[ppt]me421 heat exchanger and steam generator...
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Design of Condensers/Condensing Zones
P M V SubbaraoProfessor
Mechanical Engineering DepartmentI I T Delhi
Lowest Shell side Thermal Resistance !!!
HP CFWH
HP CFWH No. 8
Thermodynamic Layout of HP Closed Feed Water Heater
Desuperheater
Condensing Shell Drain Cooler
HP Turbine
TRAP
Tbi, pbi, Tbsi
Tfi+1Tfi
DS
TTD
Feedwater heater with Drain cooler and Desuperheater-TTD=Terminal temperature difference
C=Condenser
DC=Drain cooler
DS=Desuperheater
Bled steam
T
L
DCC
Condensate
CDC
Feed Water in
DS
Bleed Steam
Feed Water out
Number of Tubes • The flow rate inside the tube is a function of the density of the
fluid, the velocity of the fluid, cross-sectional flow area of the tube, and the number of tubes.
By using above Eq. and replacing Ac by di2/4, number of tubes
can be calculated as
2itt
tubet du
mN
tctttube NAum
where di is the tube inside diameter.
Tubes in Shell and Tube Hx
• The number and size of tubes in an exchanger depends on the• Fluid flow rates• Available pressure drop.• The number and size of tubes is selected such that the• Tube side velocity for water and similar liquids ranges from 0.9 to 2.4 m/s.• Shell-side velocity from 0.6 to 1.5 m/s.• The lower velocity limit corresponds to limiting the fouling,
and the• upper velocity limit corresponds to limiting the rate of
erosion.• When sand and silt are present, the velocity is kept high
enough to prevent settling.
Tube-Side Nusselt Number
For turbulent flow, the following equation developed by Petukhov-Kirillov is used:
2
322
1
28.3Reln58.1
1Pr2
7.1207.1
PrRe2
t
t
tt
tube
fWhere
f
f
Nu
Properties are evaluated at mean bulk temperature and constants are adjusted to fit experimental data.Validity range: 104 < Ret < 5 x 106 and 0.5 < Prt < 2000 with 10% error.
For laminar flow, the Sieder and Tate correlation is be used.
31
PrRe86.1
LdNu itt
tube
is applicable for 0.48 < Prt < 16700 and (Ret Prt di/L)1/3 > 2.
The heat transfer coefficient for the tube-side is expressed as follows:
i
ttt d
kNuh
Shell-diameter
2
4 Stubeprot
shell DCTPAN
A
2Ttubepro PCLA
HP Closed Feed Water Heater
Condensate Loading
This can be used to calculate a Reynolds number
Perimetercondensate of flow Mass
tubes.alfor vertic 0d
mcondensate
tubes.horiontalfor tube
condensate
Lm
filmoncondensati
4Re
General values of condensate loading for horizontal tubes: 0.01 to 0.05 kg/m.s
•Flow is considered laminar if this Reynolds number is less than 1800. •The driving force for condensation is the temperature difference between the cold wall surface and the bulk temperature of the saturated vapor
The viscosity and most other properties used in the condensing correlations are evaluated at the film temperature, a weighted mean of the cold surface (wall) temperature and the (hot) vapor saturation temperature
surfacevapourwallsatdriving TTTTT
4
343 driving
satwallsaturationsatfilm
TTTTTT
Onset of Turbulence & Turbulent Film Condensation
• The transition film Reynolds number for the tube bundle is adapted from a vertical plate turbulent transition criterion of 1600 (but also values of 1200, 1800 and 2000 have been proposed).
• Thus, the film will become turbulent on the tube bundle at ReΓ
equal to 1600 and thus when ReΓ > 1600 the following expression should be used.
• The flow is nearly always laminar on single tube because of the short cooling length around the perimeter
Wall Temperatures
• It is often necessary to calculate the wall temperature by an iterative approach.
• The summarized procedure is: 1. Assume a film temperature, Tf 2. Evaluate the fluid properties (viscosity, density, etc.) at
this temperature 3. Use the properties to calculate a condensing heat transfer
coefficient (using the correlations to be presented) 4. Calculate the wall temperature. The relationship will
typically be something like
coolantsat
oo
satwall TTAh
UATT
1
1
5. Use the wall temperature to calculate a film temperature 6. Compare the calculated film temperature to that from the
initial step. If not equal, reevaluate the properties and repeat.
Laminar Flow Outside Horizontal Tubes
When vapor condenses on the surface of horizontal tubes, the flow is almost always laminar. The flow path is too short for turbulence to develop. Again, there are two forms of the same relationship:
The constant in the second form varies from 0.725 to 0.729. The rippling condition (add 20%) is suggested for condensate Reynolds Numbers greater than 40.
31
2
3
3 Re51.1
f
vfff
oncondensaticond
gkh
41
0
3
725.0
dTghk
hdrivingf
fgvfffcond
Condenser tubes are typically arranged in banks, so that the condensate which falls off one tube will typically fall onto a tube below.
The bottom tubes in a stack thus have thicker liquid films and consequently poorer heat transfer.
The correlation is adjusted by a factor for the number of tubes, becoming for the Nth tube in the stack
4
41
0
3
725.0N
hdTNghk
h top
drivingf
fgvfffcond
The heat transfer coefficient on the Nth tube row
• The heat transfer coefficient on the Nth tube row in the bundle h(N) is
434
31
)1()(
NNh
Nh
• Kern (1958) concluded from his practice experience in designing condensers that the Jakob tube row expression was too conservative and that this resulted in condensers that were consistently over-surfaced.
• To improve his thermal designs, he replaced the exponent of (-1/4) in the Nusselt expression with a value of (-1/6) so that corresponding equations become
)1()(
hNh
N
Condensation on Horizontal Bundles: Prediction of Heat Transfer Coefficient in Nth Tube Row
Falling Film Condensation on Horizontal Tubes
• Falling-film heat exchangers are attractive because they provide good heat transfer performance and low working-fluid inventories.
• The design of falling-film heat exchangers has been largely based on empirical data.
• A thorough understanding of the falling-film flow and heat transfer interactions is important.
• An ability to predict the falling film mode would allow better data correlation and improve the modeling and analysis of heat transfer and fluid flow.
Modes of Condensation on Tube Bundle
The droplet mode The jet mode The sheet mode
Flow Rate Vs Mode of Falling Film
Condensation on Horizontal Tube Bundles : Flow Map• Hu and Jacobi (1996) proposed flow mode transition equations
with ReΓ versus Ga+ (film Reynolds number vs. the Galileo number) for the following principal flow modes: sheet flow, column flow and droplet flow.
• The mixed mode transition zones of column-sheet and droplet-column were also considered as regimes, bringing the total to five.
• Hence, they presented four flow transition expressions (valid for passing through the transitions in either direction and hence the symbol ⇔):
Range of Validity of Model
Flow Transition Map
Identification Condensation Mode
Final Correlation
Onset of Turbulence & Turbulent Film Condensation
• The transition film Reynolds number for the tube bundle is adapted from a vertical plate turbulent transition criterion of 1600 (but also values of 1200, 1800 and 2000 have been proposed).
• Thus, the film will become turbulent on the tube bundle at ReΓ
equal to 1600 and thus when ReΓ > 1600 the following expression should be used.
Condensation on Horizontal Tube Bundles : Turbulent Flow
• Turbulent flow of the condensate film may be reached in a condenser, which significantly increases heat transfer.
• Comparatively little has been published on turbulent film condensation on tube bundles compared to the information available for laminar films.
• Butterworth (1983) recommends adapting the Labuntsov expression for turbulent film condensation on a horizontal tubes for predicting local turbulent film condensation on the Nth tube row in horizontal tube bundles
h
Overall Heat Transfer Coefficient for the Heat Exchanger
The overall heat transfer coefficient for clean surface (Uc) is given by
Considering the total fouling resistance, the heat transfer coefficient for fouled surface (Uf) can be calculated from the following expression:
Outlet Temperature Calculation and Length of the Heat Exchanger
The outlet temperature for the fluid flowing through the tube is
The surface area of the heat exchanger for the fouled condition is :
and for the clean condition
where the LMTD is always for the counter flow.
The over surface design (OS) can be calculated from :
The length of the heat exchanger is calculated by
Wall Temperatures
• It is often necessary to calculate the wall temperature by an iterative approach.
• The summarized procedure is: 1. Assume a film temperature, Tf 2. Evaluate the fluid properties (viscosity, density, etc.) at
this temperature 3. Use the properties to calculate a condensing heat transfer
coefficient (using the correlations to be presented) 4. Calculate the wall temperature. The relationship will
typically be something like
coolantsat
oo
satwall TTAh
UATT
1
1
5. Use the wall temperature to calculate a film temperature 6. Compare the calculated film temperature to that from the
initial step. If not equal, reevaluate the properties and repeat.
The heat transfer coefficient on the Nth tube row
• The heat transfer coefficient on the Nth tube row in the bundle h(N) is
434
31
)1()(
NNh
Nh
• Kern (1958) concluded from his practice experience in designing condensers that the Jakob tube row expression was too conservative and that this resulted in condensers that were consistently over-surfaced.
• To improve his thermal designs, he replaced the exponent of (-1/4) in the Nusselt expression with a value of (-1/6) so that corresponding equations become
)1()(
hNh
N
Condensation on Horizontal Bundles: Prediction of Heat Transfer Coefficient in Nth Tube Row