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 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 where said tube is not in contact with the plurality of fins; and defines a thickness (tevap, tcondens) of said central portion along a thickness direction; characterized in that the value of the ratio (tcondens/tevap) between a thickness of the central portion of the second heat exchanger and a thickness of the central portion of the first heat exchanger is lower than 1, and a pitch of the fins of the first and the second heat exchanger is comprised between 1.8 mm and 3.3 mm and 1.4 mm and 3.3 mm respectively. 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 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 where said tube is not in contact with the plurality of fins; and defines a thickness (tevap, tcondens) of said central portion along a thickness direction; 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 where said tube is not in contact with the plurality of fins; and defines a thickness (tevap, tcondens) of said central portion along a thickness direction; characterized in that the value of the ratio (tcondens/tevap) between a thickness of the central portion of the second heat exchanger and a thickness of the central portion of the first heat exchanger is lower than 1, and a pitch of the fins of the first and the second heat exchanger is comprised between 1.8 mm and 3.3 mm and 1.4 mm and 3.3 mm respectively.