Enclosed Rooms
10.4.7.1 General
There are many different combinations of supply inlets and exhaust enclosures, where the system has been designed as a whole. Two of these enclosu res are described in some detail: abrasive blasting rooms with a person working inside the enclosure, and hospital isolation rooms.
Many other combinations exist but will not be described in the following sections. For example, in the electronic and medical industries it is quite common to use so-called unidirectional (UDI) rooms.53 These utilize the same airflow principles as those used in abrasive blasting rooms (Section i 0.4.7) and in cabinets with filtered air (Section 10.3.4), i. e., the air is supplied through one entire wall or the ceiling and is exhausted through the entire opposite wall or the floor.14 In these cases, it is necessary to design the supply distribution carefully to avoid large turbulent eddies and so that the air velocities created are not uncomfortably high for the workers inside the room.
Most of the combined systems could be designed using information provided in Sections 10.2 and 10.3 and taking into account the mutual influence ot the supply and the exhaust system. Many of these local ventiiation systems for small enclosures are very similar to general ventilation systems for rooms and could be designed using the methods described in Chapter 8.
It should be noted that the primary purpose of the ventilation systems described for abrasive blasting rooms and hospital isolation rooms is to prevent or minimize exposure to hazardous substances in those persons working outside the blasting or isolation room. The ventilation system may also reduce exposure for workers inside these rooms, but often the reduction is not sufficient to eliminate the need for respiratory protection.
10.4.7.2 Abrasive Blasting Rooms with Person Inside
General
Abrasive blasting normally takes place inside a special room, cabin, or exhaust hood, but if very large equipment must be treated it is usually done without a protective surrounding. When using exhaust hoods and small cabins, where the worker is located outside the hood or cabin, it is possible to work without further protective equipment. Inside an abrasive blasting room, it is always necessary for the worker to wear a protective helmet supplied with breathing air, since the concentrations of the blasting material and other contaminants are very high, irrespective of the ventilation.
The ventilation in an abrasive blasting room has three main functions. The first is to transport the generated dust to the exhaust during rhe work in such a way that good visibility is achieved. The second is to eliminate, as fast as possible, the dust in the room after the work has ended. The third is to prevent unrestricted dispersal and backwash on the blast operator and the machinery and to keep the dust inside the room, preventing exposure to personnel working outside the room.
The blasting equipment and the air cleaning equipment may be placed outside or inside the room.
Principle
The air is supplied by a ventilation system or from the surrounding area. In both cases, the air is supplied through a diffusing device that covers the larger part of either one wall or the ceiling. The air is exhausted through another wall or the floor. All required openings on abrasive blasting enclosures should be designed with baffling to prevent unrestricted dust leakage. The air flow is intended to be unidirectional and directed from one wall to the opposite wall or from the ceiling to the floor. However, it is not necessary to have unidirectional flow and there are some advantages to good mixing of the air in the room. Good mixing may be achieved, for ex ample, by having the air enter through the ceiling and exit through slots along both longer sides of the floor. Figure 10.103 shows different ways of supplying and exhausting air. Several recommended configurations are shown, as well as two configurations that are not recommended and should be avoided.34
The configuration B1 uses the same flow distribution inside the room as is common in horizontal flow cleanrooms.
Applicability of Sources
The size of the room determines the size of objects that can be treated in the room. The main contaminant source is the blasting equipment and the abrasive material that impacts on the treated surface. The object being blasted can also be a source of hazardous contaminants. For example, removal of paint, which contains lead, cadmium, chromates, or other metals, may result in hazardous concentrations of these contaminants. Removal of fused sand from new castings may result in elevated silica exposure even when non-silica-containing abrasives are used. F, xcept for the size of the room, there are usually no restrictions on the use of these rooms. If the abrasive used is highly dangerous, such as silica sand, the demands on the preventive equipment and the room are higher than when using slag or steel shot. This kind of system may also be used for spray painting small and large objects.
Different Forms and Boundaries Relative to Other Types
These rooms are usually rectangular with doors on one of the short sides for entrance and exit of materials and operators. Room sizes vary from 18 mJ to more than 300 m5 and the shapes range from cubic to elongated with rectangular cross-section.
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B3 |
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There are many specific and derailed regulations related to use of blasting equipment, especially when used by an operator inside a room or when using silica sand as an abrasive. These regulations differ from country to country and include specifics on exposure limits for air contaminants (especially for silica dust) and sound levels, airflow rates in the rooms, personal protective equipment, and blast system design and maintenance. One example regarding ventilation is the German regulation that an abrasive blasting room must not be entered without a breathing mask until the room has had 50 air changes of clean air following the cessation of blasting.55
Design Equations And/or Parameters
There are very few design recommendations. The following ones are taken from the literature. The NIOSH reference covers nearly all aspects of how to design an abrasive blasting room.56
Table 10.13 gives recommended ventilation rates for Germany. »
For these rooms, the air should flow horizontally from one end to the other with a velocity greater than 1.3 m s_1. The necessary airflow rate is of course calculated as the volume times the number of air changes per hour.
According to INRS,54 the supply air velocity should be 2 m s_1. For vertical ventilation (from ceiling to floor), a flow rate minimum of 0.11 m3 s-1 m 2 should be used. The cross-sectional area used in the calculation is the room length times its width. When using horizontal ventilation, the flow rate should be greater than 0.28 m3 s~* nr2. In this case, the cross-sectional area is equal to the room width times its height. These values should be increased by 40% when very dusty operations are performed, when the generated dust is dangerous, or when the room is extremely long (longer than 15 m). According to the reference, these flow rates will result in a contaminant concentration equal to the concentration before work in approximately 100-120 seconds after blasting has stopped.54
According to NIOSH,56 ventilation should be from ceiling to floor or horizontal, but never from floor to ceiling. Vertical (downdraft) ventilation is preferred to prevent leakage through cracks and holes in the room’s walls. The recommendation is to use a flow rate of 0.1 m3 s"4 m-2 when using nonsilica blasting material and 0.4 ni3 s ’ m-2 if the abrasive contains silica,
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TABLE 10.13 Recommended Air Changes for Abrasive Blasting Rooms5
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Horizontal ventilation is acceptable if the flow rate is equal to or greater than that recommended for vertically downward ventilation. A minimum entrance velocity for the supply air of 1.2-1.5 m s_1 is also recommended. To provide good visibility during blasting, an air change rate of 50 h 1 is also recommended, even though the recommended flow rates result in higher air change rates,
10.4.7.3 Hospital Isolation Rooms
General
Infectious patients present a difficult challenge when trying to protect health care workers. These patients must be isolated from the health care workers as well as from the other patients in the hospital. Special isolation rooms are used for this purpose. These rooms are generally used for isolation of infectious tuberculosis (TB) patients, but could be used for patients with other airborne-transmitted diseases. In the United States, there were 22 812 new cases of tuberculosis in 1993, equal to 8.7 per 100
0 population. This represents a 2.8% increase since 1985, following a 6-7% annual decline from 1981-1984.57>5fi Several studies have documented higher than expected tuberculin skin test (TST) conversion rates in hospital personnel.59-63 The National Institute for Occupational Safety and Health64 reports that multiple-drug-resistant (MDR) strains of TB have been reported in 40 states and have caused outbreaks in at least 21 hospitals, with 18-35% of exposed workers having documented TST conversions.
A number of these outbreaks gained national attention in the U. S. in the late 1980s and early 1990s. In response to these outbreaks and the identified risk of TB infection in health care workers, the U. S. Centers for Disease Control and Prevention (CDC) issued a series of guidelines for preventing TB transmission in health care facilities.65’66 These guidelines include the traditional hierarchy of control for occupational hazards which puts primary emphasis on engineering and work practice controls and secondary emphasis on personal protective equipment. The engineering controls used for TB include isolation of the source, in this case an infective patient, and dilution ventilation. Administrative controls include a written infection control program, training of health care workers, and medical surveillance of at-risk workers. For workers who must enter isolation rooms housing infective patients or who are present during cough-inducing procedures, respiratory protection is necessary.
Principle
Specific ventilation recommendations were included in the CDC guidelines. The recommendations for isolation rooms serve two purposes. The first is dilution of contaminant, in this case droplet nuclei in the isolation room. The second is isolation of the space occupied by known or suspected TB patients from adjacent areas. The CDC65,66 and Conroy, et al.67 present recommended operating conditions for isolation rooms. These three conditions are presented in the Different Forms and Boundaries Relative to Other Types section below.
The principle behind all of the conditions is to use the direction of airflow to prevent contaminated air from traveling out of the isolation room to other areas of the hospital. The direction of the airflow is controlled by creating and maintaining a pressure differential between the space containing the contaminated air and adjacent areas. Additionally, sufficient air must flow through the space to dilute the contaminant concentration to as low’ as feasible.
The sources for this type of control are infectious hospital or clinic patients, where the infection can be transmitted through the airborne route. The most common application is for control of the spread of tuberculosis, but it could be used for other airborne infections such as varicella or influenza/18
Different Forms and Boundaries Relative to Other Types
The different types of infectious isolation rooms are shown in Fig. 10.104. This figure shows three designs which all use an anteroom. It is possible to have isolation without the use of an anteroom, but that is not the preferred method. The three examples shown here are (1) strong negative pressure in room, balanced pressure in anteroom; (2) balanced or positive pressure in room, negative pressure in anteroom; (3) balanced or negative pressure in room, positive pressure in anteroom.
The CDC-recommended ventilation strategy is condition 1, with negative pressure isolation driven by excess exhaust air in the patient room. This is the most effective strategy for protecting workers and other patients. However, the pressurized and depressurized anteroom strategies (conditions 2 and 3} can provide reasonable isolation as well as airflow protection for certain pa tients who need it (e. g., immunosuppressed patients). The limitations of these latter two strategies (contamination of anteroom, loss of isolation when doors are opened, reduced dilution airflow) must be understood by the hospital workers tor their protection. When protection of the patient is an overriding concern, these two strategies are reasonable alternatives.
Good preventive maintenance programs are needed at the hospitals. Egg — crate-style flow straighteners or other duct obstructions should not be used because they collect lint and block air flow. Exhaust discharge screens and induct fan motors also accumulate lint and reduce airflow. Leaky pre-filters quickly clog high-efficiency particulate air (HEPA) filters with lint and can negate the excess exhaust capacities in the rooms. Airflow and pressure gauges that are not maintained can deceive the hospital and maintenance staff about the performance of the isolation rooms. Windows that are open or ajar have uncontrollable and detrimental effects on room isolation. Maintenance and wrard workers who are well informed about the operation and care of the Isolation room systems are necessary for effective control.
Door sweeps were effective in controlling isolation of the rooms as long as negative pressurization of the room existed. Although they are a useful supplement to ventilation controls, they can be damaged so that they do not close effectively. Therefore, the preferred way to test these rooms would be with the door sweep retracted so that door gap velocity could be measured.
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Condition 1: Negative pressure isolation room strategy
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Omdition 2: Negative pressure anteroom strategy
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Condition 3: Positive pressure anteroom strategy
FIGURE 10.104 Isolation strategies using anterooms.
Design Equations and/or Parameters
A dilution ventilation rate of at least 6 air changes per hour (ach) is recommended, with 12 or more ach recommended for new construction or renovation. This may not provide sufficient dilution to allow workers to enter without respiratory protection, but it is considered a feasible dilution rate that will reduce the risk of infection for those workers who must enter the room with respiratory protection. Dilution also reduces the contaminant concentration and therefore the risk when temporary leakage from the room occurs such as when doors are opened or closed.
CDC recommendations for isolation are shown in Table 10.14. Conroy et al.67 present a modified version of these criteria to take into account different isolation conditions (Table 10.15).
In all cases, it is necessary to supply adequate makeup air while still maintaining appropriate pressure differences. A study of five hospitals in
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> 0.024 m3 s-> > I).,5 m s ‘1 < -0.25 Pa Anteroom or hallway to patient room |
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Chicago67 showed that lack of mechanical makeup air resulted in contaminated air reentering the hospital, and therefore these rooms were not isolated from adjacent spaces.
The same study showed that negative pressure rooms with balanced anterooms (condition 1) generally had sufficient negative pressure when there was at least 24 L s_1 excess exhaust in the patient room. Rooms using pressurized anterooms (condition 3) provided an excess of supply air to the anteroom, thereby pushing system air into the patient room and into the hall. The advan —
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TABLE 10.15 Isolation Room Criteria for Three Different Anteroom Conditions67
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Tage of this strategy is that it protects the patient from hallway air contaminants. However, most of these rooms failed to provide the recommended dilution airflow rate and necessary excess anteroom supply air. Condition 2, although not observed in this study, uses a strongly depressurized anteroom to induce airflows from both the hallway and the patient room. By design, these rooms fail the CDC guidelines66 for isolation because of the reversed airflow into the anteroom. However, the rooms provide isolation at the anteroom barrier and protection for the patient from all adjoining spaces. The disadvantage of this strategy is the need to don respiratory protection before entering the anteroom.
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