Consider Open-Rack Seawater Heat Exchangers

Such units often offer an attractive way to take advantage of the resource

By Amin Almasi, mechanical consultant

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Heat exchangers that use seawater are common worldwide because they provide a compelling benefit — seawater is a free heat-transfer medium that doesn’t produce emissions. The units exchange heat between a process fluid (cryogenic or hot) and seawater of ambient temperature. In warm sea conditions (i.e., water at >14°C or, better, >18°C), such a seawater stream can serve as the heat source in vaporizer/heater equipment. On the other hand, cold seawater (preferably <12°C) can handle cooling of hot process streams; even seawater in tropical climes might suffice to cool some process streams.

Many seawater heat exchangers are open-rack designs because they typically boast better economics and shorter payback periods than other options such as shell-and-tube units. This advantage holds particularly true for large high-flow and high-capacity equipment. Another key plus is that open-rack heat exchange is considered a very safe and reliable technology. The units provide a simple and cost-efficient heat exchange process that’s easy to operate and maintain. Plants usually use parallel operating exchangers with spares.

Mechanical Details

In an open-rack heat exchanger, the seawater enters at the top and runs downward in a film-like manner over tubes containing the process fluid. The seawater collects in a basin at the bottom of the tubes and then is returned to the sea.

Tubes are fabricated from suitable materials with proper thermal, mechanical, corrosion and erosion characteristics. For example, in cryogenic services (vaporizers/heaters), aluminum alloys nearly always are used.

The tubes require an appropriate coating (such as a zinc aluminum alloy one for aluminum alloys) to ward off different mechanisms of seawater corrosion and erosion. Typically spray applied, the coating usually serves as a sacrificial anode to protect the base material from corrosion by seawater. As a rough indication, such a coating should have an expected life of 25,000–50,000 total operating hours (say, 3–5 years) — but this depends on seawater quality and operation pattern as well as cleaning and maintenance work. For cheap coatings or poorly maintained open-rack exchangers, recoating should take place every one or two years, particularly when polluted seawaters are involved. As a very rough estimate, a 2-block 200-t/h unit takes 2–3 weeks, excluding preparation period, to recoat.

Tubes usually have fins to increase heat transfer area. In addition, the inside of each tube most often has a cruciate profile, spirally twisted and fixed through the entire length, or another arrangement to further enhance heat transfer. Such a structure promotes turbulent flow, which improves heat exchange.

The tubes are arranged in panels. Each panel generally holds close to a hundred heat-transfer tubes. The distance between the panels often is 550–700 mm. Several of these panels (say, four to eight) are unified into a block by a manifold pipe and hung from a ceiling frame placed over a concrete structure at the installation site. A slide-type support provided under the block absorbs thermal movements. Increasing or decreasing the number of panels or blocks easily allows a design appropriate for the necessary heating/cooling capacity.

Often, it’s possible to isolate each block individually using proper valving systems. For instance, a 3-block design might operate only one or two blocks at low turndown. However, some manufacturers recommend using all blocks for part-load.

The units require regular inspections and maintenance to keep the finned tube surface clean. So, it’s important to ensure the seawater distribution facility over the heat exchanging panels is easily accessible, adjustable and designed to permit cleaning, via methods such as a high-pressure water jet or a rodding brush.

Although the design might seem simple, some components need great care and proper choice. For example, the type of connection for the process fluid inlet to the equipment is critical because this point experiences the highest thermal gradient (temperature difference) at startup.

Ideally, opt for flange connections instead of transition joints; for maintenance and panel re-coating (if you don’t plan on in-situ recoating), choose panels that are removable without cutting . Flange leakage at the inlet for very low or very high temperature applications usually is a concern. In such cases, consider the use of a one or two class higher ASME flange or any other suitable alternative to eliminate the potential leakage.

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