Understanding Dew Point

Quick facts:

• Dew Point represents the absolute moisture content of the air.
• Dew Point determines which temperature will give you which %RH.
• If your building does not have humidification or dehumidification, the indoor dew point is the same as the outdoor dew point.
• If your building has cooling-based dehumidification, the indoor dew point temperature is within a few degrees of the temperature of the cooling coil of your mechanical system.

What is Dew Point?

The Short Answer:

The Dew Point temperature is the temperature at which the relative humidity will reach 100% and moisture will begin to condense out of the air; in other words, it is the temperature at which dew will begin to form.

The Longer Answer:

In practice, the better way to understand dew point is as an indication of the absolute moisture content of the air.

It may seem strange to talk about the dew point as the amount of moisture present in the air when it is expressed as a temperature. This measurement seems appropriate, however, when you understand that the amount of moisture that air can hold depends on the temperature of the air. At any given temperature, air can hold a certain amount of moisture. The warmer the temperature of the air, the more moisture the air can hold. The cooler the temperature of the air, the less moisture the air can hold. This relationship is also echoed in dew point temperatures. The higher the dew point temperature, the more moisture there is in the air. The lower the dew point temperature, the less moisture there is in the air.

The Dew Point determines what combinations of Temperature and RH will be possible in your environment.

Every environment is defined by a unique combination of three variables: Temperature (T), Relative Humidity (RH), and Dew Point (DP). These three variables are interrelated. If you set values for two of the variables, the third value will automatically be determined. By extension, if you change any one of these variables, a second variable will also have to change.

Explore the relationship of T, RH, and DP using IPI’s online tool, the Dew Point Calculator. Observe:

• At constant Temperature:
  • the RH and Dew Point will increase/decrease together.

• At constant RH:
  • Temperature and Dew Point will increase/decrease together.

• At constant Dew Point:
  • Temperature and RH will move in opposite directions:
    • as Temperature increases, RH will decrease.
    • as Temperature decreases, RH will increase.

Therefore, when the dew point is constant, the dew point is also responsible for determining which temperature will give you which RH. For example, say you would like to have an environment with a temperature of 55ºF (13ºC) and an RH of 45%. What dew point would you need to accomplish this environment? In order to have these conditions, you would need a dew point of 34ºF (1ºC).
 

How do I control Dew Point?

In buildings with humidity control, the dew point of the indoor air is controlled by the building’s mechanical system. When the outdoor air is brought into the building, the mechanical system modifies the air to obtain more desirable conditions of the indoor air. In order to change the dew point of the outdoor air, the mechanical system must have the capacity to add or remove moisture.

To raise the dew point:

There are two basic methods for mechanical systems to humidify the air, and thus raise the dew point:

1. Introduce steam (water vapor) into the air stream:
   • High-pressure steam created by the mechanical system is injected into the air stream as the last stage in conditioning the supply air within the AHU.
2. Evaporate liquid water directly into the air stream:
   • Liquid water is atomized into the air stream by being forced through a small nozzle, or else is evaporated from a cascading stream of water or from a foam pad. Because evaporation of water consumes heat, this method of humidification also cools the air as well as humidifying it.

To lower the dew point:

There are two basic methods for mechanical systems to dehumidify the air, and thus lower the dew point:

1. Sub-cool and Reheat:
   • Sub-cooling followed by reheating is the most common method of dehumidification in large building systems. This method involves first “sub-cooling” the air – cooling the air below its dew point – and then reheating it to the desired temperature. Passing air over a cooling coil whose surface temperature is below the dew point of the moving air stream causes condensation, which drips off the coil and runs down a drain. Air leaving the cooling coil has a lower moisture content than before (a lower dew point), but it is also cold (near the coil surface temperature) and has an RH near 100%. This is why after sub-cooling the air, it must typically be reheated, raising its temperature and lowering its RH, before it is delivered to the building spaces.
2. Desiccant Wheel:
   • Less common but recently gaining in popularity, desiccant dehumidification makes use of the property of some chemical substances (silica gel, lithium chloride) to absorb moisture from the air at moderate temperatures, and then to release that moisture when heated to high temperatures. Desiccant dehumidification systems typically involve passing air over a moving, honeycombed wheel containing water-absorbing chemicals. As the wheel rotates, it then passes through a separate stream of heated “regeneration” air that is vented to the outdoors. This “regeneration” stream causes the chemicals to release the moisture so by the time the wheel reaches the supply air, the chemicals are again ready to absorb moisture.