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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
When tube is finned what happens to As
As = A total = Afin + A unfinned for short fins of high thermal conductivity As = Aunfinned + fineff Afin
26
why does performance of heat exchangers decreases with time
accumulation of deposits on heat transfer surfaces | additional resistance to heat transfer
27
how is accumulations treated
with a fouling factor R
28
With fouling factor 1/UAs =
R = 1/hiAi + Rfi/Ai + lnDo/Di / 2pi()kL + Rfo/Ao + 1/hoAo
29
fouling factor increases with
operating temperature, length of service and decreases with velocity of the fluids
30
If fluids stream have same capacity rates
experience the same temperature change in well insulated heat exchanger
31
Symbol of heat capacity rate
Ch or Cc for hot or cold
32
C =
m. *c mass flow rate * heat capacity of fluid
33
Heat transfer in heat exchanger
Q. = Cc (Tcout - Tcin) Q. = Ch (Thin - Thout)
34
Ideal scenario what does Q. =
UAs * LMTD
35
How is U found
1/U = 1/hi + 1/ho, found from Nu of internal and external flow from Re and Pr number
36
Non ideal scenario what does Q. =
F*U*As * LMTD what F is a geom effect
37
In a condenser or boiler tube what does Q.
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
Graph of condenser or Boiler heat transfer
see book
39
Derive the equation for LMTD
see book
40
what is LMTD
exact representation of the average temperature difference between the hot and cold fluids
41
DELTAT1 for parallel and counter flow
parallel Thin - Tcin | Counter Thin - Tcout
42
DELTAT2 for parallel and counter flow
parallel Thout - Tcout | Counter Thout - Tcin
43
Difference between LMTD and arithmetic mean temp difference
AMTD overestimate rate of heat transfer | when DETLAT1 and DELTAT2 differ by no more than 40% less than 1% error in using AMTD
44
What will never happen in counter flow heat exchangers
outlet temperature of cold fluid can never exceed inlet temperature of the hot fluid
45
for specified inlet and outlet temps
LMTD for counter flow is always greater than parallel therefore greater heat transfer with sme area
46
when heat capacity is equal in counter flow
LMTD = DETLAT1 = DETLAT2
47
F= 1 for
a condenser or boiler
48
How is F found
using the charts
49
LMTD method
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
Whats NTU method used fo
determining heat transfer rate and outlet temps
51
heat transfer effectiveness
Q. /Q.max = actual heat transfer rate / Maximum possible het transfer rate
52
DELTATmax for NTU =
Thin - Tcin
53
Q.max =
Cmin (Thin - Tcin) where Cmin is smaller of Ch and Cc
54
Why is Cmin used
smaller heat capacity = larger temp jump see book
55
If Ch = Cc
then DELTATh = DELTATc
56
if Cc = Cmin
eff = Q/Qmax = Cc (Tcout - Tcin)/Cc(Thin - Tcin) = | Tcout - Tcin/Thin - Tcin
57
if Ch = Cmin
eff = Q/Qmax = Ch (Thin - Thout)/Ch(Thin - Tcout) = | Thin - Thout)/(Thin - Tcout
58
NTU (Number of transfer units)
UAs/Cmin
59
what is NTU
a measure of the surface area As thus the larger the NTU the larger the heat exchanger
60
capacity ratio
c =Cmin/Cmax
61
the effectiveness of a heat exchanger is a function of
NTU and capacity ratio
62
how does effectiveness vary with NTU
rapid increase at low values, at high values more gradual, large size cannot be justified large increase in NTU small increase in effectiveness
63
effectiveness max when c =
0, boiler or condenser | minimum for c = 1
64
highest effectiveness
Counter flow heat exchanger, then cross flow unmixed

MEC203 (12 decks)

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