Compiled by our team of technical experts, this Q&A section contains answers to frequently asked questions and commonly used definitions in the heat transfer industry.
- Elevated viscosity resulting in impaired low-temperature fluid performance
- Elevated quantities of high-molecular-weight degradation products approaching (or, in the worst cases, exceeding) their limit of solubility in the subject fluid
- Contamination of the heat transfer fluid with foreign compounds
Glossary of terms
The film temperature is the maximum temperature of the thin layer of fluid in contact with the metal wall in tubes or pipes. The fluid in this layer is not in turbulent flow and, in a heater, often has a temperature 20°–30°C (30°–50°F) higher than the bulk fluid temperature. Although very little fluid is present in the film, if the film temperature exceeds the maximum recommended, the contribution to the degradation of that fluid volume can be high and can be estimated for individual cases.
Film temperature can be calculated by the ratio of the maximum total heat flux density of a system to the heat transfer coefficient.
Nu = 0.025 * (Re^0.79) * (Pr^0.42) * phi where
Nu = Nusselt number = h * D/k
h = Heat transfer coefficient, W/(m²•K)
D = Inside diameter, m
k = Thermal conductivity, W/(m•K)
Re = Reynolds number = ρ * V * D/µ
ρ = Fluid density, kg/m³
V = Bulk fluid velocity, m/s
µ = Fluid viscosity, Pa•s
muw = Fluid viscosity at the wall, Pa•s
Pr = Prandtl number = cp * µ/k
cp = Fluid heat capacity, kJ/(kg•K)
and the factor phi = (µ/muw)^0.11 is given the fixed value of 1.023, which corresponds to a film temperature difference of about 30°C (50°F) for liquids at common use temperatures.
Delta P = f * (L/D) * (ρ * V²)/2
1/√f = -0.86 * ln (e/(3.7 * D) + 2.51/(Re * √f))
Delta P = Pressure drop, Pa/m
f = Friction factor, from Colebrook
L = Pipe length, m
√f= Square root of f
e = Wall roughness, m
ln = Natural logarithm
Nu = θ * Nu2 + (1 – θ) * Nu10
where Nu2 is the laminar-based Nu computed at Re = 2,000, and Nu10 is the turbulent-based Nu computed at Re = 10,000:
Nu2 = 2 + 20 * (1/3)^4 + 1.45 * ( (3.14/4) * 2,000 * Pr /(L/D) )^(1/3)
Nu10 = 0.025 * (10,000^0.79) * (Pr^0.42) * 1.023
θ = 1.25 – Re/8,000
Negligible natural convection, negligible entrance effect, and negligible viscosity gradient correction are assumed, and L/D = 100 is taken as typical.
High acidity generally indicates contamination from material added to the system inadvertently or leaked from the process side. High acidity may also indicate severe fluid oxidation if the system is not protected with inert gas in the expansion tank vapor space.
If the acid condition becomes excessive, the system expansion tank is at increased risk of corrosion and failure. Corrosive products form sludge and deposits that decrease the heat transfer rate. Contamination or oxidation of this nature may require removing the fluid for disposal, system flushing to remove acidic or contaminant residues, and refilling with new heat transfer fluid while ensuring the correction of the identified root cause of acidity.
Viscosity changes generally indicate contamination, thermal stress, or oxidation degradation. Viscosity is related to the molecular weights of the fluid components. Generally, lower-molecular-weight components decrease viscosity and higher-molecular-weight components increase viscosity.
Contamination from leaked process streams, incorrect fluid added to the system, solvents from system cleanout, thermal stress, or oxidation may be the source of materials that increase or decrease viscosity. Operational problems may result from either high or low viscosity. If the viscosity is high, the circulating system may have difficulty starting up, resulting in heater burnout. Heat transfer rates may be reduced. High-viscosity fluid generally requires draining and replacement with fresh fluid. Extended use of high-viscosity fluid may contribute to fouling, thereby requiring a system flush before refilling. Sometimes, however, the problem may be corrected by significant dilution with fresh fluid.
If viscosity is low, low-boiling components will be more volatile and can result in pump cavitation and reduced flow. To remove low-boiling components, the heated fluid should be circulated through the expansion tank with an inert gas purge of the vapor space. The cause of viscosity changes should be determined no matter what action is taken.
Moisture generally indicates that either there is a system leak on the process side or wet fluid has been added to the system. New systems or systems cleaned using aqueous solutions can contain residual water. Water can also infiltrate through open vents, expansion tanks, or storage tanks. Moisture can cause corrosion, high system pressures, pump cavitation, and vapor lock. If hot fluid contacts a water pocket, steam may develop which can cause fluid from the system to erupt and its components to fail.
Corrective action includes careful and gradual start-up of a potentially wet system with circulation through the expansion tank where the vapor space is slowly purged with inert gas to sweep moisture from the system. If a large amount of water contamination is present, it may be necessary to remove the fluid for external drying. Leaks from the process side should be corrected, and new heat transfer fluid should be stored to minimize water entry. When stored outside, new sealed drums should be turned on their sides and adequately covered to prevent moisture contamination from rain.
The flash point is the lowest temperature at which a fluid gives off sufficient vapor to burn when ignited; however, the rate of evolution of vapor at the flash point is insufficient to maintain a flame.
The flash point is determined by two techniques:
1. In an open crucible (Cleveland open-cup (COC) method), ASTM D92 or DIN ISO 2592
2. In a closed crucible (Pensky-Martens closed-cup method), ASTM D93 or DIN 22719
The values found by the Pensky-Martens method are about 20°–30°C (30°–50°F) lower* than by the COC method because the gases are kept together and are not diluted by air addition.
*For fresh product; in-service tested fluid difference may be larger.
Low- and high-boiling components are measured by a gas chromatography technique. This method, based on ASTM D7213 (DIN 51435), determines the boiling range distribution or distillation curve of organic heat transfer fluids.
This technique helps to assess the thermal stability of heat transfer fluids and is offered as a routine test as part of the testing service.
Heat transfer system equipment—descriptions
Here you will find a list of equipment descriptions providing recommendations for equipment and components suitable for use with Therminol products within their respective acceptable design limitations.
Usually, the expansion tank is installed at the highest point of the system and is connected to the suction side of the pump. It may also be connected to the main circulating loop at the lowest pressure point. It should serve as the main venting point of the system as well as provide for fluid expansion, which can be 25%-30% of the total system volume. Actual fluid expansion volume depends on the physical properties of the fluid selected and the operating temperature range.
All expansion tank vent lines must be routed, preferably via a cooled condenser, to a safe external location so that vapor may not enter working areas. The normal design choice will be a double-leg expansion tank which provides higher flexibility in normal operation than a single-leg expansion tank with degassing tank and temperature buffer tank. With careful attention to design, particularly to venting systems for noncondensable and water, both single-leg and double-leg designs may be used and can provide satisfactory service.
Low boilers and moisture should be collected in either a vent condensate or cold-seal trap and should be periodically discarded as part of routine operating procedures.
An effective way to minimize fluid oxidation is to blanket the system with an inert gas such as nitrogen. In small systems, the nitrogen may be replaced by a cold-seal trap or an expansion leg filled with system fluid maintained at a low temperature.
Before a new system is started, a wire mesh strainer should be installed in the pump section. These strainer baskets may be removed after debris removal from the start-up.
When operating where solids or contaminants might enter or be generated in the heat transfer system, it is advisable to install a high-temperature filter bypass line that can be positively isolated with valves for periodic cleaning or replacement.
Filter elements are commonly glass fiber string-wound cartridges or sintered metal filters in the 5–20 micron range. These filters require a significant pressure drop between the inlet and outlet of the bypass.
For high-temperature heat transfer fluid systems, spiral-wound or graphite types of flange gasketing conforming to API 601 and DIN 4754 specifications are recommended.
Standard materials for spiral-wound flange gaskets are type 304 stainless steel and pure graphite. To avoid leaking with spiral-wound gaskets, it is important to use raised-face flanges, allowing steel bolting and even compression of the gasket during bolt tightening. Graphite gaskets are an acceptable alternative for many applications.
Generally, sheet gasketing with various binders is unacceptable for Therminol 66 and some other fluids because of incompatibility of the binders with these fluids.
The heater may be electrical, fuel-oil, or gas-fired and is the most critical component in designing a heat transfer system for use with Therminol fluids. With the proper balance of heating capacity, temperatures, and fluid velocity, the service life of the heat transfer fluid is increased to an optimum level. Another important factor for good life is that systems must be protected from contamination by foreign materials.
Two basic designs of fired heaters for use with Therminol fluids are liquid tubes and fired tubes. In liquid-tube heaters, the heat transfer fluid is pumped through the tubes as it is heated. The hot gases pass outside the tubes. In fired-tube heaters, the fluid flows through the heater shell with hot gases passing through the tubes.
When bulk fluid temperatures higher than about 240°C (460°F) are required, a liquid-tube heater must be used unless a specific heater design is devised to force a uniformly steady turbulent flow of liquid over the fired-tube surfaces.
Most Therminol fluids are liquid when transferring heat. To avoid hot spots in the heater, the fluid should be pumped over or through the heating surfaces at sufficient velocity so that no area of fluid stagnation occurs. Since heating is not perfectly uniform in fired-tube heaters, the maximum heat stress conditions must be calculated to determine what film temperatures will be encountered.
Fluid velocities over heat transfer surfaces must be relatively high to develop turbulent flow. This helps to avoid excessive film temperatures that may be detrimental to heat transfer surfaces and the fluid. The heater manufacturer should be consulted for the required flow velocities.
Organic heat transfer fluids, such as Therminol fluids, have a slow oxidation reaction with air in the presence of insulation materials when the fluid temperature is above 260°C (500°F). Porous insulation, such as calcium silicate, offers a larger reaction surface with poor heat dissipation which, along with possible catalysis from the insulation material, can cause a temperature buildup. This temperature rise may result in ignition of the fluid when the saturated insulation is exposed to air, such as for repairs.
This phenomenon is not fully understood but appears not to occur with cellular glass, possibly because of its closed cell structure. Cellular glass should be used in all areas where leakage is a possibility. The principal leakage areas are usually near instrument connections, valve packing glands, flanges, and other sealed surfaces. As a precaution, eliminate any source of leakage promptly. Replace leaky gaskets and oil-soaked insulation, and repack valve stems. Cover insulation where leaks might occur with metal covers. Where possible, install valves with the stems in a horizontal position so that leaks will drip away from the insulation.
The piping layout for systems using Therminol heat transfer fluids should be sized to provide the normal required flow rate at an economical pressure drop.
Because the system will undergo temperature changes, adequate flexibility to relieve thermal expansion and contraction stresses is essential. Schedule 40 carbon steel pipe or equivalent should be used throughout the system. The tendency to leak through joints and fittings is a characteristic of most organic fluids unless the fittings are very tight.
The best way to prevent piping leakage is to weld all connections. Where access is necessary, raised-face flanges with weld neck joints are recommended.
To help insure good seating and sealing of the spiral-wound gaskets recommended for Therminol fluid piping, the following procedure should be followed:
- Clean flange faces free of loose rust and dirt. Remove any weld spatter. Assure that the flange faces have no gouges or grooves and are aligned properly, since gaskets cannot correct for these problems.
- Check alloy stud bolts and nuts to assure they are clean and free of rust and thread shavings, and lubricate the threads. The bolting stress and torque are defined by the supplier of the gasket. The torque is also a function of the diameter and the thickness of the gasket.
- Torquing is performed by tightening opposite studs to the required torque values using small increments. Tighten studs in the sequence 9, 3, 6, and 12 o'clock and repeat with adjoining studs.
Pumps must have enough capacity and pressure head to circulate the fluid at the required rate through the system. Pumps are generally centrifugal, seal-canned, glandless, or magnetically driven. They must conform to appropriate standards. The pump housing may be cast steel for most systems but may be made of other appropriate materials for very low or high temperatures.
For temperatures higher than 200°C (390°F), pump manufacturers usually specify either water-cooled ring seals or, preferably, fluid-cooled stuffing or air-cooled, extended-shaft seal and bearing.
On pumps with a stuffing box, at least five rings of laminar graphite packing should be provided. Inert blanketing of the seal with steam or nitrogen eliminates deposit formation from oxidation, which can lead to seal leakage. A secondary seal provides additional safety in case of sudden seal failure.
Regardless of the type of pump selected, the flow rate should be checked regularly against the pump characteristic performance curve originally supplied. To prevent alignment problems and seal leakage, it is important to avoid pipe support stresses on the body of the pump. Each pump should be fitted with a control device to switch off the heat source in case of pump failure. If expansion loops are used in the pump section piping, they should be horizontal or vertically downward. Loops should not be vertically upward because this forms a trap which can collect air and vapor that seriously hampers pump performance.
Forged-steel valves with deep stuffing boxes are satisfactory for systems utilizing Therminol fluids. Gate and globe valves with an outside screw should be used throughout the heat transfer system. Gate valves do not always provide an absolutely tight shutoff.
Various types of packing are used to seal valve stems on high-temperature systems, and generally five rings are specified on valve stems to assure a good seal. Valve stem bellows will provide virtually leak-free operation.