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Heating Ventilation and Air Conditioning Full Course - HVAC Course (Part 27)


            In this part of full HVAC course, we are going to see about Evaporators and Different types of Evaporators in HVAC.

Evaporators in HVAC

An evaporator is used in air conditioning systems to remove moisture from the air before conditioning. A typical commercial unit can have many small coils (called “evaporator pans”) in parallel. Each coil represents a single stage of cooling. The coolant flows through each pan and removes heat from the surrounding area. When the air flow passes over these pans, the cold surface of the pans conducts the heat out of the air passing over them. This cooling effect occurs whether the fan or blower is operating or not.

Natural and Forced convection type Evaporator

Convection air conditioners have been around since the 1960's and have now become almost universal. Air conditioning systems use forced convection to move cool air over the condensing coil where water vapor is condensed out of the air flow and then discharged back into the room via a return grille. In this way, a constant supply of fresh, dry air is circulated throughout the space. Since they first became popular, many manufacturers have created various types of evaporation coils for different purposes. These include: natural convection evaporators (NCAE), forced convection evaporators (FCE), counter-flow cooling towers (CCT) and solar cooling systems. The basic concept behind these designs is to increase the surface area of the evaporative coil in order to minimize the amount of water necessary to create a certain amount of sensible heat output.

Evaporators work best in situations where the ambient temperature is relatively low. When operating at high ambient temperatures, the efficiency of the system drops significantly. However, if the ambient temperature is consistently below 40 degrees F, evaporators will not only produce less heat, but also reduce electricity consumption. There are two major categories of evaporating systems; natural and forced convection. A natural evaporator consists of a flat pan containing water, a fan and a drain pipe. As the fan turns, the warm air passes over the pan, causing the water to evaporate, producing steam. To maximize performance, the pan should be placed directly under the blower blades. This means that the hot air is passing over the pan first before being passed over the condenser coil. The downside of natural evaporators is that they require a lot of maintenance due to dirt collecting in the bottom of the pan and increasing the pressure drop across the evaporator.

The second category consists of forced convection evaporators. These evaporators consist of a vertically mounted coil surrounded by a jacket filled with water. Fans pull in cold outside air and blow it upward over the coil. As the air hits the surface of the coil, some of its moisture is removed and becomes concentrated in the surrounding liquid. This causes the temperature of the air to decrease until it reaches the dew point. At this point, the remaining water drips off and is collected in a sump to be reused. This design makes sense for high humidity climates. Natural evaporators work well in dry climates, while forced evaporators perform adequately in humid environments.

There are several factors that affect the cost effectiveness of an evaporator system. First, the size of the evaporator affects the amount of heat produced and thus the size of the water usage. Second, the quantity of electrical power consumed increases as the number of fans employed increases. Third, the length of the ducting system between the evaporator and the return air plenum will determine how much heat is lost to the environment. Fourth, the height of the ceiling above the evaporator will determine whether the unit can be installed, and fifth, the price of the equipment.

It is often difficult to find a balance between good and cheap. If you want maximum efficiency and minimum costs, you may need to look at a commercial evaporator. These units are designed specifically for indoor applications. Their cost ranges anywhere from $300-$900 per ton. While they offer great efficiency, they are also large, bulky and expensive. Alternatively, smaller residential units range from $150-$700 per ton. Most units can be purchased for less than $100 and are ideal for both small spaces such as apartments and larger rooms in a house. Smaller units are ideal for those who do not mind having a relatively small footprint, while larger units are excellent for rooms with ceilings that are higher than 10 feet. An air handler with a single stage will generally provide sufficient capacity for a small space. However, if the room is larger, two or three stages may be added to meet the volume requirement.

If you plan to cover a lot of ground or install a large unit in a basement, a multi-stage air handling unit will provide additional capacity. Multi-stage units may also come equipped with a variable speed motor which can be controlled manually, electronically or automatically depending on your preferences.
                    The air handler's evaporator coil is responsible for cooling the room while the fan circulates air throughout the entire space. However, different types of evaporators use different methods of removing moisture from the air. 

Why do we need a forced convection evaporator over natural circulation?

- A forced convective evaporator is used for heating and cooling air flow inside homes and commercial buildings, and even a car's cabin. It consists of a blower unit that forces hot air out and cool air into the building through duct work. The air flows through a series of tubes and fins before being released back into the room. As the warm air travels through the tube, it becomes cooler. However, the difference between the temperature of the air inside and outside the tubes causes the air to lose its volume. The greater the air loss, the greater the pressure drop. As a result, the air pressure decreases and the air is forced to exit the tubes at a higher velocity. This type of evaporative equipment uses much less electricity than conventional systems because it does not require any compressors to force the air through the system.

What is the difference between a natural convection evaporator and forced convection evaporator?

- Natural convection happens naturally in closed spaces, whereas forced convection occurs when air passes through a heater or cooler. In a natural convection system, the air is heated or cooled by contact with the outer surface of the material. Hotter air rises while colder air sinks, keeping things just right. When you turn on your heater in winter, the air inside your home moves toward the warmer vents. There is no fan blowing air around a space, only the natural movement of air.

In forced convection, the air is blown past a heat exchanger (which is basically a radiator) to cool the air down. You'll also find a fan pushing the air throughout the house. In both cases, the air eventually exits the heater or cooler where it picks up some of the heat or cold again. In the case of natural ventilation, the air will cycle completely once and then exit the building. In forced ventilation, the air will continue circulating until it reaches a certain threshold. That means the air stays in the house longer.

Flooded and Dry Type Evaporators

The flooded evaporator represents a cooling system that uses water or some other liquid coolant to remove heat from a refrigeration circuit. In many cases, the liquid coolant is sprayed onto tubes inside the condenser coil (a unit containing fins separated by small air gaps), thereby removing heat and turning the coolant's temperature into vapor. The cooled, pressurized vapour is then sent back to the compressor where the cycle begins again.

A dry-type evaporator operates somewhat differently. Instead of spraying liquid coolant over the coils, it has a fan blowing air across them. Thus, the air is cooled and its humidity increased before being expelled into the atmosphere. Airflow across the fins of the condenser coil causes the coolant in the air to become saturated with moisture and is thus turned into vapour, which is collected and compressed by the compressor. The compressed vapour is condensed and returned to the radiator or directly to the compressor. A dry-type evaporator requires less energy than a wet-type evaporator.

The typical evaporator consists of two parts: a housing and a blower/fan assembly. The housing contains the coils and controls the airflow. The fan assembly provides the force to move air around the coils. These two components are connected either by ductwork or tubing.

How does a HVAC technician know if the evaporator is flooding?

If the outside temperature is above 74°F, then you have a problem. You may need to add a dehumidifier. If the problem persists, you may need to replace the evaporator instead of repairing it.

You should check the evaporator for leaks using a black silicone grease. Make sure that the evaporator cover is open and the fan is running. Put a small amount of grease on each blade of the motor. Check both sides of the blades for any leakage. Clean the area thoroughly with soap and water.

What is a flooded evapotranspiration?

A flooded evaporator is a system where water is introduced directly into the air stream before it enters the evaporator coil. Water is added to the air stream to reduce the temperature of the incoming air to the evaporator coil.

How does flooding work?

The purpose of adding water to the air stream is to take advantage of latent heat released by condensation (heat transfer). Water added to the air stream lowers the temperature of the air entering the collector hood, thus lowering the temperature of the water vapor being expelled from the evaporator coils. In addition, the lower humidity air exiting the evaporator reduces the amount of moisture carried back to the compressor.

Why should I use flooded evaporators?

By introducing water into the air stream just before it enters the evaporating unit, the cooling effect is maximized. When using a dryer, the air flow is relatively cool compared to the air flow coming out of the fan. By pre-cooling the air, the air leaving the fan is much warmer, reducing the efficiency of the cooling system. Adding water to the air stream increases the relative humidity of the air before entering the fan, increasing the capacity of the fan to move air throughout the building.

When should I use flooded evaporator systems?

Dryer systems are not recommended if the outside temperature exceeds 70 F (21 C) because the dew point will approach 0 F (-18 C) at these temperatures. Flooded evaporator systems are generally best used in climates where the outside ambient temperature rarely exceeds 75F (24C) and the relative humidity is less than 60%. These conditions prevent condensation problems associated with low relative humidity.

Where do I install a flooded evaporator system?

Install a flooded evaporator system near the ceiling, to avoid direct sunlight. Install the system away from mechanical components such as motors, compressors, fans, etc., that could damage the unit.

 What is Dry Type Evaporator?

A dry type evaporator is used as a cooling system where water is removed from refrigerated air to cool the air before it enters the space being cooled.

How does a dry type evaporator work?

The dry type evaporator works based off of evaporation. As warm air passes over cold coils, moisture condenses out of the warm air and falls back down to the coil where it is reabsorbed into the air stream. At the same time, the cold coil heats the air passing over them causing the air to lose its latent capacity. Thus, the warmer air becomes drier and cooler. Once the air reaches the desired temperature, the remaining liquid is filtered out using a filter to keep the air clean.

 Why use a dry type evaporator?

A dry-type evaporator is much more effective than traditional vapor-compression unit (VCU) systems since it doesn’t require any moving parts or pumps. In addition, it’s extremely low maintenance and requires less power consumption. Since it doesn’t need any moving parts, it eliminates the possibility of breakdowns and repairs, while lower power consumption means no expensive monthly electricity bills. A dry-type evaporator provides a greater efficiency rate since it removes nearly 100% of the humidity and contaminants from the air.

 What are some of the disadvantages of using a dry type evaporators?

There are drawbacks to using a dry-type evaporator, however. For starters, they take longer to heat up and cool down than VCUs, meaning that they are not well suited for applications where quick heating and cooling cycles are necessary. Also, if the fan fails, the entire room may become uncomfortably humid. There are also some safety concerns. For instance, if a fan or compressor breaks down, the equipment could cause carbon monoxide buildup.

How do I know if my space demands a dry type evaporator or a traditional vapor compression unit?

If your space currently has a traditional vapor compression unit, you may want to consider switching to a dry-type evaporators. If you have a space that lacks windows and/or requires a lot of ventilation, then a dry-type evaporations would be best. If your space is small, then a dry-typed evaporator would be ideal. Another good option would be to install two different units – one for each zone.

Shell and Tube type Evaporator

The shell-and-tube type evaporator is an air conditioning equipment consisting of a refrigeration system with water cooling coils inside a tube, similar to the shell of a chicken egg. Shell-and-tube evaporators are designed for small spaces such as kitchens and bathrooms where space to install traditional split systems is at a premium, but still require heating and/or cooling.

Benefits of Shell-and-Tube Type Evaporators over Split Systems

• Can be installed in tight, awkward spaces where split systems cannot

• No ductwork to install

• Lower installation costs than a typical split system

• Requires less maintenance and repairs than split systems

• Less expensive to operate


• Installers need only to remove the old unit and replace it with the new unit

• Installation can be completed in a single day


• Must clean the condenser coil regularly (usually weekly)

• No fan motor requires regular replacement

What do I need?

We recommend having at least two different types of evaporators in your home or work space. One of them should be shell & tube and the other should be a cross flow system. We would suggest that the shell & tube evaporator be located outside near the air intake where the temperature ranges will be lower. Another consideration is if this will be a small room, we may not want to put the evaporator directly inside the room since the fan may cause noise issues.

Where do they go?

The shell & tube type evaporator goes outside the area where the air is being pulled in. The exhaust goes out either towards the back of the building or towards the front depending on how the ductwork is designed. If the ductwork is tight fitting, then the exhaust could exit in the same room, but if it's open, the exhaust might have to travel further away.

 How do they work?

A shell & tube type evaporating unit works by forcing hot air and moisture vapor (water) into a water-filled chamber called a wet bulb. As the heat and moisture move through the wet bulb, they cool down until the point where condensation forms on the surface of the wet bulb. The condensate drains from the wet bulb into a collection tank placed below the wet bulb.

 Are these units safe?

Yes! These units are UL listed and approved for use in residential environments.

 Is a crossflow evaporating system safe?

No! A crossflow evaporator uses cold air rather than heat to draw air across the coil. Cold air does not warm up quickly enough to remove moisture and therefore the evaporation rate is much slower. You can still get mold build up on the coil but it won't be as concentrated due to the time factor involved. The best thing about a cross flow evaporator is that it is quieter than shell & tube systems.

Flooded Type Shell and Tube Evaporators

Flooded type shell and tube evaporators have been around for decades. They have traditionally been used in industrial applications where high purity refrigeration was desired. They work great for small scale residential applications as well. The advantage of these units over traditional air conditioner systems is their efficiency and price. All aspects of the unit are designed for maximum efficiency and minimum maintenance. The only downside to them is they do not work with all types of refrigerants. Typical refrigerants for flooded type shell and tube evaporator are R 1234ze, R 22, and R 500a.

 Another type of evaporator is the flooded type tube exchanger. These units utilize copper tubing that runs through the evaporator coils. Often times, these tubes are connected to additional piping system that extends outside for further cooling. Tubing exchangers take less space than shell and tube evaporators and are usually used in commercial settings. The advantage of tube exchangers is its compact size and ease of installation. Typical refrigerants for tube exchanger are R 1234ze and R 22.

 There are many other types of evaporators out there, including multi-purpose condensing units (MPUs) and integrated recirculating vapor compression units. I am not going to go too deep into these units here. Just know that if you want a larger capacity system you may need to look elsewhere.

Shell and Coil Type Evaporator

The shell-and-coil evaporator was designed specifically for high-volume applications where space and weight constraints make conventional evaporators impracticable. Shell-and-Coil evaporators are capable of delivering high temperatures with low air flow rates, which makes them ideal for use in the agricultural industry where temperature control and precise accuracy are critical. In addition, because they have no moving parts, shell-and-coils can operate continuously without maintenance.


• High efficiency (up to 90%)

• Can handle high humidity levels

• Available in 4 sizes

• Suitable for 1 ton to 200 tons capacity

• No moving parts

• Economical to maintain

The shell and coil type evaporator consists of several parts. The shell has many tubes inside of it that run parallel to each other. These tubes are responsible for transferring air from the outside environment into the room where the evaporator is located. The coils are what actually cools and condenses the steam produced by the cooling water.

What is Shell and Coil?

The shell-and-coil evaporator combines the advantages of both evaporative cooling and traditional air conditioning systems. In this system, water is pumped through the coils (the tubes inside the shell) while warm air flows over them. As the water evaporates, the latent heat released by evaporation lowers the temperature of the flowing hot air. This cooled air then passes across the condensing surface and is returned to the room. All of these processes occur simultaneously, resulting in the same cooling effect as conventional air conditioners without having to use refrigerants or compressors.

How does Shell-Coil Work?

In a shell and coil evaporator, warm air moves over the top of the coils, while cold water circulates through the bottom of the shell. Cold water is circulated through the shell by a pump, while the temperature of the flow of air passing over the coils is reduced by the latent heat absorbed by the cooling water. A fan moves air across the coils, drawing in cool air through the ductwork and out of the house. Since air movement occurs only at the top of the shell and not at its base, no additional fans are required.

 Benefits of a Shell-Coil System

An HVAC shell and coil system uses fewer moving parts than those requiring a compressor and refrigerant. This makes maintenance easier for homeowners and lessens the risk of mechanical failures. Also, since there is no need for moving parts, the heating and cooling loads placed on the electrical power grid are greatly reduced.

Types of Shell Coils

There are many different types of shell and coil units available today on the market, ranging in size from small residential units to large commercial models. Each type accomplishes the purpose of cooling using a unique method. Here are some of the most popular options:

 Horizontal Flow Shell-Coil Units

Horizontal flow evaporators have horizontal tubes running throughout their entire length. These horizontal tubes may be either copper tubing or plastic tubing. The horizontal tubes take cold water and circulate it around the outside of the unit, where it absorbs heat from the air passing over the coils. Air enters the unit through the top and exits the unit through the bottom. They produce little noise and require minimal maintenance.

Vertical Flow Shell-Coil Unit

Vertical flow evaporators have vertical tubes running throughout their entire lengths. This design takes advantage of gravity's force and forces cold water down the center of the tubes, absorbing heat and becoming heated. Air enters the unit at the top and exits the bottom of the unit. They also generate little noise and require minimal upkeep.

Counterflow Shell-Coil Units

Counterflow evaporators are similar to vertical flow designs, except instead of flowing down the center of the tube, cold water flows up the center and releases heat as it returns to the base of the unit. Air enters the top of the unit and exits the bottom.

 Multi-Flow Shell-Coil Unit

Multi-flow evaporators combine features from the previous three styles of shell and coil units.

Double Pipe Type Evaporator

The Double Pipe Type Evaporator consists of two parallel pipes that are joined together. One pipe is located on top of the other pipe. Both the bottom pipe and the upper pipe have a heating coil attached to them. The heat generated by current flowing through the coils heats the air inside the evaporator. The warm air then rises up the lower pipe while still being heated. As the air passes over the top of the upper pipe, the hot air loses its heat and becomes cooler. The cool air moves down the upper pipe to the bottom pipe where it is reheated again. Once the air reaches the bottom pipe, it enters the return section of the ducting system. From here, the air returns back to the supply fan. A Double Pipe Type Evaporater can be useful in cooling rooms that require a lot of space for the same amount of airflow. In addition to that, the evaporative cooler is not only effective in cooling, but it also increases humidity.

The double pipe evaporator design is specifically designed for heating and cooling systems that are not air-cooled. In a typical air-cooled system, a condenser coil collects the cool water vapor that comes off of the refrigeration coils, and then sends it back to the compressor via a second circuit. The purpose of the double piping evaporator is to eliminate the need for a second circuit and allow the water vapor to travel directly back to the compressor.

In order to maintain proper humidity levels in greenhouses, the double pipe evaporator design was created. By routing the hot humid air to the interior of the greenhouse, it creates a positive pressure inside the structure. This helps keep the environment warm and moist, and prevents the build-up of bacteria and mold. The ideal temperatures for the space should ideally be between 65°F – 70°F (18°C – 22°C), and the relative humidity level should stay around 50%. If either of those conditions are exceeded, the risk for the growth of mold and disease increases significantly.

Although the double pipe evaporator is considered a unique solution for reducing ventilation costs, it often does require additional equipment and some modifications to existing ductwork. Depending on what size greenhouse you have, we recommend considering upgrading your existing system to a double pipe evaporator design.

The cost of installing a double pipe evaporator varies depending upon the amount of work involved, and the type of insulation installed. Typically, the installation consists of two holes being drilled at each end of the existing ductwork, and attaching the appropriate hardware. An electrician will wire the unit for operation and make sure it is properly grounded.

Once the unit is operating correctly, the owner will want to check on the temperature and relative humidity levels regularly. It may take several months before the unit’s performance is maximized, but once the greenhouse reaches its target temperature, the unit will provide long term value to the owner over time.

Baudelot type Evaporator

This type of evaporator uses two separate hot water circuits. In the first circuit, fresh air enters at ambient temperature (approximately 25 degrees Celsius) and passes over the refrigerant coil where it picks up moisture. As the air continues to pass over the coils, however, the high temperature of the refrigerant vapor causes some of the water contained in the air to condense on the cold surface of the coil and then drip back down to the water pan below. After passing through the second circuit, the dehumidified air exits as dry air.

The Baudelot system was created by French engineer Emile Baudelot in 1847. Today, his name is synonymous with evaporative cooling technology. The name “Baudelot” comes from the Latin word baudillo, meaning “to protect” or “to shield”. The design features a series of tubes connected to a central manifold through which water flows, forming a closed loop of piping through which air circulates. A fan pulls air through the tubing.

A Baudelot evaporator is often used in medical facilities and research centers where it provides an effective way of maintaining sterile conditions inside a controlled-environment room. One of the first uses of the Baudelot system came about after World War I, when it was used to cool rooms filled with mustard gas victims. By using only air and water, these sickened people could breathe without fear of suffering any ill effects from the chemicals surrounding them.

One of the biggest advantages of this particular type of evaporator is its ability to make use of low temperatures without necessitating the need for a compressor. Other types of evaporators require higher temperatures than this type, making them difficult to install and operate under lower temperatures. When the temperature drops, the efficiency of the unit decreases as well.

This method requires no compressor, just a small electric motor to run a pump, which forces a continuous flow of water through the unit. Because it has no moving parts, the unit operates quietly and does not cause any noise. Another advantage is that it works equally well indoors or out.

Direct Expansion Fin and Tube type Evaporator 

 What is Direct Expansion Fin & Tube?

The direct expansion fin and tube (DX) type evaporator is a refrigeration system commonly used in residential applications. DX units are built using copper tubing, fins, and an aluminum alloy shell. A fan blows air across the water coils, cooling the air inside the unit. The cooled air then flows back out of the DX evaporator and into the room.

What are the advantages of this type of system compared to others?

The DX evaporator offers several advantages over traditional vapor coolers. First, they have higher efficiency ratings than conventional vapor coolers. Second, they allow for smaller, lighter cooling devices. Third, they provide increased airflow control, making them ideal for small spaces. Lastly, they offer greater durability than traditional vapor coolers. Since these systems use copper tubing and copper heat exchangers, they have been proven to last longer than other types of air conditioning systems.

The Direct Expansion Fin and Tube (DET) evaporators are used commonly in industrial applications where they are exposed to extreme conditions such as high temperature, humidity, and air flow rates. These units are designed to provide maximum efficiency and performance over long periods of time. DETs are manufactured using stainless steel tubing and finned copper plate to maximize the evaporation rate and reduce total system pressure drop. Because of their use in these harsh environments, these units require periodic maintenance to ensure proper operation and prevent damage due to corrosion. In addition, regular cleaning of the inside surface of the tubes to remove any build-up of algae or other impurities is necessary to prevent clogging.

Plate Surface Type Evaporators

Plate Surface evaporators are commonly used in residential applications where cooling is not critical. Such as offices, medical facilities, educational institutions, RVs, and home offices. If you decide to install a plate surface evaporator in any of these applications then they should be sized for your specific application.

Dry-type evaporation system

Dry-type systems use water as a refrigerant instead of air. In these systems, water is sprayed directly onto a metal coil, which then condenses the water into a liquid state. Once the water is condensed, it flows down the coil and into a collection area where it drains back into the supply line. Since dry-type evaporators do not use air, they are less prone to corrosion than plate surface evaporators.

 Wetted air-type evaporator

Wetted air-type evaporators work similarly to dry-type evaporators except that instead of spraying water directly onto the metal coil, it sprays water droplets onto a wick, which absorbs the water and distributes it evenly across the coil. A fan blows the air over the wetted coil, drawing the moisture out of the air. The advantage to this method is that it does not require any power input; however, since the air is already moistened, the efficiency of the system is lower than that of dry- and plate surface evaporators.

Plate Type Evaporators

Plate-type evaporators are commonly used in forced air systems for both cooling and heating purposes. In these applications, they are often referred to as plate-and-fin heat exchangers. These heat exchangers consist of two metal plates called “plates” and fins between those plates. Each plate contains holes, through which hot water enters or cold water exits. While passing through the hole(s), the water heats up or cools down. When the water reaches the end of the conduit, the temperature difference causes it to condense and collect on the fin, which then transfers the heat to the room or space being heated. When using water, the colder side of the exchanger provides both the input and output. When using refrigerants, the hotter side provides the input and the cooler side provides the output.

 Advantages to Plate Evaporators

Plate-type evaporators offer many advantages over traditional coil-type evaporators. Because they use less copper than coil systems, they can reduce electrical cost by almost half. Also, since they provide a smaller surface area for condensation, they do not require as much water to create the same amount of vapor. Finally, they are easier to install and maintain. Traditional coil-type evaporators have a relatively small footprint, while plate-type evaporators are able to maximize space utilization.

Disadvantages of Plate-Type Heat Exchangers

The disadvantages of plate-type heat exchangers lie primarily in their performance. As previously mentioned, they produce less heat transfer due to their small surface area. Therefore, they must run at higher temperatures (typically around 250°F) than either coil units or flat plate units. Additionally, they often need to be maintained by cleaning out clogs, which requires shutting off power to the system. Since they are less expensive than coil units, they may be prone to corrosion, especially if exposed to saltwater.

 What are the advantages of using plate type evaporators over finned tube type evaporators?

A plate type evaporator provides a larger surface area than a finned tube type evaporator. As a result, smaller amounts of water vapor per unit volume of air and cooler temperatures can be achieved. Plate type evaporators are more cost effective than finned tube types, due to the low pressure drop across the plates compared to tubes. Also, plate type evaporators are easier to maintain than finned tube types.

How does the design of a plate type evaporator affect performance?

The design of a plate type evaporation system affects performance mainly in two ways. First, the number and size of the plates determines how much heat transfer area is provided. Second, the type of material used to construct the plates influences the efficiency of heat transfer between the plates and the refrigerant. A metal alloy can provide better thermal conductivity than plastic, for example.

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