Heat transfer between the fluid and the pipe’s surroundings is an important aspect of piping system design and a computer program is often a useful tool to use to help understand the effects of heat transfer. The addition or removal of heat to/from the pipe flow stream can change the fluid viscosity, change the fluid phase-state, i.e. change from liquid to solid phase, cause cavitation, it can cause the surroundings to freeze or melt and in the case of gas flow, can cause significant changes to the fluid density.
Pipe heat transfer can be quite a challenging task and can become increasingly difficult as layers such as insulation are added to the pipe.
Heat transfer should be combined with fluid flow analysis to achieve an accurate estimation of the fluid physical properties related to fluid flow such as density, viscosity and in two-phase flow, liquid surface tension. Also, fluid flow thermodynamics such as vapor-to-liquid ratio which is an important factor in two-phase flow of gases and liquids is a function of the fluid temperature.
Pipe heat transfer to or from the surroundings can occur by one or a combination of the following three known heat transfer processes, convection, conduction and radiation.
Convection: This refers to the transfer of heat energy by movement of fluids. Convection heat transfer is a function of the fluid physical properties, fluid velocity and system geometry (pipe diameter). Pipe flow is normally subjected to simultaneous heat transfer by convection and conduction. Convection heat transfer can be divided into two types; free (natural) or forced. Free convection refers to heat transfer which is driven by buoyancy, i.e. it occurs due to the natural movement of fluid caused by a density difference induced by a temperature difference. Forced convection arises when the fluid is forced to move by an external source such as a fan.
Conduction: This refers to the mechanism whereby an energy exchange occurs between the fluid and the pipe wall, insulation and soil (if the pipe is buried) due to direct contact.
Radiation: Radiation heat transfer is the electromagnetic transfer of energy from a hot surface to a colder surface and represents heat transfer without a physical medium. Although radiation heat transfer is often minimal for short pipe lengths, it is an important aspect of heat transfer analysis of onshore oil and gas transmission lines.
Pipe heat transfer calculations are often based on heat transfer rate per linear “M” or “ft” rather than the area of the length of pipe under consideration. In carrying out pipe heat transfer analysis, the various diameters of pipe insulation must also be considered, i.e. the inner area of the insulation is lower than the outer area and this difference must be considered in the analysis.
Heat transfer in piping systems, in its simplest form, can be for a single uninsulated pipe. However, it is often the case that the pipe has a layer or several layers (insulation etc) which the complicates the analysis. Figure 1 gives an overview of a cross-section through a pipe with a number of concentric, cylindrical layers.
Q is the heat transfer rate (btu/hr).
k is the thermal conductivity of the material (btu/h ft F).
R is the radius (ft).
T is the temperature (F).
L is the pipe linear length (ft).
Performing pipe heat transfer calculations allows designers to better understand system operating conditions and select the most suitable and economic thickness of pipe insulation required for a given application.
Below are a number of points which highlight some of the main benefits of insulating piping systems:
- Insulation helps reduce and delay the heat transfer into or out of a process which in turn helps maintain a stable fluid temperature, allow chemical reactions to proceed normally and safely.
- Uninsulated pipes permit the uncontrollable release of thermal energy to atmosphere which wastes millions of dollars.
- Insulation offers protection against freezing. If piping systems such as steam condensate, aqueous solutions, fire water systems were uninsulated in many cases, the would be a likelihood of freezing thereby preventing the operation of the system. Freezing can also cause failure of components as a result of water expansion when it freezes.
- Insulation prevents moisture condensing from the air onto cold pipe surfaces which can cause damage to surrounding materials and equipment.
- Insulation also offers protection against burns. In industries such as petrochemical, chemical process etc, pipelines can operate at very high temperatures and in locations where personnel need access and need to work safely on a daily basis. Providing insulation to pipes and fittings in these hazardous areas therefore gives protection to the employees allowing them to work safely and effectively in these zones.
- Since insulation retards uncontrolled thermal energy transfer from pipes, it reduces wasteful heat loss which has a knock-on effect of reducing wasteful energy consumption and associated emissions.
Insulation of piping systems serves an important function in the operation of oil and gas facilities, petrochemical processes, chemical processes, oil refining plant as well as chilled water systems, district heating system and refrigeration plant, to name a few. In some industries, refrigeration systems can account for a significant proportion of overall site energy costs. The correct selection and use of insulation on these piping systems can significantly improve system efficiency, with thermal performance and condensation prevention being the key considerations.
Economic Thickness of Insulation
When choosing insulation thickness for piping systems it is important to note that there is a limit beyond which it is not prudent to increase the insulation thickness. Increasing the thickness of insulation will reduce the heat loss from the pipe and thus give a saving in terms of operating costs. The cost of insulation increases with increase in thickness and there is an optimum thickness when a further increase does not save sufficient heat to justify the cost. In general, the smaller the pipe, the smaller the insulation required. So, a well-designed and optimised piping system will help with the selection of the economics of insulation thickness required.
There are various heat transfer functions available in FluidFlow. As well as performing heat transfer analysis at heat exchangers etc, pipe heat transfer analysis can be completed using a number of different approaches. Firstly, above-ground or buried-pipe heat transfer calculation can be considered. When modelling above-ground scenarios, you can perform heat transfer calculations for uninsulated or insulated pipes. You can set a fixed temperature change across the pipe or a fixed heat energy transfer across the pipe.