Arkansas HVACR NewsMagazine January 2022

HVACR NewsMagazine January 2022

Tech News

built. Outside of those roadblocks, is the fact today’s systems move air more efficiently than older systems. Today’s variable speed blowers insure air flow. Older fans simply slipped in the airstream when air met resistance. Two objects must be addressed on every system as the requirements are never the same. CFM or volume of air is the object we generally understand. Velocity or speed at each point the system, not so much. Quantity (CFM) will remain constant only if the velocity is designed in at each juncture. Even more important than air speed through the return grille, is velocity through the filter. The general standard for efficient filter flow is 300 FPM. This limit requires significant increases in surface area beyond what the bottom of the furnace provides. If necessary, please review part one of this series by clicking on the link below. https://user-czmy2oa.cld.bz/arkansas- hvacr-newsmagazine-may-2021/52/ Many, if not most one-inch fiber glass filters require velocities as low as 200 FPM. If you follow one-inch filters to their logical end, you’d find 10 square feet of filters necessary on a five-ton nominal system. We seldom pay attention these values and wonder why evaporators become restricted in short order. In other words, one-inch filters have no place in today’s HVAC market. Even highly regarded media filters often advertise 2000 CFM air handling capacities with a handicapped static pressure of .5 IWC or more. It is up to the contractor to understand these statistics are for reference and our design criteria from the field is critical. Systems of over 3-ton capacity, always require more than

one media filter. Efficient air flow in some cases will require multiple filters on smaller systems. These facts are not facts of just economy. They are facts of operation and laws of physics. If you want the correct air flow and filtering you must design for each application.

Path from terminal device to return area- From every terminal device, there must be an unimpeded route back to the air handler. We sometimes forget the largest damper in the HVAC system, are closed doors. No air flow out of the room, equals no air flow into the room. When sizing returns, jumper ducts or returns to the plenum we must remember the supplies are fan driven and returns are static driven. Generally, any room that requires a supply should have a 10” return at a minimum. 8” supply requires a 12” and a 1 0” supply requires a 14” static return. These sizes insure flows back to the