A system for roasting coffee grains comprising at least: - a combustion chamber equipped with a unit for generating heat which is able to heat air; - a rotary drum for roasting coffee grains; - a first conduit for connecting between the combustion chamber and the rotary drum for the passage of hot air from the combustion chamber to the rotary drum in a feed direction; - a tubular body with two channels (11, 12) positioned on the first connecting conduit between the combustion chamber and the rotary drum; the tubular body being equipped with a first and second valve in such a way as to intercept, on a first channel and with a first valve, the flow of hot air coming from the combustion chamber and with the second channel and the second valve configured for intercepting air outside the system, at a temperature less than the flow of air arriving from the combustion chamber, and feeding it into the first conduit downstream of the first valve, with respect to the feed direction, in such a way as to modify the temperature of the mixed flow of air entering the rotary drum; - a control device being connected to the first and second valve for modifying their position inside the corresponding first and second channel in such a way as to modify the corresponding flow rate of the air flows as a function of the temperature required inside the rotary drum; characterised in that the control device has means for connecting to the first and second valve in such a way as to simultaneously move the first and second valve between several intermediate operational positions included between a first end operating position, wherein the first valve is positioned in such a way as to completely close the first conduit for feeding the flow of air from the combustion chamber and, simultaneously, the second valve is positioned in such a way as to keep completely open the second channel, and a second end operating position, wherein the first valve is positioned in such a way as to keep fully open the first conduit for feeding the flow of air from the combustion chamber and, simultaneously, the second valve is positioned in such a way as to completely close the second channel. - a combustion chamber equipped with a unit for generating heat which is able to heat air; - a rotary drum for roasting coffee grains; - a first conduit for connecting between the combustion chamber and the rotary drum for the passage of hot air from the combustion chamber to the rotary drum in a feed direction; - a tubular body with two channels (11, 12) positioned on the first connecting conduit between the combustion chamber and the rotary drum; the tubular body being equipped with a first and second valve in such a way as to intercept, on a first channel and with a first valve, the flow of hot air coming from the combustion chamber and with the second channel and the second valve configured for intercepting air outside the system, at a temperature less than the flow of air arriving from the combustion chamber, and feeding it into the first conduit downstream of the first valve, with respect to the feed direction, in such a way as to modify the temperature of the mixed flow of air entering the rotary drum; - a control device being connected to the first and second valve for modifying their position inside the corresponding first and second channel in such a way as to modify the corresponding flow rate of the air flows as a function of the temperature required inside the rotary drum;