タグアーカイブ Cross Flow Heat Exchanger

A crossflow heat exchanger works by allowing two fluids to flow at right angles (perpendicular) to each other, typically with one fluid flowing through tubes and the other flowing across the outside of the tubes. The key principle is that heat is transferred from one fluid to the other through the walls of the tubes. Here's a step-by-step breakdown of how it works:

Components:

1. Tube Side: One of the fluids flows through the tubes.
2. Shell Side: The other fluid flows over the tubes, across the tube bundle, in a direction perpendicular to the flow of the fluid inside the tubes.

Working Process:

1. Fluid Inlet: Both fluids (hot and cold) enter the heat exchanger at different inlets. One fluid (let's say the hot fluid) enters through the tubes, and the other fluid (cold fluid) enters the space outside the tubes.
2. Fluid Flow:
   • The fluid flowing inside the tubes moves in a straight or slightly twisted path.
   • The fluid flowing outside the tubes crosses over them in a perpendicular direction. The path of this fluid can be either crossflow (directly across the tubes) or have a more complex configuration, like a combination of crossflow and counterflow.
3. Heat Transfer:
   • Heat from the hot fluid is transferred to the tube walls and then to the cold fluid flowing across the tubes.
   • The efficiency of heat transfer depends on the temperature difference between the two fluids. The larger the temperature difference, the more efficient the heat transfer.
4. Outlet: After heat transfer, the now cooler hot fluid exits through one outlet, and the now warmer cold fluid exits through another outlet. The heat exchange process results in a temperature change in both fluids as they flow through the heat exchanger.

Design Variations:

• Single-pass crossflow: One fluid flows in a single direction across the tubes, and the other fluid moves through the tubes.
• Multi-pass crossflow: The fluid inside the tubes can flow in multiple passes to increase the contact time with the fluid outside, improving heat transfer.

Efficiency Considerations:

• Crossflow heat exchangers are generally less efficient than counterflow heat exchangers because the temperature gradient between the two fluids decreases along the length of the heat exchanger. In counterflow, the fluids maintain a more consistent temperature difference, which makes it more effective for heat transfer.
• However, crossflow heat exchangers are easier to design and are often used in situations where space is limited or where fluids need to be separated (like in air-to-air heat exchangers).

Applications:

• Air-cooled heat exchangers (like in HVAC systems or car radiators).
• Cooling of electronic equipment.
• Heat exchangers for ventilation systems.

So, while not as thermally efficient as counterflow heat exchangers, crossflow designs are versatile and commonly used when simplicity or space-saving is important.

Here’s a breakdown of the temperature profile for a cross flow heat exchanger, specifically when both fluids are unmixed:

🔥 Cross Flow Heat Exchanger – Both Fluids Unmixed

➤ Flow Arrangement:

• One fluid flows horizontally (say, hot fluid in tubes).
• The other flows vertically (say, cold air across the tubes).
• No mixing within or between the fluids.

📈 Temperature Profile Description:

▪ Hot Fluid:

• Inlet temperature: High.
• As it flows, it loses heat to the cold fluid.
• Outlet temperature: Lower than inlet, but not uniform across the exchanger due to varying contact time.

▪ Cold Fluid:

• Inlet temperature: Low.
• Gains heat as it flows across the hot tubes.
• Outlet temperature: Higher, but also varies across the exchanger.

🌀 Because of the crossflow and no mixing:

• Each point on the exchanger sees a different temperature gradient, depending on how long each fluid has been in contact with the surface.
• The temperature distribution is nonlinear and more complex than in counterflow or parallel flow exchangers.

📊 Typical Temperature Profile (schematic layout):

                ↑ Cold fluid in
        │
  High  │       ┌──────────────┐
  Temp  │       │              │
        │       │              │ → Hot fluid in (right side)
        │       │              │
        ↓       └──────────────┘
      Cold fluid out           ← Hot fluid out

⬇ Temperature Curves:

• Cold fluid gradually heats up — the curve starts low and arcs upward.
• Hot fluid cools down — starts high and arcs downward.
• The curves are not parallel, and not symmetrical due to crossflow geometry and varying heat exchange rate.

🔍 Efficiency:

• The effectiveness depends on the heat capacity ratio and the NTU (Number of Transfer Units).
• Generally less efficient than counterflow but more efficient than parallel flow.

A cross flow heat exchanger with both fluids unmixed refers to a type of heat exchanger where two fluids (hot and cold) flow perpendicular (at 90°) to each other, and neither fluid mixes internally or with the other. This configuration is common in applications like air-to-air heat recovery or automotive radiators.

Key Features:

• Cross flow: The two fluids move at right angles to each other.
• Unmixed fluids: Both the hot and cold fluids are confined to their respective flow passages by solid walls or fins, preventing any mixing.
• Heat transfer: Occurs across the solid wall or surface separating the fluids.

Construction:

Typically includes:

Enclosed channels for the second fluid (e.g., water or refrigerant) to flow inside the tubes.

Tubes or finned surfaces where one fluid (e.g., air) flows across the tubes.

Common Applications:

• Radiators in cars
• Air-conditioning systems
• Industrial HVAC systems
• Heat recovery ventilators (HRVs)

Advantages:

• No contamination between fluids
• Simple maintenance and cleaning
• Good for gases and fluids that must remain separate