HVAC Heat Exchangers Explained The basics working principle how heat exchanger works

Hey there guys. Paul here from theengineeringmindset.com. In this articleWe’re going to be discussing the different types of heat exchangers used in HVAC and building services applications for both residential as well as commercial properties. We’ll also look at how these are applied to system components to condition the build environment. Before we dive into HVAC heat exchangers and how they work, I just want to take a moment to thank our partner Danfoss for sponsoring this article.

Danfoss has a wide range of heat exchangers including micro plate and micro website heat exchangers. Whatever HVAC system you’re working with, they’ve got what you need to boost efficiency, reduce refrigerant charge and save space. Head on over to their website to see all the options available to you and how they can improve your HVAC system. You can find the link in the article description below.

Okay, I have a quick test view and I want you to tell me your answers in the comment section below. I’ll give the answers at the end of the article. There are three heat exchangers explainn on the screen. Now, can you name all of them? No cheating, just give you your best shot and see how you get on.

Firstly, what is a heat exchanger? A heat exchanger is exactly what the name implies, a device used to transfer thermal energy. Heat exchangers are either given a hot fluid to provide heating or a cold fluid to provide cooling.

A fluid can be either a liquid or a gas. Heat always flows from hot to cold and there must be a temperature difference for heat to flow. How is heat exchanged? Thermal energy is transferred via three methods, conduction, convection and radiation. Most heat exchangers for HVAC purposes uses convection and conduction.

Radiation heat transfer does occur but it makes up only a small percent. Conduction occurs when two materials of different temperatures physically touch. For example, if we place a hot cup of coffee onto a table for a few minutes and remove the cup, the table will have conducted some of this thermal energy. Convection occurs when fluids move and carry the thermal energy away. This can occur naturally or by mechanical force such as using a fan.

An example of this is when you blow onto a hot spoon of soup, you blow onto the spoon to cool the soup down and the air carries this heat away. Radiation occurs when a surface emits electromagnetic waves. Everything including you, emits some thermal radiation. The hotter the surface is, the more thermal radiation it will emit. An example of this would be the sun, the heat from the sun travels with electromagnetic waves through space and reaches us with nothing in between.

Fluids used, the fluids used in HVAC systems typically include water, steam, air, refrigerant or oil as the transfer mediums. HVAC heat exchangers usually do one of two things. They either heat or cool water or air.

Some are used to cool or heat equipment for performance reasons but the majority are used just to conditioned air or water. Types of heat exchangers. Most heat exchangers follow one of two designs. Either coil or plate design. Let’s have a look at the basics of how both of these work and then see how they’re applied to common heat exchangers and systems.

Coil heat exchangers in their simplest form use one or more tubes which run back and forth a number of times. The tube separates the two fluids. One fluid flows in the inside of the tube and another flows on the outside. Let’s have a look at a heating example. Heat is transferred from the hot inner fluid to the tube wall via convection.

It then conducts through the pipe wall to the other side and the outer fluid which is cooler then carries this away through convection. Plate heat exchangers use a thin plates of metal to separate the two fluids. The fluids generally flow in opposite directions to improve the heat transfer.

The heat of the hottest fluid is convected onto the plate wall and conducted through to the other side.
The other fluid which is entering as a lower temperature then carries this away from convection. Thin tube coil for fluids. Thin tubes are often referred to simply as the coil, eg the heating or cooling coil.

These are extremely common. You’ll find these in air handling units, fan coil units, ductwork systems, evaporators and condensers of air conditioning systems, on the back of refrigerators, in trench heaters, the list really goes on and on. For these heat exchangers, water refrigerant or steam usually flows through the inside and air flows on the outside. For example, when used for heating air using heated water the hot water flows inside the tube and transfers its thermal energy via convection to the tube wall. There is a temperature difference between the hot water and the air so that the heat is conducted through the tube wall.

The air passing on the outside carries this away via convection. The fins usually connect between all the pipes. These sit directly in the path of the flow of air and helps pull the heat out of the pipe and get it into the air because this acts as an extension to the surface area of the pipe. Duct plate the heat exchanger.

Duct plate heat exchangers are used in air handling units to exchange thermal energy between the intake and exhaust air streams without moisture being transferred and without air streams being mixed. The heat exchanger is made from thin sheets of metal, typically aluminium with the two fluids of different temperatures flowing in opposite diagonal directions. Usually, air is used in both but the exhaust gases from something like a CHP engine can also be used. The heat from one stream is convected into the thin sheets of metal which separates the streams.

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This is then conducted through the metal where it is carried away by forced convection into the other stream.

Trench heaters, trench heaters are installed around the perimeter of a building, usually under a window or glass wall and are very common in new commercial buildings. Trench heaters are installed into the floor and their purpose is to reduce the heat loss through the glass as well as preventing condensation forming. They do this by creating a wall of convecting air currents. Trench heaters usually use hot water or electric heating elements to heat the air.

Their position at the floor level means they have access to the coldest air in the room. The heat exchanger transfers the heat into this via the thin tube. This causes the cold air to heat up and rise towards the ceiling.

As it rises up, cold air in the room will rush in to take its place. This creates a convective current and a thermal boundary between the glass and the room. Duct electric heater, open coil element. Open coil heating elements are used mostly in ductwork applications, furnaces and sometimes fan coils.

These operate using exposed live coils of highly resistive metal to generate heat. These heat exchanges are placed directly into the flow of air and as the air passes across the coils, the thermal energy is transferred via convection. These provide uniform heating across the air stream, although these are only used where it is safe to do so and the coils cannot easily be accessed.

Microchannel heat exchangers. Microchannel heat exchangers are an advancement on the thin tube coil, providing superior heat exchange, although these are only used for refrigeration and air conditioning systems. You can find this type of heat exchangers on air-cooled chillers, condensing units, residential AC, air dryers, cabinet cooling and rooftop units et etcera et cetera.

These type of heat exchangers also work using convection as their main method of heat transfer.
The micro website heat exchanger has a simple design. On each side is a header. Running between each header are some flat tubes with fins in between.

Air passes through the fins between the gaps to carry the thermal energy away. The refrigerant enters through the header and then passes through the flat tubes until it reaches the other header. The headers contain baffles which control the direction of flow of refrigerant and are used to loop the refrigerant through the tubes a number of times to increase the time spent inside and thus increase the opportunity to transfer thermal energy. Inside each flat tube are a number of small holes and then there’s microchannels which run the entire length of each flat tube.

These microchannels dramatically increase the surface area of the heat exchanger which allows more thermal energy to get out of the refrigerant and into the heat exchanger’s metal casing. The temperature difference between the refrigerant and air cause the heat to conduct through the flat tube casing and into the fins. As the air passes through the gaps, it carries the thermal energy away via convection. Furnace evaporator coil.

Furnace evaporators are commonly found in large homes and small commercial properties with small ducted systems. You can get larger coils that work on a similar principle but for larger systems, mostly for AHUs in medium to large commercial buildings. The coil inside the furnace evaporator works the same as a thin tube heat exchanger and uses a refrigerant on the inside with ducted air on the outside. The air passes across the tubes and transfers its heat via force convection. This is then transferred through the tube wall through conduction.

The refrigerant on the inside carries this heat away through force convection. The refrigerant boils and evaporates away to the compressor. Radiators, these are very common especially across Europe and North America in homes and older commercial buildings.

They are mounted two walls, typically under a window to provide space heating. Their function is very simple, they are usually connected to a hot water pipe which is fed water from a boiler.
The water enters through a small diameter pipe and flows to the inside of the radiator. Internal area of the radiator is larger than the pipe which slows the water velocity down and allows more time for the heat to be transferred. The heat of the water is transferred by conduction to the metal walls of the radiator.

On the outside of the radiator is the air of the room. When this air comes into contact with the hot surface of the radiator, the heat will transfer into the air and this will cause the air to expand and rise. Colder air then moves in to replace this air causing a continuous cycle of moving air which heats the room. This moving air is therefore convection heat transfer. Water heating element.

The water heating element is usually found in calorifiers and water heaters. It’s also sometimes used in the basin of open cooling towers to prevent the water from freezing in winter. These use a metal coil along the tube which has a high resistance value. This resistance generates heat. The coil is insulated to contain the flow of current but permit the flow of thermal energy.

The heating element is submerged in a tank of water and the heat is conducted out of the element and into the water. The water which comes into contact with the heating element is therefore heated and this causes it to rise within the tank. Cooler water then flows in to replace this this heated water where this cycle will then continue. Rotary wheel, this type of heat exchangers are usually found within the air handling units between the supply and extract ducted air streams. They work by using a small electric motor connected to a pulley belt to slowly rotate the heat exchanger disc which sits directly in the airflow between both the exhaust and fresh air intake.

The air passes straight through the disc but as it does so it comes into contact with the material of the wheel. The material of the heat exchanger disc absorbs the thermal energy from one stream of air and as it rotates and enters the second stream of air where it will release this absorbed thermal energy. This type of heat exchanger will result in a small amount of fluid mixing between the intake and exhaust air streams due to the small gaps present where the wheel rotates.

Therefore, it can’t be used where strong odors or toxic fumes are used. These heat exchangers can be used in the winter months to reclaim heat from the building’s exhaust system. This heat is captured by the thermal wheel and transferred into the fresh air intake stream which will be much cooler than the air inside the building.

These heat exchangers can also be used in the summer months to recover cold air from the building’s exhaust and use this to cool down the fresh air intake. Water boiler, you find large boilers like this mostly in medium to large commercial buildings in cooler climates. Homes and smaller buildings will use much smaller versions which are usually wall-mounted. Both have many variations but this type is very common.

Fuel is combusted in the combustion chamber, usually through gas or oil and the hot exhaust gases are forced through a number of tubes until they reach the flue and are released to the atmosphere. The tubes and the combustion chamber are surrounded by water. The heat convex to the tube walls and is then conducted through into the water.

This is then carried away by convection. Depending on the system design, the water then leaves as either heated water or as steam. The water is forced through by a pump, the speed of the pump as well as the amount of fuel combusted can be varied to change the temperature as well as the flow rate.
Heat pipe, you’ll find these in solar thermal water heaters and some heat recovery AHU coils. If we look at the solar thermal application, we have a tube made from special glass which is evacuated of all air to create a vacuum and is then sealed.

The inner layer of the tube has a special coating. The coating and the vacuum work together to prevent the heat from being able to leave, once it enters the tube. It then helps move this to the heat pipe at the center. The heat pipe has a fin on each side connected to the tube coating to pick up the thermal energy. The heat pipe is a sealed long hollow copper pipe which runs the length of the glass tube and has a protruding bulb at the top.

The bulb is connected into a header and cool water passes through the header to pass across the bulb head. Inside the heat pipe is a water mixture held at very low pressure. temperature difference between
This low pressure allows the water to evaporate into steam of little heat. The steam then rises up into the bulb where it will give up this heat into the water flowing through the header.

As the steam gives up its heat, it will condense and fall back down to repeat the cycle. The tube absorbs thermal radiation. This is then conducted into the tube. The water inside convexes up to the bulb, the heat is conducted through the pipe wall and is carried away by convection into the stream of water. Chilled beam, there are two types of chilled beam used, passive and active.

Both are used mostly in commercial buildings. Active chilled beams work by passing a cool liquid, typically water, through a thin tube heat exchanger. Air is then ducted into the chilled beam and it exits through a specially positioned nozzle. The air moves over the thin tube and blows the cold air into the room. Therefore, it uses forced convection.

Passive chilled beams will also use thin tube heat exchangers but they will not have a ducted air supply connected. Instead, they create a natural convection current by cooling down the warm air at the ceiling level. This air then sinks and is replaced by warmer air where the cycle will repeat.

Furnace heater, furnace heaters are common in homes with ducted air conditioning. These are very common in North America. Furnace heaters use a heat exchanger placed directly into the ducted air stream.

Fuel is combusted and the hot gas is sent through the heat exchanger. The heat of this is convected into the walls of the heat exchanger. The cooler ducted air passes across the other side, causing a temperature difference. So, the heat of the gas is conducted through the wall and will be carried away by convection. Plate heat exchanger.

There are two main types of plate heat exchangers, gasket type and brazed plate type. These are both very effective at transferring thermal energy but for even greater efficiency and compact design, you can use a micro plate heat exchanger for many applications. We’ve covered all of these heat exchangers in great detail, previously. Links are in the article description below.

The basic things to know about these two types of heat exchangers is that gasket type can be dismantled. Its heating or cooling capacity can be increased or decreased simply by adding or removing heat transfer plates. You’ll find these use especially in large or high-rise commercial properties to indirectly connect chillers, boilers and cooling towers to the heating and cooling circuits or to connect buildings to district energy networks. Brazed plate heat exchangers are sealed units which cannot be dismantled. Their heating or cooling capacity is fixed.

These are used for applications such as heat pumps, combi boilers, heat interface units, connecting calorifires indirectly et cetera. Both work the same however and pass fluids, usually in opposite directions in adjacent channels. The fluids are usually water and/or refrigerant.

The thermal energy is convected onto the plate and then conduct through the plate and the fluid on the other side carries this away through convection. Shell and tube, shell and tube heat exchangers are typically found on chillers for the evaporator and/or condenser, sometimes are also used for lubricating oil cooler. These are possibly the simplest design of heat exchanger. They have an outer container known as the shell. Sitting inside the shell are a number of pipes known as the tubes.

The tubes contain one fluid and the shell contains another fluid. The two fluids are always separated by the tube walls, they never meet or mix. The fluids will be at different temperatures which causes a thermal energy to be transferred between the fluids and this thermal energy will pass through the tube walls. When using the evaporator or condenser, the two fluids will be water and refrigerant.

Depending on the design, the water can be in the shell or the tube and the refrigerant will be in the other. Chillers, a chiller will use either a shell and tube heat exchanger, a plate heat exchanger or a thin tube heat exchanger. Many chillers will actually use a combination of all of these. For example, an air cooled chiller may use a shell and tube heat exchanger for the evaporator, a thin tube or microchannel heat exchanger for the condenser, a brazed plate heat exchanger for the compressors oil lubrication cooling and a gasket plate heat exchanger to indirectly connect the chiller to the central cooling circuit. brazed plate
Okay, before I wrap things up, I just want to say thanks to Danfoss one last time for sponsoring this article.

Don’t forget to check out all their heat exchanger solutions over on their website. Just click the link in the article description below.
The answer to the questions at the beginning of the article is a, shell and tube heat exchanger, b, brazed plate heat exchanger and c, micro website heat exchanger. Okay guys, that’s it for this article. Thank you very much for reading.

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