Apparatus for heating an air stream

An apparatus for heating an air current, includes a heating coil being heated until it glows providing radiation in a given region, a device for conducting an air current past the heating coil, an electronic heating power control device connected to the heating coil, and a control sensor connected to the control device in the form of a light radiation sensitive circuit element disposed in the given region.

The invention relates to an apparatus for heating an air stream, especially 
for coffee roasting machines, wherein coffee beans are heated and thereby 
roasted by air flowing through, including a heating coil which is heated 
to the glow region, and an air stream conducted past the heating coil. 
Methods and devices for roasting coffee beans, especially small quantities 
of green coffee beans are known. In these conventional devices, such as is 
described in German Published, Non-Prosecuted Application DE-OS No. 32 17 
055, the coffee beans to be roasted in the roasting chamber are whirled up 
by a heated air stream, which is conducted through the roasting chamber 
vertically from the bottom up. The coffee beans are thereby heated and 
roasted. The aroma content and aroma development are to a great degree 
dependent on the degree to which the coffee beans have been roasted. In 
turn, the degree of roasting results from the roasting temperature and the 
duration of the roasting process. In order to achieve a constant and 
reproduceable roasting result, it is therefore necessary to provide for 
the exact determination of the roasting time and to provide for a constant 
temperature as well. The time control can be provided without difficulty 
by techniques which can be taker: from the existing state of the art. 
Temperature sensors would have to be used for holding the roasting 
temperature constant. Temperature sensors for suitably high temperatures 
and sufficiently accurate regulation characteristics are very costly. 
Their cost is disposed proportionately high for household roasting devices 
for coffee beans. 
It is accordingly an object of the invention to provide an apparatus for 
heating an air stream, which overcomes the hereinafore-mer:tioned 
disadvantages of the heretofore-known devices of this general type, and to 
provide an apparatus for roasting coffee beans, especially in small 
quantities, through the use of an available heated air stream, which has a 
low manufacturing cost and yet guarantees the required temperature 
constancy during the roasting operation. It therefore must be considered 
that the temperature of the environment, the appliance temperature, the 
supply voltage and the different filling states are also parameters which 
disturb the constancy of the roasting temperature. 
With the foregoing and other objects in view there is provided, in 
accordance with the invention, an apparatus for heating an air current, 
especially for coffee roasting machines, in which coffee beans are heated 
and roasted by an air flow, comprising a heating coil being heated until 
it glows providing emission or radiation in a given region, means for 
conducting an air current past the heating coil, an electronic heating 
power control device connected to the heating coil, and a control sensor 
connected to the control device in the form of a light radiation sensitive 
circuit element disposed in the given region. 
An apparatus constructed according to the invention fulfills the 
requirements mentioned above to an outstanding degree. 
In accordance with another feature of the invention, the lightsensitive 
circuit element is a light-dependent variable resistor (LDR). 
A light-dependent variable resistance (LDR) is available at advantageous 
prices. The current supply of the heating coil is regulated by this 
circuit element in dependence on the glow-image of the heating coil, such 
as by using full-wave control. The image of the glowing heating coil 
corresponds to the air temperature with sufficient accuracy. Even 
different filling quantities in the roasting chamber are taken into 
account, because the air flow rate depends on the degree to which the 
roasting chamber is filled, and this in turn influences the glow-image of 
the heating spiral. 
In accordance with a further feature of the invention, there are provided 
means for conducting the air current toward the heating coil, the 
light-sensitive circuit element being disposed in vicinity of the air 
current conducted toward the heating coil. 
In accordance with an added feature of the invention, there is provided a 
light-radiation permeable cap covering the light-sensitive circuit 
element. 
In accordance with an additional feature of the invention, there is 
provided a housing in which the heating coil and the light-sensitive 
circuit element are disposed, and an additional temperature sensor being 
connected to the control device and being disposed in the housing and 
outside the given region. 
In accordance with again another feature of the invention, the temperature 
sensor is a temperature dependent variable resistor (NTC). 
In accordance with again a further feature of the invention, the air 
current conducting means includes a heating channel in which the heating 
coil is disposed and a guide channel conducting the air current laterally 
into the heating channel, the guide channel including a step in vicinity 
of the heating coil distributing air uniformly in the heating channel 
In accordance with a concomitant feature of the invention, the heating 
power control device is in the form of a control using half-wave doses and 
a triac. 
Other features which are considered as characteristic for the invention are 
set forth in the appended claims 
Although the invention is illustrated and described herein as embodied in 
an apparatus for heating an air stream, it is nevertheless not intended to 
be limited to the details shown, since various modifications and 
structural changes may be made therein without departing from the spirit 
of the invention and within the scope and range of equivalents of the 
claims 
The construction and method of operation of the invention, however, 
together with additional objects and advantages thereof will be best 
understood from the following description of specific embodiments when 
read in connection with the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to the figures of the drawings in detail and first, 
particularly to FIG. 1 thereof, there is seen a roasting chamber 2 which 
is mounted onto a housing 1 of the coffee roasting machine A hot air 
current 3 which is heated by a heating coil 4, is blown into the roasting 
chamber 2. The air current 3 is generated by a fan 6 which is driven by a 
motor 5. A cold air stream 7 which is sucked into the interior of the 
device is conducted to the heating coil 4 by the fan 6. A step 8 which is 
formed in the air flow channel, serves the function of distributing the 
air current uniformly around the heating coil 4 in the heating channel. A 
light-sensitive variable resistance 9 is disposed in vicinity of the air 
stream 7 which is conducted to the heating coil 4. The resistance 9 
monitors the glow-image of the initial turns of the heating coil 4. The 
light-sensitive variable resistance 9 is temperature protected, and still 
can furnish a measured value representative of the existing air current 
temperature to an electronic temperature regulator 10, viewing the 
glow-image of the heating coil. Additionally, a temperature-dependent 
variable resistor 11 (NTC) which senses the temperature in the interior of 
the housing, is disposed in the housing. A bi-metal switch 12 is provided 
as an overload safety device for the heating coil. 
The circuit for operating the coffee roaster can be seen from FIG. 2. The 
operating current is supplied at the terminals U. Switches S1 and S2 are 
controlled by a timer mechanism 13. If only the switch S1 is closed, only 
the motor 5 is driven, which creates the air flow by means of the fan 6. 
When the switch S2 is also closed, the heater coil 4 and its control are 
also in operation. The heating current flows through a triac 14 of the 
regulator 10. The triac is controlled by an integrated circuit 15 of the 
regulator 10 with half-wave doses, units or batches or groups of cut out 
half waves controlling. The control criteria depends on the resistance 
value of the light-sensitive or light-dependent variable resistance 9 (LDR 
and on the resistance value of the temperature-dependent variable resistor 
11. 
The foregoing is a description corresponding in substance to German 
application No. P 34 19 267.0, filed May 23, 1984, the International 
priority of which is being claimed for the instant application, and which 
is hereby made part of this application. Any material discrepancies 
between the foregoing specification and the aforementioned corresponding 
German application are to be resolved in favor of the latter.