Both direct and indirect evaporative cooling use water evaporation as the cooling process. Energy is required to change water from a liquid state to a vapor state. Water evaporating from a warm surface of an object will cool the object. This process is called direct evaporative cooling. Indirect cooling is accomplished by evaporating water into the air to reduce the air temperature. During indirect cooling, the actual energy level in the air remains the same, but how the energy is stored has changed. The energy in the air is composed of both sensible heat energy and latent heat energy. The actual air temperature indicates the level of sensible heat energy. The moisture level in the air indicates the level of latent heat energy. Indirect cooling occurs because sensible heat energy is converted to latent heat energy by evaporating water into the air. The reduced level of sensible heat energy results in a reduced air temperature. However, the latent heat energy increases proportionally causing the moisture level in the air to increase.
If sprinklers are used, direct evaporative cooling is developed when wet pigs are allowed to dry. The water evaporating from the pig's skin cools the pig because energy is removed from the pig to evaporate the water. About 1050 BTU's of energy is absorbed from a pig for every pound of water that evaporates from the pig's skin surface. So, a significant amount of cooling for pigs (relief from heat) can be accomplished if water evaporation can easily occur. In order for evaporation to easily occur, air movement over the pig should be significant, and the building must be well ventilated to remove the moisture that is being evaporated.
Indirect cooling to reduce the incoming air temperature is typically accomplished in swine facilities through the use of evaporative cooling pads. As air is drawn through wet cooling pads, water is evaporated into the air causing the temperature to be reduced while increasing the air moisture level. The increased moisture level in the air will eventually limit the amount of indirect cooling that is possible in any given situation. One measure of the air moisture level is relative humidity. Relative humidity is defined as percent saturation of air at a given air temperature. Or in other words, relative humidity is a measure of how full of moisture the air is at a given air temperature. During the indirect cooling process inside an evaporative cooling pad, the air temperature will reduce until the relative humidity reaches about 85%. Air leaving the cooling pad will cooler than the outside air and have a relative humidity at approximately 85%. Since the moisture level in the air limits indirect cooling, the amount of moisture initially in the outside air will directly impact how much reduction in air temperature might be expected. When the outside air is initially very humid (relative humidity greater than 70%), the reduction in air temperature will be minimal (less than 5 to 10 F) due to indirect cooling. When the outside air is relatively dry (relative humidity less than 55%), a significant reduction in incoming air temperature (at least 15 F) should occur due to indirect cooling. When the outside relative humidity is between 55% and 70%, using indirect cooling is potentially beneficial because a reasonable air temperature reduction should be expected. The actual temperature reduction that can be provided by indirect cooling will depend upon your given situation.
If you are located in a humid region, you may think that indirect cooling is not beneficial to consider; however, it may very well be beneficial. During stable weather patterns, the outside moisture level, measured by the dewpoint temperature, typically remains fairly constant throughout a day, and the air temperature will typically increase during the day and decrease in the evening. By definition, the relative humidity does not and will not remain constant throughout the day. As the air temperature rises during the day, the relative humidity will decrease even though the actual moisture level in the air remains constant. So, during the hottest portion of the day, the outside relative humidity will probably be at a level (often below 60% in the afternoon) that operating an indirect evaporative cooling system (i.e. cooling pads) will be beneficial in reducing heat stress for your pigs. If you are in a region where the relative humidity actually does stay high all the time, use of direct evaporative cooling (sprinkling your pigs and letting them dry) should still be capable of providing heat stress relief as long as the relative humidity stays below 100% near your pigs.
The table gives target air temperatures for pigs at various sizes and ages to help you determine if you are providing adequate cooling. Increased air movement at pig level will increase the pig comfort by making the air temperature feel cooler than the actual air temperature. Providing air movement at pig level with a velocity of 90 ft/min provides about a 10 F decrease in air temperature due to cooling from air movement. Increasing the air movement to about 300 ft/min will provide up to about 15 to 20 F decrease in air temperature due to cooling from air movement. Increased air movement at pig level when using sprinklers will not only improve the pig comfort but also ensure adequate ventilation will occur at pig level to remove the moisture evaporating from the pig's skin surface.
Table 1.
Target Temperature Ranges for Growing and Adult Pigs
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