A DISCUSSION ON EVAPORATIVE COOLING

 

How does evaporative cooling work?

    When water evaporates from a surface, that surface becomes cooler because heat is expended to change the water from a liquid into a vapor. A nice breeze on a hot day cools us because the current of air makes perspiration evaporate quickly. The heat needed for this evaporation is taken from our body surfaces.

 

    As water comes in contact with air, it evaporates to become moisture in the air. The amount of water the air can hold depends largely on how much water is already in the air. The term humidity describes the amount of water in the air. At any given temperature, there is a maximum about of water that air can hold.

 

What Is Relative Humidity?

    Humidity is said to be high if the air contains large amounts of moisture and low if the air contains only a small amount of moisture. When the air holds as much moisture as possible at a given temperature, the air is saturated. The warmer the air, the more moisture it can hold. Relative humidity (RH) gauges the amount of water in the air relative to the amount needed for saturation. If the air contains half the amount of moisture it can hold, the relative humidity is 50%.

 

    In order for water to evaporate, heat is required. To evaporate one gallon of water requires almost 8,700 BTUs (2,192 kilocalories, .580 kilocalories per gram at skin temperature) of heat. This heat is taken from whatever the water is in contact with, cooling the object as it evaporates. This heat can be taken from your body, from the air itself, or from a CoolTek garment made from Hydroweave™.

 

Does Cool Water Absorb More Heat Than Hot Water?

    Water temperature does not have a great effect upon the cooling produced through evaporation. At 90º F (32º C) it takes 9,000 BTUs (2.268 kilocalories) to evaporate a gallon of 50° F (10º C) water and 8,700 BTU (2.192 kilocalories) to evaporate a gallon of 90º F. water. In this example, the water is 180% warmer and results in only a 3% reduction in the amount of heat absorbed.

 

The Higher The Temperature, The Better Hydroweave Works

    The hotter the day, the dryer the air and the greater the evaporative cooling effect. As the day gets hotter, the relative humidity becomes lower and evaporative cooling efficiency increases.

 

    Stated another way, RH decreases as air temperature increases. For every 20° F (11º C) rise in temperature, the moisture-holding ability of air doubles. For instance, if the RH is 50% at 70° F (21º C) the RH would only be 25% at 90º F (32º C).

 

    The extent to which relative humidity decreases throughout the day can be affected by weather systems and proximity to large bodies of water. If a warm weather system moves in, but has a lot of water associated with it already, the decrease in humidity will not be as great.

 

Evaporation

    Evaporation is the conversion of a liquid substance into the gaseous state. This change in state from a liquid to a gas causes a decrease in the temperature of the remaining liquid. To maintain the liquid at a constant temperature, surrounding heat must be absorbed.

 

Heat Transfer by Vaporization

    The body’s normal response to heat is sweating. The secretion and evaporation of sweat is the principal mechanism by which the human body gets rid of excess heat.

 

    In the course of doing work, the body generates heat though its heart, lungs, and muscles. When the air temperature is higher than body temperature, then radiation, conduction and convection, all transfer heat into the body rather than out. The body must overcome this heat or become overheated, with serious health consequences.

 

    The heat expelled through evaporation can come from sweat or from surrounding heat sources. The more heat that is absorbed, the faster evaporation and cooling occur.

 

    The moisture lost through evaporation creates a second problem, dehydration. As little as 2% dehydration by weight can seriously impair a person’s ability to react and think. Physiology texts state that about 600 grams of insensible moisture loss occurs per day from the skin.

 

    The effectiveness of evaporative cooling is determined by the area of the wet surface and by the closeness of its contact with the dry air. The greater the surface area, the more effective the cooling. To get the largest air contact possible, CoolTek garments (made with Hydroweave) suspend thousands of tiny water absorbing (hydrophilic) fibers in a field of water resistant (hydrophobic) threads. Each hydrophilic fiber is held loosely within the batting, increasing air circulation and the effective wetted surface.

 

Calculating Cooling

    The temperature drop can be calculated. Assuming 80% efficiency, Hydroweave will reduce the wet bulb depression temperature by 80% (the difference between Wet Bulb and Dry Bulb temperatures).

 

    95° Dry Bulb Temperature

-   75° Wet Bulb Temperature

    20° Wet Bulb Depression

 

    20° X 80% = 16° Temperature Drop

 

    95° - 16° = 79° Dry Bulb Temperature

 

Active Evaporative Cooling Vs.
Passive Heat-Sink

    To demonstrate cooling effectiveness, CoolTek vests were tested as a passive heat sink beneath an encapsulated polyethylene-coated Tyvek barrier suit. The tests were conducted by Auburn University and measured the physiological changes of the wearer (e.g., core temperature) and tolerance times. In these tests, CoolTek vests using Hydroweave were proven as an effective deterrent to heat stress while increasing work time by an average of 16.4 %.

 

    As these tests demonstrated, Hydroweave can be used to make an effective ‘passive’ heat sink. Now let’s examine how much heat can be absorbed. If it takes 1 BTU to raise one pound of water 1º F (or 1 calorie to raise 1 gram 1º C) then a cooling vest with 2 pounds (907 grams) of water at 70º F (21º C) in ‘passive’ heat sink mode would absorb 57.2 BTUs (14.512 kilocalories) before reaching 98.6 F (37º C).

 

    However, heat absorption characteristics are dramatically different when evaporation can take place. Again looking at heat absorption, it requires approximately 8700 BTUs to evaporate 1 gallon of water or 2,192. kilocalories (580 calories/gram). A CoolTek will hold about ¼ of a gallon or 2 pounds (907 grams) of water and in ‘active’ evaporative mode will remove 2088 BTUs (526 kilocalories) of heat before all the water has evaporated.

 

    You can see that while passive heat absorption is effective, the heat absorption capacity is more than 36 times greater through evaporation. While you can increase the capacity in passive mode by refrigerating the garment prior to wear, this increase proves to be only a minor improvement compared to the increase evaporation brings (starting at 40º F a two pound vest would absorb 117 BTUs before reaching 98.6 F). 

 

    Further, when Auburn University tested with refrigerated vest, the improvement in heat absorption was offset by an initial 20-minute increase in core body temperature. This initial warming of the core temperature was attributed to vascular constriction and strongly suggests that refrigerated garments may not be the best choice for cooling.

Auburn University tested Hydroweave to evaluate how long it cooled. Samples were heated in an oven, starting at 70º F (21º C) with 50% RH and gradually increasing to 170º F (76º C) and 1.5% RH. When compared to a conventional fabric, the Hydroweave samples lasted five times longer.