Heat-transfer fluid selection can be intimidating when considering the long-term impact on costs and operability. The right fluid will provide optimal performance and lifecycle costs, while freeing engineers for other assignments. Most engineers do not routinely consider fluid selection, so making the proper choice is a challenge. This one-page reference offers guidance in this area, and can serve as a basis for taking advantage of the expertise of heat-transfer fluid manufacturers.
The process will define the temperature and heat-duty requirements that must be met by the fluid and the supporting heat-transfer system. The heat-transfer fluid must have the requisite thermal stability to meet the process demands, while providing long fluid life expectancy.
The list of potential fluid candidates may be shortened considerably by defining the required operating temperatures. The fluid’s bulk operating temperature must be substantially above the process temperature required in order to provide the driving force for heat transfer.
Next to consider are the remaining physical properties that are key to heat-transfer efficiency. These include: viscosity, density, thermal conductivity and heat capacity of the liquid. These terms combine to determine the fluid-side heat-transfer coefficient [Equation (1)].
Where, h is the heat-transfer coefficient (within a tube), k is thermal conductivity, cp is heat capacity, μ is the viscosity and C is a constant incorporating length, velocity and pipe diameter. Note the exponent for each term, which indicates the relative influence on the resulting heat-transfer coefficient.
Next, the overall heat transfer coefficient, U, is determined. This calculation considers process-side heat-transfer coefficient ( hi or ho), wall (rw) and fouling (rf) resistances [Equation (2)].
This equation shows that a high process-side heat-transfer coefficient can make the fluid selection very important in increasing U and potentially decreasing heat-exchange surface area requirements.
The combined effects of fluid properties under design conditions should be considered when comparing fluid performance.
Cost of ownership
Cost of fluid ownership is more than just the purchase price — lifecycle costs should be carefully considered. These include the following factors:
- Purchase price
- Impact on capital costs
- Operating costs
- Make-up addition rate and volume
- Fluid replacement frequency
- Impact on equipment costs
- System cleaning requirements
Consider an example 20-yr comparison of mineral oil with a synthetic heat-transfer fluid (Figure 1).
Product and data support
To support the necessary engineering calculations for initial system design and for later troubleshooting or modifications, complete and accurate physical properties for their fluid are necessary. These property data should be better than those found on graphs with wide plot lines, which can be ambiguous, and should fill gaps often found on material safety data sheets. These data should be detailed measurements and correlations capable of supporting the desired engineering accuracy. Usually the best way to access these data is through a heat-transfer fluid manufacturer with stringent quality controls, and one that has capably measured the properties of representative samples. This enables lower “safety factors” in designs.
Thermal oxidative stability
High-temperature organic fluids must be resistant to thermal and oxidative stress. Oxidation can be prevented by use of inert-gas blanketing of the system. Fluids with oxidation-stabilizing additives can become depleted and laden with solids, leaving the system vulnerable to sludge formation and fouling. Top-up with more additives exacerbates the fouling potential, which has a negative impact on the cost of ownership. Thermal stability is a characteristic of bond strength and fluid composition with minimal impurities. Thermal stability can be measured in the laboratory using ASTM D-6743 [ 1] (Standard Test Method for Thermal Stability of Organic Heat Transfer Fluids), followed by proper quantification of the degradation products formed. Fluid manufacturers may have decades of experience in field-verification of fluid life in a variety of applications and at different operating temperatures. Those with ISO-9001-certified processes for quality-assurance-management systems are recommended, to ensure that lot-to-lot product performance and reliability are consistent.
1. ASTM D-6743, Standard Test Method for Thermal Stability of Organic Heat Transfer Fluids.
2. Gamble, C. and Schopf, M., “Optimizing heat transfer fluid performance: How to avoid costly consequences,” Eastman Chemical Co., white paper, 2014.
Content for this column was provided by Conrad Gamble, senior engineering associate at Eastman Chemical Co. (Kingsport, Tenn.; www.eastman.com).
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