Hey there guys. Paul here from theengineeringmindset.com. In this video, we'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 video. Danfoss has a wide
range of heat exchangers including micro plate and
micro channel 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 video 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 video. There are three heat
exchangers shown 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. 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 channel 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. 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 video 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. Okay, before I wrap things up, I just want to say thanks
to Danfoss one last time for sponsoring this video. Don't forget to check out all
their heat exchanger solutions over on their website. Just click the link in the
video description below. The answer to the questions
at the beginning of the video is a, shell and tube heat exchanger, b, brazed plate heat exchanger and c, micro channel heat exchanger. Okay guys, that's it for this video. Thank you very much for watching. I hope you've enjoyed this
and it has helped you. If so, please don't forget
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theengineeringmindset.com. Once again, thanks for watching.
