Variable Air Volume System:
A Variable Air Volume, or VAV, system can control the amount of air supplied to a specific zone within a building at a constant temperature. This type of system ideal for office building that contain many different spaces with varying HVAC demands. This is different than a CAV, or Constant Air Volume, system where the air supplied is only to change the temperature of a room. Because of this, a VAV system is considered to be more efficient in cost and air supply than a CAV system. Although it is more efficient, a VAV system does not have the ability to control the humidity and pressure of the spaces which it is supplying air to. Also, a VAV system does have a higher initial cost than many other systems due to necessary duct work to supply the air, which is typically a driving cost in the selection of a system. This type of system is likely the most ideal system to be implemented in the ASCE National Headquarters because of its ability to supply different demands of air to varying spaces within the building such as large offices, conference rooms, and general work areas such as cubicle spaces. Optimum comfort in these work areas is important to provide the occupants with appropriate air flow and creating a stable and comfortable work environment.
Figure 1) VAV Sytem Diagram
Constant Air Volume System:
A CAV, or Constant Air Volume, system works similar to a VAV system, except instead of varying the air volume, it is set at a constant air flow. The temperature of the air is then varied to control the levels in the space. This system works better for large open spaces rather than small multiple spaces, which is typically how an office building is laid out. Additionally, a CAV system also gives the user great control over the temperature of the space as well as control the pressure within a space. This system would work very well given the large areas allocated for the cubicles, however, it would not be ideal for the offices and conference rooms surrounding the cubicle areas. The constant air flow set throughout the whole building would likely need to be different for the individual spaces when compared to the large open spaces. Depending on where the sensor is located, this could cause either the offices to become over-cooled or over-heated while the large open areas remain comfortable, or the large open area being under cooled or heated while the individual spaces remain comfortable.
Figure 2) CAV System Diagram
Chilled Beam System:
A Chilled Beam system is a type a convection HVAC system. Pipes of water are passed through a heat exchanger (beam) that is suspended from the ceiling. As the beam chills the air around it, the air becomes denser and falls to the floor. It is replaced by warmer air moving up from below, causing a constant flow of convection and cooling the room. This system typically has a low operating cost compared to others as well as typically using less ceiling space than forced-air HVAC systems. Downsides to this system are in its heating capabilities, where subsidiary heating systems are often needed in addition to the chilled beam system. The chilled beam system includes passive and active forms. The system has low operating costs that the fans can run at lower speed and the air is well distributed causing a uniform temperature throughout the space. The system can reduce typical HVAC load costs since it lacks the need to constantly pull and treat outside air. Because of limited space between our drop ceilings and the floors above, chilled beam systems should not be utilized in the design of the ASCE Headquarters. Additional systems would have to be implemented in the design of the HVAC system within the headquarters and would cause a higher operation cost.
Figure 3) Chilled Beam System Diagram
Air Handling Units:
Rooftop HVAC units work along the same lines as Air Handling Units. These units are typically implemented in the HVAC design of large commercial buildings. Rooftop units are also often used alongside CAV or VAV units to enhance efficiency. These rooftop units are also, in most cases, simpler to install than other units but also utilized ductwork to circulate the air supplied to the spaces within the building. Because of their high initial and operating costs, simply using rooftop units would not be sufficient to supply air to the 20 story ASCE National Headquarters buiding. Because the building will contain additional office spaces for lease, they will be considered in the design of the overall HVAC system in conjunction with an additional VAV HVAC system.
Figure 4) Rooftop Air Handling Unit Layout
System Selection:
Based on the spaces created for the mechanical systems by the structural group, it was decided that a VAV system would be the most beneficial system to by implemented in the design of the ASCE National Headquarter in Chicago, Illinois along with additional Rooftop Units and Air Handling Units on Floors 5, 10, and 15 to supply adequate airflow to the various spaces throughout the building. This combination of the VAV and Rooftop AHU systems should supply sufficient air supply to the various spaces within the building by reducing the distances that the controlled air must travel to the target areas from the HVAC components. It was determined that the basement units supply air to floors 1-8, which will be the designated space for the ASCE National Headquarters, and that the rooftop air handling units will supply air to floors 9- 19 which will be available for leasable offices.
After analyzing the pros and cons of the possible systems to be implemented, it was determined that for the 20 Story ASCE National Headquarters in Chicago, a VAV system with Reheating Coils should be used rather than a more basic VAV system without reheating.
After analyzing the pros and cons of the possible systems to be implemented, it was determined that for the 20 Story ASCE National Headquarters in Chicago, a VAV system with Reheating Coils should be used rather than a more basic VAV system without reheating.