the impact of facility hvac systems on biological … · facility hvac systems on biological safety...

THE IMPACT OF FACILITY HVAC SYSTEMS

ON BIOLOGICAL SAFETY CABINETS

Prepared for the: RPIC/SLCan Sustainable Labs Regional Workshop

Presented by:

BSC, Biosafety Cabinet, Bio-Hood and Class II Laminar Flow Hood

Biological Safety Cabinets are referred to by several different names, some of the more common

are:

Unfortunately these cabinets are also commonly called and labeled “Fume Hoods”

PROCEDURES INVOLVING THE USE OF HAZARDOUS GASES AND VAPOURS WHICH REQUIRE THE USE OF A FUME HOOD

ARE SPECIFICALLY PROHIBITED IN BIOLOGICAL SAFETY CABINETS

BSC Biosafety Cabinet Bio-Hood Class II Laminar Flow Hood

NSF/ANSI 49 – 2010 NSF International Standard /ANSI American National Standard Biosafety Cabinetry: Design, Construction, Performance, and

Field Certification

This Standard, usually referred to as NSF 49, provides the following definition for Class II Biological Safety Cabinets.

“A ventilated cabinet for personnel, product, and environmental protection having an open front with inward airflow for personnel protection, downward HEPA/ULPA filtered laminar airflow for product protection, and HEPA/ULPA filtered exhausted air for environmental protection.”

This class of cabinet is sub-divided into four types

Class II:

NSF/ANSI 49 – 2010 NSF International Standard /ANSI American National Standard Biosafety Cabinetry: Design, Construction, Performance, and

Field Certification

NSF/ANSI 49 – 2010 DEFINITIONS

Class II type A1 (formerly designated Type A)

Class II type A2 (formerly designated Type B3)

Class II type B1

Class II type B2


Class II:


While any of these cabinets can be connected to an external exhaust system, there are distinct differences between them that need to be understood before design and/or adjustment of the exhaust system can be made without affecting the performance and safety of the cabinet.

Class II Biological Safety Cabinets AIRFLOW CHARACTERISTICS

While any of these cabinets can be connected to an external exhaust system, there are distinct differences between them that need to be understood before design and/or adjustment of the exhaust system can be made without affecting the performance and safety of the cabinet.



Like fume hoods Class II cabinets use low velocity air flow into the cabinet to provide containment Unlike fume hoods they also provide HEPA filtered downflow air over the work surface These airflows are determined by the manufacturers but are tested for performance by NSF. The permitted airflow ranges are very small typically no more than 10 fpm from maximum to minimum for either inflow or downflow


One of the more common causes of Biological Safety Cabinet performance failure is relatively high velocity air discharge from room supply diffusers disrupting the low velocity Biological Safety Cabinet's inflow air barrier. Low velocity, large area supply air diffusers should be used wherever Biological Safety Cabinets are to be used regardless of cabinet type


The following examples are specifications taken from the

certification requirements provided by NSF

Averaged air velocities as small

as 1 fpm above or below the indicated ranges are not

acceptable for certification to NSF49


NuAire 407-424 Class II type A1

Inflow Velocity 75 - 85 fpm Inflow Volume 242 - 274 cfm Inflow Variance 32 cfm Exhaust Volume 242 – 274 cfm


NuAire 425-400 Class II type A2



ThermoForma 1142 Class II type B1



ThermoForma 1148 Class II type B2 (Total Exhaust)

Inflow Velocity 100 - 110 fpm Inflow Volume 258 - 283 cfm Inflow Variance 20 cfm Downflow Velocity 60 - 70 fpm Downflow Volume 504 - 588 cfm Downflow Variance 84 cfm Exhaust Volume 762 – 871 cfm


Class II type A1 and A2:

The only significant difference between these Class II Cabinets is the permitted minimum inflow velocity. Type A1 cabinets have a minimum inflow of 75 fpm. Type A2 cabinets have a minimum inflow of 100 fpm. In both cabinets approximately 70% of the air is internally recirculated and 30% is exhausted. They are designed to be free standing and connection to an external exhaust canopy system is not recommended but is permitted in some circumstances such as for odour removal. The most recent edition of the NSF49 standard prohibits “hard duct” exhaust connections to type A cabinets.



Supply HEPA filter

Exhaust HEPA filter

Exhaust Canopy


In this configuration the exhaust should be set to draw additional air from the room without affecting the cabinet inflow. Typically 110% to 120% of the cab ine t exhaus t volume will be appropriate


Exhaust Canopy



In this configuration the exhaust should be set to draw additional air from the room without affecting the cabinet inflow. Typically 110% to 120% of the cab ine t exhaus t volume will be appropriate


Type A Biological Safety Cabinets should never be “Hard Ducted” since even minor fluctuations in exhaust volume can compromise containment



These cabinets have very stringent velocity requirements for certification. The range for both inflow and downflow is usually no more than ± 5 fpm from the nominal set point

An increase in external exhaust volume will increase inflow velocity and decrease downflow velocity. Conversely a decrease in external exhaust volume will decrease inflow velocity and increase downflow velocity



Class II type A1 and A2: Class II type B1 and B2:


Class II type B1 and B2:

These Cabinets are significantly different from type A cabinets. Both types require a hard ducted external exhaust system to operate They both require a minimum of 100 fpm inflow velocity B1 Cabinets recirculate all the inflow air through the supply system, typically 50% of the air is recirculated. B2 Cabinets exhaust all the inflow and supply air without recirculation


Class II type B1 and B2:

These types of cabinet should have dedicated ductwork and exhaust blower for each BSC. There should be a minimum of 1.5” wg. Negative static duct pressure at the cabinet. Both types should have low flow interlocks to the supply system and alarms to indicate exhaust system failure. The exhaust blower should be direct drive. The exhaust system should have a stack extending straight upward at least 10ft above the roof surface or have a stack with a smaller diameter trailing end to produce higher velocity flow.


Class II type B1 and B2: Class II type B1:


Class II type B1:


Class II type B1:

Fluctuation of the exhaust volume will affect only the inflow. The inflow limits are similar to those of Class II A2 cabinets. Relatively small excursions from the exhaust set point can compromise containment of the cabinet.

The cabinet certifier will test the inflow velocity or volume at the access opening of the cabinet. Air volume data collected from the exhaust system may not agree with those measured at the access opening. Regardless, only the inflow measurement data can be used for type B1 cabinet certification


Class II type B1:

The cabinet certifier will test the inflow velocity or volume at the access opening of the cabinet. Air volume data collected from the exhaust system may not agree with those measured at the access opening. Regardless, only the inflow measurement data can be used for type B1 cabinet certification

Fluctuation of the exhaust volume will affect only the inflow. The inflow limits are similar to those of Class II A2 cabinets. Relatively small excursions from the exhaust set point can compromise containment of the cabinet.


Class II type B1:

These cabinets are also required to have the supply air system interlocked with the exhaust system. The interlock is set to shut down the supply blower if the exhaust volume drops by 20% or more. This prevents the escape of contaminated supply air from the cabinet

Class II type B1: Class II type B2:


Class II type B2


Class II type B2


The air flow through this type of cabinet is the easiest to understand Unlike the type A and B1 cabinets in which the exhaust volume is exactly equal to the inflow volume, the exhaust system must have sufficient capacity to capture the volume of the supply system as well as the inflow volume.

Class II type B2


Although the operation of these cabinets is very simple they are the most sensitive to exhaust fluctuations as these are magnified at the intake of the cabinet. A change of 1% of the exhaust volume typically results in a change of about 3% of the inflow volume

Class II type B2


To illustrate this phenomenon we will use the specifications for the Class II B2 cabinet displayed earlier.

Although the operation of these cabinets is very simple they are the most sensitive to exhaust fluctuations as these are magnified at the intake of the cabinet. A change of 1% of the exhaust volume typically results in a change of about 3% of the inflow volume

Class II type B2


To illustrate this phenomenon we will use the specifications for the Class II B2 cabinet displayed earlier.



Inflow Volume 258 - 283 cfm

Downflow Volume 504 - 588 cfm


Class II type B2


Exhaust Volume 762 – 871 cfm




Class II type B2





504 - 588 cfm 271 cfm

Class II type B2






258 - 283 cfm

762 – 871 cfm

546 cfm

817 cfm

Class II type B2


546 cfm

271 cfm

817 cfm

The cabinet downflow and exhaust system have been adjusted to the center of the specified range With a dedicated exhaust system the stability of the inflow will be dependant on the pressure differential of the area in which the cabinet has been installed relative to the exterior of the facility

Because of the push for energy savings and other economies it is not unusual for these cabinets to be installed with an exhaust system shared with other type B cabinets and even VAV controlled fume hoods and other types of exhaust system. This condition is not recommended in the NSF 49 Standard

Class II type B2


546 cfm

271 cfm

817 cfm

Constant Volume Valve

If this cabinet is connected to a shared exhaust system a constant volume valve is usually installed to control the exhaust fluctuations that will occur.

Class II type B2


546 cfm

271 cfm

817 cfm

With current technology the tightest control specifications for a CVV is typically ± 5% of the set point Therefore with a set point of 817 cfm the CVV is still within specification at 858 cfm and 776 cfm But is the cabinet?

Class II type B2


546 cfm

271 cfm

817 cfm 858 cfm

The exhaust flow has increased by 41cfm to 858 cfm The supply volume will not be affected by the increase in exhaust volume Consequently the inflow will increase by 41 cfm to 312 cfm

312 cfm

858 cfm

312 cfm

Class II type B2


546 cfm

Inflow range 258 - 283 cfm

The exhaust flow has increased by 41cfm to 858 cfm The supply volume will not be affected by the increase in exhaust volume Consequently the inflow will increase by 41 cfm to 312 cfm

This is 29 cfm above the maximum permitted inflow and 15% above the inflow set point of 271 cfm

271 cfm

858 cfm

312 cfm

817 cfm Class II type B2


546 cfm



Class II type B2


546 cfm

271 cfm

817 cfm 776 cfm

The exhaust flow has decreased by 41cfm to 776 cfm The supply volume will not be affected by the decrease in exhaust volume Consequently the inflow will decrease by 41 cfm to 230 cfm

230 cfm


776 cfm

230 cfm

Class II type B2


546 cfm


The exhaust flow has decreased by 41cfm to 776 cfm The supply volume will not be affected by the decrease in exhaust volume Consequently the inflow will decrease by 41 cfm to 230 cfm

This is 28 cfm below the minimum permitted inflow and 15% below the inflow set point of 271 cfm

Class II type B2


776 cfm

230 cfm

546 cfm



Class II type B2


The preceeding configuration often creates conditions that are usually difficult and sometimes impossible to resolve.

Unfortunately there is a more complex installation option that

compounds the problems for cabinet certification.

Class II type B2


546 cfm

271 cfm

817 cfm

Ducted supply from Building HVAC system

The downflow volume can now fluctuate by ± 5% of the set point if a CVV has been installed. Greater excursions than this have been measured when the CVV is omitted. Let us look at the extremes of the possible flow values


Class II type B2


546 cfm

271 cfm

817 cfm

The downflow volume can now fluctuate by ± 5% of the set point if a CVV has been installed. Greater excursions than this have been measured when the CVV is omitted. Let us look at the extremes of the possible flow values


546 cfm

271 cfm

817 cfm 858 cfm

The exhaust flow has increased by 41cfm to 858 cfm The supply volume has decreased by 42 cfm to 504 cfm. This is still within both CVVs specifications The inflow will increase by 83 cfm to 354 cfm

354 cfm

504 cfm

This is 71 cfm above the maximum permitted inflow and 31% above the inflow set point


Class II type B2


858 cfm

504 cfm

354 cfm

The exhaust flow has increased by 41cfm to 858 cfm The supply volume has decreased by 42 cfm to 504 cfm. This is still within both CVVs specifications The inflow will increase by 83 cfm to 354 cfm



Class II type B2


188 cfm

588 cfm

776 cfm 858 cfm

504 cfm

354 cfm


The exhaust flow has decreased by 82 cfm to 776 cfm The supply volume has increased by 84 cfm to 588 cfm. This is still within both CVVs specifications The inflow will decrease by 166 cfm to 188 cfm


Class II type B2


188 cfm

588 cfm

776 cfm


The exhaust flow has decreased by 82 cfm to 776 cfm The supply volume has increased by 84 cfm to 588 cfm. This is still within both CVVs specifications The inflow will decrease by 166 cfm to 188 cfm


Class II type B2


188 cfm

588 cfm

776 cfm


This installation configuration also effectively compromises the interlock safety system. In the event of a reduction of more than 20% of the exhaust volume the interlock is required to be set to disable the supply fan in order to prevent potentially contaminated air from flowing out of the internal work space. Unless the ducted supply system air flow is also shut off when the interlock is activated air will continue to pass through the cabinet supply filter and will flow out of the cabinet if the exhaust has decreased in volume to less than that of the supply system.

Class II type B2

This installation configuration also effectively compromises the interlock safety system. In the event of a reduction of more than 20% of the exhaust volume the interlock is required to be set to disable the supply fan in order to prevent potentially contaminated air from flowing out of the internal work space. Unless the ducted supply system air flow is also shut off when the interlock is activated air will continue to pass through the cabinet supply filter and will flow out of the cabinet if the exhaust has decreased in volume to less than that of the supply system.


Class II type B2

Class II type B2


In order to keep the exhaust air volume of a B2 cabinet within specifications the CVV would have to maintain a set point with no more than ±1.5% variance. Apparently this is not possible with current CVV technology. This is why dedicated exhaust systems are indicated for these cabinets and why multiple cabinets connected to a single exhaust system create many problems for facility designers, cabinet certifiers and air balancers. Current concerns for energy saving will almost guarantee the requirement for dedicated cabinet exhaust systems will be ignored. Failure prone systems will continue to be designed and built and as long as CVV devices are marketed and sold as appropriate for use with Class II type B Biological Safety Cabinets and these cabinets are considered to be merely fume hood variants.

Class II type B2


In order to keep the exhaust air volume of a B2 cabinet within specifications the CVV would have to maintain a set point with no more than ±1.5% variance. Apparently this is not possible with current CVV technology. This is why dedicated exhaust systems are indicated for these cabinets and why multiple cabinets connected to a single exhaust system create many problems for facility designers, cabinet certifiers and air balancers. Current concerns for energy saving will almost guarantee the requirement for dedicated cabinet exhaust systems will be ignored. Failure prone systems will continue to be designed and built and as long as CVV devices are marketed and sold as appropriate for use with Class II type B Biological Safety Cabinets and these cabinets are considered to be merely fume hood variants.

Class II Biological Safety Cabinets Class II Biological Safety Cabinets AIRFLOW CHARACTERISTICS

Class II Biological Safety Cabinets


The following illustration is from a highly regarded and respected organization's laboratory design guideline

document

Laboratory HVAC Systems - CAV

Dedicated exhaust systems will not eliminate the problems that may be encountered when a hard ducted Class II type B1 or B2 is installed in a facility. The inflow of any hard ducted cabinet will be influenced by any change in room pressure differential relative to the exterior of the facility. Some of the causes for changes in room differential pressure include: Opening doors to adjacent areas of different pressure. Starting or stopping local exhaust systems. Variable volume fume hood exhaust systems. Economizers on room supply systems. Slow reacting facility HVAC control system valves. Filter changes and other regular maintenance on facility HVAC systems.

Class II type B Biological Safety Cabinets

There have been cautionary statements regarding the exhaust system requirements for Class II B cabinets in early versions of the NSF 49 Standard. Typically these cautions have been ignored!


1987 "It is recommended that each type B cabinet have its own exhaust system"


2009 "It is recognized that there is interest in utilizing the increasingly sophisticated modulated flow exhaust ventilation systems where the exhaust from Type B1 or B2 cabinets, chemical fume hoods, flexible exhaust hoses, and/or room exhausts are modulated based on use to optimize containment, maintain appropriate pressure differentials, and maximize energy savings by reducing overall exhaust volume. These systems are required to maintain a high level of control of many complex factors over a number of years. Although the potential cost savings are great, the severity of the hazards contained by the biosafety cabinets requires the use of simpler and more reliable constant flow systems for the cabinet exhaust."


The following Class II type B biological Safety Cabinet information is taken from the most current standard

NSF 49 - 2012


Class II Biological Safety Cabinets OTHER DUCTED HOOD CONSIDERATIONS

There should be a damper, accessible from the hood location, to allow adjustment of the exhaust air flow by the certification technician. On Hard ducted hoods, the technician is required by the NSF standard to set up/test the interlock in the hood for the supply system by reducing the exhaust flow below the set point of the interlock. Also the damper and the duct connection to hood must be air-tight if the hood does not provide an integral damper to allow gaseous decontamination of the hood. The exhaust system for the hood must be able to be shut down to allow HEPA filter changes and other servicing needs. Hood exhaust should not be used as the principal exhaust for the laboratory.

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