# Air Handling Unit (AHU) Design & Cleanroom Sizer Documentation

# 1. Executive Summary & Objective

The Air Handling Unit (AHU) Design & Cleanroom Sizer Calculator provides a rigorous thermodynamic and aerodynamic design platform for pharmaceutical cleanroom HVAC systems. It sizes supply airflows, chilled water cooling coil capacities (kW / Tons of Refrigeration), sensible and latent heat loads, reheat duty, 3-stage cleanroom filtration pressure drop, and total static fan motor horsepower (HP).

The design engine complies with ISO 14644-1/4, EU GMP Annex 1 (2022 Revision), ISPE Good Practice Guide: Heating, Ventilation, and Air Conditioning (HVAC), and ASHRAE Fundamentals Handbook.

# 2. Governing Equations & Psychrometric Thermodynamics

# 2.1 Saturation Vapor Pressure & Carrier Equation

Saturation pressure of water vapor $p_{ws}(T)$ is computed using the ASHRAE / Sonntag formulation (accurate between $-40^\circ\text{C}$ and $+60^\circ\text{C}$ ):

p_{ws}(T) = 611.213 \cdot \exp\left( \frac{17.5043 \cdot T}{241.2 + T} \right) \quad [\text{Pa}]

When outdoor wet bulb temperature $T_{wb}$ is given, water vapor partial pressure $p_v$ is determined using the Carrier / Stoecker psychrometric wet-bulb formulation:

p_v = p_{ws}(T_{wb}) - \frac{(P_{atm} - p_{ws}(T_{wb}))(T_{db} - T_{wb})}{1555.56 - 0.722 \cdot T_{wb}} \quad [\text{Pa}]

# 2.2 Humidity Ratio & Enthalpy

Humidity Ratio ( $W$ ):

W = 0.62198 \cdot \frac{p_v}{P_{atm} - p_v} \quad [\text{kg water / kg dry air}]

Specific Enthalpy ( $h$ ):

h = 1.006 \cdot T_{db} + W \cdot (2501 + 1.86 \cdot T_{db}) \quad [\text{kJ / kg dry air}]

Moist Air Density ( $\rho$ ):

\rho = \frac{P_{atm}(1 + W)}{R_{da}(T_{db} + 273.15)(1 + 1.6078 W)} \quad [\text{kg/m}^3]

# 3. Cleanroom Airflow Criteria & Governing Driver

For each cleanroom zone $i$ , supply airflow $V_{supply,i}$ is determined by the maximum of:

Air Changes per Hour (ACPH) Criterion:

V_{ACH,i} = \frac{\text{Volume}_i \times \text{ACH}_i}{3600} \quad [\text{m}^3/\text{s}]

Thermal Sensible Heat Removal Criterion:

V_{thermal,i} = \frac{Q_{sensible,total,i}}{\rho \cdot c_p \cdot (T_{room,i} - T_{supply})} \quad [\text{m}^3/\text{s}]

Minimum Ventilation / Pressurization Leakage:

V_{OA,i} = \max\left( \frac{\text{Occupants}_i \cdot \text{OA}_{person}}{1000}, \quad V_{exhaust,i} + V_{leakage,i} \right)

\mathbf{V_{supply,total} = \sum_{i} \max(V_{ACH,i}, V_{thermal,i}, V_{OA,i})}

# 4. Coil Capacities & Dehumidification

# 4.1 Air Mixing State

T_{mixed} = \frac{V_{OA} T_{OA} + V_{RA} T_{RA}}{V_{supply}}, \quad W_{mixed} = \frac{V_{OA} W_{OA} + V_{RA} W_{RA}}{V_{supply}}

# 4.2 Cooling Coil Duty & Chilled Water Flow

Using Apparatus Dew Point ( $ADP$ ) and Coil Bypass Factor ( $BF = 0.08$ for deep 8-row cleanroom coils):

Total Cooling Capacity ( $Q_c$ ):

Q_{cooling} = \dot{m}_{air} \cdot (h_{mixed} - h_{off-coil}) \quad [\text{kW}]

\text{Tonnage} = \frac{Q_{cooling}}{3.51685} \quad [\text{TR}]

Sensible Cooling Capacity:

Q_{cs} = \dot{m}_{air} \cdot c_p \cdot (T_{mixed} - T_{off-coil}) \quad [\text{kW}]

Latent Dehumidification:

Q_{cl} = Q_{cooling} - Q_{cs} \quad [\text{kW}]

Chilled Water Flow Rate ( $6.5^\circ\text{C} \to 12.0^\circ\text{C}$ ):

\dot{V}_{CHW} = \frac{Q_{cooling}}{4.186 \cdot (T_{CHW,return} - T_{CHW,supply})} \quad [\text{L/s}]

Condensate Drainage Rate:

\dot{m}_{condensate} = \dot{m}_{air} \cdot (W_{mixed} - W_{off-coil}) \cdot 3600 \quad [\text{L/h}]

# 4.3 Reheat Coil Duty

In pharmaceutical HVAC, air off the cooling coil ( $10^\circ\text{C} - 12^\circ\text{C}$ ) is cold and saturated. It is reheated to design supply air temperature ( $15^\circ\text{C} - 17^\circ\text{C}$ ) to prevent room overcooling:

Q_{reheat} = \dot{m}_{air} \cdot c_p \cdot (T_{supply} - T_{off-coil}) \quad [\text{kW}]

# 5. Multi-Stage Filtration & Fan Aerodynamic Sizing

# 5.1 Pressure Drop Budget (Loaded Condition)

Pre-Filter (G4 / MERV 8): $150\text{ Pa}$
Fine Secondary Filter (F9 / MERV 15): $250\text{ Pa}$
Terminal HEPA Filter (H14 / 99.995% EN 1822): $450\text{ Pa}$
Internal Coils & Dampers: $130\text{ Pa} + 60\text{ Pa} + 40\text{ Pa} = 230\text{ Pa}$
External Static Pressure (ESP - Ducts & Silencer): $280\text{ Pa} + 50\text{ Pa} = 330\text{ Pa}$

\mathbf{TSP = (Internal\,\Delta P + ESP) \cdot (1 + \text{Safety Margin})}

# 5.2 Fan Shaft & Motor Sizing

\text{Fan Shaft Power } P_{shaft} = \frac{V_{supply} \cdot TSP}{1000 \cdot \eta_{fan}} \quad [\text{kW}]

\text{Motor Required } P_{motor} = \frac{P_{shaft}}{\eta_{motor}} \quad [\text{kW}]

\text{Motor Horsepower } HP = P_{motor} \cdot 1.34102 \quad [\text{HP}]

# 5.3 Fan Air Temperature Rise

\Delta T_{fan} = \frac{TSP}{\rho \cdot c_p \cdot 1000 \cdot \eta_{fan}} \approx +0.8^\circ\text{C} \text{ to } +1.4^\circ\text{C}

# 6. Cleanroom Standards Reference (ISO 14644-1 / EU GMP)

Grade / Class	Airflow Pattern	Target ACPH	Max Particles $\ge 0.5\,\mu\text{m}$ (Rest)	Max Particles $\ge 0.5\,\mu\text{m}$ (Action)	Differential Pressure
Grade A (ISO 5)	Unidirectional ( $0.45\text{ m/s}$ )	60+ ACPH	3,520 / m³	3,520 / m³	$+15\text{ Pa}$ to $+20\text{ Pa}$
Grade B (ISO 5)	Turbulent / Mixed	40 – 60 ACPH	3,520 / m³	352,000 / m³	$+15\text{ Pa}$ cascade
Grade C (ISO 7)	Turbulent	20 – 30 ACPH	352,000 / m³	3,520,000 / m³	$+15\text{ Pa}$ cascade
Grade D (ISO 8)	Turbulent	10 – 20 ACPH	3,520,000 / m³	Not Defined	$+10\text{ Pa}$ cascade

# 7. Verification & Reference Standards

ASHRAE Handbook — Fundamentals (SI Edition): Chapter 1 Psychrometrics and Chapter 17 Clean Spaces.
ISO 14644-1:2015: Cleanrooms and associated controlled environments — Part 1: Classification of air cleanliness.
ISO 14644-4:2001: Cleanrooms — Part 4: Design, construction and start-up.
EU GMP Annex 1 (2022): Manufacture of Sterile Medicinal Products.
ISPE Good Practice Guide: Heating, Ventilation, and Air Conditioning (HVAC).
EN 1822 / ISO 29463: High efficiency air filters (EPA, HEPA and ULPA).