ME 421 Heat Exchanger and Steam Generator Design

ME 421 Heat Exchanger and Steam Generator Design Lecture Notes 6 Double-Pipe Heat Exchangers

Introduction

Introduction • DP HEX: one pipe placed concentrically inside another • One fluid flows through inner pipe, the other through the annulus • Outer pipe is sometimes called the shell • Inner pipe connected by U-shaped return bends enclosed in a return-bend housing to make up a hairpin, so DP HEX = hairpin HEX • Hairpins are based on modular principles: they can be arranged in series, parallel, or series-parallel combinations to meet pressure drop and MTD requirements; add-remove as necessary

Usage Areas / Advantages • Sensible heating / cooling, small HT areas (up to 50 m 2) • High pressure fluids, due to small tube diameters • Suitable for gas / viscous liquid (small volume fluids) • Suitable for severe fouling conditions (easy to clean and maintain) • Finned tubes can be used to increase HT surface per unit length, thus reduce length and Nhp • Outside-finned inner tubes most efficient when low h fluid (oil or gas) flows through annulus • Multiple tubes can be used inside the shell • Used as counterflow HEX, so they can be used as an alternative to shell-and-tube HEX

Thermal / Hydraulic Design Inner Tube • Use correlations to find HT coefficient and friction factor • Total pressure drop Annulus • Same procedure as above, but use – Hydraulic diameter, Dh = 4 Ac/Pw for Re calculation – Equivalent diameter, De = 4 Ac/Ph for Nu calculation • For a hairpin HEX with Bare Inner Tube, – Dh = Di - d o – De = (Di 2 - do 2)/d Study Example 6. 1 (detailed analysis) o

Thermal / Hydraulic Design (continued) • For a hairpin HEX with Multitube Longitudinal Finned Inner Tubes – Get Dh and De using – Unfinned, and total outside HT surface areas

Thermal / Hydraulic Design (continued) – Overall HT coefficient based on outer area of inner tubes where is the overall surface efficiency Area ratios At/Ai and Af/At are needed Rw is for bare tube wall hf is the efficiency of a rectangular continuous longitudinal fin (for other types of fins, use references) h affects fin efficiency; have the fluid with the poorest HT properties on the finned side *

Thermal / Hydraulic Design (continued) • The heat transfer equation is (heat duty equation) • The design problem, in general, includes determining the total outer surface area of the inner tubes from the above equation. • If the length of hairpins is fixed, then Nhp can be calculated. • U can also be based on the inner area of the inner tubes, Ai • For counterflow and parallel flow arrangements, no correction is necessary for Tm. However, if hairpins are arranged in series/parallel, a correction must be made (later). • Study Example 6. 2 (detailed analysis)

Parallel / Series Arrangement of Hairpins • If the design indicates large Nhp, it may not be practical to connect them all in series for pure counterflow. A large quantity of fluid through pipes may result in p > pallowable • Solution: Separate mass flow into parallel streams, then connect smaller mass flow rate side in series. This is a parallel-series arrangement. • If such a combination is used, the temperature difference of the inner pipe fluid will be different for each hairpin. • Thus, in each hairpin section, different amounts of heat will be transferred and true mean temperature difference, Tm will be different from the LMTD.

• The true mean temperature difference in becomes dimensionless quantity S is • For n hairpins, S depends on the number of hot-cold streams and their series-parallel arrangement. • Simplest case is to either divide the cold fluid equally between n hairpins in parallel or to divide the hot fluid equally between n hairpins in parallel.

• For one-series hot fluid and n 1 -parallel cold streams, • For one-series cold fluid and n 2 -parallel hot streams, • Then, the total heat transfer rate is

• In the previous equations, it is assumed that U and cp of the fluids are constant, and the heat transfer rates of the two units are equal. • Graphs are available in literature for LMTD correction factor F as well. • If number of tube-side parallel paths is equal to the number of shell-side parallel paths, regular LMTD should be used.

Total Pressure Drop • Total pressure drop includes frictional pressure drop, entrance and exit pressure drops, static-head, and the momentum-change pressure drop. • Frictional pressure drop is • For frictional pressure drop, use correlations from Chapter 4 or Moody diagram. Add equivalent length of the U-bend to the L in tube-side (Dh = di) pressure drop. • You may need to account for the effect of property variations on friction factor.

Total Pressure Drop (continued) • Entrance and exit pressure drops through inlet and outlet nozzles is evaluated from where Kc = 1. 0 at the inlet and 0. 5 at the outlet nozzle. • Static head is pf = H, where H is the elevation difference between inlet and outlet nozzles. • For fully developed conditions, momentum-change pressure drop is • In all pressure drop calculations for design, allowable p must be considered. • Cut-and-twist technique increases h in longitudinal finnedtube HEX. See book for p details.

Design and Operational Features • In hairpin HEX, two double pipes are joined at one end by a U-tube bend welded to the inner pipes, and a return bend housing on the shell-side. The housing has a removable cover to allow removal of inner tubes. • Double-pipe HEX have four key design components – shell nozzles – tube nozzles – return-bend housing and cover plate on U-bend side – shell-to-tube closure on other side of hairpin(s) • The longitudinal fins made from steel are welded onto the inner pipe. Other materials can be joined by soldering. • Multiple units can be joined by bolts and gaskets. • For low heat duty applications, simple constructions, easy assembly, lightweight elements and minimum number of parts contribute to minimizing costs.

IPS: inch per second (unit system) NFA: net flow area