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

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.

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Drybulb dosent take the humidity content of the air into consideration. Wet bulb does.
Air Conditioning dosent actually cool you. It lowers the temperature and humidity of the environment which lowers the dew point… It is the evaporation off your sweaty body that cools you.
You want to make the temperature and humidity of the room at such a point that the humidity level of the room is above the dew point so the sweat will evaporate.
Many places dont take it into consideration and have high room humidity levels resulting in uncomfortable but cold rooms. However to properly deal with dewpoint, you need to have the proper equipment. Generally it is hotdeck / colddeck / reheat with a full size outdoor air damper for a AHU and the proper sized unit with a outdoor air damper when dealing with a RTU or split system.
This is one of the reasons that an oversized ac unit is a really bad idea in humid climates. The oversized a/c unit will cycle on and off many times and never remove the humidity. The correct or slightly undersized unit will run all the time constantly stripping the humidity from the air. The temperature of the room may be higher but the humidity level is lower and the room feels comfortable.

In response to John above.
Outside air is ALWAYS used in buildings that require enthalpy control. Perhaps not all year round. It is even a requirement in places like California where it is coded into law.
When properly implemented cool humid air is brought from outside for a few hours every morning. Of course less humidity is best but you get what you get. As the people arrive their body temp forces the temperature up and as the temperature rises the relative humidity drops.
At some point during the day, the humidity levels of the building exceed the established setpoint and the A/C starts.