Fluid bed Roasting

Fluid-bed Roasting – Sivetz Type – Profiling the roast

In the 1960’s Michael Sivetz a Chemical engineer, who worked in the Polyurethane

industry invented Fluid-bed roasting. If one knows a bit about the History of the

Roaster one would know they were built in a church. If one had spoken to Michael

one would know how seriously he took this industry and how much work, he did to

design these incredible machines. If one read his book one would know he was a

Genius. One would also know that he loved coffee with a black heart.

Fluid-bed roasting is the process whereby superheated air is blown into a specifically

designed chamber, to roast coffee. Basically, hot air does all the work. More on his

later.

This invention saw quick uptake as it offered some serious advantages in an

immature / unrefined industry. Fluid-bed roasting was much more productive, this

equipment was simpler to use and had fewer moving parts, which also made it

simpler to maintain.

Fluid-bed roasting saw a massive uptake in the 70’s into the early 80’s. I joined the

industry the late 90’s in its flavoured, syrupy adolescence, by this time, the Fluid-bed

roasting process was falling out of favour as the speciality coffee industry grew and

this roasting process remained unrefined. Its clear advantage in design, cost and

capacity did not speak to the enormous focus on quality and the roasting process

(managing ROR -Rate of Rise and profile roasting) Not enough smaller roasters

were introduced to the technology and almost none of “these” smaller roasters had

the inhouse capacity to modify and modulate the Fluid-bed roasting process. Most of

the talent entering the speciality industry was only exposed to the Drum roasting

process and most of these spent all their time and effort developing their skillset on a

Drum Roaster. Competitors drove each other to improve the equipment used. This

part of industry saw an enormous improvement in the control of the roasting process.

Drum roasters moved from single fan, single flame units to multi-fan, multi-flame

units with airflow control and profiling systems. In this time very little to no

improvement was done to the Sivetz type Fluid-bed roaster. Drum manufacturers

simply ignored the technology. It became apparent that fluid-bed roasting could

deliver a good product, but it could not be refined or managed to deliver something

truly special.

This is worth relooking. A well modified Sivetz Type Roaster can produce amazing

coffees. Somethings one can do with a Sivetz simply can’t be done on a Drum

Roaster. A good technical background and an understanding of the roasting process

can overcome or work around the inherent design challenges. Just like a drum

roaster a Sivetz can be modified and “tamed”.

The clear difference between the two roasting methods is the method of heat

application. Drum Roasters use conductive heat (from the steel drum), radiative heat

(from the drum, end plate and steel agitators) and convective heat (from the hot air

being sucked through the drum from the burners) Fluid-bed roasters only use

convective heat – the only component being heated is the air passing through the

 

specially designed chamber, this super-heated air then heats the beans and

because of the force of the movement keeps the beans agitated in a “fluid state”.

NB – It is important to note that about 60% of the heat in a drum roaster is

convective, depending on the roaster. So, to a very limited degree all roasters are Air

Roasters…

The Fluid-bed Sivetz type roaster was the first to really attempt to managed and

focus this convective energy. It has only one moving part, a ferro-bronze high-speed

high-temperature fan.

What really has happened in the roasting fraternity is that we have associated

convection with average roasting results. A lot of smaller fluid-bed roasters, in

particular “home” roasters, are just plain bad roasters as they can’t transfer enough

heat via the air to the beans or they simply can’t move the coffee fast enough. This

has nothing to do with the technology. It is simply a failure to apply adequate design

to the system.

Fluid-bed Roasters may not be every roasters favourite, but one can’t argue that

they can produce awesome coffee.

The Problems

All Fluid-bed roasters use the flow of air to both agitate and heat the coffee beans.

This air flow must carry enough energy to lift and agitate these beans. If this is not

the case, you will create all sorts of havoc in the roasting chamber. This air must also

have enough heat to transfer to the beans and constantly raise this temperature. If

there isn’t enough of a differential in the temperature of the air and the temperature

of the beans you will not have a good rate of rise nor will one have any room to

managed this.

At some point, the difference between the air and bean temperature could become

so small that the rate of rise could be close to zero. At this point your roast has

stalled and you are no longer roasting coffee.

The variables one can use to manage the roasting process are; the temperature of

the air going into the chamber, the speed of the air and the amount of backpressure

in the system. All three of these are incredibly entwined and change as one or the

other changes. A fluid-bed roaster uses upward of 5 times of pressure in the system

compared to a drum roaster.

There are challenges due to the high pressures one is dealing with. The first problem

would be leaks, if the system has a leak, this would affect airflow and back pressure

which would affect the roasting process in a few ways.

Depending on where a leak in the system is, it would create smoke and reduce back

pressure, or it would be dissipating heat and pressure before the roasting chamber.

One cannot roast coffee in a Smokey environment. The loss of energy in the roasting

process due to a leak would clearly affect the product.

Controlling the roasting process

 

The biggest factor leading to poor results in any roaster is poor control.

If you want to profile roast coffee in a fluid-bed roaster one needs to be able to

control the variables. An adjustable butterfly valve on the exhaust system, speed

control on the fan motor and an adjustable flame through orifice size or adjustable

gas pressure allows one control of these variables. Even being able to control one of

these factors would improve control of the roasting process as these components

are so entwined.

(Thermodynamics 101 - Heat is not temperature. Temperature is how hot something

is and is measured in Celsius. Heat is how much energy is being added and is

measured in Calories.)

The amount of heat available to a batch of coffee depends on the speed and

temperature of the air flowing over the coffee.

As speed increases less heat is imparted to the air and as the flow of air is reduced

more heat is imparted to the air. Too little air and the beans stall - Havoc. Too much

air and the beans pass through the chamber into the chaff collector - Havoc.

As the same time, when flame size is increased more heat is available to the air

passing over the flame and as this flame is reduced less so. A flame too big could be

troublesome as there are inherent problems with the coffee “tipping” as the coffee is

roasted too fast and metal distorting with too much energy. A flame that isn’t big

enough would simply be snuffed by the air passing over it.

Back pressure can be used to finesse these two variables. In that a little adjustment

on this pressure would slow the air sufficiently to affect the available heat without

affecting the airflow enormously.

If one can control something one can measure it. Drill holes for temperature probes

measure the inlet air temperature measure the exhaust temperature, measure the

speed of the fan. Measure your gas pressure. Keep these probes clean.

Mismeasurement can cause some anxiety in the roasting process…

Measuring the bean temperature and the air temperature during the roast (every 30

seconds) lets one work out the rate of rise etc.

Once you have this, one can adjust all the variables to bring the roast under “control”

ensuring that the starting point, mid-point and end points are the same. Once the

roaster has been tamed one can play and finesse the variables with much the same

control as a profiling system on a profile roaster.

Simply adjusting the speed of the air by as little as 3% can make a 5 degree C

difference to the air temperature (depending on the setup), without stalling the roast

or the beans. This type of control is where true profile roasting starts.

 

The fun really starts when one starts playing with big flat beans, Peaberry’s, and

Long berry Harrah. It is amazing how these coffees are so different in a Sivetz type

Fluid-bed roaster.

In trying to make this article interesting, it might have become a little technical – light

reading for coffee tech-heads (My wife says nerd).

 

I learnt my trade on a Sivetz and have spent 20 years roasting on many versions of

the roaster. I have modified and played with these machines. I was lucky enough to

build a business that could afford my mistakes and happy enough to keep playing. I

have also worked on drum roasters from a basic single burner, through multi-burner,

multi-fan and profile roasters. I am still learning.

 

https://coffeetechniciansguild.org/blog/2018/4/10/fluid-bed-roasting-the-path-less-taken

 

Author Michael Bayer