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