Does anyone know if cold propane produces less BTU's

[karneson]

I am trying to end the debate in my neighborhood as to why everone uses so much propane in winter.

I realize the obvious, the heater and fireplace is usually running but i think there is more to it than that. Here is why.

I run a seasonal catering business cooking crawfish. I have two large burners that produce about 500,000 btu's a piece. They can boil 40 gallons of cold water in about 10 minutes. If i have one hooked up to a small propane cylendar at 80 degrees ambient temprature i can freeze the tank in under two minutes and it will stop producing gas. To resolve this problem i have created a manifold and strung 3 100lb cylanders together. By doing this i have spread the demand for gas over multiple cylanders. I still have the same problem but i can resolve it by pouring boiling water over the tank which raises the temperature of the gas in the tanks.

Now that you undersand the context of my question and my logic am I on the right track.

My theory is this: In winter the ambient temprature depending on where you live will have a dramatic impact on the BTU rating of propane because the colder the gas is the more it takes to produce the same amount of BTU's with warm gas. Keeping in mind everyone has external above ground tanks.

 

[titan]

As the ambient temperature drops, so too does the gas pressure within the cylinder. This lessens the supply of gas to your appliance and is why you see evaporators in use with larger propane tanks.

 

[karneson]

I am unfamilliar with evaporators. Are they used in residental applications say with a 500 gallon tank?

 

[begreen]

There's a difference between flow rate and energy content. The energy content remains the same, but it the propane stays as a liquid, it doesn't flow well causing a pressure drop.

How big are these burners? 1 million btus is a lot of energy. Perhaps they are 50,000 btus each? That's still a big ole burner.

 

[karneson]

Well this is what i know about the burners that i have. i have two of the fish fry 160,000 btu burners and two of these multi-jet burners. the best i can tell you is the 160,000 btu burner produces a blue flame that is about 6 inches at the base and about 7 inches tall. the multi-jet burners produce a flame about 16 inches across and about 30 inches tall. They get so hot that i have to reinforce the 3/8 inch angle iron burner stands so they won't buckel under the weight when they are hot.

Really what i am attempting to understand is the efficency of propane at different ambient temperatures. Does 1 gallon of propane produce the same amount of BTU's at 20 degrees ambient temp vs. 85 degrees ambient temp. or would you need more gas at the colder temp?

Hey thanks for your help on this cause as you can see for an amature i am fumbling my way through this. Kind of expense trial and error thing going on here.

 

[begreen]

The heat content of propane is around 91,300 btus per gallon I believe, regardless of temperature of the fuel. Though the scientists on the board may be able to provide more info.

However, the cold 40 gallon of water will need to be brought up many more degrees in the winter than in the summer and that will take more btus (more propane).

 

[karneson]

how do i contact them?

 

[Andre B.]

Really what i am attempting to understand is the efficency of propane at different ambient temperatures. Does 1 gallon of propane produce the same amount of BTU's at 20 degrees ambient temp vs. 85 degrees ambient temp. or would you need more gas at the colder temp?

Since propane is more dense at lower temps there will be more BTUs in the cold gallon so you should need less gas.

It does take a little extra energy to heat the propane from 20 degrees to 85 degrees but it is not going to be very much.

However if the change in temperature affects the gas pressure seen by the burner there could be a change in the ratio of gas to air which could affect the efficiency of the burner but that is a problem with the burner not the propane.

 

[karneson]

Andre, thanks for joining.

Humm.... that contradicts my logic.

Here is what i experience I have a mainfold that has a oil filled pressure gage that reads from about 5 psi to 200. When the ambient temp is 85 degrees one 100lb cylindar will produce about 100 psi. Once i kick in the multi-jet burner and let it run for a while i notice frost about an 1/8 inch thick on the outside of the cylindar. I know that this is due to the conversion of the gas in the tank from a liquid to a gas (evaporation) which in turn lowers the temperature of the remaining gas thus dropping my head pressure to about 60 psi. If i raise the temp of the gas in the cylinder by poring hot water on it the head pressure raises.

I think the issue here is flow. My burners require a high volume of gas to operate and when that gas rate decreases the burner shuts down and becomes less efficent.

So i am still stuck on this gas temp vs. burner efficency relationship. I am working on a solution to externally heat the tanks to prevent the freeze up's.

 

[TMonter]

This is why you typically see oversized tanks in high demand application. A larger tank has more surface area and therefore can make more use of the ambient heat during colder weather without requiring a vaporizer.

It sounds like you need a vaporizer or possibly find a way to put the tanks in a mildly heated enclosure of some type.

As for the energy content, this is why you always want to work in pounds (mass) and not in gallons (volume) with compressible fuels. Energy content per pound will not change over temperature, but efficiency of combustion certainly can.

 

[Gooserider]

I don't know the inside details, but a couple years ago my GF and her brother took their parents on a Hot Air ballon ride a couple years ago for their 50th Anniv. It was a cool trip, hot air balloons are FUN!

However the relevant issue is they use propane for fuel, but obviously are limited in how much they can carry. I believe the pilot said it was about a 3 million BTU burner, I know it was shooting a flame 10 or 15 feet tall into the balloon. I don't know just how they did the fuel, but there was a supply line that ran through the flame area before going into the burner body. The tanks, which were in the balloon basket with us, didn't get noticeably cold. What I suspect is that the tank was set up to deliver liquid propane up to the burner, and that the supply line through the flame area acted like the "generator" on a Coleman white gas lantern and vaporised the gas just as it was being fed to the burner...

Presumably this would approach would solve many of the OP's problems as there would be very little evaporation going on in the tank, just enough to maintain the head pressure as the liquid level dropped. This wouldn't be enough to cause a noticeable temperature change. The challenge would be figuring out how to plumb things so that the liquid flow rate into the vaporizer would match the needed gas output without over heating the gas...

Gooserider

 

[par38lamp]

The reason your tanks are getting cold (freezing) and your pressure drops is governed by the ideal gas law, PV=nRT, and evaporation. As the LPG liquid vaporizes and turns into a gas, it also cools (evaporative cooling). That cooling causes the T in the PV=nRT to drop. To keep the equation equal, the P must drop as well (since the V, volume of the tank, stays the same). Same goes for cold weather. The T is low, therefore the P (pressure) must be lower, too.

To effectively consume a lot of propane, it is best to draw it off the tank as a liquid, and then vaporize it into a gas for burning. That way, your tank pressure is unaffected by cold weather (low T). That is what they are doing in hot air balloons. Gasoline engines that are converted to burn propane do this as well.

Blacksmiths who use propane for their forges run into this problem. The struggle to get the propane out of the tank fast enough without the tank getting too cold and the pressure dropping.

http://www.anvilfire.com/FAQs/gasfacts.htm

Like BeGreen mentioned, the propane still contains the same amount of energy. If the gas is colder, it will be more dense, and have more BTU's for the same volume at a lower temperature. But if the pressure pushing that more dense gas is lower, you are back where you started.

I stunk at chemistry, but cooking with a high pressure burner and having my tank freeze on me one cold day inspired me to read up on what I forgot in Chemisty class

 

[karneson]

You have solved the mystery for me. Thanks. Now i need to figure out how to use that to solve my problems.