Part 2: Exhaust methods and UV lights By Dave Phillips
Today’s biological safety cabinets (BSCs) are among the most effective and commonly used primary containment devices in laboratories, and paying for energy to power these cabinets is an essential outlay for all laboratories. Energy consumption of laboratories may significantly impact the environment over time, contributing to the majority of the global warming potential of greenhouse gases that are released into the atmosphere. Therefore, key decisions made by building owners, designers, engineers, and facilities personnel can influence and determine our rates of energy consumption for many years to come.
In Part 1 (January, page 1), we looked at two considerations in achieving maximum energy efficiency and safety: usage pattern (duration/frequency) and method for maintaining the sample chamber. Part One reviewed efficiencies available in modern cabinets using DC motor technology and the impact on consumption with different usage patterns. This second article examines the benefits and energy costs associated with different exhaust methods and optimizing the use of UV germicidal lights within BSCs.
Table 1. Estimated annual cost of exhausting one Class II, Type A2 BSC with canopy exhaust vs. a Class II, Type B2 BSC with direct duct. Click to enlarge.
Externally ducted BSC exhaust The third consideration in achieving maximum energy efficiency is the use, or lack, of external ducting for exhaust. The key reason for labs’ high energy consumption is the typically high ventilation rates and the associated air conditioning loads. The ventilation rates are typically required to maintain safety and containment levels and meet the relevant authority and risk-management guidelines. This supply air is normally conditioned to meet user comfort expectations and deal with internal heat gains.
All Class II BSCs will filter the BSC exhaust before expelling it from the cabinet. HEPA filters do not filter gases, and users may occasionally need to work with toxic or irritating gases. In these cases, the exhaust from the BSC can be conveyed outside through a thimble/canopy or direct connection. However, this greatly increases the energy consumption of the safety cabinets.
Not all Class II BSCs are the same. The size and type of Class II BSC can significantly impact the amount of air exhausted. The table shown above shows the comparative exhaust volumes in ft3/min (cfm) and estimated cost of the exhaust for different sizes and both Class II, Type A2 and B2 BSCs from an example manufacturer. The estimated costs of exhaust were determined using the average annual cost of $4.50 per cfm provided in “Energy Use and Savings Potential for Laboratory Fume Hoods” by Mills and Sartor (Energy 30 (2005) 1859–1864).
The quantity and cost of exhaust increases with the width of the cabinet and the height of the window opening. A Class II, Type A2 BSC with a canopy connection requires less exhaust than the Class II, Type B2 BSC. Both types of Class II BSC exhaust air outside the laboratory, but the Class II, Type B2 BSC does not recirculate any air within the cabinet.
While energy considerations appear to favor the canopy-connected A2, there may be safety, product or regulatory issues requiring the Class II, Type B2 BSC. The proper cabinet and type of exhaust must be selected to provide the necessary protection without excessive exhaust.
Further energy-saving design measures that should be considered within laboratories include the number and size of safety cabinets, as this will directly influence the laboratory’s energy performance. Where feasible, variable air volume (VAV) supply and exhaust fans should be employed in any energy-conscious facility over traditional constant volume systems. The air volume extracted is continuously varied depending on use as well as the conditioned supply air, through pressure controls, while maintaining the velocities required to achieve containment levels. While VAV systems can be used effectively for laboratory supply and exhaust and even laboratory fume hoods, they are not recommended for use with BSCs.
Cost-effective UV lighting The fourth usage element is the use of ultraviolet (UV) radiation to decontaminate the sample chamber. Ultraviolet lights are now a common accessory of many BSCs, and these lamps are regarded as biocidal devices, protecting the operator from exposure to infectious agents and experimental materials from contamination.
The majority of protocols for working in a BSC require the surface decontamination or disinfection of the work surface and area with an appropriate liquid decontaminant before beginning work and after completion of work. Some users prefer the additional protection of a germicidal or UV light. After finishing work, the window of the BSC will be closed and the UV light will be activated. The UV light will sometimes be left on until the user returns the following day.
Recently, more and more cabinets are being provided with timers for the UV light. The effectiveness of the UV is a function of the quantity of energy on the biological contamination and the length of exposure. Although NSF International (www.nsf.org), formerly the National Sanitation Foundation, no longer addresses the use of UV light in a BSC, they formerly published guidelines on its usage. At that time, they required a minimum of 40 µW per cm2 on the work surface for effective disinfection. Assuming that the 40 µW per cm2 level of radiation is used, most experts indicate that even the most resistant organisms will succumb after only 3 hr of exposure.
In a typical research lab, the UV light is used for more than 3 hr per day. If the user turns on the UV light in the BSC at 5 p.m. and turns it off at 8 a.m., 15 hr of total exposure time will occur. A timer would allow the UV fixture to operate for the recommended 3 hr or less, and save 12 hr of energy. The savings in electricity would be less than ~200 kW/yr.
In addition to reducing energy consumed, the lab will benefit from increased bulb life. Many UV bulbs used in BSCs have an estimated life of 7,500 hr. A bulb used 15 hr per night, every work night, and left on over the weekend would reach 7,500 hr in 14 months. The same bulb with a timer allowing only 3 hr of operation, would last more than 9.5 years.
Fig. 1. Graphic representation of the exhaust and operating cost differences between BSC types and ducting methods reported in Table 1.Click to enlarge.
Wise choices yield rewards Comfort, health, and safety requirements are no longer the only objectives when choosing and using a BSC, and environmental conditions for experimental work are equally important. Laboratories are high consumers of energy, so reducing overall energy usage is critical for a cost-efficient lab. The chart presented in Fig. 1 (left), shows the estimated costs of electrical energy and exhaust as discussed previously.
The first consideration in selecting the proper BSC for effective and energy efficient operation is size and type. The most common widths are 4 ft and 6 ft. Usually a nominal width of 4 ft is sufficient for one user at a time. When it is necessary for two people to use the BSC at the same time, a nominal width of 6 ft is most commonly selected.
The most common operating window heights for Class II BSCs are 8 in. and 10 in. Since most BSC windows can be raised for placement and removal of equipment, the selection of height is dependent on operator comfort and familiarity.
The characteristic having the greatest impact is whether the BSC requires external exhaust and if so, what type of externally exhausted BSC is required. After the width, window height, and exhaust of the BSC are selected, the type of motor technology and usage must be considered (see Part 1).
Using BSCs with the latest energy-efficient technologies and optimizing design specifications will also maximize performance and capacity. Modern equipment incorporating these strategies uses ~60% less energy and emits ~60% less heat than traditional BSCs. A reduction in energy consumption allows relative reductions in CO2 emissions, benefiting the environment as well as optimizing lab efficiency and operation.
Dave Phillips is a technical applications specialist—laminar flow with Thermo Fisher Scientific. The company provides a complete range of lab products and solutions, including biological safety cabinets (www.thermo.com/bsc).
