Laboratory Ventilation Systems: An Analysis of the Function and Flow Stability of Different Ductwork Design Alternatives
Abstract: One of the prerequisites for high safety in laboratories for chemical and microbiological research is a well-functioning ventilation system. The most typical safety measure in these laboratories is to use a fume hood as a place for activities that can cause dangerous emissions to the room air. The protective function of fume hoods must never be jeopardised. One of the prerequisites for this is that the exhaust air flow through the fume hood must be kept stable. It is known that the pressure fluctuations in the ventilation system can cause leakage of contaminated air from the fume hood. The laboratory ventilation systems have ductworks for air supply and air exhaust. In order to achieve good stability in the exhaust system, the airflows in both systems and the premises should be well-controlled and the correct air flow balance must prevail in the laboratory space. Most of the contemporary laboratories are multi-storey buildings with the centralised VAV ventilation systems. A principal layout of their ductwork can vary. In traditional systems the ducts to individual floors are routed separately. An alternative design, which is common vertical duct instead of multiple separated ducts, is attractive due to somewhat decreased need of space and has been applied in a few new laboratories. This thesis presents a comparison of these design alternatives based on computerised isothermal empirical models of the systems. The models were developed in an internal fluid flow network simulation software (Flowmaster) and the verified. The verification procedure was carried out in two stages: first a test rig with a single fume hoodwas tested and simulated, then the ventilation system in a large functioning chemical laboratory was investigated. The software provided satisfactory prediction of the transient processes that were the consequences of system's excitation with fume hoods during the field experiments session. The field tests proved also that rigid duct model provides a good estimation of pressure wave propagation in the ventilation systems. After the verification of the model, a sensivity analysis of various design, tuning and operation disturbances that might occur in the exhaust systems was performed. As the total number of factors was 10, a fractional factorial design of two levels of the simulation runs was applied. The sensivity analysis showed that an exhaust system with separated vertical ducts and that with common vertical ducts have no significant differences in their performance. Furthermore, the air volume in the common duct has a damping effect and increases the stability of the system. Also the complexity of the control system must be in accordance with the size of the ventilation system. In large systems, a partition of the system into multiple control levels is recommended.
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