HVAC Cooling Load Calculation — CLTD Method for Indian Projects
Cooling load calculation is the foundation of every HVAC design. Get it wrong and you face oversized equipment, poor part-load performance, and wasted capital — or undersized equipment and uncomfortable buildings. The CLTD (Cooling Load Temperature Difference) method remains the most widely used hand-calculation approach in Indian practice, and understanding it thoroughly is essential for every HVAC engineer.
1. What is Cooling Load?
Cooling load is the rate at which heat must be removed from a space to maintain desired temperature and humidity conditions. It differs from heat gain — not all heat entering a space immediately becomes cooling load. Thermal mass in walls and slabs absorbs heat and releases it later, creating a time lag and peak load reduction.
Load Component | Description | Time Lag |
Solar through glass (SHGF) | Direct and diffuse solar radiation through windows | Immediate |
Conduction through walls/roof (CLTD) | Heat conducted through opaque envelope | 1–8 hours depending on wall mass |
Internal — people | Sensible + latent heat from occupants | Some lag — depends on mass |
Internal — lighting | Heat from light fixtures | 0.5–2 hour lag — ASHRAE CLF |
Internal — equipment | Heat from computers, motors, appliances | Similar to lighting |
Fresh air / ventilation | Sensible + latent load from outdoor air | Immediate |
Infiltration | Uncontrolled air leakage | Immediate |
2. CLTD Method — Step by Step
Step 1: Solar Load Through Glass
Qs = A × U × CLTD_glass (conduction component)
Qs_solar = A × SC × SHGF × CLF (solar radiation component)
Where: A = window area (m²), U = overall heat transfer coefficient (W/m²K), CLTD_glass = cooling load temperature difference for glass, SC = shading coefficient, SHGF = solar heat gain factor (W/m²), CLF = cooling load factor
Orientation | SHGF (W/m²) — June 21, 23°N Latitude | Peak Time (IST) |
North | 100–120 | 10:00–14:00 (diffuse) |
East | 500–580 | 07:00–09:00 |
South | 200–280 | 12:00–14:00 |
West | 520–600 | 15:00–17:00 |
Horizontal (roof) | 800–950 | 12:00–13:00 |
Step 2: Conduction Through Walls and Roof
Q_wall = A × U × CLTD_corrected
CLTD values are tabulated in ASHRAE Fundamentals or the ISHRAE handbook for different wall constructions and orientations. Correction factors are applied for:
- Latitude and month (Indian cities: use 23°N for Mumbai/Ahmedabad, 13°N for Chennai, 28°N for Delhi)
- Indoor and outdoor design temperatures (correct CLTD tables to local conditions)
- Colour of external surface (dark surface increases CLTD by 2–5°C)
Wall/Roof Type | U-value (W/m²K) | Time Lag (hours) | Notes |
230mm brick plastered both sides | 2.1 | 4–5 hours | Common Indian exterior wall |
230mm brick + 50mm insulation | 0.6 | 5–6 hours | ECBC 2017 compliant |
AAC block 200mm plastered | 1.2 | 4–5 hours | Lightweight — less mass than brick |
RCC roof 150mm no insulation | 3.8 | 2–3 hours | Very poor — common in India |
RCC roof + 75mm EPS insulation | 0.4 | 3–4 hours | ECBC compliant — standard green build |
Double glazing (6+12+6mm) | 2.7 | Minimal | Improved vs single — still significant load |
Step 3: Internal Heat Gains — People
Activity Level | Sensible Heat (W/person) | Latent Heat (W/person) | Total (W/person) |
Seated — office, restaurant | 60 | 45 | 105 |
Standing — light work, retail | 70 | 55 | 125 |
Walking — hotel lobby | 75 | 65 | 140 |
Light work — kitchen, factory | 80 | 80 | 160 |
Heavy work — laundry, gym | 90 | 170 | 260 |
Step 4: Internal Heat Gains — Lighting
Q_lighting = W_installed × CLF_light × ballast factor
CLF for lighting depends on hours of operation and room construction. For a continuously operated office: CLF ≈ 1.0 at peak. For an office operating 10 hours/day, CLF ≈ 0.8 for intermediate mass construction.
Step 5: Fresh Air Load
Qs_FA = 1.23 × Q × (To – Ti) [sensible, kW with L/s and °C]
Ql_FA = 3.0 × Q × (Wo – Wi) [latent, kW with L/s and kg/kg]
Where Q = fresh air flow rate (L/s), To/Ti = outdoor/indoor dry bulb temp, Wo/Wi = outdoor/indoor humidity ratio
3. Design Conditions — Key Indian Cities
City | Summer DBT (°C) | Summer WBT (°C) | Humidity Ratio (g/kg) | Indoor Design |
Mumbai | 35 | 27 | 18.5 | 24°C / 55% RH |
Delhi | 43 | 25 | 11.2 | 24°C / 50% RH |
Chennai | 38 | 29 | 22.1 | 24°C / 55% RH |
Bangalore | 33 | 22 | 12.0 | 24°C / 50% RH |
Hyderabad | 40 | 24 | 12.8 | 24°C / 50% RH |
Ahmedabad | 42 | 24 | 11.0 | 24°C / 50% RH |
Kolkata | 36 | 29 | 21.5 | 24°C / 55% RH |
Pune | 36 | 24 | 13.5 | 24°C / 50% RH |
4. Worked Example — Small Office, Mumbai
Parameter | Value |
Floor area | 200 m² (20m × 10m, east-west orientation) |
Floor height | 3.2m floor to ceiling |
Occupancy | 40 persons (seated office work) |
Lighting | 15 W/m² installed = 3000W |
Equipment (computers) | 3000W |
West glass | 20 m² single glazing, SC=0.65 |
South glass | 10 m² single glazing, SC=0.65 |
Roof | 150mm RCC, no insulation, dark colour |
Walls | 230mm brick, plastered both sides |
Fresh air | 10 L/s per person = 400 L/s total |
Peak cooling load components (3:00 PM June, Mumbai):
Load Component | Calculation | Load (kW) |
West glass — solar | 20 × 0.65 × 580 × 0.85 / 1000 | 6.42 |
West glass — conduction | 20 × 5.7 × 14 / 1000 | 1.60 |
South glass — solar | 10 × 0.65 × 210 × 0.5 / 1000 | 0.68 |
Roof conduction | 200 × 3.8 × 22 / 1000 | 16.72 |
Wall — west | 32 × 2.1 × 18 / 1000 | 1.21 |
People — sensible | 40 × 60 / 1000 | 2.40 |
People — latent | 40 × 45 / 1000 | 1.80 |
Lighting | 3000 × 0.9 / 1000 | 2.70 |
Equipment | 3000 / 1000 | 3.00 |
Fresh air — sensible | 1.23 × 400 × (35-24) / 1000 | 5.41 |
Fresh air — latent | 3.0 × 400 × (0.0185-0.0095) | 10.80 |
TOTAL COOLING LOAD | — | 52.74 kW ≈ 15 TR |
5. Common Errors in Indian Practice
- Using thumb rules (TR per m²) without calculation — error can be 30–50% in either direction
- Ignoring latent load — critical in humid coastal cities like Mumbai, Chennai, Kolkata
- Not applying diversity factors — calculated peak load assumes everything on simultaneously
- Using ASHRAE US design data — always use Indian city-specific conditions from NBC/ISHRAE
- Ignoring roof load — heavily underestimated for top floor in Indian climate (uninsulated RCC roof = 15–25% of total load)
Published by MEPVault — India's MEP Engineering Platform | mepvault.com/
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