Heat Exchangers: Design Considerations
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Transcript Heat Exchangers: Design Considerations
Heat Exchangers:
Design Considerations
Types
Heat Exchanger Types
Heat exchangers are ubiquitous to energy conversion and utilization. They involve
heat exchange between two fluids separated by a solid and encompass a wide
range of flow configurations.
• Concentric-Tube Heat Exchangers
Parallel Flow
Counterflow
Simplest configuration.
Superior performance associated with counter flow.
Types (cont.)
• Cross-flow Heat Exchangers
Finned-Both Fluids
Unmixed
Unfinned-One Fluid Mixed
the Other Unmixed
For cross-flow over the tubes, fluid motion, and hence mixing, in the transverse
direction (y) is prevented for the finned tubes, but occurs for the unfinned condition.
Heat exchanger performance is influenced by mixing.
Types (cont.)
• Shell-and-Tube Heat Exchangers
One Shell Pass and One Tube Pass
Baffles are used to establish a cross-flow and to induce turbulent mixing of the
shell-side fluid, both of which enhance convection.
The number of tube and shell passes may be varied, e.g.:
One Shell Pass,
Two Tube Passes
Two Shell Passes,
Four Tube Passes
Types (cont.)
• Compact Heat Exchangers
Widely used to achieve large heat rates per unit volume, particularly when
one or both fluids is a gas.
Characterized by large heat transfer surface areas per unit volume, small
flow passages, and laminar flow.
(a)
(b)
(c)
(d)
(e)
Fin-tube (flat tubes, continuous plate fins)
Fin-tube (circular tubes, continuous plate fins)
Fin-tube (circular tubes, circular fins)
Plate-fin (single pass)
Plate-fin (multipass)
Tubular Exchanger Manufacturers Association
Overall Coefficient
Overall Heat Transfer Coefficient
1 / U A = 1 / h1 A1 + dxw / k A + 1 / h2 A2
where
U = the overall heat transfer coefficient (W/m2K)
A = the contact area for each fluid side (m2)
k = the thermal conductivity of the material (W/mK)
h = the individual convection heat transfer coefficient for each fluid (W/m2K)
dxw = the wall thickness (m)
Overall Coefficient
where
kw - Thermal Conductivity of Fluid
DH - Hydraulic Diameter
Nu - Nusselt Number
where:
•ν : kinematic viscosity, ν = μ / ρ, (SI units : m2/s)
•α : thermal diffusivity, α = k / (ρcp), (SI units : m2/s)
•μ : dynamic viscosity, (SI units : Pa s)
•k: thermal conductivity, (SI units : W/(m K) )
•cp : specific heat, (SI units : J/(kg K) )
•ρ : density, (SI units : kg/m3 ).
•V : Fluid velocity (SI units m/s)
•L : Pipe Internal Diameter (SI units m)
n = 0.4 for heating (wall hotter than the bulk fluid) and 0.33 for cooling
(wall cooler than the bulk fluid)
Thermal Conductivity
Thermal conductivity
Material
(W/m K)*
Diamond
1000
Silver
406
Copper
385
Gold
314
Brass
109
Aluminum
205
Iron
79.5
Steel
50.2
LMTD Method
A Methodology for Heat Exchanger
Design Calculations
- The Log Mean Temperature Difference (LMTD) Method • A form of Newton’s Law of Cooling may be applied to heat exchangers by
using a log-mean value of the temperature difference between the two fluids:
q U A T1m
T1m
T1 T2
1n T1 / T2
Evaluation of T1 and T2 depends on the heat exchanger type.
• Counter-Flow Heat Exchanger:
T1 Th ,1 Tc ,1
Th ,i Tc , o
T2 Th ,2 Tc ,2
Th , o Tc ,i
LMTD Method (cont.)
• Parallel-Flow Heat Exchanger:
T1 Th ,1 Tc ,1
Th ,i Tc ,i
T2 Th,2 Tc,2
Th,o Tc,o
Note that Tc,o can not exceed Th,o for a PF HX, but can do so for a CF HX.
For equivalent values of UA and inlet temperatures,
T1m,CF T1m, PF
• Shell-and-Tube and Cross-Flow Heat Exchangers:
T1m F T1m,CF
F Figures 11.10 - 11.13
Energy Balance
Overall Energy Balance
• Application to the hot (h) and cold (c) fluids:
• Assume negligible heat transfer between the exchanger and its surroundings
and negligible potential and kinetic energy changes for each fluid.
q m h ih,i ih,o
q m c ic,o ic,i
i fluid enthalpy
• Assuming no l/v phase change and constant specific heats,
q m h c p, h Th,i Th,o Ch Th,i Th,o
q mc c p,c Tc,o Tc,i Cc Tc,o Tc,i
Ch,Cc Heat capacity rates
Special Conditions
Special Operating Conditions
Case (a): Ch>>Cc or h is a condensing vapor Ch .
– Negligible or no change in Th Th,o Th,i .
Case (b): Cc>>Ch or c is an evaporating liquid Cc .
– Negligible or no change in Tc Tc,o Tc,i .
Case (c): Ch=Cc.
– T1 T2 T1m