HVAC Heat Exchangers Explained

How heat exchangers work

Explanation of HVAC heat exchangers. In this article, we are going to discuss the different types of heat exchangers used in HVAC applications and building services for residential and commercial properties. We will also see how they are applied to system components to condition the built environment by covering the operating principle of common HVAC heat exchangers with animations.

Scroll down for video tutorial - includes detailed animations for each heat exchanger!

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What is a heat exchanger?

A heat exchanger is exactly what its name suggests, a device used to transfer (exchange) heat or thermal energy. The heat exchangers receive a hot fluid to heat or a cold fluid to cool.

· A fluid can be a liquid or a gas.

· Heat always changes from hot to cold.

· There must be a temperature difference for heat to circulate.

How is heat exchanged?

Thermal energy is transferred through three methods.

· Driving

· Convection

· Radiation

Most HVAC heat exchangers use both convection and conduction. Radiant heat transfer does occur, but it is only a few percent.

conduction heat transfer

Thermal image conduction heat transfer - hvac heat exchangers

Conduction occurs when two materials of different temperatures physically touch each other. For example, if we put a cup of hot coffee on a table for a few minutes and then remove the cup, the table will have conducted some of this heat energy.

convective heat transfer

Convection occurs when fluids move and carry away heat energy. This can happen naturally or through mechanical force, such as the use of a fan. An example of this is when you blow on a spoonful of hot soup. You blow on the spoon to cool the soup and the air carries away the heat.

radiant heat transfer

Radiation occurs when a surface emits electromagnetic waves. Everything, including you, emits heat radiation. The hotter a surface is, the more thermal radiation it will emit. An example of this would be the sun. Heat from the sun travels as electromagnetic waves through space and reaches us with nothing in between.

fluids used

The fluids used in the HVAC system generally include water, steam, air, coolant, or oil as transfer fluids. HVAC heat exchangers typically do one of two things: they heat or cool air or water. Some are used to cool or heat equipment for performance reasons, but most are used to condition air or water.

Types of heat exchangers.

Most heat exchangers follow one of two designs. Coil or plate design. Let's take a look at the basics of how these two work and then see how they apply to common heat exchangers in systems.

Coil Heat Exchangers - Simplified

Basic coil heat exchanger

Coil heat exchangers, in their simplest form, use one or more tubes that go back and forth several times. The tube separates the two fluids. One fluid flows into the tube and another flows out. Let's take an example of heating. Heat is transferred from the hot internal fluid to the tube wall by convection, then it is conducted through the tube wall to the other side and is also carried away by the external fluid by convection.

Plate heat exchangers – simplified

base plate heat exchanger

Plate heat exchangers use thin metal plates to separate the two fluids. The fluids normally flow in opposite directions to improve heat transfer. Heat from the hotter fluid convects into the plate wall and is then conducted to the other side. The other fluid, which enters at a lower temperature, is then carried away by convection.

Let's take a closer look at how these types of heat exchangers apply to HVAC applications.

Finned Tube Coil (Fluid)

Finned Tube Coil Heat Exchanger

Finned tubes are often referred to simply as a coil, eg heating or cooling coil. These are extremely common. You'll find them on air handlers, fan coil units, ductwork systems, air conditioning system evaporators and condensers, on the back of refrigerators, on trench heaters, and the list goes on. .

In these heat exchangers, water, coolant or steam usually circulate inside and air circulates outside.

For example, when used to heat air, using hot water, the hot water circulates inside the tube and transfers its heat energy by convection to the tube wall, there is a temperature difference between the hot water and the air, so the heat is conducted through the tube. tube wall The air that passes to the outside is carried away by convection.

Fins typically connect between all the pipes, sit directly in the airflow path, and help draw heat from the pipe into the air as it acts as an extension of the pipe surface. More surface area = more space for heat transfer.

Duct Plate Heat Exchanger

Ductable plate heat exchanger

Plate and duct heat exchangers are used in air handling units to exchange heat energy between inlet and outlet air streams without transferring moisture and without mixing the air streams. The heat exchanger is made of thin sheets of metal, usually aluminum, and the two fluids of different temperatures flow in opposite diagonal directions. Usually air is used in both, but exhaust from something like a CHP engine can also be used.

Heat from one stream convects toward the thin sheets of metal that separate the streams, then is conducted through the metal where it is forced by convection toward the other stream.

trench heater

Trench heaters are installed around the perimeter of a building, usually under a glass window or wall, and are very common in new commercial buildings. Trench heaters are installed in the ground and their purpose is to reduce heat loss through the glass, as well as prevent the formation of condensation.

They do this by creating a wall of convective air currents. Trench heaters generally use hot water or electric heating elements to heat the air. Their position at ground level allows them to access the coldest air in the room. The heat exchanger transfers heat to it through a finned tube, causing cold air to warm up and rise to the ceiling. As this warm air rises, cooler air in the room rushes to take its place. This creates a convective current and a thermal boundary between the glass and the part.

Electric Duct Heater - Open Coil Element

Electric Duct Heater

Open coil heaters are primarily used in duct, furnace and sometimes fan coil unit applications. These operate by using exposed energized coils made of highly resistive metal to generate heat. These heat exchangers are placed directly in the airflow, and as the air passes through the coils, heat energy is transferred by convection. These provide even heating through the airflow, though they are only used where it is safe to do so and not easily accessible.

Microchannel heat exchangers

Microchannel heat exchanger

Microchannel heat exchangers are an improvement over the finned tube coil offering superior heat exchange, although they are only used for refrigeration and air conditioning systems. You can find these types of heat exchangers in air-cooled chillers, condensing units, residential air conditioners, air dryers, cooling cabinets, and rooftop units, etc.

These types of heat exchangers also work with convection as the primary method of heat transfer. The microchannel heat exchanger has a simple design. On each side is a header, between each header are flat tubes with fins in the middle. The air passes through the interstices of the fins to evacuate the thermal energy.

The refrigerant enters the header and then passes through the flat tubes until it reaches the other header. The headers contain baffles that control the direction of coolant flow and are used to circulate the coolant through the tubes multiple times to increase the time it spends inside and therefore increase the chance of transferring heat energy.

Inside each flat tube are a series of tiny holes called microchannels that run the entire length of each flat tube. These microchannels greatly increase the surface area of the heat exchanger, allowing more heat energy to leave the coolant and enter the metal shell of the heat exchanger. The temperature difference between the coolant and the air causes heat to be conducted through the flat tube shell to the fins. When the air passes through the interstices, it evacuates this thermal energy by convection.

Furnace Evaporator Coil

how furnace evaporator air conditioning works hvac heat exchangers explanation

Furnace Evaporator Coil

Furnace evaporators are commonly found in large homes and small commercial properties with small duct systems. You can get larger coils that work on similar principles, but for larger systems, primarily for AHUs in medium to large commercial buildings. The coil inside a furnace evaporator works similar to a finned tube heat exchanger and uses refrigerant on the inside with ducted air on the outside. Air passing through the tubes transfers its heat by forced convection, this is then transferred through the tube wall by conduction, the refrigerant inside removes this heat by forced convection, the refrigerant boils and evaporates to the compressor .

radiators

These are very common, especially in Europe and North America, in older houses and commercial buildings. They are mounted on the walls, usually under a window, to heat the space. Their function is very simple, they are normally connected to a hot water pipe to which hot water is fed by a boiler.

The water enters through a small diameter tube and flows inside the radiator. The internal area of the radiator is wider than the pipe, which slows down the velocity of the water to allow more time for heat transfer.

The heat of the water is transmitted by conduction to the metal walls of the radiator. Outside the radiator is the air from the room. When this air comes into contact with the hot surface of the radiator, heat is transferred to the air, causing the air to expand and rise. Cooler air then moves in to replace this air, causing a continuous cycle of moving air that warms the room; therefore, this moving air is convective heat transfer. The radiator usually has fins attached to the back or between the panels, especially on newer ones, these are there to extend the surface area of the radiator and provide more opportunity for heat transfer to the air. Radiators are misnamed because they are mainly transferred by convection.

You will sometimes find purpose built heaters connected to steam systems but this is becoming less common, oil was also used but this is quite rare these days.

water heating element

The water heating element is usually found in water heaters and heaters, it is also sometimes used in the basin of outdoor cooling towers to prevent water from freezing in winter. These use a metal coil along the length of the tube that has a high resistance value. This resistance generates heat. The coil is insulated to contain the flow of current but allow the flow of thermal energy. The heating element is immersed in a tank of water and heat is conducted out of the element into the water. Therefore, the water that comes into contact with the heating element heats up and this causes it to rise inside the tank, cooler water then flows to replace this hot water where this cycle will continue.

rotating wheel

Rotating Wheel Heat Exchanger

These types of heat exchangers are generally located in the air handling unit between the supply and extract ducted air streams. They work by using a small electric motor attached to a pulley belt to slowly rotate the heat exchanger disc that sits directly in the airflow between the exhaust and fresh air intake. The air passes directly through the disc but in doing so it comes into contact with the material of the wheel. The heat exchanger disc material absorbs heat energy from one air stream and as it rotates it enters the second air stream where it releases this absorbed heat energy. This type of heat exchanger will result in a small amount of fluid mixing between the intake and exhaust airflow due to the small gaps present where the impeller rotates, so it cannot be used where strong odors or toxic gases.

These heat exchangers can be used during the winter months to recover heat from the buildings exhaust stream, this heat is captured by the heat wheel and transferred to the fresh air intake stream which will be much colder than the air inside the building.

These heat exchangers can also be used during the summer months to capture cool exhaust air from the building and use it to cool incoming fresh air.

Kettle

Explanation of how an HVAC boiler heat exchanger works

You'll find large boilers like this mostly in medium to large commercial buildings in colder climates. Smaller homes and buildings will use much smaller versions, usually wall-mounted. Both have many variations, but this type is very common.

Fuel is burned in the combustion chamber (usually gas or oil) and the hot exhaust gases are forced through a series of tubes until they reach the chimney and are discharged into the combustion chamber. The tubes and the combustion chamber are surrounded by water. The heat is convected towards the tube walls and is then conducted to the water, which is then evacuated by convection. Depending on the system design, the water exits as heated water or steam. This water is forced through a pump, the speed of the pump and the amount of fuel burned can be changed to change the temperature and flow rate.

heat pipe

You'll find them in solar thermal water heaters and some UTA heat recovery coils. If you look at the application of solar thermal energy, we have a special glass tube that is evacuated of all the air to create a vacuum and then sealed. The inner layer of the tube has a special coating. The liner and vacuum work together to prevent heat from being able to escape once it enters the tube, then help move it into the heat pipe in the center.

The heat pipe has a fin on each side connected to the tube liner to capture heat energy.

The heat pipe is a long, sealed, hollow copper tube that runs the full length of the glass tube and has a protruding nub at the top. The bulb is connected to a manifold and cold water is passed through the manifold to pass through the head of the bulb.

Inside the heat pipe is a mixture of water maintained at very low pressure. This low pressure allows the water to evaporate as steam with little additional heat. The steam then rises in the bulb where it releases its heat to the water circulating in the collector. As the vapor gives up its heat, it condenses and falls back down to repeat the cycle. The tube absorbs heat radiation, which is then conducted into the tube. The water inside rises by convection to the bulb, the heat is conducted through the wall of the pipe and carried by convection in the water current.

cold beam

Chilled Beam HVAC Heat Exchangers Explained

Cold Beam HVAC Heat Exchangers

Two types of cold beams are used, passive and active. Both are mainly used in commercial buildings.

Active chilled beams work by passing a cold liquid, usually water, through a finned tube heat exchanger. The air is then channeled into the cold beam and out through specially located nozzles. This air travels over the finned tube and blows cool air into the room. Therefore, using forced convection.