This two-part article is on a subject very close to my heart. I have spent a significant amount of time working on chiller and refrigeration systems in petrochemical plants. This column deals with what one of my colleagues calls, “First things first” — or fluid management. Next month’s column will focus on minimal or low-cost measures in these systems that provide some quick energy efficiency wins.
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Generally, my work at a plant starts not because someone calls to improve the refrigeration and chiller systems’ energy efficiency but because of an over-arching issue, e.g., system capacity not being met; inability to maintain temperatures; system failures, etc. Irrespective of the issues and problems in the refrigeration and chiller systems, methodology starts with a systems-approach-based data collection and an energy assessment.
So what does fluid management mean? The best explanation and analogy I can give you is our own body. We go for a physical exam periodically. We give blood and urine samples, which are sent to labs for analysis. Then at the doctor’s visit, we get our vitals monitored. The doctor combines the information from the lab reports and the vitals to provide a health assessment complete with potential issues and improvements to make for a long and healthy life.
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This process is identical to what happens in our refrigeration and chiller systems. The fluids that circulate through the system — refrigerant, oil, water, brine, glycol, etc. —should be managed; they reveal information about the system’s performance and reliability. Understanding the sample test results of these fluids can provide a wealth of valuable information for a root-cause analysis. Combining this information with operating data from the system provides the cause-effect relationship, which often gets overlooked.
Refrigerants are heat transfer fluids. Industry standard AHRI 700 establishes composition and purity specifications for fluorocarbon refrigerants. Similar standards exist for hydrocarbon and inorganic refrigerants. The main point of interest: if the refrigerant isn’t up to specifications, then its performance and the system can become compromised. This results in degradation of energy efficiency and capacity, and sometimes, costly unplanned shutdowns. Impurities in the refrigerants could be water (from tube leaks or moisture), chlorides and acids (from breakdown of refrigerant), high boiling residue (oil), particulate matter (from corrosion, degradation of equipment), non-condensables (air and volatile components), etc. Several other parameters come into play, including operating temperatures and equipment type. For example, a shell-and-tube evaporator with refrigerant boiling on the shell side serves as a collection pot for contaminants, particulate, high boiling residue, water, etc. and the evaporator is extremely susceptible to fouling, leading to a sharp drop in energy efficiency and cooling capacity. Hence, it’s very important to periodically sample the refrigerant in the evaporator to ensure it’s always close to specifications and free of contaminants.
Now, let’s talk about oil. Typically, we check periodically for viscosity, water and elemental chemicals in the oil. This is a good best practice. Nevertheless, we shouldn’t stop there. Oil carries a significant amount of information related to equipment wear and rotating equipment health. Wear and tear in the compressor bearings and seals can lead to lower compressor efficiencies and higher temperatures. Using state-of-the-art ferrographic analysis and viewing the oil sample through an electron microscope can provide details of where wear could be occurring, which would allow for immediate corrective action.
Water, brine and glycol all circulate through heat exchangers and process units. Their properties can impact heat transfer coefficients, pressure drops and pumping power. If cooling tower water is used, then fouling can be a significant concern in the condensers. Maintain proper water chemistry to minimize scaling, microbiological growth, etc. Other secondary coolants and heat transfer fluids can develop process contamination, which, if left unchecked, could result in an unplanned shutdown in addition to heat transfer inefficiencies.
In conclusion, understanding fluids and their properties is fundamental to energy- efficient operations. Set up periodic sampling programs and trend the results to manage the fluids in your refrigeration and chiller systems.
