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Laboratory Ventilation ControlVarious laboratory ventilation control schemes have been devised in efforts to save energy and at the same time maintain or improve worker safety levels. This paper examines some of these schemes with a view to helping a system designer deal with the often confusing and conflicting array of information available from a number of sources, and then specify what is appropriate for a particular application.
IntroductionThis paper is about laboratory ventilation control, specifically laboratory fume hood and space pressurization control. It does not cover other aspects of laboratory ventilation, such as operating procedures or construction methods and materials. Biological and chemical hazards are classified according to level of risk. There are four generally accepted levels with Level 1 representing the lowest and 4 the highest. This paper deals only with levels 1 and 2. It does not get into levels 3 and 4, which are very special applications. According to the Laboratory Biosafety Guidelines published by Health Canada, summary definitions of biosafety levels 1, 2, 3, and 4, along with their corresponding basic physical containment requirements are as follows: Level 1 - Low individual and community risk. Room separated from public areas by a closed door. Work on bench tops and without containment cabinets is allowed. Level 2 - Moderate individual risk, limited community risk. Self-closing doors. Work involving aerosols confined to Class I or II biological safety cabinets. Level 3 - High individual risk, low community risk. Away from public areas. Controlled access through lockable changing room. No recirculation of laboratory air. Negatively pressurized at all times. Written use protocols. Event recording system. Level 4 - High individual risk, high community risk. Geographically isolated and functionally independent. Air lock entry and exit. Class III biological safety cabinets. Dedicated ventilation system. Although the Laboratory Biosafety Guidelines give a definition of Class I, II, and III biological safety cabinets, this paper does not deal with them because they are essentially constant flow devices with no flow control. Of interest here is the ordinary fume hood with sliding vertical or horizontal sash and commonly used in chemical or biochemical laboratories. Though the Scientific Apparatus and Manufacturers' Association (SAMA) no longer publishes standards, its standard on fume hoods is often referred to. It classifies fume hoods according to chemical risk as follows: Class A - Used for materials of extreme toxicity. Recommended average face velocity is 125 to 150 fpm. Class B - Used for materials of moderate toxicity, which includes most materials and operations. Recommended average face velocity is 100 fpm. Class C - Used for materials of low toxicity. Recommended average face velocity is 75 to 80 fpm. It takes an expert in the field of industrial hygiene to know which type of hood is needed for any particular application. The decision is based on the substance involved and what effect exposure of a given duration to the substance has on a human being. Only in rare instances are Level 3 and 4 laboratories involved, so the majority of applications are governed by the guidelines applying to Level 1 and 2, whether for biological or chemical applications. A question of paramount importance facing the designer is what has to be done to meet the various laws, regulations, standards, codes, and industry practices that apply? In fact, what is a law, a regulation, a standard, and an industry practice? For the purposes of this paper, they are defined as follows: Law - This is a resolution passed in a session of government at any level. Regulation - This is the detailed wording that indicates how the law is to be implemented in practice. Standard - This is a recommended practice written by an organization recognized as a standards writing body. Code - This is a law or regulation that is sometimes an adaptation of a standard written in "code language." Industry practice - This is a practice used by experts in a particular field and recognized by the industry in general as what should be done even though there is no law, regulation, or code specifically stipulating the use of such a practice. Standards and industry practices may become as enforceable as though they were an actual part of the law, if they are invoked by a law or regulation under certain conditions, or if they are invoked by a specification as part of a contractual document. It is the responsibility of all the parties involved in the design and construction of a laboratory to be familiar with the various things that MUST be done or SHOULD be done. Fume HoodsAccording to the Canadian Standards Association (CSA), a fume hood is "an enclosed work space ventilated by an induced flow of air through the face opening intended to capture and contain gas, vapor or aerosol generated within the enclosure." The types of fume hoods discussed in this paper are: standard, bypass, 2-position, and variable air volume (VAV). Characteristics of a standard hood are:
Characteristics of a bypass hood are:
Characteristics of a 2-position hood are:
Characteristics of a VAV hood are:
Hood ControlAccording to various standards, fume hoods should be equipped with the minimum instrumentation needed to give visual and audible alarms of conditions of low or no flow. Some hoods have on-off switches to shut them off when they are not in use; however, hoods should always be on. Otherwise they cannot contain the dangerous substances the use of which called for hoods in the first place. Logically, if the substances in use are not considered dangerous, a fume hood is not needed. In addition to the basics, other instrumentation may be used, depending on the type of hood. For a constant-volume (CV) hood, the exhaust air flow is intended to be constant at all times. If the total supply and exhaust flow with which the hoods are associated remains constant at all times, there will be very little variation in the duct static pressure systems. Consequently, there will be very little variation in hood exhaust flow, which is proportional to the square root of the pressure drop across the hood. On the other hand, duct static pressure variations can be considerable if variable air flow devices (including CV hoods that can be turned off) are connected to it. For example, a static pressure change from 1.0 to 2.0 inches w.g. would cause the hood exhaust flow to increase by 50 percent. This would cause a corresponding change in face velocity, perhaps enough to invalidate a certification. To avoid such flow and face velocity variations, a CV hood can be equipped with a flow control device. This may be a crude device such as a manual balancing damper set to deal with average conditions. It may also be a Venturi type air valve, which maintains the flow constant regardless of the static pressure drop across the valve. The range of pressure over which the valve is pressure-independent is typically 0.5 to 3.0 inches w.g. For a 2-position hood, there are two possibilities. One is to operate a two-speed fan dedicated to the hood. Even if a fan serves only one hood, the fan should be mounted on the roof of the building such that there is no pressurized exhaust ductwork in the building. The second possibility is to control the flow with a two-position Venturi valve. This is a more desirable arrangement because the valve is pressure independent and because it is less likely that the exhaust ductwork will be under positive pressure. For a VAV hood, the exhaust flow is varied continuously to maintain a constant face velocity by either direct closed-loop control or indirect open-loop control. In direct closed-loop control, the exhaust flow is controlled according to a measurement representative of face velocity. This measurement may be the velocity of air entering an opening in the side of the hood (the sidewall sensor) or the velocity of the air passing through the lower airfoil of the hood (the airfoil pitot tube). There is no known way to measure the actual face velocity directly without having measuring instruments interfere with the operation of the sash. In indirect open-loop control, the exhaust flow is varied according to a measurement of sash position, which is effectively the open sash area. The face velocity is related to the open sash area and the exhaust flow by the formula FV = Q/A, where FV is the face velocity in feet per second, Q is the flow in cubic feet per second, and A is the area in square feet. With sash position control, the exhaust flow is controlled by either a closed-loop flow control loop or by an open-loop flow control device. In the closed-loop flow control method, a direct measurement of air flow is used in a flow control loop, consisting of a flow sensor (such as a pitot tube), a controller, and a final control element (such as a butterfly damper). In this method, the set point of the loop is varied according to the sash position measurement. In the open-loop flow control device method, there is no direct measurement of air flow. Instead, a flow control device (such as a Venturi valve or a variable speed drive operating a dedicated hood fan) is repositioned according to the sash position measurement. Since the flow control device has a known and repeatable relationship between flow and sash position signal, the exhaust flow can be regulated according to sash position to within five per cent of the desired value. When a Venturi air valve is used, it can maintain this level of flow accuracy over a wide range of static pressure fluctuations in the exhaust duct. Laboratory Pressurization ControlLaboratory pressurization is controlled either directly from a measurement of space static pressure or indirectly by air flow tracking. In the case of direct static pressure control, the static pressure in the space is maintained at a constant value, while the infiltration rate varies. For example, if a door is opened, the control system detects a momentary decrease in space pressure and corrects by decreasing the supply air flow rate, which in turn increases the rate of infiltration to maintain the set static pressure. A variation of this method involves controlling the velocity of air entering the laboratory through the main entry door. In the case of air flow tracking, the rate of infiltration into the laboratory is maintained at a constant value, while the actual static pressure in the space varies. If a door is opened, the space static pressure will decrease as the supply air flow rate is maintained constant. As with sash position control, the supply air flow is controlled by either a closed-loop flow control loop or by an open-loop flow control device. In the closed-loop flow control method used for supply air flow control, the set point of the loop is varied according to the measurement of the sum of all the exhaust flows from the laboratory. In the open-loop flow control device method, there is no direct measurement of air flow. Instead, a flow control device such as a Venturi air valve is repositioned according to the measurement of the sum of all the exhaust flows from the laboratory. Again, since the Venturi air valve has a known and repeatable relationship between flow and valve stem position, the supply air flow can be regulated according to exhaust air flow to within five per cent of the desired value, over a wide range of static pressure fluctuations in the supply duct. Although not impacting the control system directly, an important factor in laboratory ventilation is the ventilation rate, measured in air changes per hour (ACH). This is the number of times per hour that the air in the space is replaced. It has long been believed that increasing the ventilation rate increases the level of safety. To a degree, this is true; however, there are diminishing returns beyond six ACH, according to the Handbook of Laboratory Safety. In addition, recent studies have shown that placing air diffusers and return grilles so that there is a sweeping effect throughout the space is more efficient than simply increasing the laboratory ventilation rate. Control System Performance The performance of control systems in general has been analyzed using advanced mathematical techniques. In industrial process control, many processes have even been modeled and simulated on computer. In addition, there is much empirical data available on the behavior of process control systems of all types. This is not the case for laboratory ventilation control, which appears to be more of an evolution of the techniques used for ordinary heating, ventilating, and air conditioning (HVAC). HVAC control has always taken a distant second place to process control in terms of quality and reliability. This is probably because a failure in a control system does not have the same disastrous economic consequences in HVAC as it could have in a manufacturing process where an off-spec product might cost a company millions. Nevertheless, laboratory ventilation control systems affect human safety. Consequently, they will probably someday be subject to the same analytical scrutiny as process control systems. In the meantime, there is a convenient way to understand the behavior of control systems by means of simplified block diagrams. Each block represents one of the various elements affecting a control loop and facilitates discussion of the key concepts:
StandardsAlthough much can be said about laboratory fume hood and pressurization controls, the real issue to focus on is containment of hazardous substances. In the case of Level 1 and 2 laboratories, hood performance is of primary concern since Level 1 and 2 laboratories do not have the stringent space pressurization control requirements of Level 3 and 4. It comes down to evaluating how well the hood does its job of containment with its various configurations, control systems, and applications. Of major concern is how containment is measured. With a smoke bomb? With a tracer gas? And under what conditions - factory or installed - static or dynamic? The only performance standard that seems to be in use in North America is ASHRAE 110-1985R, which measures the performance of the hood under static conditions. When this test is done it is usually in the factory of the hood manufacturer. This test is rarely done once a hood is installed, because of the cost and the difficulty in duplicating the test conditions stipulated by the standard. A revised version of the ASHRAE 110 standard has been issued for public review and acceptance of it is expected at the January 1995 ASHRAE meeting. To quote from the foreword of the standard, "There is a need for a performance test that can be used in the field to establish an as used performance rating, including the influences of the laboratory arrangement and its ventilation system." Nevertheless, the revised standard does not deal with the following important factors in the safe operation of laboratory fume hoods:
The ASHRAE 110 standard does not stipulate required performance levels. Rather, it describes a test procedure for determining the performance under a set of specified conditions. The standard could be part of a specification once the required performance level is determined. The types of conditions for which the standard can be used are: as manufactured (AM), as installed (AI), and as used (AU). Whether AM, AI, or AU, the method outlined comprises three tests:
Flow visualization consists of two tests - a local challenge whereby small amounts of smoke are released at specific points inside and outside the hood and a gross challenge whereby large quantities of smoke are released inside the hood. These tests are meant to give a qualitative visual indication of hood performance, except in the case of the local challenge where the hood is deemed to fail the test if any smoke is seen to leave the hood. Face velocity measurements are meant to give a quantitative indication of hood performance. The method for measuring face velocity is described in the standard. Tracer gas containment tests measure the containment ability of the hood under specified conditions. This is primarily a test of the hood under static conditions, except for the one which measures the containment while the hood sash is moved according to specified parameters. With respect to laboratory pressurization for Level 1 and 2 applications, the Health Canada Biosafety Guidelines prescribe no written requirement, except as called for by local building codes. The reason for this is that containment of hazardous substances is controlled by biological cabinets rather than by ventilation of the laboratory. This is very significant because it implies that safe containment is determined much more by the hood than it is by laboratory pressurization or ventilation rates. Nevertheless, negative pressurization is recommended by many sources, for example:
NFPA 45, which deals with fire hazards and is often included in building regulations, states (p45-12, S6-4.2), "Laboratory units and laboratory work areas in which hazardous substances are present shall be maintained at an air pressure that is negative relative to the corridors or adjacent non-laboratory areas." This good advice applies equally to biological and chemical laboratories. In principle, if there is a recognized requirement for a fume hood, this means that workers are dealing with substances too hazardous to be experimented with on an open bench top. Thus, it follows that any space with fume hoods should be negatively pressurized in conformance with NFPA 45. As respected as it is, NFPA is not specific about transient behavior of laboratory pressurization. For example, it does not address the possibility of the negative pressurization being momentarily lost or reversed. It is left to the judgement and common sense of the designer. For Level 1 and 2 applications, this would not appear to be a problem, since containment is afforded primarily by the hood. Value EngineeringGiven that the most current standard deals with only a limited set of conditions compared to the actual working environment of a fume hood, it is frequently a challenge for a designer to know exactly what to do in terms of fume hood control. One method of dealing with the often conflicting advice from experts in the field and from suppliers' marketing departments is to use a form of value engineering called functional analysis, which involves the following steps:
As an example, in the case of fume hood containment, factors affecting performance could be listed in random order as follows:
Additional factors, classified as dynamic challenges, could be as follows:
In the value engineering exercise, these factors are usually shown as a number of smaller circles surrounding a larger circle, which represents the objective - in this case containment. Following is a list of typical factors that apply to containment:
AuthorDonald A. Coggan, PE, is recognized internationally as an expert in the field of control systems design and training. In addition to consulting directly to clients in the United States and Canada, he has addressed groups throughout North America as well and in Europe and Asia. He is the originator of a design evaluation technique called "Specifying for Maximum Value" based on principles set out by the Society of American Value Engineers (SAVE). Mr. Coggan has authored numerous technical publications including a training system and accompanying software for instrumentation technician evaluation for the Instrument Society of America (ISA). He has also co-edited Fundamentals of Industrial Control, the flagship volume of the ISA's Practical Guide Series. Return from this laboratory ventilation page
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