Section 4.12 addressed for the reciprocating compressor the precaution that there will always be some…
Economic Pressure Drop and Velocity
Pressure drop is required to change fluid momentum (increase velocity and/or change direction), and to overcome friction. Friction pressure loss is the dominant cause of pressure drop in most single- and two-phase heat exchangers. Acceleration (momentum) loss is most important in boilers and is usually provided by differential static head. Momentum and friction losses dominate in the inlet regions of condensers (and overall), with some vapor deceleration pressure recovery occurring in the bundle. Both heat transfer and pressure drop increase with increasing fluid velocity.
Optimum pressure drop for single-phase heat exchangers and some two-phase heat exchangers (excluding boilers and condensers) is determined by balancing the cost of pressure drop (e.g., pumping power) against the cost of heat transfer surface area.
Inevitable pressure losses associated with nozzles, including entrance and exit zones of exchangers, do not substantially affect heat transfer. Exchanger nozzles usually match the piping size and pipe size is usually based on the same principle of balancing pipe costs against pumping costs. These pressure losses are therefore a more-or-less constant fraction of the exchanger friction losses.
For pumped liquids, compressed vapors, or both (two-phase) in carbon steel exchangers, the economic expenditure of pumping power on each side of the exchanger (including nozzle, entrance, and exit losses) is about 5 hp per 1000 ft2 of heat exchanger surface area. The optimum is relatively flat. Power expenditures between 3 and 7 hp/1000 ft2 have a significant effect on exchanger and pump or compressor costs, but result in approximately the same overall cost (5%).
Designs anywhere in this range are reasonable. Optimum power expenditure per unit area is approximately the same for any fluid density and tube size.
The above economic range of power expenditure can be expressed in terms of velocity and pressure gradient.
Economic velocity, ft/sec (independent of tube size):
(28 to 36)/(Density, lb/ft3)1/3 Tube side
( 7 to 10)/(Density, lb/ft3)1/3 Shell side
The shell side velocity is for the “B” stream (crossflow) used in the HTRI programs and is shown in Figure 200-3. “B” stream flow is usually about 70% of the total shell side flow. Typical values for low viscosity hydrocarbon liquids and water are:
7 to 9 ft/sec Tube side
2 to 3 ft/sec Shell side
Economic pressure gradient, psi/ft (for 3/4-inch, 13 BWG tubes):
(0.05 to 0.08) ×(Density, lb/ft3)1/3 Tube side
(0.1 to 0.16) ×(Density, lb/ft3)1/3 Shell side
The tube side flow path is the straight tube length times the number of tube passes. The shell side flow path is the axial shell length times the number of shell passes. Typical values for low viscosity hydrocarbon liquids and water are:
0.2 to 0.3 psi/ft Tube side
0.4 to 0.6 psi/axial ft Shell side
The factor of two difference between shell and tube side pressure gradients simply reflects the fact that actual shell side flow path is about twice the axial length. Economic pressure gradient for 1-inch tubes is about 70% of that for 3/4-inch tubes.
These simple rules-of-thumb are insensitive to wide variations in energy costs, because exchanger costs are energy intensive and track well with energy costs.
Exchangers with similar tube side and shell side heat transfer coefficients (the norm) should be designed to the economic parameters particular to each side. If one side of the exchanger limits (much lower coefficient than the other side), the above economic parameters should be used for the limiting side, and less power expended on the side that does not limit. Power expenditures on the high side, or more, are justified for expensive alloy exchangers.
In some cases (e.g., desalter effluent water coolers), available pressure drop is dictated by other process requirements (e.g., suppressing vaporization in the desalter) and should be used to the maximum practical extent to reduce exchanger size.
These guidelines do not apply to reboilers and condensers. Reboilers are usually driven by natural circulation rather than pumps. Pressure drop in condensers impacts column and reboiler design as well pumping/compression costs. At atmospheric pressure or above, condenser pressure drop is typically the smaller of about 5 psi or 10% of the absolute pressure. At very low pressures, the type and performance of vacuum equipment governs condenser pressure drop.
The economics of pumping/compression costs versus exchanger cost is one aspect of optimum exchanger utilization. Equally important is balancing the value of heat exchange against the cost of achieving it (ie., determining the appropriate duty of the exchanger). Determination of exchanger duty is beyond the scope of this manual.
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