Gloveboxes and Class III BSCs: No Room For Error
Glovebox ventilation design must be carefully performed and reviewed to ensure that during both design conditions, as well as upset conditions, the glovebox ventilation system continues to operate in a safe manner to ensure that workers and the surrounding confinement area do not become contaminated.
Following is a summary of how to determine the design requirements (operating pressure and flowrate) for a glovebox as well as the local filter, instrumentation, and controls for a typical glovebox.
Design RequirementsOperating pressure. The design pressure within the glovebox is typically within the range of -0.3 in. to -0.5 in. wg relative to the operating space in which the glovebox is located. This pressure differential ensures that any leakage is from the area of least contamination (the surrounding room) into the area of highest probable contamination. While the glovebox is designed and constructed to be airtight, pressure transients are often experienced within the glovebox during the transfer process of materials in/out of the box, opening and closing of adjacent gloveboxes, as well as general operation of the glovebox glove ports (pumping of the box).
The design pressure within the glovebox should not exceed -0.8 in. wg with respect to the surrounding space, because the gloves will become stiff and very difficult for an operator to manipulate.
Ventilation flowrate. The ventilation flowrate for a glovebox is critical and must be carefully evaluated on a case-by-case basis. Below is a summary of the required criteria that need to be individually evaluated when determining the design flowrate for a glovebox.
- Credible breach flowrate;
- Glovebox temperature control; and
- Explosive gas/corrosive gas control.
Credible breach flowrate. This flowrate is based upon the most probable breach of the glovebox confinement which is the failure of a glove. Glove port breaches occur with the highest frequency due to the physical manipulation of the gloves and the delicate process of changing gloves on a glove port.
To maintain confinement, the face velocity across the breach is designed for a minimum of 125 fpm. The standard glove port has a diameter of 8 in., the minimum exhaust flowrate is 44 cfm. Typically, a design value of 50 cfm is used as the minimum ventilation flowrate for a glovebox based upon a credible breach evaluation.
Glovebox temperature control. This flowrate is determined to evaluation of maximum allowable glovebox temperature with respect to the inlet air temperature. While the maximum allowable glovebox temperature may be process dependent (and must be reviewed and documented), the design temperature often is restricted to between 10° and 15°F above room temperature for operator comfort and to prevent excessive perspiration within the gloves. A list of all of the heat producing equipment is necessary for each glovebox to accurately determine this ventilation flowrate. Typical heat producing equipment includes: lights, motors, exothermic processes, hot plates, etc.
The following sensible heat equation is used to determine the temperature control flowrate:
60 xr x cr (t2-t1)
Q = airflow cfm;
H = sensible cooling load (Btuh);
60 = minutes/hour;
r = air density (0.074 lb/ft3 at sea level, 75°);
cr = specific heat (0.24 Btu/lb);
t2 = maximum glovebox space temperature (°F); and
t1 = Supply air temperature (surrounding space, °F).
At sea level, this equation typically reduces to the following:
1.08 x (t2-t1)
Note that at elevations above 2,000 ft above sea level, the reduced air density begins to have an appreciable effect on the heat capacity of the air. At these higher elevations, the amount of required airflow increases due to the lower air density. Since the ventilation flowrates for gloveboxes are relatively small for HVAC applications, this has the potential for creating unacceptably warm enclosures if the actual air density is not utilized in the calculation.
If the resultant flowrates are unacceptably high (creating excessive airflow within the box, which would disturb glovebox process operations or require large inlet filters, etc.), then the addition of internal cooling systems and better process/equipment insulation must be evaluated to reduce the cooling load carried by the ventilation flowrate.
Explosive gas/corrosive gas control. This flowrate is determined based upon review of the processes and materials used within the glovebox to prevent (through dilution) unacceptably high concentrations of gases. The maximum gas generation rate must be determined or obtained (and documented) to analyze the dilution flowrate requirements. Below is the equation used to determine the minimum safe ventilation rate:
Q= R x 106 X S
Q = minimum airflow rate for dilution (cfm);
R = gas generation rate (cfm);
S = safety factor (between 4 and 10 depending on volatility, degree of mixing, and risk); and
L = limiting value of containment in volume parts per million (vpm).
Note: For toxic vapors, use the threshold limit value (TLV), and use the lower explosive limit (LEL) for combustible vapors which are converted to vpm.
Once all three (breach, temperature control, and gas dilution) criteria are evaluated, the worst case flowrate is obtained and becomes the design basis for the glovebox flowrate.
Design PhilosophyAfter determining the design operating pressure (and allowable operating range) and the design flowrate, the physical implementation of the criteria must be properly utilized to properly size and select the system components and controls.
System components. The minimum inlet configuration for a glovebox is comprised of an inlet air array, which consists of a prefilter, a HEPA filter, and a manual-balancing valve. Depending on the application and customer requirements, automatic isolation valves/dampers and an airflow measuring station may be added.
The minimum exhaust configuration for a glovebox is comprised of a prefilter, a HEPA filter, and a balancing valve. Similarly, depending on the application and customer requirements, a fire screen, automatic control valve, and an airflow measuring station may be added. Figure 1 is a simplified sketch of a glovebox with its basic HVAC components.
Inlet/supply filters. The inlet filters for a glovebox are typically "oversized" due to the low differential pressure available to create flow. Most glovebox installations draw air from the surrounding room rather than from a pressurized supply (supply fan, booster fan, etc.) to remove the possibility of over pressurizing the glovebox due to a mechanical, or more likely, a control system error/failure. The inlet/supply filter's main purpose is to prevent workers from coming in contact with any products within the glovebox should the box become positively pressurized.
As previously discussed, the design pressure for a glovebox is approximately -0.5 in. wg with respect to the surrounding room. This low-pressure differential is often overlooked in the design of the inlet equipment, which results in undersized filters and low flowrates through the glovebox.
Most published filter flowrate vs. pressure drop information provides the published filter flowrates at 1 in. wc differential pressure. Since the available differential pressure is appreciably less than 1 in. wc, the corresponding required filter area increases proportionally.
Prefilters are typically only 30% ASHRAE-rated filters used to collect any large particulates before the inlet HEPA filter. Higher prefilter efficiencies are not usually employed due their high clean pressure drops. The HEPA filter(s) are of a grade and construction as required for the application. Depending upon the application, the filters are either installed within a bag-in/bag-out filter housing or, more commonly, integrally mounted on the top of the glovebox. This integral mounting allows for the replacement of inlet filters by pushing the spent inlet filter into the glovebox during the installation of the new, clean filter.
Exhaust filters. A prefilter (60% to 90% ASHRAE rated) is typically employed as well as the HEPA filter to satisfy the design exhaust flowrate. Since the exhaust system is provided with an exhaust pressure source (exhaust fan), the exhaust filter sizing issue is not as critical as with the inlet filters. An inlet (suction) pressure, typically available for the exhaust filters, is approximately 2 in wc. Similar to the inlet/supply filters, the exhaust filters are typically housed within a bag-in/bag-out filter housing or directly mounted on the glovebox.
Ductwork. Since the ductwork becomes a physical extension of the confinement created by the glovebox, it must be designed, specified, constructed, and tested to maintain the same low leakage requirements. On the inlet/supply side, the high quality of construction must be maintained up to the inlet HEPA filter(s), since the ductwork up to the filters may be contaminated during an upset condition. The exhaust ductwork is typically sized for a minimum of 2,500 fpm to ensure that any contamination in the ductwork does not settle in the ductwork and is captured on the filter media. This causes the ductwork to be relatively small (2.5 to 3 in. dia). The ductwork must be of all welded sheet metal construction or schedule 10S pipe.
Finally, the glovebox exhaust system must not be integrated into a fume hood exhaust or general exhaust system (unless they have exhaust HEPA filters) since during an upset condition, there would be a free (unfiltered) path back into occupied spaces.
Controls/instrumentation. The pressure within the glovebox is controlled by the exhaust valve. This control is either performed automatically by a control system or through manual balancing. The flowrate through the glovebox is controlled by the inlet/supply valve/damper. If the valve is automatically controlled, it should be through a PI-1/D algorithm to prevent hunting when the gloves are being pumped during normal operation.
A shielded static pressure sensor should be installed within the glovebox to accurately sense the static pressure and to exclude any velocity pressure reading, which is caused due to the relatively small volume of the box. Local pressure differential indicators and switches (or transmitters) are necessary for proper monitoring of the glovebox pressure. Local high-pressure (approaching room ambient) alarms should be included to warn personnel of a potentially unsafe condition.
The pressure differential across the exhaust filters must be monitored and alarmed as an indication of required rebalancing and/or filter replacement. Pressure differential gauges across the inlet filters are not recommended since they will not provide an indication of filter loading, but merely the differential pressure between the glovebox and the surrounding space. It is important to remember that any utility connections, which interface with the glovebox or its ductwork, must maintain the confinement boundary. For example, on pressure differential gauges, isolation snubbers may be required, and depending upon the application, bellow-sealed valves may also be required.
It is recommended that an airflow measuring station be installed in the exhaust ductwork to monitor the exhaust air flowrate. The airflow measuring station should be installed downstream of the local exhaust filters to ensure that the sensing ports do not become plugged and that there is sufficient velocity (2,500 to 3,000 fpm) to obtain an accurate flow reading.