Today’s post will be included in the Spring 2021 Flow in the Know quarterly newsletter. It is the feature of the reoccurring section, “Pick of the Quarter,” where we like to highlight one of our 19 manufacturers and the engineered HVAC equipment solutions we provide. In this post you’ll learn several ways to combat air stratification and promote good air mixing.
What is a blender and when would I use it?
In mixed-air handling systems where warm return air mixes with cold outdoor air during winter months, air stratification (separation of airstreams due to temperature differential) is possible. This can cause cold air to set off freeze stats or worse, damage downstream air handling components like coils. If the coils freeze and get damaged, other negative effects such as water damage outside the air handling unit are possible.
There are several ways to combat air stratification and promote good air mixing, of which we will highlight a few.
Perpendicular dampers: Using dampers at 90 degrees with parallel blades to force mixing. This is relatively easy and inexpensive to do (you need to bring air into the unit and to control with dampers anyway) but is not very effective at forcing airstreams to mix, especially when temperature differential is high, and one damper is much more open than the other (think minimum OA on a very cold day).
Parallel dampers (shelf mixing): Instead of having dampers at right angles to each other, put them directly across from each other, close together and at high velocity. The shelf mixing design configures the outside air and return air dampers discharging directly into each other with a mixing plenum between the outside air and return air dampers is sized for a velocity of 1500 FPM providing a reasonable controllability. Parallel blade dampers are used to aid mixing by imparting some directionality towards the rear of the mix box. Additionally, the outside air damper is split into a minimum OA damper (located at the back of mix box) and an economizer OA damper. This allows the economizer damper to remain closed during winter operation thus forcing the minimum outside air to enter the back of the mix box and mix with the return air. It’s important to note that the economizer damper must remain closed during cold weather operation for this damper arrangement to be effective. This can be very effective for mixing but, depending on the application can create higher static pressure requirements (system effect) and can add significant length to unit.
Air Enterprises has been successfully using the shelf mixing arrangement for nearly 40 years. This method of mixing incorporates design features as listed in the 2012 ASHRAE Handbook – HVAC Systems and Equipment.
Static blenders: An effective way to help airstreams mix passively (no moving parts), typically used in conjunction with option 1 above (perpendicular dampers). This option has a low pressure drop but can add significant length to unit.
Blender Products Channel Blender: An innovative solution that actively mixes air. This option has a lower pressure drop and requires about half the length of air handling tunnel space that a static blender requires. This option is more expensive but is well worth the added cost for most applications.
We’d love to discuss any of the above options for your air mixing needs. We are a proud representative for Blender Products Channel Blender and engineered air mixing equipment in Connecticut and MA’s Franklin, Hampshire and Hampden counties. To learn more about how this HVAC solution can improve the comfort and efficiency of your facility, contact us or visit our website. You can always stay up-to-date on the latest Flow Tech happenings by following us on social media: Facebook, LinkedIn, Twitter and YouTube.
Michael O. Davis, PE has been with Flow Tech, Inc. since 2009. As one of Flow Tech’s Outside Sales Engineers, Michael is responsible for selling some of Flow Tech’s more technically complex systems in their territory covering Connecticut and parts of Massachusetts and New York. Michael specializes in custom air handling applications, air-to-air heat recovery exchangers, and controls-related products including gas detection, airflow measuring products and control dampers.
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.