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, also called evaporator, where the refrigerant is heated up, a second heat exchanger, also called condenser, 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; 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 and characterised 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 lower than 0.95, and in that a pitch of the fins of the evaporator is comprised between 1.8 mm and 3.3 mm, and a pitch of the fins of the condenser is comprised between 1.4 and 3.3 mm. - 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, also called evaporator, where the refrigerant is heated up, a second heat exchanger, also called condenser, 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; is a finned tube heat exchanger comprising a tube having multiple sections one above the other and a plurality of fins; 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 and characterised 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 lower than 0.95, and in that a pitch of the fins of the evaporator is comprised between 1.8 mm and 3.3 mm, and a pitch of the fins of the condenser is comprised between 1.4 and 3.3 mm.