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Vaporization of Propane

This Chapter contains information on:

• Vaporization rates and ASME Storage Tanks
• Vaporization Charts and Formulas

The amount of vaporization in BTU’s of a container is controlled by three factors as follows:

1. Size and shape of the container

The standard storage tanks are manufactured as follows:

• 500 gallon capacity, 9’5" long X 37" in diameter.
• 1,000 gallon capacity, 16’1" long X 41" diameter.

(always check tank length and diameter for exact measurements)

2. The wetted surface of the container

The wetted surface of a container is the area of the inside of the tank that is touched by the liquid propane. Therefore, the liquid level or how full the container is affects vaporization, but not as great a degree as the other two factors. In figuring tank vaporization, we use 1/2 full, which gives you a safety factor when the container is over 1/2 full.

3. The outside air temperature

The outside temperature surrounding the container is where the liquid propane touching the inside shell picks up and absorbs the heat needed to produce vaporization. . . . Liquid to vapor. The higher the heat, the larger the volume in BTU that is available to the burner. Place tank where it is in the sun, at least during the morning hours, to help it recover from the cool or cold nights.

The only thing that affects propane is heat in the container. As temperature goes up, vaporization rate of vapor pressure go up. As temperature goes down, vaporization rate and vapor pressure go down. Refer to Vapor Pressure Chart, Chapter 1.

Vaporization rates for ASME Storage Tanks

A number of assumptions were made in calculating the BTU figures listed in the table below:

1. The tank is one-half full.
2. Relative humidity is 70%.
3. The tank is under intermittent loading.

Although none of these conditions may apply, the Table below can serve as a rule-of-thumb in estimating what a particular tank size will provide under various temperatures. Continuous loading is not a very common occurrence on domestic installations, but under continuos loading the withdrawal rates in the Table should be multiplied by 0.25.

Then make sure your tank is at least this big   (If you can keep it half full or more.)

Example of 1000 gallon tank at 30° F:

What happens when the burner demand is larger than the vaporization rate of the container? As the demand is greater than the vaporization rate, a refrigerator process takes place within the container. The pressure drops, as vaporization rate and vapor pressure depends on heat. The container cannot recover, so the vapor pressure continues to drop until 32° F or lower is reached. (the pressure gauge should be about 55-60 Lbs. (Refer to Vapor Pressure Chart in this Chapter 1.) At this time, ice starts to form on the outside of the container where the liquid propane touches the inside of the container. The ice acts as an insulator and retards heat transfer into the liquid propane, which aggravates the condition dropping vaporization rates and vapor pressure. The burner demand is still the same, and the condition will become worse as the vaporization rate continues to go down, until the liquid propane reaches -44° F and both vapor pressure and vaporization will be zero.

What is the answer?

1. More wetted surface by adding tanks. . . restricted to 4000-gallon aggregate or our 2000-gallon single tank capacity, unless fire marshal approval is secured, and a 6’ fence with two 3’ gates is installed.
2. Warmer outside temperature, which we cannot control.
3. Artificial heat by using an approved external Propane vaporizer.

Model 30 Mitchell	2,760,000 BTU at -40° F
Model 70 Mitchell	6,440,000 BTU at -40° F
Model 40/40 Algas	3,391,000 BTU at -20° F
Model 80/40 Algas	6,783,000 BTU at -20° F
Ely 800 Water Bath	73,200,000 BTU at -20° F

Refer to Chapter 12 for use of Vaporizers to produce ample Propane vapor from liquid to match appliance load.

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