A dryer including a treating chamber where items are introduced and treated with a process air flow; and a heat pump system having a refrigerant circuit in which a refrigerant can flow, said refrigerant circuit including a first heat exchanger where the refrigerant is heated up, a second heat exchanger where the refrigerant is cooled off, a compressor to pressurize and circulate the refrigerant through the refrigerant circuit, and a pressure-lowering device; said first and/or second heat exchanger (32, 31) being apt to perform heat exchange between said refrigerant flowing in said refrigerant circuit and said process air; the refrigerant being a flammable refrigerant; wherein each of said first and second heat exchanger (32, 31): is a finned tube heat exchanger comprising a tube having multiple sections one above the other and a plurality of fins; and is divided in three portions: a central portion wherein said multiple sections of the tube are in contact with said plurality of fins, and a first and second end portions (61, 62) where said tube is not in contact with the plurality of fins; characterized in that a ratio between a total external volume of all sections of the tube included in the central portion in contact with the plurality of fins of the second heat exchanger and a total external volume of all sections of the tube included in the central portion in contact with the plurality of fins of the first heat exchanger has a value higher than 1.1, the total external volume of all sections of the tube included in the central portion of the second heat exchanger is comprised from 200 cm3 and 600 cm3, and a low pressure of the refrigerant in the stable phase of the heat pump cycle is comprised between 7 bar and 17 bar, wherein the low pressure of the refrigerant is measured at the inlet of the compressor, between the evaporator and the compressor, when the cycle of the heat pump is in the stable state, and wherein the refrigerant is: propane and the low pressure is comprised between 7 and 14 bar, or propylene and the low pressure is comprised between 8 and 17 bar. a treating chamber where items are introduced and treated with a process air flow; and a heat pump system having a refrigerant circuit in which a refrigerant can flow, said refrigerant circuit including a first heat exchanger where the refrigerant is heated up, a second heat exchanger where the refrigerant is cooled off, a compressor to pressurize and circulate the refrigerant through the refrigerant circuit, and a pressure-lowering device; said first and/or second heat exchanger (32, 31) being apt to perform heat exchange between said refrigerant flowing in said refrigerant circuit and said process air; the refrigerant being a flammable refrigerant; wherein each of said first and second heat exchanger (32, 31): is a finned tube heat exchanger comprising a tube having multiple sections one above the other and a plurality of fins; and is divided in three portions: a central portion wherein said multiple sections of the tube are in contact with said plurality of fins, and a first and second end portions (61, 62) where said tube is not in contact with the plurality of fins; is a finned tube heat exchanger comprising a tube having multiple sections one above the other and a plurality of fins; and is divided in three portions: a central portion wherein said multiple sections of the tube are in contact with said plurality of fins, and a first and second end portions (61, 62) where said tube is not in contact with the plurality of fins; characterized in that a ratio between a total external volume of all sections of the tube included in the central portion in contact with the plurality of fins of the second heat exchanger and a total external volume of all sections of the tube included in the central portion in contact with the plurality of fins of the first heat exchanger has a value higher than 1.1, the total external volume of all sections of the tube included in the central portion of the second heat exchanger is comprised from 200 cm3 and 600 cm3, and a low pressure of the refrigerant in the stable phase of the heat pump cycle is comprised between 7 bar and 17 bar, wherein the low pressure of the refrigerant is measured at the inlet of the compressor, between the evaporator and the compressor, when the cycle of the heat pump is in the stable state, and wherein the refrigerant is: propane and the low pressure is comprised between 7 and 14 bar, or propylene and the low pressure is comprised between 8 and 17 bar. propane and the low pressure is comprised between 7 and 14 bar, or propylene and the low pressure is comprised between 8 and 17 bar.