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Principles of Evaporative Cooling System

A. Bhatia, B.E.

Course Outline

Evaporative air-cooling is an environmentally friendly and energy efficient method of cooling air using water as cooling medium. While an evaporative cooling system can effectively meet the objectives of comfort cooling, it usually does not eliminate the need for air-conditioning except in certain arid and dry climates. Before you can decide if evaporative cooling will work in your situation you should understand exactly how it works.

This 4-hour course discusses some of the major factors you must consider when choosing an evaporative cooling system.

The course includes a glossary and multiple-choice quiz at the end, which is designed to enhance the understanding of the course materials.

Learning Objective

At the conclusion of this course, the reader will:

Intended Audience

This course is aimed at students, HVAC engineers, designers, architects, facility managers, HSE professionals, environmentalists, energy auditors, technical and sales representatives and anyone who wants a basic understanding of climate control.

Course Introduction

When warm air passes over cooler water, heat passes from the air to the water, causing water to evaporate and air is cooled. Evaporative coolers use this principle to "extract" heat from the warmer outdoor air by passing that air through a wetted media, thus causing the air to cool. This process increases humidity of the outlet air stream because moisture is added to air. Evaporative coolers use the natural relationship between relative humidity, water and air temperature.

This course reviews the above criteria in detail.

Course Content

The course content is in a PDF file Principles of Evaporative Cooling System. You need to open or download this document to study this course.

Course Summary

Evaporation, nature's air conditioner, is the most economical method for air cooling. Evaporative cooling occurs when moisture is added to air that has a relative humidity of less than 100 percent. Its effectiveness is however reduced as humidity levels rise, and eventually when the air is fully saturated the cooling effect stops altogether. Fortunately as the temperature rises, the humidity drops. So at the hottest time of day evaporative cooling is at its most effective.

The technology is a versatile and energy-efficient alternative or adjunct to compressor-based cooling. In favorable climates (most of the western United States and other dry-climate areas worldwide), evaporative cooling can fully satisfy building cooling loads using one-fourth the energy of conventional equipment. It can also be applied cost-effectively when integrated with conventional chiller systems. Evaporative cooling is especially appropriate for spaces with high outside air ventilation requirements such as workshops, locker rooms, kitchens, or shops. Compressor cooling is often too expensive to operate for these applications.

Evaporative cooling has following advantages over compression based cooling methods:

1) Evaporative air cooling provides superior indoor air quality over vapor-compression systems since 100% outdoor air is used.
2) The use of evaporative cooling instead of vapor-compression systems also helps reduce global CO2, CFC and other greenhouse gas emissions.
3) The cost of powering an evaporative cooler's fan and a small water pump is far less than running air conditioning.
4) Evaporative cooled air is perceived to be more comfortable than dried-out, air-conditioned air.
5) Compared to other methods of cooling, evaporative cooling is very simple, very effective and inexpensive to purchase and operate.

The evaporative coolers have some limitations and disadvantages:

1) Evaporative coolers are not effective in the humid regions.
2) Compared to vapor compression systems, evaporative coolers require increased air flow rates to compensate for higher supply air temperatures.

The choice of appropriate comfort system rests with the individual and it is recommended to investigate all of the alternatives and make a decision based on their lifestyle, needs and preferences.


Once you finish studying the above course content, you need to take a quiz to obtain the PDH credits.

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DISCLAIMER: The materials contained in the online course are not intended as a representation or warranty on the part of PDH Center or any other person/organization named herein. The materials are for general information only. They are not a substitute for competent professional advice. Application of this information to a specific project should be reviewed by a registered architect and/or professional engineer/surveyor. Anyone making use of the information set forth herein does so at their own risk and assumes any and all resulting liability arising therefrom.