Comfort

What affects comfort?
Apparent Temperature
How an evaporative cooler improves comfort level

What affects comfort?

Comfort is affected by 5 things:

• Temperature
• Humidity
• Airflow
• Clothing
• Work Rate

Much of the above is common sense. What is not normally appreciated is the relationship between temperature and humidity. An evaporative cooling installation affects the temperature, humidity levels and air flow. Whilst the humidity is increased it can be clearly shown that the effects of temperature reduction and air flow more than compensate for this. In order to understand this, an appreciation of the concept of ‘apparent temperature’ is required.

Apparent Temperature

The apparent temperature is a measure of relative discomfort due to combined heat and high humidity. It was developed by R.G. Steadman (1979) and is based on physiological studies of evaporative skin cooling for various combinations of ambient temperature and humidity. The apparent temperature equals the actual air temperature when the dew-point temperature is 14°C. At higher dew-points, the apparent temperature exceeds the actual temperature and measures the increased physiological heat stress and discomfort associated with higher than comfortable humidities. When the dew-point is less than 14°C, on the other hand, the apparent temperature is less than the actual air temperature and measures the reduced stress and increased comfort associated with lower humidity’s and greater evaporative skin cooling.

Apparent temperatures greater than 27°C are generally associated with some discomfort. Values approaching or exceeding 41°C are considered life-threatening, with severe heat exhaustion or heatstroke possible if exposure is prolonged or physical activity high. The degree of heat stress may vary with age, health, and body characteristics.

The following equation approximates the heat index. There are many assumptions made to produce this, far too many to list here. The equation was obtained by multiple regression analysis and there is a ±1 °C error.

HI = -42.379 + 2.04901523T + 10.14333127R - 0.22475541TR - 6.83783x10 -3 T 2 - 5.481717x10 -2 R 2 + 1.22874x10 -3 T 2R + 8.5282x10 -4 TR 2 - 1.99x10 -6 T 2 R 2

Where: T = ambient dry bulb temperature degrees Fahrenheit and R = relative humidity. This can be graphically represented as shown below.

It can be seen from the above that for 33°C conditions the stress level moves from caution below 35%RH to dangerous above 70%RH. In a UK factory it is not uncommon to see temperatures exceeding 35°C but the RH is quite low – typically about 25 – 30%. These are difficult conditions to work in and may lead to loss of productivity, increase errors or absenteeism. In extreme cases with susceptible persons this can lead to health problems associated with heat stress.

How an evaporative cooler improves comfort level

It can be seen from the diagram below how the apparent conditions of the air changes by passing through an evaporative cooler. With the air in at 35 °C and 35% RH the conditions are quite extreme. The air leaving the cooler would be 24 °C and 91% humidity. This then moves the comfort level clearly into the more comfortable zone.

It should be noted that the additional air flow on an individual can also create a wind chill effect of up to 5 °C.