The molding and manufacturing of plastic parts requires transferring considerable amounts of heat. First, heat is added to the plastic so it can be formed. Then, the heat must be extracted from the formed part so it can be handled and, ultimately, sold. The efficiency with which the heat is extracted from this process can have a tremendous impact on maximizing productivity and remaining competitive. The most common method of accomplishing this heat extraction is through the use of a portable chiller.
Portable chillers are generally sized in tons of capacity. In this use, a ton is defined as the capability to extract or reject 12,000 BTUs per hour. A 5-ton chiller, for example, can reject 60,000 BTUs per hour. Table 1 shows the relationship between the throughput rate in pounds per hour and chiller size in tons for various processes and materials. From the table, if a chiller is needed for an injection-molding machine that is processing 120 lb of HDPE per hour, a 4-ton chiller should be selected for the job. Generally, portable chillers are available in sizes ranging from fractional tonnage up to about 30 to 40 tons. Beyond that, the units are too large and cumbersome to really be considered portable.
In process cooling, a chiller is used to extract or reject heat from a process. But the chiller does not absorb the heat -- it transfers it. Again referring to the mechanical diagrams in figure 1, you can see the compressor, which is the heart of the refrigeration system, draws low-pressure, low-temperature gas from the evaporator through the suction, or low pressure, side of the refrigerant circuit. Inside the compressor, the gas is compressed, increasing its temperature and pressure. The compressor then discharges the compressed gas to the high pressure side of the system. Next, the gas flows through the condenser, where the heat from the gas is transferred to the condensing media. If the chiller is air-condensed, the heat is transferred to the ambient air. If it is water-condensed, the heat goes to the cooling tower water to be released to the air elsewhere. As the heat is transferred, the temperature of the gas is reduced and it condenses into a liquid.
After passing through a filter and a sight glass, the now-liquid refrigerant comes to the thermostatic expansion valve. This valve creates a pressure drop in the system as the liquid is injected into the evaporator. Once inside the evaporator, which is essentially a heat exchanger, the refrigerant boils off and becomes a gas again as the heat from the process fluid is transferred into it. The low temperature vapor then is drawn out of the evaporator into the compressor to complete the cycle. This cycle is continuous while the compressor is operating.
Selecting the Best SystemThe choice between an air-condensed or water-condensed chiller requires careful consideration. Because portable chillers transfer the heat from the process to its surroundings in one form or another, the environment in which the chiller will be used must be suitable. Air-condensed chillers must be located in an open, well-ventilated space to avoid overheating. If the chiller has a blower rather than fans, the heated air may be carried away via ductwork as long as adequate makeup air is provided. In addition, air-condensed chillers should not be used in areas where the ambient air temperature exceeds 95oF (35oC).
Water-condensed chillers can be operated in hot, closed areas as long as they can be supplied with 85oF cooling tower water to carry away the heat. The amount of water needed will vary based upon the capacity of the chiller. Usually 3 gal per minute per ton at 85oF will be required.
Components and features available in today's portable chillers are drastically different than those offered just a few years ago. Probably the most important change has been the introduction of scroll compressors and brazed plate evaporators. Scroll-type compressors, which use a mating pair of orbiting scroll plates to compress the refrigerant, have fewer parts than their reciprocating piston-type counterparts. They also appear to be somewhat more rugged and can tolerate some "less than perfect" operating conditions. The efficiency of brazed plate evaporators permits the transfer of significant amounts of heat in a compact size. More than anything else, this change has allowed chillers to be much more space-efficient than in the past, making better use of valuable plant floor space. Other features that are more prevalent today are nonferrous construction to minimize corrosion and water contamination and features such as microprocessor controls with SPI communication capabilities.
Figure 2 shows how a portable chiller could be incorporated into a temperature control system for a "beside the press" injection molding operation. The chiller can be used to transfer heat away from the hydraulic heat exchanger, mold temperature controller (thermolator) and feed throat of the injection-molding machine. Although some of these cooling loads could be cooled by other means (tower, well or city water), a chiller offers some distinct advantages. Because the chiller contains a precisely controlled refrigeration circuit, it is capable of supplying a consistent, accurate temperature to the process. This could result in more efficient molding with less scrap. In addition, the chilled water loop is a completely closed arrangement, which will provide the best water quality. This may result in better heat transfer, less required maintenance and decreased downtime. The final advantage is that the chiller can produce much colder water (or water/glycol mixture) than any of the other mentioned options, which may be important under some circumstances.
The array of portable chillers available today is wide and vastly different than those offered just a few years ago. Most of them can be modified easily to accommodate the specific needs of nearly any plastic processor and, with routine maintenance and water quality management, will provide years of service.