Developing an industrial process chiller system for low temperature applications has many challenges, but it can be — and has been — done. This article discusses common challenges and shares insights into research that was used to design a chiller system that operates to -45°F (-43°C) fluid temperatures. For the purpose of this article, the focus is on two-stage compression methods with a single compressor circuit.
Operating ranges on glycol-based chiller systems continue to decrease. Currently, it is possible to operate effectively down to the -5°F (-20.6°C) range. Applying injection and digital scroll technology enables designers to reach even colder fluid temperatures — to -30°F (-34.4°C) fluid ranges. But, this places limits on the heat transfer fluid and the ability to exchange the heat while protecting the system from freezing. A review of heat transfer fluids will look at fluids that enable lower operating temperatures.
Consider the Two-Stage Compressor
A two-stage reciprocating compressor is recommended to meet the operating fluid temperatures below -5°F. Utilizing a two-stage, semi-hermetic compressor introduces an intermediate compression cycle, and compression ratios are greatly improved (figure 1). Some of the compressor cylinders work to increase the suction pressure from entering the compressor into the intermediate pressure. The remaining cylinders take on the work of finishing the compression cycle to the discharge-pressure side of the system.
What About Applications Below -45°F?
It should be mentioned that for temperatures below -45°F (-43°C), a cascade refrigeration circuit is recommended. A cascade refrigeration system is an effective but also more complicated refrigeration circuit. Cascade systems also can utilize a combination of single-stage compressors and two-stage compressors, further complicating the circuit design.
There is a compression ratio between the suction and intermediate stages as well as a compression ratio between the intermediate and discharge stages. Both ratios operate within tolerable levels and ensure that the compressor works effectively at the lower operating temperatures. For example, an air-cooled, single-stage compressor circuit easily could be exposed to compression ratios greater than 10:1. By comparison, with the same air-cooled configuration using the two-stage compressor, it would operate between 3:1 and 4:1 in the two stages of compression.
Incorporating intermediate stage subcooling with the two-stage compressor lowers the liquid refrigerant temperature being supplied to the metering device. The expanded vapor is returned into the compressor intermediate stage. This can be added to increase compressor capacity and energy efficiency ratio.
Utilizing a two-stage reciprocating compressor is recommended for applications with operating fluid temperatures below -5°F.
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Heat Transfer Fluid Pros and Cons
Many types of heat transfer fluids can be used to achieve adequate freeze protection. Three common types of heat transfer fluids are propylene glycol based fluids, potassium blends and silicone blends (figure 2).
Propylene glycol reaches the recommended minimum operating temperatures at -30°F (-34°C). Propylene glycol-based mixtures in this temperature range have a viscosity of 497 centipoise, requiring larger pumps that might be impractical. Also, propylene glycol-based fluids have a thermal conductivity of 0.156.
By comparison, potassium-blended products have a viscosity of 28.3 centipoise at -50°F (-45.5°C) and a thermal conductivity of 0.615. Silicone-based heat transfer fluids have a viscosity of 244.3 centipoise and a thermal conductivity of 0.084.
The three most common types of heat transfer fluids for process chillers are propylene glycol, potassium blends and silicone blends. The viscosity and thermal conductivity values for each type are compared.
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When a fluid is stored at an operating temperature of -45°F (-43°C), one must account for the potential volumetric expansion percentage of the fluid. For example, a potassium-blend heat transfer fluid can expand as much as 3 to 5 percent when the fluid temperature is increased from -45°F (-43°C) to an ambient temperatures of 80°F (26.6°C). Yet, in facilities where low temperature chillers are employed, it would not be uncommon to place the system into a standby mode throughout the weekend, which means the heat transfer fluid in the system could realistically reach ambient temperature on a consistent basis. It is recommended to allow for this expansion range by means of an expansion vessel or pressure-relief device.
While viscosity, thermal conductivity and volumetric expansion of a heat transfer fluid are important, other factors such as polymer compatibility, corrosiveness, toxicity and flammability also should be considered. There are numerous manufacturers and products available for heat transfer fluids, so consultation with each fluid manufacturer’s application engineering documentation is necessary to identify specific brand characteristics.
Insulation thickness and sealants have a significant impact on maintaining consistent fluid supply temperature and avoiding excessive compressor cycling.
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The Importance of Insulation
Insulation thickness and sealants will have a significant effect on maintaining a consistent fluid-supply temperature and avoiding excessive cycling of the compressor. The heat losses on fluid and refrigeration piping easily can add more load to the system than the compressor can support.
At subzero operating temperatures, the heat escaping through improperly insulated piping will be below the dewpoint of the surrounding air. This will lead to moisture accumulation on the exterior of the piping systems. In extreme situations, this accumulated moisture will freeze into an ice thickness heavy enough to create damage to components.
Sealing the insulation joints with a flexible sealant that has had the water-vapor transmission properties inspected will prevent moisture accumulation at sealed joints (figure 3). Do not rely on insulation glue or adhesive as the primary sealant.
Table 1 outlines the different R-values of common closed-cell piping and sheet insulation. It is recommended to use the highest possible R-value insulation that is practical for the use.
The R-values for common closed-cell piping and sheet insulation are shown. It is recommended to use the highest possible R-value insulation that is practical for the use.
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Selecting Sensors and Controls
Many off-the-shelf temperature sensors are only certified to -40°F (-40°C). Not only are the temperature sensor limitations for the controls important, the limitations of the tools being used for testing and certification also must be confirmed. Many standard thermometer temperature sensors will demonstrate a high range of variability at temperatures below -40°F — if not also stop functioning altogether — at these lower temperatures.
Modern IIOT-enabled control platforms provide flexibility in operation, data collection and remote access and monitoring.
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Process cooling equipment users today expect a control system equipped for industrial IoT operation paired with a cellular gateway device. Such a controls scheme provides dedicated access via a mobile app or web-based portal. IIOT-enabled controls also can serve the end user’s data to additional control systems, or UI/UX devices, within their facilities. These new platforms provide flexibility in operation, data collection, remote access and monitoring from anywhere. Operating data point collection for analysis is captured and delivered to mobile devices using both cloud and edge-based devices. With machine learning and artificial intelligence development, the days of identifying issues before they occur are approaching very quickly. PC
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