = A total = Afin + A unfinned
for short fins of high thermal conductivity
As = Aunfinned + fineff Afin

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26
Q

why does performance of heat exchangers decreases with time

A

accumulation of deposits on heat transfer surfaces

additional resistance to heat transfer

27
Q

how is accumulations treated

A

with a fouling factor R

28
Q

With fouling factor 1/UAs =

A

R = 1/hiAi + Rfi/Ai + lnDo/Di / 2pi()kL + Rfo/Ao + 1/hoAo

29
Q

fouling factor increases with

A

operating temperature, length of service and decreases with velocity of the fluids

30
Q

If fluids stream have same capacity rates

A

experience the same temperature change in well insulated heat exchanger

31
Q

Symbol of heat capacity rate

A

Ch or Cc for hot or cold

32
Q

C =

A

m. *c mass flow rate * heat capacity of fluid

33
Q

Heat transfer in heat exchanger

A

Q. = Cc (Tcout - Tcin) Q. = Ch (Thin - Thout)

34
Q

Ideal scenario what does Q. =

A

UAs * LMTD

35
Q

How is U found

A

1/U = 1/hi + 1/ho, found from Nu of internal and external flow from Re and Pr number

36
Q

Non ideal scenario what does Q. =

A

FUAs * LMTD what F is a geom effect

37
Q

In a condenser or boiler tube what does Q.

A

m. * hfg where m. is the rae of evaporation or condensation of the fluid, hfg is the enthalpy of vaporization of the fluid at the specified temperature or pressure

38
Q

Graph of condenser or Boiler heat transfer

A

see book

39
Q

Derive the equation for LMTD

A

see book

40
Q

what is LMTD

A

exact representation of the average temperature difference between the hot and cold fluids

41
Q

DELTAT1 for parallel and counter flow

A

parallel Thin - Tcin

Counter Thin - Tcout

42
Q

DELTAT2 for parallel and counter flow

A

parallel Thout - Tcout

Counter Thout - Tcin

43
Q

Difference between LMTD and arithmetic mean temp difference

A

AMTD overestimate rate of heat transfer

when DETLAT1 and DELTAT2 differ by no more than 40% less than 1% error in using AMTD

44
Q

What will never happen in counter flow heat exchangers

A

outlet temperature of cold fluid can never exceed inlet temperature of the hot fluid

45
Q

for specified inlet and outlet temps

A

LMTD for counter flow is always greater than parallel therefore greater heat transfer with sme area

46
Q

when heat capacity is equal in counter flow

A

LMTD = DETLAT1 = DETLAT2

47
Q

F= 1 for

A

a condenser or boiler

48
Q

How is F found

A

using the charts

49
Q

LMTD method

A

select heat exchanger that will meet prescribed heat transfer req

  1. Select type of heat exchanger
  2. determine any unknown inlet/outlet temps and heat transfer rate using energy balance
  3. calculate LMTD and F is necessary
  4. Calculate the heat transfer surface area As
50
Q

Whats NTU method used fo

A

determining heat transfer rate and outlet temps

51
Q

heat transfer effectiveness

A

Q. /Q.max = actual heat transfer rate / Maximum possible het transfer rate

52
Q

DELTATmax for NTU =

A

Thin - Tcin

53
Q

Q.max =

A

Cmin (Thin - Tcin) where Cmin is smaller of Ch and Cc

54
Q

Why is Cmin used

A

smaller heat capacity = larger temp jump see book

55
Q

If Ch = Cc

A

then DELTATh = DELTATc

56
Q

if Cc = Cmin

A

eff = Q/Qmax = Cc (Tcout - Tcin)/Cc(Thin - Tcin) =

Tcout - Tcin/Thin - Tcin

57
Q

if Ch = Cmin

A

eff = Q/Qmax = Ch (Thin - Thout)/Ch(Thin - Tcout) =

Thin - Thout)/(Thin - Tcout

58
Q

NTU (Number of transfer units)

A

UAs/Cmin

59
Q

what is NTU

A

a measure of the surface area As thus the larger the NTU the larger the heat exchanger

60
Q

capacity ratio

A

c =Cmin/Cmax

61
Q

the effectiveness of a heat exchanger is a function of

A

NTU and capacity ratio

62
Q

how does effectiveness vary with NTU

A

rapid increase at low values, at high values more gradual, large size cannot be justified large increase in NTU small increase in effectiveness

63
Q

effectiveness max when c =

A

0, boiler or condenser

minimum for c = 1

64
Q

highest effectiveness

A

Counter flow heat exchanger, then cross flow unmixed

MEC203 (12 decks)

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