Sometimes referred to as G-fin, double-pipe or multitube heat exchangers, the hairpin heat exchanger has been used in process industries for many years. A hairpin heat exchanger can be described as a single-pass shell-and-tube unit that has been folded in half to give it a hairpin appearance. What distinguishes a hairpin exchanger from a traditional shell-and-tube exchanger are its closures. Hairpin heat exchanger closures allow for a removable tube bundle and accommodate thermal expansion without requiring expansion or packed joints.
|See the related article, "Advantages of Hairpin-Style Heat Exchangers," and learn how hairpin heat exchangers can be used in applications that require high thermal performance and a compact footprint.|
Numerous advances have been made in the last 20 years with regard to closure technology, and improvements have been made in sealing integrity, bolting and maintainability. Today's closures use independent tube-side and shell-side gasketing that prevent interstream leakage. While modern closures use all-through bolting, older designs used studs and threaded fixed flanges that were susceptible to corrosion and breakage within the flange.
Another feature used in current closure design is an external split ring located on the outside of the shell-side sealing ring, which can be removed without disturbing the tube element (figure 1). The function of the split ring is to lock the tube element to the shell. Earlier designs used an internal split ring, meaning the ring itself was confined beneath the shell closure flange, requiring the tube element be pushed forward to allow removal. The internal split ring design also meant that the ring itself came in contact with the shell-side fluid and was susceptible to corrosion and fouling.
|In current closure design, an external split ring is located on the outside of the shell-side sealing ring. The function of the split ring is to lock the tube element to the shell.|
Consider Process Benefits
Process plant personnel evaluate heat exchangers differently depending on their specific needs. Process engineers evaluate heat exchangers based on efficiency and thermal and hydraulic performance. Mechanical engineers usually are concerned with the mechanical integrity and reliability while operators and maintenance personnel usually are concerned with the ability to easily maintain and clean the equipment they use and service.
Hairpin heat exchanger design can address each of these needs. As noted, this single-pass design has tube-side and shell-side fluids running countercurrent to each other. True countercurrent flow is an efficient flow arrangement because the temperature difference between the two streams is maximized. Another benefit offered by countercurrent flow is the ability to attain closer temperature approaches and accommodate a temperature cross using a single hairpin section.
Conventional shell-and-tube heat exchangers often employ a single-pass shell-side stream with a multiple pass tube-side stream. This arrangement results in half of the tube-side passes running in countercurrent flow while the other half of the passes run in cocurrent flow. The inefficiencies inherent with this arrangement require the application of a temperature difference correction factor, which translates into an increase in the surface area required to perform a given duty. Also, cocurrent flow passes are not able to handle a temperature cross in a single shell-and-tube section. By contrast, applications requiring multiple shell-and-tube sections connected in series often can be performed in single hairpin sections. Consequently, hairpins are used where a temperature cross is present - for example, feed-to-effluent exchangers and lean-to-rich amine and glycol applications.
In addition to countercurrent flow, enhancement devices used with hairpin designs can increase heat transfer coefficients. Various inserts promote turbulence inside the tubes. Shell-side devices include fintubes for maximum surface area with relatively low pressure drop, low pressure drop tube supports, traditional segmental baffles and twisted tubes. Twisted-tube technology provides the benefits of swirl flow without having inserts inside the tubes.
Hairpin heat exchangers offer the mechanical and maintenance engineer advantages inherent with their design. The advantages include such things as:
- Independent tubesheets for high terminal temperature differences.
- Thermal shock.
- Long radius U-bends for effective thermal expansion.
- High temperature differences.
- Ease of cleaning.
- All-welded baffle cages for durability.
- High pressure closures for pressures up to 10,000 psi.
- No internal bolting.
If you need a single-pass heat exchanger, consider a hairpin design.