How Variable-Speed Portable Chillers Can Help
An alternative for process cooling applications, variable-speed compressors coupled with advanced PLCs continuously monitor the heat load and adjust the compressor speed for peak efficiency and temperature control.
A packaged system housed in a single cabinet with mounted casters, a portable chiller contains an integral tank and pumping system and uses refrigeration to cool a water circuit for industrial processes that generate heat. The removal of this heat allows for increased productivity and, therefore, increased profits. Effective cooling using portable chillers is the backbone of process industry applications such as injection molding.
Conventionally, a portable chiller uses a compressor running at a continuous speed. This type also is known as a fixed-speed compressor. Regardless of the requirements placed on it by the process load, the speed remains the same.
Fixed-speed chiller systems employ a hot gas bypass design to maintain the demand on the compressor. This design avoids wide fluctuations in the coolant temperature delivered to the process.
A hot gas bypass is an electronic valve located on the high pressure refrigerant line downstream of the compressor. The chiller’s controller — typically a microprocessor — reads the water temperature returning from the process. The controller actuates the valve as needed, allowing a portion of hot refrigerant gas to be reinserted — or bypassed — directly into the low pressure refrigerant line upstream of the evaporator. The evaporator is the heat exchanger that transfers heat from the process water to the refrigerant circuit. Introducing the hot, high pressure refrigerant to the low pressure side creates a false load on the system. This allows the compressor to continue running at 100 percent speed and maintains a stable process water temperature.
For example, a hot gas bypass system, limited to 75 percent of compressor capacity on a 10-ton chiller, delivers stable water temperature down to a 2.5-ton heat load. As the heat load falls below the 2.5-ton mark, the compressor is forced to cycle on and off as its last means of temperature control and, ultimately freeze protection. With any motor, rapid cycling is detrimental to the life expectancy. This is known as short cycling. An anti-short-cycle timer is employed to protect the compressor from premature failure.
A conventional hot gas bypass system coupled with a widely varying process heat load leads to rapid temperature swings delivered to the process. Many process cooling applications require stable incoming cooling temperatures. Avoiding fluctuations like this is crucial to consistent output from the process and eliminating scrap.
An Alternative to Fixed-Speed Chillers with Hot Gas Bypass
Variable-speed chillers offer an alternative for process cooling applications. They use variable-speed compressors coupled with advanced PLCs. The controller continuously monitors the heat load and adjusts the compressor speed for peak efficiency and temperature control. The speed adjustment eliminates the need for hot gas bypass. These chillers ensure the work done closely follows the energy required, effectively eliminating energy waste. Studies by one manufacturer have shown that a 20 percent reduction in motor speed can lead to a 51 percent energy savings.
With the variable-speed chiller design, incoming utility power is connected to an inverter control drive using frequency modulation to adjust compressor motor power output. The inverter control drive speeds up or slows down the compressor, matching system load demand accurately. The sophisticated electronics incorporated in the inverter control drive provide additional levels of compressor protection that surpass the levels of protection found in fixed-speed systems. Active protection algorithms improve reliability and safeguard the compressor and inverter control. Key features are locked rotor protection, phase protection and correction, maximum operating current detection, discharge line temperature protection and anti-short-cycling control.
Energy savings are one of the most prominent benefits of chillers designed with variable-speed compressors. Variable-speed technology offers benefits beyond energy savings, however. One of the most important is soft-start control, which provides two ancillary benefits: First, it effectively eliminates peak demand penalty costs, and second, it reduces mechanical stress on all system components. The life expectancy of all system components is extended.
What Kind of Energy Savings Can I Expect?
Compared to a typical chiller with a fixed-speed compressor, a chiller with a variable-speed compressor can provides power savings at partial loads. For instance, compare the performance of a fixed-speed vs. variable-speed compressor on a 10-ton chiller (figure 1). At a 30 percent load, for example, the variable-speed compressor consumes 24 percent of full-load power. The fixed-speed chiller consumes 85 percent of full-load power. Using a variable-speed compressor at this load would provide energy savings of more than 50 percent (table 1).
In a second example, compare the performance of a fixed-speed vs. variable-speed compressor on a 20-ton chiller (table 2). In this case, a portable chiller with two 10-hp compressors generates savings over a wider range. This specific chiller design combines a 10-ton variable-speed compressor with a 10-ton fixed-speed compressor to generate energy savings. It efficiently handling loads from 3 to 20 tons, and it delivers energy savings, especially in the 3 to 8 ton range and the 13 to 16 ton range.
Note that the energy savings are not as significant in the middle-load range where either compressor — fixed or variable — would be running near its full capacity of 10 tons. The curve for the 20-ton fixed speed chiller is a bit different too (figure 2). It has two 10-ton compressors so that it can run on just one (half power, more or less) until the second compressor needs to kick in. Still, it is clear that there are significant energy savings to be realized.
In conclusion, the energy savings generated by variable-speed compressors enable rapid payback on an investment, particularly under low and partial-load conditions.
In the end, one must weigh the benefits of employing variable-speed technology. Key benefits that support the switch include stabilizing the process, preserving the equipment investment and saving on electrical costs.
4 Questions to Consider Before Purchasing a Variable-Speed Chiller
What is my load profile? Does my heat load vary widely?
You should consider if your heat load spends a lot of time operating well below the chiller’s available capacity.
Will I require flexibility to move the chiller among various different applications?
If you have multiple processes that run intermittently with different heat loads, a single variable-speed chiller may be the best solution rather than purchasing multiple individual chillers of different sizes.
Does standardizing to one chiller size make sense?
If you can cover a variety of heat loads with one style chiller, you can benefit from commonality of parts.
Are energy savings or green initiatives important to my company?
In some instances the electrical costs alone justifies the decision. Other times, a company is working toward a more environmentally friendly culture. This is reinforced with the energy savings from the variable-speed chiller.