When organic heat transfer fluids are used in cooling systems, moisture and icing can present serious problems. Moisture can cause pump cavitation and two-phase flow during heating cycles. Icing will reduce the efficiency of the chiller and occasionally can cause blockage in the system piping. Avoiding these problems requires effective moisture removal both before the fluid is introduced to the system and during system operation.
Before Fluid IntroductionBefore the fluid is introduced to the system, all the low points in the piping system and the expansion tank should be checked for water and drained. The system should be purged with dry air or nitrogen for a period of 12 to 72 hours. The amount of moisture removal can be increased by increasing the flow rate of the purge-gas stream. A convenient hookup point for the purge-gas supply is the system-surge expansion tank with the purge stream discharging through the system low-point drains and high-point vents. (Note that all system circulation pumps should be secured to prevent rotation during the purging operation to avoid dry running.) Monitoring the purge-gas stream humidity will ensure the effectiveness of the drying operation. After purging, leaving a slight nitrogen pressure on the system will prevent moist air intrusion.
If the system is heat traced, it can be heated under vacuum while the dewpoint of the system atmosphere is monitored. A slight nitrogen gas purge into the system under vacuum will aid the moisture removal. (Be sure the vessels in the system are rated for the vacuum level.) After purging, all low points in the system should be checked again. If the system is not completely dry, the purging can be repeated. When the system is dry, a low-pressure nitrogen blanket should be left on it until it is ready to be filled with the organic heat transfer fluid.
To prevent moisture contamination of the organic heat transfer fluid in drums, the drums should be kept under cover. The change in the drums’ vapor space pressure as a result of ambient temperature changes can cause moisture intrusion of free-standing water on the drum tops.
Free Water RemovalIf moisture is present in the system after the organic heat transfer fluid is charged to the system, the moisture often is above the saturation level. This visible moisture, known as free water, is seen as water droplets on the bottom of clean glass containers. The free water often can be removed by opening the system low-point drains periodically after brief system circulations. The water should be allowed to settle for at least one hour before the low-point drains are checked for moisture.
Once the system is in operation, low levels of free water in the chiller system can be removed by filtering ice particles through 100-mesh strainers. Dual strainer systems are recommended for continuous ice-particle removal until the leakage source can be plugged. The presence of ice particles in the strainers might indicate that a significant amount of ice is present on the chiller heat-exchanger tubes, so that part of the system also should be checked.
Saturated Water RemovalAfter the free water is removed, the moisture level in the organic heat transfer fluid will be at the saturation level. Although the organic heat transfer fluid moisture saturation level usually is low, the soluble moisture still might cause icing problems in the cooling system.
Saturated water can be removed from organic heat transfer fluids with molecular sieves, azeotropic separation or a nitrogen purge.
Molecular Sieves. Placing molecular sieve towers in a side stream provides bypass flow control. As the fluid circulates over the sieves, the moisture is removed. The sieves used in this application typically are 8 to 12 mesh and have the capacity to remove approximately 2 lb of moisture per 100 lb of sieves. Moisture can be removed down to the 1 ppm concentration level by this method.
Azeotropic Separation (Dean-Stark trap).In this method, a collector/separator and condenser are added to the system (figure 1). To start, a low-pressure nitrogen blanket is placed on the system. The organic heat transfer fluid and water mixture is heated initially to the saturation temperature of water, and the temperature gradually is increased to the boiling point of the organic heat transfer fluid as the water is removed from the system. The water and organic heat transfer fluid vapors are taken overhead from the separator, condensed and collected in the collector until the visible water/organic heat transfer fluid interface is seen high in the sight gauge, with the water below the interface. The water then is drained from the collector for disposal, and the organic heat transfer fluid is passed back to the separator. This procedure should be repeated until only clear organic heat transfer fluid collects in the collector. The dry organic heat transfer fluid is returned to the system. This method is particularly effective for large systems and those that experience frequent moisture leakage to the cooling system.
Nitrogen Purge. By running the system expansion tank hot, the moisture can be removed by purging nitrogen through the expansion tank vapor space. The fluid can be circulated through the expansion tank from 160 to 340°F (71 to 171°C). Nitrogen is purged through the vapor space in the expansion tank for a period of time ranging from four to 24 hours. Lower tank operating temperatures and slower nitrogen flow rates will increase the time required for drying. Equal portions of organic heat transfer fluid and water can be removed during this operation.
This method is convenient for removing large quantities of free moisture from small to medium-size systems that can be heated to higher temperatures. Since organic heat transfer fluid can be combustible, the purge stream must be vented to a safe area and collected for disposal. The molecular sieve procedure described previously can be used in conjunction with this method to remove the moisture that is soluble down to 32°F (0°C).
Efficient OperationMoisture in the system can cause a loss of chiller efficiency, increasing pressure drops across the strainers, cavitation in pumps operating near the boiling point of water and other operational problems. Purging the system before introducing the organic heat transfer fluid and implementing the appropriate filtration and moisture removal procedures during operation can keep the system running smoothly and prevent shutdowns due to ice fouling.
SIDEBAR:Another way to prevent moisture contamination in the cooling system is through good design practices. Cooling water heat exchangers should use double tubesheets and seal welds. The nitrogen inerting gas supply on the system expansion tanks should be maintained at a low dewpoint, and the use of dryers should be considered as a way to ensure a dry nitrogen supply.
Preventing Moisture Problems Through Good System Design
Additionally, effective moisture traps can be designed into the system. For example, many systems use low-temperature thermal surge tanks to store large quantities of cold organic heat transfer fluid. The discharge from these tanks can employ a stand-off pipe in the bottom of the tank where free moisture can accumulate on startup. A drain can be placed in the bottom of the tank near the stand-off pipe to remove free moisture effectively.
And don’t forget vents - high-point vents in process equipment are necessary for insoluble gas removal, and vent valves might need to be used for continuous removal.