Evaporative Cooling Basics

SummaryEvaporative cooling has made summers more bearable for thousands of years and with 21st century technology provides effective, economical, environmentally friendly, and healthy cooling. Evaporative cooling comes in five flavors: direct, indirect, indirect/direct, indirect/indirect, and indirect/DX. Evaporative cooling works well in the Pacific Northwest, alone or as a supplement to a chiller or DX system.

What is evaporative cooling?Evaporative cooling is responsible for the chill you feel when a breeze strikes your skin—-the air evaporates the water on your skin, with your body heat providing the energy. The ancient Egyptians hung wet mats in their doors and windows, and wind blowing through the mats cooled the air—-the first attempt at air conditioning. This basic idea was refined through the centuries: mechanical fans to provide air movement in the 16th century, cooling towers with fans that blew water-cooled air inside factories in the early 19th century, swamp coolers in the 20th century.

These simple examples illustrate direct evaporative cooling. Modern technology has dramatically increased the efficiency and effectiveness of direct evaporative cooling and made possible four other types of evaporative cooling: indirect evaporative cooling, indirect/direct evaporative cooling, indirect/indirect evaporative cooling, and indirect/DX evaporative cooling.

Direct evaporative cooling

With direct evaporative cooling, outside air is blown through a water-saturated medium (usually cellulose) and cooled by evaporation. The cooled air is circulated by a blower.

Direct evaporative cooling adds moisture to the air stream until the air stream is close to saturation. The dry bulb temperature* is reduced, while the wet bulb temperature** stays the same.

*dry bulb: Sensible air temperature (as measured by a hermometer).**wet bulb: The lowest air temperature achievable by evaporating water into the air to bring the air to saturation.

Indirect evaporative cooling

With indirect evaporative cooling, a secondary (scavenger) air stream is cooled by water. The cooled secondary air stream goes through a heat exchanger, where it cools the primary air stream. The cooled primary air stream is circulated by a blower.

Indirect evaporative cooling does not add moisture to the primary air stream. Both the dry bulb and wet bulb temperatures are reduced.

During the heating season, an indirect system’s heat exchanger can preheat outside air if exhaust air is used as the secondary air stream.

Indirect/direct evaporative cooling

With indirect/direct evaporative cooling, the primary air stream is cooled first with indirect evaporative cooling and then cooled further with direct evaporative cooling.

Indirect/indirect evaporative cooling

In first stage, the primary air stream is cooled by indirect evaporative cooling.

In second stage, water used in first-stage cooling passes through the wet side of a coil. Additional sensible heat is removed from the primary air stream, and no moisture is added to the primary air.

Indirect evaporative cooling/DX

With indirect evaporative cooling with DX back-up, the primary air stream is cooled first with

indirect evaporative cooling. Most of the time, this cools the primary air stream to the desired temperature. When more cooling is required, the supplemental DX module cools the air further to reach the desired temperature.

This unit is in beta release, and achievable energy savings are still being tested.

What’s so great about evaporative cooling?Evaporative cooling is economical, effective, environmentally friendly, and healthy.

Economical

Evaporative cooling is economical because it:

Reduces DX/chilled water cooling requirements for fresh air. Cuts mechanical cooling costs 25% to 65%. Increases existing equipment cooling capacities without adding mechanical cooling. Increases compressor life. Increases heat exchanger life.

Effective

Evaporative cooling actually becomes more effective as the temperature increases—just when DX air conditioning becomes less effective.

Evaporative cooling works in all areas of the country, not just in hot, dry climates. Although the Pacific Northwest is certainly damp in winter, it is dry in summer. In fact, humidity in this region of the country almost always decreases proportionally as the temperature increases. So the cooling power of evaporative systems increases as the temperature increases.

Environmentally friendly

Because evaporative cooling does not use chlorofluorocarbons (CFCs), it does not contribute to ozone depletion.

Healthy

Evaporative cooling is healthy and comfortable because it:

Brings in outside air and exhausts stale air, smoke, odors, and germs. Helps maintain natural humidity levels, which benefits both people and furniture and cuts

static electricity.

Does not need an air-tight structure for maximum efficiency, so building occupants can open doors and windows.

What kind of temperature reductions can I expect with evaporative cooling?The greater the difference between the wet bulb and dry bulb temperatures, the greater the achievable temperature reduction. Here’s how to calculate temperature reductions achievable with direct, indirect, indirect/direct, and indirect/indirect evaporative cooling. These examples use a starting dry bulb (DB) temperature of 86 degrees F and wet bulb (WB) temperature of 66 degrees F.

Temperature reduction achievable using direct evaporative cooling

NOTEWith direct evaporative cooling, the dry bulb temperature is reduced while the web bulb temperature remains the same.

1. Temp drop achievable = (dry bulb - wet bulb ) x (efficiency* of the media)Example: (86 degrees - 66 degrees) x .9 = 18 degrees

2. Achievable temp = dry bulb - temp drop achievableExample: 86 degrees - 18 degrees = 68 degrees DB/66 degrees WB**

3. Starting DB: 86 degreesEnding DB: 68 degrees

*Efficiency is usually 90%**Because cooling is achieved by adding moisture to the supply air stream, the new dry bulb/web bulb temperatures are found on the wet bulb gradient.Refer toAppendix Afor more information on using psychrometric charts to calculate temperature reduction using evaporative cooling.

Temperature reduction achievable using indirect evaporative cooling

NOTEWith indirect evaporative cooling, both the dry bulb and web bulb temperatures are reduced.

1. Temp drop achievable = (dry bulb – wet bulb) x (efficiency* of indirect module)Example: (86 degrees - 66 degrees) x .7 = 14 degrees

2. Achievable temp = dry bulb – temp drop achievableExample: 86 degrees – 14 degrees = 72 degrees DB/61.4 degrees WB**

3. Starting DB: 86 degreesEnding DB: 72 degrees

*Efficiency is usually 60%-70%**Because no moisture is added to the supply air stream, the new dry bulb/wet bulb temperatures are found on the dry bulb gradient.

Temperature reduction achievable using indirect/direct evaporative cooling

First calculate the dry bulb and wet bulb temperatures achievable with indirect evaporative cooling:

1. Temp drop achievable = (dry bulb - wet bulb ) x (efficiency of indirect module)Example: (86 degrees - 66 degrees) x .7 = 14 degrees

2. Achievable temp = dry bulb - temp drop achievableExample: 86 degrees - 14 degrees = 72 degrees DB/61.4 degrees WB

3. Starting DB: 86 degreesEnding DB: 72 degrees

Then use the dry bulb/wet bulb values from step 3 to calculate the dry bulb/wet bulb temperatures achievable with direct evaporative cooling:

4. Temp drop achievable: (dry bulb - wet bulb ) x (efficiency of the media)Example: (72 degrees - 61.4 degrees) x .9 = 9.5 degrees

5. Achievable temp = dry bulb - temp drop achievableExample: 72 degrees - 9.5 degrees = 62.5 degrees DB/61.4 degrees WB

6. Total temperature reduction using indirect/direct evaporative cooling:Starting DB: 86 degreesEnding DB: 62.5 degrees

Temperature reduction achievable using indirect/indirect evaporative cooling

First calculate the dry bulb and wet bulb temperatures achievable with the first stage of indirect evaporative cooling:

1. Temp drop achievable = (dry bulb - wet bulb ) x (efficiency of indirect module)Example: (86 degrees - 66 degrees) x .7 = 14 degrees

2. Achievable temp = dry bulb - temp drop achievableExample: 86 degrees - 14 degrees = 72 degrees DB/61.4 degrees WB

3. Starting DB: 86 degreesEnding DB: 72 degrees

Then use the dry bulb/wet bulb values from step 3 to calculate the dry bulb/wet bulb temperatures achievable with the second stage of indirect evaporative cooling, assuming 50%

effectiveness:

4. Temp drop achievable: (dry bulb - wet bulb ) x (efficiency of the media )Example: (72 degrees - 61.4 degrees) x .5 = 5.3 degrees

5. Achievable temp = dry bulb - temp drop achievableExample: 72 degrees - 5.3 degrees = 66.7 degrees DB/59.5 degrees WB

6. Total temperature reduction using indirect/indirect evaporative cooling:Starting DB: 86 degreesEnding DB: 66.7 degrees

Where can I use evaporative cooling?In many locations and for many applications, evaporative cooling is all the cooling required to maintain a comfortable indoor environment. In hotter areas or where cooling loads are high, such as in office buildings, one of the most useful applications for indirect/indirect evaporative cooling is supplementing a chiller or DX system. By cooling the air stream before it reaches the cooling coil, an indirect/indirect evaporative unit extends chiller life, cuts energy costs, and provides the boost the chiller needs to function effectively on hot days.

You can add an indirect/indirect evaporative cooling unit to an existing system or design a new cooling system that incorporates the indirect/indirect unit with the chiller or a standard roof-top DX system.

How much tonnage can I save using supplementary indirect/indirect evaporative cooling?

Example conditions

Required outside air volume: 10,000 CFM Required refrigeration tonnage to meet building cooling load: 25 tons

Formula

Tons saved = [1.08][1% Design Delta T][efficiency][CFM]/12,000

Examples

Oregon

Portland: [1.08][86-66][.90][10,000]/12,000 = 16.2 tons savedBend: [1.08][89-61][.90][10,000]/12,000 = 22.7 tons savedMedford: [1.08][95-66][.90][10,000]/12,000 = 23.49 tons saved

Washington

Seattle: [1.08][81-64][.90][10,000]/12,000 = 13.8 tons savedSpokane: [1.08][89-61][.90][10,000]/12,000 = 22.7 tons savedWallaWalla: 1.08[[[95-65][.90][10,000]/12,000 = 24.3 tons saved

Idaho

Boise: [1.08][94-63][.90][10,000]/12,000 = 25.11 tons savedIdaho Falls: [1.08][89-60][.90][10,000]/12,000 = 23.49 tons savedZPocatello: [1.08][90-60][.90][10,000]/12,000 = 24.3 tons saved

Seecalculations for additional Northwest cities.

Approximate costs for indirect, indirect/direct, and indirect/indirect evaporative modules

Indirect evaporative module: $1.95 to $2.30 per CFM Indirect/direct evaporative module: $3.25 to $3.50 per CFM Indirect/indirect evaporative module: $4.00 to $5.00 per CFM

Appendix APsychrometric Chart 101 Psychrometry is the study of moist air and the changes in its conditions. The psychrometric chart graphically represents the relationship between air temperature and moisture content and is a basic design tool for mechanical engineers and designers.

You can represent psychrometric processes (that is, any changes in the condition of the atmosphere) on the psychrometric chart. Common processes include:

Sensible cooling/sensible heating Cooling and dehumidification/heating and humidification Humidification/dehumidification Evaporative cooling/chemical dehydration

Figure 1 shows a basic psychrometric chart.

Figure 1 Psychrometric chart

The following sections explain using a psychrometric chart to calculate how much you can reduce dry bulb temperature using direct, indirect, indirect/direct, and indirect/indirect evaporative cooling.

Using a psychrometric chart to calculate the dry bulb temperature possible with direct evaporative cooling

NOTEWith direct evaporative cooling, the dry bulb temperature is reduced while the web bulb temperature remains constant.

1. Start with the dry bulb (DB) and wet bulb (WB) design conditions for the location you are interested in. For example, Portland’s 1% design conditions are 86 degrees DB and 66 degrees WB.

2. Find where 86 degrees on the dry bulb line intersects with 66 degrees on the wet bulb line (see Figure 2). That is the starting point.

Figure 2 Starting point

3. Calculate the temperature drop achievable using the following formulas. Figure 3 graphically represents the process.

Temperature drop achievable = (dry bulb - wet bulb) x (efficiency of the media)Example: (86 degrees - 66 degrees) x .9 = 18 degrees

Achievable temperature = dry bulb - temp drop achievableExample: 86 degrees - 18 degrees = 68 degrees DB

Because cooling is achieved by adding moisture to the supply air stream, the new dry bulb/wet bulb temperatures are found on the wet bulb gradient.

Figure 3 Direct evaporative cooling

4. Starting temperatures: 86 degrees DB/66 degrees WB5. Ending temperatures: 68 degrees DB/66 degrees WB

Using a psychrometric chart to calculate the dry bulb temperature possible with indirect evaporative cooling

NOTEWith indirect evaporative cooling, both the dry bulb and web bulb temperatures are reduced.

1. Start with the dry bulb (DB) and wet bulb (WB) design conditions for the location you are interested in. For example, Portland’s 1% design conditions are 86 degrees DB and 66 degrees WB.

2. Find where 86 degrees on the dry bulb line intersects with 66 degrees on the wet bulb line (see Figure 2 in the previous section). That is the starting point.

3. The efficiency of the indirect module determines the percentage of the possible temperature drop you can achieve with indirect evaporative cooling. Efficiency of the indirect module is usually 60% to 70%.

Calculate the new dry bulb temperature using the following formulas. Figure 4 graphically represents the process.

Temp drop achievable = (dry bulb – wet bulb) x (efficiency of indirect module)Example: (86 degrees - 66 degrees) x .7 = 14 degrees

Achievable temperature = dry bulb – temp dropExample: 86 degrees – 14 degrees = 72 degrees DB

4. Indirect evaporative cooling decreases the wet bulb temperature as well as the dry bulb temperature. Figure 4 shows how to use the psychrometric chart to calculate the new wet

bulb temperature.

Because no moisture is added to the supply air strean, the new dry bulb/wet bulb temperatures are found on the dry bulb gradient.

Figure 4 Indirect evaporative cooling: new wet bulb temperature

5. Starting temperatures: 86 degrees DB/66 degrees WB6. Ending temperatures: 72 degrees DB/61.4 degrees WB

Using a psychrometric chart to calculate the dry bulb temperature possible with indirect/direct evaporative cooling

First calculate the dry bulb and wet bulb temperatures achievable with indirect evaporative cooling:

1. With starting temperatures of 86 degrees DB/ 66 degrees WB, the achievable temperature using indirect evaporative cooling is 72 degrees DB and 61.4 degrees WB (refer to the previous section).

Then use the dry bulb/wet bulb values from step 1 to calculate the dry bulb/wet bulb temperatures achievable with direct evaporative cooling:

2. After being reduced by indirect evaporative cooling, the new starting point is 72/61.4.3. From the new starting point, use the following formulas to calculate the temperature drop

achievable with direct evaporative cooling. Figure 5 illustrates the process.

Temp drop achievable: (dry bulb - wet bulb) x (efficiency of the media)Example: (72 degrees - 61.4 degrees) x .9 = 9.5 degrees

Achievable temp = dry bulb - temp drop achievableExample: 72 degrees - 9.5 degrees = 62.5 degrees DB/61.4 degrees WB

Figure 5 Indirect/direct evaporative cooling temperature drop achievable

4. Starting termperatures: 86 degrees DB/66 degrees WB5. Ending temperatures: 62.5 degrees DB/61.4 degrees WB

Using a psychrometric chart to calculate the dry bulb temperature possible with indirect/indirect evaporative cooling

First calculate the dry bulb and wet bulb temperatures achievable with the first stage of indirect evaporative cooling:

1. With starting temperatures of 86 degrees DB and 66 degrees WB, the achievable temperature using indirect evaporative cooling is 72 degrees DB and 61.4 degrees WB (refer to the section on indirect evaporative cooling). Efficiency of 70% is used for the first indirect stage.

Then use the new dry bulb/wet bulb values from step 1 to calculate the dry bulb/wet bulb temperatures achievable with the second stage of indirect evaporative cooling:

2. After being reduced by indirect evaporative cooling, the new starting point is 72 degrees DB and 61.4 degrees WB.

3. From the new starting point, use the following formulas to calculate the temperature drop achievable with the second-stage of indirect evaporative cooling. Efficiency of 50% is used for the second indirect stage. Figure 6 illustrates the process.

Temp drop achievable: (dry bulb - wet bulb) x (efficiency of the media)Example: (72 degrees - 61.4 degrees) x .5 = 5.3 degrees

Achievable temp = dry bulb - temp drop achievableExample: 72 degrees - 5.3 degrees = 66.7 degrees DB/59.5 degrees WB

Figure 6 Indirect/indirect evaporative cooling temperature drop achievable

4. Starting temperatures: 86 degrees DB/66 degrees WB5. Ending temperatures: 66.7 degrees DB/59.5 degrees WB