Evaporative Cooling Introduction

The Science of Evaporative Cooling

Evaporative cooling is based on a physical phenomenon in which evaporation of a liquid (usually water) into surrounding air cools an object or a liquid in contact with it.  As the liquid turns to a gas, the phase change absorbs heat.  Technically, this is called the “latent heat of evaporation”.   Water is an excellent coolant because it is plentiful, non-toxic, and evaporates easily in most climates.  Six gallons (22.7 L) of water evaporating has the same cooling effect as a typical (3.5 ton-hour) home central air-conditioner.

The most common example we all experience is perspiration, or sweat, which the body secretes in order to cool itself.   The thin layer of water evaporates; the water vapor absorbs heat as it evaporates off our skin.  The amount of heat transfer depends on the evaporation rate, which in turn depends on the humidity of the air and its temperature.  When the air humidity is very high, sweat evaporates slowly.  This is why you feel hotter in humid climates compared to dry climates of the same temperature: the sweat cannot evaporate as fast to cool your body in a humid climate.

Evaporative cooling systems use the same laws of physics to cool machinery and buildings.  Evaporative cooling is a very common form of cooling buildings because it is relatively inexpensive and requires less energy than many other forms of cooling. Unfortunately, evaporative cooling requires an abundant water source, and is most effective in climates with low humidity.  In arid climates, homes and small business use direct evaporative cooling systems; often referred to as “swamp coolers” or “desert coolers”.  In almost all climates, large buildings often use indirect evaporative cooling in cooling towers for the chillers in the HVAC system.  Manufacturing and industry often use evaporative cooling technology to remove excess heat from machines, compressors and other equipment.  In dry climates, the installation and operating cost of an evaporative cooler can be much lower than air conditioning, often by 80% or more.

In moderate humidity locations there are many cost-effective uses for evaporative cooling, in addition to their widespread use in dry climates. For example, commercial kitchens, laundries, dry cleaners, greenhouses, loading docks, warehouses, factories, construction sites, athletic events, workshops, garages, kennels and confinement farming (poultry ranches, hog, and dairy) often employ evaporative cooling.  

Direct Evaporative Coolers

Also known as swamp coolers, sump coolers or desert coolers, Evaporative Coolers (EC) are air cooling devices which use simple evaporation of water into the air.   These are most commonly found in homes and small businesses located in dry hot climates.  They differ from refrigeration or absorption air conditioning, which use the vapor-compression or absorption refrigeration cycles (using Freon or other refrigerants in closed systems) to cool air re-circulated in the home.  ECs draw outside air across a wet pad or mesh into the home (while pushing an equal amount of air out of the home).  As air passes through the wet mesh, the water in the mesh evaporates which cools the air about 20F (-6.7).   Where the dry outside air is 85F (29.4 C), the EC will provide fresh cool 65F (18.3) air into the home.  This process also humidifies the air before it enters the home, and can sometimes lead to too much humidity, creating a “swamp” feel and aroma in the home if improperly used.

The ECs traditionally use potable water sources from local water utilities.  The water use varies by size and type of equipment, quality of supply water, and the local climate.  A typical EC will consume 20 to 100  gallons (75.7 L to 378.4 L)of water per day, if it is maintained properly.  An EC that is in disrepair, or improperly maintained might waste more than 1,000 gallons (3.8 m3) per day.

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The EC contains a small water reservoir where water is pumped across the mesh pad.  As the water evaporates, the minerals collect in the reservoir.  The EC will dump this water and replace it with new water from the water supply, referred to as “blow-down”.  The frequency of blow-down is supposed to be determined by the hours of use of the EC. And the level of minerals in the fresh water supply.  Unfortunately, many EC are set for the maximum blow-down or run off, which wastes more than 3 times the water the EC actually needs to operate effectively.

The EC contains an array of refill valves, pumps, tubes, pipes, and sensors.  All of these mechanical devices can fail; resulting in excessive water use.  Though the equipment is relatively simple, it requires regular maintenance.  Malfunctions can cause excessive water waste and cause expensive water damage to the home or building it is located.  The EC should be inspected by a trained technician at least once per year.

Recommendation for Water Efficiency

The water efficiency of direct evaporative coolers varies greatly by make, model, and installation.  A perfectly efficient EC would use only 1.6 gallons (6.1 L) of water per ton-hour of cooling.  (Most single family homes require 3.5 tons of cooling per hour).  Current makes and models range in water efficiency from 3 to 15 gallons (11.3 L to 56.8 L) per ton-hour.   It is reasonable to set “5 gallons (18.9 L) per ton-hour” as a minimum water efficiency standard for this equipment.  Unfortunately, there is not yet standardized testing protocol for water efficiency of direct evaporative coolers.

Evaporative Cooled Air-Conditioners

Evaporative cooled A/C s have recently been introduced in the residential market.  They are similar to typical central A/Cs, except for the condenser coils (outside of the house) use water to enhance cooling, instead of air only.  The evaporation of the water on the coils greatly increases the energy efficiency of the system compared to an air only method.  This technology uses engineering principles similar to large cooling towers, and has similar water waste potential as cooling towers.   Essentially, the energy efficiency trades water use for reduced energy use.   Field studies have not yet verified the amount of energy saved per gallon of water consumed.

There is not enough experience with the systems to verify long-term water use.  The manufacturers’ literature suggests 30 gallons (113.5 L) per day (GPD) of water use is likely.  The technical information suggests peak day use would probably be approximately 50 (189.2 L) to 60 (227.1 L) GPD - in a perfect installation.  In some areas, the water use might be as high as 100 (378.4 L) GPD on peak use days. In most climates, the peak day use would coincide with the peak water use days (July and August) when water suppliers are often operating at full capacity and often pleading for customers to curtail usage.  This 50 (189.2 L) to 100 (378.4 L) GPD usage represents an increase demand of approximately 25% for the typical home.   If the equipment is not properly installed and maintained, the water use can exceed 500 (1.9 m3) GPD.

The value of the energy savings versus the increased water use is determined by the local water and energy resources.   There is much debate about the merits of this new technology, and more research is needed to determine the proper application for evaporative cooled A/Cs.