HEAT EXCHANGER NETWORK sYnThESIS 1: TARGETING OBJECTIVES 1 Determine the minimum energy inputs required ∫ or a process 2 Determine the minimum number of heat exchangers required to transfer this energy 3 Develop a methodology for determining where these heat exchangers should be placed in the process 2/21/99 HENS 1: Targeting
2/21/99 HENS 1: Targeting 1 HEAT EXCHANGER NETWORK SYNTHESIS 1: TARGETING OBJECTIVES: 1) Determine the minimum energy inputs required for a process 2) Determine the minimum number of heat exchangers required to transfer this energy 3) Develop a methodology for determining where these heat exchangers should be placed in the process
SINGLE HEAT EXCHANGER- REVIEW W COLD T COLDin W HOT HOTin HOTout HOLDout Stream W(kg/h) Cp(kJ/kg-K Tin(K) Tout(K) Q(gJ/h Hot20,000 4.2 180112.5-567 Cod15,00 4.2 60150+567 2199 HENS 1: Targeting
2/21/99 HENS 1: Targeting 2 SINGLE HEAT EXCHANGER - REVIEW W HOT T HOTin T COLDout W COLD T COLDin T HOTout Stream W(kg/h) Cp (kJ/kg-K) Tin(K) Tout(K) Q(gJ/h) Hot 20,000 4.2 180 112.5 -5.67 Cold 15,000 4.2 60 150 +5.67
Heat exchanger Q-T diagram 200 180 2 160 14 120 s40 Q(gJ/h) 一Thot-Tcol 2199 HENS 1: Targeting
2/21/99 HENS 1: Targeting 3 Heat Exchanger Q-T Diagram 0 20 40 60 80 100 120 140 160 180 200 0 1 2 3 4 5 6 Q (gJ/h) Temperature (K) Thot Tcold
IMPORTANT POINTS 1 The First law says that the heat transferred from the hot stream must equal that transferred to the cold stream Therefore, of the six process variables (two flow rates and four temperatures), only five can be specified independently 2) The second law says that heat can only be transferred from a hotter fluid to a colder one. Therefore, the temperature of the cold fluid must be less than that of the hot fluid at all points along the leng th of the exchanger 2/21/99 HENS 1: Targeting
2/21/99 HENS 1: Targeting 4 IMPORTANT POINTS 1) The First Law says that the heat transferred from the hot stream must equal that transferred to the cold stream. Therefore, of the six process variables (two flow rates and four temperatures), only five can be specified independently. 2) The Second Law says that heat can only be transferred from a hotter fluid to a colder one. Therefore, the temperature of the cold fluid must be less than that of the hot fluid at all points along the length of the exchanger
IMPORTANT POINTS (Contd. 3) As a practical matter, for heat to be transferred at an acceptable rate, the temperature of the cold fluid must be less than that of the hot fluid at each point in the exchanger by a reasonable amount. We refer to the minimum acceptable temperature difference as the MINIMUM APPROACH TEMPERATURE or ATmir This is also known in the literature as the pinch TEMPERATURE. The methodology for synthesizing heat exchanger networks is sometimes called PINCH TECHNOLOGY 2199 HENS 1: Targeting 5
2/21/99 HENS 1: Targeting 5 IMPORTANT POINTS (Cont’d.) 3) As a practical matter, for heat to be transferred at an acceptable rate, the temperature of the cold fluid must be less than that of the hot fluid at each point in the exchanger by a reasonable amount. We refer to the minimum acceptable temperature difference as the MINIMUM APPROACH TEMPERATURE or DTmin. This is also known in the literature as the PINCH TEMPERATURE. The methodology for synthesizing heat exchanger networks is sometimes called PINCH TECHNOLOGY