Heat Exchanger Networks

Khetam Alwan Abbass
Prof. Dr. Nada B. Nakkash

    This study deals with the recovery of the energy available in hot and cold streams that exchanging heat. This can be done by heat exchanger network, to minimize the cost and the use of utilities. The transfer of energy from the hot stream to the cold stream depends on the rate of flow, area of the exchanger, the heat transfer coefficient and temperature gradient along each stream. Heat exchanger networks were considered for three systems A, B and C.System A with four streams and systems B and C with six streams, all systems are in liquid phase only.   Heuristics, TI and pinch methods for heat exchanger networks were considered. Three heuristics which are Rudd, Kobayashi and Linnhoff were used, these heuristics are applied on system A first, which gives four possibilities when Rudd heuristic was used, the minimum configuration cost is the 2possibility which have a cost 36.2×106 ID/y. Eight possibilities where obtained when Kobayashi heuristic was used, the configuration which have a minimum cost is of 4th  possibility where the cost was 36.2×106 ID/y, while Linnhoff heuristic  gives one structure with cost =113×106  ID/y. For system A, Rudd heuristic is the best, it gives the minimum cost structure in shortest way. For system B, Rudd heuristic gives 5 possibilities, the minimum cost is for the 3 rd possibility which is 107×106  ID/y.  Kobayashi gives 25 possibility, the minimum possibility cost is the 1st  which is 58.1×106 ID/y. Linnhoff possibility cost was 60.8×106  ID/y and it is close to the minimum cost structure. These heuristics were applied on system C but it gives unreasonable results.Ind TI method was considered on system A and C; a single structure was obtained for each system. For System A the cost was 47.5×106 ID/y and for system C cost was 565*10 6 ID/y.  Pinch method is applied on systems A and C and it gives the same possibilities and same costs as for TI method.The minimum approach temperature was selected to be 11 oC (20F) for all above cases, because it is the most appropriate value for the shell and tube heat exchangers when the minimum approach temperature reduced to 5.5F)for solving system C, the cost obtained by this value for the single structure of this system is equal to 1,015×106 ID/y and 565×106ID/y if ∆Tmin=110C The results obtained from this work was compared with the results of the previous works for the same systems, a difference about 46% in the value of the cost will be notice, as in the cost for the 2nd  possibility in system A (Kobayashi heuristic) which is 60.9×10 ID/YR in 1975 and 792×10 ID/YR in the present years after correcting the costs for the utilities (steam and cooling water) by the cost index to the last year and because of the change of the cost of materials for the heat exchangers.