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What I've Learned About Propane and Portable Cylinders

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MountainDon:
I've know for a very long time that temperature affects propane, that at -44 degrees F or colder it will not turn to a vapor. I also knew that some propane fueled construction heaters would perform poorly or maybe not at all when used on 20 pound cylinders. I did not realize just how seriously the low temperatures affect the propane output from small cylinders. So I've complied some information in the form of tables and have placed them below.

First, a chart with the relationship between temperature and the vapor pressure of propane. Most propane appliances require 11" to operate properly. The chart states "Outside Temperature". I should have labeled that "Temperature of Liquid Propane" to be 100% correct, but since the tank should be outside it is in all likelihood the same.



The next table shows the performance of 100 pound cylinders at various temperatures. The left column shows the 100 pound tank with various amounts of propane, measured in pounds. The body of the table shows the maximum continuous draw measured in BTU/hr, at zero degrees and at 20 defrees F. There are columns for single, dual and triple tanks, connected together with a manifold of some kind. Parts are available to make up whatever is needed.



The third table is similar to the above, but is based on what the maximum output would be when a tank is 25% full. The maximum continuous output of different size cylinders at a range of temperatures is shown in the body of the table.



From the two above tables one should be able to get a good idea of how they may fare if using small cylinders with devices that use various amounts of propane. Many heaters require an input of 10,000 BTU/hr or greater to operate properly. With lower volumes they will perform poorly or perhaps not at all. It is readily apparent that 20 pound cylinders are not adequate to power many devices when the temperature gets down to 10 degrees F or lower.

The last table is a compilation of propane properties.



Notes:

Propane is stored as a liquid under pressure and boils to produce a vapor that is drawn off at the top for the heater or other device to use as fuel.  Because propane boils at -44° (below zero), the gas will freeze if it can not absorb enough ambient heat to compensate for the boiling process.  The bigger the cylinder is compared to the amount of load, the warmer it is outside, the warmer the cylinder is kept, all are a determining factor in the likelihood of a cylinder freezing up.

If a sweat or frost line forms around the cylinder at the level of the fuel, this is a telltale sign that the cylinder is over worked and is in the process of freeze up.  If the gas does freeze, it will stop producing vapor and the pressure inside the cylinder will drop to as low as zero psi which will cause the device to stop running.

If a tank of cold propane is warmed it will perform better. The exhaust from a propane fueled generator could be used to warm a tank. Proper cautions to prevent overheating must be taken as propane expands when heated. That is why propane tanks are filled to 80%.

bayview:

   A lot of great information!   

   I’ve noticed that the propane flame is more yellow in the winter.   Now I see why. . .    The appliance is receiving less than one third of the pressure compared to a 100 degree summer day.   (Even with a pressure regulator)

Thanks.

/.

ajbremer:
Thanks MountainDon and BayView. I have been using those little, what are they called, 5-gallon tanks - the same ones barbecues use. I was wondering, would it be better to have one big one and then 'Y' off to different heaters and appliances from it?

MountainDon:
5 gallon = 20 lb, more or less

What to use depends a lot on the outdoor temperature and the volume (BTU's/hour) needed by the appliance. It seems to me the 20# cylinders begin to get into the potential trouble zone around +30 F.

My propane dealer has fittings for manifolding two or more tanks together. It's similar to connecting batteries in parallel to increase electrical capacity. As the charts show, larger tanks combined, equals more available vaporized gas.

Note the charts are for vertical tanks. Horizontal tanks perform better as the surface area of the liquid propane is increased. But beware, a cylinder designed for vertical use will not work properly. Do not try that.

Shawn B:
Yep known about the small tank and torpedo heater issues for awhile now. MtnDon mentions that appliances need 11" w.c.  Keep in mind that this is burner manifold pressure, or outlet of the gas valve/regulator. Most appliances, especially central forced air furnaces need 14" w.c. on the inlet pressure to appliance. Most house regulators are set at 14" w.c. Not sure if R.V. appliances are the same. Also keep in mind that "propane" or L.P. gas is not an exact mixture region to region or batch to batch. It is a man made gas of several natural and other man made gases. This is one reason why propane has a history of burning dirtier and with more CO2 an CO than natural gas. This is one reason the price changes so much and in some areas of the Nation electric is a cheaper energy source.  In colder climates such as the northern tier states it is common to bury the propane tanks to avoid the temp issues. Also colder areas use a different mixture of butane and iso-butane.

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