MARINE Archives - Cressall

WATER VERSUS CONVECTION COOLING — WHAT’S THE DIFFERENCE?

CHOOSING THE RIGHT COOLING SYSTEM FOR YOUR APPLICATION

For applications relying on an electrical drive system, such as those in the marine and automotive sectors, overheating poses serious risks to equipment performance. Employing an appropriate cooling system helps to safeguard equipment, but knowing what to look for can be tricky. Here, Mike Torbitt, managing director at resistor manufacturer Cressall, explains the differences between water and convection cooling and how to determine the best-suited system.

When engines operate significantly above their optimum working temperature for long periods of time, they are at risk of engine failure due to overheating. Excessive heat not only reduces the ability of lubricants to protect engine parts from wear and tear, but it can also lead to thermal shock. This phenomenon occurs during rapid temperature changes, causing application components to expand and contract at different rates, resulting in cracks and fractures.

There are several steps you can take to protect against overheating. Firstly, it’s important to understand that optimum operating temperatures differ between applications. For example, the ideal temperature for car engines ranges between 75 and 105 degrees Celsius, while for boats this can vary depending on engine type. Selecting dynamic braking resistors (DBRs) with insulated components can help to prevent thermal shock, but employing an effective cooling system is also essential in avoiding overheating.

However, with both water and convection cooling options to choose from, it can be difficult to select the right cooling system for your resistor. So, what’s the difference?

CONVECTION VERSUS WATER-COOLED DBRS

While both options require minimal maintenance and are cost-effective to run, there are several key differences between air and water-cooled resistors.

Convection cooling, also known as air cooling, refers to the transfer of heat into the ambient air using airflow. There are two types of convection cooling systems available: natural and forced convection.

Natural convection relies on the buoyancy effect to cool the application. Since warm air is less dense than cool air, it naturally rises upwards away from the heat source and is replaced by cool air. Natural convection is therefore able to generate a consistent air flow without the need for ventilation mechanisms in applications where the ambient airflow meets thermal demands.

However, where increased heat transfer is required, forced convection cooling is preferable. Since forced convection uses fans, more air can be moved through the system in the same amount of time.

Despite generally being more effective than natural cooling, forced convection also has its limitations. Fans can be noisy and take up a lot of space, meaning they are not well suited to compact applications.

Consequently, water cooling often provides a more effective solution. Not only does the water-cooling method use less space and energy than convection cooling, but it is also better suited to applications with higher continuous power requirements. Since liquid has a higher density than air, it has a higher capacity for heat carrying.

MARINE COOLING

Water-cooled DBRs are especially useful in maritime applications. Cooling often proves difficult as the drive system is usually placed within the ship’s innermost parts and surrounded by heat-sensitive equipment.

To tackle this, most vessels use a chilled water system for machinery cooling and air conditioning. Adding resistors into systems such as the closed air/ closed water (CACW) is relatively simple and allows for up to 95 per cent of the energy from a DBR to be transferred to the ship’s water supply. This recirculation protects the equipment in the machinery room from detrimental ambient temperature increases.

Some marine applications also utilise a sea water cooling system. Provided the DBR is coated in a suitable material such as titanium to safeguard against erosion, this method is a sustainable way of reducing fresh water usage.
Cressall has several decades of experience in designing and manufacturing convection and water-cooled DBRs for applications ranging from automotive and railways to cranes and maritime. In addition to matching continuous power requirements of up to 1500 kilowatts (kW) for convection-cooled DBRs and up to 1800 kW for water-cooled DBRs, Cressall also offers custom options tailored to individual applications.

Safeguarding your application from heat is crucial but proves challenging without a thorough understanding of the different cooling methods available. Choosing a reliable convection or water-cooled DBR provides the assurance that your equipment is protected against overheating.

To discuss the right cooling system for your application, get in touch with our expert team.

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Diving into marine resistor design

DIVING INTO MARINE RESISTOR DESIGN

DESIGN CONSIDERATIONS FOR OFFSHORE ELECTRICAL COMPONENTS

Covering over 70 per cent of the Earth’s surface, the oceans are a vital element of our planet’s ecosystem. However, for the millions of vessels that cross them, the aquatic environment can present a problem. Vessels are increasingly using electrical systems to power across oceans, but a component’s design must account for these extreme conditions.

Whether for main propulsion propellors, crane or lifting systems, or cable laying, electrical drives can be found at the heart of many marine operations, offering increased control, reliability and mechanical simplicity. Dynamic braking resistors (DBRs) are an essential part of an electric drive system that remove excess energy from the system when braking to either dissipate as heat if system is not receptive to regeneration or if system is receptive, but energy level goes beyond the system limits, so needs to be removed.

When designing electrical components for offshore applications, material selection is key from the start of the process to guarantee that equipment will perform under harsh conditions, including saline atmosphere, high wind loadings and corrosive sea water.

Engineers tasked with designing resistors for marine applications must consider material choice, structural stability and cooling method.

CORROSION-RESISTANT MATERIALS

Sea water and the saline atmosphere is corrosive, which could leave equipment inoperable. Due to this, stainless steel, combined with special paint systems, is typically used for the enclosure metalwork for resistor elements. With materials containing at least 10.5 per cent chromium, stainless steel reacts with oxygen in the air to produce a protective layer on its surface to prevent corrosion if not painted.

There are many grades of stainless steel that can offer high corrosion resistance, which can be further enhanced by the addition of extra elements. For below-deck applications, 316 and 304 stainless steel contain nickel to broaden the protective layer created by the chromium, and can be used in unpainted condition.

However, for above-deck components, 316 stainless steel has a higher nickel quantity and added molybdenum, so the resistor unit’s metalwork receives optimum protection against the marine atmosphere, but in some conditions, painting will also be required. Cressall’s resistor enclosures for the EV2 resistor terminal cover boast at least an IP56 ingress protection rating, certifying that sea water cannot enter the unit to cause harm.

In addition to the exterior, it is important that the resistor’s element can withstand the harsh conditions. For these applications, Alloy 825 sheathed mineral-insulated elements are less vulnerable to atmospheric corrosion. As the element in encased within the mineral insulated sheathing, the sheath is at earth potential, so if water or high humidity is present this will prevent accidental contact with the live element, making them a much safer choice for marine applications.

STRUCTURAL STABILITY

Weather at sea is unpredictable, so vessels must be able to withstand the large variance in wind and harsh sea conditions found worldwide. Many offshore structures such as wind turbines are located in areas with high winds, so if the system requires resistors to help provide stability to their electrical components these considerations must be considered within a resistor’s design.

Considering the impact of a vessel’s rotational motions — its side-to-side motion, or pitch, and its front-to-back motion, or roll, is crucial. Design engineers need to ensure that there is enough mechanical support in the structure to stabilise the resistors for safe operation when it is subjected to these forces.

Cressall can conduct finite element analysis (FEA) to help ensure structural stability. FEA allows design engineers to predict a product’s performance in the real world, then see the impact of forces and make changes accordingly. This ensures the resistor performs well in the potentially extreme weather conditions.

It’s also important to consider the size constraints of marine applications. In contrast to onshore units, offshore electrical components must fit into a compact area, so the size of the unit’s support structures must be minimised without compromising durability.

COOLING METHOD

An essential part of a resistor is its cooling system. Since the resistor dissipates excess energy as heat, the cooling system is responsible for cooling the resistor element to ensure continued operation. Depending on the layout and resources of the system, resistors can be naturally or forced air or water-cooled.

Air-cooled resistors come in two types — forced and naturally cooled systems. Forced cooling systems use a fan to dissipate heat in a compact space. These units are suitable for deck mounting and can be secured using anti-vibration mounts. Natural cooling is the most common in marine applications, offering a higher power rating and can be mounted in machinery spaces, protected environments or on deck. For machinery spaces or protected areas, consideration should be given to how the hot air released from the resistors should be evacuated to ensure other equipment mounted locally does not overheat.

Alternatively, the cooling system can use the vessel’s chilled water system, which circulates cool water for air conditioning and equipment cooling. If the chilled system uses sea water, titanium-sheathed elements with super duplex steel metalwork can be incorporated, for continuous use in acidic, tropical sea water and downgraded to 316 stainless steel for freshwater systems.

The ocean is a valuable asset for energy, transport and trade. Ongoing development of electric drives for marine applications can be challenging, but taking these conditions and energy savings into account makes them a viable and advantageous option for powering vessel and for use in offshore structures.

When required Cressall can design the resistors to help with your application. Contact us here.

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Breaking the ice

THE ROLE OF RESISTORS IN POWERING ICEBREAKERS

Sea ice covers around twelve per cent of the world’s oceans, blocking the path for ships attempting to travel across the Earth’s coldest regions. Although they aren’t a new concept, icebreaker ships consistently play a crucial role in clearing these routes for trade, research projects and travel.

Icebreaker ships are a special class of vessel designed to break through even the thickest of ice sheets. Initially developed to open up trade routes that experience either seasonal or permanent ice conditions, ice breakers are commonly found in areas like the Barents Sea, Artic Ocean and the Saint Lawrence Seaway. More recently, they’ve also been used to support scientific research projects in the Arctic and Antarctic.

DETAILED DESIGN

To meet the challenges of ice-covered waters, icebreaker ships have a very specific, carefully considered design. The bow of an icebreaker has a unique shape that is smoother and rounder than a standard vessel, to allow it to easily glide over thick ice sheets with minimal opposing force. As it glides over the ice in this way, the weight of the ship descends onto the ice, crushing it and clearing the path.

To power the icebreakers to smoothly move over this difficult seascape, the vessels also require a significantly enhanced electric propulsion system that matches the power requirements for the icebreaker’s thrusters to break through the ice.

As the sole enabler of transportation through these ice-covered waters, it’s essential that the propulsion system — and all of the components that it includes — are reliable, effective and safeguarded. If an icebreaker were to fail in transit, there could be major disruption to the global supply chain in the repair time. Think the Suez Canal fiasco in 2021, but much colder.

RELIABLE RESISTORS

One component that plays a crucial role in ensuring the safe operation of an icebreaker’s electric propulsion system is a dynamic braking resistor (DBR). When there is no ice in the vessel’s path, there’s less load on the system, meaning that any excess energy produced is surplus to requirements. To dissipate this excess energy, a DBR is integrated into the system, which acts as a load dump during propulsion and icebreaking activities. This load dump activity stabilises the power system, giving a constant load to the vessel’s gas engines.

It’s important to include a DBR in the electric drive system of an icebreaker for several reasons. Without the DBR, the power system would destabilise, risking potential damage to other components of the power circuit. If this continued, it could eventually lead to the loss of the vessel’s icebreaking function and complete failure of the power system.

Therefore, integrating a DBR is an absolute essential for icebreaker vessel design engineers. However, it’s not as simple as just selecting a DBR. There are several design elements for this specific application that must be considered to ensure the drive’s optimal performance.

MARINE MATTERS

When designing electrical components, like resistors, for use on icebreakers, there are several application-specific factors to consider. Each component needs to be able to withstand the salty, cold and unstable conditions that are common at sea.

In terms of structural stability, conducting rigorous testing procedures like finite element analysis (FEA) provides evidence of a component’s ability to withstand unpredictable, inhospitable conditions. It’s also important to design in line with standards outlined by the global testing, inspection and certification specialists Bureau Veritas, for global compliance.

The saline atmosphere at sea is corrosive, so selecting the right material is essential to prevent salty sea water from leaving equipment inoperable. For metal components, it’s important to use stainless steel with a chromium content of at least 10.5 per cent. This enables the stainless steel to react with oxygen to produce a protective layer that prevent corrosion, even in an unpainted condition.

Icebreaker vessels are an indispensable part of the marine transport system. While their function is simple, having the right electrical components, including DBRs, designed specifically for icebreaking applications, is crucial to their safe and successful operation, making even the most treacherous of routes in the Polar regions accessible all year round.

Cressall designs and manufactures DBRs specifically for icebreaker vessels. Our team of expert engineers works together with our customers to develop the ideal, customer DBR solution for each application. For more information, please get in touch here.

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Marine resistor design