SINGLE-DUCT – Variable Air Volume
A VAV system, as shown in Figure 10, controls temperature in a space by varying the quantity of supply air rather than varying the supply air temperature. A VAV terminal unit at the zone varies the quantity of supply air to the space. The supply air temperature is held relatively constant: while supply air temperature can be moderately reset depending on the season, it must always be low enough to meet the cooling load in the most demanding zone, and to maintain appropriate humidity. Variable air volume systems can be applied to interior or perimeter zones, with common or separate fans, with common or separate air temperature control, and with or without auxiliary heating devices. The greatest energy saving associated with VAV occurs at the perimeter zones, where variations in solar load and outside temperature allow the supply air quantity to be reduced. If the peak room load is not determined accurately, an oversized VAV system will partially throttle at full load and may become excessively noisy throttling at part loads.
Humidity control is a potential problem with VAV systems. If humidity is critical, as in certain laboratories, process work, etc., systems may have to be limited to constant volume airflow. Particular care should be taken in areas where the sensible heat ratio (ratio of sensible heat to sensible plus latent heat to be removed) is low, such as in conference rooms. In these situations, the minimum set point of the VAV terminal unit can be set at about 50% and reheat added as necessary to keep humidity low during reduced load conditions.
Other measures may also be used to maintain enough air circulation through the room to absorb sufficient moisture to achieve acceptable humidity levels. The human body is more sensitive to elevated air temperatures when there is little air movement. Minimum air circulation can be maintained during reduced load by (1) raising the supply air temperature of the entire system, which increases space humidity, or supplying reheat on a zone-by-zone basis; (2) providing auxiliary heat in each room independent of the air system; (3) using individual zone recirculation and blending varying amounts of supply and room air or supply and ceiling plenum air with fan-powered VAV terminal units, or, if the design permits, at the air-handling unit; (4) recirculating air with a VAV induction unit, or (5) providing a dedicated recirculation fan to increase airflow.
Variable Diffuser. The discharge aperture of this diffuser is educed to keep the discharge velocity relatively constant while reducing the conditioned supply airflow. Under these conditions, the induction effect of the diffuser is kept high, cold air mixes in the space, and the room air distribution pattern is more nearly maintained at reduced loads. These devices are of two basic types—one has a flexible bladder that expands to reduce the aperture, and the other has a diffuser plate that physically moves. Both devices are pressure-dependent, which must be considered in the design of the duct -distribution system. They are either powered by the system or pneumatically or electrically driven.
SINGLE-DUCT – Constant Volume
While maintaining constant airflow, single-duct constant volume systems change the supply air temperature in response to the space load (Figure 9). Single-Zone Systems. The simplest all-air system is a supply unit serving a single-temperature control zone. The unit can be installed either in or remote from the space it serves, and it may operate with or without distribution ductwork. Ideally, this system responds completely to the space needs, and well-designed control systems maintain temperature and humidity closely and efficiently. Single zone systems often involve short ductwork with low pressure drop and thus low fan energy, and single-zone systems can be shut down when not required without affecting the operation of adjacent areas, offering further energy savings. A return or relief fan may be needed, depending on the capacity of the system and whether 100% outdoor air is used for cooling as part of an economizer cycle. Relief fans can be eliminated if provisions are made to relieve over pressurization by other means, such as gravity dampers.
Multiple-Zone Reheat. The multiple-zone reheat system is a modification of the single-zone system. It provides (1) zone or space control for areas of unequal loading; (2) simultaneous heating or cooling of perimeter areas with different exposures; and (3) close tolerance of control for process or comfort applications. As the word reheat implies, heat is added as a secondary simultaneous process to either preconditioned (cooled, humidified, etc.) primary air or recirculated room air. Relatively small low-pressure systems place reheat coils in the ductwork at each zone. More complex designs include high-pressure primary distribution ducts to reduce their size and cost and pressure reduction devices to maintain a constant volume for each reheat zone.
The system uses conditioned air from a central unit generally at a fixed cold air temperature that is low enough to meet the maximum cooling load. Thus, all supply air is always cooled the maximum amount, regardless of the current load. Heat is added to the airstream in each zone to match the cooling capacity to the current load in that zone. The result is very high energy use. However, the supply air temperature from the unit can be varied, with proper control, to reduce the amount of reheat required and the associated energy consumption. Care must be taken to avoid high internal humidity when the temperature of air leaving the cooling coil is permitted to rise during the spring and fall.
When a reheat system heats a space with an exterior exposure in cold weather, the reheat coil must not only replace the heat lost from the space, but also must offset the cooling of the supply air (enough cooling to meet the peak load for the space), further increasing energy consumption compared to other systems. If a constant volume system is oversized, the reheat cost becomes excessive.
Bypass. A variation of the constant volume reheat system is the use of a bypass terminal unit instead of reheat. This system is essentially a constant volume primary system with a VAV secondary system. The quantity of room supply air is varied to match the space load by dumping excess supply air into the return ceiling plenum or return air duct, i.e., by bypassing the room. When bypass terminal units dump to a return air plenum, the return air plenum temperature is reduced and the plenum must be kept at a lower pressure than the room so that cooler air does not spill into the room through the return air grilles. A return fan is often used to create that negative pressure. While this reduces the air volume supplied to the space, the system air volume and fan energy remains constant. Refrigeration or heating at the air-handling unit is reduced due to the lower return air temperature. A bypass system is generally restricted to small installations where a simple method of temperature control is desired, a modest initial cost is desired, and energy conservation is less important.
AHU – Equipment Isolation
Vibration and sound isolation equipment is required for most central fan installations. Standard mountings of fiberglass, ribbed rubber, neoprene mounts, and springs are available for most fans and prefabricated units. The designer must account for seismic restraint requirements for the seismic zone in which the particular project is located. In some applications, the fans may require concrete inertia blocks in addition to non-enclosed spring mountings. Steel springs require sound-absorbing material inserted between the springs and the foundation. Horizontal discharge fans operating at a high static pressure frequently require thrust arrestors. Ductwork connections to fans should be made with fireproof fiber cloth sleeves having considerable slack, but without offset between the fan outlet and rigid duct. Misalignment between the duct and fan outlet can cause turbulence, generate noise, and reduce system efficiency. Electrical and piping connections to vibration-isolated equipment should be made with flexible conduit and flexible connections. Special considerations are required in seismic zones.
Equipment noise transmitted through the ductwork can be reduced by sound-absorbing units, acoustical lining, and other means of attenuation. Sound transmitted through the return and relief ducts should not be overlooked. Acoustical lining sufficient to adequately attenuate any objectionable system noise or locally generated noise should be considered. Chapter 46 of the 1999 ASHRAE Handbook—Applications, Chapter 7 of the 1997 ASHRAE Handbook—Fundamentals, and ASHRAE Standard 68 have further information on sound and vibration control. Noise control, both in the occupied spaces and outside near intake or relief louvers, must be considered. Some local ordinances may limit external noise produced by these devices.
AHU – Outside Air Requirements
common complaint regarding buildings is the lack of outside air. This problem is especially a concern in VAV systems where outside air quantities are established for peak loads and are then reduced in proportion to the air supplied during periods of reduced load. A simple control added to the outside air damper can eliminate this problem and keep the amount of outside air constant, regardless of the operation of the VAV system. However, the need to preheat the outside air must be considered if this control is added.
Another problem is that some codes require as little as 2.4 L/s per person [about 0.25 L/(s·m2)] of outside air. This amount is far too low for a building in which modern construction materials are used. Higher outside air quantities may be required to reduce odors, VOC’s, and potentially dangerous pollutants. ASHRAE Standard 62 provides information on ventilation for acceptable indoor air quality. Air quality (i.e., the control or reduction of contaminants such as volatile organic compounds, formaldehyde from furnishings, and dust) must be reviewed by the engineer.
Heat recovery devices are becoming more popular as the requirements for outside air increase. They are used extensively in research and development facilities and in hospitals and laboratories where HVAC systems supply 100% outside air. Many types are available, and the type of facility usually determines which is most suitable. Many countries with extreme climates provide heat exchangers on outside/relief air, even for private homes. This trend is now appearing in larger commercial buildings worldwide. Heat recovery devices such as air-to-air plate heat exchangers can save energy and reduce the required capacity of primary cooling and heating plants by 20% and more under certain circumstances.
The location of intake and exhaust louvers should be carefully considered; in some jurisdictions, location is governed by codes. Louvers must be separated enough to avoid short-circuiting of air. Furthermore, intake louvers should not be near a potential source of contaminated air such as a boiler stack or hood exhaust. Relief air should also not interfere with other systems. If heat recovery devices are used, intake and exhaust airstreams may need to run together, such as through air-to-air plate heat exchangers.
AHU – Supply Air Fan
Either axial flow, centrifugal, or plug fans may be chosen as supply air fans for straight-through flow applications. In factory-fabricated units, more than one centrifugal fan may be tied to the same shaft. If headroom permits, a single-inlet fan should be chosen when air enters at right angles to the flow of air through the equipment. This permits a direct flow of air from the fan wheel into the supply duct without abrupt change in direction and loss in efficiency. It also permits a more gradual transition from the fan to the duct and increases the static regain in the velocity pressure conversion. To minimize inlet losses, the distance between the casing walls and the fan inlet should be at least the diameter of the fan wheel. With a single-inlet fan, the length of the transition section should be at least half the width or height of the casing, whichever is longer. If fans blow through the equipment, the air distribution through the downstream components needs analyzing, and baffles should be used to ensure uniform air distribution.
AHU – Humidifiers
Humidifiers may be installed as part of the central station airhandling unit, or in terminals at the point of use, or both. Where close humidity control of selected spaces is required, the entire supply airstream may be humidified to a lower humidity level in the air handler, with terminal humidifiers located in the supply ducts serving just those selected spaces bringing them up to their required humidity levels. For comfort installations not requiring close control, moisture can be added to the air by mechanical atomizers or point-of-use electric or ultrasonic humidifiers. Proper location of this equipment prevents stratification of moist air in the system.
In this application, the heat of evaporation should be replaced by heating the recirculated water, rather than by increasing the size of the preheat coil. Steam grid humidifiers with dew-point control usually are used for accurate humidity control. It is not possible to add moisture to saturated air, even with a steam grid humidifier. Air in a laboratory or other application that requires close humidity control must be reheated after leaving a cooling coil before moisture can be added. The capacity of the humidifying equipment should not exceed the expected peak load by more than 10%. If the humidity is controlled from the room or the return air, a limiting humidistat and fan interlock may be needed in the supply duct. This prevents condensation and mold or mildew growth in the ductwork when temperature controls call for cooler air. Humidifiers add some sensible heat that should be accounted for in the psychrometric evaluation.
AHU – Reheat Coil
Reheat systems are strongly discouraged, unless recovered energy is used (see ASHRAE Standard 90.1). Reheating is limited to laboratory, health care, or similar applications where temperature and relative humidity must be controlled accurately. Heating coils located in the reheat position, as shown in Figure 1, are frequently used for warm-up, although a coil in the preheat position is preferable. Hot water heating coils provide the highest degree of control. Oversized coils, particularly steam, can stratify the airflow; thus, where cost-effective, inner distributing coils are preferable for steam applications. Electric coils may also be used.
AHU – Cooling Coil
In this section, sensible and latent heat are removed from the air. In all finned coils, some air passes through without contacting the fins or tubes. The amount of this bypass can vary from 30% for a four-row coil at 3.5 m/s to less than 2% for an eight-row coil at 1.5 m/s. The dew point of the air mixture leaving a four-row coil might satisfy a comfort installation with 25% or less outdoor air, a small internal latent load, and sensible temperature control only. For close control of room conditions for precision work, a deeper coil may be required.
Coil freezing can be a serious problem with chilled water coils. Full flow circulation of chilled water during freezing weather, or even reduced flow with a small recirculating pump, minimizes coil freezing and eliminates stratification. Further, continuous full flow circulation can provide a source of off-season chilled water in airand-water systems. Antifreeze solutions or complete coil draining also prevent coil freezing. However, because it is difficult, if not impossible, to drain most cooling coils completely, caution should be exercised if this option is considered.
AHU – Preheat Coil
The preheat coil should have wide fin spacing, be accessible for easy cleaning, and be protected by filters. If the preheat coil is located in the minimum outdoor airstream rather than in the mixed airstream as shown in Figure 1, it should not heat the air to an exit temperature above 2 to 7°C; preferably, it should become inoperative at outdoor temperatures of 7°C. Inner distributing tube or integral face and bypass coils are preferable with steam. Hot water preheat coils should have a constant flow recirculating pump and should be piped for parallel flow so that the coldest air will contact the warmest part of the coil surface first.
AHU – Mixing Plenum
If the equipment is closely coupled to outdoor louvers in a wall,
the minimum outdoor air damper should be located as close as possible to the return damper connection. An outside air damper sized for 7.5 m/s gives good control. Low-leakage outdoor air dampers minimize leakage when closed during shutdown. The pressure difference between the relief plenum and outdoor intake plenum must be measured through the return damper section. A higher velocity through the return air damper—high enough to cause this loss at its full open position—facilitates air balance and creates good mixing. To create maximum turbulence and mixing, return air dampers should be set so that any deflection of air is toward the outside air. Parallel blade dampers may aid mixing. Mixing dampers should be placed across the full width of the unit, even though the location of the return duct makes it more convenient to return air through the side. When return dampers are placed at one side, return air passes through one side of a double-inlet fan, and cold outdoor air passes through the other. If the air return must enter the side, some form of air blender should be used.
Although opposed blade dampers offer better control, properly proportioned parallel blade dampers are more effective for mixing airstreams of different temperatures. If parallel blades are used, each damper should be mounted so that its partially opened blades direct the airstreams toward the other damper for maximum mixing. Baffles that direct the two airstreams to impinge on each other at right angles and in multiple jets create the turbulence required to mix the air thoroughly. In some instances, unit heaters or propeller fans have been used for mixing, regardless of the final type and configuration of dampers. Otherwise, the preheat coil will waste heat, or the cooling coil may freeze.