So, what kind of cooling system is best for plastic extrusion -- a central chilled water system or multiple portable chillers? Both approaches offer certain benefits. Portable chillers require plumbing that is less complicated and less costly; they allow the processor to accommodate widely variable processing temperatures; and they are “safer” than a central system, in that the failure of one unit will not shut down an entire plant.
By contrast, for operations that use similar temperatures for all applications, a central system will require less operating tonnage for a comparably sized plant, occupy less floor space, and involve less equipment that needs to be maintained. As a general rule, any plant in which 80 percent of the materials are processed within a ±5oF (±3oC) temperature range can consider a central chilling system. However, keep in mind that the chiller must be sized to deliver the lowest required temperature. If just one material runs 10oF (5.5oC) below the others, the system must be sized to deliver that temperature and will actually be oversized for the vast majority of processing requirements. Also, a system will require 2 percent more capacity per degree when operating below the nominal rating of 50oF (10oC). In other words, running a material at 40oF (4.4oC) will require 20 percent more (essentially wasted) capacity than running the material at 50oF. In such circumstances, it might be more sensible to size the central system for all but the lowest temperatures and use portable chillers for those special materials.
Running The NumbersConsider this scenario at an imaginary profile extrusion house, Hypothetical Extrusions Inc., to better understand the logistic and economic issues that come into play when selecting a chiller system. HEI has 16 extruders: two 6"; six 4.5"; six 3.5"; and two 2.5" lines. The company processes four different materials: polystyrene (PS), polypropylene (PP), high-density polyethylene (HDPE) and polyvinyl chloride (PVC). While in this example HEI is a profile extruder, the following analysis would be the same for a film or sheet extruder. Chill rolls and bubble cooling would present slightly different loads, but the outcome would be much the same.
Admittedly, it would be unusual for a plant of this size to consider only portable chillers, but we will make that assumption to illustrate the tradeoffs between portable and central units. In addition, a variety of chiller options might be considered in an actual installation, depending on where the plant is location and the availability and cost of water. However, to simplify the analysis, assume that HEI is considering just two options: a system of air-cooled portable chillers, or a central water system that incorporates a single air-cooled, central chiller.
Capacity. As HEI's management begins to weigh various cooling options, it needs to look at the total chilling load of the extrusions. Different materials run at different temperatures and have different specific heats -- that is, they give up the heat required for processing at different rates. Therefore, HDPE, for instance, will require a larger chiller than PS running at the same production rate. These rates and capacities are well established. Table 1 shows the chiller capacity required by the machines and materials being run at Hypothetical Extrusions.
When determining the required chiller capacity, it is necessary to specify what materials will likely be run on which extruders. For the purpose of this illustration, assume that each line will require an average of all possible capacities. Using these averages, the calculation for total cooling required would be:
(12.4 x 2) + (5.8 x 6) + (4.6 x 6) + (4.3 x 2) = 95.8 tons of chiller capacity
The capacity calculations have shown that HEI needs 95.8 tons of chiller capacity when operating at full capacity. Of course, HEI will rarely, if ever, operate at full capacity across the plant. Like most typical extrusion operations, HEI often will have one or more lines down for maintenance or a product change, and even those lines that are running might be operating at a slower speed or with a material requiring less than peak cooling.
However, while it is unlikely that all machines will run at peak load all the time, it is quite likely that any one of the lines will be operating at peak at any given time. For this reason, if portable chillers are used, they must be sized for peak load. By contrast, if a central water system is chosen, it can safely be sized to match whatever load may be reasonably expected -- say, 90 percent of theoretical peak load.
Hot-Gas Bypass. Because portable chillers can be moved easily from place to place, a smaller chiller theoretically could be used any time a given extrusion line is not operating at peak load. This would be true if the processor has a sufficient number of portables of various capacities to match the changing loads around the plant. Even under the best of circumstances, however, chances are good that the chiller will be somewhat oversized and will need to unload some of its capacity. To maintain the proper temperature, therefore, portable chillers usually are designed to induce an artificial load instead. In other words, if the process requires only 85 percent of rated cooling capacity, the chiller will induce the other 15 percent using a hot-gas bypass.
For example, if HEI has a portable chiller that is sized for a 12-ton load, and the process actually only requires 9 tons, then the chiller needs to “lose” 3 tons of cooling by using a hot-gas bypass. But, operating costs are based on actual capacity, so there is a cost associated with those “lost” tons. For this air-cooled chiller, the annual cost of those 3 extra tons would be roughly $1,125/year.
By contrast, a central chiller usually is designed for compressor unloading and can more accurately match output to demand at any given time. Thus, the chiller only needs to handle the actual process load -- not the process load plus the load induced by hot-gas bypass.
Hypothetical Extrusions' InvestmentHaving looked at some of the factors that enter into a portable vs. central decision, it is time to consider some equipment and operating costs that HEI might face. Assume that material cooling must be accomplished using either individual air-cooled portable chillers or a central air-cooled chiller.
Portable Chiller Analysis. Table 2 summarizes the cooling loads presented by the different machines in the HEI plant, while Table 3 details the costs associated with installing and operating portable air-cooled chillers. All figures are approximate, and installation costs are factored in as 10 percent of the equipment cost.
Central Chiller Analysis. Several other factors come into play when considering how a central chiller should be sized.
First, as previously noted, a central chiller usually does not need to be sized for 100 percent of theoretical load. Most companies will still size the chiller for 100 percent or more, but for the HEI example, assume that operating costs will never reflect 100 percent loading.
Finally, the extra load caused by providing 35oF (1.67oC) water, which is required for PVC extrusion, must be taken into account. Factoring in the 2 percent per degree below 50oF rule mentioned earlier, the central chiller must be de-rated by approximately 30 percent.
So, starting with an anticipated process load of 95.8 tons, HEI needs to add 6 tons for the pump load and then reduce the total by 10 percent on the assumption that the full load will never be realized. Then, assuming the chiller capacity must be de-rated by 30 percent, HEI can conclude that it would require a 156-ton air-cooled central chiller. This air-cooled chiller with an associated pump tank would cost roughly $92,000 to operate annually.
Like Hypothetical Extrusions, some companies might find that a central system is less expensive to purchase and less costly to operate than individual portable chillers. However, this will not always be the case. While this particular scenario appears to favor a central water system, it is just as likely that multiple portable chillers could be the right choice in another plant. Additionally, almost all plants that use a central system will have at least a few portable chillers for emergencies or to handle processes that run outside the temperature range of the central system.
Determining which combination of central or portable chiller capacity is right for a given plant can be a difficult task. Water flow and pressure, for instance, are at least as important as cooling capacity, and plant layout and piping distances need to be analyzed for optimum efficiency. For this reason, it is always best to work closely with an experienced consultant or system supplier that can interpret the plant's objectives and design a system that meets current needs and allows for future growth. PCE