CM3110 Transport I Part II: Heat Transfer Heat Transfer with Phase Change Evaporators and Condensers Professor Faith Morrison Department of Chemical Engineering Michigan Technological University 1 Heat Transfer with Phase Change So far we have discussed heat transfer at a boundary due to a temperature difference between bulk temperatures q x htb Tw A 1. forced convection laminar Newton s law of cooling turbulent 2. natural convection 2 1
Heat Transfer with Phase Change So far we have discussed heat transfer at a boundary due to a temperature difference between bulk temperatures q x htb Tw A 1. forced convection laminar Newton s law of cooling turbulent 2. natural convection 3. phase change When a phase change takes place, the temperature on one side is CONSTANT, but the presence of boiling/condensing fluids produces heat transfer. Important in evaporation, distillation LARGE h It s important to know in which regime you operate Each regime has different correlations 3 T sat resistance to heat transfer Boiling T T sat heat liquid phase At low DT, few bubbles form, and heat transfer is by natural convection. As DT increases, more bubbles form, increasing convection (flow) in the liquid phase, increasing h. vapor phase 4 2
resistance to heat transfer T sat Film Boiling T T sat heat liquid phase As Δ increases further, a film appears and grows, increasing resistance to heat transfer vapor phase 5 Boiling natural convection nucleate boiling transition boiling film boiling radiation becomes important There are correlations for h for each regime. 0.25 T Geankoplis 4 th ed, p284 6 3
Boiling For example: Nucleate boiling, horizontal surfaces 1043 ΔT 16 5.56ΔT 16 240 Equations good for these units: Δ Geankoplis 4 th ed, p284 7 Boiling For example: Nucleate boiling, vertical surfaces 537 ΔT 7.95ΔT 3 3 63 Equations good for these units: Δ Geankoplis 4 th ed, p285 8 4
Boiling For example: Nucleate boiling, forced convection inside tubes 2.55Δ Equations good for these units: Δ Geankoplis 4 th ed, p285 9 Boiling For example: Film boiling, horizontal tubes Geankoplis 4 th ed, p285 0.62 Δ 0.4, Δ Δ Equations good for these units: Δ, Δ / 2 (All material properties at the film temperature) 10 5
Condensation T T sat T sat heat vapor phase At low Δ, few droplets form, As Δ increases, more droplets form, increasing convection (flow). Vertical plates; horizontal tubes important condensed phase 11 Dropwise Condensation high h hard to maintain not used in practice heat T T sat T sat vapor phase condensed phase 12 6
Film Condensation film reduces h very stable often used T T sat T sat heat vapor phase heat condensed phase 13 Condensation For example: Film condensation, vertical surfaces, laminar flow Geankoplis 4 th ed, p289 1.13 Δ Δ Re 4 1800 Equations good for these units: Δ, Δ / 2 (All material properties at the film temperature) 14 7
Condensation For example: Film condensation, vertical surfaces, turbulent flow Geankoplis 4 th ed, p289 0.0077 L Re. Re 4 1800 Equations good for these units: / 2 (All material properties at the film temperature) 15 Condensation Geankoplis 4 th ed, p285 For example: Film condensation, outside horizontal cylinders, laminar flow 0.725 Δ Δ Re 4 1800 Equations good for these units: Δ, Δ / 2 (All material properties at the film temperature) 16 8
Evaporators heat hot fluid (steam) boiling liquor Issues: viscosity, m scale formation liquor velocity time in evaporator T sat, steam h s negligible heat-transfer resistance due to wall h l large heat-transfer resistance on liquorside T sat, liquor 17 steam in tubes liquor on outside inexpensive, but poor liquid circulation good for nondepositing, low fluids steam liquor in tubes steam on outside liquid circulates by natural convection steam downcomer steam horizontal-tube evaporator vertical-tube evaporator hydrostatic head in tubes prevents boiling in tubes Geankoplis, p492 18 9
liquor in tubes steam on outside liquid circulates by natural convection single pass high liquid velocities liquor in tubes steam on outside liquid circulates by forced convection good for high fluids steam condensate Geankoplis, p492 long-tube vertical evaporator forced-circulation evaporator 19 Long-tube vertical evaporators boiling, 2-phase zone Decreasing Increasing g non-boiling zone For discussion of how to estimate, see Geankoplis 4 th edition, p533 20 10
Greater efficiency may be obtained by operating several evaporators in series: Multiple-Effect Evaporation T1 T2 T3 T f P1 P2 P3 T s 21 Norbert Rillieux: Inventor of Multiple-Effect Evaporation 11
Single-Effect Evaporators T b vapor feed steam T f T s T b T b steam condensate concentrated liquor T b 23 Multiple-Effect Evaporators P P P 1 2 3 For each effect the vapor product becomes the source of heat for the subsequent effect T1 T2 T3 T1 T2 T3 T f P1 P2 P3 T s pressure, temperature decrease (made white sugar possible) 24 12
Compare Single- and Multiple-Effect Evaporators T 3 Q UAT s T 3 T f T s T 3 T1 T2 T3 T 3 T f T s P1 P2 P3 Q q1 UAT s T1 q2 UAT 1 T2 q3 UAT 2 T3 qi UAT s T 3 same capacity = same amount of heat transferred (but we did not have to pay for it all = more efficient) (used in sugar production, for example) 25 Summary Boiling/Evaporation Industrially we operate either in nucleate boiling or film boiling regimes There are different correlations for each regime and for different geometries Nucleate boiling, horizontal surfaces Nucleate boiling, vertical surfaces Nucleate boiling, forced convection Film boiling, horizontal tube Condensation Industrially we operate in film condensation There are different correlations for different geometries Vertical surfaces, laminar or turbulent Outside stack of horizontal cylinders Evaporators designed with the boiling regimes in mind 26 13
Next: Heat transfer with phase change Evaporators Radiation DONE 27 14