Propane Vapor vs Liquid Draw
Why this matters
A 22 kW propane standby generator on a 500-gallon tank in northern Minnesota runs flawlessly in October and inexplicably fails in February. The customer blames the generator; the actual cause is propane vaporization. Propane in a standard tank delivers vapor (gas) to the generator. Vaporization absorbs heat from the surrounding tank; in extreme cold, vaporization slows or stops, and the generator starves for fuel even though the tank reads "full". The fix is sizing for cold-weather vaporization OR converting to liquid draw with an external vaporizer OR specifying natural gas. The contractor who understands vapor-vs-liquid draw and quotes appropriate tank sizing for the climate avoids the most common cold-weather propane failure.
How propane delivery works
Propane is stored as a liquid under pressure inside the tank. At normal temperatures, the pressure inside the tank is 100 to 150 psi (varies with temperature). The tank's pressure relief and regulator keep this in spec.
When fuel is drawn, two methods:
Vapor draw (standard for residential)
The tank withdraws gas from the vapor space above the liquid. As gas is withdrawn:
- Tank pressure drops slightly
- More liquid propane evaporates to replace withdrawn vapor
- Evaporation absorbs heat from the surrounding tank metal
- Tank temperature drops
- At lower temperature, propane has lower vapor pressure
- Delivery rate is limited by how fast the tank can evaporate
Liquid draw (commercial / specialized)
The tank withdraws liquid propane from below the surface:
- A dedicated dip tube extends below the liquid line
- Liquid is delivered to an external vaporizer (typically electric or generator exhaust-heated)
- The vaporizer converts liquid to gas under controlled conditions
- Delivery rate is not limited by tank evaporation
- Cold-weather vaporization issue is solved
Most residential generators are vapor draw. Liquid draw with an external vaporizer is used for very large commercial generators or cold-climate installations where vapor draw cannot keep up.
Vapor delivery rate (vaporization capacity)
The vaporization rate of a propane tank depends on:
- Tank size (larger = more surface area for evaporation)
- Tank temperature (warmer = faster vaporization)
- Liquid level in the tank (more liquid = more vaporizing surface; below 25 percent, vaporization area is reduced)
Manufacturer vaporization charts:
| Tank size (gallons) | Vaporization rate at 70 F (BTU/hr) | At 32 F | At 0 F | At -20 F |
|---|---|---|---|---|
| 120 (vertical) | 195,000 | 95,000 | 50,000 | 15,000 |
| 250 (horizontal) | 350,000 | 175,000 | 90,000 | 30,000 |
| 500 (horizontal) | 800,000 | 400,000 | 215,000 | 75,000 |
| 1000 (horizontal) | 1,500,000 | 750,000 | 400,000 | 140,000 |
These are at "75 percent full" tank level. Below 25 percent full, vaporization drops to roughly 50 percent of the charted rate.
Generator fuel consumption
A residential standby generator's fuel consumption:
| Generator size | Fuel consumption at full load (BTU/hr) |
|---|---|
| 7 kW | 96,000 |
| 10 kW | 145,000 |
| 14 kW | 195,000 |
| 18 kW | 245,000 |
| 22 kW | 305,000 |
| 26 kW | 365,000 |
| 36 kW | 510,000 |
| 48 kW | 670,000 |
For propane (91,500 BTU/gallon), divide the BTU/hr by 91,500 to get gallons per hour at full load.
A 22 kW generator at full load consumes 3.3 gph propane.
The cold-weather sizing problem
For a 22 kW generator (305,000 BTU/hr full load) on a 250-gallon tank:
- At 70 F: tank vaporizes 350,000 BTU/hr - sufficient
- At 32 F: tank vaporizes 175,000 BTU/hr - generator starves
- At 0 F: tank vaporizes 90,000 BTU/hr - generator at 30 percent of nameplate
- At -20 F: tank vaporizes 30,000 BTU/hr - generator essentially non-functional
The customer's expectation: backup power during the storm that drove them to buy a generator. The reality: the system delivers 30 percent capacity in extreme cold, and that 30 percent decreases as the tank level drops.
The fix: oversized tank.
For a 22 kW generator in cold climate:
| Tank size | Vaporization at 0 F | Match to 22 kW (305k BTU/hr) |
|---|---|---|
| 250 | 90,000 | 30 percent |
| 500 | 215,000 | 70 percent (marginal) |
| 1000 | 400,000 | 130 percent (good) |
| Two 500s | 430,000 | 140 percent (good) |
The 1000-gallon tank or dual 500-gallon configuration provides margin for the cold-weather rated capacity.
Underground tank advantages
An underground (buried) tank stays at ground temperature year-round. Ground temperature at 4 ft depth in the US ranges:
- Southern US: 60 to 70 F year-round
- Mid-Atlantic / Midwest: 45 to 60 F
- Northeast / Mountain: 40 to 55 F
- Far North: 35 to 45 F
A 500-gallon underground tank in upstate New York at 50 F ground temperature delivers vaporization closer to the 32 F surface chart than the -20 F surface (because ground is at 50 F regardless of weather).
Underground tanks are common in cold-climate residential propane install. Cost is higher (excavation, tank), but performance is dramatically better.
Tank wrap (vapor-flow blanket)
For an above-ground tank in extreme cold, a manufacturer-supplied tank wrap (insulated jacket around the tank) reduces heat loss to ambient and improves vaporization. The wrap typically increases vaporization by 20 to 30 percent.
Wraps cost a moderate per tank. For a customer with marginal vaporization, this is a cost-effective improvement.
Dual tank configuration
Two propane tanks plumbed together effectively double the vaporization capacity. Common in cold-climate residential.
Configurations:
- Two 500-gallon tanks: vaporization roughly 2x single 500-gallon
- One large tank: same total vaporization as two smaller (related to total surface area)
- Multiple smaller tanks (4x 120-gallon): slightly less efficient but cheaper for some installs
Two-stage regulators
A two-stage regulator system uses:
- First-stage regulator at the tank: drops pressure from tank pressure to ~10 psi
- Second-stage regulator near the appliance: drops further to ~11 inches water column (the appliance operating pressure)
Two-stage regulators handle cold-weather pressure drop better than single-stage. The first-stage maintains adequate pressure at the second-stage even when tank pressure drops due to cold.
For cold-climate residential install, two-stage is the standard. Single-stage is acceptable for warm-climate only.
When to specify liquid draw
For very large generators (over 50 kW residential or commercial), the vapor-draw approach reaches its limits even with large tanks. Liquid draw with an external vaporizer becomes economical:
References
- NFPA 58 (Liquefied Petroleum Gas Code).
- NFPA 54 / ANSI Z223.1 (National Fuel Gas Code).
- NFPA 37 (Standard for the Installation and Use of Stationary Combustion Engines and Gas Turbines).
- Propane Education and Research Council (PERC) technical literature on vaporization.
- Manufacturer literature: Generac, Kohler, Cummins, Briggs and Stratton propane installation manuals.
- Manuall internal: Generator Cold Weather Operation, Generator Fuel Options, Generator Generator Sizing.