by Marc Ward
Clay Times Nov. 2003
Last issue, I talked about treating symptoms instead of treating the underlying problem. It’s a common response to a situation,… especially if we don’t really understand the nature of the problem. When you read this, one of my symptom seasons will be about over. The season is the raku season during the spring, summer, and early fall. The symptom is propane tank freeze-up. The only reason it isn’t as much of a problem in the winter is that the majority of people firing raku live where it’s unpleasantly cold to stand outside for hours at a time. Otherwise, the problem would be worse.
So, if tank freeze-up is the symptom, what exactly is the problem?
Before we get to the problem, I just want to make one point in regards to a popular treatment of the symptom. I guarantee Manchester Tank (one of the leading manufacturer of propane cylinders) doesn’t design or want people to use their cylinders as flotation devices in tubes of water! This is treating the symptom. It’s like drinking lots of whiskey for a toothache to dull the symptoms. I’ve got nothing against whiskey, but a trip to the dentist is really the appropriate course of action. One other point that’s just an anal preoccupation with semantics; a tank is an open or non-sealed container that usually holds a liquid (like the tub you unfortunately float your propane cylinder in). A cylinder is a closed vessel that usually holds a pressurized gas and/or liquid. For the sake of familiarity, we’ll just call ‘em a propane tank because that’s what everyone is used to calling them.
As propane gas is pressurized, it becomes a liquid. We all know that that tank is full of liquid and it turns into a gas when we let it out. What happens is that when you open the tank valve, some of the pressure in the tank escapes (in the form of gas) and causes the liquid to boil which releases more gas. Different liquids have different boiling points at different pressures. We all know from junior high that water boils at 212°F…. AT SEA LEVEL. This is all about the pressure exerted on the liquid. In Death Valley, which is below sea level, water boils at a slightly higher temperature. At the top of Mt Everest, it’s tough to cook anything, because boiling water is at a much lower temperature (less air pressure). So most of us have heard about this, but those folks in junior high didn’t tell us why water only could reach 212°F at sea level. Or, if they did, we weren’t really paying attention. Why can’t it get hotter while it’s boiling?
Moving from solid to liquid to gas is a change of phase. When you change phase, you lose energy. The faster you change, the faster you lose energy. That’s why a light simmer is the same temperature as a rapid boil. Inside your propane tank, the system is losing energy (temperature) every time you release pressure. Our pot on the stove keeps having energy added by the burner. Our propane tank isn’t so lucky. The more you release, the more temperature you lose… the colder it gets, the less rapidly the liquid is going to boil which in turn produces less gas. A vicious cycle. But phase change isn’t the only thing in the wonderful world of gas physics that’s working against us.
During the beginning of the industrial revolution, a French scientist named Jacques Charles was kind enough to give us Charles’ Law. Basically it says that the amount of change in volume or pressure in a given volume is directly proportional to the change in temperature. Translated into walking around language, it means tanks get colder when pressure is released and they get hotter when pressure is increased. And, old Jacques’ law can really have a chilling affect.
I’m fortunate to be part of an equally whacky group of people besides potters. Scuba divers. I’m a certified gas blender for exotic breathing mixtures. One of the tools used is a booster pump that transfers lower pressure gasses into higher-pressure cylinders. Inside this pump, there are very sudden and severe drops in pressure. It is not uncommon to see thick layers of frost on these pumps during a summer day in Florida. We’re not talking phase change here, which has a loss of energy going from liquid to gas; this is just the effect of changes in pressure. Cool huh?
Guess what? You have both of these things (Phase change & Charles’ Law) working against you with propane tank freeze-up. So why is this such a problem with raku and not with a stoneware kiln?
Raku generally uses 2 to 3 times more Btu’s per cubic foot than a stoneware kiln uses. Granted, a raku kiln is generally much smaller, but the tank many people chose to use with raku is HUGELY SMALLER (there’s a great oxymoron!,… but, you know what I mean). You have a 50 cubic foot kiln that uses 12,000 Btu’s per hour with a 500-gallon tank. No problem. Now, you have a 5 cubic foot raku kiln that uses 30,000 BTU’s with a 20 lb Bar-B-Que tank (4.5 gallons). Can you see the problem? You are simply trying to take too much out of the raku tank in relation to how much is there. The stoneware kiln’s tank has more than 100 times the fuel that your Bar-B-Que tank has. Yes, your total Btu’s per hour for the raku kiln is much smaller (150,000 Btu’s per hour for the raku kiln vs. 600,000 Btu’s per hour for the stoneware kiln, a factor of 4:1), but your tank size for the stoneware kiln is greater than 100:1. You see what the answer to the problem is… you need a bigger tank for your raku….that is unless you just want to keep drinking whiskey for your bad tooth.
Next issue I’ll tell you about another variable in our tank symptom/problem. That’s the added affect of outside temperature…. Yea, I know, I have to use a lot of cheap literary techniques like the “tease” to keep people reading about stuff that involves math. ‘Till next issue…..
This issue’s article is a continuation of last issue’s subject. Last time I wrote about what was happening inside a propane tank (its really a cylinder, but we already know that from last issue). Pressure and phase change were the brunt of the column… this time I’m going to talk about temperature and its affect on ‘ole C3H8 (that’s chemistry speak for propane).
At normal ambient temperatures, propane is a gas. Like water, it rapidly becomes a gas at a certain temperature. With water, that temperature is 212°F. With propane, it quickly becomes a gas above its boiling point. The thing is, propane has a boiling point that’s really low. How low? How about –44°F. Because propane is in a sealed container, it is forced to stay a liquid because of the pressure in the tank. As we get closer to the boiling point of propane, (that really cold –44°F I just spoke about) it takes less and less pressure to keep the propane in a liquid state. When that tank valve is closed the propane will seek a state where the pressure builds and builds until the propane stops boiling and turning into gas, which in turn stops the rise in pressure. So, the colder it is outside the less pressure needs to build up to stop the boiling. This can have profound effects.
The colder it is outside, the less boiling and accompanying pressure is needed to hold our liquid state. The bottom line: the colder it is, the less pressure in the tank. The less pressure in the tank, the less gas you can extract. Let me give you some examples of what outside temperature can do to the internal pressure of a propane tank. Remember, propane could care less what size tank its in… it will still behave the same whether it’s a Bar-B-Que tank or a small submarine sized tank.
We have a traveling propane tank vacationing in the desert sun in Arizona. It might very well have 200 psi (pounds per square inch) of pressure. Take the same tank and follow it to visit me in Tennessee in the fall. Its 50°F outside. Our traveling tank now has 77 psi. Nothing has changed except our outside temperature. Our traveling tank now wants to check out the beauty of Maine in the winter. It’s 0°F outside and our tank is mellowing out… it’s feeling a whole lot less pressure… about 23 psi. If it were to venture to Barrow, Alaska and the outside temperature was –30°F our tank, regardless of size and amount of propane inside, would have only about 5 psi of pressure. Now it’s not mellow, it’s closer to dead.
You can see what happens as things get colder. You get less pressure. And, with this drop off in pressure, you automatically can get less gas out of the tank. Couple this with the phase change dynamics and Charles’ Law that I regaled you with last issue and you can see that we are walking a fine line. Another thing that becomes evident from our traveling tank is the need for a regulator. A regulator does just as its name implies… it regulates the pressure to keep it constant. But, a regulator can only do so much. All it can do is bring down higher, fluctuating pressures to a constant lower pressure. It can’t make pressures higher. This is one of the problems with weed burners that run straight off tank pressure. Their output is dependent on the outside temperature, pressure drops in the tank, and the chilling effect of Charles’ Law. That weed burner that is rated at 500,000 Btu’s per hour is rated at that if you are in the desert in summer just starting with a full tank. Otherwise, it’s a crapshoot as to what you are getting (other than a way to burn weeds). This would be the same as driving your car without an accelerator. You have no control over how fast your car goes. The hotter it is, the faster your car goes, the longer you drive, the colder the gas tank gets and you start slowing down. Stop… let the tank warm back up and you can go fast again… for a while, but you start slowing down quicker. In the winter, you can never drive fast and sometimes the car just stalls out.
So, what’s the answer? You need to size your propane supply to take in all these variables. To give you an idea, we’ll take a 500-gallon tank: It has certain maximum Btu outputs based on how full it is (its ability to boil and make gas) and the outside temperature. At 0°F a 500 gallon tank that is 60% full can produce about 425,000 Btu’s of gas. As the gas level falls it starts to lose gas production capacity, but temperature plays a more crucial role. At 20°F, our same 60% full tank can produce 850,000 Btu’s of gas. This temperature thing is a big deal! After all this you should know one thing… if your tank freezes up, you need a bigger tank for the prevailing circumstances. Most of you will be surprised to read the following warning, but I’ve heard and seen things that are damn scary. Here’s the warning: NEVER try to heat a propane tank with anything that could provide a source of ignition. No car exhausts (ignition backfires). No hair dyers or heat guns (electrical motor sparking). My God, I know of people that have built wood fires under their LP tanks. This is courting inclusion in the Darwin Awards. I’ll end this with a fun new word for your word games… BLEVE. This is the acronym for “boiling liquid, expanding vapor explosion” I guarantee you never want to be around one. It is what can happen if a propane tank explodes from a leak and a source of ignition. Stop and think. Be smart. Have fun. Talk to you next issue.