Where do you start in selecting the right heat transfer fluid?

Heat transfer fluids are far more efficient at heating or cooling equipment than air cooling. Because of this, most high performance machines use some form of fluid heating or cooling, resulting in a plethora of different heat transfer fluids to choose from.

Selecting the correct high temperature heat transfer fluid.

The process of selecting the optimum heat transfer fluid
should begin once the energy transfer required by the process and the planned/actual service ratings of the mechanical components of the heat transfer system have been calculated and thoroughly researched.

Since there are a number companies specializing in heat transfer fluids and a wide range of fluid products available, the knowledge of this key element of the system's operating requirements can help to create a set of criteria that can be used to compare various fluids and allow rapid elimination of fluids that are not best suited for the application.

However, before comparing various individual heat transfer fluids, a lot of time and effort can be saved in the selection process can by comparing the various chemistries of the fluids.

Once a fluid chemistry is selected that best meets the performance properties, and other criteria required by the application, detailed apples-to-apples comparisons are a lot easier.

The different chemical make-up of heat transfer fluids.

Fluid Chemistry. High temperature heat transfer fluids can be categorized by chemical structure into three primary groups:

1. Synthetics 
2. Hot Oils  
3. "Others" including silicones 

The synthetics, also referred to as "aromatics", consist of benzene-based structures and include the diphenyl oxide/biphenyl fluids, the diphenylethanes, dibenzyltoluenes, and terphenyls. Depending on the specific product, the overall bulk fluid temperature operating range of the synthetics is from -70°F to 750°F.

The hot oils are petroleum-based and most consist of paraffinic and/or napthenic hydrocarbons. The overall bulk fluid temperature operating range of the petroleum-based fluids is from -10°F to 600°F, with the high-grade hydrogenated white oils strongly recommended for applications requiring bulk fluid temperatures in the 575°F to 600°F range.

Silicone-based fluids, and to a greater extent hybrid glycol fluids, are used primarily in specialized applications requiring process/product compatibility should a heat exchanger leak occur. This group's performance and cost factor disadvantages in the comparative temperature ranges of the synthetics and hot oils make silicone-based and other specialty fluids unlikely choices for most process applications.

When researching a potential heat transfer fluid, consider the following basic criteria:

Thermal Stability. Relative thermal stability testing of heat transfer fluids measures a particular fluid's molecular bond strength at a specific temperature versus another particular heat transfer fluid at the same temperature and under identical testing conditions. 'Relative' is the key word- since the tests are run under ideal laboratory conditions and do not factor in real-world fluid stresses such as mechanical malfunctions, design flaws, oxidation, etc., the data generated is useful for comparative purposes only. Accurate predictions of fluid life in actual processing applications should not be implied from thermal stability data alone.

Therefore, in selecting a fluid chemistry, the first step in the selection process is to determine the maximum bulk operating temperature required by the process. Most hot oils have a recommended maximum bulk fluid temperature of 550°F - 600°F, while the aromatics have recommended maximum bulk fluid temperatures between 600°F- 750°F, depending on the fluid.

Heat Transfer Efficiency.
Heat transfer efficiency comparisons between heat transfer fluids are made using heat transfer coefficients. At a specific temperature, a fluid's overall heat transfer coefficient can be calculated using its density, viscosity, thermal conductivity and specific heat at a determined flow velocity and pipe diameter.

Most of the aromatic-based fluids have a significant advantage in heat transfer efficiency over hot oils from 300°F to 500°F. Above this temperature range (up to 600°F) petroleum fluids narrow the difference somewhat with a select number of highly refined paraffinic/napthenic white oils having a slight efficiency advantage over the mid-range aromatics.

Pumpability Point.
The pumpability point is defined as the temperature at which the viscosity of the fluid reaches a point (typically 2000 cP) where centrifugal pumps can no longer circulate the fluid.

Generally, most of the hot oils offer adequate start-up protection down to the 0°F to +25°F range. The mid-temperature aromatics (with 650°F maximum bulk temperatures) offer protection down to -70°F to -20°F, while the top-end temperature aromatics (with 700°F- 750°F maximum bulk temperatures) are at +40°F to +60°F.

Fluid Serviceability.
Fluid replacement, reprocessing, or filtration may be required from time to time due to unexpected temperature excursions, system upsets, or contamination. Because of the relatively low cost of petroleum-based fluids, very few suppliers offer reprocessing services for hot oils.

Reprocessing synthetics using fractional distillation is an economical alternative to disposal and replacement; hence, most synthetic fluid suppliers offer this service at a nominal cost.

Filtration versus reprocessing or fluid replacement is a cost effective method of removing carbon and coke suspended in the heat transfer fluid. Most fluid suppliers recommend slipstream filter loops permanently installed and closely maintained on both hot oil and synthetic systems. .

Cost.
As a general rule, the higher the bulk fluid temperature a fluid is rated, the higher the cost of the fluid. The synthetics rated for use above 650°F are two to three times more expensive than the average hot oil rated to 600°F, while aromatics rated from 600°F to 650°F are one and a half to two times the cost of the average hot oil.

Which heat transfer fluid Is The Best? It would be difficult to pronounce a clear winner; the aromatics offer superior heat transfer efficiency and stability at elevated temperatures coupled with serviceability and adequate pumpability, while the hot oils have a significant cost.

If you’re in doubt about selecting the correct heat transfer fluids for your application, give the professionals at Habot Synthetic Lubricants a call.


The figures quoted in this article are by: Michael R. Damiani of Radco Industries, Inc.