Comprehensive Study Notes on Heat Exchangers and Cooling Systems

Double Pipe Heat Exchangers

A double pipe heat exchanger is a type of exchanger where the hot and cold fluids do not come into direct contact with each other, often described as a "Pipe in a Pipe" arrangement.

Flow Patterns

The fluid flows can be arranged in two primary ways:

• Co-current (Concurrent) Flow: Both fluids enter from the same side and move in the same direction.
• Counter-current Flow: The fluids enter from opposite ends and move in opposite directions. This is the more efficient arrangement for heat transfer.

Construction and Arrangement

• Jacketed Pipe: This is the simplest arrangement consisting of an outer pipe welded to an inner pipe.
• Layout: It consists of an inner pipe surrounded by another larger diameter pipe. This creates a space between them known as the Annulus.
• Bundles: Some double pipe heat exchangers may contain a bundle of inner tubes instead of a single tube to increase the surface area.
• Pipe Types: Inner pipes can be plain bare tubes or may be fitted with various elements to increase surface efficiency, such as:
  • Longitudinal Fins
  • Helicoidal Fins
  • Studs (Stud Fins)

Concentric Pipe and Hairpin Heat Exchangers

Concentric Pipe Heat Exchanger

This is a simple form of a double pipe heat exchanger consisting of a single pipe placed inside a larger pipe. The inner pipe can have fins or studs attached to enhance heat transfer efficiency.

Hairpin Heat Exchangers

Also known as U-Tube heat exchangers, these refer to a specific arrangement of double pipe exchangers.

• Structure: They have two sets of concentric pipes connected by various fittings: specialized tees and a return bend.
• Connection Type: The inner pipes are joined by a return bend, while the outer pipes are connected using tees.
• Serpentine Arrangement: Multiple hairpins can be joined together in series; this specific configuration is often referred to as "Serpentine."

Characteristics of Double Pipe Exchangers

Advantages

• Construction: Simple and easy to manufacture.
• Suitability for Pressure: Small diameters allow for stronger construction, making them very well suited for high-pressure applications.
• Flexibility: Units can be easily added or removed as process requirements change.
• Maintenance: Maintenance and the replacement of individual sections are relatively simple.
• Compactness: They are footprint-efficient.
• Fluid Utility: Suited for processes requiring significant temperature changes in both fluids.

Disadvantages

• Leakage Risk: The single biggest disadvantage is the high number of joints required, which increases the statistical possibility of leaks.

Shell and Tube Heat Exchangers

The shell and tube heat exchanger is the most common type of heat transfer equipment in industrial environments. It is versatile, suited for high-pressure applications, and can be custom-designed for a wide range of industrial needs.

General Features

• Large surface areas for heat transfer.
• Capability to handle high temperatures and high pressures.
• Two principal sub-types are used: U-tube and Fixed Tube Sheet (Straight Tube).

U-Tube Heat Exchangers

• Head Attachment: The head is bolted to the open end of the shell. The tube sheet is held in place between the flanges.
• Radial Restrictions: A major problem is that tubes can only be bent to a minimum bend radius. This creates a space between the arms of the "U" that cannot be filled with tubes, reducing potential heat transfer area.
• Fouling Management: These units should always have the fouling fluid on the shell side for easier cleaning.
• Repair: Damaged tubes are usually plugged off, as tube replacement is restricted to those on the outside of the bundle.

Straight Tube / Fixed Tube Sheet Heat Exchangers

• Construction: Tube sheets are welded directly to the shell at both ends. Tubes are straight and can completely fill the shell area.
• Cleaning: These are much easier to clean than U-tube designs.
• Expansion: Because the tubes and shell are fixed, expansion joints may be required to accommodate thermal expansion and prevent tube sheet stress depending on the length of the exchanger.
• Fluid Placement: Fouling fluid should be placed on the tube side.
• Limitations: They are prone to leaks due to the high number of joints, gaskets, and fasteners. High $\Delta T$ between fluids can cause unacceptable stresses.

Floating Tube Sheet Heat Exchangers

• Design: This type features one fixed head and tube sheet, and one internal head/tube sheet that is free to slide or "float" horizontally within the shell.
• Thermal Expansion: The floating design allows for different rates of expansion between the tubes and the shell without causing stress.
• Removability: The entire tube bundle can be removed for inspection, cleaning, and repair.
• Maintenance: Leaking tubes can be replaced rather than just plugged.
• Disadvantage: The potential for leaks at the floating end.

Baffles in Heat Exchangers

Baffles are plates installed on the shell side to enhance heat transfer efficiency.

Functionality

1. Flow Direction: Without baffles, the fluid takes the shortest path between the inlet and outlet, leaving much of the heating surface unused. Baffles force the fluid to flow back and forth across the tubes.
2. Support: They provide mechanical support for the tubes.

Common Types

• Segmental Baffles: Flat plates with a segment cut out. They are staggered on opposite sides to cause a zig-zag flow.
• Annular Ring Baffles: Circular plates that control flow concentrically.
• Disc and Doughnut Baffles: Alternating discs and ring-shaped (doughnut) plates.

Baffle Pitch

• Definition: The center-to-center distance between baffles is called the baffle pitch or baffle spacing.
• Optimization: Designers determine spacing to maximize heat transfer while ensuring the pressure drop does not increase significantly.

Reboilers

Reboilers are specialized shell and tube exchangers used primarily in gas processing and petrochemical plants to provide heat to the bottom of distillation columns.

Types of Reboilers

1. Kettle Type:

   • Provides a large liquid volume and vapor disengagement space (the space above the tube bundle).
   • Easy to control and maintain.
   • Operates based on gravity for liquid circulation.
   • Includes a weir plate to control liquid level; excess liquid overflows the weir to storage or further processing.

2. Horizontal Thermosyphon:

   • Designed with no vapor space above the tubes (unlike the Kettle type).
   • Must be installed at an elevation lower than the liquid level in the tower bottom to ensure the shell remains completely flooded.
   • Circulation is driven by density differences: the heated fluid/vapor mixture in the reboiler is less dense than the liquid in the tower.

3. Vertical Thermosyphon:

   • Positioned vertically and attached closely to the side of the tower.
   • Usually features the heated process liquid on the tube side and the heating medium (steam) on the shell side.

Steam Side Operation

The heat duty of a reboiler can be controlled by varying the level of condensate in the tubes using a Level Controller (LC). Raising the condensate level covers more tube surface, reducing the amount of surface area available for steam to contact and transfer heat. This is managed via local/field controllers or a hand/auto station in the control room.

Feed Water Heaters

These are utilized in steam cycle generating stations and industrial plants to improve cycle efficiency. They heat the boiler feed water ($\text{BFW}$) using bleed steam from the turbine before it enters the economizer section of the steam generator. Most are of the U-tube design.

Plate and Frame Heat Exchangers (PHE)

Construction

• Consists of a number of pressed metal plates aligned on a frame and clamped together.
• Gaskets: Placed between plates to direct flow, prevent leakage to the atmosphere, and prevent cross-contamination between fluids.
• Corrugation: Plates have alternating ridges and grooves to create turbulence and increase surface area.
• Spacing: Plates are spaced with nominal gaps of $2\,mm$ to $5\,mm$.

Principal Parts

• Pressed Plates: Create the heat transfer surface.
• Ports: Located in the corners for fluid inlet and outlet.
• Carrying Bars: The upper bar carries the weight of the plates; plates are installed by sliding them along these bars.
• Compression/Stud Bolts: Used to tighten the end plates and seal the gaskets.
• Movable and Fixed Covers: Provide the structure to compress the plate pack.

Advantages

• Low Fouling: High turbulence and low residence times prevent buildup.
• High Efficiency: Transfer coefficients are so high that they require only $\frac{1}{3}$ to $\frac{1}{2}$ the area of a shell and tube exchanger for the same duty.
• Detection: Leaks are easily detected as they move to the exterior.
• Modularity: Capacity can be changed after installation by adding or removing plates.
• Rapid Start-up: Small liquid hold-up allows for quick thermal response.

Disadvantages

• Temperature/Pressure Limits: Typically limited to $175\,^\circ\text{C}$ and $2.07\,MPa$.
• Viscosity: Not recommended for thick or highly viscous fluids.
• Particulates: Small clearances ($2$-$5\,mm$) trap particles and suspended fibers.
• Velocity: Flow rates must be sufficient to maintain velocities above $0.1\,m/s$ to prevent poor distribution.
• Materials: Not suitable for toxic or explosive fluids as leakage is external to the atmosphere.

Aerial Coolers and Condensers

Basic Principles

• Uses atmospheric air as the cooling medium.
• Often called finned tube exchangers due to the fins attached to the exterior of the tubes. These fins are welded, wrapped mechanically, or expanded onto the tubes.
• Preferred when process fluids are at least $55\,^\circ\text{C}$ or when fouling from cooling water is a concern.

Configurations

• Forced Draft: Fans are located below the tube bundles and force air upwards.
• Induced Draft: Fans are located above the bundles and pull air through them.
• Plenum: The ducting that directs airflow from the fan through the coils.

Layout Terminology

• Bundle: A single set of tubes attached to headers.
• Bay: A section containing one or more bundles and dedicated fans.
• Bank: A group of bays serving the same purpose.

Control and Maintenance

• Temperature Control: Managed by varying fan speed, starting/stopping fans, or adjusting louvers (dampers).
• Cleaning: External fins collect airborne debris. Headers have threaded plugs opposite each tube to allow for manual lancing, jetting, or chemical cleaning.
• Freezing: In cold climates, hot air can be recirculated to prevent condensate from freezing and damaging tubes.

Bundle Shapes

• Horizontal: Most common.
• Vertical: Used when floor space is limited; affected by wind direction.
• A-Frame / V-Frame: Require less room than horizontal and are more wind-resistant than vertical. Often used in steam condensing.

Cooling Towers

Natural Draft Cooling Towers

1. Atmospheric Towers: Depend on natural wind currents. Large open sides with louvers help contain water while allowing air to flow across the tower.
2. Hyperbolic Towers: Large chimney-like concrete structures (up to $150\,m$ high). Airflow is produced by the density differential between heated air inside and cooler air outside. They contain packing only in the bottom $10$ to $15\,m$. They are expensive to build but have zero fan power costs.

Mechanical Draft Cooling Towers

1. Forced Draft:
   • Fans at the base force air into the tower.
   • Fans handle dry ambient air, reducing erosion.
   • Less suited for cold weather as moisture can recirculate to the fan and cause icing/vibration.
2. Induced Draft:
   • Fans at the top pull air through the tower.
   • Counterflow: Air moves vertically upward against downward water. They are taller and occupy less floor space.
   • Crossflow: Air moves horizontally across the falling water. They have lower air pressure drops and lower fan power requirements.

Tower Components and Operations

• Drift Eliminators: Remove entrained water droplets from the air stream before it exits the tower.
• Fill (Packing): The material that promotes contact between air and water.
• Splash Type Fill: Usually made of $PVC$; breaks water into droplets as it falls.
• Film Type Fill: Thin $PVC$ sheets spaced closely. Water spreads into a thin film. This provides more cooling than splash fill for the same volume.

Operating Variables

• Water Quality: Must be monitored via chemical treatment.
• Drift: Water loss caused by wind or broken louvers.
• Basin Level: High levels lead to overflow; low levels risk pump cavitation and decreased flow.