Heat Transfer 传热学 Li xuemei Welcome to

Chapter 9 Heat Exchangers Outline Overall Heat Transfer Coefficient Types of Heat Exchangers The Log Mean Temperature Difference Effectiveness-NTU Method

The Overall Heat-transfer Coefficient  Overall heat transfer through a plane wall k: over-all heat transfer coefficient ( 传热系数） Review…

T B h 2 T A h 1 0  x k =constant x T 1 T 2 The Overall Heat-transfer Coefficient

hihi hoho Hollow cylinder heat transfer

The Overall Heat-transfer Coefficient  Double-pipe heat transfer

Based on the outside area of the inner tube The Overall Heat-transfer Coefficient Based on the inside area of the inner tube

Fouling Factor (fouling resistance) various deposits corroded surfaces additional resistance decreased performance fouling factor:

Critical Thickness of Insulation Review… the critical radius of insulation:

Types of Heat Exchangers What are heat exchangers? Heat exchangers are devices that provide the flow of thermal energy between two or more fluids at different temperatures.

double-pipe heat exchanger parallel flow Heat exchangers can take a variety of shapes. Types of Heat Exchangers

counterflow Types of Heat Exchangers

Shell-and-Tube Heat Exchangers

Exchanger with 1 shell pass and 1 tube pass 1-1 型换热器

Exchanger with 1 shell pass 2 tube passes 1-2 型换热器

one shell pass two tube pass

Exchanger with 2 shell passes 4 tube passes 2-4 型换热器

Both fluids unmixed

Plate-fin heat exchanger common in air conditioning and refrigeration systems

Plate heat exchanger

Plate Heat Exchanger

Condenser （冷凝器）、 evaporator （蒸发器） for heat pump （热泵）

Spiral heat exchanger for wastewater use

Air-Cooled Heat Exchangers come in compact sizes

In a binary cycle power plant (binary means two), the heat from geothermal water is used to vaporize a "working fluid" in separate adjacent pipes. The vapor, like steam, powers the turbine generator. In the heat exchanger, heat is transferred from the geothermal water to a second liquid. The geothermal water is never exposed to the air and is injected back into the periphery of the reservoir.

供 暖 器 的 加 热 器 芯供 暖 器 的 加 热 器 芯

蒸 汽 发 生 器蒸 汽 发 生 器

Newly developed heat exchangers Hot air inletHot air outlet Cold air inlet Cold air outlet

Baffles （隔板） Tubes in matrix （矩阵） Newly developed heat exchangers

The Log Mean Temperature Difference Double-pipe heat exchanger Temperature profiles for parallel flow and counterflow

The Log Mean Temperature Difference dt h dt c thth tctc parallel flow:

The Log Mean Temperature Difference

(1) (2) (3) (1)+(2)+(3) log mean temperature difference

counterflow: The Log Mean Temperature Difference

parallel flow: counterflow:

If 0.5<  T 1 /  T 2 <2, then (  T m ) counter flow > (  T m ) parallel flows

The Log Mean Temperature Difference For heat exchangers other than the double-pipe type: correction factor

Thermal Calculation of Heat Exchanger The Basic Equations of the LMTD method

Thermal Calculation of Heat Exchanger The LMTD method can be used in either of the following: Design Calculations: To determine the heat transfer area needed; Check Calculations: To check if a heat exchanger can accomplish the given task.

Heat-Exchanger Design Considerations Heat-transfer requirements; Cost or economy considerations; Physical size; Pressure-drop characteristics (pumping costs). Discussion: What should be considered when designing a heat exchanger?

Effectiveness-NTU Method for self-study and discussion

Thank you!