AHU for a 1200 m² Building HVAC System

AHU for a 1200 m² Building HVAC System

1. First: Clarify Building & Load Basics (Prerequisites for AHU Sizing)

① Building Use Case (Critical for Load Calculation)

② Calculate Key Loads & Air Volume

• Fresh Air Volume (V_fresh):
  Based on occupancy. For example, a 1,200 m² office with 100 people (12 m²/person) requires:
  V_fresh = 100 people × 40 m³/person/hour = 4,000 m³/hour.
  This typically accounts for 20–30% of the total supply air volume (V_supply).
• Total Supply Air Volume (V_supply):
  Determined by the sensible heat load (Q_sensible) and the allowable temperature difference between supply air and room air (ΔT, usually 8–12°C for comfort).
  Formula: V_supply = Q_sensible / (1.2 × ΔT)
  (Note: 1.2 = air density × specific heat capacity of air, in kJ/(m³·°C))
  For a 1,200 m² office with a sensible load of 60 kW (50 W/m², typical for offices), and ΔT = 10°C:
  V_supply = 60,000 W / (1.2 × 10°C) = 5,000 m³/hour (fresh air accounts for 80% here, which may need adjustment based on recirculation).
• Cooling/Heating Capacity:
  Total cooling capacity (Q_cooling) = sensible load + latent load. For most comfort applications, latent load is 20–30% of sensible load. Using the office example:
  Q_cooling = 60 kW (sensible) + 15 kW (latent) = 75 kW.
  Heating capacity (Q_heating) depends on local winter design temperature.

2. AHU Type Selection for 1,200 m² Buildings

① rooftop Packaged AHU (All-in-One Type)

• Structure: Combines fans, filters, cooling coil, heating coil, and sometimes a refrigeration compressor (if it’s a "packaged rooftop unit, RTU") into a single outdoor unit, installed on the building’s roof.

• Advantages:

• No need for a dedicated mechanical room (saves indoor space, critical for retail/office buildings).

• Easy installation and maintenance (all components are centralized).

• Reduces indoor noise (no fan/coil units inside the building).

• Disadvantages:

• Affected by ambient weather (performance drops in extreme heat/cold).

• Higher energy consumption than water-cooled systems in hot climates.

• Best For: 1,200 m² buildings with no mechanical room (e.g., low-rise offices, retail stores) and moderate climate conditions.

② Split AHU (Indoor Air Handler + Outdoor Condenser)

• Structure: The indoor unit (AHU) contains fans, filters, coils (cooling/heating), and humidity control components (e.g., 加湿器); the outdoor unit houses the compressor and condenser (for cooling) or heat exchanger (for heating, e.g., with a boiler).

• Advantages:

• Flexible installation (indoor AHU can be placed in a small mechanical room or basement; outdoor unit on the ground/roof).

• Better energy efficiency than rooftop RTUs (especially with variable-speed fans/compressors).

• Easier to zone (can connect multiple indoor AHUs to one outdoor unit for different areas).

• Disadvantages:

• Requires a small mechanical room (≥10–15 m² for a 1,200 m² building).

• More complex piping (between indoor and outdoor units).

• Best For: Office buildings, schools, or industrial spaces that need zoned control and have space for a mechanical room.

③ Modular AHU (For Expandable Needs)

• Structure: Composed of multiple standardized modules (e.g., filtration module, cooling module, heating module, fan module) that can be combined or added later.

• Advantages:

• Scalable (add modules if the building’s load increases, e.g., from 1,200 m² to 1,500 m²).

• Easy maintenance (replace individual modules without shutting down the entire system).

• Customizable (select only the modules needed, e.g., skip humidity control for dry climates).

• Disadvantages:

• Higher initial cost than non-modular units.

• Requires more installation space for modules.

• Best For: Buildings with future expansion plans (e.g., retail malls adding floors) or variable load demands (e.g., seasonal retail peaks).

3. Key AHU Components & Specifications (For 1,200 m² Buildings)

4. Optimization Strategies for Efficiency & Reliability

① Energy Efficiency Improvements

• VSD Fans/Compressors: Adjust speed based on real-time load (e.g., lower fan speed at night when occupancy is low) to reduce energy use by 20–40%.

• Heat Recovery: Install a heat recovery wheel or plate exchanger to transfer heat from exhaust air to fresh air (saves 30–50% of heating/cooling energy in winter/summer).

• High-Efficiency Coils: Use microchannel coils (instead of traditional finned tubes) for better heat transfer and lower pressure drop.

② Maintenance & Durability

• Regular Filter Replacement: Clogged filters increase fan energy use by 15–20% and reduce indoor air quality. Schedule replacements every 3–6 months (or use pressure sensors to trigger alerts).

• Coil Cleaning: Clean cooling/heating coils annually (with compressed air or chemical cleaners) to remove dust and debris—this maintains 90% of heat transfer efficiency.

• Corrosion Protection: For outdoor AHUs (e.g., rooftop units), use galvanized steel casings or anti-corrosion coatings to resist rain/salt (critical for coastal areas).

③ Zoning Control (For Uneven Loads)

• Use multiple small AHUs (e.g., 2×4,000 m³/hour units) instead of one large unit.

• Install zone dampers to adjust airflow to each area based on temperature/humidity sensors.

• This reduces energy waste (e.g., no need to cool unoccupied zones) and improves comfort.

Summary

1. Define Loads: Calculate fresh air volume, sensible/latent heat loads based on building type and occupancy.

2. Choose Type: Rooftop AHU (no mechanical room), split AHU (zoning), or modular AHU (expansion) based on space and needs.

3. Optimize: Prioritize VSD components, heat recovery, and regular maintenance to balance efficiency and reliability.