HAIRPIN TYPE HEAT EXCHANGERS
Hairpin Heat Exchangers are used in various applications in most any industry. The units normally operate in true counter current flow. This allows the design to be based on the full log mean temperature difference without considering the reducing correction factors generally necessary in shell and tube equipment. Applications requiring a temperature cross (hot medium cooled below outlet temperature of coolant) are particularly suited to this design. The hairpin section consists of two fintubes or multitube bundles connected with a return bend inside of a bare outer tube or shell.
The hairpin sections are designed in a manner that allows a number of hairpin sections to be arranged in the proper series, parallel or series-parallel combinations to perform a given duty. Using multiples of
hairpin heat exchanger modules makes it possible to mass produce the section. Standard hairpin sections provide a high degree of flexibility. For any given duty, the required heat exchanger is simply made up of the necessary number of sections arranged in proper series.
As the shells are relatively small the hairpin sections lend themselves to high pressure applications. Shell side design pressures are available to 2,000 PSIG and tube side pressure to 10,000 PSIG. All sections meet or exceed the requirements of the ASME Code and are Code Stamped. Generally hairpin sections utilize extended heat transfer surface. This is accomplished by welding several U-shaped channels to the tubes. Steel or steel alloy fins are welded to the tube with continuous electric resistance weld which assures high heat transfer efficiency throughout the life of the hairpin section. In the case of copper alloys a
continuous brazing process is used. Fintube Heat Exchangers have proven to be highly effective in a variety of situations. A few general conditions where their use should be considered almost mandatory are:
1. Where fluids have different heat transfer film coefficients: When a fluid is used to heat or cool air the air probably will have a lower heat transfer film coefficient.
2. Where fluids have different viscosities: Exchanging heat between a viscous liquid and a light fluid is suited to fintube design because viscous fluids usually do not have very good heat transfer characteristics.
3. Where pressure drop is important: Fintube Exchangers can be effective with gases that are identical in all their heat transfer characteristics, with the exception of pressure. Usually, the pressure drop restriction on the low pressure gas is more severe. Since the density of the high pressure gas is greater, better velocities and a better film coefficient can be attained for the high pressure gas.
4. Where fluids are heat-sensitive: Fluids that are heat-sensitive will be exposed to lower skin temperatures in a fintube heater. In heating a fluid with steam, for example, the added surface on the fintube transfers heat to the heat sensitive fluid at lower temperatures than a bare tube. Conversely, when cooling with fintubes, the cooling is done at higher temperatures than with bare tubes, which helps to prevent solidification on the tube wall.
An important fundamental rule to remember is this: When using fintubes, always place the fluid having the lower heat transfer characteristics in contact with the fins and the fluid having the higher heat transfer characteristics inside the tube. In this way the additional surface afforded by the fins helps to compensate for the low heat transfer of the fluid with which they are in contact.
SPECIFICATIONS AND DIMENSIONS
Hairpin sections are available in shell sizes from 2" IPS through 16" IPS; single tube sections are available in shell sizes through 6" IPS and multi-tube sections are available in shell sizes from 4", 6", 8" and 16" IPS. The charts on Pages 3 and 4 list all the section types available, the material they can be fabricated from, and the approximate overall dimensions.
All Dimensions in inches, unless otherwise marked.
(1) Not Available. BW = Beveled for Welding. SW = Socket Weld.
(2) Not required on Single Tube Units.
(3) Available with bare tubes only.
(4) When Tube Side Design Pressure is over 1,800 PSI, add 2-1/2" to dimension shown.
(5) Flange rating may be increased to suit design conditions.
(6) High Pressure Tube side lens ring construction shown. Available with other type gaskets for lower
(7) Up to 3,500 PSI @ 650F. Above this pressure connection will be SW.
(8) With Patented Interstream Leakage Prevention Feature.
With heat transfer surface requirements known refer to the chart below to determine the quantity and length of hairpin sections required. Standard hairpin sections are fabricated in lengths of 10, 20 and 25 feet. When the heat transfer surface area required exceeds a single hairpin section, use multiple hairpin sections.
When the equipment is received at the project site it will be necessary to unload, inspect, transport to point of installation, install and perform the cleanup work occasioned by the foregoing.
The Direct Labor Manhours required for the work described may be estimated as follows:
The Manhours above assume the transporting distance will not be in excess of 200'0" and that site or other obstructions do not exist.
MANHOURS EXTENSION TO DETERMINE DIRECT LABOR COST FOR INSTALLATION
The Manhours should be extended by a Composite Crew Rate determined from the Wage Rates in effect at the project site.