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 (V); - a tubular body with two channels 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 (V), 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 (V); - a tubular body with two channels 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 (V), 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 ; <EOS>