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FAHU - Fresh Air Handling Units

 


Fresh Air Handling Unit (FAHU) Overview

A Fresh Air Handling Unit (FAHU) is a type of air handling unit (AHU) used in HVAC (Heating, Ventilation, and Air Conditioning) systems. It is designed to supply fresh, filtered, and conditioned air to a building by bringing in outside air, treating it (filtering, heating, cooling, humidifying, or dehumidifying), and then distributing it into the indoor spaces.


1. Components of a FAHU

A typical FAHU consists of:

✅ Intake Louver – Allows fresh outdoor air intake.
✅ Filters – Removes dust, pollutants, and airborne particles (e.g., HEPA or carbon filters).
✅ Cooling/Heating Coils – Cools or heats the incoming air using chilled or hot water.
✅ Humidifier/Dehumidifier – Controls air moisture levels.
✅ Fans/Blowers – Helps in air circulation and distribution.
✅ Dampers – Controls air volume and direction.
✅ Heat Recovery System (optional) – Reclaims energy from exhaust air to improve efficiency.

2. How FAHU Works

1️⃣ Fresh Air Intake – The unit draws in fresh outdoor air.
2️⃣ Filtration – The air passes through filters to remove contaminants.
3️⃣ Temperature & Humidity Control – Cooling or heating coils adjust air temperature, and humidifiers/dehumidifiers regulate moisture.
4️⃣ Air Distribution – The treated air is supplied to the indoor spaces via ductwork.



3. Differences Between FAHU and AHU



4. Applications of FAHU

📌 Hospitals & Healthcare Facilities – Ensures a sterile and clean air supply.
📌 Hotels & Commercial Buildings – Provides fresh, comfortable air for occupants.
📌 Industrial Facilities – Controls contaminants in clean rooms and production areas.
📌 Data Centers – Maintains temperature and air quality for equipment cooling.


Fresh Air Handling Unit (FAHU) - Detailed Explanation

A Fresh Air Handling Unit (FAHU) is a critical component in HVAC (Heating, Ventilation, and Air Conditioning) systems designed to introduce 100% fresh outdoor air into a building while filtering, cooling, heating, and sometimes dehumidifying it. This ensures improved indoor air quality (IAQ) and proper ventilation in various commercial, industrial, and healthcare applications.


1. Working Principle of FAHU

The FAHU takes in fresh air from outside, processes it through a series of filters and conditioning components, and then supplies it into the building. The process typically includes the following steps:

Step-by-Step Air Handling Process:

  1. Fresh Air Intake

    • Outside air is drawn into the FAHU through intake louvers.
    • Dampers control the air volume to maintain proper airflow.

  2. Air Filtration

    • The air passes through pre-filters and fine filters to remove dust, pollen, and pollutants.
    • In specialized applications (like hospitals or cleanrooms), HEPA filters or activated carbon filters are used.

  3. Cooling or Heating the Air

    • The air then flows through cooling coils (chilled water or DX coil) in warm climates or heating coils (electric, hot water, or steam coils) in colder climates.
    • This ensures that the fresh air is delivered at a comfortable temperature.

  4. Humidity Control (Optional)

    • A humidifier adds moisture if the air is too dry.
    • A dehumidifier removes excess moisture if the air is too humid.

  5. Air Distribution

    • A powerful blower fan moves the conditioned fresh air into the building through ductwork.
    • It may be supplied directly to rooms or integrated with an AHU (Air Handling Unit) for further mixing with recirculated air.

  6. Heat Recovery (Energy Efficiency Feature - Optional)

    • A heat recovery unit (HRU) or heat exchanger transfers heat between incoming fresh air and outgoing exhaust air.
    • This reduces energy consumption by recovering cooling or heating energy.

2. Key Components of FAHU

FAHUs are made up of several important components, each playing a crucial role in air handling:


3. Difference Between FAHU & AHU

Many people confuse FAHU and AHU, but they have distinct roles:


4. Applications of FAHU

FAHUs are commonly used in areas that require strict ventilation control and high indoor air quality, such as:

  • Healthcare Facilities (Hospitals, Labs, Operating Rooms)
    • Ensures a clean, sterile environment by supplying filtered fresh air.
  • Hotels, Theaters, Shopping Malls
    • Provides fresh air to prevent stale indoor air.
  • Factories, Pharmaceutical & Cleanrooms
    • Maintains controlled air purity for sensitive manufacturing processes.
  • Airports, Office Buildings & Schools
    • Prevents the buildup of CO₂ and airborne contaminants.
  • Kitchens & Industrial Workspaces
    • Removes cooking fumes and chemical vapors by supplying fresh air.


5. Advantages of FAHU

✅ Improves Air Quality – Removes dust, pollutants, and stale air.
✅ Increases Oxygen Levels – Ensures occupants have a healthy air supply.
✅ Reduces Odors & Contaminants – Essential for hospitals, kitchens, and factories.
✅ Energy Efficiency – Heat recovery systems help save cooling/heating energy.
✅ Humidity Control – Ensures comfortable indoor conditions.
✅ Complies with Ventilation Regulations – Required in buildings with strict IAQ standards.


6. Challenges of FAHU

❌ Higher Energy Consumption – Cooling and heating 100% fresh air requires more power than recirculating indoor air.
❌ Larger Space Requirement – FAHUs are bulkier than standard AHUs.
❌ High Initial Cost – More expensive than conventional air handling units.
❌ Frequent Maintenance – Filters and heat exchangers require regular cleaning/replacement.


7. How to Select a FAHU?

When choosing a Fresh Air Handling Unit (FAHU), consider the following factors:

✔ Airflow Capacity – Measure in CFM (Cubic Feet per Minute) based on building size.
✔ Filtration Level – HEPA, Carbon, or standard filters depending on air purity requirements.
✔ Cooling/Heating Load – Ensure coils are sized properly for climate conditions.
✔ Humidity Control – Include humidifiers or dehumidifiers if needed.
✔ Energy Efficiency – Consider heat recovery options to save energy.
✔ Noise Levels – Select low-noise fans and dampers for quiet operation.

More Technical

A Fresh Air Handling Unit (FAHU) is a type of HVAC (Heating, Ventilation, and Air Conditioning) system component designed specifically to handle and condition fresh (outside) air before it is supplied into a building's ventilation system. It is particularly important in buildings where indoor air quality is critical, such as hospitals, laboratories, commercial complexes, and clean rooms.

1. Function and Purpose

The primary function of a FAHU is to:

  • Bring in outdoor (fresh) air

  • Filter it to remove dust, pollen, and contaminants

  • Condition it (cool, heat, or humidify/dehumidify)

  • Deliver the air to indoor spaces, either directly or via the Air Handling Unit (AHU)

This improves indoor air quality (IAQ) by diluting and removing indoor pollutants like CO₂, VOCs, and pathogens.



2. Main Components of a FAHU


3. Working Principle

  1. Air Intake: Outside air enters through the intake louver and damper.

  2. Filtration: Air passes through filters to remove contaminants.

  3. Cooling or Heating:

    • If cooling is required, the air passes through a cooling coil.

    • If heating is needed (especially in cold climates), it passes through a heating coil.

  4. Humidity Control (if necessary): Air is humidified or dehumidified depending on the room requirements.

  5. Air Supply: The blower pushes the conditioned fresh air into the duct system or directly to the space.

4. Control Systems

  • Thermostats and Humidistats: Measure and regulate temperature and humidity.

  • Sensors: Monitor pressure drop across filters, air flow rate, CO₂ levels, etc.

  • VFDs (Variable Frequency Drives): Adjust fan speed for energy efficiency and optimal performance.

  • BMS Integration: FAHU can be integrated into a Building Management System for centralized monitoring and control.

5. Design Considerations

  • Air Changes per Hour (ACH): Determines how frequently air is replaced in a room.

  • Location: FAHUs are typically installed in mechanical rooms, rooftops, or dedicated plant spaces.

  • Thermal Load Calculations: Required to size coils and fans correctly.

  • Noise Control: Sound attenuators or acoustic insulation may be needed.


6. FAHU vs AHU


7. Benefits of Using FAHU

  • Ensures healthy and breathable indoor air

  • Helps maintain positive or negative pressure in critical environments

  • Reduces CO₂ buildup and odors

  • Supports LEED and green building standards


2. Sample FAHU Design Calculation

Let’s walk through a basic design example:

🔸 Project Requirement:

  • Fresh air for a hospital room.

  • Room size: 6 m x 5 m x 3 m (H)

  • Required air changes: 15 ACH (air changes per hour)

  • Outdoor summer condition: 38°C DB, 28°C WB

  • Required supply air condition: 22°C DB, 50% RH

  • Cooling coil with chilled water at 7°C in / 12°C out

🔸 Step 1: Calculate Air Volume Flow (CFM)

Room Volume = 6 × 5 × 3 = 90 m³

Airflow required = ACH × Volume / 60

Airflow=15×9060=22.5 m³/min=375 m³/hr\text{Airflow} = \frac{15 × 90}{60} = 22.5 \, \text{m³/min} = 375 \, \text{m³/hr}

Convert to CFM (Cubic Feet per Minute):

1 m³/hr≈0.589 CFM⇒375 m³/hr≈221 CFM1 \, \text{m³/hr} ≈ 0.589 \, \text{CFM} \Rightarrow 375 \, \text{m³/hr} ≈ 221 \, \text{CFM}

🔸 Step 2: Select Filters

  • Pre-filter: G4 panel filter (90% arrestance)

  • Fine filter: F9 bag filter (ePM1 > 80%)

🔸 Step 3: Coil Sizing (Simplified)

Estimate required cooling capacity using the formula:

Q=1.2×CFM×ΔT (kW)Q = 1.2 × \text{CFM} × ΔT \, (\text{kW})

Assume ΔT = 16°C (38°C → 22°C):

Q=1.2×221×16=4243 kcal/hr=4.93 kWQ = 1.2 × 221 × 16 = 4243 \, \text{kcal/hr} = 4.93 \, \text{kW}

So, a cooling coil capable of ~5 kW is needed.

🔸 Step 4: Fan Selection

  • Static pressure: 500 Pa

  • Volume: 221 CFM

  • Fan power: Use manufacturer selection charts for centrifugal fans; likely ~0.25 kW for this size.

🔸 Step 5: Accessories

  • Drain pan under coil

  • Motorized damper for automation

  • Thermostat and humidity sensor

  • VFD for fan speed control (optional)

  • BMS Interface: for monitoring and alarms


Conclusion

A Fresh Air Handling Unit (FAHU) is a crucial part of modern HVAC systems, ensuring better indoor air quality (IAQ) by providing 100% fresh, filtered, and conditioned air to a building. It is essential in healthcare, cleanrooms, industrial spaces, and commercial buildings where ventilation, filtration, and energy efficiency are priorities.


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