How to characterize the thermal properties of fluids
Physical properties of working fluids
SACOME R+D+i Department continues to offer our clients a service to analyze the physical properties of their work fluids. The main objective of this analysis is the correct characterization of those properties that have a direct influence on the heat transfer process: specific heat, thermal conductivity and viscosity.
The starting point in the design of any heat exchange system is the definition of how these thermal properties evolve throughout the process, whether heating or cooling. An incorrect definition of properties can lead to an inefficient or inadequate design.
Design parameters affected by thermal properties
In terms of tubular heat exchangers there are two design parameters that are directly affected by the thermal properties that have been considered::
- Required Exchange Area. The thermal properties determine (along with other parameters of the process) the operating regime of the equipment, which is obtained from different ‘dimensionless numbers’ such as the Reynolds number, the Prandtl number or the Rayleigh number, among others. The heat transfer coefficients are very different depending on whether the flow is laminar, in transition or in turbulent regime. Therefore, an adequate characterization of the properties allows to optimize the exchange area of the equipment, that is, it helps the designer to adjust the size of the heat exchanger and, consequently, its price.
- Estimated Pressure Drop. Although its also depends on the operating regime (laminar, transition or turbulent), in this case the determining parameter is the viscosity. In general, the viscosity is heavily dependent on the temperature, so the assumption of a constant temperature value can imply an erroneous pressure drop calculation when the heating or cooling range are wide. The direct consequence will be a bad sizing of the heat exchanger, as well as the rest of the elements of the installation, such as pumps, valves, etc.
Characterization of the thermal properties of fluids
In SACOME we have access to measurement devices and highly skilled staff, that allows us to characterize the thermal properties reliably and accurately, starting from a small sample of product:
- Specific Heat. Thanks to a differential scanning calorimeter we can apply a modulated temperature method in which the sample is heated in different stages of a few degrees, separated by isothermal periods, and with a heating ramp expressed in ºC / min. The calibration of the heat flow allows an exact measurement of the specific heat at low and high temperatures.
- Thermal Conductivity. It can be obtained by indirect measurement of the thermal diffusivity, by applying the Laser Flash (LFA) technique, or by direct measurement by contact using an inverted sensor, which also performs effusivity measurements. The equipment also allows measurements at different temperatures.
- Viscosity. Many products processed in the Food Industry, as well as many waste compounds from all types of industries, show a non-Newtonian behavior: that is, the viscosity varies not only with temperature, but also with the shear stress (directly related with the flowrate and diameter of the pipe). For a correct characterization of these fluids a rheological study needs to be performed, using a rheometer with the appropriate geometry, and a shear rate scan can also be done. This study can be performed in a wide range of temperatures whenever the mandatory precautions to avoid the drying out of the sample are met.
The importance of rheology in the design of any processing plant is crucial since the sizing of many of the elements composing it is very dependent on this “resistance”, also taking special relevance with food products: heat exchangers, pipes, valves, pumps, mixers, etc.
To design a heat exchanger it is necessary to have certain data, such as the process flow rate, the temperature and the physical properties of products.
Hairpin heat exchangers have a more efficient and economical design compared to a multiple pass heat exchanger when the process requires a temperature crossing between the cold and hot fluid.