Wet Bulb Temperature and Evaporative Cooling

Using evaporative cooling to keep birds cool during hot weather is relatively simple. Water is circulated over paper pads. Tunnel fans pull hot outside air through the pads and into the house. As the air moves through the wetted pads, its temperature decreases. Fairly straightforward. On the other hand, understanding how evaporative cooling pads lower the temperature of the incoming air can be rather difficult to understand fully, which often leads to mismanagement.

One of the most important things to keep in mind is that it's not the temperature of the water flowing over the pads that is reducing the temperature of the incoming air, but rather, it's the evaporation of water from the pads that produces the cooling of the incoming air. No matter the temperature of the water pumped to the top of a pad system, by the time it flows a few inches over the pad's surface, it will warm or cool to the wet bulb temperature (Figure 1). The wet bulb temperature is the lowest air temperature achievable through evaporative cooling and is determined by outside temperature and relative humidity. You can pump 50°F or 80°F water to the top of a pad; it doesn't matter, the water will change to the wet bulb temperature long before it reaches the bottom.

Measuring wet bulb temperature is easy. Simply placing a small wet cloth over a thermometer/temperature sensor and blow air over it. As water evaporates from the cloth, heat is "removed" from the cloth, and the temperature measured by the thermometer/temperature sensor will decrease. The lower the relative humidity, the greater the amount of water that evaporates, the greater the cooling produced, and the lower the temperature that a thermometer/temperature sensor will indicate. Figure 2 illustrates the wet bulb temperature as a function of air temperature and relative humidity.

Evaporative cooling pads produce cooling by simply evaporating moisture into the incoming air. As water evaporates from the pad into the air, heat is removed from the incoming air, thereby lowering its temperature. The greater the amount of water that can be evaporated into the air as it passes through a pad, the greater the cooling produced. It is very important to realize that it's not the fact that the wet pad is relatively cool that is reducing the temperature of the incoming air but rather it is the evaporation of the water into the incoming air that is reducing its temperature.

Evaporating water requires a lot of energy and, therefore, has a tremendous potential to cool hot air. For every ten gallons of water you evaporate into the air entering a poultry house, approximately 87,900 Btu's of heat will be "removed" from the air, which is very close to the amount of heat produced by burning a gallon of propane (91,500 Btu's). On the other hand, it doesn't take much energy to heat or cool water. For instance, if the 10 gallons of water is 30°F cooler than the wet bulb temperature, only an additional 2,500 BTUs of heat will be removed from the incoming air, an increase in cooling of around 3%. As a result, if a pad system normally reduces the incoming air temperature by 20°F, and the circulating water is chilled down by 30°F, it would only increase cooling by less than one degree. But, keep in mind by the time the water returned to the sump, it would have been warmed to the wet bulb temperature and would need to be discarded and replaced with 30°F cooler water, which would be very wasteful. The net result is that the cooling produced by a pad system is determined by outside temperature and relative humidity, and the water temperature has no practical effect on the amount of cooling produced by a pad system.

It is important to note that pads are not designed to decrease the incoming air temperature to the wet bulb temperature. Though this would result in maximum cooling of the incoming air, it would also result in the maximum humidification of the incoming air, meaning that the relative humidity of the incoming air would be 100%. Regardless of the amount of air cooling, bringing in air saturated with moisture would compromise the bird's ability to regulate their body temperatures, resulting in severe heat stress/possible death even at traditional target temperatures.

To avoid saturating the incoming air with moisture, evaporative cooling pads are typically designed for a cooling efficiency of between 70 and 75%. This means that they will decrease the incoming air temperature to within 75% of the wet bulb temperature. For instance, if it is 90°F outside and the wet bulb temperature is 70°F (RH=35%), the typical evaporative cooling pad would decrease the incoming air temperature to approximately 75°F ( (90°F - 70°F) X 0.75 = 15oF cooling). Since the relative humidity will increase by approximately 2.5% for every 1oF cooling, the relative humidity of the incoming air would increase to approximately 73% (35% + (2.5% X 15°F) = 73%).

You can estimate how much cooling a pad system should produce by simply knowing the nighttime low temperature. This is because in humid climates the wet bulb temperature throughout the day will be very close to the nighttime low air temperature. Simply take the outside air temperature, subtract the nighttime low air temperature, and multiply by 0.75, and this should be close to the cooling that will occur as the hot outside air moves through a pad. Over time, you will notice that the drier the weather, the lower the nighttime temperature, the lower the wet bulb temperature, and the greater the cooling your evaporative cooling pad system will produce.

Though understanding the physics of evaporative cooling can be difficult to grasp, all you have to keep in mind is that, like their name implies, evaporative cooling pads reduce the incoming air's temperature through water's physical evaporation into the air. The hotter and the drier the air is, the greater the amount of water that can be evaporated into the air, and the greater the cooling produced. The temperature of the water circulating over a pad is not something you can realistically control, and even if you did, it wouldn't significantly affect the temperature of the incoming air.