What makes a pipe smoke great? This question mark has been haunting me for quite a few years ever since I became a pipe smoker starting with Stanwell, Peterson, Castello and Dunhill pipes, through late days with mostly high end pipes made by just a handful of carvers in the world. I have tried most of the available pipe brands and carvers' pipes. All of them may smoke good or sometimes not, or on the other hand occasionally good but mostly not. Indeed, there are thousands of smoking habits (e. g. tobacco choice, packing method, puffing pace, to name a few) and theories as it comes to pipes. But we can still list a few things in common that we expect from a good pipe, for instance: stay lit, taste good, easy draw, burns cool, no gurgles, and so on. I truly admire the high-end pipe makers not just for the beautiful outlook of their pipes; they do surpass the other ones in every aspects in terms of smoking quality by far. What's the secret hidden in their workrooms?
As a former physicist I could not resist the temptation of trying to understand the mechanism behind this "mysterious" topic. I once tried to code something aerodynamic to simulate the airflow in pipes. However as I went deeper into technical details I found it so labor intensive. It was like doing a ph. D. so I had to give up the simulation idea. Then I tried to make other approaches but could not find a way into it, until recently I threw away everything but the very basic and general facts in gas and fluid dynamics. So here comes this small article addressing these facts, how they apply to the airflow in pipes and finally the role bowl shape plays in the smoking quality of a pipe.
Let's start with something familiar. The picture below depicts something known to most of us. This diagram roughly shows the temperature distribution in a smoldering pipe. Anyway we would not partition the bowl into the burning, combustion and the condensation zones here. We just say that the temperature is not constant in the whole chamber. Now it is time to talk about the two facts that play essential roles in pipe engineering. They are both directly related to the temperature of the smoke.
Fact one, for the same gas, the higher the temperature, the higher the viscosity is. Or in simple words, hotter gas is more difficult to flow. The figure below shows the viscosity of air versus temperature at standard atmosphere. As we may see that at 600 ℃ which is the typical temperature of the hottest part in the chamber, the viscosity is significantly higher than the stem end and by the chamber wall. You may want to prove this by puffing into a smoldering pipe. In most cases the smoke would rise from a region close to the wall, or even a smoke ring by coincidence. Please be noted that gas is on the contrary to liquid (hotter liquid flows easier).
Fact two, hot air contains more moisture. And at standard atmosphere the amount of moisture air can carry drops more than half as the temperature drops by 20℃.
And immediately we may realize that the shape/size of the bowl and the specific spot the airway connects to it may determine the airflow in the chamber and the quality of smoke that being drawn into the airway. That is shown in the figure below.
Two extreme cases are shown in the diagrams: a narrow bottom V-shaped bowl (left panel) and a flat bottom square bowl (right panel). Since airflow is always easier through cooler tobacco, the air drawn into the chamber would prefer to go through the cooler part, which is, the space surrounds the ember. In the first case, the ember is tightly confined by the wall thus less cooler space is available. The air flow through the ember is stronger than the latter case. The airflow is shown in the figure by black arrows with the size in correspondence with the flow intensity. This would result in the promotion of combustion and a hotter smoke. Particularly, as the tobacco burned over half way, this effect would be overwhelming as there is very little space surrounding the ember. The smoke may get hot and harsh. All the rich flavors to be released at cooler burning conditions would be gone.
While for the latter case, there is ample space surrounding the ember. Therefore the air would tend to skirt the central hot spot and flows easily along the wall and the coolest bottom edge of the chamber, as shown by the hollow arrows (So no worries about uneven burning here. I have experienced quite a few badly designed V-bowl pipes burning uneven, but never a wide bottom had such problem. The air flows by nature laws, not gut guess.) As a result the ember is less pushed downward by the airflow and the burning would be slower and cooler, and the smoke enters the airway would of course be cooler too, but maybe less flavorful due to very little airflow through the combustion.
And we may go a bit further along the smoke temperature line if we take the two facts mentioned above into consideration. Obviously the smoke in the first case is hotter (and the bowl engineering may push the ember even bigger). So, a hotter smoke, more difficult airflow, more moisture it carries ... Well, a compatible wider airway might be desired here. The reason is two-fold: easier airflow for the higher viscosity and more briar surface absorption for the moisture. As for the latter case, the cooler and drier smoke, a smaller drill size would be enough. The only attention needed here is the bowl size; it shall remain small to maintain a good smoke intensity. If you see a late Jørn Micke pipe, you will know what I am talking about.
An ideal pipe design would be individually quite different, but in general it should be something between the two. The airflow into the ember and the surroundings are well balanced. And the smoke drawn out should be as cool as possible. Remember, the bowl shape and the airway position constitute the selecting philosophy that determines the qualities of the smoke we draw into the airway. Then the airway engineering shall be compatible with the provided smoke. My personal taste is always strongly against the V-bowl engineering. I like the bowl to be small but wide enough at the bottom. In his book Rick Newcombe wrote about a criterion of telling if a pipe would smoke good or not. It was called the "four square test". Inserting a pipe cleaner with white/blue (or white/red) bars from the stem end. If at least four squares of the pipe cleaner can be seen in the tobacco chamber the pipe would probably smoke great. I think this is another version of "ample space at the bottom". And besides this, I would add another advice - do not drill the airway right into the bottom center of the bowl, make it further away in the corner to collect cooler smoke. Well, this is my two cents on some tiny aspects in the engineering department. Happy puffs!
As a former physicist I could not resist the temptation of trying to understand the mechanism behind this "mysterious" topic. I once tried to code something aerodynamic to simulate the airflow in pipes. However as I went deeper into technical details I found it so labor intensive. It was like doing a ph. D. so I had to give up the simulation idea. Then I tried to make other approaches but could not find a way into it, until recently I threw away everything but the very basic and general facts in gas and fluid dynamics. So here comes this small article addressing these facts, how they apply to the airflow in pipes and finally the role bowl shape plays in the smoking quality of a pipe.
Let's start with something familiar. The picture below depicts something known to most of us. This diagram roughly shows the temperature distribution in a smoldering pipe. Anyway we would not partition the bowl into the burning, combustion and the condensation zones here. We just say that the temperature is not constant in the whole chamber. Now it is time to talk about the two facts that play essential roles in pipe engineering. They are both directly related to the temperature of the smoke.
Fact one, for the same gas, the higher the temperature, the higher the viscosity is. Or in simple words, hotter gas is more difficult to flow. The figure below shows the viscosity of air versus temperature at standard atmosphere. As we may see that at 600 ℃ which is the typical temperature of the hottest part in the chamber, the viscosity is significantly higher than the stem end and by the chamber wall. You may want to prove this by puffing into a smoldering pipe. In most cases the smoke would rise from a region close to the wall, or even a smoke ring by coincidence. Please be noted that gas is on the contrary to liquid (hotter liquid flows easier).
Fact two, hot air contains more moisture. And at standard atmosphere the amount of moisture air can carry drops more than half as the temperature drops by 20℃.
And immediately we may realize that the shape/size of the bowl and the specific spot the airway connects to it may determine the airflow in the chamber and the quality of smoke that being drawn into the airway. That is shown in the figure below.
Two extreme cases are shown in the diagrams: a narrow bottom V-shaped bowl (left panel) and a flat bottom square bowl (right panel). Since airflow is always easier through cooler tobacco, the air drawn into the chamber would prefer to go through the cooler part, which is, the space surrounds the ember. In the first case, the ember is tightly confined by the wall thus less cooler space is available. The air flow through the ember is stronger than the latter case. The airflow is shown in the figure by black arrows with the size in correspondence with the flow intensity. This would result in the promotion of combustion and a hotter smoke. Particularly, as the tobacco burned over half way, this effect would be overwhelming as there is very little space surrounding the ember. The smoke may get hot and harsh. All the rich flavors to be released at cooler burning conditions would be gone.
While for the latter case, there is ample space surrounding the ember. Therefore the air would tend to skirt the central hot spot and flows easily along the wall and the coolest bottom edge of the chamber, as shown by the hollow arrows (So no worries about uneven burning here. I have experienced quite a few badly designed V-bowl pipes burning uneven, but never a wide bottom had such problem. The air flows by nature laws, not gut guess.) As a result the ember is less pushed downward by the airflow and the burning would be slower and cooler, and the smoke enters the airway would of course be cooler too, but maybe less flavorful due to very little airflow through the combustion.
And we may go a bit further along the smoke temperature line if we take the two facts mentioned above into consideration. Obviously the smoke in the first case is hotter (and the bowl engineering may push the ember even bigger). So, a hotter smoke, more difficult airflow, more moisture it carries ... Well, a compatible wider airway might be desired here. The reason is two-fold: easier airflow for the higher viscosity and more briar surface absorption for the moisture. As for the latter case, the cooler and drier smoke, a smaller drill size would be enough. The only attention needed here is the bowl size; it shall remain small to maintain a good smoke intensity. If you see a late Jørn Micke pipe, you will know what I am talking about.
An ideal pipe design would be individually quite different, but in general it should be something between the two. The airflow into the ember and the surroundings are well balanced. And the smoke drawn out should be as cool as possible. Remember, the bowl shape and the airway position constitute the selecting philosophy that determines the qualities of the smoke we draw into the airway. Then the airway engineering shall be compatible with the provided smoke. My personal taste is always strongly against the V-bowl engineering. I like the bowl to be small but wide enough at the bottom. In his book Rick Newcombe wrote about a criterion of telling if a pipe would smoke good or not. It was called the "four square test". Inserting a pipe cleaner with white/blue (or white/red) bars from the stem end. If at least four squares of the pipe cleaner can be seen in the tobacco chamber the pipe would probably smoke great. I think this is another version of "ample space at the bottom". And besides this, I would add another advice - do not drill the airway right into the bottom center of the bowl, make it further away in the corner to collect cooler smoke. Well, this is my two cents on some tiny aspects in the engineering department. Happy puffs!