Taken from http://www.cemag.us/ thankx.
When planning a cleanroom laundry, it is important to determine the correct size and type of washer and dryer needed. It is also important to consider the options available and determine which options will be most beneficial.
Once the momentous decision has been made to launder cleanroom garments on site, rather than contract for outside cleaning services—and space set aside for that function—it is necessary to specify equipment for installation.
Washer-Extractors
Most cleanroom garments are made of 100% polyester, proprietary materials such as GoreTex™, or a combination of materials. Because these fabrics are relatively light, the use of the washer’s loading ratio and volume (in cubic feet or liters) to determine its production capability is recommended. Loading ratio is the volume required to wash 1 kg (or 1 lb.) of garments.
For a typical non-cleanroom laundry, the loading ratio is usually 10. For cleanrooms, however, the typical loading ratio is 12 for ISO 5 (Class 100), 14 for ISO 4 (Class 10), and 16 for ISO 3 (Class 1). The loading of a 600-liter washer is cylinder volume (in liters) ÷ loading ratio (liter/kg). An example using the loading ratio to determine the load size for a 600-liter washer is shown in Table 1.
The same information for the above 21-cu. ft.(600-liter) washer, but based on cubic feet and pounds, is shown in Table 2, in which the load size is: cylinder volume (in cubic feet) ÷ loading ratio or cubic feet/pounds.
It takes 40 to 60 minutes to load, wash, and unload a load, depending on the number of wash and rinse cycles. A 600- liter (21-cu. ft.) washer will process 4000 to 5000 ISO 5 (Class 100) garments per week, based on garments weighing 1 lb. each and 40 hours of use.
Split pocket washers are the most popular because the inner cylinder is divided into two compartments, making the washer much easier to load and unload. Washers should be sized so that the contents of one pocket would go to one dryer when unloaded, while the contents of the other pocket would go to the other dryer. The garments in one 22-cu. ft. pocket of a 44-cu. ft. split pocket washer, for example, would go into one 50-cu. ft. cleanroom dryer, while garments from the other pocket would go into another dryer. This doesn’t necessarily double the number of dryers required, since the drying time of a cleanroom dryer is only approximately 50% of the wash time.
Water Consumption. Water consumption is determined by multiplying the load size in kg (based on the above calculation) times the water/workware ratio (liters/kg). Typical water/workware ratios for a cleanroom washer process are shown in Table 3. For a 600-liter (21-cu. ft.) washer using only a pre-wash, wash, and two-rinse cycle the water consumption would be as shown in Table 4.
Pass-through (side-loader) vs. front-loader washers . A pass-through washer allows soiled garments to be loaded into the washer one side and unloaded on the clean side. With pass-through washers, no soiled garments are ever in the cleanroom. (This is also possible with some “top” loading washers, which can be automated and are used in high-volume commercial laundries.) If a front-loader is used, as is common in ISO 6 (Class 10,000) and higher laundries, soiled garments must be brought into the cleanroom in a covered laundry cart. It is important that cleanrooms in which front-loaders are installed have fast recovery rates.
Soft-mount washers. A soft-mount washer is relatively easy to install because it does not require a thick, reinforced, isolated concrete pad. Depending on the size of the washer, most can be bolted to a 4 to 8-in. reinforced concrete floor. Small washers can be installed on reinforced wooden floors. A soft-mount washer is required for installations where the washer is not installed on the lowest (ground) level.
Washer-Extractor Options Early cleanroom laundries used pass-through washers designed for hospitals. Today, washers are available that truly meet the needs of a cleanroom laundry. In addition to choices of various types of ss, options include the following:
Electropolishing. Electropolishing, which is recommended for ISO 3 and 4 (formerly Class 1 and 10) cleanroom laundries, enhances the surface by leveling the metal and making it smooth and shiny, minimizing areas where particles, chemicals, and bacteria can be trapped. Electropolishing also removes any free iron on the surface and passivates the ss. Although electropolishing is expensive, it removes all the surface contaminants, passivates the surface to eliminate oxidation, and leaves a smooth surface (typically 16 RMS) that is easy to keep clean.
Dimpled cylinder perforations. Cylinder perforations should be dimpled (embossed) to the outside so that garments never contact the sharp edges of the cylinder perforations. This reduces chafing and wear of the fabric and increases the life of the garment.
Variable speed drive. Most modern cleanroom washers allow the washing and extraction speeds to be programmed for maximum g’s that will not damage the fibers of the fabric. It is important to program the correct extraction speed for the item being washed and the type of material from which the garment is made. It is desirable to have an extraction speed that is high, but not so high that the fabric is damaged. This is especially important in processing ESD garments. Using a microscope, examine the fabric for stress marks, nicks, or small tears.
Bearing lubrication relief. A washer’s main bearings require regular lubrication. When too much is added, excess pressure may build up in the bearing housing, forcing grease out of the seals, past the shaft seal, and into the washer, contaminating the garments and the stainless steel interior. Repairs, which are expensive, cause costly downtime. When considering washers for purchase, look for bearing housings designed so that any excess lubrication will be released to the outside of the bearing housing. All seals should be Viton®.
Auto weighting. An optional automatic weighting system weighs each load as it is placed into the washer, simplifying the even loading of compartments. The amount of water and chemicals entering the washer will then be based on the actual weight of each load. High-end washers go a step further by adjusting the size of the cylinder to match the load size. Now a small load receives the same mechanical action as a full load and prevents excessive abrasion during washing.
Cleanroom Dryers
The use of loading ratio is actually more important in the specification of cleanroom dryers than washers because dryer efficiency is rated using UL Cloth, which is much heavier than polyester or Goretex™. For cleanroom garments, the recommended loading ratio for dryers is 0.036 for ISO 5 (Class 100); 0.031 for ISO 4 (Class 10), and 0.026 for ISO 3 (Class 1). The loading ratio for a 1400 liter (50 cu. ft.) dryer (cylinder volume in liters x loading ratio in kg/l) or (cylinder volume in cu. ft. x loading ratio in lb/cu. ft.) would be as shown in Table 5.
It takes 20 to 40 minutes to dry a load, depending on the extraction speed of the washer and the amount of cool-down time required. When the supply air is cold and/or humid, drying time will increase. Drying time will decrease as the washer extraction speed and time are increased.
Because most of the particles—up to 80%—are removed in the dryer, and most of those during cool-down, the cool-dawn cycle is normally extended on cleanroom dryers. Adequate cylinder size is also critical.
Cleanroom dryers use HEPA, ULPA, or VLSI filters to provide clean air. All air entering the dryer during the drying and cool-down modes must pass through the filter system. In a ISO 3-4 (Class 1-10) dryer, all moving parts or materials that can shed particles must be located upstream of the filter.
Dryer Options
Air sample port. The air sample port provides access to the interior of the dryer cylinder through the loading door. A particle counter can be used to measure the quality of the air going through the dryer; because garments can never be cleaner than the air flowing through the dryer, this should be part of the quality control protocol for each dryer. In some installations it is also helpful to have an air sample port at the downstream side of the HEPA filter.
Hinged center front panel for cleanout. The inside of the dryer must be kept clean, especially in ISO 3-5 (Class 1-100) operations. When the dryer stops, normal air flow stops and random pulses occur that tend to free resident particles inside the dryer, contaminating the garments being dried.
Turbo box. The turbo box is an ss transition that mounts to the discharge of the dryer’s outer shell and connects to the inlet of the blower. The velocity of air is maintained to eliminate particle drop-out, eliminating the need to clean the lint compartment from the cleanroom side.
Pressure gauge across the filter assembly. Because the HEPA filter system is mounted above the dryer, the pressure drop across the filters should be read from the floor level. This reading is usually done weekly as part of the QC protocol. The pre-filters should be replaced when the pressure differential exceeds 1 inch of water. If the pressure differential continues to be greater than 0.8 inch water after replacing the pre-filters, the HEPA filter should also be replaced. (These are typical readings; readings can vary depending on the dryer and filtration system being used.)
Ionization system. Static electricity attracts and holds particles to the fabric. The fabric also clings together. When this occurs, the clean air flowing through the dryer cannot carry the particles away. The ionizer neutralizes the static electricity, allowing the particles to be released and lowering the particle count of the cleanroom garment. This option is most common on ISO 3-4 (Class 1-10) dryers.
Two-stage blower. As noted above, when the dryer stops, the airflow loses its momentum and random pulses occur, freeing particles inside the dryer. Some of these particles enter the cylinder (basket) and contaminate the garments. With a two-stage blower, the basic airflow is maintained at a minimum cfm. This minimizes cross contamination and almost eliminates contamination from random air currents at the end of the drying cycle when the blower would normally shut off.
Filtration system. It is relatively easy to wash and dry cleanroom garments; removing particles is the challenge. As noted above, the garments cannot be any cleaner than the air flowing through the dryer. So a true Class 1-10 dryer requires a first-class filtration system and must be clean, air tight, and particle-free.
Once the size and options for the cleanroom washer and dryer have been determined, it is important to integrate process flow, material handling, quality control, and cleanroom design. This will require close communication between the cleanroom design engineer and the process engineer. Many changes are easy to make, but some can increase the cost of a cleanroom substantially. If the cleanroom design stays simple and there is adequate space for the washer and the dryer and for material handling, the basis of an excellent cleanroom laundry will be complete.
No comments:
Post a Comment