A simple, closed-loop temperature control system typically involves the input of heat, a sensor and a controller. Heat is added to the process using an external heat source like gas burners or electric heaters, and it is removed from the system either by the product as it flows through or by ambient heat losses. The sensor reads the heat loss in the form of temperature and, as needed, the controller turns the heaters on and off.
Some processes, however, generate additional heat — either through mechanical forces or chemical reactions. They require cooling as well as heat to maintain process control. This article will explore the two primary mediums used for cooling and use the plastics extrusion to illustrate the differences.
The two primary mediums used in the cooling process are water and air. The physical characteristics that affect the cooling capabilities of each are specific heat, density and thermal conductivity (table 1).
- Specific heat is a measure of a material’s capability to absorb heat. Specifically, it is the amount of energy (in Joules) required to increase the temperature of 1 gram of mass by 1.8°F (1°C).
- Thermal conductivity is a measure of a material’s ability to transfer heat. It is measured in watts per meter Kelvin.
- Density is a measure of mass. The more dense a material is, the better it can hold heat.
For the purpose of this article, the units of measure are not as important as the relative differences in the numbers. As shown in table 1, water has four times the capacity of air to absorb heat; 21 times greater capability to conduct the heat; and nearly 1,000 times the ability to hold the heat and move it away from the process. This translates to a significantly higher capability for water to remove heat and to do it quickly. In fact, it can be argued that water removes heat while air simply dissipates it.
Two primary processes are used for molding plastic parts: injection molding and extrusion. In extrusion, heat is generated not only by the electric resistance heaters on the outside of the barrel but also through shear forces created by the screw inside the barrel. As the screw turns and moves the plastic pellets down the length of the barrel, the flights of the screw shear the pellets against the inside surface of the barrel. The friction creates heat that, if left unchecked, can cause the process to overheat and burn the material. Most extruders use either air cooling or water cooling to remove the excess heat created by shear.
In an air-cooled system, the air is delivered by electric blowers that are attached to a sheet metal shroud. The shroud directs the airflow over the heaters and barrel before the heated airflow is vented into the surrounding area. The controller turns the blowers on and off as needed.
The heaters used in plastic extrusion are either ceramic band heaters or finned cast heaters. The advantages of cast heaters are durability and greater surface area — for a higher rate of cooling. The benefits of ceramic bands are that they heat up faster and are more energy efficient. To offset the surface area advantage of cast heaters, many ceramic systems can be supplied with sheet metal fins that also help to wick heat away from the process. The fins create more surface area over which the air can flow.
This is an important concept and can be applied to other processes. In effect, the two ways to increase the efficacy of air cooling are to increase the airflow and to increase the surface area of the part or process being cooled.
In a water-cooled system, the water flows through cooling tubes that are either cast into the heaters or placed into grooves machined directly into the barrel. The cooling tubes are plumbed into a closed-loop system that also includes a pump and chiller or heat exchanger. Unlike an air-cooled system, the pump runs continuously, adding energy costs (see below). Water must be keep moving so it does not flash to steam, which can cause instability in the process. The water also will flow through a chiller or heat exchanger so the heat can be removed.
What, then, are the decision factors for picking one system over the other? First, the differences in the costs of the two systems are significant. Air cooling is done with a shroud and blower assembly. The shrouds are made of standard sheet metal with an attached blower. Ceramic band heaters with added fins are the least expensive option at a cost of 50 to 60 percent of cast heaters. Finned cast heaters tend to run 25 to 35 percent more than water-cooled cast. The biggest added cost driver of water-cooled systems is the pump, the plumbing and the heat exchanger or chiller. Overall, the cost to purchase and install a water-cooled system can be as much as one-and-a-half to two times the cost of an air-cooled system
From a maintenance standpoint, water-cooled systems can leak, and the tubing can get clogged with mineral deposits or biological accumulations. Replacing cast heaters or tubing is time consuming and can be costly. It is highly recommended to use only distilled water.
Operationally, significant energy cost differences exist between the two systems. Depending on the resin, water-cooled systems use anywhere from 7 percent (for LDPE) to 80 percent (for PET) more energy than air-cooled systems. As an example, one study tested an air-cooled system on a 3.5” extruder running HDPE. The study found it used 22 percent less energy than the heat/cool system. Assuming an energy cost of $0.08 per KW and that the extruder is running seven days per week and 52 weeks per year, the savings would be $2,684 per year. The output was higher for two (HDPE and PP) of the four resins tested using a water-cooled system. This study did not measure the impact on air-conditioning costs of the hot air being vented into the surrounding work area. A potential opportunity for energy savings would be to reclaim the heat from either the water-cooled or air-cooled systems and use it for general workspace heating.
Lastly, it should be noted that water-cooled systems can be more unstable than air cooled. Besides the potential for water to flash to steam, water cooling gives the operator an opportunity to overcool individual zones. In either case, the temperature profile along the length of the barrel may be uneven.
Clearly, cost and operating benefits can exists for air-cooled systems versus water-cooled systems. However, at a certain point, air cooling may not be able to dissipate enough heat to provide the necessary control. For plastics extrusion, that point is at barrel sizes 4.5 to 6”. As the outside diameter of a barrel gets bigger, the wall thickness increases and, accordingly, the overall mass.
A future study perhaps will develop a surface-area-to-mass ratio that will more definitively mark the boundary between the effectiveness of air versus water cooling. Regardless, the principles described can be applied to many processes requiring cooling. Each process will need to be evaluated to see which cooling method is most efficient with regard to cost and output.