By Michael Czarick
& Dr. Brian Fairchild
Special to Poultry Times
ATHENS, Ga. — Knowing how much water an evaporative cooling system will use during hot weather can prove to be a very useful piece of information. For instance, to properly size well pumps, pressure tanks and water distribution pipes, designers must know the peak demand of pad systems on the farm because they can be responsible for 90 percent of the water usage on a hot summer afternoon. Furthermore, if a producer wants to install a water treatment system on their farm they must know what the peak water usage is to insure the system is properly sized. Though knowing peak demand is important, knowing how much water the pads will use on a typical afternoon can also prove useful, especially if the farm is being supplied by municipal water system where the producer is being charged for every gallon of water used.
The amount of water the evaporative cooling pads will use is primarily dependent upon three factors: outside temperature, outside humidity and the amount of air being drawn through the evaporative cooling pads. The amount of evaporative cooling pad on a house, within reason, has a minimal effect on water usage. For instance, increasing pad area by 20 percent over what is typically recommended (1 ft2 per 350 cfm of tunnel fan capacity) will increase water usage by less than 5 percent. While in contrast, increasing the number of tunnel fans operating on a hot summer day 20 percent will increase pad water usage by 20 percent.
The amount of water a pad system will use over the course of day will change as both outside conditions and the number of exhaust fans operating change. As outside temperature increases, relative humidity will decrease and the amount of water evaporating off the pads will increase. In addition, the number of fans operating will tend to increase as outside temperatures increase, which further increases the amount of water evaporating from a pad system. For example, the evaporative cooling pads might only use 1.5 gals/min (per 100,000 cfm of tunnel fan capacity) in the morning when it is 85 degrees F and 65 percent humidity, but in the afternoon this could increase to 2.6 gals/min (per 100,000 cfm of tunnel fan capacity) when the outside temperature increases to 95 degrees F and the humidity falls to 50 percent. If in the same house the number of tunnel fans operating increases by 30 percent from morning to afternoon the water evaporating from a pad system can increase from 1.5 gals/min to 3.4 gals/min (per 100,000 cfm of tunnel fan capacity) from morning to afternoon. But, during very hot dry weather when temperatures climb above 100 degrees F, and humidity falls well below 30 percent Rh, pad water usage can increase from 1.5 gals/min to 7 gals/min (per 100,000 cfm of tunnel fan capacity) from morning to afternoon.
On a typical summer day the fact is outside conditions will vary from day to day, week to week and year to year and as a result so will the cooling produced and water consumed by a house’s evaporative cooling system. A chart was created using ten years (2007-2017) of hourly weather data for Athens, Georgia. The chart illustrates the range of relative humidifies that existed when the outside air temperature was 80 degrees F and higher. For instance, over the 10-year period when it was 90 degrees F outside relative humidity ranged from a low 20 percent to a high of 65 percent. The variation in relative humidity would result in a variation in the cooling produced by a pad system at 80 degrees F ranging of a low of 8 degrees F when the relative humidity is 65 percent and a maximum of 20 degrees F when the relative humidity was 20 percent. At higher temperatures, i.e. 100 degrees F, the outside relative humidity tended to be lower as was the variation in humidity. The combination of the higher temperatures and generally lower humidifies resulted in an increase in cooling ranging between 16 degrees F and 21 degrees. Large variations in the amount of cooling produced by an evaporative cooling pad system would of course result in large variations in pad water usage.
Another chart illustrates the expected water usage of a pad system, per 100,000 cfm of operating tunnel fan capacity, based on the same historical weather data (Athens, Georgia 2007-2017). From this chart we can see that at an outside air temperature of 90 degrees F pad water usage varies between 1.5 to 4.5 gals/min. If outside temperature increases to 100 degrees F we can expect water usage to vary between 3.5 to 5.5 gals/min. For a tunnel-ventilated house with 250,000 cfm of tunnel fan capacity this means that the pads typically use between 8.75 (3.5 gals/min x 2.5) and 13.75 gals/min (5.5 gals/min x 2.5) when it is 100 degrees F outside and 3.75 and 11.25 gals/min when it is 90 degrees F outside.
From the second chart, it becomes clear that when outside air temperatures are 80 degrees F or higher the potential water usage of a pad system can vary dramatically from a low of approximately 0.5 gals/min to high of 7.5 gals/min (per 100,000 cfm of tunnel fan capacity). But what isn’t clear is what is typical pad water usage. Yes, the pads can use between 0.5 and 7.5 gals/min but how common are these water usage rates and what would be an average/typical water evaporation rate? A histogram of pad water usage (gals/min per 100,000 cfm of tunnel fan capacity) illustrates that the theoretical number of hours each summer a producer would expect various water evaporation rates from their pad systems. This assumes the pads are operating all summer long whenever the outside temperature is above 80 degrees F, which of course would be the worst case scenario. In most instances, the birds are not present in a house the entire summer or the birds are too young to require evaporative cooling and/or the evaporative cooling system may not be operated until the outside/house temperature is much higher than 80 degrees F. The most important aspect is not precise number of hours indicated but rather the relative frequency of the various water evaporation rates. For instance, though pad water can vary between 0.5 and 7.5 gals/min (per 100,000 cfm of tunnel fan capacity) the fact is the vast majority of the time it ranges between 1.5 and 4 gals/min with an average of approximately 2.5 gals/min. Furthermore, it illustrates how rare the peak water usage rates are. Yes, the peak water usage would be approximately 7 gals/min, but it only occurs a couple of hours each year. The more typical maximum pad system water usage rate would be closer to 4 to 5 gals/min at around 50 hours per year.
How much does an evaporative cooling pad water usage profile vary from location to location? Not as much as you may think. For instance, there are minimal differences in the both the peak and the average pad water usage for various poultry growing areas around Georgia. The most significant difference between the profiles is, as you might expect, that the total number of hours of operation is higher in the southern areas of the state where temperatures tend to be higher. Even when you compare most major poultry growing areas across the U.S. pad water usage profiles tend to be similar in shape. The most notable differences in water usage profiles are for the far western and northern states. In California water usage will tend to be dramatically higher than most poultry growing areas due to very hot and dry summertime conditions. In Minnesota, though the shape of the profile is similar to other regions, evaporative cooling pad water usage will tend to be very low because of the relatively low summertime temperatures.
Michael Czarick is an Extension engineer and Dr. Brian Fairchild is an Extension poultry scientist, both with the University of Georgia College of Agricultural & Environmental Sciences Cooperative Extension in Athens, Ga. More information can be obtained at www.poultryventilation.com
