In plastics processing, water is everywhere as it touches virtually every aspect of plastics processing operations. That makes water management an important issue.
Consider a plant with three 165-ton and three 120-ton hydraulic injection-molding machines. In total, those machines typically require a continuous water supply at a rate of 100 gal/min to regulate the hydraulic system and mold temperature. Assuming a two-shift operation, that is 96,000 gal a day and more than 30 million gal a year -- a virtual river -- circulating throughout even a medium-sized injection-molding plant. Water circulation for a comparable extrusion operation can be even greater.
With that much water going through a plant, its quality has an important impact on the direct costs of water, water treatment, disposal and water-related maintenance. Water’s involvement is critical in maintaining consistent resin temperatures, plasticating rates, mold temperatures and cycle times.
A processor typically uses one of three types of water sources:
- City (municipal) water.
- On-site well water.
- Bulk process water.
In the past (and to some extent today), processors simply sent the water where it needed to go, then dumped it down the drain. This method is not only costly but introduces impurities into the plant.
Water RecyclingOver time, companies realized that recycled water could save on direct water costs, and that, with minimal treatment, recycled water could be kept relatively clean of large debris. Recycling is accomplished by closing the loop between the process and the source. This can be done with a cooling tower or a water chiller, whether in central or portable format. The choice depends on what temperatures are required to properly control the process.
If the process calls for water around 85°F (29°C), then a cooling tower can be extremely efficient, exploiting natural evaporation to offer a 10°F (6°C) drop for a relatively low investment. However, a typical induced-draft cooling tower will scrub the air of debris -- dirt, leaves, cottonwood fluff, feathers and microbials -- and may contaminate the water. Also, because cooling towers achieve cooling by evaporation, they call for makeup water (evaporation plus blow-down) of approximately 3 percent of the total flow going through the tower.
By contrast, chilled water systems do not expose the cooling water to the outside air. These systems can be set up in closed loop or semi-open loop configurations. In a closed loop system, the water is not exposed to any air; in a semi-open loop, the cooling water is exposed to the air inside the plant. A closed loop system is well-suited for use with bulk process water. Once the system is filled, the amount to be made up is small compared to the total volume.
No matter what type of process cooling system you choose for your facility, the recycled water needs to be treated. Beyond cleaning the water of any large debris that could plug up the system, many other types of contaminants will precipitate, or fall out of, the solution and cause processing problems.
Just 0.0625" (1.6 mm) of calcium carbonate (scale) that plates out of solution onto equipment surfaces results in a heat transfer loss of 20 percent. This means that hydraulic systems will run hotter and processing machines will have slower cycle times, diminishing productivity and eventually requiring maintenance. If the water is left untreated, the calcium carbonate can break off in chunks and find its way to rotating mechanical seals in pumps that supply the water, causing the seals to wear prematurely.
Water Treatment OptionsWhat is considered good water quality? This question has as many answers as there are water chemists, but a good rule of thumb is to keep alkaline pH levels between 8 to 10 (7 is neutral, less is acidic), total hardness (calcium carbonate content) to less than 500 mg/liter, iron content to less than 1 mg/liter, and a total dissolved solids figure of less than 50 mg/liter.
Minerals that are naturally present in the water are a problem, but a more serious matter is microbial growth, especiallyLegionnella pneumophila. This bacterium has been found in ground water and needs to be treated in any type of plant water system -- even closed loop systems. Make sure your plan of action for water treatment includes dealing with microbial growth.
Water can be chemically treated or mechanically filtered. If you choose to chemically treat the water, contact your local certified water chemist to assist with your needs. For non-chemical ways to treat the process water, three basic types (ozone generation, reverse osmosis and mechanical filtration) have demonstrated effectiveness over time.
The use of water has become an important topic around the world, and the more that can be recycled in process cooling applications, the better off everyone is going to be. Contact a reputable water chemistry agent to help keep your facility working at highest efficiency it can.