The Dry BulbWet Bulb and Dew Point temperatures are important to determine the state of humid air. The knowledge of only two of these values is enough to determine the state - including the content of water vapor and the sensible and latent energy (enthalpy).

Dry Bulb Temperature - Tdb

The Dry Bulb temperature, usually referred to as air temperature, is the air property that is most common used. When people refer to the temperature of the air, they are normally referring to its dry bulb temperature.

The Dry Bulb Temperature refers basically to the ambient air temperature. It is called "Dry Bulb" because the air temperature is indicated by a thermometer not affected by the moisture of the air.

Dry-bulb temperature - Tdb, can be measured using a normal thermometer freely exposed to the air but shielded from radiation and moisture. The temperature is usually given in degrees Celsius (oC) or degrees Fahrenheit (oF). The SI unit is Kelvin (K). Zero Kelvin equals to -273oC.

The dry-bulb temperature is an indicator of heat content.

Wet Bulb Temperature - Twb

The Wet Bulb temperature is the temperature of adiabatic saturation. This is the temperature indicated by a moistened thermometer bulb exposed to the air flow.

Wet Bulb temperature can be measured by using a thermometer with the bulb wrapped in wet muslin. The adiabatic evaporation of water from the thermometer and the cooling effect is indicated by a "wet bulb temperature" lower than the "dry bulb temperature" in the air.

The rate of evaporation from the wet bandage on the bulb, and the temperature difference between the dry bulb and wet bulb, depends on the humidity of the air. The evaporation is reduced when the air contains more water vapor.

The wet bulb temperature is always lower than the dry bulb temperature but will be identical with 100% relative humidity (the air is at the saturation line).

Dew Point Temperature - Tdp

The Dew Point is the temperature at which water vapor starts to condense out of the air (the temperature at which air becomes completely saturated). Above this temperature the moisture will stay in the air.

  • if the dew-point temperature is close to the dry air temperature -  the relative humidity is high
  • if the dew point is well below the dry air temperature - the relative humidity is low
If moisture condenses on a cold bottle taken from the refrigerator, the dew-point temperature of the air is above the temperature in the refrigerator.

The Dew Point temperature can be measured by filling a metal can with water and some ice cubes. Stir by a thermometer and watch the outside of the can. When the vapor in the air starts to condensate on the outside of the can, the temperature on the thermometer is pretty close to the dew point of the actual air.

Sensible heat

When an object is heated, its temperature rises as heat is added. The increase in heat is called sensible heat. Similarly, when heat is removed from an object and its temperature falls, the heat removed is also called sensible heat. Heat that causes a change in temperature in an object is called sensible heat.

Latent heat

All pure substances in nature are able to change their state. Solids can become liquids (ice to water) and liquids can become gases (water to vapor) but changes such as these require the addition or removal of heat. The heat that causes these changes is called latent heat.

Latent heat however, does not affect the temperature of a substance - for example, water remains at 100�C while boiling. The heat added to keep the water boiling is latent heat. Heat that causes a change of state with no change in temperature is called latent heat.

Appreciating this difference is fundamental to understanding why refrigerant is used in cooling systems. It also explains why the terms 'total capacity' (sensible & latent heat) and 'sensible capacity' are used to define a unit's cooling capacity.

During the cooling cycling, condensation forms within the unit due to the removal of latent heat from the air. Sensible capacity is the capacity required to lower the temperature and latent capacity is the capacity to remove the moisture from the air.

Latent heat - HVAC Refrigeration

Latent heat is the moisture content in the air.

There is water vapor in the air. If you want to take it out, you must condense it out by passing it across a surface which is cold enough (at or below dew point) to cause water to form (like the mirror in the bathroom when you take a shower, or a window on a winter day).

This condensation occurs at 100 % relative humidity. When the air can no longer hold any greater concentration of water vapor, the vapor will change to a liquid. When a vapor changes to a liquid, it gives up heat. This is latent heat, because the energy came from changing vapor into liquid, not from lowering the temperature. All refrigeration units have some effect on both latent and sensible heat, assuming the evaporator is below dewpoint.

The proportion will vary according to the amount that the coil temperature falls below dewpoint, and the time the air (with water vapor) gets to sit on that coil and lose energy.

In other words, the temperature (sensible) does not change, but rather the energy is given up by the process of changing state from a gas to a liquid.

Sensible heat is that heat which you can sense (standard temperature readings). Latent heat is that heat which you can not sense ( the heat energy that it took to turn that water into vapor in the first place ), although the energy is still there.

When you remove water from the air without changing the temperature, your RELATIVE humidity goes down. You have changed the RELATIVE amount of LATENT HEAT (moisture) to SENSIBLE HEAT (temperature).

When you raise the (Sensible) temperature without adding water vapor (latent heat), your RELATIVE humidity goes down, because the RELATIONSHIP between temperature and water content has changed, in favor of temperature.

If you add BOTH LATENT and SENSIBLE heat, your RELATIVE humidity may stay the same, if you have maintained the same proportions.

During the last few years, there has been an increase in the use of total heat evaporator selections offered for sale as a result of the globalization within the market. Global manufacturers, who have historically designed and rated their equipment for use in what is often referred to as the commercial refrigeration and light industrial markets, have been pursuing business in the industrialrefrigeration market. Because evaporator coil capacity is strongly affected by entering air moisture content, it has become important to realize the potential consequences of a total heat evaporator selection.

What is "total heat"? Total heat is the sum of the sensible heat and latent heat portions of the evaporator load as shown in the equation:

Qt = Qs + Ql

In conclusion, there must be an understanding of the components when designing and sizing HVAC cooling systems. There are many factors that must be considered. The final design must be able to address both the sensible and the latent heating loads to provide a quality environment for the occupants.

If you would like more information on this, please contact our office and we would gladly discuss this further.