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.
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.
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.
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.
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).
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.
Air-handling equipment is available as packaged equipment in many configurations using any desired method of cooling, heating, humidification, filtration, etc. In large systems (over 25 m3/s), airhandling equipment is usually custom-designed and fabricated to suit a particular application. Air handlers may be either centrally located or decentralized.
Central Mechanical Equipment Rooms (MER). Usually the type of facility determines where the air-handling equipment is located. Central fan rooms today are more common in laboratory or industrial facilities, where maintenance is kept isolated from the conditioned space.
Decentralized Mechanical Equipment Rooms. Many office buildings locate air-handling equipment at each floor. This not only saves floor space for equipment but minimizes the space required for distribution ductwork and shafts. The reduced size of equipment as a result of duplicated systems allows the use of less expensive packaged equipment and reduces the need for experienced operating and maintenance personnel.
The basic secondary system is an all-air, single-zone, air-conditioning system consisting of an air-handling unit and an air distribution system. The air-handling unit may be designed to supply a constant air volume or a variable air volume for low-, medium-, or high-velocity air distribution. Normally, the equipment is located outside the conditioned area in a basement, penthouse, or service area. It can, however, be installed in the area if conditions permit. The equipment can be adjacent to the primary heating and refrigeration equipment or at considerable distance from it by circulating refrigerant, chilled water, hot water, or steam for energy transfer.
Figure 1 shows a typical draw-through central system that supplies conditioned air to a single zone or to multiple zones. A blowthrough configuration may also be used if space or other conditions dictate. The quantity and quality of supplied air are fixed by space requirements and determined as indicated in Chapters 27 and 28 of the 1997 ASHRAE Handbook—Fundamentals. Air gains and loses heat by contacting heat transfer surfaces and by mixing with air of another condition. Some of this mixing is intentional, as at the outdoor air intake; other mixing is the result of the physical characteristics of a particular component, as when untreated air passes through a coil without contacting the fins (bypass factor).
All treated and untreated air must be well mixed for maximum performance of heat transfer surfaces and for uniform temperatures in the airstream. Stratified, parallel paths of treated and untreated air must be avoided, particularly in the vertical plane of systems using double inlet or multiple-wheel fans. Because these fans do not completely mix the air, different temperatures can occur in branches coming from opposite sides of the supply duct.
Designers have considerable flexibility in selecting the supply air temperature and corresponding air quantity within the limitations of the procedures for determining heating and cooling loads. ASHRAE Standard 55 also addresses the effect of these variables on comfort. In establishing the supply air temperature, the initial cost of lower airflow and low air temperature (smaller fan and duct systems) must be calculated against the potential problems of distribution, condensation, air movement, and the presence of increased odors and gaseous or particulate contaminants. Terminal devices that use low-temperature air can reduce the air distribution cost. These devices mix room and primary air to maintain reasonable air movement in the occupied space. Because the amount of outside air needed is the same for any system, the percentage in low temperature systems is high, requiring special care in design to avoid freezing of preheat or cooling coils. Also, the low-temperature air supply reduces humidity in the space. Lower humidity during cooling cycles costs more in energy because the equipment runs longer. Also, if the humidity is too low, it may cause respiratory problems.
Although steam is an acceptable medium for central system preheat or reheat coils, low-temperature hot water provides a simple and more uniform means of perimeter and general space heating. Individual automatic control of each terminal provides the ideal space comfort. A control system that varies the water temperature inversely with the change in outdoor temperature provides water temperatures that produce acceptable results in most applications. To produce the best results, the most satisfactory ratio can be set after the installation is completed and actual operating conditions are ascertained.
Multiple perimeter spaces on one exposure served by a central system may be heated by supplying warm air from the central system. Areas that have heat gain from lights and occupants and no heat loss require cooling in winter, as well as in summer. In some systems, very little heating of the return and outdoor air is required when the space is occupied. Local codes dictate the amount of outside air required (see ASHRAE Standard 62 for recommended optimum outside air ventilation). For example, with return air at 24°C and outside air at -18°C, the temperature of a 25% outdoor/75% return air mixture would be 13°C, which is close to the temperature of the air supplied to cool such a space in summer. In this instance, a preheat coil installed in the minimum outdoor airstream to warm the outdoor air can produce overheating, unless it is sized so that it does not heat the air above 2 to 5°C. Assuming good mixing, a preheat coil located in the mixed airstream, prevents this problem. The outdoor air damper should be kept closed until room temperatures are reached during warm-up. A return air thermostat can terminate the warm-up period.
When a central air-handling unit supplies both perimeter and interior spaces, the supply air must be cool to handle the interior zones. Additional control is needed to heat the perimeter spaces properly. Reheating the air is the simplest solution, but it is not acceptable by most energy codes. An acceptable solution is to vary the volume of air to the perimeter and combine it with a terminal heating coil or a separate perimeter heating system, either baseboard, overhead air heating, or a fan-powered terminal unit with supplemental heat. The perimeter heating should be individually controlled and integrated with the cooling control. Resetting the supply water temperature downward when less heat is required generally improves temperature control.