化工传热英文讲义-换热器(heat exchangers).docx

Heat exchangersClassification:(a). According to flow arrangement (1). Parallel flow (co-current flow) (2). Counter flow (counter-current flow) (3). Cross flow(b). According to type of construction (1). Finned (mainly for gas/liquid systems), un-finned (2). Shell and tube (probably most commonly used), very flexible,relatively cheap, used at high pressure (3). Plate heat exchangers (when cleaning is important, foodindustry, milk processing), only at low pressure (4). Spiral heat exchangers (5). Compact heat exchangersTypes of heat exchangers(1). Concentric tube heat exchangerParallel flow, (b) counter-flow. This kind of heat exchanger has low heat transfer rate and thus very limited applications. (2). Shell and tube heat exchangers-very common in liquid-liquid systems, also as condensers. One shell pass and one tube pass (cross-counterflow)One shell pass and two tube passesTwo shell passes and four tube passesCross flow heat exchangers: (b)Finned with both fluids unmixed, (b) un-finned with one fluid mixed and the other unmixedCompact heat exchanger(a) Fin-tube (flat tubes, continuous plate fins), (b) Fin-tube (circulartubes, continuous plate fins), (c) Fin-tube (circular tubes, circular fins),(d) plate-fin (single pass), (e) plate-fin (single pas).Large specific surface area 700 m2 /m3 but laminar flow because ofsmall size of channels (low heat transfer coefficients).Overall heat transfer coefficient:Where: -fouling factor (additional thermal resistance)-thermal resistance in the wall separating hot and cold fluid (corresponding to conduction) depends on material and geometry-overall surface efficiency defined Typical values of fouling factor:Typical values of overall heat transfer coefficientsEnergy balance (for all types of heat exchangers)Heat load –net change of internal energy in hot and cold fluid:Subscript h – hot f