Volume XXVI: Hairpin Heat Exchangers

ASME PRESSURE VESSELS

The scope of this presentation is to present basic information and understanding of the ASME code for the design of pressure vessels for the chemical and process industry as applicable in the United States and most of North and South America. For more information about our products, heavy plate & custom fabrication services or fabrication capabilities contact us today! 

HAIRPIN HEAT EXCHANGERS

INTRODUCTION

In Refinery & Petrochemical Complex Heat Exchangers plays an important role by exchanging heat energy. A hairpin heat exchanger can be described as a single-pass S&T unit that has been folded in half to give it a hairpin appearance. Below image shows the hairpin and simple hairpin heat exchanger.

This article and its contents will be useful to engineers working with engineering consultancies and engineering team of EPC companies.

APPLICATION:

For following conditions Hairpin Heat Exchangers can be an alternative to S&T exchangers;

• Duty < 500 kW
• Low Flow rates
• Large Temperature cross
• Diameter of S&T exchanger is 8" (200 mm) or less.

Conventional TEMA type S&T exchanger becomes uneconomical for above conditions because a large number of small diameter shells in series are required to provide adequate velocities and near counter-current flow.

CONSTRUCTION:

It is classified based on construction as follows;

a) Double Pipe Heat Exchanger:

The basic double-pipe construction consists of a single pipe or tube, placed concentrically inside a larger pipe or tube (Outer shell). Double pipe exchangers can have multiple configurations. Some typical configurations are shown below in Figure 3 & Figure 4.

Configuration shown in figure 3 utilizes a front end closure at tube side inlet outlet nozzles, with independent tube side and shell-side gasketing that prevent inter stream leakage. The closure arrangement is also provided with external split ring that locks the tube element to the shell. At rear end two inner tubes are joined together by a welded on U-bend, while the two outer shells are joined together by a dished head and a flanged joint. The depth of dished head is sized to allow free axial thermal movement of inner tubes & bend within the shell. This type of configuration is costly and may need a special vendor.

Configuration shown in figure 4 is the simplest & cheapest construction which uses pipes, nozzles, flange, standard 90º bend, gaskets and bolting. The disadvantage here is it does not utilize the bend portion in heat transfer. So for the same heat duty requirement, it may need more no of shells.

b) Multitube Heat Exchanger:

Multitube heat exchanger contains bundle of U-tubes instead of a single inner tube as double pipe. Each leg of the U-bundle has its own tubesheet and design permits free thermal movement of the bundle and its withdrawal from the shell. The front end and rear end construction of multitube heat exchanger is similar to that of double pipe exchanger as per Figure 3. Plain tubes are supported by segmental baffles inside shell. Baffles are held in place with the use of tie rods.

PROCESS BENEFITS:

Hair pin heat exchanger has tube-side & shell-side fluids running countercurrent to each other. True counter current flow is an efficient flow arrangement because the temperature difference between the two streams is maximized. Another benefit offered by counter current flow is the ability to attain closer temperature approaches and accommodate a temperature cross using a single hairpin section. Long radius U-bends takes care of thermal expansion in the tubes hence eliminating the requirement of expansion joints / bellows.

MECHANICAL ASPECTS:

a) Procurement:

Hairpin exchangers are considered as special equipments especially the configuration explained in Figure 3 & Figure 5 has to be procured from special vendors. Most of the time Owner indicates approved vendors for hairpin exchangers separately in the bid. For exchangers as per Figure 4 can be procured from normal exchanger suppliers. Following are some of the known vendors for hairpin exchangers in the world;

• Brown Fintube France
• Koch Heat Transfer Company
• Alco Products

b) Design:

Thermal design of hairpin exchanger is done & guaranteed by Vendor most of the time. The reason is, hairpin exchanger is considered as modular equipment i.e. vendor selects the equipment to match their standard size. Hairpin exchanger can be very well designed using HTRI Software but vendor prefers to do it in their own software and most of the time vendor does not provide any detailed thermal calculation. As far as Mechanical design is concerned, most components (shell, head, flange, nozzles, split ring, tubes, baffles, tie rods etc.) of hairpin exchanger can be designed using ASME Sec. VIII and TEMA (partly). Special vendors prefer to use excel spread sheet for the calculation purpose instead of commercial software.

c) Material of Construction:

Hairpin exchangers can be manufactured from all possible materials. The limiting factor may be fabrication of cast return bend closure. Due to its odd shape for exotic material it may have to be fabricated from plate.

d) Range:

Hairpin exchangers are available in nominal shell size of 2" to 30". Hairpin Exchangers are used for temperature range of -200 ºC to +700 ºC and up to pressure of 350 bars. It can withstand pressure up to 680 bars with special designs. Based on the service, the unit is available with bare tubes, finned tubes and Twisted tubes construction.

e) Return bend closure of hairpin exchanger has a special shape. It is usual practice of special vendor’s to use casting material due to ease of fabrication. Most owners also accept the use of casting for return bend closure.

f) Material of gaskets must be compatible to process fluid.

g) Unlike conventional S&T exchangers, Tube bundle of hairpin exchanger is removed from return bend closure side. This information is very important for piping engineers as they have to allocate space for bundle removal in plant layout.

h) Supports: Most special vendors provide special bracket type supports for hairpin exchangers. This support contains sleeves thru which the shell passes. The shell is welded to sleeve at one place and other places are kept loose to take care of thermal expansion. Refer Figure 7. Such arrangement eliminates the need of sliding plate.

i) Modular construction: The smallest hairpin exchanger may comprise only a single two leg section, but greater capacities may be achieved by stacking additional section in parallel or series or combination of both. The modular construction allows it to do so very easily. The stacking arrangement shall be decided in conjunction with piping engineer based on plot availability & piping routing feasibility.

j) Nozzle Loads: Hairpin exchangers are compact in size and have comparatively small size inlet and outlet nozzles. Due to this fact nozzle loads/moments of conventional S&T heat exchangers cannot be applied to hairpin exchangers. Further tube side nozzles are attached to tube closure so large amount of load can crush the gasket which can result in leakage. Hence, it is always advisable to get the maximum allowable nozzle loads from hairpin exchanger vendor and design the connected piping system accordingly.

MAINTENANCE OF HAIRPIN HEAT EXCHANGERS:

Hairpin heat exchangers are very easy to dismantle and maintenance can be performed easily during plant shutdown. Specially designed closure arrangement permits the tube side connections to be removed. These enable the inner tube or tube bundle & bend to be removed as a unit from the shells, from the rear end, after first removing the rear end assembly.

API-663 / ISO 12212:

With the wide demand of hairpin exchanger in oil & gas industry, in order to standardize, hairpin exchanger standard is under preparation by API & ISO committee along with renowned hairpin exchanger vendors. 1^st edition of API 663 / ISO 12212 will be released in 2013.

Sources:

1. Heat Exchangers – Selection, Design & Construction by E.A.D. Saunders
2. Website of Koch Heat Transfer Company, Brown Fintube France and Alco Products
3. Internet

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The sizes of these projects are up to 200’ in length, 350 tons, 16’ diameter and 4” thick.

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