CHE4450_Lecture14_Contemporary Optimization.ppt

# Note that heat exchanger design shall comply with

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Note that heat exchanger design shall comply with TEMA standard (we will discuss about TEMA in detail later). The diameter of shell size determines how many tubes can be put in a heat exchanger. This rule allows you to quickly estimate the heat exchange area. 7

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Cont’d Tube side is for corrosive, fouling, scaling, and high pressure fluids. Shell side is for viscous and condensing fluids. Mechanical cleaning is easier on tube side than shell side. The critical Reynolds number for the shell side is about 200. For a highly viscous medium or a low flowrate turbulent flow can be obtained easier at the shell side. Condensing fluids drain better on the shell side. 8
Cont’d Pressure drops are 0.1 bar (1.5 psi) for boiling and 0.2-0.62 bar (3-9 psi) for other services. Note that there is pressure drop (not increase) in boilers. If pressure drop is too high in a shell-and-tube heat exchanger, you should change the design, e.g., using more tubes or increasing baffle spacing. 9

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Cont’d Minimum temperature approach is 10ºC (20ºF) for fluids and 5ºC (10ºF) for refrigerants. Approach temperature determines the heat transfer area, hence the cost of the heat exchanger. If approach is too small, the heat transfer area will be large. 10
Cont’d Cooling water inlet is 30ºC (90ºF), maximum outlet is 45ºC (115ºF). For cooling water, the outlet temperature can never be close to its boiling point. This is because a high temperature will cause severe fouling issue in heat exchangers. 11

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Cont’d Heat transfer coefficients for estimating purposes, W/m2ºC (Btu/hr ft2ºF): water to liquid, 850 (150); condensers, 850 (150); liquid to liquid, 280 (50); liquid to gas, 60 (10); gas to gas, 30 (5); reboiler, 1140 (200). Maximum flux in reboiler 31.5 kW/m2 (10,000 Btu/hr ft2). When phase changes occur, use a zoned analysis with appropriate coefficient for each zone. You would notice that liquid would have a higher heat transfer coefficient than gas. This is mainly because of the physical property such as density. Note that Nu increases when Re increases. In detailed design, U is solved by empirical correlations, but this rule would give you a good starting point. 12
Cont’d Double-pipe exchanger is competitive at duties requiring 9.3-18.6 m 2 (100-200 ft 2 ). When the heat transfer area is small, you will not use a shell and tube heat exchanger. 13

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Cont’d Compact (plate and fin) exchangers have 1150 m2/m3 (350 ft2/ft3), and about four times the heat transfer per cut of shell-and-tube units.
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