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:

1. Air Changes per Hour (ACPH) Criterion: $$V_{ACH,i} = \frac{\text{Volume}_i \times \text{ACH}_i}{3600} \quad [\text{m}^3/\text{s}]$$
2. Thermal Sensible Heat Removal Criterion: $$V_{thermal,i} = \frac{Q_{sensible,total,i}}{\rho \cdot c_p \cdot (T_{room,i} - T_{supply})} \quad [\text{m}^3/\text{s}]$$
3. 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)

7. Verification & Reference Standards

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