Single Duct Systems

The majority of all-air HVAC systems employ a single network of supply air ducts which provide a continuous supply of either warmed or cooled air to the occupied areas of the building.

Single Zone—The single duct, single-zone system is the simplest of the all-air HVAC systems. It is one of the most energy-efficient systems as well as one of the least expensive to install. It uses a minimum of distribution energy,* since equipment is typically located within or immediately adjacent to the area which it conditions. The system is directly controlled by a thermostat which turns the AHU on and off as required by the space temperature. The system shown in Figure 10.3 is a single zone system.

Single zone systems can provide either heating or cooling, but provide supply air at the same volume and temperature to the entire zone which they serve. This limits their applicability to large open areas with few windows and uniform heating and cooling loads. Typical applications are department stores, factory spaces, arenas and exhibit halls, and auditoriums.

Variable Air Volume—The variable air volume (VAV) HVAC system shown in Figure 10.4 functions much like the single zone system, with the exception that the temperature of individual zones is controlled by a thermostat which regulates the volume of air that is discharged into the space. This arrangement allows a

*Distribution energy includes all of the energy used to move heat within the system by fans and pumps. Distribution energy is typically electrical energy.

Air Conditioning Duct System For Civil
Figure 10.3 Elements of an all-air air-conditioning system.

high degree of local temperature control at a moderate cost. Both installation cost and operating costs are only slightly greater than the single-zone system.

The distribution energy consumed is increased slightly over that of a single-zone system due to the friction losses in VAV control devices, as well as the fact that the fan in the AHU must be regulated to balance the overall air volume requirements of the system. Fan regulation by inlet vanes or outlet dampers forces the fan to operate at less than its optimum efficiency much of the time (see Figure 10.5) Consequently a variable speed fan drive is necessary to regulate output volume of the fan. For the system to function properly, it is necessary that air be supplied at a constant temperature, usually about 55°F (13°C). This requires indirect control of the supply air temperature with an accompanying decrease in control efficiency.

Single-duct VAV systems can often provide limited heating by varying the amount of constant temperature air to the space. By reducing the cooling airflow,

Cooling Filter coil

Cooling Filter coil

Figure 10.4 Variable air volume system schematic diagram.

the space utilizes the lights, people and miscellaneous equipment to maintain the required space temperature. However, if the space requires more heat than can be supplied by internal heat gains, a separate or supplemental heating system must be employed.

Single-duct VAV systems are the most versatile and have become the most widely used of all systems for heating and cooling large buildings. They are appropriate for almost any application except those requiring a high degree of control over humidity or air exchange.

Reheat systems—Both the single-zone and single duct VAV systems can be modified into systems which provide simultaneous heating and cooling of multiple zones with the addition of reheat coils for each zone (Figure 10.6). These systems are identical in design to the foregoing systems up to the point where air enters the local ductwork for each zone. In a reheat system supply air passes through a reheat coil which usually contains hot water from a boiler. In a less efficient option, an electrical resistance coil can also be used for reheat. (See comments regarding the efficiency of electric resistance heating in §10.5.6.)

A local thermostat in each zone controls the temperature of the reheat coil, providing excellent control of the zone space temperature. Constant air volume (CAV) reheat systems are typically used in situations which

Figure 10.5 Fan power vs. discharge volume characteristics.
Figure 10.6 Reheat system schematic diagram.

require precise control of room temperature and/or humidity, often with constant airflow requirements, such as laboratories and medical facilities.

Both the CAV and VAV reheat systems are inherently inefficient, representing the highest level of energy consumption of the all-air systems. This is due to the fact that energy is consumed to cool the supply air and then additional energy is consumed to reheat it. In VAV reheat systems, the reheat coil is not activated unless the VAV controls are unable to meet local requirements for temperature control, and they are therefore somewhat more energy efficient than CAV reheat systems.

Both CAV and VAV reheat systems can also be used with specialized controls to condition spaces with extremely rigid requirements for humidity control, such as museums, printing plants, textile mills and industrial process settings.

Multizone—Although commonly misused to indicate any system with thermostatically controlled air-conditioning zones, the multizone system is actually a specific type of HVAC system which is a variation of the single-duct CAV reheat system. In a multizone system, each zone is served by a dedicated supply duct which connects it directly to a central air handling unit (Figure 10.7).

In the most common type of multizone system, the AHU produces warm air at a temperature of about 100°F (38°C) as well as cool air at about 55°F (13°C) which are blended with dampers to adjust the supply air temperature to that called for by zone thermostats. In a variation of this system, a third neutral deck uses outside air as an economizer to replace warm air in the summer or cool air in the winter. In another variation, the AHU produces only cool air which is tempered by reheat coils located in the fan room. In this case, the hot deck may be used as a preheat coil.

Multizone systems are among the least energy efficient, sharing the inherent inefficiency of reheat systems since energy is consumed to simultaneously heat and cool air which is mixed to optimize the supply air temperature. Since a constant volume of air is supplied to each zone, blended conditioned air must be supplied even when no heating or cooling is required.

In addition, multizone systems require a great deal of space for ducts in the proximity of the AHU which restricts the number of zones. They also consume a great deal of energy in distribution, due to the large quantity of constant volume air required to meet space loads. These drawbacks have made multizone systems nearly obsolete except in relatively small buildings with only a few zones and short duct runs.

Figure 10.7 Multizone system schematic diagram.

Dual Duct Systems

Dual duct systems are similar to the multizone concept in that both cool supply air and warm supply air are produced by a central AHU. But instead of blending the air in the fan room, separate hot-air ducts and cold-air ducts run parallel throughout the distribution network and air is mixed at terminal mixing boxes in each zone (Figure 10.8). The mixing boxes may include an outlet for delivering air directly to the space, or a duct may connect a branch network with air mixed to a common requirement.

Dual duct systems require the greatest amount of space for distribution ductwork. In order to offset the spatial limitations imposed by this problem, dual duct systems often employ high velocity/high pressure supply ducts, which reduce the size (and cost) of ductwork, as well as the required floor-to-floor height. However this option increases the fan energy required for distribution. Their use is usually limited to buildings with very strict requirements for temperature and or humidity control.

Constant Volume Dual Duct—For a long time, the only variation of the dual duct system was a CAV system, which functioned very much like the multizone system. This system exhibits the greatest energy consumption of any all-air system. In addition to the energy required to mix conditioned air even when no heating or cooling is required, it requires a great amount of distribution energy even when normal pressure and low air velocities are used. For these reasons it has become nearly obsolete, being replaced with dual duct VAV or other systems.

Dual Duct VAV—Although the dual duct VAV system looks very much like its CAV counterpart, it is far more efficient. Instead of providing a constant volume of supply air at all times, the primary method of responding to thermostatic requirements is through adjusting the volume of either cool or warm supply air.

The properly designed dual duct VAV system functions essentially as two single duct VAV systems operating side by side; one for heating and one for cooling. Except when humidity control is required it is usually possible to provide comfort at all temperatures without actually mixing the two air streams. Even when humidity adjustment is required a good control system can minimize the amount of air mixing required.

The dual-duct VAV system still requires more distribution energy and space than most other systems. The level of indirect control which is necessary to produce heated and cooled air also increases energy consumption. Consequently its use should be restricted to applications which benefit from its ability to provide ex

Figure 10.8 Dual duct system schematic diagram.

ceptional temperature and humidity control and which do not require a constant supply of ventilation air.

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