Randy G. wrote:I am thinking that the mass of metal of a commercial drum roaster supplies its heat from the metal of the roaster itself so the heat energy from the gas flame has to be turned down a bit during the roast. heck- the front cover of a small Probat weighs more than ten Hottops! In the home roaster the heating element has to be turned up (or more accurately, left on) during most of the roast because there is not enough mass of metal to store any heat. Active vs. passive if you like...
In either case, if the temperature in the drum is decreased (or more accurately, if the temperature of the beans is allowed to drop) during the roast, the coffee will suffer. The same can be said if you tease the roast (bring the temperature up too slowly). I learned that first hand (Thanks, Barry). Balsa wood flavored coffee...Yuchhh!
In a large pro, gas-fired commercial roaster, if you leave the heat on all through the roast you burn or tip the beans. if you turn the heat down in a small home roaster you stall the roast and end up with bad coffee. . . .
First caveat: I've noticed that folks around here can demand a high level of accuracy and get very "technical" about the central and side issues. The explanation is pretty much "Physics for Poets," with all its limitations. Not only am I not Greg Scace, but I'm neither a physicist nor a poet.
Second caveat: Heat is transferred in (basically) three ways: a) conduction; b) convection; and c) radiance. In the culinary world, as in most practical applications, some mixture is involved.
The heat the bean sees in a drum roaster is almost entirely conductive. The exterior of the drum is heated, and the heat moves through the drum material itself (mostly via
free electron diffusion). Remember there's more to a drum roaster than just the drum itself, the energy going into and out of the drum is effected by other parts of the system to a respectively greater or lesser extent. But because heat transference to the beans is (largely) conduction, and the instrument of transference is the drum -- the drum is the most important part of the system from the beans' perspective. The drum exists in an operating environment including not only the heat source but the rest of the system, and the modalities of the drum's heat gain and loss is more system dependent than the beans'.
The drum in a commercial drum roaster has relatively high thermal mass compared to the thermal mass of the beans being roasted. Besides the drum, other parts of a commercial roaster also have relatively high thermal masses contributing to the system via any and all of the transference modes. Consequently when the beans enter the drum, the drum roaster doesn't give up a high proportion of its stored thermal energy. The roaster can effectively "coast" on previously banked energy. Compared to the amount of energy already stored, little additional energy is required to maintain the drum's temperature.
Think of "thermal mass" as just plain mass and temperature change as motion. Higher masses have higher momentum at a given velocity, but also greater inertia. Similarly greater thermal mass is better able to maintain temperature but less able to change temperature. Commercial roasters are built for batch operation, so high thermal mass chassis and drums are efficient. However, these roasters respond relatively slowly to additional heat unless supplied in relatively large quantities. Returning to the drum, it also takes a while for heat energy applied to the outside of the drum to make its way to the interior.
The Hottop's drum and chassis have a much lower thermal mass relative to the mass of the beans. Consequently when the beans enter the drum, they absorb a relatively higher proportion of the energy stored in the system during the preheat period. During the roasting process, the exterior of the drum requires constant heating in order to replace energy the beans absorb through conduction and the system loses by radiance. (Why do you think they call it Hottop?) More energy still is needed if the roast master wants to raise the drum's temperature. Because the system is so conductive and loses such a high percentage of its energy to the surrounding environment (under normal ambient conditions), heat has to be pumped into the system more or less constantly. The Hottop is a lot more agile than a commercial style roaster in how quickly drum temperature responds to relatively slight increases of new energy.
It might or might not to be of interest to consider that the electric elements in the Hottop transfer heat to the drum almost entirely by radiance, while the gas burner in a commercial roaster transfers heat through all three modes.
No matter how you slice it, the energy supplied to the coffee in either drum, as well as to each drum itself, is both "positive" and "active." The operant principles are the "Zeroth" and Second Laws of Thermodynamics.
Does this help at all?
Rich
Other than the excellent cappuccino, that is.. 
) Take, Fourier's law, describing contact heat conductance: q=-kA (dT/dx) where q is the heat flow, k is the thermal conductivity, A is the cross sectional area and dT / dx is the temperature gradient in the direction of flow. Now check your ocular movement. See what I mean.
