Evaporative (evaporative) coolers are cooling systems that use only water and a blower (fan) to circulate air. When warm, dry (unsaturated) air is drawn from a water-soaked pad, the water evaporates and is sucked into the air as water vapour. In the process, the air is cooled and the humidity increased.
Evaporative cooling technology is an energy efficient alternative to compressor-based cooling. In dry and arid regions, evaporative cooling can meet most or all building cooling loads using a quarter of the energy of conventional equipment. It can also be cost-effectively implemented when integrated with traditional chiller systems, which can greatly improve a facility’s load profile.
Evaporative cooling requires a large water supply and is only effective when the relative humidity is low, limiting its efficient use to dry climates.
Part 1. Principles of Evaporative Cooling
The first basis for understanding any air conditioning, dehumidification and evaporative cooling is psychrometry. Psychrometry consists of interactions between heat, humidity and air. It is basically the study of air-water mixtures and is a basis for understanding how to change air from one state to another. As the air temperature increases, the moisture holding capacity also increases; and the warmer air becomes less dense. This makes humidity a very influential factor for both comfort and heat gain in calculations.
Knowledge of systems consisting of dry air and water vapor is essential for the design and analysis of air conditioners, cooling towers and industrial processes that require close control of the vapor content in the air. The interactions of air humidity and heat are quite complex; Fortunately, these interactions can be combined into a single diagram.
Cooling Processes in the Psychrometric Chart
The cooling processes in the Psychrometric Chart are shown below:
Sensible Cooling – In the sensible cooling process, the temperature of the air changes from ‘A’ to ‘C’ while maintaining a constant humidity. The temperature is lowered by [T(A) – T(C)] and the wet bulb temperature is also lowered. Since there is no addition or loss of moisture, the amount of moisture remains the same.
Evaporative Cooling – In evaporative cooling, both the temperature and humidity of the air change along the lines of the constant wet bulb temperature (shown as line AB). There is no change in heat content and energy is only converted from sensible to latent energy.
In the evaporative cooling process, changes occur in the dry bulb temperature, specific volume, relative humidity, humidity rate, dew point temperature and vapor pressure of the moist air. There is no change in wet bulb temperature and enthalpy. Evaporative cooling is a constant enthalpy process (technically called the adiabatic process).
Evaporative Cooling Key Terms
1) Wet Bulb Depression: It is the difference between dry bulb and wet bulb temperatures.
2) Temperature Drop: The difference between the inlet dry bulb temperature and the outlet dry bulb temperature.
Example: If the dry bulb temperature entering an evaporative cooler is 38°C and the leaving dry bulb temperature is 23°F, the range is 38–23= 15°C.
3) Saturation or Cooling Efficiency (SE or CE): The difference between the inlet and outlet dry bulb temperatures over the wet bulb depression.
Example: If the Wet Bulb Depression is 17 ºC and the actual measured temperature drop in the cooling environment is 15 ºC (as in the example above), the cooling efficiency is 88%. (15/17 = .88).
4) Evaporative Cooling Performance Factors: Evaporative cooler performance is directly related to its ability to evaporate (cool) water at a given relative humidity. The drier the air, the better the performance. Humidity may be high in the morning, but as the day progresses and the temperature rises, the relative humidity will naturally decrease.
The hotter the day, the drier the air and the more cooling occurs through the evaporation of water. The temperature of the water does not have a large effect on the cooling produced by evaporation. For example, a gallon of water at 10°C will produce 9,000 BTUs of cooling, while a gallon of 32°CF water will produce 8,700 BTUs of cooling.
5) Evaporation Rate: Evaporation rate is a measure of the moisture absorption capacity of the air. The amount of absorption is largely dependent on four factors;
- Humidity of the air: Dry air has a greater ability to retain moisture. If the air already has a high moisture content, its moisture absorption capacity will be low.
- Temperature of the air: The warmer the air, the more water can evaporate.
- Flow rate of air: Molecules in motion promote evaporation. The stronger the air flow, the greater the evaporation power of the air.
- Cooling medium saturation efficiency: Cooling pads must be of high thickness (12 inches) to provide large contact area or low bypass. For practical purposes, this rate is measured in gallons of water per hour (or minute).
5) Air Changes Per Hour (ACH): Air change is the number of times the air in a space is changed over a specified period of time, such as an hour or a minute. It is usually expressed in changes in the hour or minute. A good rule of thumb is to do 1 air change every 3 minutes in the north, 1 air change every 2 minutes in the middle, and 1 air change every 1-2 minutes in the southern states.
6) British Thermal Unit (BTU): A British thermal unit or BTU is a unit used to measure heat. The heat of vaporization (evaporation) of water is 1043 BTU/lb and evaporation of 1 gallon of water requires approximately 8700 BTUs of heat (8700 conversion factor based on 8.34 lb. water/gallon and 1043 BTU/lb). 1000 BTU = 0.29 kW
7) CFM: It is a measure of the air flow per minute. It is often simply referred to as CFM. This is a necessary ingredient in any formula that includes evaporative cooling. The air volume can be calculated using the sensible heat load equation or the air exchange method. Both the sensible heat and the air exchange method are discussed in the next sections. 100 CFM = 170 m3/h
8) Surface Velocity: Expressed as FPM, it is the ratio of airflow (in CFM) divided by the face area of the cooling medium.
Evaporative Cooling Example
- Dry Bulb Temperature: 88°F
- Wet Bulb Temperature: 68 °F
- Relative Humidity: 36%
The maximum reduction possible with an evaporative cooling system is the difference between the dry bulb and wet bulb temperature. Since no equipment is perfect, there will be certain losses in the evaporative cooler; these losses will be due to the efficiency of the cooling medium. If we say that the evaporative cooler is 90% efficient, the process will take place along a constant wet bulb temperature line for 90% of the distance from point “A” to the saturation line (100% relative humidity).
Reachable temperature drop = (dry bulb vs wet bulb) x (efficiency of the media) Example: (88° to 68°) x .9 = 18° F
Reachable temperature = dry bulb – achievable temperature drop. Example: 88° – 18° = 70°F
Point B represents the conditions of the air leaving the evaporative cooler.
Dry Bulb Temperature: 70°F
Wet Bulb Temperature: 68 °F
Relative Humidity: 92%
The conditions obtained after evaporative cooling can be expressed as “low sensible heat energy” and “high latent heat energy”.
Final Conditions of Air in the Refrigerated Area
The final conditions of the air in the room cooled by the evaporative cooler will depend on the heat load in the space. If the area is not affected by any other heat conditions, the air condition in the room will be very close to the conditions of the air leaving the evaporative cooler. If there are additional heat loads in the room (eg many people or machines), then the temperature of the air leaving the evaporative cooler will tend to rise by a few degrees. This situation will be represented by a horizontal movement from condition B to condition C on the psychrometric chart.
- Evaporative cooling is represented in the “Psychrometric Chart” by constant wet bulb temperature lines. With direct evaporative cooling, the dry bulb temperature is lowered while the wet bulb temperature remains constant.
- The higher the difference between the dry bulb and wet bulb temperature, the more effective the evaporative cooling will be. Evaporative cooling efficiency will decrease as the wet bulb temperature approaches the dry bulb temperature.
- The wet bulb temperature is the lowest air temperature that can be achieved at 100% cooling efficiency. This corresponds to point D in the psychrometric chart. At this point the air is completely saturated – it has reached its dew point – and cannot hold any additional moisture. At this point, KTS=YTS.
Part 2. Why Evaporative Cooling
1) Evaporative coolers do not use compressors, condensers, chiller coils, cooling towers or heavily insulated pipes. Therefore, the cost to purchase and operate is a fraction of conventional air conditioning and mechanical refrigeration systems.
2) The initial cost is less than 1/2 of the cost of the refrigerated air conditioner and the operating costs are less than 1/3 of the running cost of the refrigerated air conditioner. Maintenance costs are minimal, requiring simpler procedures and less skilled maintenance personnel.
There is a 1500 square meter house in Ankara with a dry bulb temperature of 42 ºC and a wet bulb temperature of 22 ºC.
- Air Conditioning – Assuming that the air conditioner provides 500 square feet of cooling per tonne, 3 tons of air conditioner will be required to cool a 1500 square feet house. Since an air conditioner will likely operate in this climate most of the time, the electricity usage will be around 3.6 kWh per hour. (1 ton = 3.5 kW)
- Evaporative Cooler – The only power consuming components of the evaporative cooler are the fans and small water pumps. When using an evaporative cooler with a minimum 5000 CFM and ¾ horsepower motor, the total power consumption (motor and pump combined) will be approximately 0.991kW per hour.
Result – If the air conditioner and the evaporative cooler run for a comparable period of time, it means that the evaporative cooler will use 72% ½ less power than an air conditioner to cool the same space in the same environment.
Evaporative air cooling creates cooler temperatures in several ways:
1) Lower the effective temperature (the temperature you feel) by at least another 4º to 6º. In some cases, the temperature will drop further depending on the relative humidity. The rapid movement of cold air increases evaporation on the skin surface, causing body heat loss. ASHRAE Handbook, 1995, chapter 47, notes on Evaporative Air Cooling “…reduction in dry bulb temperature due to evaporation of water always results in a lower effective temperature regardless of the relative humidity level” and “. . evaporative cooling can provide: almost relief cooling of factories regardless of geographic location.”
2) Reduces radiated heat – The constant flow of cold air absorbs heat from all exposed surfaces, resulting in a reduction of radiated heat to the human body.
In many industries, it is normal to experience hot weather, increased temperature-related illnesses, low productivity, and increased absenteeism among workers. While most of these industries can afford evaporative cooling, they cannot afford the enormous costs of conventional mechanical cooling or air conditioning. Quoting again from the ASHRAE Handbook, 1995, Chapter 47, Evaporative Air Cooling, “evaporative cooling can alleviate this heat problem and contribute to worker productivity along with improved morale.
Health and Environmental Benefits
Evaporative cooling is healthy and comfortable because:
- Brings 100% fresh outdoor air that is cooled and cleaned through filter pads
- Comfort is improved by air movement.
- The continuous air movement of the evaporative cooler removes dust, pollen, smoke, odor and pollution, pushing hot air out and replacing it with cool fresh air.
- It helps maintain natural humidity levels that benefit both people and furniture and cut static electricity.
- Unlike air conditioning, evaporative cooling does not require an airtight structure to operate at maximum efficiency. In fact, building occupants can open doors and windows.
- Evaporative cooling is an environmentally friendly alternative to air conditioning as it has no CFCs or HCFCs.
In summary, evaporative coolers have a low initial cost, use much less electricity than conventional air conditioners, and do not use refrigerants such as chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs) that can damage the ozone layer. It is economical, effective and provides a much needed alternative to traditional mechanical cooling.
Limitations and Disadvantages
Evaporative coolers have some limitations and disadvantages:
- Evaporative coolers are not effective in humid areas.
- High humidity conditions reduce the cooling capacity of the evaporative cooler.
- The air supplied by the evaporative cooler is approximately 100% humid. Very humid air prevents sweaty or wet skin from evaporating and cooling. High humidity in the air accelerates corrosion. This can significantly shorten the life of electronic equipment. High humidity in the air can cause condensation (which can be extremely dangerous if it happens inside electrical equipment).
- The chilled air can bring dust and pollen to the area causing discomfort for allergy sufferers. The growth of microorganisms such as mold on the cooling pads can cause allergy problems in sensitive people.
- Evaporative coolers use in-situ water.
- Refrigerants are not aesthetically appealing and if not maintained, the build-up of concentrated salts can damage the building surface.
- Compared to vapor compression systems, evaporative coolers require higher airflow rates to compensate for higher supply air temperatures.
- Airspeed can cause annoying noise when operating at high speed. Vents that allow air to exit the building can pose a safety risk.
Part 3. Types of Evaporative Cooling System
The two basic methods of evaporative cooling are:
1) Direct Cooling: In direct cooling, water evaporates directly into the air stream, thus lowering the dry bulb temperature of the air while humidifying the air.
2) Indirect Cooling: In indirect cooling, an air stream called primary air is cooled perceptibly (without adding moisture) by a heat exchanger, while secondary air carries heat energy from the primary air.
Direct and indirect cooling can be combined. The effectiveness of any of these methods is directly dependent on the low wet bulb temperature in the supply air stream.
Direct Evaporative Cooling (Open Loop)
Direct evaporative cooling delivers water directly to the supply air stream (usually by a spray or some form of wet medium). As water absorbs heat from the air, it evaporates and cools the air.
In direct evaporative cooling, the dry bulb temperature is lowered, but the wet bulb temperature does not change.
As it works, a blower draws air through a permeable, water-soaked pad. As the air passes through the pad, it is filtered, cooled and humidified. A recirculation pump keeps the medium (a woven fiber or corrugated paper pad) wet, while air travels through the pad. To ensure that the entire environment is wet, more water is usually pumped out than can be evaporated, and excess water flows into a chamber from the bottom. The automatic refill system replaces the evaporated water.
The efficiency of direct cooling depends on the pad environment. A quality hard cellulose pad provides up to 90% efficiency, while a loose poplar fiber pad provides 50 to 60% contact efficiency.
Indirect Evaporative Cooling (Closed Loop)
Indirect evaporative cooling lowers the temperature of the air through a type of heat exchanger arrangement where a secondary airflow is cooled by water, which then cools the primary airflow. The cooled air never comes into direct contact with water or the environment.
In the indirect evaporative cooling system, both dry bulb and wet bulb temperatures are reduced.
Indirect evaporative coolers do not add moisture to the air, but they cost more and operate at lower efficiency than direct coolers.
The efficiency of indirect cooling is in the range of 60-70%.
Two Stage Indirect/Direct Evaporative Cooling
Two-stage evaporative coolers combine indirect evaporative cooling with direct evaporative cooling. This is accomplished by passing air through a heat exchanger cooled by evaporation outside. In the second stage, pre-chilled air passes through a water-soaked pad and absorbs moisture as it cools. Because the air supply to the second stage evaporator is pre-cooled, less moisture is added to the air, whose moisture affinity is directly related to temperature. Two-stage evaporative cooling provides cooler air directly or indirectly than a single-stage system can supply separately. In many cases, these two-stage systems provide better comfort than a compressor-based system as they maintain a more favorable indoor humidity range.
The advanced two-stage evaporative cooler uses 100 percent outside air and a variable speed fan to circulate cool air. According to the American Society of Heating and Engineers (ASHRAE), two-stage evaporative coolers can reduce energy consumption by 60 to 75 percent over conventional air conditioning systems. However, this relative improvement is dependent on location and application.
Comparison of Direct, Indirect and Two-Stage Cooling
Note: The following temperature units are °F.
In the examples below we will see the temperature drop that can be achieved with three different approaches. These examples would be an initial dry bulb temperature of 86° and a wet bulb temperature of 66°. Cooling efficiency; we will assume 90% for direct cooling and 70% for indirect cooling.
1) Temperature reduction achievable using Direct Evaporative Cooling
A. Obtainable temperature drop = (dry bulb – wet bulb) x (efficiency of media) Example: (86° – 66°) x .9 = 18°
B.Achievable temperature = dry bulb – achievable temperature drop Example: 86° – 18° = 68°DB
- Starting LT: 86°
- End KT: 68°
- Starting YT: 66°
- End YT: 66°
*Since cooling is achieved by adding moisture to the supply air stream, the new dry bulb/wet bulb temperatures are located on the wet bulb gradient.
2) Temperature reduction achievable using Indirect Evaporative Cooling
A. Obtainable temperature drop = (dry bulb – wet bulb) x (efficiency of the indirect module) Example: (86° – 66°) x .7 = 14°
B. Reachable temperature = dry bulb – achievable temperature drop Example: 86° – 14° = 72° DB/61.4°YT
- Starting LT: 86°
- End KT: 72°
- Starting YT: 66°
- End YT: 61.4°
*New dry bulb/wet bulb temperatures are located on the dry bulb gradient, as moisture is not added to the supply air stream.
3) Temperature reduction achievable using Two-Stage Indirect/Direct Evaporative Cooling
First, calculate the dry bulb and wet bulb temperatures, which can be obtained with the following formula. Indirect evaporative cooling:
A. Obtainable temperature drop=(dry bulb – wet bulb)x(efficiency of indirect module) Example: (86° – 66° ) x .7 = 14°
B. Reachable temperature = dry bulb – achievable temperature drop Example: 86° – 14° = 72°DB
Stage 1. Results after indirect cooling
- Start LT / HT: 86° / 66°
- End LT / HT: 72° / 61.4°
Then use the dry bulb/wet bulb values from step 3 to calculate the dry bulb/wet bulb temperatures achievable with direct evaporative cooling:
C. Obtainable temperature drop: (dry bulb – wet bulb) x (efficiency of direct module medium) Example: (72° – 61.4°) x .9 = 9.5°
D. Reachable temperature = dry bulb – achievable temperature drop Example: 72° – 9.5° = 62.5°DB
Results after stage 2, direct cooling
- Starting LT / HT: 72° / 61.4°
- Ending LT / HT: 62.5° / 61.4°
Net results after 1st and 2nd stage cooling
- Starting LT / HT: 86° / 66°
- End KT / HT: 62.5° / 61.4°