Cooling Principles

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PRINCIPLES OF EVAPORATIVE COOLING

To understand evaporative cooling, it is important to know the relationship between dry air and water vapor, as well as some technical terms.

♦ Dry bulb temperature (db) is the degree (not amount) of heat measured with a regular thermometer.

♦ Wet bulb temperature (wb) is the degree of heat measured with a “wet sock” attached to the bulb.
The Wb is also the temperature at which water evaporates when air is blown over the wet surface.

♦ Relative humidity (rh) is the ratio of the amount of water (as vapor) actually contained in the air to the maximum amount of water vapor that could be contained at the same temperature (saturation).
For example, at 50% rh, the air contains 50% of the maximum (100%) amount of moisture (water vapor) that it could contain at the same temperature.

♦ Adiabatic cooling is the direct evaporative cooling process, whereby the dry bulb temperature is lowered without altering the amount of heat in the air.

♦ Latent Heat is the heat absorbed by the moisture as it changes from liquid to vapor during evaporation. This heat is absorbed by the vapor without any increase in its temperature - an important physical phenomenon.

♦ Media or Filter Pad or Evaporation Pad is the screen through which air must pass to come in contact with the water.

Basic considerations of airflow are volume and velocity.
Volume is the amount of air (expressed in cubic feet per minute (CFM)), and velocity is the speed of air (expressed in feet per second (FPS)).

♦ The volume of air through the media affects the cooling capacity of the Evaporative Cooler along with the evaporation rate of the water. The volume of air also affects the ability of the Evaporative Cooler to reduce the temperature of a room.

♦ The velocity of the air through the media is an important criterion for determining efficiency. Velocity is also important in preventing water carry-over and blow-off. Warm air can hold more moisture than cold air, and therefore when air is heated without changing the moisture content, the relative humidity drops. Conversely, when air is cooled, the relative humidity rises.

In an Evaporative Cooler the latent heat, which is absorbed by the water as it changes from liquid to vapor, is taken from both the air passing through the media and the water left in the media - so both the air and water are cooled and their temperature falls. The water does not concern us, but the cooled air is what we want.

The above illustrates that when water passes over the media and air is blown through it, water evaporates and the air is cooled so that the dry bulb temperature of the cooled air approaches the original air wet bulb temperature.

Note that the total amount of heat has not changed, but its nature has changed. The small amount of latent heat in the original water vapor in the air has become larger because of an increase in the amount of water vapor, due to evaporation.

CALCULATING SUPPLY AIR TEMPERATURE

Evaporative cooling effectiveness is the expected reduction of air temperature based on the efficiency of the media and the air movement system. “Wet bulb depression” (wbd) is the difference between dry bulb temperature and wet bulb temperature for the entering (ambient) air.

For example,: 95°F db and 77°F wb equals a 18°F wet bulb depression.
To determine the expected discharge dry bulb air temperature, :
Discharge temperature db = entering db - [efficiency x wbd]; Example :Entering (ambient) air condition (from outside)
Db = 95°F and Wb = 77°F
Efficiency = 80% (see note below)
Hence wbd = 18°F
Therefore discharge air temperature = 95 - (80% x 18) = 80.6°F

Note:

1. Db = 95°F, Wb = 77°F = Relative Humidity of 44%
2. The Efficiency must be provided by the manufacturer of the Evaporative Cooler. When brand new, it may be as high as 85% for Aspen fiber pads but reduces as the media deteriorates. A well-cleaned and maintained media will be about 80% efficient if the machine is well designed. Many machines operate at lower efficiencies

TEMPERATURE DROP CHART

Below represents the approximate supply air temperature based on a minimum pad saturation efficiency of 80%
Sea Level - 80% pad saturation efficiency (most aspen pad type coolers)

below represents the approximate supply air temperature based on a minimum pad saturation efficiency of 90% ( Chill Cell media Breezair units)

Source:Seeley International

How does a evaporative cooler work

Why use Evaporative cooling

Evaporative Water consumption

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