August 2021

Heat Transfer

Spiral heat exchanger technology

Heat exchangers are some of the most critical equipment in the oil and gas industry, as they provide the required temperature of fluid to sustain the plant’s operation.

Heat exchangers are some of the most critical equipment in the oil and gas industry, as they provide the required temperature of fluid to sustain the plant’s operation.

At the inlet of process plant facilities, accumulation of high sludge (high fouling) is a perennial issue that leads to inefficient performance of exchangers: shell-and-tube heat exchangers, to be precise. This also eventually leads to a drastic reduction in equipment reliability and an increase in the number of maintenance intervals, as well as high turnaround duration. Maintenance costs will be high over time, as additional inspection and maintenance are required due to the anticipated tube failures.

Even in such fouling environments, shell-and-tube heat exchangers are widely used due to their well-known designs and ease of fabrication with low manufacturing cost. Shell-and-tube heat exchanges always suffer from scaling formation created by high fouling. This results in frequent shutdowns to perform proper turnaround inspection and maintenance.

To overcome these issues, the spiral heat exchanger (SPHE) is a technology best suited for such applications with the potential for high fouling; an SPHE’s self-cleaning design can remove sludge due to the turbulent flow created during the SPHE’s operation.

As mentioned, the SPHE design feature creates a turbulent flow, preventing scaling within the equipment internal. Compared to the shell and tube, an SPHE is smaller, saving considerable space within the plant layout. The mechanical handling and cleaning of an SPHE is also much easier, as there are no internals like bundles within shell-and-tube exchangers. An SPHE consists of a shell and plates (which are welded to the shell) in a spiral shape that are separated by spacing studs to accommodate both hot and cold fluids in a countercurrent flow. This inherent design feature of an SPHE results in a compact design and enables cleaning of the internals by a hydro-jetting process and chemical cleaning.

Shell-and-tube heat exchangers often require a capital spare bundle to avoid long procurement lead times when in need of bundle replacement. Therefore, stocking spare bundles increases the CAPEX and requires long-term preservation and storage within the warehouse.

Based on the design features explained here and a case study, insight is provided of the advantages of the SPHE in terms of minimized plant footprint utilization, operation, mechanical handling, maintainability and ease of inspection.

DESIGN FEATURES

The SPHE’s distinct design features overcome historical issues related to high fouling. The SPHE is internally shaped as a spiral—hence the name—that provides a continuous path for both fluids, with no hard corners or edges. The equipment is self-cleaning due to the turbulence created by the spacing studs between the spiral plates. SPHE design features are detailed here.

Compact design

Due to its compact design, the SPHE has a small footprint compared to shell-and-tube heat exchangers. The overall space requirements and complexities within the SPHE unit are greatly reduced. With this compact design and absence of tubes bundle, a heavy lift crane, tube bundle puller and extra space are not required during installation, operation and maintenance activities. Additionally, turnaround inspection duration and labor requirements are reduced.

Self-cleaning

Flow turbulence created by the curved pathway of the fluid acting with the spacing studs pushes any deposits as they form (FIG. 1), facilitating a self-cleaning process within the SPHE.

FIG. 1. Flow turbulence created by the curved pathway of the fluid acting with the spacing studs pushes any deposits as they form. Source: NEXSON.

High overall heat transfer coefficient

With the spacing studs and the resultant fluid turbulence created within the spiral channels, the design provides a higher overall heat transfer coefficient compared to other heat transfer designs.

Spacing studs

The design and construction of an SPHE are made up of concentric channels that are separated by spacer studs. The studs, shown in FIG. 2, are welded on channels at specific locations and heights to form the channel gaps. The channel gaps and width are designed to meet requirements and working conditions unique to the end users. This versality in design allows the SPHE to be customized easily based on process conditions provided by the end users.

FIG. 2. Spacer studs are welded on channels at specific locations and heights to form the channel gaps.

This approach permits the design to account for flowrates, the sizes of the particles for fouling fluids, and pressure drops. Spacing studs facilitate turbulent flow in each channel, enabling a high overall heat transfer coefficient. It is worth mentioning that the number of studs and their spacing and arrangement are proprietary designs by SPHE manufacturers.

True countercurrent flow

As shown in FIG. 3, both hot and cold fluids flow through channels from one end to the other in opposite directions, contributing to a true countercurrent flow and resulting in high heat transfer efficiency, as well as a high overall heat transfer coefficient for SPHEs.

FIG. 3. Hot and cold fluids flowing in opposite directions contribute to a true countercurrent flow and result in high heat transfer efficiency. (Source: NEXSON).

Ease of inspection and maintenance

Due to its compact size, an SPHE can be easily accessed from both ends for required inspections and maintenance, including cleaning by hydro-jetting. Additionally, SPHEs can be chemically cleaned without removing any internals. This is in stark contrast to shell-and-tube exchangers, which require a cumbersome arrangement of a tube bundle puller device, crane, support arrangements and their associated safety risks, and logistics to transport the bundle to the maintenance shop for refurbishing.

Construction materials

Since the advent of spiral heat exchangers in the refining, petrochemicals, and oil and gas industries, manufacturers have been fabricating these heat exchangers in various metallurgies, such as carbon steels, stainless-steels, duplex, super-duplex, titanium, nickel alloys, etc., depending on the process and end-user requirements. Therefore, no limitations exist in terms of construction materials, which depends on the process fluid and conditions provided by the end users.

Spiral vs. shell-and-tube

TABLE 1 summarizes the advantages of SHEs compared to shell-and-tube heat exchangers based on design features explained in the previous section. With fewer turnaround inspection and cleaning activities, the end user will avoid production losses and significantly decrease maintenance costs.

Technology implementation

For the Saudi Aramco Tanajib Gas plant project (part of the Saudi Aramco Marjan Increment Program), the licensor selected SPHEs for the three services here:

  • Reclaimer loop heater
  • Slurry cooler
  • Lean/rich ethylene glycol (MEG) exchanger.

The SPHE was selected due to the high fouling phenomena that would be eliminated with this technology. Furthermore, due to the services’ highly corrosive nature, the licensor specified that the material of construction must be stainless-steel alloy 6MO to sustain the reliability of the exchanger throughout its anticipated lifetime of 25 yr in service. Inspection and maintenance activities will be minimized for these services within the plant, which is a time and cost-effective proposition.

A case study performed using an HTRI run for a slurry cooler considering SPHEs and shell-and-tube heat exchangers (TABLE 2) provides a good indication of the size, weight and use of plant layout for the exchanger design with identical duties and temperature differences requirements. Slurry is a concentrated MEG solution saturated with salts and containing up to 20 wt% solid salts particles. The total salts content is 25 wt%, as per H&MB Rev.H. Salts are mainly composed of sodium chloride (NaCl), so the chloride content is high. pH is maintained between 10 and 11.5.

Based on TABLE 2, the shell-and-tube design would be eight times larger in length compared to an SPHE (compacted), confirming the advantage of having a smaller footprint in the industrial plant, excluding the need of additional space for tube bundle removal utilizing crane and truck for loading and unloading.

Another advantage is the higher velocity in the SPHE design, which in turns provides a self-cleaning effect due to the higher velocity and the turbulence created with spacing studs.

SPHE technology has been in existence since 1930; with advancements made over the years, it has proven to be the best-suited technology for such services.

This technology was implemented in the Fadhili Gas Plant in 2019 with an indication of satisfactory performance and reliability in operation with no adverse reports, including the performance of SHE implementations within applications at a petrochemical complex and another chemicals manufacturing facility.

Takeaway

Fouling is a chronic problem in the oil and gas industry, particularly for exchangers operating in harsh environments. The SPHE is a reliable technology that can overcome such conditions.

The SPHE’s compact, true countercurrent flow design with self-cleaning effect and high heat transfer coefficients provide easy access to interior heat transfer services for field inspection, routine maintenance, or manual and chemical cleaning as required, with no involvement of heavy equipment.

The authors’ company is adopting the International Standard API 664 for material selection, design, fabrication, inspection and testing. Developing a company specification, standard and special inspection requirement formed in collaboration with the company’s approved spiral heat exchanger manufacturer will be the next step to govern design, fabrication and inspection for future purchase requests. HP

The Authors

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