CAV Systems

 

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Constant Air Volume (CAV)

In Constant Air Volume (CAV) a master thermostat automatically regulates the quality of conditioned air to each zone. Constant airflow volume is maintained, but the heating and cooling coil inlet temperature is varied.

 

There are many factors in the CAV system, below are the following main topics which will be discussed:

The System

Typical Uses

Limitations

Costs

 

Typical Uses

CAV systems are usually used in spaces with large open areas, few windows and uniform loads. Examples of theses types of spaces include lobbies, department stores, theaters, auditoriums and exhibition halls. In addition, CAV reheat systems are used in spaces that require specific temperature control and constant airflow. Spaces such as laboratories, hospital operating rooms or specialized industrial processes utilize the CAV reheat system.

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The System

In CAV systems, the most basic type is for a single-zone and the more complex include the dual-duct CAV system and the multizone CAV system.  In addition there are other alterations which can be added to the system such as a furnace or reheat system. Below is a picture of a single zone CAV system which has optional components shown in dashed boxes.

 

Single Zone CAV System

Heating and Cooling of Buildings

As can be seen in the diagram, air flows through the filter, coiling coil, heating coil, humidifier and fan to the conditioned space. In CAV systems, it is important that the heating and cooling coils do not operate at the same time to avoid energy waste. As a result, if only the heating or coiling coil is allowed to operate by the control system, the CAV single zone system can be energy-efficient. In a single-zone system there are several optional components. For example, a humidifier can be added to avoid excess air dryness or an exhaust fan can be added for local exhaust from a fume hood, stove or toilet. Also, a return fan is not needed if air is relieved from space in a low-pressure drop return system, but if large outdoor air fractions are used, a return fan should be used to avoid overpressuring the space. Another alternative is the recovery system, which uses the conditioned exhaust air to precondition the entering outside air. The secondary system equipment as well as a return/outside air mixing chamber and flow control damper are normally kept in a air handling unit (AHU), normally near the central plant for easy pluming connections. Through insulated ducts, the conditioned air is brought to the zone.

Another variation to the CAV system is the reheat system. It can be used for multiple or single zone space conditioning and is operated with a fixed volume flow rate and fixed supply fan outlet temperature (normally 55F) which are determined to meet the peak sensible and latent cooling load. The CAV reheat system is the same as the normal CAV until the point where the air enters the ductwork to be carried to the zone(s). At this point, the the air enters a reheat coil which may carry hot water, steam piped from the boiler room or an electrical resistance coil. While the reheat system allows for close temperature control of a space, it is energy-inefficient in the CAV system. By adding heat to the air at each zone with a reheat coil, the variation of load below the peak is corrected. Due to the fact the cold air is reheated prior to the release of a zone, there is energy waste.

The furnace is another alteration to the CAV system, which is normally used in single-family houses and other very small buildings. Multiple furnaces can be used to heat and cool larger buildings. A furnace is an indoor unit that incorporates an air circulating fan and source of heat, such as a gas burner, an oil burner, an electric resistance coil or a heat pump coil, into a single metal box. Cooling coils can also be integrated.

 

Single Duct, CAV System

   

The Architect's Studio Companion

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In the CAV system instead of using a single-duct system, the dual-duct constant-volume system can also be used. The dual-duct system has a single supply and return fan but two sets of duct (one for cold and one for hot). The heating duct and the cooling duct merge in a mixing box controlled by a reverse-acting damper to combine the two different air types to reach the desired temperature of thermostat in each zone. Some benefits  of using a dual-duct CAV system are good humidity control and very quick and precise temperature control. However, there are also a lot of disadvantages. One of the important disadvantages is that the system is not energy-efficient since there is both heating and cooling of the air. Below is a diagram of a dual-duct CAV system where mixing occurs in a zone mixing box.

 

Dual-Duct CAV system

Heating and Cooling of Buildings

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A way to simplify the dual-duct system and reduce cost is to mix the hot and cold airstreams at the central air handler, instead of in the mixing boxes in each zone. As a result, only one duct is needed for distribution to the zones. The system is called the multizone CAV system.  Multizone systems are normally limited to a small number of zones with short runs of ductwork do to the large amounts of space are needed for ductwork in the vicinity of the fan.  Below is a diagram of a multizone CAV system showing the connections to three zones. Cold and hot air are mixed at the air handler and transported to zones in a single duct.

 

Multizone CAV system

Heating and Cooling of Buildings

Below is an example of a multizone CAV system with reheat coils in the fan room to control the temperature of the air supplied to each zone.

 

Multizone CAV System

The Architect's Studio Companion

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Components

As can be seen in the different diagrams for the types of CAV systems, there are many components, including filters, coiling coils, heating coils, humidifies, fans and dampers. Boilers and chimney, chilled water plant, cooling tower, fan room, outdoor fresh air and exhaust louvers, vertical supply and return ducts, horizontal supply and return ducts, supply diffusers and return grills. Below are examples of grills diffusers.  

 

The Architect's Studio Companion

In medium-sized buildings, a package system can replace all of the components except for the ducts, diffusers and grills. Examples of package systems include, single-package system and split-package system. The single-package system incorporates all central components in a single metal box. The split-package system is divided into two parts, an outdoor package with the compressor and condensing coils and an indoor package with the cooling and heating coils and circulating fan. Multiple packaged systems can be used in buildings with a large horizontal extent.

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Calculations

 The load of a zone depends on internal loads as well as loads driven by external conditions such as solar gain and ambient temperature which affect both the transmission load and ventilation load. The fixed airflow is determined by the heating and cooling load. The cooling load normally requires the greater flow and since there is only one supply fan, the heating airflow is the same as the cooling airflow. Below are equations used to find the airflow rate (V) under different loads (Q). When taking at just sensible cooling load or heating load into consideration, the temperature of the room and coil outlet temperature are included. However, when both the latent and cooling loads are considered, enthalpy is important instead of temperature.

 

               Sensible Cooling Load           Sensible and Latent Cooling Load                Heating Load

 

                                  

ρair= density of zone air, lbm/ft3 (kg/m3)
cp,air= specific heat of air, Btu/(lbm ˚F)
Tcoil,i= room temperature ˚F (˚C)
Tcoil,o= coiling coil outlet temperature ˚F (˚C)
hcoil,i= cooling coil inlet air enthalpy, Btu/lbm (KJ/kg)
hcoil,o= cooling coil outlet enthalpy, Btu/lbm (KJ/kg)

 

Below is a chart showing the average cooling loads in different spaces for different ventilation system, including the CAV system.

 

Type of building

Ventilation system Ventilation principles and
Cooling loads (W/m2)
Typical rooms and cooling loads (W/m2)
Mixing Displacement

Offices

CAV 0-20 10-30

Normal offices without automatic

0-30

Normal offices with automatic

30-50

Conference rooms

20-75

Data rooms

>60

VAV 20-60 20-60
Fan-coil 40-70  
Cooling ceiling 60-100 60-100

Hotels

CAV 0-20 10-30

Guest rooms, normal standard

0-25

Guest rooms, high standard

25-50

VAV 20-50 20-50
Fan-coil 20-50  

Hospitals

CAV 0-20 10-30

Patients rooms

0-20

Treatment rooms

20-60

Intensive rooms

>50

VAV 20-60 20-60
Cooling ceiling 60-100 60-100

Public buildings

CAV 0-20 10-30

Conference rooms

20-75

Theatre, cinema

40-60

Restaurants

30-70

Class rooms

http://Engineeringtoolbox.com

In designing a CAV system, it is also important to calculate the necessary spaces needed for heating and cooling equipment. Below is a chart, which gives the cooling capacity, total space for a boiler room and chiller water plant as well as the space for a cooling tower as long as the floor area served and type of building are known.

 

The Architect's Studio Companion

In addition, it is also necessary to approximate the sizes of the air-handling components . As can be seen as long as the floor area served and type of building is known, different components can be found, such as the cooling air volume in cfm, area of main supply or return ducts (ft2), area of branch supply or return ducts (ft2), area of fan rooms (ft2) area of fresh air louvers (ft2) and area of exhaust air louvers (ft2).

 

 

Using the two above charts, the heating and cooling equipment for a 100,000 ft2 apartment can be calculated. If this is done, it is calculated that approximately 200 tons are needed for cooling capacity. A space of around 1750 ft2 is needed for a boiler room and chilled water plant as well as a space of around 300 ft2 for the cooling tower. In addition, approximately 100,000 cfm of cooling air volume are needed. This will required 60 ft2 of main supply or return ducts and 100 ft2 of branch supply or return ducts. In addition, an area of 2500 ft2 for the fan rooms, 250 ft2 for fresh air louvers and 200 ft2 for exhaust air louvers.   

 

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Limitations

One of the biggest limitations is that there is no individual temperature control since the entire area served by the system is a single zone. This is effective for certain situations, such as open areas with no windows, however, this is not effective in areas subdivided into smaller sections where different temperatures are desired. Adding a reheat coil just before air enters a zone fixes that problem, but is very energy ineffective. The multizone CAV system also minimizes this problem since it allows for a different thermostat in each zone. However, this system requires large amounts of space for ductwork, especially near  the fan.

Another limitation is the energy inefficiency caused by the heating and cooling coils operating at the same time. This is one of the biggest problem with the reheat and dual-duct CAV systems. While both systems give  precise temperature control for a zone, they are very energy inefficient with high operational cost due to the cooling and heating of the air.

 

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Costs

Compared to the Variable air volume (VAV) system, the CAV system has a lower initial cost, but much higher operational costs. These higher expenses are due to the CAV systems operating at a constant peak flow rate, even when this is not needed. One way of decreasing these inefficiencies is to reduce the airflow at part-load conditions. This is the VAV system, which varies the volume by  reducing the system airflow from full-load levels whenever loads are less than peak loads. As a result of the reduction in flow, the transfer at the air handler coils are reduced, as well as the fan electric energy, cooling electric energy and boiler thermal energy use.

To see the differences between the two systems, calculations were made in a two-zone building using both the CAV and VAV systems. The bin method was used with the same given conditions. Details of the individual numbers and calculations can be seen be going to the two links below. The overall results can be seen in the following chart.

 

Annual Primary
Energy Use
Fan Electric
Energy (kWh)
Cooling Electric
Energy (kWh)
Boiler Thermal
Energy (kBtu)
CAV 102, 027 260,272 2,521,721
VAV 74,153 124,179 364,747

Calculations provide by Ashley Kenyon

 

                               CAV System                                                              VAV System

 

Looking at the results, the difference in the two systems energy is huge. In all the categories, the VAV uses less energy and there is a great difference in the fan energy (27874 kWh), chiller energy (136093 Btu/h) and boiler energy results (2,156,974 kBtu). These differences are enormous and can lead to huge cost differences. As a result, for this system, VAV is much more energy efficient. This situation occurs in many cases, which is one of the main reasons why VAV systems are more commonly used today than CAV systems.

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This site was last updated 05/04/05