The parameters of a CAV system are based upon the size, capacity, target temperature, and the function of a room. The number of occupants, amount of machinery, and function of the room all contribute to determining the load on the system in terms of a heating or cooling load. These loads are also influenced by the outdoor air condition, as less energy must be used to condition the air if outdoor conditions are already similar to the indoor target conditions.

The number of air changes per hour that the system is required to perform is represented by the formula:

 

N=(60*Q)/Vol

 

Where Q stands for the flow rate of air. N refers to the number of times all of the air in the room must be cycled out within an hour. A typical value for N is 1.3 AHC, but this number can change based on the use of the room. Thus, this formula can determine how quickly air must be supplied by the CAV system.

The loads caused by people and room function can be determined by the formula:

 

Q=qs*n*CLF

 

Where Q stands for heating load in this case, qs refers to the heat gains per person, n stands for the number of occupants, and CLF refers to the cooling load factor per person. Common values can be found in this table:

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Typical indoor conditions describe a comfort zone with a relative humidity between 40 and 60 percent. A comfortable temperature has been found to be between 22 and 27 degrees Celsius. These values vary between summer and winter, as occupants' clothing and expectations adapt based on the outdoor conditions. This parameters can be seen in the following graph:

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

The system parameters are based upon the individual systems that make up overall system. The American Society of Heating, Refrigerating and Air Conditioning Engineers (ASHRAE) provides the following guide on selecting the components of a CAV system (ASHRAE STD 90.1-1999):

 

• Fan Sizing: in general, ASHRAE limits constant air volume fans to 1.2 hp/1,000 cfm (ft³/min.) for systems under 20,000 cfm, and 1.1 hp/1,000 cfm for systems over 20,000 cfm. Demand Controlled Ventilation, on the other hand, is required for systems of more than 3,000 cfm and occupant density of 100 people/ 1000 sf. The following figure shows the chart provided by ASHRAE to aid in fan selection. The chart shows the typical fan performance curves that can be used to select the fan horse power. Fans are chosen based on the required air flow rate and pressure.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

• Duct Sizing: using the parameters determined from fan selection, air ducts can then be selected using the following chart, ducts are classified according to the maximum velocity at which it delivers air. For Class Types 1 - 7, the maximum velocity ranges from 2000 to 5000 cfm:

   

   

   

   

   

   

   

   

   

 

 

 

 

• Chiller Selection: as previously stated, the capacity of chillers depend on the outdoor air temperature and desired indoor air temperature. The following chart shows the typically used chillers for CAV, their capacity, and performance.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

• Boiler Selection: similar to chillers, boilers are selected based on heating loads determined by the outdoor air temperature and desired indoor air temperature, as well as heat source, application and operating cost, which usually controls more than the initial cost. The following chart shows the typically used heat sources used in boilers, and the amount of energy produced by a unit quantity of fuel.

   

   

   

   

   

   

   

   

   

 

 

 

 

 

 

 

In the case of a mixed-air system, the final two figures show the appropriate proportions of the mixed air to be used, and how the supplied air temperature changes based on the proportion of mixed air used: