Evaporative Cooler Operation and Application
The previous two lessons introduced the evaporative cooling process and the construction of a typical direct evaporative cooler. Understanding the cooler itself is only part of understanding the system. Successful evaporative cooling also depends on outdoor air conditions, proper airflow through the building, adequate water supply, water quality, and regular maintenance.
This lesson examines how a direct evaporative cooler operates as part of the complete building. We will look at where evaporative cooling works well, why air must be allowed to leave the building, how changing outdoor conditions affect performance, and what technicians should consider when operating and maintaining these systems.
Learning Objectives
Evaluate Climate Conditions
Explain why outdoor temperature and humidity determine whether direct evaporative cooling is an appropriate cooling method.
Understand Building Airflow
Explain why supply air entering the building requires an adequate relief or exhaust path.
Recognize Water Requirements
Understand how evaporation, mineral concentration, water quality, and water-management practices affect system operation.
Apply the System Correctly
Recognize applications where evaporative cooling can be effective and situations where mechanical refrigeration may be more appropriate.
The Cooler and Building Must Work Together
A direct evaporative cooler continually brings outdoor air into the building. Unlike a conventional central refrigeration system that normally recirculates a substantial amount of indoor air, the evaporative cooler depends on a continuous movement of air through the building.
Hot outdoor air enters the cooler, passes through the wetted evaporative media, and leaves the cooler at a lower dry-bulb temperature and higher moisture content. The blower supplies that air to the occupied space. For additional cooled outdoor air to enter, approximately the same amount of air must be able to leave the building.
The Incoming Air Needs Somewhere to Go
When an evaporative cooler supplies air to a building, the building becomes slightly pressurized. If there is no adequate path for indoor air to leave, pressure increases and the amount of air the cooler can move into the building decreases.
Relief air can be provided through appropriately opened windows or doors, dedicated relief openings, up-ducts, exhaust openings, or another system designed for the particular building. The important principle is that the air being supplied must have a suitable path out of the conditioned space.
Do Not Operate It Like Conventional Refrigerated Air Conditioning
Closing the building as tightly as possible is normally desirable when operating conventional refrigeration-based air conditioning. A direct evaporative cooler is different. It requires a relief-air path so that the continuous supply of cooled outdoor air can move through the building.
Relief Openings Can Help Direct Cooling
The location of relief openings affects the path that cooled air takes through the building. Air tends to move from the evaporative cooler supply toward the available relief openings.
This means that windows or other relief openings can sometimes be used strategically to encourage airflow through occupied rooms. A room with no practical airflow path may receive less cooling even though the evaporative cooler itself is operating correctly.
Outdoor Air Enters the Cooler
The blower draws hot outdoor air through the wetted evaporative media.
Cooled Air Enters the Building
The cooler supplies lower-temperature, higher-moisture air to the occupied space.
Air Moves Through the Building
Positive pressure and the location of relief openings encourage the supply air to move through rooms and occupied areas.
Air Leaves the Building
Indoor air exits through the intended windows, vents, relief openings, or exhaust paths.
Hot and Dry Is the Ideal Combination
Direct evaporative cooling is particularly well suited to climates where summer air is hot and relatively dry. Under those conditions, the air can accept substantial additional water vapor and a significant amount of evaporation can occur.
As outdoor humidity increases, the air has less capacity to accept additional moisture. Less water can evaporate from the media, reducing the amount of temperature reduction that the cooler can produce.
Hot + Dry
Excellent conditions for evaporation. A relatively large temperature reduction may be possible because the air can accept substantial additional moisture.
Warm + Dry
Evaporative cooling can still be useful, and fan-only ventilation may sometimes be adequate when outdoor temperatures are mild enough.
Hot + Humid
Cooling capability is greatly reduced because the outdoor air already contains substantial moisture.
Cool Outdoor Conditions
Mechanical cooling may not be necessary. Depending on the equipment and building, ventilation alone may provide acceptable comfort.
The Practical Limit on Direct Evaporative Cooling
As discussed in Lesson 8, the difference between outdoor dry-bulb temperature and wet-bulb temperature provides a useful indication of evaporative cooling potential. When the air is dry, wet-bulb temperature is substantially lower than dry-bulb temperature. As humidity increases, the two temperatures move closer together.
A direct evaporative cooler cannot simply continue reducing air temperature without limit. The leaving-air temperature approaches the entering-air wet-bulb temperature, but an actual direct evaporative cooler does not normally reach that temperature exactly because the media and air-contact process are not 100 percent effective.
Do Not Expect Refrigeration Supply-Air Temperatures
The temperature leaving an evaporative cooler depends strongly on outdoor dry-bulb temperature, outdoor wet-bulb temperature, media effectiveness, airflow, and water distribution. The system should be evaluated according to evaporative-cooling principles rather than compared directly with the expected temperature drop across a refrigeration evaporator coil.
Performance Can Change During the Same Day
An evaporative cooler that performs very well during a hot, dry afternoon may provide less temperature reduction when outdoor humidity rises. The equipment may be operating normally even though the leaving-air temperature is warmer than it was under drier conditions.
Technicians should therefore consider outdoor dry-bulb and wet-bulb conditions when evaluating evaporative-cooler performance. Looking only at supply-air temperature can lead to an incorrect diagnosis.
Do Not Diagnose the Cooler Without Checking Outdoor Conditions
A smaller temperature reduction does not automatically indicate failed media, poor water circulation, or inadequate airflow. Increased outdoor humidity can reduce the cooling available from a properly operating direct evaporative cooler.
Evaporation Requires a Continuing Water Supply
Water is an operating resource for an evaporative cooler. Some of the water supplied to the media changes into vapor and leaves the cooler with the supply air. That water must be replaced through the makeup-water system.
Additional water may also be intentionally discharged through a bleed-off, purge, drain, or other manufacturer-approved water-management process. This can help control the concentration of dissolved minerals in the recirculating water.
Water Evaporates — Minerals Do Not
When water evaporates from the media, dissolved minerals remain behind. As the same water is repeatedly circulated and additional water evaporates, the concentration of minerals in the reservoir can increase.
Those minerals can form deposits on the evaporative media, reservoir, pump, water-distribution system, and other wetted surfaces. Heavy mineral buildup can restrict water distribution and airflow and reduce cooler performance.
Makeup Water Enters
Fresh water enters the reservoir and contains whatever dissolved minerals are present in the local water supply.
Water Evaporates
Water vapor enters the supply air while dissolved minerals remain in the cooler.
Minerals Concentrate
Repeated evaporation can increase the mineral concentration of the recirculating water.
Deposits Can Form
Minerals can accumulate on the media and other wetted components and interfere with proper operation.
Controlling Mineral Buildup
Some evaporative coolers intentionally remove a portion of the recirculating water so that highly concentrated water is replaced by fresh makeup water. Depending on the equipment, this may be called bleed-off, purge, dump, or another manufacturer-specific term.
The correct water-management method depends on the equipment design, local water quality, operating conditions, and manufacturer requirements. Excessive bleed-off wastes water, while inadequate water management can contribute to mineral accumulation.
Follow the Equipment Manufacturer’s Requirements
Do not assume that every evaporative cooler should have the same bleed rate or water-management arrangement. Determine how the specific equipment is designed to control mineral concentration and adjust or service it according to the manufacturer’s instructions.
Cooling Is Not Always Necessary
Many evaporative coolers provide more than one operating mode. Depending on the equipment, the blower and water pump may be controlled separately, and the blower may have multiple speeds.
Cool Mode
The water pump wets the evaporative media while the blower moves outdoor air through the media and into the building.
Vent or Fan Mode
The blower operates without the water pump. Outdoor air is moved through the building without intentional evaporative cooling.
Low Speed
Where provided, a lower blower speed can reduce airflow and may be appropriate for lower cooling loads or other operating conditions.
High Speed
A higher blower speed moves more outdoor air through the cooler and building when greater airflow is required.
Match the Cooling Method to the Application
Direct evaporative cooling can be an effective choice where the climate is sufficiently dry and the building can accommodate the required outdoor and relief airflow. It is particularly associated with the dry regions of the western United States, although actual suitability depends on local design conditions rather than simply the state or region.
Hot, Dry Climates
Low outdoor humidity provides the evaporative potential needed for useful temperature reduction.
Buildings With Suitable Relief Air
The building must allow the large supply-air volume to move through the occupied spaces and leave through appropriate relief openings.
Applications Benefiting From Outdoor Air
The continuous introduction of outdoor air can be useful in applications where substantial ventilation is desirable and outdoor air quality is acceptable.
Applications Where Added Humidity Is Acceptable
Because direct evaporative cooling adds moisture, the resulting indoor humidity must be suitable for the occupants, building, materials, and processes.
Evaporative Cooling Is Not a Universal Replacement for AC
A direct evaporative cooler should not automatically be selected simply because it uses less complex mechanical equipment. Climate, building construction, water availability, ventilation requirements, indoor humidity requirements, and the needs of the occupants or processes must all be considered.
Humid Climates
High outdoor humidity limits evaporation and therefore limits the available cooling effect.
Humidity-Sensitive Spaces
Buildings or processes requiring low indoor humidity may not be suitable for direct evaporative cooling because the process adds moisture.
Limited Water Supply
Water consumption can be an important consideration in arid regions where evaporative cooling otherwise performs well.
Poor Outdoor Air Quality
Because the system continuously introduces outdoor air, smoke, dust, pollution, or other outdoor contaminants can affect system operation and indoor conditions.
Prepare the Cooler for the Cooling Season
Evaporative coolers are commonly used seasonally. Equipment that has been shut down through the winter should be inspected and prepared before returning it to cooling operation.
Clean the Reservoir
Remove accumulated dirt, debris, and mineral deposits before beginning regular seasonal operation.
Inspect the Media
Check for deterioration, mineral buildup, blockage, damage, and conditions that could prevent uniform wetting or proper airflow.
Check the Pump and Distribution System
Verify that the pump operates correctly and that water reaches the required areas of the evaporative media.
Check the Float and Water Level
Confirm that makeup water enters correctly and that the reservoir maintains the manufacturer’s specified operating level.
Inspect the Blower
Check the motor, blower wheel, belt where used, bearings where applicable, and other air-moving components according to manufacturer instructions.
Check Water Connections
Inspect the water supply and cooler for leakage before leaving the equipment in regular operation.
Protect Equipment During the Off Season
In climates where freezing occurs, evaporative coolers and exposed water piping must be prepared for winter according to the equipment manufacturer’s instructions and local conditions. Water left in exposed components can freeze and damage piping, valves, pumps, reservoirs, and other parts.
Typical seasonal shutdown may include turning off the water supply, draining water from the cooler and exposed piping, cleaning the reservoir, servicing the media, and protecting the equipment from weather. Exact procedures depend on the installation.
Winterization Is Installation Specific
Follow the equipment manufacturer’s instructions and account for the local climate and piping arrangement. Do not assume that simply turning the electrical power off adequately prepares a water-containing evaporative cooler for freezing weather.
Not Every Evaporative System Adds Moisture to the Supply Air
The evaporative coolers studied in these lessons are direct evaporative coolers. In a direct system, supply air contacts the wetted media and gains moisture as it is cooled.
Other evaporative technologies also exist. Indirect evaporative systems use evaporation to cool a secondary air stream or heat exchanger so that the primary supply air can be cooled without directly adding that evaporated water to the supply air. Indirect/direct systems combine both approaches, and hybrid equipment can combine evaporative cooling with mechanical refrigeration.
Keep the Categories Separate
When this course refers to a conventional swamp cooler or direct evaporative cooler, the supply air passes through wetted media and gains moisture. Do not apply that description automatically to indirect or hybrid evaporative equipment.
Sometimes the Cooler Is Not the Problem
A technician should evaluate the complete installation rather than assuming that poor comfort always means the evaporative cooler has failed. Outdoor conditions, relief-air openings, water distribution, airflow, mineral buildup, and building use can all affect performance.
Cooler Runs but Building Is Uncomfortable
Check outdoor dry-bulb and wet-bulb conditions and verify that the climate currently provides enough evaporative cooling potential.
Good Supply Air but Poor Room Cooling
Check whether adequate relief openings are allowing cooled air to move through the affected rooms.
Airflow Seems Low
Check the media, blower, ductwork, air openings, and building relief path for restrictions.
Supply Air Is Too Warm
Check outdoor conditions, media wetting, water distribution, airflow, and the physical condition of the evaporative media.
Excessive Mineral Deposits
Evaluate water quality, water-management operation, media condition, and the manufacturer’s recommended cleaning and bleed or purge procedures.
Water Consumption Seems Excessive
Inspect for leaks, improper water level, excessive bleed or purge flow, and other unnecessary water losses before assuming normal evaporation is responsible.
Avoid These Application Mistakes
“Close Every Window to Keep the Cool Air In”
Incorrect. A direct evaporative cooler requires an appropriate path for air to leave the building as cooled outdoor air enters.
“The Same Supply Temperature Should Occur Every Day”
Incorrect. Evaporative-cooler performance changes with outdoor dry-bulb and wet-bulb conditions.
“If the Air Is Not Cold Enough, the Cooler Is Broken”
Not necessarily. High outdoor humidity can substantially reduce the available evaporative cooling effect.
“Water Quality Does Not Affect Cooling”
Incorrect. Mineral deposits can restrict media airflow and water distribution and can affect pumps, reservoirs, and other wetted components.
“Evaporative Cooling Is Appropriate Anywhere”
Incorrect. Direct evaporative cooling is highly dependent on climate and is best suited to conditions where outdoor air is sufficiently dry.
“Every Evaporative Cooler Works the Same Way”
Incorrect. Direct, indirect, indirect/direct, and hybrid evaporative systems use different arrangements and should not be treated as identical equipment.
Review Questions
1. Why must a building have relief openings when a direct evaporative cooler is operating?
Answer: The cooler continuously supplies outdoor air to the building. Existing indoor air must have an appropriate path to leave so additional cooled supply air can enter and move through the building.
2. What happens if the building does not have adequate relief airflow?
Answer: Building pressure increases and the amount of supply air the evaporative cooler can move into the building can decrease.
3. Why does direct evaporative cooling work best in hot, dry climates?
Answer: Dry air can accept more additional water vapor, allowing more evaporation and a greater reduction in dry-bulb temperature.
4. Why might the same cooler provide less cooling on a humid day?
Answer: Humid outdoor air has less capacity to accept additional moisture, so less water evaporates and the available temperature reduction decreases.
5. What happens to dissolved minerals when water evaporates?
Answer: The minerals remain behind in the water system and can become increasingly concentrated.
6. Why might some evaporative coolers intentionally discharge a portion of their recirculating water?
Answer: Removing concentrated water and replacing it with makeup water can help control mineral concentration and scale formation.
7. What is vent or fan-only operation?
Answer: The blower moves outdoor air through the building without operating the water pump for intentional evaporative cooling.
8. Why should outdoor dry-bulb and wet-bulb conditions be considered when troubleshooting?
Answer: The amount of temperature reduction available from direct evaporative cooling depends strongly on current outdoor air conditions.
9. Why can water availability be a concern even in climates where evaporative cooling works especially well?
Answer: The cooling process consumes water through evaporation, and many hot, dry regions also have limited water resources.
10. Does every evaporative cooling system add moisture directly to the building supply air?
Answer: No. Direct evaporative cooling adds moisture to the supply air, while indirect systems use a separate evaporative process so the primary supply air does not directly contact the water.
Lesson 10 Summary
- A direct evaporative cooler is a flow-through cooling system that continually introduces outdoor air into the building.
- Air supplied by the cooler requires an adequate relief or exhaust path from the building.
- The location of relief openings can influence how cooled air moves through rooms and occupied spaces.
- Direct evaporative cooling performs best when outdoor conditions are hot and dry.
- Increasing outdoor humidity reduces the amount of evaporation and therefore reduces the available cooling effect.
- Outdoor dry-bulb and wet-bulb conditions should be considered when evaluating evaporative-cooler performance.
- Leaving-air temperature can approach the entering-air wet-bulb temperature but is limited by the effectiveness of the actual cooler.
- Evaporative coolers consume water because part of the circulating water leaves the system as water vapor.
- Evaporation leaves dissolved minerals behind, which can cause increasing mineral concentration and deposits.
- Bleed-off, purge, draining, or other manufacturer-approved water-management methods may be used to control mineral accumulation.
- Some evaporative coolers can operate in fan-only or ventilation modes when evaporative cooling is not required.
- Evaporative cooling may not be appropriate in humid climates, humidity-sensitive applications, or locations where adequate water or relief airflow is unavailable.
- Seasonal startup should include inspection of the reservoir, media, pump, distribution system, float valve, blower, and water connections.
- Equipment exposed to freezing conditions must be properly winterized according to the manufacturer and installation requirements.
- Direct, indirect, indirect/direct, and hybrid evaporative cooling systems should be recognized as different equipment arrangements.
- Successful troubleshooting requires evaluating the cooler, outdoor conditions, water system, airflow, and building relief-air path together.