Arkansas HVACR NewsMagazine November 2019

S tate, National, Chapter News Tech News

Spread is the measurement in degrees of how wide the delivery of air in a particular space is. Generally speaking

flow can result in drafty conditions. As air temperatures are reduced velocities must also be reduced to avoid discomfort. This fact emphasizes the idea air flow must be managed in the occupied zone of individual rooms to avoid drafts. Air velocity of 150 fpm or more will blow light objects about. Air at a velocity of 65 fpm can blow paper off a desk. At 15 fpm smoke hangs in the air. Our guide for comfort from ASHRAE states summer conditions should be 76 degrees to 82 degrees and winter conditions should be 69 degrees to 78 degrees at 30% relative humidity. At 60% relative humidity those temperatures fall to 74 degrees to 78 degrees in the summer and 68 degrees to 75 degrees in the winter. Air motion in the occupied zone should be no more than 30 fpm in the winter and 50 fpm in the summer. It is also important to realize there may be a temperature gradient of 5 degrees between 4 inches above the floor to 67 inches above the floor during winter conditions. Our targets for comfort when analyzing grilles placement will include the following; a. An acceptable noise level b. No stagnant air in the occupied zone c. Velocities between 20 to 50 fpm in the occupied zone d. Supply air mixes with room air completely e. No high velocity interference between air supplies

most rooms only require one way grilles due to the fact air naturally spreads at approximately 22 degrees. After years of studies and comparisons, use of high sidewall grilles is the most effective method of distributing air unless cold floor loads are anticipated. These terms are necessary and useful to diagnose issues with existing systems. All too often we fail to analyze the quantity and velocity of air delivered to a space. We deal in generalized terms and fail to identify room to room differences. Some consumers can identify differences in air temperature down to as little as two degrees. In winter humidity can be a factor to comfort however, the human body is more sensitive to changes in air and surface temperatures. During cooling season humidity management become critical. The effects of high velocity air