A heat exchanger works by allowing heat from one fluid to pass another, cooler, fluid without them mixing or coming into direct contact.
For example, imagine a pipe with another pipe around it. The inner pipe could allow a hot fluid to pass through it while a cooler fluid is passed simultaneously through the outer pipe. This would allow the cooler fluid to reduce the temperature of the warmer one, as the warmer fluid simultaneously increased the heat of the cooler one. Of course, this is a very basic example ofheat exchange, and there are a number of other factors to consider when investigating heat exchangers:
1. Passes
By curving the pipes, for example into an ‘S’ shape, you can allow the fluids to make more than one ‘pass’ before leaving the heat exchanger. A single pass is a straight pipe, where the fluid enters at one end and exits at the other end of the heat exchanger fairly quickly. A double pass uses a U-shape, so that the fluid enters and leaves the heat exchanger at the same end, prolonging the time that the fluids are passing one-another in the heat exchanger. A triple pass uses an ‘S’ shaped formation that allows the fluid to travel along the length of the heat exchanger three time before exiting. The greater the number of passes, the greater the amount of heat transfer is available – simply because the fluids are together in the system for longer – although this can also lead to pressure drops and a loss of velocity.
2. Temperature Cross-Over
Temperature cross-over occurs when the heat of the cooler fluid begins to cross over with the temperature of the hot fluid in the heat exchanger. For example, oil entering a heat exchanger at 80⁰C alongside water at a temperature of 30⁰C, could see their temperatures cross-over should the oil reduce to 50⁰C as the water reach 51⁰C. At this point the cooler fluid (water) has become hotter than the oil. Temperature cross-over can significantly reduce the efficiency of a heat exchanger, particularly when cooling. This can be avoided by increasing the coolant’s flow rate (see, ‘Flow Rate’ below), so there is more coolant in the system. Where temperature cross-over is unavoidable, using a plate heat exchanger (see ‘Types of Heat Exchanger, below) is the best solution.
3. Temperature differential
This refers to the difference in temperature between the coolant and the hot fluid, which is important in a heat exchanger, as shown by ‘temperature cross-over’ (above). The coolant should be kept at a lower temperature than the hot fluid, and, the colder the coolant, the more effective it will be at taking heat out of the hot fluid.
4. Flow Rate
This is the amount of fluid that is passing through a cross-section of a pipe in a specific period of time. It is worked out as the volume of fluid per the time the fluid has flowed - with a greater flow rate potentially increasing the heat exchanger’s capability to transfer heat. However, more fluid also means a greater mass to transport, as well as increasing pressure loss and velocity.
Heat exchangers can be classified according to the flow arrangement or by the design of the heat exchanger itself.
The three main classifications of flow arrangement found in heat exchangers are parallel-flow, counter flow, and cross-flow:
- Parallel Flow: The two fluids enter at the same end of the heat exchanger and travel in parallel before exiting together
- Counter Flow: The fluids enter at opposite sides of the heat exchanger and travel through the system in opposite directions. This is the most efficient flow arrangement, as the average temperature difference between the fluids remains higher throughout the system
- Cross-Flow: In this form of heat exchanger, the fluids travel perpendicular (at right angles) to each other
As well as the flow arrangement, heat exchangers can be categorised by type, according to the physical design of the heat exchanger:
- Double Pipe: This is the most straightforward type of heat exchanger used by industry. As the name suggests, these consist of two tubes through which fluids can flow. The flow configuration can be parallel or counter flow, with counter flow being more efficient while parallel flow is better should the two fluids need to be taken to the same temperature. These heat exchangers are easy and cheap to design and maintain but have a relatively low level of efficiency compared to other designs
- Shell and Tube: Shell and tube heat exchangers use a set of metal tubes through which one fluid flows, surrounded by a sealed shell through which the second fluid flows. This type of heat exchanger works with all types of flow – parallel, counter and cross flow- and can be found in steam locomotives. Baffles are used to direct fluid flow, induce vibrations in the fluid and support the tubes, while fins can also be used with air-cooled technologies (such as combustion engine inter-coolers) to increase the area of heat transfer
- Plate/Fin: This type of heat exchanger uses stacks of thin metal plates that are held apart by fins to form levels, much like the floors of a building. However, each of these ‘floors’ is self-contained and separated from the one above or below, creating a sealed series of tubes through which fluids can flow. This creates a large surface area that is able to exchange heat rapidly while also guaranteeing fluid flow across the entire heat transfer surface, preventing fluid stagnation and accumulation. The plates can be created in different sizes, depths and shapes, including corrugated plates, plate and frame, plate and shell, or spiral plates. A high flow turbulence between the plates creates greater heat transfer and a decrease in pressure. This type of heat exchanger is used in applications such as gas furnaces and boilers.
- Air Cooled: This type of heat exchanger is commonly found in vehicles and other moving applications where cool water is not available. Instead of two liquids, air cooled systems use cool air from a fan or air flow caused by the movement of the vehicle itself.
- Condensers, Boilers and Evaporators: This type of heat exchange deserves a special mention as it works in a slightly different manner to those already described. Rather than inserting two fluids (hot and cold), these use the cooling and condensation of a hot gas into liquid form (and vice versa) to create a heat transfer cycle. This is the process used by steam turbines and steam generators, which use the cycle of vaporising water into steam and its subsequent cooling back into a liquid.
- Recuperators: A recuperator is a heat exchanger designed to capture heat that would otherwise be lost, such as heat being vented from a building. The warm air is directed out through a channel alongside a separate, incoming channel of cool fluid to create a counter flow. This is how a heat-recovery ventilation system works, allowing a building to be ventilated with fresh air without losing all of its heat from the outgoing warmer air
- Regenerators: These type of heat exchanger allow both the incoming and outgoing fluids to move through the same channel in opposite directions at different times. As the warm fluid flows out it gives up some of its heat then, as the cool fluid flows in, it picks up some of the residual heat that was left there. This type of heat exchange is used in a Stirling engine, that uses a piston to push trapped gas back and forth between a heat source (like a fire) and a cooler area, or ‘sink,’ where the heat is lost. This type of heat exchanger reduces heat loss in a system by ‘regenerating’ the heat.
Heat exchangers can be made out of a range of different materials, although metals, such as stainless steel, are commonly used because of their ability to absorb and conduct heat. As well as metals, ceramics, metal and ceramic-based composites, and plastics can be used to make heat exchangers. Each of these materials offers their own advantages, making them suitable for different applications.
Metals offer good thermal conductivity, heat absorption and high temperature resistance, but ceramics can be used for higher temperature applications of over 1000°C, which would melt metals like copper, iron and steel. Ceramics are also used with high and low temperature abrasive and corrosive fluids.
Plastics, or polymers, can also resist corrosion and fouling, as well as being less expensive and lighter than metals. Although they can be engineered to have good thermal conductivity, plastics are not generally suited to high temperature applications. However, with cooler applications, such as heating a swimming pool or shower, plastic heat exchangers work perfectly well.
Composite heat exchangers deliver the advantages of their parent materials, such as the lower weight and corrosion resistance of plastic with the thermal conductivity and heat resistance of a ceramic or metal.
Other materials are also being explored for use in heat exchangers, such as tiny carbon nanotubes, which are being used as heat removing heat sinks for electronic devices due to their excellent heat conducting properties.
Heat exchangers are used for a wide range of applications across industry and for consumer products like air conditioners and refrigerators as well as in cars, aeroplanes and ships, gas boilers, and more.
They are used to save and reuse energy that could otherwise be wasted, saving costs for factory managers and other building operators. Of course, it is not just about saving money, but also saving the environment by not wasting energy if it can be recycled and reused instead. Heat exchangers also improve the efficiency of different processes including heating water or cooling down buildings in hot weather.
For example, power plants often create heated exhaust gases that are vented out into the air. However, a heat exchanger positioned inside chimneys can take the heat from these gases to warm water flowing through tubes, which then takes the heat back into the plant. This warmed water can then be used to heat office space or even to warm cooler gases to start the process once again. While they don’t reuse all of the energy that would otherwise be wasted, these type of heat recovery systems still ensure a significant amount doesn’t go to waste.
Heat generated by the engines in buses can take the fluids used to cool the engine, once heated, and use them to warm cold air from outside the bus as it is pumped into the inside of the bus, saving the need for separate electric heaters in the bus.
Energy efficient showers have a heat exchanger that takes the hot water as it goes down the plug and uses it to warm cold water as it feeds through up to the shower head, without the dirty plug water coming into contact with the clean water. This means that the shower doesn’t have to fully heat all of the water it uses.
Heat exchangers are used for a wide range of applications in both industrial and everyday uses.
While heat exchangers come in a range of different designs and flow systems, they all work in a similar manner; allowing thermal contact between a hotter and a cooler fluid (without actually mixing), so that the two fluids heat or cool one-another accordingly.
This improves the efficiency of vehicles and other units as well as saving costs and lowering energy consumption (and any associated environmental impact).
Can you clean a heat exchanger?
Different types of heat exchanger have different cleaning requirements, depending on design, materials and uses. However, one method is submerge the heat exchanger into an ultrasonic cleaner with appropriate chemicals to remove any unwanted material build up without damage.
Can a heat exchanger be replaced?
Heat exchangers can be replaced, but this can be expensive and so it may be more economical to remove and replace any damaged components. In some cases, professionals may recommend that it is cheaper and easier to replace an entire furnace unit rather than just replacing the furnace heat exchanger.
Which type of heat exchanger is best?
While all heat exchangers are able to serve a purpose, it is generally accepted that the most effective heat exchangers are plate heat exchangers, as there is a greater surface area in contact between the fluids, creating better rates of heat transfer than with other forms of heat exchanger. Plate heat exchangers are, however, often more expensive than other designs.
Who invented heat exchangers?
Heat exchangers were first investigated in the latter part of the 19th Century by the food and beverage industries. The first recorded patent for a plate heat exchanger was given to Albrecht Dracke of Germany in 1878. However, the first commercially-viable plate heat exchanger did not reach the market until 1923, with Dr Richard Seligman revolutionising the indirect heating and cooling of fluids.
Why are heat exchangers important?
Heat exchangers are used to provide heating and cooling for processes and products ranging from aeroplanes to refrigerators and power stations to buses. They are used in everything from energy and oil refining, to manufacturing, transportation, air conditioning, cryogenics and recovery systems to save costs, reduce energy consumption and improve efficiency.