Box-Type Cooler, also known as an Outboard Cooler, is a cooler composed of multiple bundles of copper tubes, directly installed in the seawater tank, saving engine room space and operational costs.
The box-type cooler is generally used for cooling purposes in systems such as main engines, generators, and auxiliary equipment. Suitable ship types include medium and small cargo ships, oil tankers, tugboats, barges, fishing boats, ferries, supply vessels, refrigerated ships, icebreakers, etc.
This article takes the 17,500-ton chemical/oil tanker built by Taizhou Port Shipyard as an example to introduce the working principle, product features, installation design points, and other content of the box-type cooler.
Working Principle
The box-type cooler consists of parts such as the water tank, spacer pads, tube sheets, installation pads, installation flanges, and tube bundles. The integrated U-shaped tube bundle is expanded and connected to the tube sheet, which is then fixed to the installation flange on the top plate of the seawater tank with bolts.
The cooling effect of the box-type cooler is achieved through forced circulation during the ship's voyage or natural convection when the ship is stationary.
When the ship is in motion, the cooling seawater enters the seawater tank through the bottom grid, then flows out along the outside of the U-shaped tube bundle through the top grid. The forced circulation of cooling seawater generated by the ship's movement achieves the cooling effect. When the ship is stationary, such as at the dock or anchorage, the cooling seawater in the seawater tank exchanges heat with the ship's fresh water cooling system. The seawater, which warms up and decreases in density, flows upwards and is discharged from the seawater tank, while cooler seawater at the bottom enters through the bottom grid, forming a natural upward circulation process, eventually achieving the cooling effect (see Figure 1).
Figure 1: Schematic of the Heat Exchange Process of the Box-Type Cooler
2. Product Features
In summary, the box-type cooler has the following advantages compared to conventional plate-type heat exchangers (or shell-and-tube heat exchangers):
No need for cooling seawater piping
The box-type cooler is installed in the seawater tank, directly exchanging heat with the outboard cooling seawater. Unlike conventional cooling seawater systems, it does not require components such as seawater pumps, filters, valves, and seamless steel pipes, which are corrosion-resistant and have high maintenance costs. Additionally, there is no need to introduce seawater into the engine room, simplifying system and piping layout.
Saves engine room space
Using a box-type cooler saves significant engine room space. Without changing the hull structure, more space can be obtained for equipment and piping layout, which is especially important for medium and small ships. The simple system layout also reduces the workload of crew members for daily operations and later maintenance.
Saves power station capacity
Traditional cooling seawater systems require secondary pumps, consuming about 30 kW and burning about 6 L/h of fuel. By using a box-type cooler, a savings of 30,000 L of fuel can be achieved, assuming an average operation of 5000 hours per year. Additionally, operational and maintenance costs are reduced due to the absence of secondary cooling pipeline equipment.
No limitations on operational waters
Since outboard seawater is not introduced into the engine room, there is no issue of clogged filters or cooling system attachments caused by poor water quality. This makes it suitable for ships operating in ice regions, shallow draft, or polluted waters.
Simple maintenance
The box-type cooler is simple in structure, and the tube bundle is made from corrosion-resistant materials that are not prone to scaling. Internal inspection and testing do not require removal of the tube bundle.
3. Design Considerations
Before starting the design of a system using a box-type cooler, shipyard designers should maintain close communication with the design company, relevant manufacturers, and shipowners to understand the design requirements of the box-type cooler manufacturer and the layout characteristics of related system equipment. The following aspects should be focused on:
3.1 Design of the Cooling Water System
The cooling water system using a box-type cooler is generally set up as an independent loop, typically divided into three independent loops: high-temperature water for the main engine, low-temperature water for the main engine, and low-temperature water for the generator and auxiliary equipment.
The flow resistance pressure drop of the box-type cooler should be calculated, and the rated head of the diesel engine-driven water pump and the electric water pump should be adjusted to avoid issues during equipment mooring tests.
The normal operating pressure of the box-type cooler should be within the range accepted by the manufacturer. The box-type cooler is installed in the seawater tank at the bottom of the engine room. During normal operation of the cooling water system, the normal operating pressure of the box-type cooler is generated by the pressure from the water pump and the static head generated by the height of the expansion tank. If the box-type cooler operates under overpressure conditions for a long time, its service life will be significantly shortened.
3.2 Design of Seawater Tank Layout
To ensure the heat exchange performance of the box-type cooler, the layout of the cooler and the area of the grid openings in the seawater tank are critical design considerations:
The actual grid opening area of the seawater tank must be larger than the grid opening area clearly specified in the technical agreement by the manufacturer, with some margin considered. If multiple coolers are placed in one seawater tank, the required total grid opening area should be greater than the cumulative area needed for all the coolers.
To ensure adequate space around the tube bundle for effective cooling, the distance between the tube bundle and the ship's structure should be greater than 100 mm, as shown in Figure 2.
Figure 2: Required Free Flow Distance Around the Tube Bundle
(3) The Direction of the Grid Openings
As shown in Figure 3, the grid inlet openings are aligned along the ship's width, while the grid outlet openings are aligned along the ship's length. This arrangement ensures better seawater cooling circulation during the ship's voyage.
3.3 Design of the Heat Exchange Area of the Cooler
For ships using box-type coolers, the outboard seawater system does not introduce seawater into the engine room, meaning there is no forced cooling circulation function. During the technical agreement phase, the manufacturer should be required to determine the cooler's heat exchange area based on the natural flow of water when the ship is stationary, ensuring that the ship can achieve the design heat exchange efficiency when stationary at the dock or anchorage. The 17,500-ton chemical/oil tanker is designed according to the following requirements:
Figure 3: Required Direction of Grid Openings
Main Engine High-Temperature Box-Type Cooler: Minimum speed 3 kn.
Main Engine Low-Temperature Box-Type Cooler: Minimum speed 3 kn.
Generator and Auxiliary Equipment Low-Temperature Box-Type Cooler: Stationary state 0 kn.
Cleaning Margin of the Box-Type Cooler: To be executed according to the specifications.
3.4 Shape Design and Installation Method of the Cooler
The shape design and layout of the box-type cooler are flexible and can be optimized based on the shape of the seawater tank or the system characteristics:
The cooling tube bundle can be designed as circular or square.
If the seawater tank space is limited, the cooling tube bundle can also be designed in a stepped structure.
Multiple coolers can be designed in series or parallel.
A combined cooler design is also possible (i.e., using the same cooler mounting plate for both high- and low-temperature water).
The most common installation method for the box-type cooler is top-mounted, meaning it is suspended downward from the top of the seawater tank. If the shipowner has special requirements, a bottom-mounted installation method can also be used, where the cooler is lifted upward from the bottom of the seawater tank. The main differences between the two installation methods are shown in Table 1, Figure 4, and Figure 5 on the following page.
Table 1: Comparison of Advantages and Disadvantages of Installation Methods
Figure 4: Top-Mounted Box-Type Cooler
Figure 5: Bottom-Mounted Box-Type Cooler
3.5 Hull Section Division
Before installing the box-type cooler, it is essential to ensure that the internal fittings, paints, and other components in the seawater tank are completed. Since the internal space of the seawater tank is limited, once the cooler is installed, subsequent tasks such as grinding, welding, and final painting cannot be performed.
When dividing the sections at the design stage, construction convenience should be fully considered. The seawater tank for installing the box-type cooler should be located within the same section to avoid issues like large welding seams that may affect subsequent equipment installation.
4. Corrosion Protection Measures for the Body
The cooler's tube bundles are fully immersed in seawater during operation. Although the tube material is highly resistant to seawater corrosion, if the external surface of the tube bundles is not coated, copper rust will form after six months of use, and biofouling, such as barnacles, mollusks, and algae, may also develop. At first, it is difficult to observe the impact on the cooling performance when biofouling starts; only when the surface is fully covered with biofouling, blocking the flow channels between the copper tubes, can its effect be visually judged.
The simplest and most effective external corrosion protection method is to use seawater-resistant materials for the tube bundles and all parts in contact with seawater. The surface should also be coated with phenolphthalein or phenolic resin paint to prevent galvanic corrosion between the cooler and the hull. Additionally, sacrificial zinc anodes or impressed current cathodic protection (ICCP) electrodes can be installed in the seawater tank to enhance corrosion resistance.
Internally, corrosion prevention is achieved by adding cooling water inhibitors. These inhibitors should be selected based on the purpose and temperature of the cooling water system. Without inhibitors, microorganisms in the cooling system may proliferate, leading to the corrosion of pipes and equipment. The metabolic byproducts of these microorganisms can form a sticky layer inside the pipes, which may affect cooling efficiency or cause blockages in the copper tubes.
5. Measures to Prevent Biofouling
Compared to traditional impressed current antifouling devices (ICAF), the Thermal Antifouling System (TAS) has no environmental impact and does not require additional electrodes, which further reduces operational and maintenance costs.
For preventing biofouling on the exterior of the tube bundles, either an impressed current antifouling device (ICAF) or a Thermal Antifouling System (TAS) can be used.
The ICAF releases toxic copper ions from an anode copper rod installed at the bottom of the cooler in the seawater tank, creating an environment harmful to marine life and inhibiting their growth. The released toxic Cu(I) ions only last for a short time and quickly convert to Cu(II), so the environmental pollution is minimal. The copper rod typically lasts for 3-5 years, with its dissolution rate controlled by adjusting the current.
The TAS operates based on the heat intolerance of marine organisms. When the ship is docked or at the pier, the louver mechanism in the seawater tank is closed, creating a relatively sealed environment. Heat from excess steam or high-temperature water from auxiliary machinery is used to raise the temperature of the cooling water inside the cooler. This heat treatment removes or kills the biofouling on the tube bundle's exterior. The TAS system requires an additional heat exchanger, pump, and independent piping system, with the low-temperature freshwater system and external heat sources being separate.
It is important to note that the TAS system design depends entirely on the heat capacity of the auxiliary machinery's high-temperature cooling water system, making the calculation of system heat balance and coordination between equipment critical. The TAS system should only be used when ensuring the normal and safe operation of one auxiliary engine.
6. Construction Preparation and Process Control
Due to the special installation location of the box-type cooler (e.g., if problems arise after the ship is launched, it must be dry-docked again), controlling and protecting the installation process is extremely important.
Since the box-type cooler is not widely used in China, and many designers are unfamiliar with it, it is necessary to thoroughly review the manufacturer's installation manual and prepare the "Box-Type Cooler Installation Process" document, tailored to the actual conditions of the shipyard. The process should include transport, hoisting, mounting plate welding, installation steps, bolt tightening, and protective measures.
Before installation, a technical briefing should be held with the production department and construction unit to ensure all relevant personnel are informed of and understand the installation requirements.
