Why JUCA Sides Slope At 17 Degrees

When any fuel burns, a large quantity of hot effluent gas (smoke) is produced. In the case of wood, about six pounds or 72 SCF (Standard Cubic Feet) is produced for each pound of wood consumed. There is generally a good deal of "excess air" that also goes up the chimney that had nothing to do with the actual combustion. It is necessary to provide such excess air to improve the probability of combustion occurring for each molecule of fuel since there might not be a perfect matchup of oxygen molecules in the air with the exact number of fuel molecules. This excess air increases the smoke volume per pound to over 100 SCF. If 10 pounds of wood are burnt per hour, this represents a volume flow of 10 x 100 or 1000 SCFH or almost 20 SCFM (per minute). (In the case of traditional fireplaces, excess air can be 50 times the amount of air used by the fire, which sometimes sucks so much house air out to create NEGATIVE overall efficiencies! In the case of air-tight woodstoves, excess air can be as low as 10%, which minimizes house air lost but also badly affects combustion efficiency, since each complicated molecule of fuel may never be near enough available oxygen molecules, and so could not complete the combustion process.

This mixture of hot smoke and air leaves the fire vicinity at high temperature. The actual flame temperature of wood can vary over a wide range (approximately 900°F to 2500°F). For our example, we will assume the mixture temperature as it leaves the immediate vicinity of the flame tips to be 1650°F. At this temperature each 'standard' cubic foot actually displaces four cubic feet because gases expand when they are heated according to the PV=nRT ideal gas law from chemistry. Therefore, we now have about 80 CFM of hot gases leaving the fire. These gases will tend to rise, accelerating at a rate dependent on the density differential between these "expanded" gases and the air outside the chimney that is at ambient temperature. Of course, many other factors can also affect the final smoke velocity but those are left for other papers in this series.

In nearly all the wood burning appliances on the market; in fact, in virtually all conventional fueled burners as well; the firebox cross-sectional area is constant as the smoke rises through the heat exchangers. This is not an ideal configuration and actually hurts the overall effectiveness of the heat exchange process. As the hot smoke passes each portion of the heat exchanger, it loses some of its heat and therefore tends to decrease in volume again in accordance with the perfect gas law. It takes up less space, and tends to "rattle around" in the heat exchanger, rather than consistently contacting the exchange surfaces.

It turns out that the atmospheric pressure of the smoke is affected when the heat exchanger smoke passageways don't reduce in area in an appropriate proportion. This anomaly of pressure causes unpredictable local accelerations and decelerations in the smoke velocity thereby making the heat exchange itself somewhat unpredictable.

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Another unfortunate effect of a constant-section exchanger is that as the smoke volume decreases it tends to fill only part of the upper exchanger smoke chambers. It may flow along one wall or another or it may pass right up the middle of the passageway bypassing some of the exchanger surface. If the latter condition results, then we experience a laminar flow near the exchanger surface with a resultant of a much reduced heat transfer coefficient. The desired condition of controlled turbulence for maximum coefficient will not regularly occur. Now, all this sounds simple enough to incorporate into existing designs. Unfortunately, in actual practice the geometry and aerodynamics of the situation are very complicated especially in light of the wide range of firing conditions present in domestic wood burners. The problem is more straightforward for a constant firing appliance such as a gas-fired furnace. It seems peculiar that the designers of such devices have not incorporated (to our knowledge) this into their designs. The improvement in overall efficiency would be substantial (several per cent).

For the design of the JUCA heat exchangers, a number of different firing conditions were assumed in the initial design analysis. In 1973 and 1974, analysis was done "by hand." When a computer became available in 1977, much more comprehensive analysis was possible. The results showed that for hot fires the JUCA configuration would produce a maximum effectiveness with an average wall angle of about 15.5 degrees. For a slow fire, the ideal angle came out to be 19 degrees. We use wall angles of about 17 degrees for various of the JUCA models to cover this range effectively. (Different models have slightly different wall angles due to our assumptions of consumer usage)

Since the walls slope in at this angle, as the smoke cools and contracts it continues to completely fill the heat exchange passageways thereby allowing predictable air flows and maximum heat transfer coefficients. The smoke tends to rise without unplanned accelerations and decelerations. The resultant local atmospheric pressures in different parts of the heat exchangers (except right at the tubular exchangers) is very constant for which we gave the name "isobaric equilibrium" to this characteristic of the exchanger.

In the vicinity of the tubular exchangers, we applied slightly different constraints in the design to ensure a certain level of turbulence in the smoke so there are localized areas of higher and lower pressures right near these exchangers. However, the general isobaric equilibrium concept still applies to the overall exchanger system.

You may note that the built-in versions of JUCAs do not have this principle incorporated into them. Their inner chambers are rather squarish. Early units did have tapered chambers, but we found that the owners of built-in fireplaces tended to want to see a more roaring fire. With the great total net efficiency of the JUCA unit, many owners felt that they were faced with the choice of building smaller fires or over-heating their houses. After numerous requests, we re-designed the built-in models to be a little less efficient and one way we accomplished this was by not applying parts of the isobaric equilibrium concept to the newer built-in models. Owners of the current models seem pleased with the balance of heating ability and fire appearance.

(By the way, we DO still have the patterns and capability of making the old style (F-3). It's structure and dimensions are based on the B-3D unit. Even though the visible part is the same size as the newer F-9A, the hidden body is almost two feet taller. The price is also based on the B-3D, so the F-3 is about \$500 higher than the newer style F-9A.)