Latent vs. Sensible Loads
When you read about our Awesome Air® product for your air conditioner,
you'll see references to latent and sensible loads. The following may help
to explain the process. (Courtesy of the University of Florida Cooperative
Extension Service)
A building's air conditioning system is responsible for removing moisture
from the air in order to provide for both human comfort and mold-and-mildew
control. Inside the air conditioner, warm moist air is blown through a
cooling coil. In the coil, the air is cooled below its dew point temperature.
The dew point temperature is defined as the temperature of the air when the
relative humidity is 100 percent. Relative humidity is defined as the amount
of moisture in the air relative to the most moisture the air can hold at the
same temperature. As air is cooled it loses its ability to hold moisture.
So, relative humidity is increased by cooling the air, as well as by adding
moisture to it. For example, as the air cools on a muggy night the relative
humidity increases. When the relative humidity reaches 100%, the air has been
cooled to its dew point and dew forms on surfaces.
Similarly for the air conditioner, once the air is cooled below the dew point,
the air releases moisture which collects in a drain pan, and drains out of
the system. The cooled and dried air is delivered to the building. The air
now has a lower dew point called the exit dew point.
Many air conditioning systems do not remove adequate amounts of moisture for
Florida's climate (or other areas as well). Most AC systems are designed to
handle peak load conditions -- The hottest afternoon of the summer.
Accordingly, they work best during the hottest times of the year but not so
well at other times.
AC systems are designed to remove a certain amount of moisture at peak
conditions. This is called the latent heat ratio of the system. Sensible
heat is heat in terms of degrees one reads on a common thermometer. Latent
heat, the other kind of heat, is heat in terms of moisture removed. Sensible
plus latent is the total heat removed. The latent heat ratio of an AC system
is the portion of latent heat it can remove out of the total heat it can
remove. It is typically around 30 percent at peak conditions (95 degrees F
outdoors) and a few percent higher at night conditions (75 degrees F outdoors).
The building load also has a latent heat ratio: it is the portion of latent
heat that needs to be removed from the building out of the total heat that
needs to be removed. At peak conditions there is much more sensible heat than
latent heat. At night and on cooler days, the amount of sensible heat shrinks
but the amount of latent heat does not. And, on wet days, the amount of
latent heat grows. The latent heat ratio may rise to 50 percent or more.
During humid and/or cool weather, the AC system indeed cools the building,
but it can't dehumidify adequately.
To make things even worse, some new air conditioning units have sacrificed
latent (moisture removal) capacity in order to increase their nameplate SEER
ratings. (SEER stands for Seasonal Energy Efficiency Ratio and is a measure
of energy efficiency.) One way manufacturers increase SEER is to raise the
cooling coil temperature. Unfortunately, this means that the air blown
through the coil does not reach a low dew point temperature. Some of these
high efficiency units have a latent heat ratio of 15 percent or less at
design conditions.
Awesome Air® is designed to remove the latent heat, improving the
performance of all air conditioners.
