Understanding how heat is transferred efficiently is essential in many commercial and residential applications. At the heart of this process lies the heat exchanger, that facilitates the exchange of thermal energy between fluids. Whether heating our homes during winter, cooling them during the hot seasons, heating our pool or recover heat from grey water, heat exchangers play an important role in the process.
In this article, we will explore the following facets in detail, shedding light on how heat exchangers enable efficient thermal management together looking at the different types of heat exchangers, their materials, sizing considerations, and diverse applications.
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Principle of Heat Transfer
The theory of heat transfer delves into the core principles and mechanisms underlying the exchange of thermal energy between diverse objects or areas. According to the laws of physics, energy within a system naturally gravitates towards equilibrium, prompting the flow of driving energy until a state of balance is attained. Concerning heat transfer, this entails the movement of heat from a warmer source or fluid to a cooler one, until temperature uniformity is achieved.
The transfer of heat from one medium to another, or from one fluid to another, is governed by several fundamental rules.
- Heat will always be transferred from a hot medium to a cold medium.
- There must always be a temperature difference between the media.
- The heat lost by the hot medium is equal to the amount of heat gained by the cold medium, except for losses to the surroundings.
There are three methods by which heat can be transferred.
- Conduction
- Convection
- Radiation
Conduction
Conduction is the transfer of heat by direct contact between bodies or through the same body. conduction, there is no transfer of matter, only energy.
At higher temperatures, molecules exhibit increased vibration or movement. When these molecules come into contact with adjacent molecules at a lower temperature, they transfer some of their energy. This energy transfer occurs within the same object or between two objects in contact.
Convection
Convection – Energy is transferred by mixing part of a medium with another part. Heat is transfer via the interchange of hot and cold molecules. It occurs when a surface at a certain temperature is in contact with a fluid moving at a different temperature.
There is 2 ways that convection can happen.
- Natural convection, in which the fluid motion is entirely because of differences in the density of the fluid temperature due to the variation between two points.
- Forced convection where the fluid movement is due to some external factor. The transfer of heat is better with forced convection, since the movement – the speed – is much higher, as there is assistance from the external factor (e.g., pump, fan, wind or stirrer) in addition to the density difference.
Radiation
Radiation is heat transfer via electromagnetic waves. It could be termed as molecular transport, as energy is produced by changes in the electronic configurations of constituent molecules or atoms and transported by electromagnetic waves or photons. There is no direct contact between the two media and the intermediary or interface does not participate in the exchange functions; in most cases this is air, although there is also heat transfer in a vacuum.
| Conduction | Convection | Radiation |
| Energy transferred by direct contact | Occurs in gasses and liquid | Energy Transferred by electromagnetic waves such as light, microwaves and infra red radiation |
| Energy flow directly from warmer object to cooler object | Movement of large number of particles in same direction | All objects radiate energy |
| Can occur within one object | Occurs due to difference in density | Can transfer energy through empty space |
| Continues until object temperatures are equal | Cycle occurs while temperature differences exist |
Heat Exchangers
Understanding the principles of heat transfer lays, the foundation to comprehend the function of a heat exchanger.
Heat exchangers are devices designed to efficiently transfer heat between two or more fluids, or between a fluid and a solid surface, while keeping them physically separated. Notably, the term "fluid" encompasses substances in liquid, gas, or vapor form. They operate based on principles of thermodynamics and fluid dynamics, utilizing conduction, convection, and sometimes radiation to facilitate the exchange of thermal energy.
Heat exchangers come in various designs and configurations, but their basic principle remains that is transferring heat from a hot fluid or surface to a cooler one.
Types of Heat Exchanger
There are several common types of heat exchangers, each designed to suit specific applications based on factors like the fluids involved, space constraints, efficiency requirements, and operating conditions. Here are some of the most prevalent types:
Shell and Tube Heat Exchanger
Shell and tube Heat Exchanger Consists of a series of tubes housed within a cylindrical shell. One fluid flow through the tubes (tube side), while the other flows over the tubes within the shell (shell side), allowing for efficient heat transfer. This Type of Heat exchanger is widely used in industrial processes, power generation, and HVAC systems.
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Plate Heat Exchanger
A Plate Heat Exchanger is Composed of multiple thin, slightly separated plates that have fluid passages between them. The Fluids flow alternatively through the channels formed between the plates, facilitating heat exchange through the plate surfaces. This type of heat exchanger is now used in many types of hydronic heating and cooling systems, as well as for the evaporator and condenser in some refrigeration systems.
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Air Cooled Heat Exchanger
The Air-Cooled Heat Exchanger uses ambient air as the cooling medium to remove heat from a process fluid. the Process fluid passes through finned tubes while air flows over the external surface of the tubes, carrying away heat. It is often used in refineries, petrochemical plants, and other industries where water availability or contamination concerns limit the use of water-cooled systems.
Double Pipe Heat Exchanger
In its Simplest form the Double Pipe Heat Exchanger consist of one pipe inside another larger pipe (annular space between them). One fluid flow inside the inner pipe while the other flows in the annular space between the pipes, enabling heat transfer. It is Suitable for small-scale applications or where space is limited, such as laboratories or small process systems.
Plate-Fin Heat Exchanger
Plate-Fin Heat Exchanger Utilizes a series of fins to increase the surface area for heat transfer. The Fluids flow in alternate channels formed by finned plates, enhancing heat exchange efficiency. Commonly used in aerospace, cryogenics, and HVAC systems due to their lightweight design and efficient heat transfer capabilities.
Each type of heat exchanger has its advantages and limitations, depending on factors such as heat transfer efficiency, pressure drop, maintenance requirements, and cost considerations. The selection of a heat exchanger type is crucial in optimizing the overall performance and energy efficiency of industrial processes and systems.
Sizing
Properly sizing and configuring a heat exchanger is crucial to ensure its effectiveness and cost-efficiency. If the exchanger is undersized, it won't transfer enough heat, leading to poor performance. This can also cause high pressure drops and fluid speeds that might damage the equipment through erosion or vibration. On the other hand, if the exchanger is oversized, it becomes inefficient and may lead to low fluid velocities that increase the risk of fouling.
Factors to Consider when sizing a heat exchanger:
When determining the appropriate size for heat exchangers, several factors must be carefully considered. This includes the following:
- calculating the heat load - the amount of heat that needs to be transferred between the fluids or surfaces
- terminal (inlet and outlet) temperatures
- the surface area
- pressure drops - difference (loss) in pressure between the inlet and outlet of a single process stream as it goes through the heat exchanger
- flow rates
- viscosities
Heat exchanger materials by purpose
Selecting the right material for a heat exchanger depends on factors such as the operating environment, fluid characteristics, temperature, pressure, and corrosion resistance requirements.
- Corrosion-resistant heat exchanger: Hastelloy, Inconel, Tantalum, Titanium, Zirconium
- high-temperature heat exchangers: Nickel-based alloys, Advanced carbon and silicon carbide composites
- Heat exchangers used in seawater environments: Nickel-based alloys, Tantalum, Titanium
Stainless steel is also a material that is often used to manufacture heat exchanger due to its exceptional combination of mechanical, thermal, and corrosion-resistant properties.
Stainless steel alloys, particularly grades like 304 (AISI 304) and 316 (AISI 316), offer excellent resistance to corrosion from various chemical environments, including acids, alkalis, and chloride-containing solutions
Certain grades of stainless steel, such as 304H and 316H, are designed to withstand elevated temperatures without significant deformation or oxidation.
Stainless steel heat exchangers find use across various industries, including HVAC chemical processing, oil and gas.
Application
Snow & ice melting (sim) systems
Snow and ice Melting system can either designed solely for snow and ice melting or can also be used to served multiple load such as space heating and domestic water heating. The ratio of antifreeze in a Snow and ice Melting system is not the same as the one in space heating. Even some space heating systems are designed without the use of anti freeze.
In case that Snow & ice melting system are couple with space heating and domestic water heating, then then a heat exchanger is often used to separate the Snow and ice Melting system “subsystem” from the remainder of the system. This allows the majority of the system to operate with water only or a different antifreeze ratio.
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Pool Heating
In setups where a boiler or hydronic heat pump provides winter space heating and continuous domestic hot water throughout the year, there is typically surplus heating capacity during the offseason. If the property includes a swimming pool, this excess heat can be utilized to maintain comfortable water temperatures for swimming.
Tube and shell are the heat exchange type the mostly used as it is better suited for the higher flow rates needed for pool filter systems.
Water from the heat source flows through the outer shell of the heat exchanger, which is made from materials such as steel, cast iron, or cast brass, forming part of a closed-loop system. Meanwhile, pool water circulates through the tube bundle within the heat exchanger. These tubes are usually made from titanium or a titanium alloy combined with stainless steel to withstand the corrosive nature of heavily chlorinated pool water.
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Domestic water heating
In hydronic systems that provide both space heating and domestic hot water, the most common approach for heating domestic water is through the use of an indirect water heater. Indirect water heaters use internal coil heat exchangers.
Another approach is to use a side arm tube heat exchanger.
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On-demand domestic water heating
Another method for heating domestic water involves utilizing a brazed plate heat exchanger, which draws heat from either a buffer tank or a boiler system. This setup ensures that hot water is generated almost instantaneously when needed at any faucet. This method is suitable for residential and small commercial applications that incorporate a heated buffer tank. the tank can be linked to sustainable energy sources like solar thermal collectors, biomass boilers, or heat pumps, or alternatively, it can be heated by any conventional fossil fuel boiler.
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Hydronic Cooling
Hydronic systems offer a wide range of benefits and the least common one but which its coming more and more popular is hydronic cooling. Water not only conducts heat efficiently but also serves as an excellent medium for cooling. It has the capacity to absorb more than 3000 times the heat energy compared to an equivalent volume of air.
In cooling mode, the hydronic system circulates cold water through the pipes instead of warm water. This cold water absorbs heat from the space, acting as the medium for heat exchange.
Numerous products have been designed for these applications, focusing on forced convection heat transfer on both the water and air sides of the heat exchanger. These products can be classified into the following categories:
Fan Coils
A fan-coil unit is essentially an assembly consisting of a water-to-air heat exchanger, commonly referred to as a "coil," paired with a fan or blower that generates forced convection on the air side of the coil. Typically, fan-coil units are intended to either heat or cool specific areas within a building. By strategically placing multiple fan-coil units, heating and cooling zones can be established throughout the building.
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Air Handler
Air handlers are units specifically designed to integrate with standard ducting systems. Alternatively, there are specialized "high velocity" models equipped with blowers capable of generating significant static pressure. These units typically feature a single trunk duct that distributes air through several flexible branch ducts, each around two inches in diameter.
These smaller branch ducts can be routed through partitions or along framing cavities and terminate at small orifices installed on ceilings or walls. Normally, about six to seven branch ducts are required per ton (12,000 Btu/h) of cooling capacity. Branch ducts typically have a maximum allowable length of 25 feet from their connection point to the trunk duct.
Plumbing
Heat exchanger are also used in plumbing contexts, such as Recovery heat from domestic hot water that has been used in fixtures such as sinks, or shower. This water, known as greywater still retains much of its heat as it drains. Typically, this heat is lost into the sewer system in many buildings. However, using a greywater heat exchanger can recover as much as 50% of this otherwise wasted heat
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Conclusion
In conclusion, the heat exchanger plays an important role in contemporary heating systems, providing efficiency and dependability for both home and business applications. Whether you're upgrading an existing system or embarking on a new project, the right heat exchanger can make all the difference in performance and energy savings.
We invite you to visit our physical store or give us a call for advice. Alternatively, browse our online store for detailed product descriptions, customer reviews, and convenient ordering. Enhance your heating system today with our quality heat exchangers.
