Containment Negative Air Pressure Calculator

ANA›Life Services Authority›National Calculator Authority›Containment Negative Air Pressure Calculator

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Containment Negative Air Pressure Calculator

Calculate the required exhaust airflow (CFM) to maintain negative air pressure within a containment area, accounting for room volume, air changes per hour, leakage, and pressure differential requirements.

### Room Dimensions

Room Length (ft)

Room Width (ft)

Room Height (ft)

### Ventilation Parameters

Required Air Changes per Hour (ACH)

Typical: 6–12 ACH for general containment; 12–20 ACH for infection control

Envelope Leakage Factor (%)

Extra airflow added to compensate for envelope leakage (typically 10–20%)

Target Negative Pressure Differential (Pa)

Typical: 2.5–8 Pa (0.01–0.03 in. w.g.) below adjacent spaces

### Openings / Leakage Area

Total Crack / Gap Area (in²)

Estimated total area of gaps around doors, penetrations, etc.

Discharge Coefficient (Cd)

Typical value: 0.60–0.70 for cracks and gaps

Calculate Results will appear here.

function conCalc() { const resultDiv = document.getElementById('con-result');

// --- Inputs --- const length = parseFloat(document.getElementById('con-length').value); const width = parseFloat(document.getElementById('con-width').value); const height = parseFloat(document.getElementById('con-height').value); const ach = parseFloat(document.getElementById('con-ach').value); const leakagePct = parseFloat(document.getElementById('con-leakage').value); const pressurePa = parseFloat(document.getElementById('con-pressure').value); const crackAreaIn2 = parseFloat(document.getElementById('con-crack-area').value); const Cd = parseFloat(document.getElementById('con-discharge-coeff').value);

// --- Validation --- const errors = []; if (isNaN(length) || length 50) errors.push("Leakage Factor must be between 0% and 50%."); if (isNaN(pressurePa) || pressurePa 1) errors.push("Discharge Coefficient must be between 0.1 and 1.0.");

if (errors.length > 0) { resultDiv.innerHTML = 'Input Errors:' + errors.map(e => '').join('') + ''; return; }

// ============================================================ // STEP 1: Room Volume // V (ft³) = L × W × H // ============================================================ const volumeFt3 = length * width * height;

// ============================================================ // STEP 2: Base Exhaust CFM from ACH // Q_base (CFM) = (V × ACH) / 60 // ============================================================ const Q_base = (volumeFt3 * ach) / 60.0;

// ============================================================ // STEP 3: Leakage Compensation // Q_exhaust (CFM) = Q_base × (1 + leakagePct / 100) // This ensures net negative pressure is maintained even with // envelope leakage allowing air infiltration. // ============================================================ const Q_exhaust = Q_base * (1.0 + leakagePct / 100.0);

// ============================================================ // STEP 4: Leakage Airflow through Cracks (Orifice Equation) // Convert crack area from in² to ft² // A_ft2 = crackAreaIn2 / 144 // // Air density at standard conditions: ρ = 0.0765 lb/ft³ // Pressure differential: ΔP (Pa) → convert to lb/ft² // 1 Pa = 0.020885 lb/ft² // // Velocity through crack (ft/s): // v = Cd × sqrt(2 × ΔP_lbft2 / ρ) // where ΔP_lbft2 = pressurePa × 0.020885 // ρ = 0.0765 lb/ft³ // // Leakage flow (CFM): // Q_leak = v × A_ft2 × 60 // ============================================================ const rho = 0.0765; // lb/ft³, standard air density const deltaP_lbft2 = pressurePa * 0.020885; // Pa → lb/ft² const crackAreaFt2 = crackAreaIn2 / 144.0; // in² → ft² const velocity = Cd * Math.sqrt(2.0 * deltaP_lbft2 / rho); // ft/s const Q_leak_cfm = velocity * crackAreaFt2 * 60.0; // ft³/s → CFM

// ============================================================ // STEP 5: Pressure Differential in other units // 1 Pa = 0.004015 in. w.g. // ============================================================ const pressureInWG = pressurePa * 0.004015;

// ============================================================ // STEP 6: Supply CFM (must be less than exhaust to maintain negative pressure) // Q_supply = Q_exhaust - Q_leak_cfm // If Q_leak already provides enough infiltration, supply may be zero. // ============================================================ const Q_supply = Math.max(0, Q_exhaust - Q_leak_cfm);

// ============================================================ // STEP 7: Net Negative Pressure Verification // Actual pressure differential achievable given exhaust vs supply: // ΔP_actual (Pa) = ((Q_exhaust - Q_supply - Q_leak_cfm) / (Cd × A_ft2 × 60))² × ρ / 2 × (1/0.020885) // Simplified check: net outflow = Q_exhaust - Q_supply - Q_leak_cfm // ============================================================ const netOutflow = Q_exhaust - Q_supply - Q_leak_cfm;

// Room air changes actually achieved const achActual = (Q_exhaust * 60.0) / volumeFt3;

// Format numbers function fmt(n, d=2) { return isNaN(n) ? 'N/A' : n.toFixed(d); }

resultDiv.innerHTML = ` ### Results

ParameterValue Room Volume${fmt(volumeFt3, 1)} ft³ Base Exhaust CFM (from ACH)${fmt(Q_base, 1)} CFM Leakage Compensation (${fmt(leakagePct, 0)}%)+${fmt(Q_base * leakagePct / 100, 1)} CFM Required Exhaust CFM${fmt(Q_exhaust, 1)} CFM Crack Leakage Airflow (infiltration)${fmt(Q_leak_cfm, 2)} CFM Maximum Supply CFM${fmt(Q_supply, 1)} CFM Target Pressure Differential${fmt(pressurePa, 1)} Pa (${fmt(pressureInWG, 4)} in. w.g.) Actual ACH Achieved${fmt(achActual, 1)} ACH Net Outflow (Exhaust − Supply − Leakage)${fmt(netOutflow, 2)} CFM ${netOutflow >= 0 ? '✅ Negative pressure maintained' : '⚠️ Check inputs'} Air Velocity Through Cracks${fmt(velocity, 2)} ft/s

Note: Exhaust must exceed supply by at least the leakage infiltration flow to sustain negative pressure. Ensure HVAC equipment is sized for ${fmt(Q_exhaust, 1)} CFM exhaust minimum.

`; }

#### Formulas Used

1. Room Volume: V = L × W × H  (ft³)

2. Base Exhaust CFM (ACH Method): Qbase = (V × ACH) / 60  (CFM)

3. Exhaust with Leakage Compensation: Qexhaust = Qbase × (1 + Leakage% / 100)  (CFM)

4. Crack Leakage Flow (Orifice / Bernoulli Equation): v = Cd × √(2 × ΔPlb/ft² / ρ)  (ft/s) Qleak = v × Acrack × 60  (CFM) where ΔPlb/ft² = ΔPPa × 0.020885,  ρ = 0.0765 lb/ft³

5. Maximum Allowable Supply CFM: Qsupply = max(0, Qexhaust − Qleak)  (CFM)

6. Negative Pressure Verification: Net Outflow = Qexhaust − Qsupply − Qleak ≥ 0  → Negative pressure maintained

#### Assumptions & References

• Discharge coefficient Cd = 0.60–0.70 is typical for building envelope cracks and gaps (ASHRAE Fundamentals).

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References

• ANA
• Life Services Authority