An Overview of Push—Pull Ventilation Characteristics

Abstract

Push—pull ventilation has been used as an engineering control technique for a number of years. It can provide efficient and cost effective control in conjunction with local exhaust. It is effective where control by local exhaust alone is not possible. However, its use has been limited due to a lack of design criteria and, in a large part, due to a lack of knowledge of the physical characteristics of the push—pull system components and their interaction with each other. Recent work by National Institute for Occupational Safety and Health (NIOSH) researchers has led to the development of design criteria for a number of processes including open surface vessels, foundry casting cleaning, roller mills used for rubber and plastics manufacture, and multiple opening presses used for laminating wood. In addition to providing the design criteria for these processes, the research has provided observations and data regarding the characteristics and limitations of the push—pull technique. For example, the belief that the push jet must have a high nozzle exit velocity can lead to a jet with excessive momentum and flow rate which can overpower the exhaust flow and spill contaminated air into the workplace. It also has been shown that low momentum push jets are very effective and are relatively insensitive to obstacles in the path of the jet. Many times push—pull has not been utilized due to the belief that it is effective only over short distances. However, a push jet can be used effectively over distances of 6 m (20 ft) or more. This article provides generic guidelines for the design of new push—pull ventilation systems and for the evaluation or correction of existing ones.