The choice of evaporator feed and oil cooling methods provides ways to reduce ammonia (R717) charge in industrial systems, with glycol offering the lowest-charge scenarios, according to Brian McMillan, Director of Preconstruction, InterCool USA.
McMillan delivered these remarks during his presentation for the Contractors Panel at the ATMOsphere (ATMO) America Summit 2023 on natural refrigerants. The conference took place June 12–13 in Washington, D.C., and was organized by ATMOsphere, publisher of R717.com.
Based in Carrollton, Texas, InterCool USA is a national design/build/service industrial refrigeration contractor primarily focused on the cold storage, food and beverage, and pharmaceutical market segments.
McMillan’s presentation primarily delved into low-charge options in central ammonia systems, emphasizing the potential for reduced ammonia charges in different scenarios. With concerns regarding environmental impact and safety growing, minimizing the amount of ammonia in systems without compromising efficiency is a significant step forward, he noted.
“The quest of low ammonia charges isn’t just a desirable outcome – it’s becoming an imperative,” said McMillan.
Screw compressor oil cooling
McMillan presented a comparative analysis among different types of oil cooling systems for screw compressors, including liquid injection oil cooling (LIOC), glycol/water oil cooling (GOC/WOC) and thermosyphon oil cooling (TSOC).
In LIOC, a high-pressure liquid refrigerant undergoes expansion through a valve, transforming into a gas-cooled, low-pressure liquid. This cooled liquid is then directly injected into the compressor. As it meets the hot screw threads during compression, it evaporates, cooling both the oil and the suction refrigerant vapor in the process.
GOC is an external type of cooling. A separate coolant – glycol or water – is circulated through an external heat exchanger. This exchanger removes heat from the compressor oil.
TSOC relies on the natural phenomenon of hot fluids’ rising and cooler ones’ sinking. TSOC allows the hot oil from the compressor to rise into an oil cooler. As it gets cooled, the oil then descends back to the compressor. The cooling effect is achieved through the evaporation of the refrigerant in the oil cooler.
When it comes to its ammonia charge in refrigeration systems, there were contrasts among the three examined systems. LIOC carries an ammonia charge of 240lbs (108.8kg), making it a mid-range option. GOC systems stand as the most preferred system by achieving a zero ammonia charge. The TSOC has the highest charge, weighing 400lbs (181.4kg).
On the financial front, the liquid injection system is the baseline with a relative cost of 1.0, said McMillan. The glycol oil cooling system comes with slightly higher relative cost (1.05) “due to additional lines or separate hydronic circuit that carries water or glycol.” TSOC comes in slightly below glycol at 1.04.
Overall, the glycol oil cooling system presents a promising future as it offers enhanced performance and safety by eliminating the necessity for ammonia charges in high-pressure circuits, according to McMillan. “Initial costs may be higher with glycol systems, but the long-term benefits – both environmental and operational – cannot be underestimated,” he added.
Analysis of evaporator feeds
McMillan also presented a comparison among a direct expansion (DX), glycol-fed and recirculated (pump-overfed) evaporator systems.
“Each evaporator feed system offers its own set of advantages and challenges,” he said.
But in terms of ammonia charge, the DX and recirculated systems come with a notable ammonia charge, whereas the glycol system, despite initial costs and complexities, “stands out for its commitment to safety and environmental benefits by keeping zero ammonia charge,” said McMillan.
In his comparison, the DX system, with its compact design, has a charge of 8.6lbs (3.90kg). This method is mostly favored for its smaller line sizes, avoiding complexities related to the suction riser and two-phase flow, and resulting in less pressure drop, he noted. However, it has several challenges, like larger coils, liquid subcooling, liquid management and complex valves and controls.
The glycol-fed system offers a completely zero ammonia charge in the evaporator, improving safety by keeping ammonia confined to the machine room. However, adopting the system has some challenges, like higher initial cost, complexity in setup, more robust piping and coil and being mostly suitable for low-temperature applications.
The recirculated system closely mirrors the DX system in terms of ammonia charge at 8.7lbs (3.91kg). Its design elements focus on efficiency, but like the DX system, it doesn’t offer complete ammonia elimination in the evaporator.
“The quest of low ammonia charges isn’t just a desirable outcome – it’s becoming an imperative.”Brian McMillan, Director of Preconstruction, InterCool USA