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.
AHU – Economizers
An economizer uses outside air to reduce the refrigeration requirement. A logic circuit maintains a fixed minimum of ventilation outside air. The air side economizer is an attractive option for reducing energy costs when the climate allows. The air-side economizer takes advantage of cool outdoor air to either assist mechanical cooling or, if the outdoor air is cool enough, provide total system cooling. It is necessary to include some method of variable volume relief when air-side economizers are employed, to exhaust the extra outdoor air intake to outdoors. The relief volume may be controlled by several different methods, including fan tracking (operating the supply and return fans to maintain a constant difference in airflow between them,) or relief air discharge dampers which modulate in response to building space pressure. The relief system is off and relief dampers are closed when the air-side economizer is inactive. In systems with large return air static requirements, return fans or exhaust fans may be necessary to properly exhaust building air and take in outside air.
Advantages of Air-Side Economizers
• Substantially reduces compressor, cooling tower, and condenser water pump energy requirements
• Has a lower air-side pressure drop than a water-side economizer
• Has a higher annual energy savings than a water-side economizer
• Reduces tower makeup water and related water treatment
Disadvantages of Air-Side Economizers
• Humidification may be required during winter operation
• Equipment room is generally placed along the building’s exterior wall
• Installed cost may be higher than that for a water-side economizer if the cost of providing the exhaust system requirements exceeds the costs of piping, pump, and heat exchanger
AHU – Outdoor Air Intakes
Resistance through outdoor intakes varies widely, depending on construction. Frequently, architectural considerations dictate the type and style of louver. The designer should ensure that the louvers selected offer minimum pressure loss, preferably not more than 25 Pa. High-efficiency, low-pressure louvers that effectively limit carryover of rain are available. Flashing installed at the outside wall and weep holes or a floor drain will carry away rain and melted snow entering the intake. Cold regions may require a snow baffle to direct fine snow particles to a low-velocity area below the dampers. Outdoor dampers should be low-leakage types with special gasketed edges and special end treatment. Separate damper sections with separate damper operators are strongly recommended for the minimum outdoor air needed for ventilation. The maximum outdoor air needed for economizer cycles is then drawn through the entire outside air damper.
AHU – Return Air Dampers
The negative pressure in the outdoor air intake plenum is a function of the resistance or static pressure loss through the outside air louvers, damper, and duct. The positive pressure in the relief air plenum is, likewise, a function of the static pressure loss through the exhaust or relief damper, the exhaust duct between the plenum and outside, and the relief louver. The pressure drop through the return air damper must accommodate the pressure difference between the positive-pressure relief air plenum and the negative pressure outside air plenum. Proper sizing of this damper facilitates both air balancing and mixing. An additional manual damper may be required for proper air balancing.
AHU – Relief Openings
Relief openings in large buildings should be constructed similarly to outdoor air intakes, but they may require motorized or selfacting backdraft dampers to prevent high wind pressure or stack action from causing the airflow to reverse when the automatic dampers are open. The pressure loss through relief openings should be 25 Pa or less. Low-leakage dampers, such as those for outdoor intakes, prevent rattling and minimize leakage. Relief dampers sized for the same air velocity as the maximum outdoor air dampers facilitate control when an air economizer cycle is used. The relief air opening should be located so that the exhaust air does not short-circuit to the outdoor air intake.