Recycling process cooling water with a closed-loop cooling tower or water chiller can help prevent water quality problems in plastics manufacturing.

Water recycling can be accomplished by closing the loop between the process and the water source with a closed-loop cooling tower (pictured) or a water chiller.

As process cooling systems have grown larger and more complex, higher volumes of water move at faster speeds. For example, a plastics plant with three 165-ton hydraulic injection-molding machines might require a continuous water supply at a rate of 100 gal/min for hydraulic system and mold-temperature regulation. Assuming a two-shift operation, that is 96,000 gal/day and more than 30 million gal/year. With that much water going through the plant, water quality has a direct, significant impact on the plant’s operations. Contamination can cause equipment damage and even failure, not to mention worker health problems.

The following conditions suggest water contamination:
  • Clogging of high-heat, low-flow areas.
  • Clogging or corrosion of process components.
  • Gelatinous deposits.
  • Worker health complaints such as fever, chills, coughs, muscle aches, headache, tiredness, loss of appetite and even pneumonia.
Most water contamination problems result from the open-loop process cooling systems common in U.S. plastics plants (figure 1). If you have an open-loop process cooling system design, you must regularly monitor and treat your water supply to control:
  • Lime, scale and other mineral deposits.
  • Corrosion.
  • Microbiological growth such as algae, bacteria, fungus and molds.
  • Suspended solids accumulations such as airborne dirt and debris that are washed into the cooling tower water.
In addition to the cost of water monitoring and treatment, an open-loop process cooling system burdens your plant with costs for sewage, disposal and equipment maintenance. These operational costs, as well as environmental regulations, have become an increasingly significant management challenge in the plastics industry.

An open-loop system is susceptible to mineral deposits, corrosion, microbiological growth and suspended solids accumulations.

Recycling: Dual Benefits

Over time, manufacturers have recognized that using recycled water will reduce water costs, and it also can forestall contamination. Water recycling is accomplished by closing the loop between the process and the water source with a closed-loop cooling tower or water chiller, in either a central or portable format.

The choice of format depends on the temperature required to properly control the process as well as the amount of water being recirculated. Portable systems typically support from 1 to 13 tons, and central systems support from 10 to 300 or more tons. Temperature ranges are determined by the process, but they can drop as low as -30oF (-34oC) leaving water temperature.

A closed-loop system is similar to a conventional (or open-loop) system, except that in a closed-loop system, the tower reservoir usually is constructed with two separate tanks. One tank holds process water that is piped to the process and back to the tank without coming in contact with the tower water. The second tank holds the tower water, which circulates to the tower cell and back to the reservoir tank in a completely separate loop. Heat is exchanged from the process water to the tower water at a plate-and-frame heat exchanger (figure 2).

The appropriate materials must be selected for all components of a closed-loop system to safeguard against contaminants. Mill-galvanized sheets offer good protection, but cutting, drilling, welding and forming these sheets exposes unprotected areas to the elements. Even small unprotected areas can lead to corrosion at rates comparable to that of unprotected mild steel. Noncorrosive materials such as stainless steel, polyvinyl chloride (PVC) or high-density polyethylene are corrosion-resistant alternative, as are PVC or copper pipes. However, they can be relatively costly. Either way, the plate-and-frame heat exchanger always should be stainless steel.

A closed-loop system separates the process water from the tower water, thereby preventing the introduction of contaminants. Heat is exchanged between the two loops at a plate-and-frame heat exchanger.

Benefits of Closed-Loop Systems

Closed-loop systems eliminate common water-related problems because the water remains unadulterated once it is initially filtered and chemically treated. Other benefits of closed-loop process cooling systems include:
  • Reduced energy costs due to improved heat transfer capacity.
  • Decreased equipment maintenance activities -- generally limited to once-a-year cleaning of the plate-and-frame heat transfer element -- along with increased manufacturing uptime and higher production rates.
  • Greater system reliability due to a lower risk of fouling.
  • Improved efficiency of chemical water treatment programs because chemical loss is decreased.
A closed-loop process cooling system constructed of corrosion-resistant materials categorically prevents the introduction of contaminants to process water and is an effective way to forestall against avoidable equipment deterioration. Moreover, once the system is filled, the amount of water required to maintain volume is a small percentage of the total, which reduces direct water costs.