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How Waterside Economizers Work

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March 29, 2023

How Waterside Economizers Work. In this video we will learn How Waterside Economizers work, also known as Free Cooling. We’ll learn how to connect an economizer into a water-cooled and air-cooled chilled water system. A 100% free cooling allows for the chiller to be shutoff and the cooling tower along with a heat exchanger to do all the cooling. We will show you three waterside economizer configurations using a plate and frame heat exchanger.

If you prefer to watch the video of this presentation than scroll to the bottom or click on this link. How Waterside Economizers Work

In our previous video we demonstrated how an air side economizer is based on the ambient dry bulb temperature, and now we’ll show you that a waterside economizer is based on the ambient wet bulb temperature using evaporative cooling. The purpose of a water-side economizer is to reduce or eliminate the hours the mechanical cooling equipment must run, such as a chiller.

Integrated vs Non-Integrated Economizers

There are several methods of operating a waterside economizer. One is for it to be integrated with the operation of the chiller, hence operating simultaneously, or non-integrated, where the chiller is completely off and the economizer has full responsibility to meet the demand of the load.

Integrated Waterside Economizer

The use of an integrated waterside economizer allows for the simultaneous use of both the chillers and the reduced energy consumption aspects of a waterside economizer as required by ASHRAE 90.1

Chilled Water Central Plant before Retrofitting with Waterside Economizer

This is a typical primary only chilled water central plant layout, where the primary chilled water pumps serve the chillers and the building load, represented here as a cooling coil.

A plate and frame heat exchanger will need to be added in series with the chillers on the chilled water side for the transfer of thermal energy between the condenser water and chilled water sides. For an explanation of How Plate and Frame Heat Exchangers Work see our previous video.

Integrated Waterside Economizer with Side Stream Pump Option.

We will tap into the chilled water return piping and connect to the heat exchanger to precool the chilled water return before sending it onto the chiller. To do this we’ll need to add a control valve (CV-1) to divert the flow, and force the chilled water return to go through the heat exchanger.

The waterside economizer can lower the chilled water return temperature before it arrives back at the chiller for mechanical cooling.

This reduces the load on the chiller in an integrated system. When in full waterside economizer mode we need to add a bypass line so that all the water avoids going through the chillers in order to save on pump energy. We’ll add a control valve (CV-2) to open fully while both chiller control valves will be closed to prevent water from going through the evaporator coils.

This completes one side of the heat exchanger which is the hot side. For the cold side of the heat exchanger we need to connect the condenser water piping in parallel with the chillers, and pipe the cold water coming from the cooling tower basins so that it flows into our heat exchanger, where it will absorb the heat through the plates from the chilled water return. The heat will then be carried by the condenser water to the top of the cooling tower where it will reject the heat to the atmosphere. We need to install a control valve (CV-3) to prevent the flow from entering the heat exchanger when it’s not in economizer mode to avoid additional pressure drop on the pumps.

At the same time, the condenser water pump can reduce its speed and water flow with the use of a Variable Frequency Drive (VFD) to compensate for the reduction in demand on the chiller because of the economizer, if the minimum flow is maintained through the chiller.

When the chiller is at partial load in an integrated economizer system, the condenser will require the manufacturers required minimum flow for head pressure control. The cooling tower will also have a requirement for minimum flow that will need to be adhered to.

There is the option to use a 3-way control valve instead of a 2-way valve, or an option to use a side stream pump to pull the chilled water return through the heat exchanger.

Adding a Waterside Economizer to an Existing Air-Cooled System

You can retrofit an existing air-cooled chilled water plant with an integrated waterside economizer as we show here. Here we have two air-cooled chillers with primary pumps and variable secondary pumps serving the building load. We can precool the returning chilled water temperature before it enters the chiller, thereby taking load off the chiller.

Air-Cooled Chillers in Central Plant before Retrofitting with Waterside Economizer

We add a plate and frame heat exchanger to the system and a pump which will pull water out of the chilled water return and send it through the heat exchanger, where it will give up some of its heat and be sent back into the chilled water return on its way to the air-cooled chillers.

Air Cooled Chiller Central Plant with Integrated Waterside Economizer

Then a cooling tower is added to the system for heat rejection along with a pump to pull cool water from the basin and send it into the heat exchanger where it will pick up heat from the chilled water return system. Then it will exit the heat exchanger warmer and be distributed at the top of the tower where it will give up the heat to the atmosphere.

Non-Integrated Waterside Economizer

The use of a non-integrated economizer strategy provides for the economizer to be either on or off, there is no simultaneous chiller function. When the economizer is operating the chiller is 100% off. The condenser water from the cooling tower will satisfy the cooling load without assistance from the chiller.

Water-Cooled Chiller Plant before Retrofitting with Waterside Economizer

Here we have a water-cooled chilled water plant served by a cooling tower that we can add a waterside economizer to in a non-integrated method just for you to see the difference.

We install a plate and frame heat exchanger and then tap into the chilled water return piping and connect to the inlet on the warm side of the heat exchanger. From the outlet on the warm side of the heat exchanger we connect into the chilled water supply feeding the load in the building, in this case we show one air handler, but this could be dozens of air handlers.

Non-integrated Waterside Economizer used in Water-cooled Chiller Plant

Next we tap into the condenser water supply coming from the cooling tower basin and feeding the chiller. This will be our inlet on the cold side of the heat exchanger. Then on the cold water outlet of the heat exchanger we’ll connect to the condenser water return piping leaving the chiller and entering the tower.

To make this all work there needs to be control valves that reroute the water when the economizer is activated. Remember this is a non-integrated economizer, so there is no simultaneous running of the chiller. This is why the piping is routed around chiller. Control valves CV-1 and CV-2 will be installed on the chilled water side of the economizer, so that when the economizer is activated and the chiller shuts off, CV-1 will close, and CV-2 will open. This allows the water from the air handler or building load to bypass the chiller and go directly to the heat exchanger.

Next there will need to be control valves on the condenser water side of the heat exchanger to bypass the chiller. Control valves CV-3 and CV-4 are installed on the condenser water supply to the chiller and the inlet piping to the heat exchanger. During waterside economizer mode, CV-3 will close preventing water from entering the chiller, and CV-4 will open allowing the flow to be diverted from the chillers condenser to the heat exchanger.

ASHRAE 90.1 (2019 Section 6.5.1.2.1)

If your local building code has adopted ASHRAE 90.1 then the use of a waterside economizer could be mandated by code under certain conditions or as one of several options to achieve energy saving goals. When the temperature drops below a set value, such as 50 F dry bulb/45 F Wet bulb or (10 Celsius DB/ 7.2 Celsius WB) mechanical cooling would shut down and 100% full waterside economizer would be active.

For those that live in the state of California, in the United States, the temperature requirement thresholds are the same as mandated in the Title-24 Energy Code. When starting up the central plant the control logic will determine whether to activate the economizer mode based on the two approach values being able to meet the chilled water supply temperature setpoint.

Important Temperatures

Some of the key temperatures to look for when determining if an economizer will work in your location is what they call the Approach Temperatures. This would take another video for an in-depth explanation but we’ll get you started with two approaches important for the integrated economizer systems we have explained in this video.

Waterside Economizer Approach Temperatures and Range

There is the Cooling Tower or Condenser Water Approach, that defines the condenser water supply temperature in relationship to how close it gets to the ambient wet bulb temperature. Then there is the Heat Exchanger Approach that compares the condenser water supply temperature to the chilled water supply temperature across the heat exchanger.

Another important value would be the temperature range of the condenser water, that’s the difference between the condenser water return minus the condenser water supply.

The three important factors in sizing the water-side economizer are the wet-bulb temperature, the temperature range, and the approach.

How Waterside Economizers Work – Free Cooling with a Fluid Economizer

How Plate Heat Exchangers Work

MEP Academy Instructor

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March 22, 2023

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How Plate Heat Exchangers Work. In this presentation we’ll learn How Plate and Frame Heat Exchangers work. These maybe referenced on engineered drawings as HEX, HX, PHX or PHE. Plate and frame heat exchangers are used in the HVAC and Plumbing industry for the transfer of heat from one system to another without the fluids meeting each other. The purpose of the heat exchanger is to transfer thermal energy from one system to another without the fluids contacting each other.

If you prefer to watch the Video of this Presentation, then scroll to the bottom or click this link. How Plate Heat Exchangers Work

The two most common types of heat exchangers are the shell and tube heat exchanger and the plate and frame type that we are going to discuss here.

Heat Exchanger Types (Shell and Tube)(Plate and Frame)

A Plate and frame heat exchangers can be used for a water side economizer, where one side is connected to the cooling towers, and the other side is connected to the chilled water distribution piping. This keeps the cooling tower water separate from the chilled water loop.

Water-side Economizer using a Plate and Frame Heat Exchanger

Parts of a Plate and Frame Heat Exchanger

There are very few parts for a Plate and Frame heat exchanger which can be used with fluids or gases. They will contain threaded or flanged inlet and outlet connections for both the primary and secondary sides of the heat exchanger. One set of inlet and outlet connection will be considered the hot side and the other the cold, as heat will move from the warmer to the colder fluid or gas, hence exchanging heat.

Plate and Frame Heat Exchanger components

The plate and frame heat exchangers are like a sandwich, where you have two slices of bread or in our case two thick sheets of mild steel that form the ends, with layers of thin, gasketed, corrugated plates sandwiched in-between. One end is a fixed plate, while the other end is movable pressure plate. The fixed plate end will have holes in it for the piping connections. For the HVAC and Plumbing trades, the use of stainless-steel plates is common and can be either 316 SS or the less expensive 304 SS.

There are top and bottom carrier bars between the two end plates from which all the gasket plates are supported from which also provides a method for their alignment. The gasketed plates are grooved along their top to fit onto the carrier bar.

Heat Exchanger with Fixed and Moveable Plates

Then there is a long clamping or tightening bolts that run the length of the heat exchanger from one end plate to the other that will tighten all the plates firmly together. There is a gasket on one side of each plate that when the bolts are tightened, all the plates are squeezed together between the two end plates ensuring a watertight seal. The gasket can be made of nitrile or EPDM, both synthetic forms of rubber.

How Plate & Frame Heat Exchanger Work

The warm fluid will enter the inlet on the primary side of the fixed end where the gasketed plates will route the fluid through the first plate and every other plate or odd numbered plate and then exit the outlet piping connection. The cold fluid will enter the inlet on the secondary side of the fixed end and be routed to the second plate and every other plate or ever even numbered plate, before exiting the outlet pipe connection on the fixed end.

The primary fluid never mixes with the secondary fluid, they just transfer their heat between the plates surface area in an alternating pattern of hot and cold plates. The hot fluid will give up some of its heat and become cooler, while the cold fluid will pick up some of that heat and become warmer.

Remember that the natural laws state that heat will leave the warmer fluid and transfer to a cooler fluid in its attempt to reach equilibrium along as there is a temperature difference between them. The heat given up by the warmer fluid is equal to the heat gained by the cooler fluid, minus any heat lost to the surroundings.

The plates are thin and close together to provide good thermal contact and heat transfer. The thin metal and large surface area provides a means for high thermal conductivity and heat transfer between the two fluids.

Heat Exchanger Plates in a Plate & Frame Heat Exchanger

The plates come stamped with many different patterns on their face and will have four holes in their corners where the main fluid or gas flows. The patterns are designed to increase turbulent flow which increases the rate of heat transfer and prevents the accumulation of mineral buildup on the plates. The various stamped metal patterns also provide rigidity to the plates.

There will be a starting and ending plate. These plates prevent the fluid or gas from getting behind them and in-between the fixed and movable end covers.

Increasing Heat Exchanger Capacity

One of the benefits for using a Plate and Frame heat exchanger is the ease by which additional capacity can be added. There are several methods that can be deployed to increase the capacity of a heat exchanger.

One method is to increase the number of plates which will increase the heat exchangers capacity, or if you want to reduce capacity, then remove some plates.

Increase the number of passes that the fluid travels through the heat exchanger. A single pass heat exchanger is where the fluid passes through a single plate and then exits. A multiple pass heat exchanger will send the fluid through more than one plate before it exits. A multiple pass heat exchanger gives the fluid more time to transfer heat.

Another way to increase capacity is to increase the flow rate through the plates.

For higher pressure systems there is the option is to use brazed, welded or fusion-bonded plates.

Flow Patterns through Heat Exchanger

There are three common methods of how the flow of fluid traverses the plates in relationship between the primary and secondary fluids. They can flow in the same direction which is considered parallel flow. They can flow in opposite direction to each other and that would be counter flow. Then there is cross flow, where one fluid travels perpendicular to the other.

Where can they be used?

Here is a possible list of places you could find plate and frame heat exchangers in the HVAC and Plumbing Industry. For areas where freezing is possible, then the use of a Water/Glycol solution can be used but will require a slight larger heat transfer area.

Heat Recovery Applications like heat from a chiller or generator
Water-side economizer
Swimming pools or Spa’s
District Heating or Cooling
Preheating of Domestic Water
Preheating of Boiler Feedwater
Process heating or cooling
Solar Heating
Water-Cooled Heat Pumps

Advantages of Plate and Frame Heat Exchangers

They take up less space.
They weigh less.
They are more efficient.
Easy to clean plates.
Longer intervals between cleanings.
Less space required for dismantling.
Easier to increase capacity with gasketed versions.
Close approach temperatures

How Plate and Frame Heat Exchangers work.

How an Air Side Economizer Works

MEP Academy Instructor

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March 15, 2023

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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

Integrated vs Non-Integrated Economizers

Integrated Waterside Economizer

Adding a Waterside Economizer to an Existing Air-Cooled System

Non-Integrated Waterside Economizer

ASHRAE 90.1 (2019 Section 6.5.1.2.1)

Important Temperatures

Parts of a Plate and Frame Heat Exchanger

How Plate & Frame Heat Exchanger Work

Increasing Heat Exchanger Capacity

Flow Patterns through Heat Exchanger

Where can they be used?

Advantages of Plate and Frame Heat Exchangers

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

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