Is Fluid Bed Roasting Just Hot Air?

Aside from the fluid bed roaster designs that have been around for a number of years, some roaster manufacturers recently have been adding machines that are fluid beds or variations of drum machines, with increased airflow. Anyone looking at new roasting equipment might want to consider fluid bed technology.

What is Fluid Bed Roasting?

Michael Sivetz patented fluid bed roasting machines in 1976 with his motto, "Keep beans moving." To understand fluid bed roasting, you need a few engineering basics.

A mass of distinct individual parts can be handled as a fluid within the right environment. This is referred to as fluid bed mechanics. Individual coffee beans within a fluid bed become a fluid mass when each is lifted and rolled in a stream of hot air.

A fluid bed roasting machine is an enclosed metal box or chamber with a vented screen over a restrictive air entrance, a blower, and a combustion chamber beneath or to the side for heat. The overall design balances air, bean load, and temperature, so that the flow of hot air will levitate green and roasted beans. The size of the roast chamber depends upon the bean mass, air, and temperature requirements. A specific amount of hot air raises the greens from room temperatures through roast temperatures. Hot air comes from cast-iron burners with hundreds of thousands of BTUs, or from electric elements in smaller unites. In one 30-pound model, temperatures are calibrated to remain below 600 degrees F. Air flow is handled by a three-horsepower motor and an industrial-pressure blower over which the intake and roast chamber are mounted.

A larger version is the half-bag model. The roast chamber stands above a massive pressure blower and seven-horsepower motor. There are two large burners and 300,000 BTUs of hot air.

In the larger fluid bed designs, roast times are between eight and twelve minutes, depending on degree of roast. Cooling times are less than five minutes.

Not all fluid bed roasters are large capacity. In the past few years, manufacturers, at the request of coffee retailers, have been building fluid bed machines of a size that is appropriate for in-store roasting.

A working example is the eight-pound model at the Alaska Coffee Company in Anchorage, Alaska. Robert Lewis (co-owner of Alaska Coffee and specialty coffee columnist for World Coffee & Tea) added the machine one year ago to the existing retail store and coffee bar. He wanted to increase freshness by roasting inhouse daily, control his own green bean and roast inventory, and create a decisive quality and exclusivity niche in a market where there are five regional roasters already and a population base of only 250,000!

The roaster sits counter high in a corner at the front end of the retail shop, behind the bar. It occupies six square feet and exhausts through a small afterburner, also within the store, at the ceiling. It roasts, empties, and cools automatically with magnificent, uniform, and aromatic roasts in eight to twelve minutes per roast.

"The beauty and intrigue of the machine as it is roasting is that the processes seen by our regular retail customers when the enter our shop," says Lewis. "We have used the system to do demo roasting in order to pick up new wholesale customers, too. We do regular in-store cupping of fresh roasts which spark sales on the spot. Because roasts are small, we turn inventory, offer fresh roasts, and are able to expand our green selections to include very high-end and exclusive estate coffees."

Roasting with a Fluid Bed Roast

As with drum roasters, green beans go into fluid bed machines from above the front. Because the roast process depends upon convection, the roast chamber preheats quickly. Fluid bed systems need correct amounts of green beans put into machines for each roast. There is a designed balance among weight, air flow, roasting temperature, and roast time. The stated capacity of each size of machine is not variable by much, or the balance is defeated.

If the load is too little, the bean mass is so light that it degenerated as air flow increases and bean movement becomes chaotic. The roast goes so long that the beans are grossly uneven. If the amount is too large, the mass is so heavy that upward air pressure from the blower and combustion is overcome. The bottom layer cooks rapidly, without air coming in to spread heat through the mass. The result is charred beans.

The interior of a Sivetz fluid bed machine is seen through a glass window, a port in the front wall. When the machine starts, you can watch the bean mass develop its roll as the fan-driven air comes up to volume and temperature. An electronic ignition starts the burner, once the blower begins creating sufficient air flow.

During start-up, temperature climbs slowly as the green mass gets lighter, starts rolling, and water comes off the beans. The mass changes color near 400 degrees F in the first four to six minutes. Temperature within the bean mass is shown on the control panel; a thermal probe extends into the roast chamber at the base of the bean flow. The indicator from the probe reflects temperature in the bean mass. Another probe above the heat chamber and blower reads the incoming air temperature. From start-up to the point at which beans lose water is the extended period of the roast, roughly six minutes.

The beans roll up and bounce off the rounded top of the roaster. Organic compounds in the beans become volatile. Fragrances flash out subtly; only the roast master knows of their short, flowery existence. Sweeter aromas track increasing temperatures as sugars caramelize.

The time from beginning of pyrolysis (the start of the deepest chemical change within the beans) until the end of the roast is shorter, between the last eight and twelve minutes. Bean volume increases rapidly. The roast chamber fills up. Bits of silverskin freed from bean creases sweep into a vortex of hot air and out the exhaust.

The roast master can control the roast by setting the terminal or end temperature. A setting of 455 degrees F produces a light roast that is bright and acidic with a sweet cup and little aftertaste. For a dark roast, at 470 degrees F, the beans become dark, lustrous brown-black, with acrid pungency; they have a penetrating aftertaste and an oily finish.

From Pyrolysis to Finished Roast

In fluid bed machines, the balance among air volume, temperature, and bean weight creates the fluid bed, and it sets limits for roasting to the desired temperature.

Roasting goes only to the selected temperature. Once the terminal temperature is set, there is little to do during roasting. When the combustion chamber shuts down, a water spray at the top of the roast chamber automatically goes on for a set time, thirty seconds for a medium roast. The pressure blower continues running, pulling room-temperature air into the roaster. The air and bean probes indicate the temperature drop from roasting to cooling.

Roasted beans are unloaded by opening a slide gate at the front of the roast chamber. The beans continue cooling in the roast chamber or in a cooling bin outside.

Roasting follows a discernible pattern. Roasts of one type of bean finish within tenths of a minute of each other. This is a good measure of the consistency of roasting. It also reveals the degree of uniformity of the green beans going into the roast.

The Technical Significance of Fluid Bed: Similarity, Uniformity, Balance

With the right amount of green beans, roasts at specific temperatures will have similarity, roast after roast. Each individual roast is uniform among the beans. Beans varying in grade or moisture content, or with defects, will leave the mass of the fluid bed and become variations in the end roast.

Fluid bed roasting depends on the unique balance among air movement, air temperature, and the rising bean temperature that triggers pyrolysis, thus finishing the roast. It can be explained in terms of the film resistance of the bean surface to heat. The bean's surface resistance gets penetrated at roughly 400 degrees F in each bean. This is the crucial range for pyrolysis and the bean's first popping.

Incoming air in fluid bed machines blows the greens gently in an increasingly hot air stream. Moisture, chaff, debris, and porous, weightless, or defective beans are lifted out and into the chaff can. The balance of increasing air temperature and decreasing internal bean resistance is reached uniformly throughout the mass.

The roast master reads development by sensing the change in aromatics, and watching the roast time and temperature. The roaster stops itself with air temperature, boosted by the exothermic development of the roasting beans, reaches the set roast temperature.

How Fluid Bed Roasting Compares to Drum Roasting

In fluid bed roasters, the temperature, air flow, and bean weights are set before roasting. It is not easy to adjust these during a roast. Fluid bed technology puffs and gases beans with surrounding heat -- with good greens going in, movement of the mass, air flow, and balance combat tipping and facing. Aromatics, color change, and the appearance of bean development in the roast chamber become the roasting benchmarks and are used to select and end temperature for roasts in fluid bed machines.

In drum roasters, heat and air flow often are designed to be varied. Sampling tubes are used to look for bean development during roasting. Corrections can be made during the roast, or beans can be dumped to coast to a finished degree in the cooling bin.

Fluid bed machines are designed so corrections are not made during the roast. before roasting, the fluid bed roast master uses art and judgment to choose greens that are uniform in size, grade, freedom from defects, sweetness, freshness, appropriate moisture, and cleanliness. The roast master needs to assess the green beans, then monitor the finished roasts for uniformity and similarity, selecting end temperatures to heighten the qualities of coffees of individual origins and blends.

Fluid Bed Maintenance and Safety

Though fluid bed roasters have no moving drum, there are motors, belts or drives, bearings exhaust pipes and roast chamber surfaces which might need servicing or cleaning. Service can be done at set intervals, depending on the amount of production or as time, temperature, and air flow changes become apparent. Afterburners with screens and burner orifices need the same attention.

Fluid bed roasters move air and debris out of roast chambers very effectively, but residue cools and collects in the exhaust route, restricting air flow and causing fire hazards. In some fluid bed machines, the water quench can be started by hand if there is a chamber, chaff can, or exhaust pipe fire. Fire in fluid bed machines is also possible if power to the main blower is interrupted of a motor fails when roasts are dark and hot. With good maintenance, fires in fluid bed machines are not likely. Yet, there is always the necessity for the roast master to be alert for accidental changes in roast operation.

Fluid Bed Technology is not Just Hot Air

The contributions of fluid bed technology to roasting are substantial. Fluid be roasters are suited to specialty roasting; they are practical and easily grasped, and employed to produce thoroughly roasted beans with developed aromas, predictable uniformity, and similarity.

© 1999 by John Gant