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向流は並流よりもなぜ効率的なのでしょうか?

熱交換器において、向流(カウンターフロー)は並流よりも効率が良いとされています。これは、熱交換器全体にわたって2つの流体間の温度差(ΔT)をより大きく一定に保ち、熱伝達を最大化するためです。詳しい説明は以下のとおりです。

1. 温度勾配と熱伝達

  • 逆流:
    • 向流では、流体は反対方向に流れます(例:高温の流体が一方の端から流入し、低温の流体が反対側の端から流入します)。これにより、熱交換器の全長にわたってほぼ一定の温度差(ΔT)が生じます。
    • 高温流体の最高温度(入口)が低温流体の出口に接触し、低温流体の最低温度(入口)が高温流体の出口に接触します。これにより、低温流体の温度が高温流体の入口温度に近づき、熱伝達が最大化されます。
    • 例: 高温の流体が 100°C で流入して 40°C で流出し、低温の流体が 20°C で流入した場合、90°C 近くで流出することができ、高い熱伝達率を実現します。
  • 並列フロー:
    • 並流では、両方の流体が同じ方向に流れるため、最大の ΔT は入口で発生しますが、両方の流体が交換器に沿って同様の温度に近づくにつれて、ΔT は急速に減少します。
    • 冷たい流体の出口温度は熱い流体の出口温度を超えることができないため、伝達される熱の総量は制限されます。
    • 例: 高温の流体が 100°C で流入し、60°C で流出する場合、20°C で流入する低温の流体は 50°C 程度にしか達せず、熱伝達が少なくなります。

なぜそれが重要なのか熱伝達率(Q)はΔTに比例します(Q = U × A × ΔT、Uは熱伝達係数、Aは表面積)。向流式ではΔTが大きく一定であるため、平均熱伝達率が高くなり、効率が向上します。

2. 対数平均温度差(LMTD)

  • 熱交換器の効率は、多くの場合、熱伝達を促進する平均温度差を表す対数平均温度差 (LMTD) を使用して定量化されます。
  • 逆流熱交換器全体にわたって温度差が比較的一定に保たれるため、LMTDが高くなります。これにより、同じ表面積でより多くの熱を伝達できます。
  • 並列フロー出口に向かって温度差が大幅に低下し、熱伝達の駆動力が減少するため、LMTD は低くなります。
  • 結果同じ熱交換器のサイズの場合、向流の方が LMTD が高いためより多くの熱を伝達します。または、同じ熱伝達を達成するために必要な表面積が小さいため、よりコンパクトで効率的です。

3. 最大限の熱回収

  • 向流では、冷たい流体は理論的には熱い流体の入口温度に到達できるため(無限に長い熱交換器内)、ほぼ完全な熱回収が可能になります(例:Holtop の 3D クロス向流熱交換器などの最新設計では 90~95% の効率)。
  • 並流の場合、冷流体の出口温度は温流体の出口温度によって制限され、キャッピング効率(通常60~80℃)に影響されます。そのため、向流はエネルギー回収換気や最大限の熱回収が重要な産業プロセスなどの用途に最適です。

4. 実用的な意味合い

  • 逆流: 安定したΔTにより必要な伝熱面積が削減され、高性能アプリケーションにおいてより小型でコスト効率の高い設計が可能になります。HVAC、産業用冷却システム、エネルギー回収システムなどで広く使用されています。
  • 並列フローΔTの急激な低下により、同等の熱伝達を実現するためにはより大きな伝熱面積が必要となり、材料とスペースの要件が増加します。これは、基本的なラジエーターや教育設備など、よりシンプルで効率がそれほど重要でない用途で使用されます。

視覚的な説明(簡略版)

  • 逆流高温流体(100℃~40℃)と低温流体(20℃~90℃)を想像してみてください。熱交換器全体で温度差が比較的高く(例えば約20~60℃)、効率的な熱伝達が促進されます。
  • 並列フロー同じ流体は大きな ΔT (100°C – 20°C = 80°C) で始まりますが、すぐに収束し (例: 60°C – 50°C = 10°C)、駆動力が低下して効率が制限されます。

結論

向流は、熱交換器全体にわたってより大きく安定した温度差(ΔT)を維持するため、より効率的です。その結果、LMTDが高くなり、同じ表面積でより多くの熱伝達が得られます。そのため、エネルギー回収や産業プロセスなど、高効率が求められる用途では向流が好まれます。一方、並流はよりシンプルですが効率は低く、要求の厳しい用途には適しています。

向流熱交換器と並流熱交換器

Counterflow and parallel flow heat exchangers are two primary configurations for heat transfer between two fluids, differing in the direction of fluid flow and their impact on efficiency, temperature profiles, and applications. Below is a concise comparison based on their design, performance, and use cases.

1. Flow Configuration

  • Counterflow Heat Exchanger:
    • Fluids flow in opposite directions (e.g., hot fluid enters at one end, cold fluid at the opposite end).
    • Example: Hot fluid flows left to right, cold fluid flows right to left.
  • Parallel Flow Heat Exchanger:
    • Fluids flow in the same direction (e.g., both hot and cold fluids enter at the same end and exit at the opposite end).
    • Example: Both fluids flow left to right.

2. Heat Transfer Efficiency

  • 逆流:
    • Higher efficiency: Maintains a larger temperature difference (ΔT) along the entire length of the exchanger, maximizing heat transfer per unit area.
    • Can achieve up to 90–95% thermal efficiency in well-designed systems (e.g., plate or tube exchangers).
    • The outlet temperature of the cold fluid can approach the inlet temperature of the hot fluid, making it ideal for applications requiring maximum heat recovery.
  • 並列フロー:
    • Lower efficiency: The temperature difference (ΔT) is highest at the inlet but decreases rapidly as both fluids approach thermal equilibrium along the exchanger.
    • Typically achieves 60–80% efficiency, as the cold fluid’s outlet temperature cannot exceed the hot fluid’s outlet temperature.
    • Less effective for applications needing near-complete heat transfer.

3. Temperature Profile

  • 逆流:
    • Temperature gradient is more uniform, with a near-constant ΔT across the exchanger.
    • Allows for a closer approach temperature (the difference between the hot fluid’s outlet and cold fluid’s inlet temperatures).
    • Example: Hot fluid enters at 100°C and exits at 40°C; cold fluid enters at 20°C and can exit close to 90°C.
  • 並列フロー:
    • Temperature difference is large at the inlet but diminishes along the exchanger, limiting heat transfer as fluids reach similar temperatures.
    • Example: Hot fluid enters at 100°C and exits at 60°C; cold fluid enters at 20°C and may only reach 50°C.

4. Design and Complexity

  • 逆流:
    • Often requires more complex piping or plate arrangements to ensure fluids flow in opposite directions, potentially increasing manufacturing costs.
    • Compact designs are possible due to higher efficiency, reducing material requirements for the same heat transfer rate.
  • 並列フロー:
    • Simpler design, as both fluids enter and exit at the same ends, reducing piping complexity.
    • May require a larger heat transfer area (longer or bigger exchanger) to achieve comparable heat transfer, increasing size and material costs.

5. Applications

  • 逆流:
    • Preferred in applications requiring high efficiency and maximum heat recovery, such as:
      • HVAC systems (e.g., energy recovery ventilators).
      • Industrial processes (e.g., chemical plants, power generation).
      • Wastewater heat recovery (e.g., shower heat exchangers).
      • Cryogenic systems where precise temperature control is critical.
    • Common in plate heat exchangers, double-pipe exchangers, and high-performance shell-and-tube designs.
  • 並列フロー:
    • Used in applications where simplicity is prioritized, or where complete heat transfer is not critical, such as:
      • Small-scale cooling systems (e.g., car radiators).
      • Processes where fluids must not exceed certain temperatures (e.g., to avoid overheating the cold fluid).
      • Educational or experimental setups due to simpler construction.
    • Common in basic tube-in-tube or shell-and-tube heat exchangers.

6. Advantages and Disadvantages

  • 逆流:
    • 利点:
      • Higher thermal efficiency, reducing energy losses.
      • Smaller size for the same heat transfer capacity.
      • Better suited for applications with large temperature differences.
    • Disadvantages:
      • More complex design and piping, potentially increasing costs.
      • May require additional measures to manage condensation or frost in cold environments.
  • 並列フロー:
    • 利点:
      • Simpler design, easier to manufacture and maintain.
      • Lower pressure drop in some cases, reducing pumping costs.
    • Disadvantages:
      • Lower efficiency, requiring larger heat transfer areas.
      • Limited by the outlet temperature constraint (cold fluid cannot exceed hot fluid’s outlet temperature).

7. Practical Considerations

  • 逆流:
    • Ideal for energy recovery systems (e.g., Holtop’s 3D cross-counterflow exchangers with 95% efficiency or RECUTECH’s RFK+ enthalpy exchangers).
    • Often equipped with features like hydrophilic coatings to manage condensation (e.g., Eri Corporation’s aluminum plate exchangers).
  • 並列フロー:
    • Used in applications where cost and simplicity outweigh efficiency needs, such as basic HVAC systems or small-scale industrial cooling.
    • Less common in modern high-efficiency designs due to performance limitations.

Summary Table

パネルルームにおける間接蒸発冷却ユニットの適用

Indirect evaporative cooling (IEC) units are increasingly used in electrical panel rooms, control rooms, and equipment enclosures to provide energy-efficient cooling without introducing additional humidity. These rooms typically house sensitive electrical and electronic equipment that generates heat during operation and requires a controlled temperature environment for reliable functioning.

Application of Cross Flow Heat Exchanger in Indirect Evaporative Cooling System of Data Center

パネルルームにおける間接蒸発冷却ユニットの適用

How It Works

An indirect evaporative cooling unit cools the air without direct contact between water and the air inside the panel room. Instead, it uses a 熱交換器 to transfer heat from the warm air inside the room to a secondary air stream that is cooled by evaporation. This process ensures that:

  • No moisture enters the panel room.

  • その internal air remains clean and dry.

  • Energy consumption is significantly lower than traditional mechanical refrigeration.

Benefits in Panel Room Applications

  1. Moisture-Free Cooling:
    Since no direct contact with water occurs, sensitive electrical components are safe from condensation and corrosion risks.

  2. エネルギー効率:
    Compared to traditional air conditioning systems, IEC units consume less power, making them ideal for continuous operation in industrial settings.

  3. Reduced Maintenance:
    With fewer mechanical components and no refrigeration cycle, the system is simple to maintain and has a longer operational life.

  4. Improved Reliability:
    Maintaining a stable and cool environment helps prolong the life of control panels and reduces the risk of equipment failure caused by overheating.

  5. Environmentally Friendly:
    No refrigerants are used, reducing the system’s environmental impact.

Typical Applications

  • Electrical panel rooms in factories

  • Server and network control cabinets

  • Inverter or PLC (programmable logic controller) rooms

  • Outdoor telecom enclosures

  • Substation control rooms

産業用熱回収ボックス、廃ガスおよび熱回収、ガス対ガス熱交換器

産業用熱回収ボックスは、様々な産業用途における排ガス流から熱を回収するために設計された、コンパクトで効率的なシステムです。ガス対ガス熱交換器を用いて、高温の排ガスから流入する新鮮な空気へ熱エネルギーを伝達し、2つの気流を混合させることなく、熱エネルギーを放出します。このプロセスにより、追加の加熱の必要性が低減され、エネルギー効率が大幅に向上し、運用コストの削減と環境への影響の軽減につながります。

アルミニウムやステンレス鋼などの耐久性の高い材料で構築されたこのシステムは、高温および腐食性環境に耐えることができます。内部の熱交換器は、多くの場合アルミ箔またはアルミ板で構成されており、高い熱伝導性と効率的な熱伝達を実現します。この設計により、汚れた排気と清浄な給気の相互汚染を防ぎ、食品加工、タバコ、印刷、化学、汚泥処理などの産業に適しています。

この省エネソリューションは、廃熱を回収するだけでなく、室内空気質の改善と安定した生産環境の維持にも役立ちます。設置とメンテナンスが容易な産業用熱回収ボックスは、持続可能性の向上と省エネ規制の遵守を目指す工場にとって賢明な選択肢です。

Industrial heat recovery box, waste gas and heat recovery, gas to gas heat exchanger

産業用熱回収ボックス、廃ガスおよび熱回収、ガス対ガス熱交換器

how does a cross flow heat exchanger work

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. 熱伝達:

    • 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.

temperature profile for cross flow heat exchanger

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.

cross flow heat exchanger with both fluids unmixed

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

向流熱交換器はどのように機能しますか?

In the counterflow heat exchanger, two neighboring aluminum plates create channels for theair to pass through. The supply air passes on one side of the plate and the exhaust air onthe other. Airflows are passed by each other along parallel aluminum plates instead ofperpendicular like in a crossflow heat exchanger. The heat in the exhaust air is transferredthrough the plate from the warmer air to the colder air.
Sometimes, the exhaust air is contaminated with humidity and pollutants, but airflows nevermix with a plate heat exchanger, leaving the supply air fresh and clean.

Plate heat recovery exchanger made in china

Heat exchangers are mainly made of materials such as aluminum foil, stainless steel foil, or polymers. When there is a temperature difference between the airflow isolated by aluminum foil and flowing in opposite directions, heat transfer occurs, achieving energy recovery. By using an air to air heat exchanger, the heat in the exhaust can be utilized to preheat the fresh air, thereby achieving the goal of energy conservation. The heat exchanger adopts a unique point surface combination sealed process, which has a long service life, high temperature conductivity, no permeation, and no secondary pollution caused by the permeation of exhaust gas.

Plate heat recovery exchanger

工業用熱リサイクルビンシリーズ

注記:

          1.排気温度が200℃以下の産業廃ガスからの熱を回収し、新鮮な空気を加熱することができる。

          2. 熱回収ボックスの構造は現場の状況に合わせて設計できます。

          3. この構造には給気ファンや排気ファンはありません。

          4. この表の熱回収効率は、給排気量と同じです。給排気量が異なる場合の熱回収効率については、弊社までお問い合わせください。

          5.熱回収ボックスは床置き型、天井型、その他の構造タイプにすることができます(一般的な風量100000m%/h)。

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