Propane question


Old 01-17-04, 09:21 PM
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Propane question

I know that you can estimate how much oil an oil furnace will use based on the nozzle size. Is there a way to estimate how much a propane furnace will use based on it's input BTU?
I'm trying to figure out if I should get a propane or electric range and dryer. The propane tank is 500 gallons, and I don't want it to run out quickly. If I would be able to estimate how many gallons of propane each unit will use per hour (roughly), I could estimate how long my tank will last.
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Old 01-18-04, 01:36 AM
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I've not done this calc before but here is one suggested method to proceed.

For propane - 1 cu ft/hr of propane produces 2500 BTU/hr. To calculate the no of cu ft of propane at the burner orifice you can use Boyle's law/Charles law. You will need to know the tank pressure of the propane tank. Also convert the 500 gallons to cu ft. You can then use PV=constant (I think it is Boyle's law if my memory serves me right). If you assume 11" WC pressure at the burner then

11" WC * Vol = 500 Gals * Propane tank pressure. If the tank pressure is in PSI don't forget to convert "WC to PSI.

Solve for Vol gives you total cu ft at 11" WC pressure. Multiply by 2500 for BTU.

In reality this calc is oversimplified since the propane tank pressure will fall as the propane is discharged and so the relationship will be exponential rather than linear. In other words the propane will last for less time that what calc will show. However, this should give you at least an approximation to work with.

This calc assumes a constant temperature. If it varies you have to use the other law.

Last edited by rav12; 01-18-04 at 01:48 AM.
Old 01-18-04, 06:05 AM
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propane consumption

try speaking to the people who sell it.... They have the experience that book knowledge tends to complicate...
Old 01-18-04, 06:15 AM
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electric vs propane

as far as I know there is no comparison.... propane is far cheaper to operate than electric unlesss you live in Canada where the Americans consumers subsidize the cost of power generated.
The initial cost of the equipment is the difference. As far as how often it has to be filled.... who's like oil... you have a scheduled delivery... it's not like you have to carry the taks somewhere to be filled. You only pay for the gallons used / pumped.
Old 01-18-04, 08:53 AM
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fuel cost

If you take and go to It will let you compare the fuel cost for ELEC. LP or Oil.

Dont have the book here But I think elec. kw-hrs ===3415 btu

oil #2 about 142,000 btu per gal and LP 91,502 btu per gal.

What you are after is out of the world. Like find out how much are you going to cook and then you will find out what to use. Then you have to get the number of degree days to say how much you will use on the furnace

As for as the oil nozzle will tell you how much oil you will burn. Also the orifice size on the stove will tell you how much Lp you will burn there. Also look at the furnace the btu rate is right on it.

I would go with a furnace on LP. The range lp or electric .The dryer electric for sure. The hot water tank Lp or electric.

We use 500 gal tanks on almost all homes and it works out just fine.
Just my .02cents ED
Old 01-18-04, 10:45 PM
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Modified calcs

Sorry – my first set of calcs only work on gases – not on liquids. The calcs below is for liquid propane.

Propane tanks are usually filled to 80% capacity. So 20% of the tank is propane vapor and 80% liquid.

Propane expands to 270 times its original volume when going from liquid to gas (physical property of propane).

80% of 500 gallons is 400 gallons liquid = 400 *270 gallons propane vapor + 100 gallons vapor already in the tank. (actually the 100 gallons is under pressure so Boyle’s law applies but I’m skipping that step since the liquid propane will dominate the volume).

Total capacity is 400*270 + 100 = 108100 gallons = 108100/7.481 cu ft = 14449 cu ft.

Total BTU is 14449 * 2500 = 36122500 BTU.

So for example a 75000 BTU furnace will last 36122500/75000 = 481 hours.

Since the energy per cu ft of propane is 2500 BTU 1 gallon of liquid propane will give (270/7.481) * 2500 = 90228 BTU which is close to the figure ed quoted.

However a direct comparison is harder and other factors also dominate. For example the efficiency of the furnace will determine how much of the input energy will actually appear as heat. With electric, efficiency should also be considered but also smaller factors such as the material with which the heating elements are made will determine how much of the input energy gets converted to heat. A 1:1 comparison is difficult but using the same units (i.e. Watts) at the output of the heater taking into account the efficiency should give a reasonable idea.

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