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.
AHU – Automatic Dampers
Opposed blade dampers for the outdoor, return, and relief airstreams provides the highest degree of control. The section on Mixing Plenum covers the conditions that dictate the use of parallel blade dampers. Pressure relationships between various sections must be considered to ensure that automatic dampers are properly sized for wide open and modulating pressure drops.
AHU – Relief Air Fan
In many situations, a relief (or exhaust) air fan may be used instead of a return fan. A relief air fan relieves ventilation air introduced during air economizer operation and operates only when this control cycle is in effect. When a relief air fan is used, the supply fan must be designed for the total supply and return static pressure in the system, since the relief air fan does not operate during the non-economizer mode of operation. During the economizer mode of operation, the relief fan must be controlled to exhaust at a rate that tracks the quantity of outside air introduced, to ensure a slight positive pressure in the conditioned space, as with the return air fan system cited previously. The section on Economizers describes the required control for relief air fans.
AHU – Return Air Fan
A return air fan is optional on small systems but is essential for the proper operation of air economizer systems for free cooling from outside air if the return path has a significant pressure drop (greater than about 75 Pa) It provides a positive return and exhaust from the conditioned area, particularly when mixing dampers permit cooling with outdoor air in intermediate seasons and winter. The return air fan ensures that the proper volume of air returns from the conditioned space. It prevents excess pressure when economizer cycles introduce more than the minimum quantity of outside air. It also reduces the static pressure the supply fan has to work against. (The use of a return fan can increase system energy use if the system is so arranged that the return fan works against more static pressure than the incremental static pressure the supply fan would have to provide to overcome the return path pressure drop) The supply fan(s) must be carefully matched with the return fans, particularly in variable air volume (VAV) systems. The return air fan should handle a slightly smaller amount of air to account for fixed exhaust systems, such as the toilet exhaust, and to ensure a slight positive pressure in the conditioned space. Chapter 45 of the 1999 ASHRAE Handbook—Applications has design details.
AHU – Air Conditioner
To determine the system’s air-handling requirement, the designer must consider the function and physical characteristics of the space to be conditioned and the air volume and thermal exchange capacities required. Then, the various components may be selected central system—equipment must be adequate, accessible for easy maintenance, and not too complex in arrangement and control to produce the required conditions. Further, the designer should consider economics in component selection. Both initial cost and operating costs affect design decisions. The designer should not arbitrarily design for a 2.5 m/s face velocity, which has been common for selection of cooling coils and other components. Filter and coil selection at 1.5 to 2 m/s, with resultant lower pressure loss, could produce a substantial payback on constant volume systems. Chapter 35 of the 1999 ASHRAE Handbook—Applications has further information on energy and life-cycle costs.
Figure 1 shows a general arrangement of the air-handling unit components for a single-zone, all-air central system suitable for year-round air conditioning. With this arrangement, close control of temperature and humidity are possible. All these components are seldom used simultaneously in a comfort application. Although Figure 1 indicates a built-up system, most of the components are available from many manufacturers completely assembled or in subassembled sections that can be bolted together in the field. When selecting central system components, specific design parameters must be evaluated to balance cost, controllability, operating expense, maintenance, noise, and space. The sizing and selection of primary air-handling units substantially affect the results obtained in the conditioned space.
AHU- Humidification
The methods used to humidify air include
• Direct spray of recirculated water into the airstream (air washer) reduces the dry-bulb temperature while maintaining an almost constant wet bulb in an adiabatic process [see Figure 3, Paths (1) to (3)]. The air may also be cooled and dehumidified, or heated and humidified by changing the temperature of the spray.
In one variation, the surface area of water exposed to the air is increased by spraying water onto a cooling/heating coil. The coil surface temperature determines the leaving air conditions. Another method is to spray or distribute water over a porous medium, such as those in evaporative coolers and commercial greenhouses. This method requires careful monitoring of the water condition to keep biological contaminants from the airstream (Figure 6).
• Compressed air that forces water through a nozzle into the airstream is essentially a constant wet-bulb (adiabatic) process. The water must be treated to keep particulates from entering the airstream and contaminating or coating equipment and furnishings.
Many types of nozzles are available.
• Steam injection, which is a constant dry-bulb process (Figure 7). owever, as the steam injected becomes superheated, the leaving dry-bulb temperature increases. If live steam is injected into the airstream, the boiler water treatment chemical must be nontoxic to the occupants and, if the air is supplying a laboratory, to the research under way.
Moisture condenses on a cooling coil when its surface temperature is below the dew point of the air, thus reducing the humidity of the air. In a similar manner, air will also be dehumidified if a fluid with a temperature below the airstream dew point is sprayed into the airstream. The process is identical to that shown in Figure 2, except that the moisture condensed from the airstream condenses on, and dissolves in, the spray droplets instead of on the solid coil surface. Chemical dehumidification involves either passing air over a solid desiccant or spraying the air with a solution of the desiccant and water. Both of these processes add heat, often called the latent heat of wetting, to the air being dehumidified. Usually about 465 J/kg of moisture is removed (Figure 8).
AHU- Cooling Process
The basic methods used for cooling include
• Direct expansion, which takes advantage of the latent heat of the fluid, as shown in the psychrometric diagram in Figure 2.
• Fluid-filled coil, where temperature differences between the fluid and the air cause an exchange of energy by the same process as in Figure 2 (see the section on Dehumidification).
• Direct spray of water in the airstream (Figure 3) in which an adiabatic process uses the latent heat of evaporation of water to reduce dry bulb temperature while increasing moisture content. Both sensible and latent cooling is also possible by spraying chilled water. Air can be cooled and greatly humidified by spraying heated water. A conventional evaporative cooler, uses the adiabatic process, by spraying or dripping recirculated water onto a filter pad (see the section on Humidification). The wetted duct or supersaturated system is a variation on direct spray. In this system, tiny droplets of free moisture are carried by the air into the conditioned space where they evaporate, providing additional cooling. This reduces the amount of air needed for a given space load (Figure 4).
• Indirect evaporative cooling adiabatically cools outdoor air or exhaust air from the conditioned space by spraying water, then passes that cooled air through one side of a heat exchanger, while the air to be supplied to the space is cooled by passing through the other side of the heat exchanger.
