As we move through this series on what defines a pipe, keep in mind that at this stage we are focusing more on the abstract geometry of the voids in the pipe (i.e. the tobacco chamber and airway). We will get to materials later. For the sake of our discussion, we will consider an idealized tobacco chamber with three potential variables: height, diameter, and shape. The generic chamber shown below would clearly make an odd pipe, but it serves to allow us a starting point to discuss the impact of these variables. It is notable that the placement of the draft (AKA draught) hole is rather unusual in these diagrams. Typically the ideal placement of the draft hole is on the rear wall dead center
and at the level of the chamber bottom. For simplicity we have placed the hole dead center at the bottom of the chamber. This is a reasonable simplification for the purposes of conveying the key concepts. We could deform these examples in a way that would place the draft hole in it’s more traditional position, but this would only lead to a loss of symmetry. On average, the airflow would be identical, just harder to explain.
The Shape of the Chamber Bottom
Keeping in mind that the first pipes were holes in the ground, it is obvious that just about any shape tobacco chamber will smoke. The question is what are the properties that will make it smoke well? In my opinion, the shape of the bottom of the chamber is the critical variable that separates the good from the bad designs. Starting with the generic chamber, let’s look at how the air would move through the chamber as the smoker puffs.
The point of highest velocity will always be right at the entrance of the draft hole. This is because the cross-sectional area of the chamber is so much larger than that of the airway. This is a consequence of something called the principal of mass continuity, but we can just think of it as what goes in (the tobacco chamber) must come out (the airway). Therefore, the airflow is downward and accelerates as it approached the draft hole. This leads to areas around the bottom edges of this generic chamber that simply do not have air drawn through them. At best, there may be some circular flows or eddy currents that develop in the corners. This leads to at least two significant problems with this design. It will be impossible to smoke the tobacco to the bottom of the bowl since these dead zones will always contain unburnt tobacco. And, the dead zones provide a place for moisture to accumulate leading to gurgles and an unpleasantly wet smoke.
Of course, very few flat bottom tobacco chambers exist in reality. This is likely because pipe makers have evolved the curved bottom chamber over time to avoid the above issues. The curved chamber bottom allows the flow to reach all parts of the chamber promoting the proper and complete combustion of the tobacco.
Curving the bottom of the chamber is a a relatively small change, but the resulting smooth airflow through the chamber has a significant impact on the smoking experience. In reality, this small feature is the most critical detail in how the chamber performs. And what that really means is that the geometry of the chamber is not all that critical as long as the bottom is well shaped.
The other sort of tobacco chambers that are somewhat common are the conical chambers popular on danish style pipes, and Dublins. This chamber design should, based on the excellent airflow pattern shown below, provide a wonderful smoking experience. If that is the case, why do so many people have trouble with conical chambers? Smokers report that conical chambers are difficult to keep lit, tend to “plug” and restrict the draw, smoke wet, can’t be smoked to the bottom of the bowl, produce uneven cake, and are difficult to ream. The last point is really the only valid criticism of the conical bowl.
The main problem that people have with conical bowls is one of technique. When packing the tobacco, it is very easy to over pack this type of chamber. The chamber geometry creates a very effective wedging action that multiplies the force applied at the top of the bowl to produce a dense pack at the bottom of the bowl. This is usually a problem of packing, but can also occur when tamping. The solution is to use a light pack method such as the Frank method, the air pocket method, or just a lighter touch. When properly packed, a conical bowl will smoke beautifully.
The second variable to consider is chamber height (or depth depending on your perspective). On average, the bowl depth will be approximately one 1.5 inch. Less on pots, more on stacks, but on average ~1.5”. This variable does not, in my experience, have a great impact on smoking quality as long as it is not taken to extremes. Larger values will have the effect of increasing the bowl volume in a linear fashion that can be reasonably calculated using the well known formula for the volume of a cylinder.
This increase in chamber volume leads to a decrease in air moving through the burning tobacco zone given the same puff. It is once again an example of “what goes in must come out.” Drawing one puff’s worth (a clearly subjective unit of measure) through a 0.5” tall chamber will move more air across the burning surface layer than it would if applied to a 1.5” tall chamber. You can think of this as being due to the volume that the chamber contributes to the puff. In the case of the taller chamber, more of the puff is occupied by air drawn from the tobacco below the burning layer, which has no impact on combustion. Therefor the taller chamber should smoke slower providing a longer and cooler smoke if all other things (diameter, chamber shape, and most importantly puffing cadence and volume) are kept constant.
The final variable is chamber diameter. Chamber diameters can vary quite a bit, but in general the diameter will be somewhere between 3/4 and 7/8 of an inch. Increasing the chamber diameter will also increase the chamber volume, so in a sense the impact of a larger diameter is similar to that of a taller chamber. However, increasing the chamber diameter produces a larger increase in volume (note the r2 term in the volume formula) and also increases the surface area of the tobacco that is burning (also by a factor involving r2). Therefore, the effect of a larger diameter chamber is more complex. It both slows the burn by increasing the volume of air that needs to be drawn through the chamber, but there is more tobacco burning. Decreasing the chamber diameter has the opposite effect with very small chamber diameters producing enough of a restriction in airflow and consequent increased air velocity. This can lead to a hotter burn across a smaller surface area.
Bringing it all Together
To understand the real world meaning of all the hypothetical geometry, we need to recall the last installment in this series where we examined the combustion process. The diagram below shows the charcoal like layer at the surface of the tobacco chamber, and the burning zone below that where fresh tobacco is undergoing combustion. There is a third zone below the burning tobacco where all sorts of interesting things are happening. It is analogous to the way that potpourri simmering on a stove can fill a room with fragrance, while the same herbs thrown directly on a fire will only produce acrid smoke. The heat from the combustion warms the unburnt leaf in this roasting zone causing it to release aromatic components. It also stoves the tobacco essentially roasting and caramelizing sugars. All of this has significant impact on the flavor arriving at the smokers pallet.
Tall chambers are often touted as being ideal for Virginia or Virginia perique blends. We now have all of the information needed to understand why this would be the case. These blends tend to be high in sugar and benefit from a slow sipping burn that caramelizes the sugars and release their sweetness. Tall chambers with moderate diameter are optimal for encouraging this sort of slow burn as long as the puffing cadence is kept at an appropriate rate.
Tall narrow diameter chambers tend to be a favorite of flake tobacco smokers. The narrowing of the chamber is the key variable here. Recall that the chamber diameter has a greater impact on total volume than the height. By narrowing the chamber, this design actually restricts airflow leading to an increase in air velocity. Therefore, the tall narrow design will tend to smoke hotter and can be puffed to a very high temperature. This is perfect to get the difficult to light flake tobacco burning. Of course, these pipes require careful puffing because they are quite easy to overheat.
Finally, squat broad chambers are often favored for burleys and english blends. These blends are not typically high in sugar and don’t benefit as much from the roasting process. However the slow burn of a broad chamber allows for a cool flavorful smoke from the large surface area of burning tobacco. And the short chamber allows for easy control of the burn rate with appropriate puffing cadence.
It is notable that we have been able to provide a theoretical reasoning for why smokers prefer a certain bowl type for a certain type of blend based solely on the abstract geometry of the bowl. We have yet to discuss airway design, stummel material, or overall shape. In the next installment we will look at airway design.