Hot Topics for Cool Running
By Jocelyn Chu
Molds can be cooled with chillers, cooling towers, fluid/air heat exchangers, or a combination. Chillers (mechanical cooling) provide cold water from 20- 65 F. Cooling towers (evaporative cooling) a...
Molds can be cooled with chillers, cooling towers, fluid/air heat exchangers, or a combination. Chillers (mechanical cooling) provide cold water from 20- 65 F. Cooling towers (evaporative cooling) and fluid/air heat exchangers provide cooling at around 85 F. Fluid coolers operate at 100-105 F.
FACTORS INFLUENCING CHILLER CHOICE:
1. Application and load: Mold, die, and material characteristics; machine hydraulics cooling requirement; other plant cooling loads; minimum to maximum loading for capacity staging; and whether the load is 24-hours.
2. Economical and environmental: Certain applications may require only a cooling tower or a sealed fluid/air heat exchanger system, resulting in lower energy consumption.
3. Existing capacity: chilling and cooling tower capacities, method of integrated expansion, ambient temperatures, and HVAC issues.
4. Future expansion.
Standard chilling systems combine pumping system, refrigeration circuit with compressor, evaporator, water/fluid cooled or air cooled condenser, and control system. Chillers are available as packaged portable, packaged central, and central systems. Packaged chillers include refrigeration and pump systems with reservoir. Central chilling systems have separate components: chiller module, tank with pumps, and sometimes, remote condenser.
Each type has its own application; as your plant grows, chilling needs evolve. Roger Lambert, president of Temperature Corp. (51), Markham, Ont., says: “Save the most money while plotting your growth curve. Keep it simple and useable by anybody — like your kitchen fridge.”
Steve Petrakis, vice-president, sales for Sterling, Inc. (In Canada, Dier International, Unionville, Ont.; Total Plastics, Delta, B.C.) notes the growing popularity of packaged chillers as a turnkey solution, saying, “They are high capacity systems that are easy to pipe and wire — ready to be plugged in for instant, incremental capacity.” Jeff Amburn, marketing manager at Thermal Care, (In Canada, Hamilton Avtec, Mississauga, Ont.) adds, “Central chillers can be custom-tailored to increase the your system’s capacity.”
There are four methods of chilling: indoor air, water, fluid, and remote air. A decision to go with air vs. water, and portable vs. central depends upon total load, the economics of future expansion, and other factors related to your application. When buying a chiller or chiller system, assess these technical issues:
DEFINITION OF LOAD AND APPLICATION
Unlike your kitchen fridge that is set up for static loads, injection molding requires a chiller made for active heat loads. Many applications have dual cooling loads: one for molds; one for the machine’s hydraulics. Often, cooling tower water or a fluid/air heat exchanger is used. Most molds require mechanical cooling by compressor (chiller). For hydraulic cooling, a chiller might be required. High-temperature extrusion can be chilled by tower water. For extrusion of other materials, a chiller is generally specified.
IDEAL SIZE AND RATING
An all-too-common, post-installation problem is failure to take into account all your loads when sizing a chiller. On what basis did the moldmaker design the cooling passes? Compute cooling needs using the sizing formula appropriate to your mold and type of plastic.
Processing machinery also places a load on your heat transfer system. In injection molding, for instance, even a temperature control unit pump adds 0.2 ton per hp. In extrusion, screws and barrels require one ton per inch of screw diameter. These loads are additive, particularly when using more than one machine. Says Bill Jones, president of Conair (In Canada, Stephen Sales Group, Markham, Ont, and Metaplast/Conair, Lachine, Que.): “It can mean the difference between having adequate capacity and not.”
Performance is rated in tons, with one ton = 12,000 Btu/hr. A cooling tower heat transfer ability is one ton = 15,000 Btu/hr. “Convert ratings to compare apples to apples,” says David Tarquini, project engineer at International Cooling Systems Inc. (Richmond Hill, Ont.). Look beyond the model number. Compare quotes not only for tonnage, but also for type and system characteristics.
Advises Lynn Wheatcraft, marketing manager at IMS Company (Chagrin Falls, OH): “To accommodate bigger molds, larger volume and faster speed, you need a 20 percent Btu oversize margin. Also, cold water passing through pipes warms. Undersized chillers can cost more over time.”
Mark Dykstra, technical services manager at TUC Inc., (Holland, MI) adds: “Don’t overly oversize. Compressors shut down due to no-load, and have to restart. Our units are not prone to premature failure.” For maximum compressor life, Advantage Engineering’s (In Canada, Chillers Inc., Newmarket, Ont.) Ice Cube and Maximum portable series, use hot gas bypass capacity control.
A new line of 10-80 ton packages chillers from Encirc, (61) Concord, Ont., is built on an all-welded tube frame, with an oversized stainless steel reservoir to even out sharp temperature fluctuations, assures Yousry Mikhail, general manager.
How hot is your plastic beforehand? How cool does it need to be afterward? Most molds require cooling to 44 -55F. Know the rule-of-thumb, “before” and “after” temperatures for materials.
When high ambient-temperature conditions exist, water-condensed chillers minimize discharge pressure, costs, and compressor wear and tear. Air-cooled chillers are not suitable for air-conditioned facilities. They pump hot air into the plant, transferring heat from process to air conditioner. Nor should air-cooled chillers be used at 95F. Use tower water where available.
FLOW, FOOTPRINT, CONTROL
Without turbulent flow in the mold’s cooling channels, the outer layer of the coolant along the channel walls insulates the centre of the flow, reducing the volume of cooling fluid. The key to turbulent flow is a system with adequate pump capacity to deliver sufficient coolant volume to the entire system. This is determined by distance the water must flow, number of loads, and pumping system layout.
Says IMS’s Wheatcraft, “With the increasing numbers of add-ons, you don’t need another dinosaur.” Petrakis says the new Sterlco SMC Series (54) achieves “big chiller features in a compact, portable design by putting central chiller features into a portable.” International Cooling Systems Inc.’s space-saving, packaged central system (55) can be located entirely outside. The unit pipes into the building; monitoring is by indoor remote control.
The low reservoir warning light on IMS’s new Summit Series (56) of mold chillers is an illustration of the rapid progress being made in cool control. “Self-correcting,” and “self-adjusting” make “set and forget” less risky than in the past. Most chillers are now microprocessor or programmed-logic controller based. Signals indicate when to turn chillers on and off. Narrow-band controllers hold temperature within one degree of accuracy. Many chillers have one-line character display. The Mid Range System Controller SC5 (57) from Berg Chilling Systems Inc., Toronto, features 2-line character display with touch button controls.
Conair’s new MPA-11 air-cooled chiller (58), like its entire portable line, features microprocessor controls with continuous, easy-to-program, PID auto-tuning for operational stability, with To Process and Set Point LED readout. As shifts in thermal lag are sensed, control constants are automatically reset. Sterlco’s new air and water-cooled portables (52) features high and low process water temperature indication and electronic cutout.
DURABILITY, MAINTENANCE, SUPPORT
Thermal Care’s SQ line of top-discharge, portable chillers (59) feature scroll technology for enhanced quiet and reliability. Of all compressor types – scroll and rotary screw have fewest moving parts. Manufacturers have incorporated scroll compressor technology into smaller-size units. Berg’s latest line central-chiller line (60) boasts screw technology. Sterling’s Petrakis says, “Expect more screw compressor options on larger chillers in future.
Regular maintenance keeps systems running efficiently and guards against freeze-up. TUC’s refrigeration, suction and discharge pressure gauges on its portables (53) are a plus vis-a-vis service and maintenance. Conair chillers (62) have durable, molded plastic reservoirs, corrosion-free evaporators, and Copeland hermetically sealed compressors – all of which reduce the need for servicing.
Encirc’s new Energy Wise System (63) uses a Fluid Cooler in place of a cooling tower for higher-temperature cooling. Under lower ambient conditions, it takes over, providing free winter cooling, and placing the chiller on stand-by. Says Mikhail, “In a recent application for an Ontario bottling plant, which required almost 100 tons of chilling, this feature afforded a potential savings of $35,000 annually.”
Michel Le Prohon, President of a Le Prohon Inc. (64), Sherbrooke, Que., says “The best approach is targeted and turnkey. We analyze the customer’s global needs for cooling from every conceivable angle, including heat recovery and lowest operating cost vs. investment.”
Jocelyn Chu is a writer-editor, illustrator, educational multimedia and web-site content specialist who shuttles between downtown Fernie, B.C. and downtown Toronto. CPL