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How an Air Side Economizer Works

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March 15, 2023

How an Air Side Economizer works. There are energy codes that mandate the use of an air-side economizer for HVAC equipment over a certain size. An economizer reduces energy consumption by using the outdoor air for cooling in mild or cold weather instead of mechanical cooling. We’ll cover how an air-side economizer works and show you three different relief air options.

If you prefer to watch the Video fo this presentation, then scroll to the bottom or click on this link How an Air Side Economizer Works.

Air-side economizers are used with Packaged Units, Split Systems and Air handlers of all sizes. When the outside air temperature is below the return air temperature, then outside air can be more energy efficient to use for cooling then mechanical cooling.

Here is one control layout of many for controlling an economizer. We have an economizer controller which can be integrated into the economizer section or as a separate controller. Next there will be a thermostat in the space, and a Supply Air Temperature sensor in the supply air discharge duct.

There is a Return Air Temperature sensor in the Return Air Duct. The Economizer Controller will need a transformer to provide 24 volt power. The system will need an outside air temperature sensor and Mixed Air Temperature sensor, and communication with the damper actuator, and the power to modulate the damper.

Economizer Relief/Exhaust Air

One of the important design considerations for an outside air system is how to control building pressure when excess outside air is brought into the space. There are three common approaches for the design of the relief air system. Some engineers may refer to this as exhaust air. The Relief or exhaust air needs to be considered to avoid over-pressurizing the space.

Packaged HVAC Unit with an Air Side Economizer

Barometric Damper (Backdraft Damper)

One method is to use relief dampers that are set to open when the building pressure reaches a certain level, such as 0.05” or 12 Pascals. This pressure will be set just below the maximum allowable for the pressure in the building space. This can be a barometric damper with an adjustable weight for varying the pressure relief setting.

As the economizer starts to open the outside air damper, it will modulate the return air damper in the opposite direction, causing the pressure in the building to increase. When the pressure reaches the preset pressure on the barometric relief damper it will begin to open, allowing air to escape the building. When the outside air damper is fully open in 100% economizer mode, the return air damper will be fully closed, causing the pressure in the building to open the barometric damper, allowing excess air to escape the building.

Building Pressure relieved with the use of a Barometric Damper

There are many sequences of operation for controlling an economizer. Each situation is unique to the geographical area and desired indoor conditions. We’ll explain the basic premise of the economizer cycle using the supply air temperature setpoint as the controlling factor.

If we set the supply air temperature to 55 degrees Fahrenheit (13 degrees Celsius), then the economizer controller will use this to determine the position of the dampers, along with all the other input points like an “Outside Temperature Sensor”, “Return Air Temperature Sensor”, Supply Air Temperature Sensor”, “Mixed Air Temperature Sensor” and the room thermostat or main controller. It will send an output signal to the damper actuator for the correct mixture of outside air and return air to meet the supply air setpoint.

If the outdoor temperature is below the return air temperature then the outdoor air can be used in situations where a differential dry bulb type of economizer would be used. If in more humid areas, then the outdoor temperature would need to be much lower than the return air temperature to compensate for the increase in humidity.

The purpose of course is to avoid using the compressor or chiller for cooling in order to save energy. If the outside air damper is 30% open, then the return air will be 70% open, so that we have 100% of the air needed by the supply fan. If the outside air damper opens to 80%, then the return air damper will close down to 20% open. If the outside air is not within the useful range of the economizer, than the outside air damper will be set to its minimum setpoint to meet ASHARE 62.1 requirements for ventilation air.

The use of a barometric relief damper is the least expensive option of the three presented here, as there is no electrical connection, sophisticated controls or motorized damper.

Air Side Economizer in a Classroom with Relief Air (Barometric Damper)

Barometric dampers are used on small systems, such as a classroom as shown here. When the classroom becomes over-pressurized, the barometric damper opens without the assistance of electrical power.

Economizer with Relief Fan/Powered Exhaust

Another method of relieving the building pressure caused by the economizer is to use a relief fan as shown here. When the economizer is operating the relief fan will be engaged and cycle on and off with the economizer. There has to be a method of controlling the relief fan according to the building pressure and the amount of outside air that is brought in during economizer mode, which can varying from 100% down to the minimum position according to ASHRAE 62.1.

Air Economizer with Powered Exhaust / Relief Fan

Remember that the economizer is modulating the volume of outside air that is coming into the building to meet the current demand which is fluctuating. This will require that the relief fan or fans be cable of some form of variable volume. This can occur by staging multiple relief fans in a large system, or with the use of a variable frequency drive to vary motor speed.

Economizer with Return Fan

The addition of a return fan is important with ducted systems that have a large pressure drop to overcome. With the addition of a return fan, the supply fan can be slightly smaller as it doesn’t need to overcome the static pressure of the return ductwork.

Controlling Economizers

Economizers can be controlled using differential dry bulb temperatures or enthalpy, which is both temperature and humidity.

Using differential dry bulb will activate the economizer when the outside air drops below the return air.

When using differential enthalpy the economizer is activated when the outside air enthalpy drops below the return air enthalpy.

We’ll use an example of a differential dry bulb economizer.

In heating mode the outside air damper is at minimum position according to ASHRAE 62.1 ventilation requirements. The minimum outside air mixes with the return air. When cooling is required and the temperature is between 35 and 55 F (2 C to 13 C) the compressor can shutoff and the outside air can mix with the return air to maintain the supply air temperature setpoint. The outside air damper will modulate from its minimum to maximum position in order to satisfy the supply air temperature setpoint.

As the outside air temperature rises further somewhere between 55 F to 75 F (13 C to 24 C), the outdoor air may not be sufficient to reach the supply air setpoint, so the mechanical system will start up and run the compressor or modulate the chilled water valve. This is considered an integrated system, one where the economizer and mechanical cooling work together to satisfy the setpoint temperature. The outside air damper is 100% open and the mechanical system modulates to reach the supply air setpoint.

The economizer will have a high-limit shutoff temperature where the economizer will reset itself to the minimum position to meet ASHRAE 62.1 for minimum ventilation. This can be 75 F (24 C) or slightly lower depending on geographical area.

ASHRAE 62.1 and (0.1 Requirements

ASHRAE 62.1 requires a minimum amount of outside air for ventilation purposes, but with the use of an economizer, outside air can be used for cooling when conditions are right and provide 100% of the air. The economizer can come as part of a packaged HVAC unit or as stand-alone components.

ASHRAE 90.1 2019 6.4.3.4 Ventilation System Controls requires that all outdoor intake and exhaust systems be equipped with motorized dampers that will automatically shut when the system or spaces served are not in use. Outdoor air and exhaust/relief dampers shall be capable of and configured to automatically shut off during preoccupancy building warm-up, cool-down, and setback, except when the supply of outdoor air reduces energy cost or when outdoor air must be supplied to meet code, except that gravity back draft dampers are acceptable for exhaust and relief in buildings less than three stories in height located in Climate Zones 0, 1, 2, and 3. This means that if your building is three stories or more than a motorized damper maybe required.

How an Air Side Economizer Works

Bypass Damper HVAC VVT System

MEP Academy Instructor

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March 7, 2023

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Bypass Damper HVAC VVT System. You might be familiar with a Variable Air Volume VAV System, but we’ll explain the less energy efficient version that tries to mimic the VAV system. A Variable Volume/Variable Temperature system uses similar components to that of a VAV but is not as effective at saving energy. We’ll show you the control strategy for a commercial and residential application using a Bypass Damper.

If you prefer to watch the Video of this presentation, then scroll to the bottom or click this link. Bypass Damper HVAC VVT System

Commercial VVT System with Bypass Damper

The constant volume air conditioner or heat pump serves several zones, with each zone having their own zone damper and controller. When the zone dampers start to close the static pressure sensor picks up an increase in the duct static pressure and sends a signal to the bypass damper controller to modulate the damper open.

VVT System with a Bypass Damper — VVT System with Bypass Damper

If we look at an example using a 3,000 CFM (84 M3/M) constant volume air conditioner with three zones each sized for 1,000 CFM (28 M3/M) at peak load, and with a Bypass Damper that is closed because all of the Air Conditioners air is being delivered to the zones.

If the controller for Zone Damper #1 required less air and the damper modulated down to deliver only 500 CFM (14 M3/M), then only 2,500 CFM (70 M3/M) of the total air from the air conditioner is needed by the zones.

VVT System using a Bypass Damper for Static Pressure Control

As the Zone Damper #1 modulates to partially closed, the pressure in the supply duct will increase. This increase in duct pressure will be picked up by the Static Pressure Sensor which will send a signal to the Bypass Damper controller to modulate open to allow the excessive air, in this case 500 CFM (14 M3/M) to pass from the supply air to the return air duct without entering any of the zones as shown above.

VVT System Diagram with Zone Dampers Closing and Bypass Damper Opening

If Zone Damper #2 was to also reduce the amount of air delivered to the space in the same amount, then the duct pressure would increase and the static pressure sensor would send a signal to the Bypass Damper to open further.

Then if Zone Damper #1 closes completely as the occupants have left the space, then the Bypass Damper will have to Bypass all the air that would have gone to this zone.

VVT System HVAC Diagram with 50% Bypass Air

Remember that the air conditioner is a constant volume unit and has no way to reduce the air delivered by the unit. This air has to go somewhere, so it is bypassed from the supply air to the return air without entering the space. What happens is that the air becomes cooler or warmer because it hasn’t rejected or absorbed heat from the space.

Residential VVT System with Bypass Damper

Anyone that has lived in a two story home knows that its best served by two separate HVAC systems. Some have tried to modify the one Air Conditioning System by adding individual zone dampers, one for the first floor and a separate one for the second floor.

Residential VVT System for HVAC Control using a Bypass Damper or Barometric Damper

When the residents go to sleep at night on the second floor, the first floor damper can close. Since this is a constant volume air conditioner the additional air will be bypassed to the return air plenum or to a dump zone. The dump zone should be a hallway or unoccupied area of the house as the extra air dumped in this area will cause temperature problems, such as excessive heating or cooling depending on the mode of operation.

HVAC VVT System in a Residence with Bypass Damper

We show a motorized bypass damper in this diagram, but a barometric damper is often used. The barometric damper is set to open when the pressure increases to a certain amount, allowing air to bypass the supply and be redirected to the return.

The other way is to directly connect the bypass duct to the return duct which avoids excessive temperature swings in a dump zone. There are many variations of this installation in order to achieve some form of zoning. The best system layout would be to have two separate HVAC systems, one for the first floor and a separate one for the second floor.

VVT Controls

The installation of the controls for a VVT System with a Bypass Damper is simple compared to a standard DDC system for a VAV system. First will install zone controllers for each zone that are connected to the zone dampers using 20ga 3 wire shielded cable.

Next we need to install a 120 volt main feeder to power all the dampers. A 120v to 24v Transformer will need to be installed to power the Bypass damper, and transformers for the zone dampers. All the zone dampers can be daisy chained together using the same 20ga 3 wire shielded cable, and then connected to the Air Conditioner. A main controller can be installed that allows for reading of all the status points of the system. And lastly there will be a static pressure sensor installed. There are other optional control items that can be installed like a CO2 sensor for carbon dioxide sensing and ventilation control, heating hot water valve actuators for Zones with Heating Hot Water. The system can also be monitored by a Building Management System.

Variable Volume

A VVT system uses zone dampers so that each zone can adjust the volume of air that it receives based on its heating or cooling load. Each zone will have its own controller that will adjust the air volume to its zone based on the demand.

What makes the VVT system different from the more efficient VAV system is the use of less expensive constant volume Air Conditioning Unit and less sophisticated controls.

The VVT system uses a bypass controller to modulate the bypass damper to allow any unused supply air to return to the system. When supply air zone dampers start to close the constant volume air delivered by the air conditioner needs to be maintained by bypassing the excessive air. The Air Conditioning Unit is sized to handle the peak load, which is only needed a few times a year. The excess air needs to be bypassed and rerouted from the supply back into the return air system.

The use of a bypass damper allows for the use of the less expensive constant volume units when compared to the cost of a VAV system. The bypass damper must ensure that the constant volume unit receives the minimum amount required for it to function properly. If the minimum amount of air is not allowed over the coil, the coil could freeze up. The bypass damper also allows the ductwork to be installed using low pressure duct, as the bypass damper prevents buildup of static pressure in the ductwork. Excessive static pressure could cause the joints or seams of the duct to come apart, creating leaks.

The bypass controller uses a duct static pressure sensor installed in the supply air ductwork. The controller is set by the user to maintain a minimum and maximum pressure in the supply duct main. As the static pressure in the duct increases due to zone dampers closing, the sensor picks up an increase in static pressure and will modulate to bypass the excess air.

There are two simple setups for the bypass air, it can either be ducted directly into the return air duct, or it can be bypassed into the return air plenum if the plenum is rated and approved for this use.

Because the fan is always running at constant speed, there is no fan energy savings when the zone dampers start closing, as opposed to a true VAV system where the fan speed is reduced.

Variable Temperature

The system temperature will also vary as the bypass damper passes excessive air from the supply back to the return. This excessive bypass air is the quantity of CFM supplied by the constant volume unit that’s above what is needed by the zones at any time. As this cold air is not sent to the zones to pick up heat from the space it returns to the air conditioner cold.

Because the volume of return air is reduced due to the zone dampers partially closing, the excess cold supply air is bypassed back to the unit without picking up heat. This raises the supply air temperature, hence the variable temperature part of the system.

Bypass Damper in a VVT System

3-Way Switch Wiring Explained

Electrical

MEP Academy Instructor

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March 1, 2023

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3-Way Switch Wiring Explained. How Three-way light switches work. In this presentation we’ll learn how to control a light using a 3-way switch, which is convenient when there are two or more entrances to a room, or an upper and lower stairwell. We’ll show several different wiring configurations.

To watch the FREE YouTube version of this presentation, scroll to the bottom or click on the following link. 3-Way Switch Wiring Explained

Electrical Safety Warning

Whenever working with electrical be sure to shutoff the power at the electrical panel, and hire a qualified electrician to do the installation if you are not confident that you can do the job safely. Electricity can kill.

3-Way Switch for Controlling a Light or Fan

How do 3-Way Light Switches Work?

If we look at a 3-way switch, we’ll see that it has four terminal screws. We have two traveler terminals, which are usually identified by their light color of bronze or copper, a ground terminal in green, and a common terminal often a dark color. A 3-way switch won’t have an on/off designation that can be found on standard switches, as either position could be on or off.

Within the switch either one of the traveler terminals is connected to the common terminal, which can make or break the circuit depending on the position of the other switches traveler terminal matching or not. Its like the game of concentration where you turn over two playing cards trying to get them to match. When the two separate 2-way switches match on their traveler wires the light goes on, when they don’t match the light goes off. They can match with both on red or both on black, but not black/red or red/black.

The two traveler terminals can be on the same side of the switch or on opposite sides of the switch depending on the switch manufacturer.

Checkout these 3-Way Switches

3-Way Switch Example #1

With this layout we have the light fixture between the two switches.

The incoming electrical power will connect to the common terminal of switch #2. Then we route the Black traveler wire for switch #2 to switch #1. We do the same thing for the Red traveler wire. From the common terminal on switch #1 we connect to the light. We bring in the neutral wire from the power source and connect directly to the light. We also bring in the ground wire from the electrical panel and connect to each of the switches.

If we remove the switch covers and look inside we can see how these 3-way light switches work. Inside is a single pole double throw switch which provides each switch with an option to connect the common terminal with either the red or black traveler terminal.

When both switches are not in the same configuration, such as shown below we have one switches common connected to a black traveler and the other switch connected to a red traveler terminal. If we follow the power from the source, the black common wire from the electrical panel connects to the common terminal on switch #2, then passes through the switch to the black traveler terminal and then onto switch #1 where it dead ends.

3-Way Switch Wiring with switches in opposing positions

If we flip switch #1 to the black traveler wire then the light comes on as the electrical circuit has a complete path through both switches to the light.

The process of turning on and off the light can occur from either switch, as we show here we now flip switch #2 to break the electrical circuit and shut the light off. In order to turn the light back on we have two options, we can flip switch #1 to the red traveler terminal so that it matches switch #2, or just flip back switch #2 to the black traveler terminal.

3-Way Switch Example #2

Depending on the layout of the space and where the wiring originates from, there are several versions of how the wiring can be done. We’ll show how to wire a 3-way switch with both switches wired before the light.

Will install a light fixture, and two switches. From the power source we’ll bring the black hot wire to switch #2 common terminal. Note, that this wire is usually black, but your wire could be another color.

Then we’ll run a black traveler wire from 3-way switch #2 to the same terminal on switch #1, this could be from left side of the switch to the left side of the other switch. Then we install an outlet box and run the black wire from the dark common terminal screw on the bottom of switch #2 to the light fixture. Now we have one complete path from the source all the way to the light.

Next, we install a red traveler wire from the right side of switch #1 to the right side of switch #2. This gives us a second optional path to the light through the red traveler wires from the black hot wire brought to the common terminal.

Checkout these 3-Way Switches

Next, we install the incoming white neutral wire which is usually white, and we connect to the light fixture. Then we install the green ground wire from our source to the outlet boxes ground terminal, and then to each of the ground terminals on the switches. The ground is usually an insulated green wire or can be bare copper wire. In the fixtures electrical box there should be a place to land the ground wire.

We have 4 wire connections on each switch. Two travelers, one common, and one ground.

When we turn on the power and both switches have the same traveler terminal connected to common then the light will come on as shown above where both black traveler wires are in the same position. This could be either both black or both red travelers in the same position.

When one of the switches is flipped so as there is no matching traveler wire that provides a complete path to the hot common wire the light goes off as shown here with switch #1 being flipped off. The 3-way switch doesn’t indicate the light is on when the switch is in the up position, and off when in the down position. The light being off or on is determined when there is a matching pair of traveler wires that provide a complete path for the power to the light.

Inside the 3-Way Switch

If we looked inside we could see that each switch has a single pole and a double throw, meaning that the switch has two options, to either have the common connected to traveler terminal #1 or 2.

When both of the switches are on the same traveler terminal as shown here, the power runs through to the light. If either of the switches changes position then the light will go off. The electrical power comes through the black common wire connected to switch #2, and uses the red wire to reach terminal #2 on switch #1, where it flows through the common wire to the light, making a complete circuit.

Stairwell Example

First we mount a light above or stairwell and install a switch at the top and bottom of the stairwell. We bring in power using a black wire and connect to the common terminal on switch #1.

From switch #1 we run a black traveler wire to switch #2’s traveler wire terminal, and from the common on switch #2 we run a black wire to the light fixture. Then we run a red, second traveler wire from switch #1 to switch #2. We install a neutral wire from the electrical panel to the light fixture. Lastly we provide a green insulated ground wire from the panel to each of the light switches.

3 way light switch

How Steam Traps Work

MEP Academy Instructor

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February 22, 2023

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How Steam Traps Work. We’ll explain the important job of steam traps and how the three most common steam traps work. We’ll also show you examples of where they’re used in the HVAC industry.

if you prefer to watch the video of this presentation scroll to the bottom or click on this link. How Steam Traps Work.

When steam gives up its heat it turns into condensate, which must be removed from the system. The rate of heat transfer through the heat exchanger is highest when the heat exchanger is full of steam. If condensate wasn’t drained out of the system, and it collected in the heat exchanger, then the rate of heat transfer would drop.

Remember that a pound (0.45 kg) of water or condensate holds 1 Btu (1.5 kj) as it gives up its heat one degree (0.55 K), and that steam gives out approximately 1,000 Btus (1,055 kj) for every pound (0.45 kg).

IThe Heat Content of Steam vs Condensate

if condensate is allowed to sit in the heat exchanger the system will lose capacity. Therefore, it’s important to make sure that the condensate is quickly removed while not allowing the steam to escape without first transferring its heat.

Condensate Water Hammer

Condensate if left to collect in the pipes could cause problems such as water hammer noises, where a high velocity slug of condensate water is slammed against fittings or equipment. This is like a tidal wave in the pipes. The energy is dissipated onto the piping components and is noticeable by the noise it makes or the rattling of the pipes.

When the steam boiler starts the system will be full of air and non-condensable gases that need to be vented out of the system. If a steam trap fails to remove air from the system, the air and non-condensable gasses will reduce the heat transfer rate of the heat exchanger. Non-condensable gases behave like an insulator, thereby reducing the heat transfer rate.

The steam traps job is to remove condensate, air, and non-condensable gases, while preventing steam from leaking past it in a steam system. For the basic understanding of a steam system see our video on Steam Heating System Basics.

There are three main types of steam traps, but they all require a differential pressure between the inlet and the outlet of the steam trap. We’ll cover how each of the three steam traps categories work.

Mechanical Steam Traps

The density difference between the steam and condensate is how a Mechanical steam trap works. Steam in the form of vapor is less dense than that of liquid condensate which makes the use of a float as an effective means of control. The float and thermostatic (F&T) trap and the inverted bucket trap are the two most common mechanical steam traps. These traps use floats that rise, and fall based on the amount of condensate, and either open or close the trap.

Float and Thermostatic Steam Trap – Mechanical Steam Trap

When the condensate level is high in a float & thermostatic trap, the float rises and opens the trap to allow condensate to flow out. When the condensate level drops, so does the float which causes the trap to close.

With an inverted bucket trap, steam will push the bucket upward closing the trap, while condensate will pull the bucket down opening the trap.

These steam traps open and close based on the density of the steam and condensate being different. With the F&T trap the heavier condensate lifts the float, while with the Inverted Bucket the heavier condensate sinks the float.

Thermostatic Steam Traps

The temperature difference between steam and condensate is how a thermostatic steam trap work. It’s based-on temperature. After the condensate forms its temperature drops below that of the steam, and after a certain drop in condensate temperature the trap opens. When steam first condenses its temperature is the same as the condensate. This will require that the condensate cool further before the trap opens.

There are various substances used as the controlling element in thermostatic steam traps, such as fluids and metals.

The bimetal type thermostatic steam traps uses the differing coefficients of expansion of various metals. When two metals are attached together, and one expands at a different rate then the other based on temperature, this can be used to open and close a steam trap. As more and more steam enters the trap, the bimetal heats up and expands while pulling up on the stem of the valve that closes the steam trap, and when it cools down it opens the trap. The presence of steam will cause the bimetal to bend in a certain direction and close the trap, while the cooler condensate will contract the bimetal to open the trap.

When the boiler starts up after being off, the air, condensable gases, and the condensate in the system will be pushed through the open steam trap.

Thermodynamic Steam Traps

The Velocity difference between steam and condensate is how a thermodynamic steam trap works. Steam moves at a greater velocity than condensate. The only moving part in this thermodynamic steam traps is the disc, which moves up and down, opening and closing the port for condensate to flow through.

When the steam system is started, the cool condensate and air is pushed by pressure into the trap, causing the disc to move upward, allowing the condensate and air to pass through.

As hot condensate moves through the steam trap some of it flashes into steam, causing the pressure to drop below the disk, which causes the disk to drop.

As the condensate heats up and flows through the trap it cause steam to flash under the disc, which drops the pressure causing the disc to drop lower towards the seat or opening. As this is occurring the flash steam on top of the disc is pushing the disc closed. This traps the flash steam in the upper chamber, which causes an equalization of pressure on both sides of the disk, preventing any more condensate from passing through.

Flash steam in the upper chamber condenses, causing the disc to be forced up, while allowing condensate to enter. This process repeats itself over and over again.

How Steam Traps Work

Economizer Relief/Exhaust Air

Barometric Damper (Backdraft Damper)

Economizer with Relief Fan/Powered Exhaust

Economizer with Return Fan

Controlling Economizers

ASHRAE 62.1 and (0.1 Requirements

Commercial VVT System with Bypass Damper

Residential VVT System with Bypass Damper

VVT Controls

Variable Volume

Variable Temperature

Electrical Safety Warning

How do 3-Way Light Switches Work?

3-Way Switch Example #1

3-Way Switch Example #2

Inside the 3-Way Switch

Stairwell Example

Condensate Water Hammer

Mechanical Steam Traps

Thermostatic Steam Traps

Thermodynamic Steam Traps

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