These systems can be inside large halls and may have no fixed limits for their in­fluence, except for some parts of the system (inlet device surface, etc.) They can also be situated inside small rooms, where walls, floors, and ceilings are the natu­ral boundaries. The systems usually consist of one exhaust hood and one supply inlet, which interact. There are also special combinations, as two or more inlets and one exhaust hood, or one supply inlet and two or more exhausts. All of these combinations need careful design and an accurate relation between supply and exhaust flow rates and velocities. Some systems also need stable temperature con­ditions to function properly. All combinations are dependent on having a defined contaminant concentration in the inlet air. This usually implies clean supply air, but some systems may use recirculated air with or without cleaning.

There are many possible combinations of supply and exhaust air. For exam­ple, a line jet could be used as a shield in an opening, as a stripping system on surfaces, for blowing contaminants into an exhaust, etc. An enclosure could be designed with a line jet in the opening, with a wall jet inside to increase effi­ciency, or with a low-momentum jet inside or outside the opening to replace the room air supply. In this section, only some basic combinations are described.

All rooms need both supply and exhaust air. The combined systems de­scribed in this section are unique in that they must be chosen and designed si­multaneously, since the aim is to get a Specific Flow field—dependent on the exhaust and the supply simultaneously—inside the specified volume. The flow rates are usually not the same in the supply and the exhaust parts of the system. This means that the same flow difference must exist in the room’s ventilation system, if the combined system takes its air from outside the room and disposes of the air to the same place. There is no general rule for how to choose between the systems, except what has been specified for exhaust hoods and supply inlets, respectively. Usually, the choice is made by tradition or the equipment available. This should not prevent the designer from exploring the relative advantages and disadvantages of these systems (which are described for each system) and apply­ing and designing a system that suits his or her demands for a specific process.

Similar to supply inlets, no measurements exist for evaluating the inlets’ spe­cific influence on contaminant concentration. The available measurements for the combinations are the same as for exhaust hoods, i. e., capture efficiencies and sim­ilar measures. Sometimes the performance of a combined system can be approxi­mated from the performance of the incoming supply inlet and exhaust hood.

Earlier descriptions for exhaust hoods and supply inlets will not be repro­duced in this section.

For a combined system it is always important to consider how the person working with a process and the contaminant source are placed in relation to the inlet and outlet. Exactly what their relative positions should be to get the highest efficiency depends on the specific system.

For a push-pull system, the source is usually an open surface tank and the airflow acts as a horizontal curtain above the surface. In this case, the person could be anywhere as long as the system works as intended and the curtain is not broken. The curtain will be broken when parts or material are lifted out of or placed into the bath and the contaminants could be spread either through convection or because the supply air blows against the material or part.

For workbenches or laboratory fume hoods with auxiliary supply it is the working person wrho could break the shielding curtain, and in that wav con­taminants are transported from the interior of the exhaust hood to the space where the person is situated.

For unidirectional rooms the placement of inlet, source, and person is as important as when using exhaust hoods. For hospital isolation rooms, it is not only the inlet, outlet, and the person who influences the contaminant concen­tration; the design of the room and the handling of the room’s door are also important.