Plate Fin Tube Heat Exchanger

What is a Plate Fin Tube Heat Exchanger and What is Its Purpose?

A plate fin tube heat exchanger, essentially a type of finned heat exchanger, is a device designed for efficient heat transfer between air and a fluid medium.

Working Principle (Medium)

The plate fin tube heat exchanger operates by allowing hot or cold media to flow inside metal tubes while the air to be treated flows over the external surfaces of the tubes, facilitating heat exchange primarily for air temperature control.

Basic Structure

The basic structure involves multiple thin metal fins (typically aluminum) spaced at regular intervals along metal base tubes (commonly copper tubes). Mechanical tube expansion or similar processes ensure tight contact between the fins and base tubes.

A complete plate fin tube heat exchanger structure also includes headers, framework, supports, flanges, and other components.

• Header Material: Galvanized steel pipe, copper pipe, or stainless steel pipe
• Framework: Galvanized sheet or stainless steel sheet

Working Modes

1. Cooling and Dehumidification

• Dry Cooling: When the surface temperature of the plate fin tube heat exchanger is lower than the air temperature but higher than its dew point, sensible cooling occurs (temperature reduction without dehumidification).
• Wet Cooling: When the surface temperature of the plate fin tube heat exchanger is below the airs dew point, cooling with dehumidification occurs (both temperature reduction and dehumidification). Condensate forms in this mode. Hydrophilic aluminum fins help condensate drain quickly.

2. Heating

When hot water or steam flows through the tubes, it can provide sensible heating to the air passing over them.

Working Media

• Heating Media: Hot water, steam
• Cooling Media: Chilled water, refrigerant

Applications

Due to their efficient heat transfer characteristics, plate fin tube heat exchangers are widely used in:

• HVAC Systems: Core components of fresh air handling units, air handling units, and fan coil units for regulating supply air temperature and humidity.
• Industrial Fields: Used in petrochemical, metallurgy, power generation, textile, printing, dyeing, drying, and other industrial processes for heating, cooling, or waste heat recovery of process air.
• Special Environments: Models using stainless steel tubes or fins can be used in corrosive working conditions.

Technical Parameters of Plate Fin Tube Heat Exchangers

1. Common Materials

• Tubes: Copper, stainless steel, cupronickel
• Fins: Aluminum, copper

2. Common Dimensions (OD, Hole Pitch, Fin Pitch, Fin Thickness)

• Tube Outer Diameter: 7mm, 7.94mm, 9.52mm, 12.7mm, 15.88mm, 16mm, 25mm
• Hole Pitch: 25mm, 25.4mm, 31.75mm, 38.1mm, 60mm
• Row Spacing: 12.7mm, 18.2mm, 19.05mm, 21.65mm, 22mm, 27.5mm, 33mm, 59.5mm
• Fin Pitch: 1.5-14mm

3. Fin Types

• Fin Types: Corrugated fin, louvered fin, sine wave fin, flat fin
• Fin Surface Treatment: Standard, hydrophilic surface, epoxy coating, nano-coating, electrophoresis, etc.

4. Number of Tube Rows:

2, 4, 6, 8, 10

Performance Enhancements

Modern plate fin tube heat exchangers incorporate various design optimizations to enhance performance:

• Fin Design: Fins are often shaped into corrugated patterns (such as sine wave or bridge-type slits) or use secondary flange processes. These designs effectively disrupt the stable boundary layer formed during airflow, enhance air turbulence, and thus improve heat transfer efficiency.
• Hydrophilic Treatment: Aluminum fin surfaces often undergo hydrophilic treatment (coated with a hydrophilic layer). This causes water vapor from the air to spread quickly into a film and drain away upon condensation, rather than forming droplets. This reduces air resistance, lowers noise, and inhibits surface corrosion and mold growth.
• Tube Enhancement: Some products feature internally grooved copper tube walls to disturb the internal fluid flow and enhance heat transfer on the tube side.

Manufacturing Process

The production of a plate fin tube heat exchanger is a process combining precision machining and manual skill. The core challenge is creating a tight, seamless bond between copper tubes and aluminum fins, which have different coefficients of thermal expansion, to ensure efficient heat exchange.

Fins and copper tubes undergo hydraulic expansion and persistent pressure leak testing to ensure tight integration, greatly enhancing heat conduction. Headers use seamless steel pipes with designs ensuring uniform flow distribution and low-pressure drop. The external frame and end plates of copper tube heat exchangers are made from galvanized steel sheets, offering high hardness, strength, long life, and easy assembly.

Relationship Between Plate Fin Tube Heat Exchanger, Evaporator, and Condenser

Feature Dimension	Plate Fin Tube Heat Exchanger	Evaporator	Condenser
Function	Heating or Cooling	Heat Absorption & Cooling	Heat Rejection & Cooling
Working Principle	Sensible heat exchange between tube-side medium (water/glycol) and air	Refrigerant evaporates inside tubes, absorbing heat, changing from liquid to gas (phase change occurs)	Refrigerant condenses inside tubes, releasing heat, changing from gas to liquid (phase change occurs)
Thermodynamic Process	Sensible Heat Exchange	Latent Heat Exchange (Primary)	Latent Heat Exchange (Primary)
Tube-side Medium	Chilled water, hot water, or glycol solution	Low-temperature, low-pressure liquid refrigerant	High-temperature, high-pressure gaseous refrigerant

A key factor in distinguishing the relationship between a plate fin tube heat exchanger, an evaporator, and a condenser is determining whether a phase change occurs.

Freeze Protection

When the unit is shut down and the plate fin tube heat exchanger is exposed to temperatures below 0°C, to prevent freeze damage, water inside the tubes should be drained, fresh air dampers closed, or antifreeze (like glycol solution) added to the circuit.

Draining Procedure:

1. Close the inlet and outlet water valves.
2. Open the ball valve on the return water pipe of the plate fin tube heat exchanger.
3. Open the ball valve on the supply water pipe of the plate fin tube heat exchanger (the main valve on the supply pipe should be closed).
4. Wait for all stored water to drain out.
5. It is often difficult to completely drain the plate fin tube heat exchanger by gravity alone. Use compressed gas (like nitrogen or compressed air) to assist in purging water from the tubes. During purging, use pulses of air (blow-pause-blow) until no moisture is visible at the drain outlet.

Recommendation: After the unit stops running, if winter temperatures are at or below 0°C, keep the water circulating within the plate fin tube heat exchanger to prevent freezing. This is a commonly used freeze protection method and should be implemented before temperatures reach 0°C to prevent cracking. If using hot water for heating in winter, the draining procedure is not necessary.

Plate Fin Tube Heat Exchanger vs. Bare Tube Heat Exchanger

The core advantage of the plate fin tube heat exchanger is its increased heat transfer area. For the same heat transfer capacity, a finned version might occupy only one-third the volume of a bare tube version, which is crucial for space-constrained equipment rooms. However, small fin pitches are prone to clogging. If the air contains oil smoke or fibers, prioritize selecting models with larger fin pitches or anti-corrosion coatings.

Type	Plate Fin Tube Heat Exchanger	Bare Tube Heat Exchanger
Core Structure	Multiple fins extended over the base tube exterior	Smooth base tubes only
Heat Transfer Area	Very large (typically 5-20 times larger than bare tube)	Small
Heat Transfer Efficiency	High (fins promote turbulence, disrupt boundary layer)	Low
Compactness	Very high, compact structure, high heat transfer per unit volume	Low, occupies more space
Air-side Resistance	Higher (requires fans to overcome resistance)	Lower
Fouling Resistance	Poorer (fin gaps prone to fouling, require maintenance)	Better (smooth tubes less prone to fouling)
Cost	Higher material and process cost, but better overall value	Lower initial material cost, but potentially higher total cost
Main Applications	Gas-liquid heat exchange (e.g., AC, ventilation, cooling towers)	Liquid-liquid heat exchange or gas-liquid applications with low compactness requirements

Why is the Plate Fin Tube Heat Exchanger Preferred for Gas-Liquid Heat Exchange?

The heat transfer bottleneck in bare tube heat exchangers lies primarily on the air side. Air has a very low thermal conductivity, and a relatively stagnant "boundary layer" forms where it contacts the tube wall. This layer acts like insulation, severely hindering heat transfer.

The plate fin tube heat exchanger perfectly addresses this issue in the following ways:

1. Greatly Increased Heat Transfer Area

This is the core advantage. Adding fins to the tubes increases the effective heat transfer area by factors of 5 to over 20 compared to bare tubes. This means air contacts much more cold surface area, dramatically increasing total heat transfer.

2. Significantly Improved Heat Transfer Coefficient

• Disrupts Boundary Layer: The design of the fins (especially corrugated or slit fins) disturbs airflow, disrupting that stagnant air boundary layer and significantly reducing thermal resistance.
• Promotes Turbulence: The narrow channels between fins increase air velocity and readily create turbulence, which has much higher heat transfer efficiency than laminar flow.

3. Compact Structure, Saves Space

For achieving the same heat transfer capacity, the volume and weight of a plate fin tube heat exchanger are far less than those of a bare tube version. This is a decisive advantage in space-critical applications like mechanical rooms or above-ceiling installations in buildings.

4. Better Overall Economy

• Material Savings: Although fins are added, the required tube length, fluid flow rate, and structural support needed for the same heat transfer capacity are greatly reduced.
• Energy Optimization: At the system level, more efficient heat transfer allows for the use of warmer cold sources (e.g., higher evaporation temperatures in AC systems), thereby improving the Coefficient of Performance (COP) of chiller units and reducing long-term operating costs.