Low-Momentum Supply System

Low-momentum air supply systems designed for local ventilation purposes mainly use vertically downward airflow. Some systems with an inlet of low-momentum horizontal airflow for a whole workroom are on the borderline between general and local ventilation and are therefore briefly described here. A more complete description will be found in chapters 7 and 8 dealing with general ventilation. 

Horizontal displacement ventilation (see Chapters 7 and 8) is a ventilation principle mainly applied to general ventilation of workrooms. In some instances, a local ventilation problem may be solved by building a separate enclosure or a room around the workstation and arranging for a general ventilation in that enclosure. An example where that principle has been utilized is the control of emissions of and worker exposure to styrene vapors during 

FIGURE 10.48 The air shower principle creates a zone of clean air around the worker and pushes the contaminant plume away from the breathing zone. 

Different Fomis and Boundaries ftelstive to Other Types 

A common supply air unit used as an air shower has a flat underside covering an area over rhc workplace with a typical size approximately 1 m (Fig. 10.49). Another type of unit is shaped as a half sphere, distributing the air over an area much bi er then the cross-section of the unit (Fig. 10.490L Other iype,s of supply air units such as a textile tube may be used for the same purpose. 

---

Since the low-momentum supply system should enhance the efficiency of an exterior hood by supplying low-momentum airflow to a source, the system can be applied to practically any sources where an exterior hood can be used. In particular, it is effective to apply the system when an exterior hood cannot be placed close to a source or the exhaust direction is different from the initial contaminant release direction. 

The low-momentum supply system could also be applied to operations inside booths. If a worker must be inside a booth, to protect the breathing zone, a supply inlet with a relatively wide area is placed above the worker and the low-momentum clean air is blown toward the worker. At the same time, the airflow could transport contaminants to the exhaust outlet. 

In the low-momentum supply system, the contaminants are emitted within the low -momentum airflow blown from the supply inlet and they are transported to near the exhaust opening. If the contaminants diffuse into the whole of the supply airflow, the exterior hood must exhaust the whole of the airflow. To diminish the exhaust flow rate, some methods to prevent the contaminants from diffusing into the whole of the airflow are required. One possible method is to supply the air as slowly as possible but with enough velocity to reach the exhaust outlet and to control the surrounding air motion. Another method is to blow supply air with uniform... 

Applying the flow ratio method to the low-momentum supply system, the required exhaust flow rate is often in excess of practical values. This is because the value of is given as the value at which all the supplied airflow should be exhausted by the exterior hood. In the low-momentum supply system, contaminant sources should usually be between the supply inlet and the exterior hood. The supply airflow is contaminated at the position of the sources and it flows to the exterior hood. Therefore, all of the airflow is not always contaminated. Unfortunately, a design method considering such cases (the diffusion of contaminants within the airflow) has not been established yet, and the appropriate exhaust flow rate has to be adjusted after the system is installed. 

FIGURE 10.87 The fundamental operation and dimensions of the low-momentum supply with exterior hood system. 

An established design method for this type of system is not available. The practical design of the low-momentum supply with exterior hood system described in the previous part of this section used the flow ratio method. How-evec, the actual exhaust flow rate was adjusted visually to the appropriate value in order to exhaust only the contaminants transported by the supply airflow. 

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. 

The discharge from both domestic and industrial wastewater treatment plants (for example, from activated sludge processes) has been a low-cost source of makeup for cooling systems for many years. Efforts to reuse water continue to gain momentum for environmental conservancy and economic reasons, but the fact that secondary use waters are, by definition, of a lower grade than other supply sources inevitably means an increased risk of deposition or fouling problems in the cooling system. 

---