The bulk fluid temperature is the average temperature of the fluid at a specified point in the heat transfer system. It is usually measured at the exit from a heater. A close relationship exists between the highest bulk temperature and the degradation rate of a fluid. A critical degradation rate, from an economic standpoint, is usually considered to be 5% per year, but also depends on the proportions of low boiling and high boiling components formed. This degradation rate should only be reached at the highest temperature recommended as the bulk temperature. In general Therminol heat transfer fluids can give long service life if the maximum bulk and film temperatures of the system do not exceed the recommended maximum limits for the fluid and if no contamination or exposure to oxygen occurs. The recommended maximum bulk temperatures of Therminol heat transfer fluids have been determined by degradation rate measurements made at high temperatures for each fluid. These measurements are made by a controlled thermal aging on a standard volume of fluid at fixed temperatures. Dynamic and static aging tests have been performed.
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.
The autoignition temperature is the minimum temperature for a substance to initiate self-combustion in air in the absence of a spark or flame.It permits grouping combustible liquids with respect to their behavior in contact with hot surfaces. This provides a basis for determining protective measures forexplosion-proof electrical or non-electrical apparatus.
Heat Transfer Coefficient
Turbulent flow inside commercial tubes and pipes is assumed. Heat transfer is computed from the HTRI correlation: Nu = 0.025 * (Re^0.79) * (Pr^0.42) * phi
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.
Pressure drop is computed from:
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
In the transition region, for 2000 < Re < 10000, an average Nusselt number is computed, from HTRI correlations:
Nu = θ * Nu2 + (1 - θ) * Nu10
where Nu2 is the laminar-based Nu computed at Re = 2000, and Nu10 is the turbulent-based Nu computed at Re = 10000:
Nu2 = 2 + 20 * (1/3)^4 + 1.45 * ( (3.14/4) * 2000 * Pr / (L/D) )^(1/3)
Nu10 = 0.025 * (10000^0.79) * (Pr^0.42) * 1.023
θ = 1.25 - Re/8000
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 inadvertentlyor leaked from the process side. High acidity may
also indicate severe fluid oxidation if the systemis 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 and 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 in 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 either that there is a system leak on the process side or that 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 startup 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 thefluid 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 D-92 or DIN ISO 2592.
2. In a closed crucible (Pensky-Martens, closed-cup method), ASTM D-93 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 D-7213 (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.