某列管换热器,用100℃水蒸气将物料由20℃加热至80 ℃,传热系数K=100 W/(m2 .℃).经半年运转后,由于污
垢的影响,在相同条件下物料出口温度仅为70℃.现欲使物料出口温度仍维持在80℃,问加热蒸气温度应取何值?
原􀄸qc(80-20)=KAΔ t 1 (1)
Δ t =(80-20)/ln(80/20)=43.28℃
半年后􀄸qc(70-20)=K AΔ t 2􀄵 (2)
Δ t= 80-30􀄵/ln(80/30)=50.98℃
􀄴2􀄵/􀄴1􀄵得􀄸
􀄴70-20􀄵/􀄴80-20=􀄺K Δ t 1/􀄴KΔ t2
􀄺(K /100)×50.98/43.28
K =70.75 w.m .K
保持原生产能力,即 KAΔ t K AΔ t (2’)
Δ t =KΔ t /K =100×43.28/70.75
􀄺=61.17℃ (3’)
Δ t =(T-20)-(T-80)/ln(T-20)/(T-80)􀄵
􀄺=61.17
∴ 60/61.17􀄺ln(T-20)􀄵/􀄴(T-80)􀄵
∴2.667=T-20􀄵/􀄴T-80􀄵
T=115.8℃