How to calculate the cooling capacity of bus air conditioner

How to calculate the cooling capacity of a bus air conditioner? This question involves the bus’s large glass area, high ceilings, and relatively poor insulation, resulting in a heat load far exceeding that of a building of the same volume. Inaccurate calculations can lead to insufficient bus air conditioner or electrical system failure.

The main heat sources of a bus, ranked by importance, are:
Solar radiation heat (through glass) > Body conduction heat (through sheet metal and insulation) > Internal heat generation (personnel, electrical appliances) > Air infiltration heat (heat leakage through gaps).

Quick estimation method (for preliminary budgeting and scheme discussion)

Formula: Required cooling capacity (BTU/h) ≈ Interior volume of the bus (cubic meters) × Coefficient
Volume calculation: Measure the length, width, and height (meters) of the living area, then multiply them.

Key coefficient selection (this is unique to buses):
1. Coefficient 70-80: Excellent insulation (≥5cm polyurethane foam throughout the bus, all windows are double-glazed or covered with high-quality insulation panels), small window area.
2. Coefficient 90-100: Standard RV insulation modifications have been made (foam insulation layer, but windows are mostly original single-pane glass). This is the most common starting coefficient.
3. Coefficient 110-130+: Original vehicle condition or simple modifications (sheet metal + interior panels, large single-pane viewing windows), commonly used in hot/high-altitude areas with strong sunlight.

Example: An 8-meter minibus has a living area volume of approximately 5m (length) × 2.2m (width) × 1.9m (height) ≈ 21 cubic meters.
1. Calculated using the common coefficient 100: 21 × 100 = 2100 BTU/h.
2. Conclusion: A cooling capacity of approximately 21,000 BTU/h is required. This typically means two 13,500 BTU RV air conditioners or one large commercial air conditioner.
How to calculate the cooling capacity of a bus air conditioner? Visit www.busclima.com or contact busclima@kingclima.com for more technical support.

How to calculate the cooling capacity of bus air conditioner

Method Two: Detailed Itemized Cost Estimation (Recommended for Final Design)

This method is more accurate, using watts (W) as the unit, and then converting the result (1W ≈ 3.41 BTU/h).

Step 1: Calculate the heat transferred through the vehicle body enclosure (Q1)

Formula: Q1 = Surface Area × Insulation Coefficient (U-value) × Temperature Difference Between Inside and Outside

Bus U-value Reference (Crucial):
1. Original sheet metal body: U-value ≈ 5.0 – 6.0 W/(m²·℃) (Very poor insulation)
2. After modification (with 3-5cm foam insulation): U-value ≈ 1.5 – 2.5 W/(m²·℃)
3. Excellent insulation (polyurethane above 5cm): U-value ≈ 0.8 – 1.2 W/(m²·℃)
4. Estimate the surface area of the roof + side walls (excluding windows), assuming 30㎡, with a U-value of 2.0 after insulation, and an inside-outside temperature difference of 12℃ (e.g., from 38℃ to 26℃). Q1 = 30 × 2.0 × 12 = 720 W

Step 2: Calculate the solar radiation heat transmitted through the glass (Q2 – a critical issue for buses)

Formula: Q2 = Net glass area (㎡) × Solar Radiation Heat Gain Coefficient (SHGC) × Solar Radiation Intensity
1. Single-pane ordinary glass: SHGC approximately 0.75-0.85 (i.e., 75%-85% of solar heat enters the vehicle).
2. After applying high-quality heat insulation film: SHGC can be reduced to 0.4-0.5.
3. Solar radiation intensity: Peak value on sunny summer days is approximately 800-1000 W/㎡.
4. Assuming an 8㎡ single-pane glass (common in minibuses), without film (SHGC=0.8), the radiation intensity is taken as 900W/㎡.
Q2 = 8 × 0.8 × 900 = 5760 W (far exceeding Q1!)

How to calculate the cooling capacity of bus air conditioner? Visit www.busclima.com or contact busclima@kingclima.com for further technical support.

Step 3: Calculate Internal Heat Generation (Q3)
1. Personnel: Approximately 100-150 W per person (seated).
2. Appliances: Refrigerators, lights, etc., estimated at 20-30% of their rated power as heat generation.
3. Calculation: 4 people + 200W of appliance heat generation.
Q3 = 4 × 150 + 200 = 800 W

Step 4: Summarize and Select Model
1. Total Heat Load Q_total = Q₁ + Q₂ + Q₃ = 720 + 5760 + 800 = 7280 W
2. Converted to Cooling Capacity: 7280 W × 3.41 ≈ 24,825 BTU/h
3. Considering a safety margin (10-20%): 24,825 × 1.15 ≈ 28,550 BTU/h
How to calculate the cooling capacity of a bus air conditioner? Visit www.busclima.com or contact busclima@kingclima.com for more technical support.

This calculation clearly shows that an 8-meter minibus requires a cooling capacity of close to 30,000 BTU/h. This explains why most large bus motorhomes need to install 2-3 rooftop air conditioners.