Heat Exchangers

Heat Transfer Fundamentals

Overview of Heat Exchangers

• Heat Exchangers are essential devices used in various thermal applications where heat transfer is required.

Overall Heat Transfer Coefficient

• Many heat transfer applications involve both conduction and convection mechanisms.

• Example: Flow through a pipe can be analyzed using temperature differential relationships.

• Equation:
  $T<em>o - T</em>i = rac{(T<em>2 - T</em>1)}{R}$
  where
  $R = rac{1}{h<em>1} + rac{1}{h</em>2}$
  and
  $R = ext{thermal resistance}$

Example Problem: Heat Loss Calculation

• Scenario: A pipe with an inside diameter of 2.5 cm conveying liquid food at 80°C; the inside convective heat transfer coefficient is 10 W/(m²°C). The pipe has a thickness of 0.5 cm and is made of stainless steel with a thermal conductivity of 43 W/(m°C). Ambient temperature is 20°C, with an outside convective heat transfer coefficient of 100 W/(m²°C).

• Objective:

  • Calculate the overall heat transfer coefficient.

  • Calculate the heat loss from a 1 m long pipe.

Heat Exchanger Designs

Plate Heat Exchanger

• Composition: Involves parallel stainless-steel plates that increase contact surface area.

• Function: Enhances heat transfer efficiency through increased turbulence patterns on the plates.

• Application: Suitable for low-viscosity liquids (<5 Pa s) with particulates less than 0.3 cm.

Example: Pasteurization Process for Liquid Food

• Schematic Overview:

  • Heating Section: Hot water at 93°C heats the liquid food (Juice).

  • Regeneration Section: Transfers heat from outgoing product to incoming product.

  • Cooling Sections: Utilizes city water, chilled water, and glycol for product cooling.

• Temperature Points:

  • Incoming product temperature: 21°C (calculating changes through various points down to 38°C).

Tubular Heat Exchanger

• Consists of a double or triple tube arrangement for efficient heat transfer, depicting a basic flow of…

  • Fluid A in and out.

  • Fluid B in and out.

Scraped-Surface Heat Exchanger

• Design Features:

  • Media zone with a polished stainless steel product tube.

  • Includes a scraper blade to enhance heat transfer by preventing fouling on surfaces.

  • Insulation surrounds the media cylinder to maintain temperatures.

Steam-Infusion Heat Exchanger

• Operation Principle: Direct contact between steam and the product ensures effective heat transfer.

Heat Exchanger Design Considerations

• Assumes Steady State Conditions.

• Flow Configurations: Countercurrent or co-current flow influence the heat transfer performance.

• Heat Transfer Coefficients: Determination of overall heat transfer coefficients relies on flow type (laminar or turbulent).

• Key Relationship:

  • $q = U imes ext{Area} imes ext{Log Mean Temperature Difference (LMTD)}$

  • Where $q$ is the heat transfer rate.

Example Calculation: LMTD and Exit Temperature

• Parameters:

  • A liquid food with a specific heat of 4.0 kJ/(kg°C) enters a double-pipe heat exchanger at 20°C and exits at 60°C, with a mass flow rate of 0.5 kg/s.

  • Hot water entering at 90°C flows countercurrently at 1 kg/s with an average specific heat of 4.18 kJ/(kg°C).

• Objectives:

  • Calculate exit temperature of hot water.

  • Calculate the log mean temperature.

  • If the average overall heat transfer coefficient is 200 W/(m²°C) and the diameter of the inner pipe is 5 cm, determine the length of the heat exchanger.

Example 2: Plate Heat Exchanger for Milk Heating

• Scenario: Raw whole milk heated to 72°C at a rate of 5000 L/h from 56°C with hot water supplied at 7500 L/h at 85°C.

• Parameters:

  • Each heat exchanger plate area is 0.79 m².

  • The overall heat transfer coefficient is 2890 W/(m².°C).

• Objective:

  • Calculate the number of plates required for heating the milk.

  • Given specific heats: Milk = 3.9 kJ/(kg°C), Hot water = 4.18 kJ/(kg°C).

  • Densities: Milk = 1030 kg/m³, Hot water = 958 kg/m³.