Plate Heat Exchanger For Marine

Marine On-board

Boat is an isolated land surrounded by sea water, you breathe the salty air, you drink the treated water and you live in a narrow space with everything you have to live with, all because of the duty you carry and safe return. Hofmann offers the most reliable and efficient thermal systems for your vessels, our device aims to provide maximum efficiency for various closed-circuit cooling systems at sea and handle other vehicle-mounted applications, including fresh water production and HVAC systems.

Ⅰ Introduction of Plate Heat Exchanger in Marine Turbine

In marine turbines, the heat exchange is undoubtedly a critical factor in ensuring the regular use of the engine. Because when the turbine is working, it needs to maintain a specific temperature to ensure the regular operation of the machine. There are many kinds of heat exchangers used in the marine engine, in which plate heat exchanger is one of the essential ones and will be the prevalent potential equipment on vessels in the future.
The heat exchanger has been one of the indispensable pieces of equipment in the marine engine, which can be applied in the main and auxiliary oil cooler, cylinder liner, freshwater cooler, fuel heater, steam condensers, water heater and other types of equipment. The unique working environment of the ship has exceptional demands on the heat exchanger, including the factors such as instability in the sea, which may encounter vibration, shock, large-angle tilt and noise. The cooling medium is usually seawater, with high salt concentration, high humidity of the working environment, and extreme plate corrosion. The marine itself is very compact in structure, having more stringent restrictions on the heat exchanger space and quality. The heat transfer medium of a traditional heat exchanger is always water, and the heat transfer medium of marine heat exchangers is generally oil and water. The emergence of a plate heat exchanger effectively solves a large part of the problem.

1. The equipments for vessel requires compact structure and small quality

At present, the plate heat exchanger used in the ship turbine is generally considered smaller than the traditional heat exchanger, with a tiny volume. For this purpose, finite element analysis is used. At the same time, to meet the requirements of marine types of equipment, the unnecessary parts of the plate are removed or reduced according to the actual stress and strain distribution, and the weak parts are locally thickened. Put gas cutting method on the whole body to reduce the influence of welding deformation on the strength, and weld together the inner plate and the panel, then eliminate the residual stress. It will make the use of equipment as much as possible on the installation in a limited space to bring more benefits; it will complete the heat transfer work to ensure the regular use of the marine.

2. The equipments for vessel must be for the impact power, large angle shaking, vibration and other issues

On the bilge, all selected parts can be used fixed connection with multi-point fixed method. Some particular critical parts also use a rib structure, which increases the stability of the equipment. Only when the installation stability is improved can the heat exchange efficiency and effect of the heat exchanger be ensured. It will affect the heat transfer effect once it occurs due to problems such as vibration,

3. The problem that the cooling medium is seawater and the concentration of nitrogen ions is high

Since many turbines are in direct contact with seawater, they substantially affect the heat transfer medium during use. Therefore, at present, the plate used in the turbine is usually made of a material with high corrosion resistance, such as titanium material, and its quality is much smaller than that of stainless steel. To ensure the quality of the titanium plate and for routine inspection, the plate is also subjected to a 100% colour penetration test to eliminate the error further when the board is sampled.

4. Heat transfer media problems

Since the specific heat capacity difference between water and oil is relatively significant, the temperature difference is more extensive during the heat transfer process. The heat transfer coefficient is also more effective. To make the product reliable and economical, it is necessary to ensure the oil-water heat transfer performance under the ship working conditions to ensure that the equipment has a specific heat exchange margin. Only when the heat medium can effectively conduct heat can the heat transfer effect be achieved.

5. Low price and operability

Plate heat exchangers are inexpensive, and their production process is very mature. During production or maintenance, we can reduce costs and increase efficiency as much as possible. It can be cleaned by loosening the screws in the mounting structure and reassemble to use again. If it is already damaged, then we can change to a new one. Its low price enables it to be prepared with emergency equipment.

Ⅱ The Role of Cooling Water System

Some of the mechanical equipment in the diesel engine power produces heat in regular operation. The heat must be distributed in time; otherwise, the heating element temperature will continue to rise more than allowable limits and will damage the reliability of mechanical equipment. In common, to distribute these calories immediately inefficiently, let a certain amount of liquid flow continuously through the heat sink and bring the heat out of the equipment. Cooling systems typically use freshwater, seawater or river water as a cooling medium.
A lot of heat taken away by the cooling system is residual heat burn done by fuel, generally about 20% to 30%. As the diesel engine is one of a heat engine, it relies on the combustion of fuel heat to work, so it is loose to take away the heat. The more intense of cooling, the greater the loss of this part is, and the more obviously the reduction of diesel economy. On the other hand, since the temperature of the heat-receiving member is high, the average temperature in the inner wall of the cylinder can reach 200 ° C to 300 ° C, and the surface temperature in contact with the cooling water is low, the thermal stress is liable to occur at such a temperature difference. The temperature difference is too significant to crack the part.
It can be seen and the diesel engine has strict requirements for the cooling process. The effect of the cooling system is to cool the diesel engine force, the temperature of each heated part is controlled within the allowable range and at the same time maintain the proper cooling water temperature and use the appropriate cooling medium to ensure its regular and reliable operation.
In determining the cooling water temperature, it is necessary to take complete account of the lack of cooling or excessive cooling. Such as the lack of cooling will make the parts overheated, resulting in mechanical properties of materials decreased, to have thermal stress and deformation, damaged the average surface gap, also resulting in wearing excessively and even damaged. Cooling will also bring too high oil temperature, shorten the service life of oil to cause oil deterioration and coking, destruct oil film to loss lubrication. On the contrary, excessive cooling will allow the coolant to take too much heat, reducing the diesel economy. While using oil with high sulfur, extreme cooling will make the cylinder form sulfuric acid and corrosion the cylinder wall and piston. Therefore, the thermal equilibrium calculation of the cooling water system is fundamental.
In the use of diesel power plant on the ship, the need for cooling machinery and equipments are:
① main and auxiliary diesel engine. Including cylinders, pistons, injectors and superchargers.
② Main and auxiliary diesel engine oil cooler, freshwater cooler and other heat exchangers.
③ shaft bearings and so on.
④ air compressor, condenser and other equipment.
⑤ other mechanical equipment or heat exchangers. Such as air conditioning, refrigeration units, deck machinery, hydraulic systems.
In these machineries and equipment, the main engine needs more heat dissipation. So the ship cooling system constitutes a cooling system with the main cooling pipe as the centre, other machinery and equipment cooling pipe, and a variety of cooling auxiliary equipments together.

Ⅲ The Basic Form of Cooling Water System

1. Open Cooling Water System

The open cooling water system refers to the diesel engine being cooled directly by outboard water or river water.
The advantage of the open system is that the unit is easy to manage and maintain. The disadvantage is that the quality of outboard water is poor. The river water contained impurities, and seawater held a variety of chlorinated salt, which will block the cooling space or cause corrosion of the parts and produce scale to reduce thermal efficiency. For salt precipitation, the seawater temperature should be below 50 ~ 55 ℃. High-temperature components cannot be used. Because the diesel engine cooling water temperature is generally above 60 ℃, the high-speed machine should reach 80 ℃ ~ 90 ℃. So now, in addition to the riverboat, the open system is not applied.

2. Closed Cooling Water System

In terms of the open system, the closed cooling water system is that the diesel engine itself is cooled with fresh water, and the outboard water cools freshwater by the heat exchanger. That is, fresh water in the system does the closed cycle and closed the cooling of freshwater is done by another open cooling system, not the diesel engine itself to cool. So it has many advantages
⑴ cycled in the machine is clean, fresh water, not easy to clog.
⑵ clean water is not accessible to fouling to ensure a good heat transfer effect and extend the service life of components.
⑶ not subject to seawater precipitation of salt temperature limit can use a higher cooling water temperature to improve thermal efficiency.
⑷ shorten the warm cylinder time, improve mobility. When the tank is warm, the freshwater does not pass through the cooler or close the seawater.

3. Central cooling water system

To make the mechanical equipment other than the hosting engine also uses freshwater, and with a system, cooling to form a central cooling water system if some of the gears are cooled with seawater alone, which is called hybrid cooling system.
In the diesel freshwater cooling system, there are high-temperature water circuits and low-temperature water circuits. For example, using high and low-temperature water system parameters is a mixed-flow central cooling system; if the high and low-temperature circuit is separated, it is the independent central cooling system. In an independent central cooling system, if a high-temperature water heat exchanger is cooled with low-temperature water freshwater, it is the independent type Ⅰ, if the use of seawater to cool high-temperature water, known as the independent type Ⅱ.
In some high-speed ships such as container ships, ships, etc., its central cooler low-temperature fresh water cooler, also known as bucket cooler, can also be designed to cool the seawater obtained by the speed of the ship’s sailing, called the self-flow central cooling system, which is a particular form of the independent type I.

Ⅳ The Principle of Cooling Water System

The cooling water system can be divided into a seawater cooling system, freshwater cooling system. And the freshwater cooling system can be divided into a low-temperature water cooling system and a high-temperature water cooling system (also known as a cylinder water cooling system). Large-scale ships mostly use a central cooling water system, so the system can also be combined, known as the cooling water system. Here we introduce the conventional seawater cooling system and the independent central cooling water system.

1 Conventional Seawater Cooling System

The conventional seawater cooling system consists of two seawater cooling pumps, oil coolers, cylinder water coolers, temperature control valves, return lines and other components.
Its working principle is the seawater pump inhale the sea from the seawater gate and seawater pipes, and then divided into two ways: one way is to the host of the air cooler, another way is to send seawater into the oil cooler, and then after the freshwater cooler flow together with discharged water from the host air cooler, through the three-way thermostat directly go outboard or back to the seawater pump inlet.
Due to the influence of the seasons or sailing areas, the temperature of the seawater is constantly changing. Therefore, a three-way thermostatic valve is installed on the pipeline of seawater discharge. When the seawater temperature is lower than the set temperature, the seawater is returned to the inlet of the seawater to improve the temperature of the seawater. The opening and closing of the three-way thermostatic valve are controlled by the temperature sensor installed on the outlet pipe of the seawater pump.
The ship usually has at least two seawater doors arranged on both sides and as low as possible. One of the three can also be a high seawater door; one can be a low seawater door. In the shallow waterway, sailing can use a tall seawater door to prevent the inhalation of sediment. But for safety reasons, large ships are set up with three seawater doors, one high seawater door, two low water doors.
Seawater should be cool lubricating oil first, and then cool freshwater by the freshwater cooler too. This is because the oil temperature is lower than freshwater, and the viscosity is high. Its heat exchange performance is worse than freshwater.

2 Independent Central Cooling Water System Type Ⅰ

The independent type Ⅰ central cooling water system consists of three separate circuits, namely the seawater circuit, the high-temperature freshwater circuit, and the low-temperature freshwater circuit.

⑴ seawater circuit

The seawater circuit is quite simple. Two seawater pumps inhaled seawater from the seawater mains sending into the central cooler. Cool low-temperature freshwater directly discharged to the outboard.

⑵ high-temperature freshwater circuit

A high-temperature freshwater circuit (the host cylinder water cooling system) is a closed-loop system. Two high-temperature freshwater pumps will send high-temperature freshwater into the host of the cylinder, piston and injector cooling, and then discharged from the highest point of the main engine, after the three-way thermostat valve, high-temperature freshwater cooler or bypass pipe, and gas cabinet back to the high-temperature freshwater pump suction port.

In the host freshwater discharge pipeline, installed parallel a water machine or a light system, using high-temperature freshwater waste heat to make freshwater, as added fresh water on board to use. It is an energy-saving device.

Installed in the high-temperature freshwater cooler inlet and outlet bypass line The three-way thermostat valve is used to control the host freshwater outlet temperature. The general requirements of the host freshwater outlet temperature are controlled at 80 ℃ or so. The adjustment of the freshwater temperature of the main machine can be achieved in three different ways. ① adjust the amount of freshwater entering the cooler. ② adjust the amount of fresh water in the main engine. ③ adjust the amount of low-temperature freshwater or seawater entering the high-temperature freshwater cooler. Although these three methods can adjust the freshwater temperature, the latter two methods will cause the host into and out of the water temperature to be too large or too slow, which are rarely used in the ship. Moreover, a three-way thermostat valve can be installed in the more excellent inlet pipe and located in the outlet pipe. The system also features a high-temperature water expansion tank, which can:

1) Allows freshwater flowing in closed circulation lines to cause swelling when the volume changes due to temperature changes

2) Expel the vaporized gas in the pipeline due to local heat out of the system. Thus, the highest point of freshwater exports is generally connected to a ventilation pipe and an expansion tank.

3) Can use the static pressure head in the expansion of the water tank so that the suction pipe is always maintained at a high water pressure, to avoid the low-pressure vaporization in the pipe, to maintain pressure stability

4) Can use the expansion tank to add freshwater loss

5) Treatment water quality in the dosing sites

⑶ low-temperature freshwater circuit

The cryogenic freshwater circuit is also a closed-loop system. The whole loop is the same as the conventional seawater cooling system but must also be cooled by seawater. In the actual ship, low-temperature water is also divided into several ways: diesel generators, air coolers, air compressors, air conditioning, refrigeration, atmospheric condensers and other cooling. The system is much more complex.

The three-way thermostatic valve in this circuit is installed in the central cooler outlet. The outlet temperature of the cryogenic freshwater cooler is generally controlled at 36 ° C. It also established a special low-temperature water expansion tank.

Where the system is located in two pumps, one of the pumps is spare and required to start automatically and make a conversion.

The difference between independent type Ⅱ central cooling water system and type Ⅰ is that the main cooler, called low-temperature freshwater cooler, is cooled by the seawater the same as high-temperature freshwater cooler, whose working principle and system are the same. In addition, the position of the cooling pump and the cooler in the closed circulatory system can be varied in two different ways. The main difference between the two arrangements is whether the freshwater pump is directly connected to the freshwater inlet line or the outlet line. The freshwater from the cooling pump first enters the main engine inlet, which can keep the cooling water in the host to maintain high pressure, so the cooling water in the cylinder cooling chamber is not easy to vaporize, which can guarantee the good cooling effect of the diesel engine. The disadvantage of this arrangement is that the cooling water pressure from the host entering the cooler has been reduced, and the seawater may leak into the freshwater at the cooler tube sheet. Freshwater through the cooling pump out, first into the freshwater cooler, then to the host of the components to cool, and finally return to the cooling pump suction port. The advantages and disadvantages of this arrangement are just the opposite.