Heat Exchanger Principles and Designs

Fundamental Heat Exchanger Concepts

Key Formulas in Heat Transfer

• Heat Exchanged (q): q = m · Cp · ΔT (Heat absorbed or released by a fluid)
• Heat Transfer Rate (Q): Q = U · A · ΔT (Overall heat transfer rate through an exchanger)
• Energy Balance for Heat Exchangers: M_c · Cp_c · (ΔT_c) = M_f · Cp_f · (ΔT_f) (Heat gained by cold fluid equals heat lost by hot fluid)
• Other Formulas (Context Dependent): ct = w1 + w2 · PC1 · CP2

Definition of a Heat Exchanger

A heat exchanger is a device designed to efficiently transfer heat from one fluid to another. Common examples include:

• Condenser: Transfers heat from a hot fluid to a colder one, causing the hot fluid to condense (e.g., steam to water).
• Evaporator: Transfers heat to a cold fluid, causing it to evaporate (e.g., refrigerant to vapor).

Heat Exchanger Flow Configurations

• Parallel Flow: Both internal and external fluids flow in the same direction.

• Countercurrent Flow: Fluids flow in opposite directions. This configuration is generally more efficient, allowing for a greater temperature change in the fluids.

• Cross Flow: One fluid flows perpendicular to the other. This configuration is typically used when there is no phase change.
• Single Pass / Multi-Pass:
  • Single Pass: Heat is exchanged only once as fluids flow through the exchanger.
  • Multi-Pass: Fluids exchange heat multiple times by flowing through several sections or passes within the exchanger.

Specialized Heat Exchanger Types

• Regenerative Heat Exchanger: A process where heat is transferred to a heat storage medium (regenerator) during one part of a cycle, and then transferred back from the medium to another fluid during a different part of the cycle.
• Rotary Regenerator: A type of regenerative heat exchanger where a warm fluid passes through a rotating matrix, followed by a cold fluid, alternating with minimal mixing between the two fluid streams.
• Direct Contact Heat Exchangers: Devices where fluids directly contact each other for heat transfer (e.g., condenser trays).

Common Tubular Heat Exchanger Designs

1. Double Pipe Heat Exchangers

   Consist of two concentric pipes. The inner tube carries the cold fluid, absorbing heat, while the outer annulus carries the warm fluid, losing heat. They offer good performance but are limited in capacity.

2. Spiral Tube Heat Exchangers

   Feature an internal spiral design. These often require high fluid pressure due to significant pressure drop.

3. Shell and Tube Heat Exchangers

   Designed for high pressures and clean industrial fluids. A shell contains a bundle of fixed tubes. Common configurations include 1-2 pass and 2-4 pass designs (referring to TEMA designations).

   Shell and Tube Naming Convention (TEMA Type Example)

   Components often referenced in naming include:

   • Shell internal diameter
   • Tube length
   • Front head type
   • Shell type
   • Rear head type

4. Plate Heat Exchangers

   Composed of multiple thin, corrugated plates pressed tightly together. Hot fluid passes through one channel, and cold fluid through an adjacent one, without mixing. They are suitable for cooling/heating fibrous products or for applications like milk pasteurization.

   Free-Flow Plate Heat Exchangers

   A variant designed for viscous fluids or those with particles, characterized by no metallic contact between plates. Specific examples might refer to dimensions like D=2 and L=5.

5. Spiral Heat Exchangers

   Consist of two concentric spirals welded together, forming two channels for fluid flow and heat transfer. Advantages include self-cleaning capabilities, easy access for maintenance, and a compact design.

6. Graphite Heat Exchangers

   Fluids circulate through holes within a compact block of graphite, offering excellent corrosion resistance.

Plate vs. Shell and Tube Heat Exchangers: A Comparison