To align with HTM 03-01 (Health Technical Memorandum) standards for healthcare ventilation, an Air Handling Unit (AHU) must follow a very specific sequence to ensure air quality, pressure gradients, and moisture control.

 To align with HTM 03-01 (Health Technical Memorandum) standards for healthcare ventilation, an Air Handling Unit (AHU) must follow a very specific sequence to ensure air quality, pressure gradients, and moisture control.

Here is a description of the components you would see in a "half-open" or sectional view of a medical-grade AHU, following the air path from the outside environment to the hospital space.
1. External Air Intake & Intake Shutter
The journey begins at the Atmospheric Air Intake. Per HTM 03-01, this must be located away from pollution sources (like boiler flues or cooling towers).
• Intake Shutter/Damper: An insulated, low-leakage damper that closes when the unit is off to prevent "stack effect" or freezing.
2. Primary Filtration (Pre-Filters)
Before reaching any coils, the air passes through G4 or F7 grade filters.
• Purpose: To protect the internal components from large dust particles and debris.
• NHS Standard: These must be fitted with differential pressure gauges (magnehelic gauges) to monitor when they become clogged.
3. Frost Coil (Pre-Heat)
In cold climates, a Frost Coil is the first thermal component.
• Function: It warms the incoming air to just above freezing (typically 5^\circ\text{C}) to prevent the downstream cooling coils or steam humidifiers from freezing and bursting.
4. Cooling Coil & Moisture Eliminator
This is a critical "Z-part" area for hygiene.
• Cooling Coil: Dehumidifies and cools the air.
• Condensate Tray: Underneath the coil, there is a stainless steel, double-sloped drainage tray. HTM 03-01 is strict here: there must be no standing water to prevent Legionella or mold growth.
• Droplet Eliminator: A "zig-zag" (Z-shaped) set of vanes that catches water droplets so they don't get blown further into the ductwork.
5. Heating Coil (Re-Heat)
After the air is cooled (and often over-dehumidified), it passes through the Re-heat Coil.
• Function: It brings the air back up to the specific "Supply Air Temperature" required for the ward or operating theater.
6. Supply Fan (The "Heart")
Usually a Plug Fan or centrifugal fan.
• NHS Requirement: In critical areas (like Operating Theaters), these often have "Run/Standby" motors so that if one motor fails, the air supply doesn't stop.
7. Secondary/HEPA Filtration
The final stage before the air enters the hospital ducting.
• Purpose: In "Ultra Clean" environments, HEPA filters remove 99.97\% of bacteria and particles.