Avoid system shutdown due to fluid freezing. Follow these five recommendations to keep your heat transfer fluid system out of trouble. Heat transfer fluids remove and carry thermal energy from one location to another. In a cooling process, the fluid removes heat from the user's process while being pumped through a cooling source, where it gives up thermal energy. Heat transfer fluids play an important role in many cooling applications, including chemical, pharmaceutical, freeze-drying, food and beverage processing and climactic chambers. Petroleum-based natural and synthetic hydrocarbons, silicone oils, aqueous brines and glycols are popular low temperature fluids for those uses.
An important characteristic of a low temperature heat transfer fluid is its freezing point, which is the highest temperature at which solid-phase starts to appear. For a pure compound, complete freezing occurs at one particular temperature, and melting usually occurs at that same temperature. For a blend consisting of two constituents, complete freezing only occurs below the eutectic point of that blend, which is the lowest temperature at which each of the components of that blend remain in a liquid state.
The composition of the fluid -- or, to put it another way, the concentration of each component -- that defines this temperature is known as the eutectic composition. If a fluid is not at its eutectic composition, then it does not freeze completely at its designated freezing point and becomes a slurry. At that temperature, known as the initial crystallization point, the first crystals appear. As the temperature drops below the initial crystallization point, more and more frozen solids form until the temperature reaches the eutectic point, where complete freezing occurs.
A fluid freezing on the surface of the cooling source in a low temperature system is analogous to fluid breakdown on a heater surface in a high temperature system. Figure 1 illustrates a heat exchanger wall and temperature distribution across it.
The cooling source is usually a refrigerant. The heat transfer fluid adjacent to the heat exchanger wall creates a stagnant liquid film, which acts like a barrier to heat transfer. The temperature at the wall/fluid interface could be several degrees below the bulk fluid temperature, and freezing can occur at the interface if the initial crystallization point of the fluid is higher than the refrigerant temperature.
Frozen Fluids Act as Insulation
Several problems can occur in a system when the heat transfer fluid freezes on the cold heat exchanger surface. The frozen fluid solids significantly reduce the heat transfer rate by acting as an insulator. Also, the accumulation of frozen fluid reduces the width of the channels in the heat exchanger, which increases pressure drop and pumping power. If the fluid freezes as slurry, the frozen particles may abrade the pump impellers and other vital components of the system. Moisture or fluid freezing inside a valve may cause it to jam.
A heat transfer fluid can freeze either totally or partially due to the following reasons:
The fluid's freezing point or initial crystallization point is higher than the refrigerant temperature.
One or more of the fluid ingredients separates out as frozen solids at low temperatures.
The composition of the fluid in an open or vented system changes over a period of time, causing a rise in the freezing point.
Moisture from the atmosphere condenses in the low temperature fluid, forming ice crystals inside a nonaqueous fluid. In an aqueous fluid, the moisture condensation dilutes the fluid, raising its freezing point.
Thermal degradation, oxidation and polymerization of the fluid over time increases the freezing point.
Following five recommendations will help operators avoid complete or partial freezing of their heat transfer fluid.
1. Choose the Right Fluid
The first and most important step is to select the right fluid. The fluid should have a freezing point well below the lowest temperature that it will experience and definitely below the lowest refrigerant temperature. Some critical questions to ask the fluid manufacturer are:
How many components does the fluid contain?
What is the expected initial crystallization point?
If the fluid is made up of more than one component, what is the eutectic point?
Glycol/water solutions and salt-based brines are two-component fluids while silicone oils and some hydrocarbon-based fluids are multi-component fluids. It is difficult to predict the initial crystallization point of a multi-component fluid, and experiments may lead to supercooling of the fluid being tested (see sidebar). There is no standardized method developed so far to determine the initial crystallization point of a fluid.
2. Test the Fluid at Regular Intervals
Fluid composition may change from time to time due to venting or degradation. In particular, venting can lead to a change in the freezing point for a multi-component fluid such as silicone oil. A representative sample of the fluid should be sent to the heat transfer fluid manufacturer at regular intervals for verification of the freezing point.
3. Use an Inert Gas in the Reservoir Tank
If possible, a dry inert gas such as nitrogen or argon should be used as a blanket in the head space of the reservoir tank. This eliminates moisture entering from the atmosphere to cause condensation and freezing.
4. Remove Moisture from Nonaqueous Fluids
Moisture in a nonaqueous fluid can separate out as a second phase and freeze. Make sure that the fluid has very low moisture content, typically less than 100 ppm of moisture in the fluid by weight. If the moisture content exceeds allowable values, install an offline desiccation system with a molecular sieve or dehydrated calcium-sulfate-based drying agent. The common practice of raising the temperature of the fluid in a system to above 212oF (100oC) to boil off the moisture is not recommended because it may remove some of the important low-boiling light-end constituents, thus changing the fluid's thermal properties.
5. Check Liquid Nitrogen System Design
The design of a liquid nitrogen system is crucial if it is used to cool a heat transfer fluid. Liquid nitrogen has a temperature of about –320oF (–195.6oC), and all the secondary heat transfer fluids available in the market have a freezing point above this temperature. In addition, a fluid's viscosity increases rapidly as the temperature approaches its freezing point, which makes the system design challenging. It is important to have a high degree of turbulence inside the heat exchanger to minimize the thickness of the stagnant liquid layer. Turbulence can be achieved by a high fluid flow rate and also by inserting static mixers inside the heat exchanger.
Given the many ways a heat transfer fluid can freeze partially or completely and knowing that the problem can force system shutdown, design engineers need to take appropriate steps to prevent freezing of the fluids in their operation.
Melting Point Is Key
This freezing point experiment involving technical grade d-limonene show that, for certain heat transfer fluids, the melting point is more important than the temperature at which solids were first observed.
Satish C. Mohapatra is vice president of engineering at Dynalene Heat Transfer Fluids, Whitehall, Pa., a manufacturer of low and high temperature heat transfer fluids for process applications. For more information from Dynalene, call (610) 262-9686 or visit www.dynalene.com