In this article we’ll discuss the basics of a chilled water central plant. The main pieces of equipment include a water-cooled chiller, cooling tower, air handler, controls, and the chilled-water and condenser water pumps.
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We covered chillers in another video, but we’ll go over the basics here. The chiller contains a refrigeration circuit with the typical components found in a standard air conditioner. There is a compressor that circulates the refrigerant, a condenser, and the evaporator.
Chilled Water Central Plant Diagram
There are two main loops of water fed from the chiller, one comes from the evaporator and the other from the condenser. The chilled water pumps circulate water from the chiller’s evaporator to the air handler through piping, this comprises the chilled water loop. Then there is the condenser water pump that circulates water from the chiller’s condenser to the cooling tower, this comprises the condenser water loop.
These makeup the two main water loops found in a water-cooled chiller plant. The chilled water loops purpose is to absorb the heat from the building. The condenser water loops purpose is to reject that heat to the atmosphere.
Chilled Water Loop
The chillers evaporator in the central plant produces chilled water. This CHW which is sent to the air handlers cooling coil and fan coils that serve the building occupants.
Chilled Water Central Plant – Chilled Water Loop
The air handlers and fan coils circulate warm air over their chilled water coils. The coils cools the air sent back to the occupied spaces. The heat from the spaces is sent back to the chillers evaporator in the chilled water return piping. That heat is absorbed into the chillers refrigerant circuit of the evaporator. The compressor then increases its pressure and temperature before sending to the condenser.
Condenser Water Loop
The cooling tower is responsible for rejecting the heat captured from the spaces. The warm refrigerant in the condenser passes the heat onto the condenser water. The warm water leaving the chillers condenser enters the cooling tower.
Central Plant – Condenser Water Loop
The warm water trickles down over the cooling tower fill. The warm water gives up its heat through evaporation to the air be drawn in by the cooling tower fans. The condenser water pump circulates this cooler water from the bottom basin of the cooling tower back to the chillers condenser to repeat the cycle.
This is the basic operation of a chilled water central plant. It includes the production of chilled water used to cool the air that serves the occupants of the building. The pumps circulate that water between the chiller and the other components. The air handlers’ job is to pick up the heat from the space by passing the warm air over the cold chilled water coil.
In this article we’ll discuss the dual duct system and mixing boxes. Dual duct mixing boxes help regulate the temperature and airflow in buildings by mixing hot and cold air to achieve the desired conditions.
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Here’s how a dual-duct system works.
Depending on the configuration, an air handler can be provided with one or more fans. Next, there will be a heating coil and boiler to provide heat, while a chilled water coil and chiller will provide cooling. There are also other sources available for heating or cooling.
There will be two main ducts leaving the air handler. One for heating, and the other for cooling, which makes this a dual-duct system.
Dual Duct System Air Handler with Dual Duct Mixing Terminals
Dual-Duct Mixing Box
Each zone will have its own dual-duct Mixing Box, that can be either constant or variable volume. This allows one zone to be in cooling, while another zone can be in heating. Dual duct boxes are made of galvanized steel and contain two dampers. Each damper has a controller located on the outside of the box. A flow sensor is in the hot and cold inlets of the box allowing for accurate measurement of the air flow.
Dual Duct System with Hot and Cold Decks
A room-based temperature sensor or thermostat will continuously monitor the indoor temperature. These sensors provide feedback to the HVAC control system. The control system analyzes the temperature data and determines the required airflow and temperature to maintain the desired conditions.
The dual duct mixing box operates by adjusting dampers within the box. These dampers control the amount of cold and hot air that is mixed before being distributed to different zones within the building.
Mixing Box Operation
There are various control strategies for dual-duct boxes. One strategy is for there to be no blending of the hot or cold air, which looks like this.
When the thermostat calls for cooling, the damper on the hot air duct closes, and the cold air damper modulates from zero to maximum. This allows cold air to enter the mixing box.
Dual Duct Mixing Box Chart – No Blending of Hot or cold duct air
Conversely, when heating is needed, the damper on the cold air duct closes, and the damper on the hot air duct opens.
Then there are various blending strategies where a mixture of cold and warm air will mix during certain conditions.
Dual-duct mixing terminal chart – Mixing of hot and cold air streams
As the temperature sensor approaches the setpoint in cooling mode, the cold air damper will modulate from maximum to a minimum setpoint. When the temperature continues to drop the hot air damper begins to open creating a mixed air flow. While the temperature continues dropping, the cold air damper completely closes, and the hot air damper modulates to maximum open position.
There are two separate ducts for supplying air. One duct carries cold air, while the other carries hot air. After mixing, the conditioned air is distributed to various areas of the building through separate ductwork.
Each zone may have its own mixing box, allowing precise control over the temperature and airflow in different parts of the building. Again, this allows any zone to be in either heating or cooling mode.
In this article we’ll show you four basic methods of whole house ventilation, which in theory explains concepts used in commercial applications. We’ll identify the advantages and disadvantages of each method of ventilation.
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Exhaust Ventilation Systems
Using exhaust as the primary method of ventilation requires that an exhaust fan creates a negative pressure within the space. The exhaust fan will pull air from the space and exhaust it outdoors. A path for outdoor air to get into the building must be created. The makeup ventilation air can come from leaks in the building construction, such as doors and windows, or from the installation of passive vents.
Whole House Exhaust Ventilation System
When the exhaust fan is running, it creates a negative pressure causing air to be drawn into the home through the cracks and vents.
Exhaust systems are simple to install and are relatively less expensive than the other methods. The fan can be installed in the attic if available or on the roof. A single location for the intake of exhaust air can be provided or from multiple locations for a much better design. Air can be exhausted from spaces that generate pollutants, such as the bathrooms or laundry rooms.
The use of passive vents for makeup air prevents the reliance on leakage into the building from the building construction. Passive vents may require larger pressure differences than that produced by the exhaust fan to work correctly.
With the use of negative pressure as the main method of ventilation, there is the concern that pollutants could be drawn into the space. Pollutants such as dust from attics, radon and mold from crawlspaces, fumes from attached garages, and flue gases from a fireplace or fossil-fuel-fired water heater and furnace. In addition to pollutants, in humid climates, moisture could be drawn into the space, so this method would not be recommended for humid climates. In cold climates the need to mix the outside air with indoor air may be required to avoid cold drafts.
Supply Fan Ventilation Systems
In stead of pulling air out of the building, a supply fan pressurizes the spaces by forcing outside air into the building. This causes the spaces to be under positive pressure, which forces air to leak out of the building through cracks, bathroom exhaust or kitchen range hood exhaust ducts, or through vents if provided. These systems are also one of the easiest and least expensive to install. Again the supply duct can supply one area or preferably several of the most occupied spaces.
Whole-house Supply Ventilation System
By pressurizing the building using a supply fan, unwanted pollutants are minimized from entering due to the pressure relationship between indoors and outdoors. A supply system can also have the outdoor air first enter through a filter to remove dust and pollen.
Supply fan ventilation systems works best in hot or mixed climate regions. In colder climates these systems can cause moisture problems because they pressurize the space. In the cold winter, the warm inside air can be forced into exterior wall and ceiling cavities. If this air is humid, then condensation could condense out causing mold and mildew.
Since there is no conditioning of the air or the removal of moisture through an air conditioning coil before entering the space, these systems could use more energy compared with an energy recovery ventilation system.
Balanced Ventilation System
The exhaust system creates a negative pressure, while the supply system creates a positive pressure. With a balanced ventilation System, the two cancel each other out to maintained balanced indoor air pressure. If supply air into the space equals the exhaust air volume leaving the space, theoretically you have a balanced system.
Whole-house Balance Ventilation System
With a balanced system you’ll have a supply and exhaust fan serving the space. The strategy is the same when it comes to where the intakes and exhaust locations should be. Supply air should be located in the most occupied spaces, while exhaust should be in areas that generate pollutants.
Having multiple fans and ductwork makes this system more expensive to install and operate than a single fan system. These first three systems do nothing to temper the incoming air or to remove moisture before entering the spaces.
Energy Recovery Ventilation Systems
This whole house ventilation system will cost the most to furnish and install. It also will require more engineering to meet the needs of the spaces. Some energy cost savings comes with the use of a heat-recovery ventilator (HRV) or Energy Recovery Ventilator (ERV). They can reduce the operating cost by transfering heat or energy between the incoming and outgoing air streams.
Energy or Heat Recovery Ventilation System
In the cold season, heat is transferred from the exhaust air into the cooler ventilation air entering the space. In summer, heat is transferred from the warm ventilation supply air entering the building to the exhaust air leaving. This helps reduce the load on the air conditioner due to the ventilation.
In this article we’ll show you how fire dampers and smoke damper work and why they are used. We’ll start with a single large air conditioner on the roof of a two-story building, but this could apply to larger buildings also.
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A main supply air duct feeds all of the first-floor spaces and is returned using an attic return from the spaces into the main return air riser. Each of the spaces will receive a connecting supply air duct and a return air grill that lets the air find its way back to the air conditioner through the return main.
Fire Dampers and Smoke Dampers, including combination Fire/Smoke Dampers
The fan in the air conditioner creates a positive pressure on the supply side pushing the air through the duct, while the return side creates a negative pressure and pulls the air back to the AC unit. You will often find fire rated walls running the length of the corridors which is the life safety pathway for occupants existing the building during a fire. In this case we show a slab-to-slab protective fire barrier with a non-rated ceiling. The fire rated wall is to prevent fire from entering the corridor, so making holes in the wall would defeat the purpose.
To air condition the corridor we’ll install a supply air duct that penetrates the fire rated wall, and we’ll also provide a return grill in the ceiling with an opening in the fire rated wall to allow the return air to make its way back to the air conditioner. Of course, the supply and return won’t be next to each other as shown here, but this is for simplicity of the example.
If we don’t protect the openings, we made in the walls for our air conditioning ducts then smoke and fire could enter the emergency exit corridor, compromising the safe exit of the occupants. Since the return air is under negative pressure, the smoke could be sucked back into the air conditioner and sent back down the supply duct to all the spaces. This would be compromising the safety of all the occupants.
Fire Dampers
Since we have penetrated the fire rated wall, we’ll need to maintain the fire rating by installing a rated fire damper at the supply and return air wall penetrations. Here is the definition of a Fire Damper according to the NFPA (National Fire Protection Association).
“A fire damper is designed to, and required to, close automatically upon detection of heat (such as a fusible link or heat detector) and to interrupt airflow and to restrict the passage of flame.”
Fire Damper
Static and Dynamic Style Fire Dampers
Fire dampers are classified for use in a Static System, (a static system is an HVAC system designed to stop the movement of air within the system at the indication of a fire). Then there is a Dynamic Systems (A dynamic systems is an HVAC system designed to maintain the movement of air within the system at the indication of a fire). So, in a static system the fan should be off, while in a dynamic system the fan will be running as part of an engineered smoke control system.
A fire damper works to protect the opening when the fusible link that holds the damper in the open position, melts. As the fire and heat builds up on one side of the wall, the fusible link reaches the melting point and releases the damper to fall and close off the opening in the duct or wall, preventing the fire from spreading to the other side.
Remember with a static style fire damper the HVAC system fan will be off, while with a dynamic style fire damper the fan will be running because it’s part of an engineered system to control pressure relationships between areas to move smoke out of the building or a protected zone. Dynamic fire dampers are designed to handle the pressure related to a fan operating.
Smoke Dampers
Smoke is an important aspect of any fire, as most people that died in the MGM Grand fire in Las Vegas, died related to smoke inhalation.
Smoke Damper or Combination Fire/Smoke Damper
The primary function of Smoke dampers according to the NFPA is to control the movement of smoke in dynamic air distribution systems, and they reduce the possibility of smoke transfer within ductwork or through wall openings. Smoke dampers are installed in corridor and shaft walls, and other barriers engineered to prevent the spread of smoke.
The smoke damper is designed to operate automatically to stop the spread of smoke throughout the air ducts. This is done with the use of smoke detectors that can be located within the duct or with the use of area detectors in smoke compartments.
Combination Fire/Smoke Dampers
There are also fire/smoke damper combinations that protect the opening using both techniques. According to the NFPA, a combination fire/smoke damper is used when a barrier is both rated for fire resistance as well as designed to restrict the transfer of smoke and will meet both the fire damper and smoke damper requirements.
Construction Trades Involved
The construction trades involved in the installation and maintenance of smoke/fire dampers and their scope of work include:
Mechanical Engineers
Mechanical engineers design the HVAC systems and smoke/fire damper layouts. They specify the types of dampers required and their locations to comply with fire safety codes and standards.
HVAC Contractors
HVAC contractors are responsible for installing the ductwork and integrating smoke/fire dampers into the duct systems. Their scope includes the physical installation of the dampers, the wiring required for damper operation, and commissioning of the system.
Electrical Contractors
Electrical contractors are involved in the electrical wiring required for smoke/fire damper activation. This may include wiring for fire alarm panels, smoke detectors, and damper actuators.
Fire Alarm and Life Safety Contractors
These specialists install and maintain fire alarm systems, including the components that detect smoke or fire and the control panels that trigger damper activation.
General Contractors and Subcontractors
General contractors oversee the coordination of all trades involved in the construction project. They ensure that the work of HVAC, electrical, and fire safety contractors is properly integrated into the building’s overall construction.
Building Inspectors
Building inspectors review and approve the installation of smoke/fire dampers to ensure compliance with local building codes and safety standards.
The installation and maintenance of smoke/fire dampers are crucial for the safety of occupants in commercial buildings, especially in the event of a fire. It’s essential that these systems are designed and installed correctly and undergo regular maintenance and testing to ensure they function as intended.
