Outside Horizontal Tubes

Outside Horizontal Tubes Condensation on the outside of banks of horizontal tubes can be predicted assuming two mechanisms laminar condensate flow and vapor shear dominated heat transfer. 

The recommended procedure is to calculate the heat transfer coefficient using both mechanisms and select the higher value as the effective heat transfer coefficient (h). For baffled condensers, the vapor shear effects vary for each typical baffle section. The condenser should be calculated in increments with the average vapor velocity (Vy) for each increment used to calculate vapor shear heat transfer coefficients. When the heat transfer coefficients for laminar flow and for vapor shear are nearly equal, the effective heat transfer coefficient (h) is increased above the higher of the two values. The table below permits the increase to be approximated  

Lowfin Tubes - For lowfin tubes, the laminar condensing coefficient can be calculated by applying an appropriate correction factor, F, to the value calculated using Equation 7.30 above. The factor F is defined below  

At = total outside surface of the fintube Aj = inside surface of the fintube Dj = inside tube diameter 

---

Figure 10-103A. Maximum heat flux boiling outside horizontal tubes kettle and internal reboilers. When using the estimate from this curve, a safety factor of 0.7 also should be used. (Used by permission Palen, J. W., and Small, W. M. Hydrocarbon Processing, V. 43, No. 11, 1964. Gulf Publishing Company, Houston, Texas. All rights reserved.)...

Boiling Nucleate Natural Circulation (Thermosiphon) Inside Vertical Tubes or Outside Horizontal Tubes... 

Ggi, = mass velocity of liquid, Ib/hr (fF). For outside horizontal tubes, use projected area (diameter X length) of the tube, not the outside surface area. This assumes that only half of the tube is effective for bubble release. This does not apply to actual heat transfer area. 

For outside horizontal tube, use outside tube surface area. For vertical tubes with inside boiling, use inside surface area of tube. A-... 

Figure 8.14. Some arrangements of shell-and-tube condensers, (a) Condensate inside tubes, vertical upflow. (b) Inside tubes, vertical downflow, (c) Outside tubes, vertical downflow, (d) Condensate outside horizontal tubes. (HEDH, 1983, 3.4.3).

Condensation Outside Horizontal Tubes. Figure 8.14(d) shows a condenser with two tube passes and a shell side provided with vertically cut baffles that promote side to side flow of vapor. The tubes may be controlled partially flooded to ensure desired subcooling of the condensate or for control of upstream pressure by regulating the rate of condensation. Low-fin tubes often are advantageous, except when the surface tension of the condensates... 

Film-type condensation of vapors Outside horizontal tubes NvDont At/ J wn/ ), . , . 3100 McAdamsf Pure saturated vapors p- < 2000 bnj Physical properties are for condensate. McAdams t Pure saturated vapors iw/rDoP/ < 2000 Physical properties are for condensate. 

Boiling liquids outside horizontal tubes Film boiling (above critical Atj) l n Tkv Pv(pL PtOffXc D. l1,, Nucleate boiling (below critical At,) Value of h depends on At/, type of surface, and materials involved (critical At/ for water as temperature drop from heating surface to liquid is approximately 45°F). Bromley t For saturated liquids on submerged surfaces film coefficient h,0 is for conduction through the vapor, no radiation effect is included. 

Design the condenser of a steam power plant that has a thermal efficiency of 40 percent and generates 10 MW of net electric power. Steam enters the condenser as saturated vapor at 10 kPa, and it is to be condensed outside horizontal tubes through which cooling water from a nearby river flows. The temperature rise of the cooling water is limited to 8°C, and the velocity of the cooling water in the pipes is limited to 6 m/s to keep tile pressure drop at an acceptable level. Specify the pipe... 

Film-type condensation of vapors Outside horizontal tubes... 

Boiling liquids outside horizontal tubes Film boiling (above critical Atj-) i AtVti(pr< Pv)q c ... 

Condensation outside Horizontal Tubes and Tube Banks.529... 

At low flow rates, a vapor may be condensed inside a horizontal or nearly horizontal tube under the condition that the condensate flhn drains down the inside surface of the tube by gravity and in laminar flow into the liquid pool at the bottom of the tube and then out the tube outlet. Kern [38] proposed a simple modification of the Nusselt equation for condensation outside horizontal tubes (see next section) that can be used for this case ... 

For shell-and-tube condensation related topics include evaporation (Section 16.11.4.1) and distillation (Section 16.11.4.2). Prefer condensation outside horizontal tubes use vertical tubes when condensing immiscible liquids to subcool the condensate. Assume pressure drop of 0.5 of the pressure drop calculated for the vapor at the inlet conditions. Baffle spacing is 0.2 to 1 times the shell diameter with the baffle window about 25%. Limit pressure drop for steam to 7 kPa on the shell side. U = 0.5 to 0.85 kW/m °C. 

Example. Find the heat transfer coefficient by condensation of methyl alcohol outside horizontal tubes. 

---