HVAC Cooling Load Calculation India — CLTD Method Explained

HVAC Design

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)

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