Handling heat transfer thermal resistances in piping heat calculations with PASS/HYDROSTSTEM

Phil Black - PII Editor 24/09/2020

282 4 minutes read

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Calculation of heat exchange with the environment is often essential in the design of pipeline systems with liquids, gases, gas-liquid, and other products. This capability is especially crucial for pipelines in which the fluid temperature and ambient temperature are very different; pipelines transporting gases and multiphase fluids with flashing and condensation, or fluids with parameters close to the saturation conditions, as well as fluids whose properties sharply depend on temperature. The designer often has to choose between “accurate” and “fast” calculations. An accurate calculation often requires a wide range of different initial data, which is difficult to obtain. PASS/HYDROSYSTEM takes into account the critical parameters of the pumped fluid thermal interaction with the environment.

The basic heat transfer equation is written in integral form as follows:

This equation shows that the heat losses from a hot object will be the greater, the larger its surface is, the higher the temperature difference is, and the lower the thermal resistance to the heat transfer process is. The total thermal resistance to the heat transfer process is the inverse value of the total heat transfer coefficient:

This quantity is in (m²K)/W and is calculated as the sum of all thermal resistances that a heat flux from a hot to a cold object encounters on its way:

When calculating heat transfer through a cylindrical surface (pipes), the curvature of the surface is taken into account, which slightly complicates the forms of these equations. Empirical criterion equations most often determine the internal heat transfer coefficient. In the general case, it depends on:

the degree of flow turbulization (Reynolds number),
the properties of the pumped fluid (Prandtl number),
and for the natural convection conditions (Grashof number).

The primary point about the heat transfer from the fluid to the piping wall is the more turbulent the flow; the higher is the heat transfer rate, and the lower the thermal resistance. Most often, with typical flows of a wide variety of liquid and gaseous products in pipelines, the value of the internal heat transfer coefficient is about 10³– 10⁴ W/(m²K), which gives a very low thermal resistance of the order of 10^-3– 10^-4 (m²K)/W.

However, in some rare cases, for example, for a laminar flow of viscous fluids or for a gas flow with small velocities when the transition from forced convection to natural convection occurs, the heat transfer rate is noticeably lower. And the heat transfer coefficient, in this case, can be of the order of hundreds or even tens of W/(m²K), and therefore, the importance of this parameter increases.

PASS/HYDROSYSTEM automatically selects a suitable correlation to calculate the internal heat transfer coefficient and calculates it for each case.

The thermal resistance of the pipe is calculated as the ratio of the pipe wall thickness to its thermal conductivity coefficient. Therefore, the thicker the wall, the higher its thermal resistance, but the higher the thermal conductivity of the wall material, the lower is the thermal resistance. The thickness of the pipeline wall is most often of the order of several millimeters or less often tens of millimeters – 10^-3– 10^-2 m. But the thermal conductivity of the pipe material can be of a different order for metallic and non-metallic materials.

The thermal conductivity of metal pipes varies in the range of 10-60 W/(mK), while the thermal conductivity of polymer pipes can be of the order of 0.5 or even 0.1 W/(mK). Therefore, the thermal resistance of metal pipes will be of the order of 10^-5 – 10^-3 (m²K)/W, non-metallic – 10^-2 – 10^-1 (m²K)/W. PASS/HYDROSYSTEM enables the user to specify the material of the pipe wall.

Most often, the heat insulation thermal resistance is the greatest of them all – that's the main reason why the pipelines are insulated. Insulation thicknesses are usually of the order of tens or less often hundreds of millimeters, and the thermal conductivity coefficients of modern insulation materials are of the order of 10^-2 W/(mK). Therefore, the thermal resistance of the heat insulation will be of the order 1 (m²K)/W. PASS/HYDROSYSTEM enables the user to specify the insulation material properties.

Thermal resistance natural convection comes into play for heat transfer from piping/insulation outer surface to the ambient air/ground. For pipelines located in the open air, with noticeable wind speed, the average external heat transfer coefficient is about 30-40 W/(m²K), and for indoor pipes and even more underground pipes, it is even lower than 10 W/(m²K). Therefore, this thermal resistance will be of the order of 10^-2 – 10^-1 (m²K)/W. PASS/HYDROSYSTEM enables the user to indicate where the pipeline is located, and the program will do the rest.

Depending on the selected location, the program will automatically determine the heat transfer coefficient and thermal resistance. For underground objects, the user will also have to enter the parameters of the. Also, for outdoor and indoor pipelines, it is recommended to indicate what material is used as a jacket layer for the thermal insulation. Since, depending on the type of its surface, the external heat transfer can be slightly different. PASS/HYDROSYSTEM can assign different locations for different piping components. The user can simulate pipelines in a mixed environment (e.g., partially outdoors and partially underground).

And now, summarizing, let's write out the sum of thermal resistances to the heat transfer process, indicating the orders of each of the terms:

PASS/HYDROSYSTEM software enables the user to calculate the thermal resistances in piping systems easily, efficiently, and accurately for even the most complex piping systems in changing environments.

The PASS Software Suite is in active use by thousands of engineers at more than 2000 customers around the world for efficient analysis and design of process piping and plant utilities, district heat transfer, water and gas networks, oil and gas field gathering piping systems, and other types of pipelines. Please check out the PASS Software Suite at www.passuite.com and https://www.youtube.com/passuite as well as the PASS contact page for an existing distributor of PASS Software Suite in your area.

PSRE Co. (Piping System Research & Engineering Co.
Email: sales@passuite.com
Tel: +7 (495) 225-9432
Website: passuite.com