Evaporative cooling efficiency is: the ratio of the actual cooling produced, to the energy required supplying it, relative to the theoretical maximum that could be produced, expressed in percentage.
When discussing the efficiency of evaporative cooling media, what you are really focused on is the actual temperature drop as the air passes through the media.
Based upon the terms defined, I'll discuss how this relates to evaporative media and how that might affect the decision regarding which media will work best for an application.
As a refresher, here are some relevant terms:
- Dry Bulb Temperature (Db). This is the ambient air temperature that surrounds you.
- Wet Bulb Temperature (Dw). This is lowest temperature the air can attain by evaporating water into the air.
Wet Bulb Depression (ΔT). This is the difference between the dry bulb and wet bulb temperature. It is typically expressed as this equation:
(Db – Dw = ΔT).
- Efficiency (eff). This is a ratio of the actual air temperature drop across the media compared to the wet bulb depression, expressed as a decimal percentage. If the wet bulb depression is 40°F and the actual temperature drop measured across the cooling media is 30°F, the cooling efficiency of the media is 75 percent (30/40 = 0.75). This cooling efficiency is also known as the saturation efficiency because it refers to the amount of moisture that the media can evaporate into the air.
- Cubic Feet per Minute (cfm). This unit of measure is used with a number to express the volume and velocity of air movement.
- Gallons per Hour (gal/hr). This unit of measure is used with a number to express volume and speed of water evaporated.
To determine the outlet air temperature of a cooler, multiply the wet bulb depression by efficiency, then subtract that from the dry bulb temperature.
Outlet Air = Db – (ΔT x eff)
That expression illustrates the importance of a higher efficiency number in that the closer the efficiency gets to 100 percent (or 1), the larger the number that is to be subtracted from the dry bulb temperature; hence, a greater the cooling effect is achieved.
To further illustrate the effects of greater efficiency, note the comparison below using typical saturation efficiencies of different media. For illustration purposes we will choose the following conditions:
- Db = 100°F
- Wb = 60°F
- ΔT = 40°F
At 75 percent efficient, the outlet air temperature will be 70°F.
100°F - (40°F x 0.75) = 70°F
8" Rigid Media
At 85 percent efficient, the outlet air temperature will be 66°F.
(100°F - (40°F x 0.85) = 66°F
12" Rigid Media
At 89 percent efficient, the outlet air temperature will be 64.4°F.
(100°F - (40°F x 0.89) = 64.4°F
4x4 Rigid Media
At 93 percent efficient, the outlet air temperature will be 62.8°F.
(100°F - (40°F x 0.93) = 62.8°F
As you can see by comparing the outlet air temperatures, the 4x4 media is the right choice in this example for maximum cooling effect. Outlet air temperature is by far the primary consideration. However, keep in mind that there may be other variables that affect an application, so it is important to do your own calculations.
See the related feature article, "Evaporative Cooling Can Save Money," to learn how evaporative cooling works to reduce cooling costs in process environments that require space cooling.