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How does an evaporative cooler work?

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Quick answer

An evaporative cooler passes hot, dry outside air through a wet pad. Water on the pad evaporates, absorbing heat from the air and lowering its temperature. A fan then blows this cooler, moister air indoors. It works best in dry climates, where evaporation is fast, and poorly in humid ones, where the air already holds much moisture.

An evaporative cooler works by pulling hot, dry air through a water-soaked pad, where water absorbs heat and evaporates into the air. This adds moisture and lowers the air temperature, so the fan delivers cooler, more humid air into the room.

Step by step

  1. A fan draws hot, dry outside air into the cooler unit.
  2. The air passes through a porous pad kept wet by a pump or wick.
  3. Water on the pad evaporates, absorbing heat from the air and cooling it.
  4. The cooled, humidified air is blown by the fan into the room.
  5. Warm, stale indoor air escapes through an open window or vent.

Key things to understand

  1. A fan draws outside air through a porous pad kept wet by a pump or capillary action.
  2. As dry air passes the wet pad, some water evaporates and absorbs heat from the air.
  3. The removed heat lowers the air temperature while adding water vapour to it.
  4. The fan then pushes this cooler, more humid air into the living space.
  5. Evaporative cooling works best in hot, dry climates and poorly in humid ones.

Labelled diagram

  1. Hot dry air

    Outside air enters the cooler, warm and low in humidity.

  2. Wet pad

    A pump keeps a porous pad soaked with water.

  3. Evaporation

    Water turns to vapour, absorbing heat from the passing air.

  4. Cooler air

    Air leaves the pad cooler but more humid.

  5. Fan

    The fan pushes this cooled air into the room.

  6. Vent

    An open window lets warm, stale air escape.

How an evaporative cooler cools air

Real-world examples

  • In Phoenix, Arizona, many homes use roof-mounted evaporative coolers, often called swamp coolers, during the dry summer months.
  • In Rajasthan, India, desert coolers are common in homes and shops where humidity is low and summer temperatures are high.
  • Large evaporative cooling systems cool warehouses and factories in dry parts of Australia and the Middle East.
  • A simple hand-held fan over a wet cloth demonstrates the same principle: the air feels cooler as water evaporates.

Common mistake

✗ Myth: An evaporative cooler works just like an air conditioner, so it can cool any room in any climate.

✓ Fact: Evaporative coolers cool by adding moisture to the air, so they only work well when the outside air is dry. In humid climates, the air is already close to saturation, evaporation is slow, and the cooler adds little cooling while making the room muggy.

People also ask

What is the difference between an evaporative cooler and an air conditioner?
An air conditioner uses a compressor and refrigerant to transfer heat from indoors to outdoors, cooling and dehumidifying the air. An evaporative cooler uses only a fan and water; it cools by evaporating water into the air, which adds humidity instead of removing it. Evaporative coolers cost less to run but only work in dry climates.
How much does an evaporative cooler cool the air?
The temperature drop depends on how dry the air is. In very dry conditions, the air can drop by around 10 to 15 degrees Celsius (18 to 27 degrees Fahrenheit). In humid conditions, the drop is much smaller. The exact amount varies with the design and the weather.
Can you use an evaporative cooler indoors?
Yes, but it must have a source of dry outside air and a way for humid air to escape, such as an open window or door. If used in a sealed room, the humidity rises and cooling stops. They are best for spaces with good cross-ventilation.
Why don't evaporative coolers work well in humid weather?
Evaporative coolers rely on water evaporating into the air. When the air already holds a lot of moisture, it cannot take up much more, so evaporation slows down and less heat is removed. That is why they are common in dry regions like Arizona or Rajasthan but rare in coastal humid cities.
Do evaporative coolers use a compressor like air conditioners?
No. Refrigerated air conditioners use a compressor and refrigerant to move heat from inside to outside. Evaporative coolers have no compressor or refrigerant; they simply use a fan, water, and a wet pad. This makes them cheaper to buy, cheaper to run, and easier to maintain.
Can an evaporative cooler make a room too humid?
Yes. Because it adds water vapour to the air, a room can become muggy if the cooler runs too long without ventilation. Most designs work best with a window or door partly open so humid air can escape and fresh dry air can enter.
What maintenance does an evaporative cooler need?
The water pad or pads must be kept clean and replaced regularly, because dust and minerals build up and block airflow. The water reservoir should be drained and refilled to prevent algae and bacteria. The fan motor may need occasional lubrication.

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