Time for a retrofit?

When condensing single-component vapours, heat transfer coefficients tend to be high. The situation changes when condensing multi-component mixtures. Under such conditions, the performance of condensers is often controlled by the additional limitations of vapour cooling and mass transport between the liquid and vapour phases, particularly when designing for low vapour velocities.

It is possible to improve several aspects of the tube-side condensation process. The flow regime can be significantly altered, allowing for potential benefits to heat and mass transfer.

This article will use an industrial case study as an example of how to ascertain which variables are important to consider in order to determine whether it would be beneficial to install tube-side enhancement technology.

Condensers: an introduction

Condensers are an important piece of equipment for the chemical processing and refining industries. They are a necessary component of many processes, i.e. overhead condensers for distillation columns, and as separators of volatile and condensable products. They are also a key feature of HVAC-R systems.

There are different options for the design of shell-and-tube condensers in particular, with condensation possible on the shell-side or tube-side, and in vertical or horizontal arrangements. When the condensing fluid is allocated to the tube-side, the presence of two-phase flow introduces complexities in the thermal design. An incremental calculation is needed, and it becomes necessary to consider how different flow regimes will affect the heat and mass transfer within the tubes. This can make the creation of an efficient and reliable design more difficult.

The application of tube-side enhancement devices can provide significant benefits in many applications. CALGAVIN’s hiTRAN® Thermal Systems technology is a form of a removable wire matrix-type enhancement device, consisting of a pattern of wire loops supported by a central core wire (see Figure 1). In general, it increases the single-phase convective heat transfer coefficients, helping to accelerate the rate of vapour cooling. Also important in multi-component condensation, the device improves mass transfer and disrupts the interface between the two phases. This can ensure that condensable vapour is continually transferred toward the cold tube wall, and that the liquid is redirected into the bulk flow.

Figure 1. A hiTRAN wire matrix element.

Pure component condensation

It is useful to first consider the simplest case of pure component condensation – where there is only a single substance present within the tubes – and how the characteristics affect heat transfer.

In pure component condensation, there is no change in the concentration of the condensing substance throughout the tube. As such, there is no mass transfer resistance preventing the vapour from reaching the cold liquid interface. Meanwhile, the bulk temperature is uniform and equal to the saturation temperature at the corresponding vapour pressure.

As the vapour condenses, a layer of liquid forms on the tube wall (see Figure 2). The thermal conductivity of the liquid film causes additional thermal resistance, reducing the rate of heat transfer from the condensing vapour. The thermal resistance through the liquid condensate often dominates. Only at near-complete condensation, where the vapour velocities are much slower, does the convective heat transfer of the vapour become a significant factor.

Figure 2. Temperature profile across a vapour-liquid interface.

Because the thermal conductivity is an intrinsic property of the liquid, applying hiTRAN in pure component condensation tends to have a minimal effect on the heat transfer rate. The increase in pressure drop also tends to decrease the saturation temperature, therefore reducing the effective temperature difference, which often offsets any heat transfer increase. An overall benefit may be realised towards the end of the condensation process, where the flow velocities are lower, by enhancing the convective heat transfer.

Written by Nathan Hill, CALGAVIN.

This article was originally published in the March 2023 issue of Hydrocarbon Engineering magazine. To read the full article, sign in or register for a free subscription.

Read the article online at: https://www.hydrocarbonengineering.com/special-reports/06032023/time-for-a-retrofit/