What Is the Difference Between Dry Bulb and Enthalpy-Based Cooling?

Originally published at: What Is the Difference Between Dry Bulb and Enthalpy-Based Cooling? - Bravo Controls

What Is the Difference Between Dry Bulb and Enthalpy-Based Cooling?

Free cooling uses outside air to cool the building rather than enabling the mechanical cooling. This has the added benefit of improving the indoor air quality, assuming the outside air is not smokey from forest fires and the like. Free cooling systems are often installed in high heat load spaces such as boiler rooms, data centers, and other facilities. Take note that free cooling is not entirely free because there is a cost of moving the air around but making use of the cool outside air is an important energy conservation strategy.

There are two common methods for judging if the outside air is worth bringing into the building. The first strategy is simpler and is based on two temperature sensors, one inside the building and one outside, if the air outside is cooler than inside then the system judges that free cooling is available. The second type of free cooling is called ‘enthalpy based’ free cooling where a temperature and humidity sensor are used to calculate the actual energy content of the inside and outside air. Air with more humidity in it will take more energy to cool down than dryer air, the enthalpy based system is able to take this latent energy into account.

Dry-Bulb-Based Free Cooling System

Dry-bulb sensing is often set at a fixed ambient changeover temperature. If the ambient temperature goes above the changeover temperature the system will not bring in the fresh air and will go directly to the DX compressor-based cooling. On the other hand, if the ambient temperature goes below the changeover temperature, the controls will activate your free cooling system. The dry bulb based system has a low initial cost and is easy to maintain so it is popular with HVAC professionals and building owners.

Enthalpy-Based Free Cooling System

It is important to note that outdoor air is a mixture of water vapor and dry air which can take more energy to cool down, not to mention the problems with humid air entering the building will make the occupants uncomfortable and can lead to mold. Enthalpy-based sensing uses both the temperature and the relative humidity to make the free cooling decisions. The ambient air is compared to the indoor air and the one with the lower energy will win the free cooling decision. There is also the minimum outdoor intake air setting and the maximum humidity level to take into account, but in between those limits the system will adapt according to the energy content of the air around a trigger point known as the ‘high limit’ enthalpy control.

An article created by John Hamlin, John Hamlin is a freelance writer who has a background in engineering. With a keen interest in technology and writing, John has been working online providing insight and direction for many years. His latest work has been on a compilation of industrial manufacturing techniques.