Apr-2015

Heat transfer systems in gas processing

There are many aspects other than temperature to consider when selecting a high temperature heat transfer fluid for gas processing.

ANNA STEWART
Dow Oil, Gas & Mining

Article Summary

When designing heat 
transfer systems, fluid temperature is the primary factor that determines what type of heat transfer fluid can be used for a specific application. However, there are several other aspects that should be considered before a specific fluid is selected for use, including fluid physical 
properties, environmental and regulatory aspects, the ability to supply material to the site, and the total cost of fluid. These items can have a significant impact on the cost and operability of the heat transfer unit and, therefore, may ultimately determine the success or failure of a project. This article discusses these additional aspects in detail and examines three case studies that highlight the importance of considering these additional factors. The focus is on design of high temperature heat transfer systems (above 350°F, 175°C).

Temperature considerations
The first decision to be made when selecting a heat transfer fluid is what type of fluid should be used. This is almost entirely based on application temperature requirements. If a heat transfer application has a maximum use temperature requirement above 350°F (175°C), operators should investigate three categories of fluids: mineral oils, synthetic organic fluids, such as Dowtherm, and silicone fluids, such as Syltherm. For efficient performance and long fluid life, a fluid with maximum recommended use temperature above the system’s anticipated bulk fluid temperature is recommended. Figure 1 shows the recommended temperature ranges for synthetic high temperature heat transfer fluids. The fluids are listed in order of decreasing maximum use temperature. As shown here, synthetic organic (Dowtherm) and silicone fluids (Syltherm) are engineered to be thermally stable at temperatures up to 750°F (400°C).

On the other hand, if the maximum bulk fluid temperature is lower than 350°F (175°C), or freeze protection for a water based system is needed, a low temperature inhibited glycol based fluid should be considered. Inhibited glycol based fluids are actually solutions of water and inhibited glycols. The concentration of glycol in the fluid directly affects its performance properties, and is specified by the user to meet application requirements, typically minimum temperature. These glycol based fluids will not be discussed in detail; however, many of the trade-offs described for high temperature heat transfer are applicable for low temperature heat transfer as well.

Once it is determined what type of fluid is required, there are several other considerations to be made before the ‘best’ heat transfer fluid can be identified for a specific application. The following should also be taken into account:
•    Fluid physical properties
•    Final fate of used fluids (recycle and disposal options)
•    Services offered by the supplier (fluid analysis, engineering and design consultation, and so on)
•    Environmental and regulatory aspects
•    Ability to supply material to the site
•    Total cost of fluid (initial cost as well as cost to use).
In addition, the significance of these items in the overall design of the system is likely to vary from project to project. It is up to the engineer to determine which characteristics are important in each situation (see Figure 2).

Fluid physical properties
Understanding the physical properties of the fluid is key to appropriately sizing equipment and ensuring reliable operability of the unit once it is up and running. For high temperature heat transfer, the physical properties that are important to consider include:
•    Viscosity
•    Vapour pressure
•    Film coefficient
•    Thermal stability/degradation rate.

In addition, these properties should be readily available to engineers so different fluids, and their impact on equipment sizing, can be evaluated. One option is to use a third-party simulator, such as Aspen Hysys. These software packages often allow the user to explore various types and brands of fluids. An alternative is to obtain fluid physical properties directly from the fluid supplier. For example, Fluidfile software enables fast, efficient evaluation of the properties and performance of heat transfer fluids under actual system conditions, and in different units of measurement. This makes choosing the right fluid easier and more convenient for both experienced professionals and first-time system designers. The properties available for high temperature heat transfer fluids include density, specific heat, thermal conductivity, viscosity, vapour pressure, Reynolds number, film coefficient, and pressure drop. In addition, all data can be exported to Microsoft Excel or as a .CSV file for easy manipulation.

Viscosity
Viscosity affects both the film coefficient of the fluid and fluid pumpability at low temperatures. Lower fluid viscosities result in more turbulent flow and, thus, higher film coefficients and lower film temperatures (the effect of this on fluid degradation will be discussed below). Lower viscosity may also be required if a system is operated while exposed to cold winter weather, causing low temperature pumpability. Low temperature pumpability is especially critical if a system is subject to shutdown, whether planned or unplanned. If the heat transfer fluid in the system is not pumpable, system start-up can be difficult, if not impossible.

The drawback of using mineral oils as a heat transfer fluid in extremely cold conditions is evident at the low end of their temperature use range, due to very high minimum pumpability temperatures (35°F or 2°C). These minimums do not often provide adequate protection to allow system start-up and operation in low temperature conditions. In addition, heat tracing, which can be costly and cumbersome, may be the only way to ensure consistent system operation. Synthetic heat transfer fluids, especially silicone fluids, are often a better option in this case. Figure 3 shows the viscosity of several synthetic heat transfer fluids across a range of temperatures. Silicone fluids offer excellent low temperature pumpability characteristics, experiencing little viscosity change down to the lower end of their recommended operating ranges. Synthetic organic fluids may also be an option as some have very low crystal points so the fluids remain pumpable in extreme cold and the potential for prolonged costly system shutdown is minimised. Viscosity considerations become quite critical for system operation, especially if a plant will be located in extreme cold.

Vapour pressure
One of the major drawbacks to using steam as a heat transfer medium is that, depending on the temperature requirements, the system must be operated under high pressure. This can lead to increased equipment cost due to the need for thicker piping, additional pressure control systems, and steam traps. Additionally, using steam can cause challenges upon system start-up, as well as additional safety hazards. Dowtherm fluids feature vapour pressures that are lower than steam (see Figure 4). Furthermore, several synthetic fluids are designed to operate in non-pressurised or low pressure heat transfer systems. This advantage decreases the safety hazards associated with high pressure operation using steam. Another reason for selecting a fluid that offers low vapour pressure is the potentially lower initial investment in expansion tanks and other specialised equipment.

Film coefficient
Film coefficient is the best measure of the overall efficiency of heat transfer fluid, as it accounts for all four critical fluid properties: viscosity, specific heat, density and thermal conductivity. Fluids with greater efficiency may enable reductions in pump and heat exchanger size, requiring less equipment and a lower overall footprint. This can lead to substantial capital cost savings while, at the same time, optimising heat transfer performance. In addition, fluids with higher film coefficient will have a lower film temperatures (when holding heat flux constant), which contributes to better thermal stability. Figure 5 shows the film coefficient for several high temperature heat transfer fluids. Note that the curves plateau and start to decrease as temperature increases. This is due to the increasing effect of thermal conductivity on the film coefficient, which decreases more rapidly as temperature increases.