Comfort is a very subjective index. It can depend on a number of factors like the amount of clothing worn, the occupant’s metabolic rate and activity level, and the temperature and humidity conditions. “Human comfort depends on factors ranging from temperature, humidity and air movement to clothing and culture. What is comfortable for one person in one society may be entirely uncomfortable for another. Someone who has long lived without refrigerated air conditioning may find an artificially air-conditioned environment uncomfortable; whereas people who take refrigerated air conditioning for granted in their homes and workplaces may avoid being outside during hot weather all together.
Standards
“Comfort zones” are often shown on standard psychrometric charts and have been developed to indicate regions where a person is “comfortable.” In the United States, the American Society of Heating, Refrigerating and Air-conditioning Engineers (ASHRAE) has developed comfort zones based on psychrometric charts. However, these standard types of comfort charts have changed over the years and now have more limited relevance related to evaporative air-conditioning. First, standard comfort zones are based on air velocities typical of vapor-compression air-conditioning systems, not the higher air velocities used with evaporative air-conditioners. Second, the traditional comfort zones used today (unlike those of the past) have horizontal, constant humidity-ratio (constant dew point) lines supposedly aimed to minimize respiratory diseases, mold growth, and similar problems. Relative humidity boundary lines are just as effective (and were previously used) and would distort comfort analysis less. Tests have shown that human comfort is a continuum, not confined between dew-point lines. Consequently, the standard comfort zones commonly used face shortcomings relative to EAC.
The Modified Comfort Standard for Evaporative Air-Conditioning
The effect of a given air stream on a person can be determined by an effective temperature chart, as is commonly used when calculating wind chill. By increasing the velocity of movement, air feels cooler. For evaporative air-conditioning, it is more reliable to consider a comfort zone bounded by relative humidity and extended to take into account the cooling effect of increased airflow, as shown in Figure 2.1.
Actual Comfort
The actual comfort derived from EAC for a given dry and wet-bulb temperature depends on the following factors:
Supply air temperature. Saturation effectiveness of the evaporative air conditioner. Only the theoretical 100 percent saturation effectiveness can reduce the leaving air temperature to the wet-bulb temperature. The EAC room temperature of an actual installation will depend on the condition and quality of the evaporative media, heat losses from the motor, fan, and pump, and heat absorbed because of the exposure of the air-conditioner cabinet to direct solar rays. Actual typical saturation efficiencies range between 60 and 90 percent for commercially available media.
Room air temperature. Heat absorption of the space to be cooled affects the radiant temperature of the interior surfaces. This depends on exposure of walls and roof to solar gain, shading, number, size, and location of windows and construction materials.
Heat generation in the space. Number of people present in the room, and the presence of heat generating equipment such as copy machines, stoves, television, and computers.
Initial sizing of the EAC unit.
Proper installation and airflow balancing. Cooled air should be properly divided and directed to most effectively “wash” the space and occupants to be cooled.
The air velocity on occupants. The air moving past the occupants’ skin adds to the cooling effect. The evaporation of small amounts of moisture on the skin is a localized evaporative cooling effect which Nature has perfected.
Activity level of the occupants. Sedentary people require less cooling than physically active persons.
In some locations, EAC may be acceptable for users willing to experience less than full comfort from the EAC for a few hours on the hottest days of the year because the slight discomfort does not outweigh the extra costs associated with VAC (vapor-compression air conditioning).”34
The American Society of Heating Refrigeration and Air Conditioning Engineers (ASHRAE) used several studies of physiological considerations to develop “comfort zones” which can be shown graphically on a psychrometric chart (see above for explanation of a “psychrometric chart”). This makes it easier to visualize the acceptable comfort limits for temperature and humidity. They have used studies with statistical connections between comfort level, temperature, humidity, sex and length of exposure.
There are different comfort charts for different conditions such as the generalized comfort chart (Fanger Chart), which is used for common refrigerated air systems, a chart for sedentary occupants and a modified evaporative air-conditioning comfort zone. This comfort zone accounts for the increased airflow of EAC systems as compared to vapor compression air-conditioning. In a smaller way, this is analogous to the wind chill factor announced on the weathercast. This wind chill changes the size of the comfort zone, which resembles a box with rounded lines, to be larger for the evaporative comfort zone, or a wider range of perceived comfort. In addition to air velocity, there are other factors such as the amount of clothing worn, and time spent in the environment. “A study of short-term adaptation to comfortable temperatures for young adults of both sexes shows men feel warmer than women on initial exposure, but later feel cooler, approaching women’s thermal sensations after 1 to 2 hours in an experimental chamber”.35