Industrial Heat Exchangers and Cooling Towers Exhaustive Study Guide
Introduction to Heat Exchangers and Cooling Towers
Heat exchange is a fundamental process in Power Engineering, used extensively across industries to either heat or cool substances. Common applications include:
Turbine Exhaust Steam: To allow water to be reused in the boiler, exhaust steam from turbines must be cooled in a heat exchanger to condense it back into water for pumping.
Refrigeration Systems: Hot, high-pressure refrigerant gas is condensed in a heat exchanger to become liquid. This liquid then passes through another heat exchanger (evaporator) to absorb heat from a space, returning to a gaseous form for recompression.
Industrial heat exchangers generally keep the medium being heated or cooled separate from the medium performing the heating or cooling.
Double-Pipe Heat Exchangers
A double-pipe heat exchanger consists of a pipe (or a bundle of tubes) situated inside a larger-diameter pipe. The hot and cold fluids remain separated, and heat is transferred from one fluid to the outer surface of the inner pipe, through the pipe wall, and into the second fluid.
Flow configurations include:
Co-current Flow: Both fluids flow in the same direction.
Counter-current Flow: Fluids flow in opposite directions.
Jacketed Pipe Design
The simplest double-pipe arrangement is the jacketed pipe. It features an inner pipe surrounded by a larger pipe that creates an annulus space. The outer pipe is usually welded to the inner pipe with end caps. A common application is a steam-jacketed line where steam in the annulus heats a process fluid in the inner pipe.
Concentric Pipe and Finned Tubes
In a concentric pipe design, the inner pipe may be plain or may feature fins or studs. These attachments increase the efficiency of the exchanger by significantly expanding the heat transfer surface area. Instead of one inner pipe, a bundle of several smaller tubes may be used, connected by tubesheets at each end, to further increase total surface area.
Hairpin (Serpentine) Exchangers
A hairpin exchanger refers to double-pipe units joined in series to resemble a hairpin or snake-like (serpentine) configuration. This arrangement provides a longer flow path for both fluids, allowing for longer retention time and greater overall heat transfer.
Advantages and Disadvantages
Advantages:
Simple construction.
Suitability for high pressures because small diameters allow for stronger materials.
Configuration flexibility; units can be added/removed to adjust heat transfer capacity.
Simple maintenance and easy replacement of sections or bundles.
Compactness due to flexible arrangement options.
Ideal for simultaneous heating and cooling of two fluids.
Disadvantage:
A high number of joints increases the potential for leaks.
Shell-and-Tube Heat Exchangers
Shell-and-tube exchangers are the most common industrial design due to their versatility in high-temperature and high-pressure applications. They consist of an outer shell surrounding a bundle of parallel tubes.
U-Tube Exchanger
The tubes are U-shaped and fixed to a single tubesheet.
Expansion: This design eliminates differential thermal expansion problems because the tubes and shell can expand at different rates independently.
Maintenance: The tube bundle and attached baffles can be unbolted and slid out of the shell for inspection and cleaning.
Disadvantages: Thinner outer walls at the tube bends require a minimum bend radius, which limits the number of tubes in a shell. Mechanical tube cleaning is difficult in the bends, and repair is restricted to the outside rows; damaged tubes are typically plugged.
Operation: Fouling fluids should be placed on the shell side to facilitate easier cleaning of the exterior tube surfaces.
Floating Head Exchanger
This design features one fixed tubesheet and one "floating" tubesheet that slides horizontally within the shell.
Benefits: Allows for differential expansion without stress. The entire bundle is removable, and both the shell and the inside of the straight tubes can be cleaned easily. Leaking tubes can be replaced individually.
Disadvantage: Increased risk of leaks at the internal floating head flange.
Fixed Tubesheet Exchanger
Tubesheets are welded directly to the shell at both ends, and straight tubes are expanded or welded into them.
Advantages: Simple, cost-effective fabrication with maximum heating surface for a given shell size. Tubes are easily cleaned with brushes or high-pressure water jets.
Disadvantages: Large temperature differences cause unacceptable stresses unless an expansion joint is installed in the shell. The shell side is extremely difficult to clean, requiring chemical means or grinding off welds.
Baffles
Baffles are plates (usually segment-shaped or annular rings) that direct shell-side fluid flow across the tubes rather than taking the shortest path.
Purpose: Increase velocity and turbulence to improve heat transfer; provide structural support for tubes.
Baffle Pitch: The horizontal spacing between baffles, designed to maximize heat transfer while minimizing pressure drop.
Reboilers and Feedwater Heaters
Reboilers
Found in gas processing and petrochemical plants, reboilers vaporize a process liquid using steam or hot oil in the tubes.
Kettle Type: Features a large liquid volume, a vapor space, and a vertical weir plate to keep tubes submerged. Circulation depends on gravity.
Horizontal Thermosyphon: The shell is completely flooded. Density differences between the liquid in the tower and the boiling mixture in the reboiler drive continuous circulation.
Vertical Thermosyphon: Attached closely to the side of a tower; typically has heated liquid in the tubes and heating medium in the shell.
Steam Side Control: A level transmitter (LT) and level controller (LRC) vary the condensate level in the tubes to expose more or less surface area to steam, regulating heat input.
Feedwater Heaters
Used to improve steam cycle efficiency by heating boiler feedwater using turbine extraction steam.
Low-Pressure (LP) Heaters: Located between hot well pumps and the deaerator.
High-Pressure (HP) Heaters: Located between feedwater pumps and the boiler.
Condensate Path: Steam condenses in the shell. Condensate passes through a liquid seal leg to a flash box, where some liquid vaporizes (flash steam) before entering the hot well.
Safety: A float-operated trip gear closes motor-operated valves (MOVs) if a tube leak causes the shell level to rise, preventing over-pressurization.
Plate-and-Frame Heat Exchangers (PHE)
A PHE consists of many thin, corrugated metal plates pressed together on a sturdy frame with gaskets between each plate.
Components
Plates: Corrugated to create turbulence and increase surface area.
Ports: Holes in the corners that align to form flow channels.
Gaskets: Rubber or synthetic (e.g., neoprene) seals that direct flow and prevent leakage to the atmosphere.
Compression Bolts and End Plates: Used to tighten the assembly and provide structural integrity.
Carrying Bars: Support and align the plates.
Performance Characteristics
Advantages: High heat transfer rates (requires only to the area of shell-and-tube units); low fouling; easy power scaling; immediate leak detection.
Limits: Pressure must not exceed ; temperature must not exceed .
Service Restrictions: Not for viscous fluids (small gaps), very dirty fluids (plugging), or toxic/explosive gases (leakage is to the atmosphere).
Flow Velocity: Velocities below cause poor distribution.
Aerial Coolers and Condensers
Aerial units use atmospheric air to cool or condense fluids inside finned tubes.
Cooler Designs
Induced Draft: A top-mounted fan pulls air across horizontal tube banks.
Forced Draft: Fans are located below the tubes, pushing air upward. Fans handle dry air, reducing erosion.
Arrangements: Organized into bundles, bays (sections with dedicated fans), and banks (groups of bays).
Control and Maintenance
Temperature Control: Managed via Variable Frequency Drives (VFD) for fan speed, starting/stopping fans, or adjustable louvres.
Recirculation: In cold climates, recirculation louvres mix warm discharge air with cold inlet air to prevent subcooling.
Extreme Heat: In undersized conditions, water may be sprayed onto the coils to utilize evaporative cooling.
Cleanliness: Screens are often installed over fan inlets to prevent airborne debris from plugging the narrow fin spaces.
Aerial Condensers
Demanding service that removes latent heat from two-phase (vapor/liquid) fluids.
Configurations: Include Horizontal, Vertical, V-frame, and A-frame (roof style). A-frame is common for high-volume steam condenser applications.
Box Headers: Fitted with threaded plugs for tube lancing. Plugs are common leak points.
Precautions: In freezing weather, condensate must be monitored to prevent ice expansion and tube rupture.
Cooling Towers
Cooling towers utilize the evaporation of water into air to achieve cooling.
Natural Draft Towers
Atmospheric: Dependent on wind. Use spray nozzles (aspiration) or wood/plastic packing to break water into droplets.
Hyperbolic: Large concrete chimneys (up to high). Airflow is driven by the density difference between heated internal air and cool external air. No power is needed for fans, but construction is expensive.
Mechanical Draft Towers
Forced Draft: Fan at the base. Vibration is minimal, but susceptible to ice buildup on blades in winter.
Induced Draft Counterflow: Fan at the top pulls air upward against falling water. Taller than crossflow but occupies less floor space.
Induced Draft Crossflow: Air flows horizontally through packing while water falls vertically. Provides low pressure drop and lower energy costs.
Cooling Tower Fill
Efficiency depends on the contact time between air and water.
Splash Fill: PVC or wood (redwood/Douglas fir) slats that break water into droplets.
Film Fill: Closely spaced thin PVC sheets. Water forms a thin film, providing much more surface area than splash fill for a given volume.
Placement and Operation
Towers should be higher than condensers to prevent water draining back during shutdown and located near the plant to minimize pumping requirements (). Monitoring water chemistry is vital to prevent packing fouling.