Abstract:
Ventilation systems in industry often undergo modifications due to process changes or to perceived need for different airflow for workers or workstations. It is difficult to determine the required modifications if one cannot predict the effects of making such changes to a system. In addition to planned modifications, there are also unauthorized alterations (by maintenance and operating personnel), adbrasive wear, and mechanical damage. Power balance coefficients computed from measured pressures and flows offer both a diagnostic tool for detecting changes to the system and to individual ducts, and means of predicting the effects of planned modifications.
The power loss coefficient (X) for a bounded volume (i.e., a section of the system such as a branch or a submain, or the entire system) is the unitless ratio of the power dissipated (LP) through the volume to the kinetic power (KP) at a reference cross-section of the volume. Earlier studies have shown that changes affecting airflow distribution can be detected by comparison of X values computed from measurements taken before and after modifications. This study tests the stability of X values in the absence of modifications and tests how accurately airflows and pressures resulting from modifications can be predicted through power loss methodology that employs values of X.
For the study five removable sharp-edged plate obstructions were designed to fit into 4 branches and 1 submain, and X values for each obstruction was estimated from preliminary data. Airflows and pressures were measured for the 5 branch ventilation system with no obstructions inserted. New airflows for each branch were predicted from the values of X from that baseline condition and from the expected changes to X values from inserting specific obstructions into specific ducts. Ten combinations of inserted obstructions were predicted. Later, obstructions were placed in the system in each of the configurations for which predictions were made, and the resulting pressures and flows were measured. Airflow predictions for each of the 10 combinations of obstructions were then made using the estimated values of X for the obstructions and measured values for the baseline condition. Airflow distributions in the system were compared to those predicted by the models.
The methodology predicted branch airflows with mean error of 0.50% and a standard deviation of 1.3%. Maximum overestimation was 3.65% and the maximum underestimation was -2.950%. This study provides direct evidence that power loss coefficients (X) defined for volumes are independence of airflows and changes to other volumes, a fundamental assumption of the power loss coefficients method.