Exhaust heat exchanger

Types of exhaust heat exchangers

By definition, an exhaust heat exchanger refers to a device designed to transfer the heat produced by exhaust gases to other fluid mediums within a mechanical system.

Based on the flow arrangement of the two fluid mediums involved, heat exchangers are generally classified into four main categories:

• Counterflow heat exchangers: In counterflow heat exchangers, both fluids flow in opposite directions to one another. This arrangement allows for a significant temperature difference between the two fluids along the entire length of the heat exchanger, which results in a very high heat transfer efficiency.
• Crossflow heat exchangers: Crossflow heat exchangers have two fluids flowing perpendicular to each other. In comparison to counterflow heat exchangers, crossflow heat exchangers can be more easily manufactured and are therefore widely used in many different types of applications.
• Co-current flow heat exchangers: Co-current flow or parallel flow heat exchangers are similar in that both fluids flow in the same direction. Although co-current flow heat exchangers are easier to manufacture like crossflow heat exchangers, they exhibit lower thermal effectiveness when compared to counterflow heat exchangers.

Furthermore, heat exchangers can also be differentiated based on their structure. Most heat exchangers for exhaust systems feature a tubular structure. According to their name, tubular heat exchanger systems use tubes as their main build components.

Plate type heat exchangers, on the other hand, use plates to facilitate heat transfer. Plate heat exchangers are suitable for high-efficiency commercial and industrial applications. They are made up of thin plates that are typically enclosed within a frame.

Specification and Maintenance

• Sizes:

  Exhaust heat exchangers come in various sizes to fit different types of vehicles. Bigger cars might need a larger heat exchanger because it has a bigger engine.

• Pipes:

  The exhaust heat exchanger has different-sized pipes for the exhaust and the coolant. The different sizes help the right amount of fluid flow. Usually, the exhaust pipe is between 25 mm and 100 mm in diameter, while the coolant pipe diameter ranges from 10 mm to 50 mm.

• Materials:

  Since the heat exchanger is exposed to high temperatures and corrosive exhaust gases, it is typically crafted using stainless steel, hastelloy, aluminum, or titanium, known for its ability to withstand heat and resist corrosion. Some are even made with a combination of materials for improved performance and durability.

• Connections:

  The heat exchanger is connected to the exhaust system and the cooling system through flanges and clips that make tight seals and prevent any leaks.

• Efficiency:

  The efficiency of an exhaust heat exchanger is the percentage of heat it transfers from the exhaust gas to another fluid. Typically, the efficiency rates between 60% and 80%.

Because of its critical function in vehicle performance, an exhaust heat exchanger needs consistent upkeep to ensure it works properly and lasts for a long time. Regular inspection is essential. Check the heat exchanger exhaust for any signs of damage, such as cracks, breaks, or blockages. Additionally, users should also ensure there's no coolant leakage from the exchanger side. If any leaks are detected, they should be promptly repaired to avoid further damage to the vehicle.

Clean the exhaust passage frequently to eliminate any carbon buildup or debris that might accumulate over time. If the heat exchanger is of the air-to-air type, it's equally important to clean the fins on a regular basis to avoid debris buildup that could affect air circulation. It's important to choose the right cleaning product for the exchanger type and not to use any abrasive products that could cause damage to the unit. For more complex units, it may be necessary to use professional products.

Checking all connectors and ensuring they're all tight is another important aspect in the maintenance procedure. If any loose connections are detected, users should first clean the connection, then use a gasket, and finally, tighten the connection with a torque wrench to the manufacturer's specifications.

Scenarios

• Industrial Boilers:

  In an industrial boiler setting, an exhaust heat exchanger plays a vital role. The boiler produces steam for various processes, and much heat is in the steam going out with the boiler's exhaust. An exhaust heat exchanger captures this leftover heat. It transfers the heat to incoming water or air to preheat it before it enters the boiler. By doing this, the heat exchanger boosts the boiler's efficiency. It reduces the need for fuel to heat water or air that is already partially warmed. This lowers energy costs and makes the entire boiler system more productive, which is beneficial for industrial facilities.

• Marine Vessels:

  On a marine vessel like a ship or boat, an exhaust heat exchanger is very useful. The vessel has engines that burn fuel to create power, and this process generates a lot of heat. An exhaust heat exchanger takes some of the heat from the engine exhaust gases. It uses this heat to warm up water through heat transfer. The heated water can be used for many things on the vessel. For example, it can provide hot water for crew showers and kitchen use. Or it can be used for heating and cooling systems to help regulate the vessel's internal temperature. This prevents the vessel from having to use extra energy to heat water. It also makes better use of the fuel burned by the engines, which conserves fuel on marine vessels.

• HVAC Systems:

  An exhaust heat exchanger can be very helpful in an HVAC (heating, ventilation, and air conditioning) system. This type of system is found in many buildings. It works to maintain comfortable indoor temperatures and good air quality. An HVAC system often has to ventilate the building to bring in fresh outdoor air, which is important. But at times, the incoming outdoor air may not need a lot of heating or cooling. An exhaust heat exchanger captures some of the thermal energy from the air being exhausted. It transfers this heat to the newly entering fresh outdoor air before it goes into the system. Because of this, the HVAC system can use less energy to warm or cool the incoming air. The heat exchanger enhances energy recovery in ventilation. This leads to improved energy efficiency for the entire HVAC equipment in the entire building.

How to choose exhaust heat exchangers

Choosing the right exhaust heat exchanger can make a big difference in performance and efficiency. First, identify the application or industry. Then determine the amount of heat that needs to be transferred, also known as the heat transfer capacity. Next, select a suitable material that will resist any corrosion from the chemical processes within the machine. A common combination is carbon steel with a zinc coating. The config of the heat exchanger must be selected according to the machine it's going into. Once these crucial factors have been addressed, the sizes of the in/exhaust pipes can now be factored in to ensure a proper fit.

The performance of an exhaust heat exchanger is indicated by its heat transfer coefficient. The higher the coefficient, the more heat will be absorbed or released by that unit. Generally speaking, finned tubes have a higher heat transfer coefficient, making them more efficient, but they also cost more than plain tubes. If budget constraints are an issue, a good way to save costs is to use finned tubes on the side where there is a high heat transfer demand and use plain tubes on the opposite site with low heat transfer demand.

The basic configuration for exhaust heat exchangers is counterflow or parallel flow. In counterflow units, the two fluids flow in opposite directions. This configuration allows for a maximum temperature difference between the fluids. Thus, it provides a higher heat transfer rate compared to parallel flow heat exchangers, where two fluids flow in the same direction. Counterflow tubes are more efficient than parallel flow, but they are also more expensive.

One way to cut down on costs is to use a combined-counter and parallel-flow heat exchanger. In this type of configuration, parallel-flow sections are used closer to the fluid entrance, while counterflow sections are further away. The configuration resembles a hybrid unit and is called a mixed counter and parallel flow heat exchanger. Although it works, it cannot provide the overall efficiency of a pure counterflow heat exchanger. Many heat exchanger manufacturers will have all the different types of configurations. Be sure to ask them to provide data sheets so that a proper comparison can be made.

Exhaust heat exchanger Q amp; A

• Q: What is the function of an exhaust heat exchanger?

  A: The primary function of exhaust heat exchangers in any given machine is to recycle and utilize the heat that is generated from the exhaust gases. By doing so, they are able to heat up another fluid or gas. In doing this, they are able to reduce the overall energy demand that the machine would have needed to function efficiently. This ultimately allows the machine to cut down on energy and power usage and improves overall energy efficiency. With the same process, the heat exchangers can also reduce the carbon footprint of the machine by eliminating the need for additional energy and power sources.

• Q: How do exhaust heat exchangers improve energy efficiency?

  A: The gas or fluid that is usually heated by an exhaust heat exchanger includes water or residual fuel in the boiler industry. Some units can heat the intake air of the engine by the gradient of the temperature between the two media. In the case of marine engines, heat from the exhaust can be used to warm up seawater for use in desalination plants.

• Q: What are the typical applications of exhaust heat exchangers?

  A: Exhaust heat exchangers are typically found in the following industries:

  Chemicals and Petrochemicals

  Power Generation

  Marine

  Oil and Gas

  Refining

  Heavy-Duty Vehicle