Chapter 9 - Evaporative Cooling Systems

When used in conjunction with air movement, evaporative cooling can be effective at alleviating or preventing poultry heat stress during hot weather. Evaporative cooling uses heat from the air to vaporize water. It results in a reduced air temperature and increased humidity. During the hottest part of the day, outdoor relative humidity drops and the potential for evaporative water increases. Thus evaporative cooling can be useful even in very humid climates.

To evaporate water, heat (energy) is required. In fact, to evaporate a gallon of water requires almost 8,500 BTUs of heat. The heat comes from whatever the water is in contact with as it evaporates. This could be a hot sidewalk, your body, a tree, or the air itself. As the heat is removed from the object, the temperature of that object is decreased. If you put you hand in a bucket of warm water and pull it out, your hand feels a slight chill as the water evaporates.

It is important to realize that the temperature of the water itself does not have a great effect on the cooling produced through its evaporation. If you were to place a gallon of 50°F water on a warm sidewalk (90°F) it would produce 9,000 BTUs of cooling. A gallon of 90°F water would produce 8,700 BTUs of cooling, only a 3% difference. Energy and equipment should not be used to either increase or decrease the water temperature prior to cooling.

A significant amount of cooling can take place by simply getting an animal wet and providing air flow over the animal. The water comes in contact with an animal’s skin, and as the water evaporates, heat is removed directly from the animal. This is not typically done with chickens for two reasons. First, the feathers insulate chickens from much of the cooling effect of the evaporative of water. Second, in order to get the chickens wet, you also have to wet the house, including litter and feed.

The goal in most poultry houses is to evaporate water directly into the air. This removes heat from the air, decreasing air temperature. Two types of evaporative cooling systems are commonly used in poultry housing:

• Pad and fan, also known as evaporative cooling pad
• Fogging or misting system

The difference between these two systems lies in the ways water and air interact. Pad and fan systems use exhaust fans to draw air through a wetted porous material at the air inlet. As air moves over the wet pad, water is evaporated off the pad, removing heat from the air.

Fogging or misting systems spray fine droplets of water directly into the indoor air. With a fogging system, water is sprayed through nozzles into the air so that very small water droplets float around the house, removing heat from the air as they evaporate. ‘Fogging-pad’ systems are evaporative cooling pads that are kept wet by fogging or misting nozzles, rather than by the more common water recirculation systems. Pad and fan systems are used in mechanically ventilated houses, while fogging or misting systems may be used in either naturally or mechanically ventilated facilities.

The key to getting the most out of any evaporative cooling system is to maximize the amount of air which comes into contact with the moisture added to the house. Evaporative cooling efficiency is the percentage of the wet-bulb depression (or evaporative cooling potential) by which an evaporative cooling system lowers the dry bulb temperature.

In general, the choice of system type should be based on:

• Benefits of cooling broilers
• Capital expense
• Operating costs
• Quality of dealer service and support
• Ability to maintain the system

Fan and Pad system

Evaporative cooling pads are currently the most effective and efficient systems for cooling broiler houses. These systems drip water downward through a porous pad while air flows across the pad into the broiler house. New pads, or those in good condition, generally have high efficiencies (70 – 90%); and when sized, installed, and maintained properly, can provide more cooling effect than fogging without the risk of wetting the broiler house interior. Recirculating-type pads use a plumbing system including a sump and pump to recirculate the water through the pads. Fogging pads are a variation in which the pad is wetted by fogging nozzle spray instead of a recirculating water delivery system.

Pads are made of corrugated cellulose, shredded fibers (e.g., ‘hog-hair’ or ‘horse-hair’), and other materials. They provide cooling by having moisture in the pad evaporate as air flows through the pad into the house. Pad systems typically produce the most evaporative cooling because they are designed to provide a maximum interaction possible between air and the water. A typical 100’ x 6’ x 4” pad has more than 20,000 square feed of surface area. This allows the air entering the house to become totally saturated with water, resulting in the maximum cooling effect.

With pad systems, it is especially important that the house be airtight without cracks, holes, or other ‘unplanned’ openings. All ventilation air should pass through the pads to achieve the cooling effect. If the house has ‘unplanned’ openings, only limited cooling is achieved. Pad systems typically cause a rather large static pressure drop of 0.05 – 0.10 inches of water. With such resistance to airflow, hot outside air will preferentially flow into the house through any other less restrictive openings such as cracks or inlets.

Pad and fan evaporative cooling systems can be very effective and are widely used in houses with more valuable chickens, such as breeders and layers. Since a static pressure drop across the pads is required, pad and fan systems cannot be used with natural wind (open curtain) ventilation and are mostly used in tunnel-ventilated poultry houses. Since pad and fan systems tend to be more expensive than fogging systems, meat bird producers have been slower to adopt them. Fogging-pad systems represent an intermediate option.

Pad selection, sizing and placement

Selection of evaporative cooling pads should involve consideration of product effectiveness, useful life, maintenance requirements, and dealer support and service, in addition to initial cost. The least expensive pad materials may not be very cost-effective, since they are generally less effective at cooling. The evaporative cooling efficiency is a good indicator of pad performance, since it represents the fraction of the potential cooling effect which the pad will provide. Evaporative cooling efficiencies of 80-90% are typical of well-designed cellulose pad systems in good condition.

Sizing of pad systems is based on preventing excessively high air velocities through the pad. High velocities can cause high static pressure drops and blow water off the pads both of which reduce system effectiveness.

Placement of the pads depends on the ventilation system design, since the pads serve as the hot weather air inlets. The tunnel ventilation inlet area should ideally be placed in the end wall farthest from the exhaust fans to avoid a dead-air space near this end wall. However, the large amount of pad area required and the large endwall access doors typically necessitates placing most, if not all pad material on the sidewalls. Place one-half the pad area in each sidewall and directly across from each other.

Pad system costs are affected by the length of pad since distribution pipe, header, and gutter increase in size and installation cost with pad length. However, the height of each pad section should be no more than 6-ft to allow uniform wetting. Since the framing and enclosure to hold the pads will obstruct a small amount of pad at the top and bottom, subtracting that area allows for a more precise calculation.

Pump and sump sizing

In order to size the pump, the pumping head (or pressure against which the pump ‘pushes’) and the water flow rate, must be known. Pump manufacturers typically specify the water flow in gal/min (gpm) for several values of total head in feet (for example, one particular ½ hp centrifugal pump delivers 47, 37, and 25 gal/min at total heads of 10, 20, and 30 feet, respectively). The pump head includes the change in elevation of the water plus the loss of head that occurs at elbows, couplings, tees, valves, and filters, as well as losses due to friction along lengths of pipe. In addition, there must be some pressure head at the top of the distribution pipe to squirt water out of the holes properly and to overcome friction down the distribution pipe. Typically, 1-2 pounds per square inch (psi), or about 4 feet of head is provided at the pad header. Dirty filters in the pipes can cause severe head losses, so good maintenance is necessary.

Water flow rate through the pads should be sufficient to flush away dirt, salts, and minerals that could otherwise foul the pads. This flow rate will be several times the actual water evaporation rate from the pads. Specific recommendations of water flow rate and sump capacities provided by pad manufacturers should be followed. When pads are installed on sidewalls, a separate sump and pump should be used for each pad to avoid excessive pumping distances.

Larger pipes and fittings are needed as the required flow rate (gpm) increases. For riser and feeder lines longer than 50 feet, pipe and fitting sizes should be increased to avoid excessive head loss or a larger pump will be needed. The same size pipe should be used from the outlet of the pump to the inlet of the distribution pipe.

Distribution pipe

For PVC distribution pipe, holes should be approximately 1/8 inch in diameter and free of burrs. The holes should be spaced evenly, 3-4 inches apart along the distribution pipe, and should face upward. Holes in the bottom of the distribution pipe will plug with dirt. The water should squirt out the holes onto the header pipe and drip down into the pad. The pad cover should have a deflector which spreads the jets of water out and back down over the top of the pad. With some older systems, water squirting upward to rectangular covers may leak out through the joints and screw holes and off the pad face. This may require caulking the joints or installing an improved deflector cover over the pads.

To ensure uniformity in pad wetting along its length, the water should squirt out uniformly along the distribution pipe. Remove the pipe cover to observe the distance that the water will squirt above the distribution pipe. Removing the cover to measure this distance can help troubleshoot water distribution problems. Holes less than 1/8 inch in diameter require more pumping head, are more likely to clog, and will deliver less water. On the other hand, holes larger than 1/8 inch in diameter are not recommended, since they can flood the pads and cause uneven water distribution.

To assure uniformity of pad wetting along the pipe for long pads, it is often desirable to split the water distribution with a tee at the center of the pad and pump water in both directions.

Since inadequate water distribution can result in ineffective cooling and shorten pad life, and excessive pumping head increases pump cost, methods of reducing pumping head, such as the following, should be considered:

• Avoid using unnecessary fittings, elbows, valves, etc., since they waste pumping head.
• Consider removing any in-line filter. If it will not be cleaned weekly it will greatly reduce flow.
• Filters with clear covers may clog with algae. Exclude light by covering with black paint or dark plastic bags.
• Make sure the distribution pipe is clean, because sediment and debris in the pipe will clog the holes. Clean by removing the end cap and flushing the pipe while the pump is running.

Maintenance of pad systems

In order to get efficient performance from a pad system, provide proper maintenance and operation. Maintaining a uniformly wet pad, cleaning scale and dirt from the pad and plumping systems, controlling algae, allowing the pads to dry out overnight, and fixing leaks will greatly extend the life of a system.

In order to avoid salt and dirt buildup in the water, it is important to bleed-off about 5-10% of the water flow continuously, or to flush the entire system periodically (e.g., weekly). Bleed-off methods include draining from the water supply line, return line, or sump tank or using a dump system to drain the entire system. Bleeding from the gutter, rather than the pump, reduces waste of clean water. Since the purpose of bleeding is to prevent salt and dirt buildup, the bleed-off drain line should not be routed to the pad gutter.

Proper water distribution is the most important factor in prolonged pad life. Necessary steps for providing efficient water distribution include ensuring adequate pressure in the distribution pipe, adjusting water flow to eliminate dry streaks, keeping the distributor pipe level, cleaning the filter, and using an adequate pump. Hard water may need to be adjusted to a pH of 6 – 8.

The following procedure is recommended for cleaning and flushing the pads on a quarterly basis:

• Completely empty the sump of water and silt
• Refill the sump with clean water
• Turn off the ventilation fans, if possible
• Manually turn pumps on to run fresh water over the pads for thirty minutes, using as much water as possible
• Open the ends of the water distribution pipes to flush out debris; replace the covers
• Remove the plug and drain the system for silt collection
• Gently hose deposits from the face of the pads (do not use a power washer)
• Completely empty the sump to remove the algae and dirt rinsed off the pads
• Disinfect the system with the proper amount of approved chemical
• Check to make sure the bleed-off is working properly
• Refill with clean water

Algae growth within the pads can clog pad pores reducing cooling efficiency. Algae require light, moisture, and nutrients to survive, and all of these can be plentiful in pad systems. In order to control algae, the following are recommended:

• Shade the pads and the pump
• Dry the pads daily (usually overnight)
• Avoid nutrient contamination (either from nutrients blowing into an uncovered sump or algaecides which degrade into nutrients)
• Drain and disinfect the sump regularly

In some tests of algaecides, those using quaternary ammonia (such as ammonium chloride) have been more effective than oxidizing-type biocides such as sodium hypochloride and calcium hypochlorite. Use the recommended concentration specified by the chemical manufacturer. Since different algaecides can have vastly different recommended concentrations, it is extremely important that manufacturers’ specifications be followed. Overdosing is easy to do and can damage the pads, pumps, and gutters.

System control

Evaporative cooling is part of the overall environmental control strategy and should be integrated into other ventilation components by using a thermostat or computer-control system. Evaporative cooling is not used when air temperatures are cool. Cooling is not normally needed between the hours of 10 PM, and 9 AM. Note that a high thermostat/controller setting of about 85°F can achieve this, as nighttime temperatures are rarely above 80-82°F. If the cooling system is to turn on at a lower temperature, a timer will be needed so that they cycle off at night. Timers should not be used to turn the evaporative cooling system on and off during the day, however.

Any practice that results in dry areas on the pads during operation will reduce the cooling effect. The pads must remain wet over their entire area when cooling is desired. In addition to reducing cooling, cycling water flow through pads during the day, with timers or other devices, will reduce pad life. Daytime cycling of water flow through pads causes increased dirt and mineral buildup because it disrupts the continuous flushing of dirt and minerals out of the pads.

Variations – Pad and plenum arrangement

Although most pad and fan systems for poultry housing are used in tunnel ventilation, a ‘pad and plenum’ arrangement may be useful for houses using conventional ventilation. The evaporative pad is placed along one or both sidewalls and the cooled air first enters a ‘plenum,’ or duct, and then enters the room through a slotted inlet. A pad and plenum system might be desirable when the building is too short for tunnel ventilation, (or too wide as is the case in some experimental facilities). Since evaporative cooling system costs are greatly affected by the length of the pads, this type of system is not generally economical for long buildings.