Single-Phase Heat Exchangers

Abstract

This chapter deals with the thermal design theory of single-phase recuperative heat exchangers. Established methods for (a) designing a heat exchanger that will yield a desired performance under specified operating conditions or (b) predicting the performance of a given heat exchanger operating under prescribed conditions are logically presented. Heat exchangers are first classified based on their construction and flow configuration. Next, basic concepts central to heat exchanger design, such as the fluid mechanics of internal flow, laminar and turbulent flow, boundary layer development, friction factor, heat transfer coefficient, overall heat transfer coefficient, fouling, etc., are discussed. Having laid the conceptual framework, two commonly encountered problems in heat exchanger design are described. Two well-established methods of designing heat exchangers, the logarithmic mean temperature difference (LMTD) and the effectiveness-NTU (ε-NTU) methods, are then explained in some detail. The chapter concludes with a discussion of the heat transfer coefficient results/correlations under various flow situations and boundary conditions, which will be helpful in the calculation of the overall heat transfer coefficient.

Nomenclature

A:

Area (m²)

C:

Heat capacity rate (W/K)

c_p :

Specific heat (J/kg K)

CF:

Counterflow

D:

Diameter (m)

D_h :

Hydraulic diameter (m)

f:

Darcy friction factor

F:

Correction factor

Gz:

Graetz number

h:

Heat transfer coefficient (W/m² K)

i:

Inlet

k:

Thermal conductivity (W/m K)

L:

Wall thickness (m)

$\dot{m}$ :

Mass flow rate (kg/s)

NTU:

Number of heat transfer units

Nu:

Nusselt number

o:

Outlet

p:

Pressure (Pa)

P:

Perimeter (m)

PF:

Parallel flow

Pr:

Prandtl number

$\dot{Q}$ :

Heat transfer rate (W)

r:

Radius (m)

R:

Conduction thermal resistance (K/W)

R´´:

Fouling resistance (m² K/W)

Re:

Reynolds number

t:

Tube side fluid temperature (K), thickness (m)

T:

Temperature (K)

U:

Overall heat transfer coefficient (W/m² K)

V:

Volume (m³)

w:

Width (m)

x:

Entry length (m)

1.1 Greek

ΔT :

Temperature difference (K)

ε:

Heat exchanger effectiveness

η:

Fin efficiency

v :

Kinematic viscosity (m²/s)

ρ:

Density (kg/m³)

1.2 Subscripts

b:

Base

c:

Cold, corrected, critical, cross section

D:

Diameter

f:

Fouling, fin

fd:

Fully developed

h:

Hot, hydrodynamic

lam:

Laminar

m:

Mean

min:

Minimum

o:

Overall

p:

Profile

r:

Ratio

t:

Thermal

turb:

Turbulent

w:

Wall