The following post was featured as the Pick of the Quarter (POQ) in the latest edition of the FLOW-in-the-KNOW newsletter.
As a self-proclaimed energy recovery nerd, I can really get lost in the weeds on this topic. Those who have heard me dive into this area of conversation before can attest to that. For the purposes of this newsletter, I’ll keep the detail level at a high level and limit my scope to our two direct offerings of energy recovery for dehumidification – passive energy wheels and wrap around heat pipes.
In most air handling systems, the bulk of the dehumidification load is put on the cooling coil. The cooling coil cools the air to a point where moisture can no longer be suspended in the airstream, at which point moisture condenses out of the air. Sometimes an engineer’s design calls for that cooled air to be reheated before being sent to the space. Two common ways to reheat this air efficiently are via a passive energy wheel or a wrap-around heat pipe.
Spoiler alert: If the cooling coil can provide the dew point (dehumidification) required, a wrap-around heat pipe will ALWAYS be more efficient than a passive wheel system. If, however, more dehumidification is needed beyond what the cooling coil can provide, a passive wheel may be the best option.
Let me explain. Wrap-around heat pipes are sensible only devices. This means they DON’T transfer moisture, only temperature energy. A wrap-around heat pipe saves energy by precooling the air going to the cooling coil, taking sensible cooling load off the coil. It takes that precooled energy and gives it right back to the air leaving the cooling coil, providing free reheat energy and helping dehumidify the air further by reducing the relative humidity.
Passive wheels on the other hand, only transfer latent, or moisture, energy. They also precool the entering air but do so by adding moisture to the airstream from the leaving side. Once air goes through the cooling coil, the air goes through the other side of the passive wheel and moisture is removed, HEATING the airstream and dehumidifying it at the same time.
So, passive wheels ADD load to the cooling coil (by adding moisture upstream, even though they’re cooling), however they dehumidify on the leaving side, getting to a lower dew point that the wrap around coil can’t achieve on its own. Furthermore, the wheels typically have higher pressure drop, 1.5” – 2” w.c. total compared to .4 – .8” w.c. total for the wrap-around heat pipe.
So, there you have it, Mike’s pick would be a wrap-around heat pipe for applications where the cooling coil can do all the dehumidification. If not, a passive wheel is a great alternative.
Please note that there are other ways to achieve dehumidification inside air handling units including plate/frame heat exchangers, energy recovery membranes, total wheels with sensible wheel reheats. All these technologies may be valid for a given application. Flow Tech can help pair these with our custom air handling unit lines or with another manufacturer’s unit – please don’t hesitate to reach out to learn more.
Michael O. Davis, PE is President of Flow Tech, Inc. In addition to leading the day-to-day operations and vision of the company, Michael is responsible for selling some of Flow Tech’s more technically complex systems in our territory covering Connecticut and parts of Massachusetts and New York. He specializes in custom air handling applications and air-to-air heat recovery exchangers.
Michael is a 2010 graduate from the University of Connecticut with a Bachelor of Science degree in Mechanical Engineering. He currently resides in South Glastonbury, CT with this wife and two children.