What to look for in today's portable chillers.
Although the basic concept of the refrigeration circuit has remained unchanged over the years, a number of relatively new features have become available on portable chillers to maximize flow of water to the process and maintain accurate temperature control. Scroll compressors, tube-in-tube evaporators, and autotuning are among the "bells and whistles" that help control the process or cut down on maintenance headaches. These standard or optional features, coupled with proven ones such as hot-gas bypass and nonferrous construction, all help keep your process running at maximum efficiency.
This article highlights key features to look for in a portable chiller in the 2-10 ton capacity range - the most popular size, which satisfies a broad range of "average" molding applications. Air-cooled chillers are slightly more expensive than the water-cooled variety, but a molder can generally expect to spend from around $4000 to $10,000 for a unit of this size.
SIZE PUMPS CORRECTLY
For portable chillers, it's important not to skimp on pump size, according to Al Fosco, sales director of Conair's Heat Transfer Group. Typical pumps for portable chillers range in size from 1/3 to 1 1/2 hp, but proper pump sizing depends on your mold's cooling requirements.
Chillers typically see 5-7 psi internal pressure loss, which must be taken into account when sizing the pump, says Tom Benson, sales and marketing v.p. of Thermal Care. One way to minimize the effect of pressure drop is to use two pumps, he says, one dedicated to circulating water through the evaporator and the other sending the water out to the process. Dual-pump systems, which are more common in larger chillers (25-ton range), may be warranted on smaller units if there is an unusually high flow requirement, according to Benson. On the other hand, drawbacks of dual pumps are increased energy consumption and greater chance of pump failure, he adds.
One relatively low-cost alternative to dual pumps for increasing flow to the mold is to design a bypass loop around the evaporator. To maintain cooling efficiency, the evaporator is designed to allow only a certain flow rate - typically 2.4 gpm/ton of cooling, explains Pat Oza, president of Cooling Technology. A bypass loop allows 2.4 gpm/ton of water to pass through the evaporator while more water flows around it. Then the two streams merge before flowing to the mold.
A CHOICE OF HEAT EXCHANGERS
Alternatives to the standard shell-in-tube evaporators in portable chillers can minimize pressure drop, increase heat transfer, or reduce a chiller's footprint.
One type is the tube-in-tube evaporator, which has a large (5/8- to 7/8-in.) orifice that minimizes pressure drop to less than 2 psi on average, maximizing flow to the process, according to Conair's Fosco. Copper surfaces within the evaporator eliminate corrosion and scale build-up that can reduce chiller efficiency.
Another alternative is brazed-plate evaporators, consisting of a series of thin plates stacked side by side, which create passages that alternately carry water and refrigerant. This design is said to offer high heat-transfer rates and to be especially forgiving of freeze-ups, says Benson.
Brazed-plate evaporators are very compact, allowing portable chillers to be designed with smaller footprints. Stainless-steel construction also makes them corrosion resistant. But there is a tradeoff: Brazed-plate evaporators have tighter passages than other types of evaporators, according to Kevin McJoynt, marketing manager for water products at AEC/Application Engineering.
COMPRESSOR EFFICIENCY IS THE KEY
Most suppliers today use hermetic (reciprocating) compressors on chillers up to 10 tons and semi-hermetic types on larger units. An up-and-coming alternative for small chillers is scroll compressors, which are offered by several suppliers as an option that costs slightly more. Compared with standard compressors, scroll types reportedly are more efficient, have fewer moving parts, and are less subject to mechanical failure. Scroll types are also said to be more tolerant of liquid refrigerant "slugging."
Scroll compressors are currently limited to small sizes - up to about 15 hp - although they can be "paired" to work together in the same refrigerant circuit on larger chillers.
For larger chillers, discus-type semi-hermetic compressors offer better efficiency than reed-style semi-hermetic compressors, according to McJoynt of AEC. The key advantage of discus compressors is up to 18% higher efficiency, he says.
As a safety feature, compressors should have an anti-cycle timer that prevents the compressor from restarting for about two and a half minutes after cycling off, according to Thermal Care's Benson. The time lag helps prevent the motor windings from overheating.
Two different approaches are commonly taken to reduce the capacity of the chiller during low-load conditions: hot-gas bypass and cylinder unloading.
Hot-gas bypass introduces an artificial load on the chiller by allowing some of the refrigerant to bypass the condenser and go directly to the evaporator. This reduces the capacity of the evaporator, resulting in less cooling. Most suppliers offer hot-gas bypass either as a standard feature or an option. Hot-gas bypass cuts wear and tear on the compressor because it reduces frequent starting and stopping. It also helps maintain accurate temperature control by holding the water temperature within a tighter window. Benson suggests that it is a good idea to ask what percentage reduction of a chiller's capacity will result from hot-gas bypass. Most suppliers offer hot-gas bypass to allow the chiller to run efficiently at 50% of maximum load; others allow the chiller to run efficiently at as little as 20% of maximum load, preventing cycling of the compressor at the lower load.
The other capacity-control method, cylinder unloading, is typically used with semi-hermetic compressors. On a four-cylinder compressor, cylinder unloading may stop two cylinders during a low-load condition and restart them as the load increases. Temperature control is not as accurate with cylinder unloading, says Benson, because it results in abrupt capacity swings, rather than a gradient. On the other hand, cylinder unloading does result in some energy savings.
WHAT CONTROLS DO YOU NEED?
Essential control functions include dual display of setpoint and process temperature, ability to change the setpoint easily, and certain alarms, according to Kevin McJoynt of AEC. "Look for basic alarming for low flow rate, high and low refrigerant pressure, abnormal temperature, and pump overload," he advises.
Some means of measuring the flow to the process is important, according to Ron Wolfe, manager of engineering at Advantage Engineering. Some sort of in-line gauge, such as a turbine or rotometer or an electromagnetic meter, is typically used. Advantage Engineering provides a built-in flowmeter that permits easy observation of water flow to the process.
A few suppliers offer autotuning temperature control, which continuously monitors PID constants under high and low loads and adjusts the constants continuously throughout the cycle. Autotuning helps to prevent the chiller from over-cooling at low load, according to Fosco of Conair, which recently introduced the feature on its chillers. Other suppliers agree that autotuning is useful, but do not consider it a must. "It's worth asking about, although this feature is not critical for most molders", says Benson of Thermal Care. "It may be useful in processes such as extrusion, which sees a major heat load, or when trying to maintain very fine tolerances."
MORE SHOPPING TIPS
Here are three more tips to keep in mind when evaluating a chiller:
* How is the chiller capacity rated? Most suppliers rate their air-cooled chillers at 95 F ambient air temperature; others rate theirs at 85 F or 90 F. That can make a chiller appear to have a higher capacity than if it were rated at the higher ambient temperature.
* Look for nonferrous parts. Bronze or stainless-steal pumps, bronze or copper piping, and stainless or plastic reservoirs are least susceptible to corrosion. Also check for filters within the chiller to separate contaminants from the water.
* Properly size all piping and hose connections. Hoses and piping between the chiller and mold should be sized to minimize pressure drop, and fittings should not restrict flow.
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|Title Annotation:||plastics industry|
|Author:||Gaspari, John de|
|Date:||Oct 1, 1997|
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