Energy Saver: Nix Non-Condensables

Several years ago, plant managers at a large petrochemical plant in Louisiana asked me to look at a complex refrigeration system experiencing an ever-increasing power demand with almost no change in the system’s process cooling load requirements. This was very baffling because operations had tried everything possible — compressor maintenance, heat-exchanger tube cleaning, etc. After spending some time with the plant’s operations and maintenance staff and doing a system walk-through, I had some ideas and needed to do some additional due diligence to confirm them. A quick data analysis and sampling of refrigerant at a few critical points in the system revealed non-condensables as the culprit.
    
As the name implies, non-condensables are substances that do not condense within systems at normal operating pressures and temperatures. Hence, non-condensables have to be mechanically removed.
    
Chiller and refrigeration systems are all closed systems, but non-condensables (sometimes called inerts) can enter at several different locations and times. Most get introduced during routine system maintenance, particularly if there’s no clear procedure for proper evacuation and purging of non-condensables from the system prior to refrigerant charging. Generally, refrigeration and chiller systems operate at positive pressures, but if processes require low temperatures then, depending on the refrigerant, the system could be operating in vacuum conditions and continuously sucking air into the refrigerant system from leak points. To put this in perspective, for an R22 system, process temperature requirements below -35°F will possibly require vacuum conditions in the evaporator of the refrigeration system. For refrigerants R134a and ammonia, the corresponding process temperature requirements are -10°F and -22°F, respectively.
    
Non-condensables in the chiller and refrigeration systems can cause several issues but the major energy efficiency penalties are due to increased head pressure for the compressor and the blocking of heat transfer surface area in the condensers. As head pressure increases, the compressor needs more power per unit of cooling delivered by the system. Because the tubes get enveloped by non-condensables, the condensers lose tube surface area and refrigerant vapor has less room to condense. This directly increases head pressure and leads to higher compressor power. Increasing levels of non-condensables also can negatively impact the chiller and refrigeration system’s capacity, creating a huge problem for operations — especially in the peak summer months.
    
A detailed system analysis can determine the true impact of non-condensables. The energy efficiency impact depends on the refrigerant used, refrigeration temperature required and whether the chiller and refrigeration system is air-cooled or water-cooled. For example, in a R134a water-cooled system providing 5°F process refrigeration, ~10% non-condensables increases energy use ~7.5%. If the same R134a water-cooled system provides 44°F chilled water, ~10% non-condensables results in an additional energy use of ~10%.
    
Once in the system, non-condensables migrate and collect at high points that have no outlets, or in condensers of the refrigeration system. Methodologies used to detect non-condensables are straightforward and should be part of the operators’ routine periodic follow up. They include:
    
1. Compare refrigerant saturation temperatures and pressures with actual operating conditions. Higher operating pressures than saturation pressures could indicate presence of non-condensables.
    2. Conduct thermal imaging on the surface temperature of the condenser. A non-uniform or a large area with an ambient temperature could represent presence of non-condensables.
    3. Collect a sample of the refrigerant and perform a gas chromatographic analysis to determine percentage of non-condensables.
    
Once the presence of non-condensables is confirmed, they should be removed immediately using proper purging units and procedures. By the time you read this column, it will be getting warmer and approaching summer in the Northern hemisphere. I am hoping you have now developed a procedure for testing for non-condensables to ensure they are purged from your systems before the warmer months ahead!