Heat Exchangers Design Considerations Types Heat Exchanger Types

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 Ø Simplest configuration. Ø Superior performance associated with counter flow.

Types (cont. ) • Cross-flow Heat Exchangers Finned-Both Fluids Unmixed Unfinned-One Fluid Mixed the Other Unmixed Ø 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. Ø Heat exchanger performance is influenced by mixing.

Types (cont. ) • Shell-and-Tube Heat Exchangers One Shell Pass and One Tube Pass Ø Baffles are used to establish a cross-flow and to induce turbulent mixing of the shell-side fluid, both of which enhance convection. Ø 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 Ø Widely used to achieve large heat rates per unit volume, particularly when one or both fluids is a gas. Ø 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, circular fins) Plate-fin (single pass) Plate-fin (multipass)

Tubular Exchanger Manufacturers Association

Overall Coefficient Overall Heat Transfer Coefficient 1 / U A = 1 / h 1 A 1 + dxw / k A + 1 / h 2 A 2 where U = the overall heat transfer coefficient (W/m 2 K) A = the contact area for each fluid side (m 2) k = thermal conductivity of the material (W/m. K) h = the individual convection heat transfer coefficient for each fluid (W/m 2 K) 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 : m 2/s) • α : thermal diffusivity, α = k / (ρcp), (SI units : m 2/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/m 3 ). • 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: Evaluation of depends on the heat exchanger type. • Counter-Flow Heat Exchanger:

LMTD Method (cont. ) • Parallel-Flow Heat Exchanger: Ø Note that Tc, o can not exceed Th, o for a PF HX, but can do so for a CF HX. Ø For equivalent values of UA and inlet temperatures, • Shell-and-Tube and Cross-Flow Heat Exchangers:

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. • Assuming no l/v phase change and constant specific heats,

Special Conditions Special Operating Conditions Ø Case (a): Ch>>Cc or h is a condensing vapor – Negligible or no change in Ø Case (b): Cc>>Ch or c is an evaporating liquid – Negligible or no change in Ø Case (c): Ch=Cc. –