In short, when designing an air conditioning system, designers must consider the size and design of the building, the local climate, energy efficiency, indoor air quality, maintenance and serviceability, cost, occupancy and use, zoning, noise, sustainability, regulatory and code requirements, and future expansion. To avoid the types of problems that occur in Honolulu hotels, air conditioning systems in buildings with humid climates must do several things to maintain humidity control. The air conditioning system must adequately condition the interior of the building, including the building envelope, and pressurize the building with dehumidified air (called exfiltration). When negative pressurization occurs in a building in humid climates, the result can be multimillion-dollar moisture and mold problems caused by intrusion and condensation of humid outdoor air.
The Florida Solar Energy Center (FSEC) found that building pressures as low as +1 pascal (Pa) relative to outdoor conditions are sufficient to prevent outdoor air infiltration problems. On the other hand, even a slightly depressurized building (-1 Pa compared to normal outdoor conditions) in hot, humid climates can cause devastating moisture and microbial growth problems when the building envelope retains this moisture. Air conditioning systems that positively pressurize a building space by providing unconditioned or only partially conditioned outdoor air will prevent the infiltration of outside air through the building envelope. However, this same situation can cause moisture loads inside the building that exceed the dehumidification capacities of the system.
of air conditioning. One of the most important causes of moisture accumulation in existing buildings in hot, humid climates is the overemphasis on ventilation at the expense of adequate dehumidification. The comparison of latent and sensitive loads in several major cities in different geographical regions (Peart and Cook, 1999) helps to illustrate the new definition. Figure 2 (ABC) shows the monthly average of latent and sensitive loads coming from outside air in Orlando (Florida), Atlanta (Georgia) and Columbus (Ohio).
During the cooling season in Orlando (Figure 2A), the latent load far exceeds the sensible load of outdoor air. The effect of these conditions, which occur for more than half a year, is that any outside air entering the building envelope or the occupied space is likely to cause moisture accumulation and microbial growth problems. In addition, since this outdoor air is used to ventilate the occupied spaces of the building, it represents an enormous dehumidification challenge for the replacement air system. Obviously, under these conditions, Orlando is very susceptible to moisture ingress problems.
Achieving adequate pressurization of buildings is sometimes difficult. The pressurization of buildings must overcome any depressurization caused by the chimney effect, the wind effect and the fan effect (figure). The design team must consider how exhaust air systems will affect space pressures. If the system cannot provide sufficient dehumidification while reacting solely to temperature control, it must continue to eliminate humidity without affecting the interior temperature or the comfort of the occupants.
One way to achieve this is by reheating, a way of simultaneously cooling and heating to continue dehumidification without overcooling the occupants. The second method of dehumidification is through the use of a desiccant that attracts moisture from the air to its surface by introducing low vapor pressure to the surface of the desiccant. The vapor pressure of moisture in the air is higher, so moisture travels from the air to the desiccant. The desiccant must then be recharged through a heating process, which allows moisture to leave the desiccant and be discharged to another location in addition to the cooling air stream.
One of the best dehumidification strategies is a combination of desiccant and cooling systems, especially for 100 percent outdoor air currents, such as systems of replacement air. Since air leaves a cooling-based system when it becomes saturated, it only moves to a lower relative humidity once it mixes with room air and heat is added to it. The desiccant, on the other hand, enters the space with a very low relative humidity, and its relative humidity increases until it reaches the relative humidity level of the room once the two air currents reach equilibrium. There are many factors to consider when choosing and designing building air conditioning systems.
Ultimately, those decisions influence the energy efficiency of the building. The main factors for optimizing energy efficiency are ventilation, zone control, heat recovery and operations. Ease of maintenance and ease of service are often overlooked, but they are essential factors in the design of HVAC systems.
