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Bus HVAC system
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HVAC System

Bus HVAC System —
R134a Refrigeration Cycle

Thermodynamic operation, component function, and flow analysis. All values in metric/SI standard.

Contents

System OverviewThermodynamic CyclePressure ZonesAirflow SystemComponent FunctionsBus-SpecificFailure ModesGauge DiagnosticsTechnical Inset

A bus HVAC system uses the vapour-compression refrigeration cycle with R134a refrigerant. The system is typically roof-mounted and consists of a compressor (engine-driven or electric), condenser, receiver/drier, thermostatic expansion valve (TXV), evaporator, and cabin air distribution ducts.

COMPRESSOREngine/Electric2-3 → 12-18 barHP Gas70-90°CCONDENSERHeat → AmbientGas → LiquidHEAT REJECTEDHP LiquidRECEIVERDRIERFilter + MoistureTXVExpansion Valveh = constLP MixEVAPORATORHeat ← Cabin AirLiquid → Gas2-4 barCOOL AIR → CABINLP GasSuctionHIGH PRESSURE12–18 barLOW PRESSURE2–4 bar
High pressure / High temperature
Low pressure / Low temperature
1

Compression

State change: Low-pressure gas → High-pressure gas
Pressure: 2–3 bar → 12–18 bar
Temperature: 5 °C → 70–90 °C

Work input increases enthalpy. The compressor raises both pressure and temperature of the refrigerant vapour.

2

Condensation

State change: High-pressure gas → High-pressure liquid
Pressure: ~12–18 bar (approx. constant)
Temperature: Decreasing to ambient + ΔT

Heat is rejected to ambient air through the condenser. The refrigerant changes phase from gas to liquid at approximately constant pressure.

3

Expansion / Throttling

State change: High-pressure liquid → Low-pressure liquid-vapour mixture
Pressure: 12–18 bar → 2–4 bar
Temperature: Drops significantly

Isenthalpic expansion through the TXV (h = constant). The TXV controls flow based on evaporator superheat.

4

Evaporation

State change: Low-pressure liquid-vapour mixture → Low-pressure gas
Pressure: 2–4 bar (approx. constant)
Temperature: ~0–5 °C

Heat is absorbed from cabin air. The cooling effect is caused by latent heat absorption during phase change. Cooling capacity is proportional to Δh_evap.

Key Formula: Q_cooling ∝ Δh_evap = h_out − h_in