Anyone who has ever exchanged an empty propane barbeque cylinder knows that propane is heavy, but how many people know that 1 gallon of liquid propane weighs 4.23 pounds? Maybe you even remember the formula for propane from high school chemistry (C3H8, by the way) but for the engineers and technicians that design and install propane heating and cooking systems, this is just the tip of the iceberg of what they need to know about propane.
In fact, understanding some of the physical characteristics of propane helps explain some of the safety rules that we adhere to and that everyone should follow diligently. Propane changes from liquid to a gas (its boiling point) at -44 degrees Fahrenheit. This is one of the reasons why our technicians wear protective gear like gloves and face shields when installing new equipment. If they were to get splashed with the liquid propane, the low temperature would instantly freeze skin, which is called a burn because of similar effects. As a gas, propane is heavier than air. This is known as its specific gravity, which is 1.52; as a comparison, air has a specific gravity of 1.0. This means that propane gas can pool in dips and low points on the floor and one of the reasons why you must leave your empty barbeque cylinder outside of the store when you are exchanging; just in case there is some propane left in the cylinder that somehow leaks out.
One of the key characteristics that engineers must design for in heating systems is the energy content in propane. Homeowners and grill masters too, should know how much propane they’ll use for their heating and cooking. One gallon of propane has 91,502 BTUs (or British thermal units). For heat system designers, they must take this into account knowing how to size the HVAC equipment and propane tanks to get the heat output that they need. But what about a new 60,000 BTU-per-hour barbeque grill (which I wish was my new grill)? It would take all of the burners (and there are six on that model) going all out for an hour and half before you’d use a whole gallon of propane (60,000 BTU/hr x 1.5 hrs = 90,000 BTUs). Larger HVAC systems, Combined Heat and Power (CHP) systems, and Gas Heat Pumps are often spec’d in units of Therms or by comparison to per MCF (thousand cubic feet) of natural gas. 1 Therm is equal to 100,000 BTU, which is 1.1 gallons of propane. 1 MCF of natural gas has 1,000,000 BTU, which is equivalent to 11 gallons of propane.
Propane characteristics referenced in this post:
Pounds per gallon of Liquid @ 60 degF 4.23
Gallons per pound of Liquid @ 60 degF 0.236
Specific Gravity of Gas (air=1) 1.52
Specific Gravity of Liquid @ 60 F (Water=1) 0.504
BTU per Gallon of Gas @ 60 F 91,502
BTU per lb. of Gas 21,548
BTU per Cubic Foot of Gas @ 60 F 2,488
Cubic Feet of Gas per Gallon of Liquid 36.38
Gallons per Therm (1 Therm = 100,000 BTU) 1.1
Gallons per MCF of Natural Gas (1 MCF = 1,000,000 BTU) 11