Stability and accuracy of variable air volume box control at low flows. Part 1: Laboratory test setup and variable air volume sensor test

Abstract

Variable air volume systems with direct digital controllers have been widely adopted in the HVAC system of commercial, industrial, and large residential buildings because they provide better energy efficiency and occupant comfort. Normally, a variable air volume terminal unit defines a minimum airflow rate to satisfy the space ventilation requirement and/or the proper operation of a terminal heating coil, if so equipped. However, it has been found that variable air volume terminal units often fail to perform as expected at the minimum airflow range (below 500 fpm [2.5 m/s]). Under such a flow range, the embedded airflow sensor becomes inaccurate, and the designed minimum airflow rate is less than the minimum controllable airflow rate. This results in a series of problems, including lack of ventilation, uneven airflow control, reduced damper and operator life, and energy waste. Through designed laboratory and field tests, this study (ASRHAE Research Project RP-1353) aims to identify the major factors that cause inaccuracy and instability issues in variable air volume terminal units and the relationship between the major factors and performance of the airflow sensor, controller, and terminal unit system. Laboratory tests performed in this study included a variable air volume sensor test, controller test, and system test. Four variable air volume boxes from three manufacturers and four controllers from four manufacturers were tested systematically. Two identical test beds with high accuracy (±0.5%) reference airflow meters were designed and constructed in the test facility. The size of the reference airflow measuring stations was carefully selected to provide maximum airflow measuring accuracy and maximum available system pressure drop. This article describes the laboratory test setup and summarizes the variable air volume sensor test results. A companion article summarizes the controller test, system test, and field test results. From the variable air volume sensor test, three factors, namely, inlet conditions, low variable air volume damper positions, and low airflow rates, are identified as strongly impacting variable air volume terminal unit performance.