Propane (LPG)

Propane, chemically designated as C3H8, is a liquefied petroleum gas (LPG) widely used in HVAC applications where natural gas is unavailable. Its high energy density, portability, and clean combustion characteristics make it a preferred alternative fuel for residential, commercial, and industrial heating systems.

Physical and Chemical Properties

Propane is a hydrocarbon with three carbon atoms and eight hydrogen atoms (C3H8). At standard atmospheric conditions, propane exists as a gas, but it readily liquefies under moderate pressure, allowing efficient storage and transportation.

Key Physical Properties:

• Molecular weight: 44.1 lb/lbmol
• Boiling point at atmospheric pressure: -44°F (-42°C)
• Specific gravity (liquid): 0.504 at 60°F
• Specific gravity (gas): 1.52 (air = 1.0)
• Vapor pressure at 70°F: 124 psig
• Vapor pressure at 100°F: 196 psig

The specific gravity of propane vapor (1.52) indicates it is heavier than air, causing unburned propane to settle in low areas, creating potential safety concerns that require proper ventilation design.

Heating Value and Combustion Characteristics

Propane delivers substantial energy content per unit volume, making it highly efficient for heating applications.

Heating Values:

• Higher heating value (HHV): 2,516 BTU/ft³ of gas
• Lower heating value (LHV): 2,316 BTU/ft³ of gas
• Liquid heating value: 91,500 BTU/gallon (typical)
• Mass heating value: 21,669 BTU/lb (HHV)

The heating value per gallon varies slightly with temperature due to density changes. At 60°F, propane contains approximately 91,500 BTU/gallon; this value decreases as temperature increases and liquid density decreases.

Stoichiometric Combustion: Complete combustion of propane follows: C3H8 + 5O2 → 3CO2 + 4H2O

Theoretical air requirement: 23.85 ft³ air per ft³ propane (15.7 lb air per lb propane). Practical burners operate at 10-50% excess air depending on application and burner design.

Storage Requirements

Propane storage systems must accommodate liquid expansion, maintain adequate vapor space, and withstand internal pressures that vary with ambient temperature.

Storage Tank Design: Storage tanks are constructed to ASME Section VIII or DOT specifications and must meet NFPA 58 (Liquefied Petroleum Gas Code) requirements. Tanks are typically designed for 250 psig working pressure with 312.5 psig test pressure.

Fill Limits: Tanks are filled to maximum 80-85% liquid capacity to provide vapor space for thermal expansion. This prevents liquid overfilling and excessive pressure rise during temperature increases.

Tank Sizing Considerations:

• Heating load (BTU/hr input required)
• Vaporization capacity at design temperature
• Delivery frequency and accessibility
• Local codes and setback requirements

Installation Requirements:

• Minimum 10 feet from property lines, buildings, and ignition sources
• Tanks over 2,000 gallons require additional setbacks
• Underground tanks require corrosion protection (cathodic protection or dielectric coating)
• Pressure relief valves sized per NFPA 58 requirements

Vaporization Rate and Capacity

Propane must vaporize from liquid to gas phase to supply fuel to burners. The vaporization rate depends on heat transfer to the liquid, which is influenced by tank surface area, liquid level, and ambient temperature.

Factors Affecting Vaporization:

1. Ambient Temperature: Lower temperatures reduce vapor pressure and vaporization rate
2. Tank Surface Area: Larger surface area exposed to ambient provides more heat transfer
3. Liquid Level: Higher fill levels provide greater surface area initially but reduce as level drops
4. Tank Material: Steel tanks absorb more heat than fiberglass or composite materials

Vaporization Capacity Calculation: Natural vaporization capacity varies with tank size and temperature:

• 500-gallon tank at 0°F: ~80,000 BTU/hr continuous
• 500-gallon tank at 70°F: ~300,000 BTU/hr continuous
• 1,000-gallon tank at 0°F: ~125,000 BTU/hr continuous

For loads exceeding natural vaporization capacity, vaporizers (heat exchangers) are required to add heat to the liquid propane, increasing evaporation rate.

Vaporizer Types:

• Direct-fired vaporizers: Use propane burner to heat liquid propane in heat exchanger
• Electric vaporizers: Use electric resistance heating elements
• Indirect vaporizers: Use steam, hot water, or engine coolant as heat source

Pressure Regulation

Propane pressure at the tank varies from 50-250 psig depending on temperature. Equipment requires regulated pressure, typically 11 inches water column (0.4 psig) for appliances.

Two-Stage Regulation: Most installations use two-stage pressure regulation:

1. First Stage Regulator (at tank): Reduces tank pressure to 10 psig intermediate pressure. Mounted at tank and often integral with service valve. Compensates for tank pressure variations with temperature.

2. Second Stage Regulator (at appliance or building): Reduces 10 psig to appliance pressure (11 inches w.c.). Provides final precise regulation for burner operation.

Single-Stage Systems: Single-stage regulators reduce tank pressure directly to appliance pressure. Used only on small installations with short piping runs (less than 100 feet) due to pressure drop sensitivity.

Regulator Capacity: Regulators must be sized for peak demand flow rate with adequate capacity margin. Undersized regulators cause pressure lockup and insufficient fuel delivery.

Propane-Air Mixtures for Standby Applications

Propane-air mixtures provide synthetic natural gas for standby service when natural gas supply is interrupted. The mixture replicates natural gas properties by diluting propane with air to match heating value and specific gravity.

Mixture Ratios: To match natural gas heating value of approximately 1,000 BTU/ft³, propane is mixed at approximately 40% propane and 60% air by volume. Exact ratio depends on desired heating value and specific gravity matching requirements.

Mixing Equipment:

• Air compressor: Provides compressed air at controlled pressure
• Propane vaporizer: Ensures adequate gas-phase propane supply
• Mixer: Combines propane and air at precise ratios using venturi or proportional control valves
• Pressure regulation: Maintains delivery pressure matching natural gas system pressure

Application Advantages:

• Fuel interchangeability without burner adjustment
• Existing natural gas burners operate without modification
• Automatic switchover during natural gas interruption
• Compliance with natural gas appliance certifications

Limitations:

• Higher equipment cost than straight propane
• Requires electrical power for air compressor
• Additional maintenance for mixing equipment
• Lower efficiency due to air compression energy

Propane versus Natural Gas Comparison

Conversion Considerations: Converting equipment from natural gas to propane requires:

• Orifice replacement (smaller orifices for propane)
• Pressure regulator adjustment or replacement
• Air shutter adjustment for proper combustion air
• Burner testing and combustion analysis
• Verification of appliance certification for propane operation

Propane requires 60% smaller orifices than natural gas due to higher heating value and different gas properties.

Safety Considerations

Propane’s properties require specific safety measures:

• Gas Detection: Propane is heavier than air; gas detectors should be located near floor level
• Odorant: Ethyl mercaptan added at 1.0 lb per 10,000 gallons for leak detection
• Ventilation: Low-level ventilation required in areas where propane is used or stored
• Ignition Sources: Electrical equipment in propane storage areas must be rated for hazardous locations (Class I, Division 2)
• Emergency Shutoff: Accessible manual shutoff valves required at tank and at building entry

Propane systems must comply with NFPA 54 (National Fuel Gas Code) and NFPA 58 (Liquefied Petroleum Gas Code).

Components

• Vapor Pressure
• Storage Requirements
• Vaporization Rate
• Propane Air Mixtures