Keeping your cool in summer

Using airflow and Effective Temperature to understand heat and humidity

By Bill Buell and Hudson Tarianto

PlantServices.com

In many industrial working environments, air conditioning for summer working comfort isn’t an option; it’s a necessity. At the same time, many industrial buildings require large openings in their envelope for day-to-day operation; that’s not conducive to air conditioning. How does one solve this conundrum?

Figure 1. Climate classification in the country.

Putting the warm air into motion offers a practical alternative in many cases. It’s energy-efficient because there’s no need to chill the air; the airflow passing over exposed skin produces the cooling sensation. The approach is particularly suitable when elevated summer air temperatures coincide with high humidity. Much of the United States has a humid environment.

Climate zones

Climatic conditions everywhere in the United States vary seasonally and from year to year. Averaging climatic data over time establishes the climate classification shown in Figure 1. Table 1, on the other hand, shows typical summer air temperature and relative humidity characteristics of the seven U.S. zones. These data can be used to evaluate summer comfort and physical performance in well-ventilated, non-air conditioned, industrial environments with no significant indoor heat sources.

ET* and thermal comfort

Zone Temp (F)  RH (percent) 1 90 57 2 94 50 3 93 47 4 91 48 5 90 45 6 87 48 7 87 44 Table 1. Summer design temperatures and  humidities for U.S. zones in Figure 1.

Probably the most common index of thermal comfort is the “new” Effective Temperature (ET*) developed by Gagge and others in 1971. This index is based on human physiological regulatory response that establishes environmental conditions equivalent to a Celsius air temperature when relative humidity is 50%. Lines of constant ET* have varying slopes when plotted on a psychrometric chart (see Figure 2). For purposes of comparison, a standard effective temperature (SET*) adopts a set of parameters. These are the use of operative temperature to accommodate radiation effects, clothing insulation value of 0.6 clo (a measure of the ability of clothing to keep you warm), an activity (metabolic rate) of one met, air movement less than 0.2 m/s and an exposure time of one hour.

Thermal comfort zone

Individual responses to the thermal environment vary. There’s no universal ET* that everyone perceives as comfortable, which explains why thermal comfort is defined as a range of ET* values bracketing the comfort zone.

Two such zones — for winter and for summer — are defined (see Figure 2). These zones account for peoples’ ability to adapt to seasonal changes by choice of clothing. Figure 2 also indicates various degrees of discomfort as ET* values.

Temp (ºF)	70	75	80	85	90	95	100	105	110	115	120
100	22.3	25.9	29.4	33.0	36.8	40.6	44.4	48.1	51.6	55.1	58.5
95	22.2	25.7	29.2	32.7	36.5	40.1	43.8	47.4	50.9	54.3	57.7
90	22.1	25.5	28.9	32.3	36.1	39.5	43.2	46.7	50.1	53.5	56.9
85	22.0	25.3	28.7	32.0	35.6	39.0	42.5	46.0	49.4	52.7	56.0
80	21.9	25.1	28.4	31.6	35.2	38.5	41.9	45.3	48.6	51.9	55.1
75	21.8	24.9	28.1	31.3	34.7	37.9	41.3	44.6	47.8	51.0	54.2
70	21.6	24.7	27.8	30.9	34.2	37.4	40.6	43.8	46.9	50.1	53.2
65	21.5	24.5	27.6	30.6	33.8	36.8	39.9	43.1	46.1	49.2	52.3
60	21.4	24.3	27.3	30.2	33.3	36.2	39.3	42.3	45.2	48.2	51.2
55	21.3	24.2	27.0	29.8	32.8	35.7	38.6	41.5	44.3	47.2	50.1
50	21.1	23.9	26.7	29.4	32.2	35.0	37.8	40.6	43.3	46.1	48.9
Table 2. Effective Temperature (ET*) look-up table.

Calculating the effective temperature is a complex procedure, but a look-up table (see Table 2) provides a close approximation.