The fume hood sash provides a containment area in case of a fire. It works as a blast shield. Along with the bypass grill located above the sash, the sash itself determines the fume hood’s airflow characteristics. Even if the position of the sash does not control the air volume passing through the fume hood, the position of the sash does control the air velocity passing through the fume hood. Plus, one factor that influences the capture of chemical vapor is air velocity. When you raise the sash, this lessens the capturing velocity which allows vapors to begin spilling over the sill into the air of the room.
Fume hoods, also known as fume closets or fume cupboards are a kind of local device for ventilation designed for limiting exposure to toxic or hazardous fumes, dust, and vapors. Typically, fume hoods are large equipment pieces that enclose a work area’s bottom and five sides, of which is located at standing height for work. There are recirculating or ducted types of fume hoods. So basically, a fume hood is a lab chemical hood. This is a local ventilation exhaust system. Typically, fume hoods are cabinets with front sashes made of safety glass you can move to close or open the hood.
The principle is to draw air from the cabinet’s open side which gets expelled out of the building or cleaned through a filter and introduced back into the room. Hoods are for protecting users from inhaling poison gas. It is for protecting the environment and the product as well.
Why Do I Have to Keep The Sash So Low When Working With A Fume Hood?
Hoods work by exhausting captured airborne chemicals into the atmosphere. You can see how well this actual process works by using dry ice. Put dry ice into the hood and if the sash is pulled down to the position marked, there is more efficient control. The vapors of the dry ice are readily captured, which causes an increase in protection. If the sash is up all the way, the vapor easily spills out of the hood. This compromises protection.
What Is The Proper Height Or Placement Of The Sash During The Use Of A Fume Hood?
The sash height is the most significant decision you will be making to control the performance of the hood. You know that the performance of the hood gets better with the decrease of the sash opening. It is good practice to keep the sash below or at the safe marker for sash-opening. You get better protection when you put the sash at the level which is the lowest for convenience in operating.
Why Do I Have To Keep The Sash Closed?
When you are not actively working at your hood, your sash should be closed. You need to be vigilant that the other occupants of the lab keep their sashes closed when they are not at their respective hoods. When an explosion occurs and the sash is not closed in a fume hood, chemicals, debris, and shards of glass could scatter through the lab. Every lab occupant, not just the person experimenting is at risk. In addition to a closed sash, you might even use a portable blast shield within the hood when you are working on explosion hazards.
Use a closed sash when working with:
• Non-hazardous and hazardous odorous materials
• Toxic gases such as NO2, H2S, C12, F2, CO, NH3
• Explosive or Reactive chemicals or materials that spatter
• Procedures that create an aerosol of toxic substances
• Particularly hazardous substances or carcinogens
• Flammable chemicals
• Formaldehyde or chloroform as well as other toxic, volatile materials
• National Fire Protection Association Chemicals rated 3 or 4
All interior baffles, sashes, and panels need to be in place before you begin work with hazardous materials.
1-Combination Horizontal and Vertical Slide Sash Hood- Locate the needed materials and equipment a minimum of six inches inside the hood. Making sure the horizontal slide sash covers the opening, lower the vertical sash. This needs to be lowered to the mark of eighteen inches.
2-Horizontal Slide Sash Hood With Velocity Screen- Locate needed materials and equipment a minimum of six inches inside the hood. Fasten and lower the velocity screen. The screen is needed to assure proper hazardous vapor containment inside the hood.
3-Horizontal Slide Sash Hood- located the needed materials and equipment a minimum of six inches inside the hood. Check to see if the horizontal sash is in place. Six-foot hoods need two sashes and four-foot hoods need one sash. The sliding sash can be positioned between the work within the hood and the operator.
4-Vertical Rising Sash Hood- locate the required materials and equipment at least six inches inside the hood. Lower the sash to below the mark or stop marker at eighteen inches.
Fume Hood Sash Types:
For the fume hood’s front sash component, many types of glass are used. People sometimes inquire as to which glass is best for a sash. The two main sash glass types are laminated safety glass and tempered safety glass. For specified applications, poly-carbonate is another option. Here are the most popular fume hood sash materials you can choose from:
1-Laminated Safety Glass Fume Hood Sash
Composed of two float glass pieces, laminated safety glass has a polyvinyl butyral layer between them. This sandwich is pressurized and then heat is applied to bond each layer together. When laminated safety glass breaks, the shards stick to the layer of PVB inside them. This protects the worker from falling or flying glass pieces flying all over the place. Both tempered and standard glass can get a laminate depending on the project budget.
2-Fume Hood Sash Comparison
There is a higher thermal resistance for tempered glass compared to laminated glass. This difference in performance is obvious during the routine use of a drying oven, fires in the fume hood and other equipment that emits heat.
There is a cracking threshold for tempered glass five to seven times more than safety laminated glass. Thus, it will fail at higher impacts and turn into less dangerous, small cube pieces. Blunt objects can be resisted by tempered glass compared to the safety laminated version. However, tempered glass will crumble or dice when it gets penetrated.
If one laminated glass layer loses integrity, the layer of plastic is exposed and under certain conditions, could melt. The glass laminate cracks with less pressure and if it is penetrated, it creates large piece jagged shards that are sharp.
3-The Third Choice Is Poly-Carbonate
for glass sashes, poly-carbonate can be used as a substitute for applications when there is work that will include Hydrofluoric Acid (HF). Glass is etched with HF. Thus, using this in a standard hood will require the replacement of the sash eventually due to getting clouded.
This barrier type can be installed on most types of fume hoods. You just want to ensure that the material that lines it is HF compatible ensuring long life out of the fume hood sash. Normally, you get a standard polycarbonate sash when you get a PVC Acid Digestion Hood.
Virtually, chemical resistance is identical to both laminates and tempered glass. However, due to its superior performance on impact and heat, tempered glass is much more superior. This is especially true if the spatters of chemicals are in combination with the semi-solid matter or hot temperatures.
Tempered Safety Glass Fume Hood Sash
Made of heating annealed glass of high quality up to six hundred degrees Celsius and then rapidly cooling this down with precisely controlled rapid blasts of cold air tempered safety glass goes through this process which cools the exterior while keeping the glass interior hot. Thus, the outside surface gets compressed and the interior tension creates a tough piece of glass.
This glass type, when used for a sash, can withstand five times stronger impact compared to ordinary glass. Once this type of glass is broken, it shatters into blunt, small pieces. This prevents serious injuries from what could otherwise be very sharp, large pieces. There is high resistance to heat shock for tempered glass.
This means that it can withstand radical temperature changes of up to two hundred degrees Celsius. This is in comparison to ordinary float glass which can only withstand radical temperatures changes of seventy degrees Celsius. When this type of glass breaks, the broken shards from round chunks that are cleaned easily and are safe for cleaning up.
Shutting The Sash
There is a campaign that is trying to promote shutting the sash. The program’s goal is to raise awareness in labs about how important it is to shut the sash when you are not using the fume. When the sash is closed, this improves the building’s energy efficiency. It is an important measure of safety. The sash needs to be pulled downwards in such a way as to protect the zone for breathing. Thus, you get safer conditions with a lowered sash.
On-campus, fume hoods will have two sticker-labels. This will prompt everyone to get reminded to close the sash when not in use. Also, another vertical sticker to indicate the vertical height for losing the sash. The sticker will also remind all workers in the lab to use the proper height for sashes. It also emphasizes the energy saved when the sash is lowered. Lab workers will be reminded to close the sash when operating the fume hood.
After all, the purpose of chemical fume hood sashes is to protect the breathing zone of the worker from hazardous pollutants and vapors released during experiments. This works by removing the vapor and getting them to exhaust outside.
Since there are large air volumes that the atmosphere exhausts, one fume hood uses as much energy is 3-4 residence homes in twenty-four hours. The design of fume hoods is that when they are not in use, the sash needs to be closed. At the bottom of the sash in all modern fume hoods, there is an airfoil sill so even when this is shut, air will get pulled from the lab to exhaust all toxic vapors.