Twisted-Tape Inserts as a strategy to improve heat transfer rates

The rate of thermal energy transfer in shell and tube heat exchangers or (chiller) condenser tubes makes them one of the most critical components, if not the heart, of the system. The efficiency of the heat exchanger is measured based on its ability to optimize heat transfer.

Factors such as scaling and fouling greatly reduce the ability of heat exchangers and condenser tubes to transfer heat; hence they affect the overall performance of the chiller. To solve this problem without necessarily halting operations to allow for tube cleanups, on-line cleaning systems were developed. One such solution is the twisted tape system.

Twisted tape enhances heat transfer in heat exchanger tubes by modifying the flow channel of the cooling fluid. In fluid dynamics, two main types of flows exist: laminar and transverse. In laminar flow, fluid particles travel in a longitudinal direction, and the only mode of heat transfer is conduction.

When twisted tape is inserted, it causes a turbulent, transverse flow inside the tube. This kind of flow activates the fluid particles and forces heat transfer through both convection and conduction.

While twisted tape has been built as an advanced online tube cleaning system, its effectiveness in heat exchanger / condenser tube cleaning has its shortcomings:

The spiral ties of the twisted tape are made of a polymer material that wears out with time. When this happens, the width of the ties reduces, thereby weakening the rotation torque of the twisted tape. Ultimately, the twisted tape cannot meet its cleaning obligations. Its surface will instead become fouled and scaled. The average lifespan of the tape is about 7500 running hours. This affects the cost of ownership (see the comparison below).

The turbulent water flow can have an increasing effect on the pressure drop of the system. If not compensated for this, the flow volume decreases, which can negatively impact the overall effectiveness of the system. This aspect needs to be factored in when considering the tape cleaning technology for retrofitting.      

Tape-equipped tubes are more likely to be blocked by debris if it can enter the tubes. Obviously, this is caused by the simple fact that the flow surface is cut in half by the tape. Special attention needs to be given to strainer performance being maintained at 100% (in the water stream from the cooling tower to the unit).

As the twisted tape breaks the retaining layer of salts and metal ions on the wall of the heat exchanger tube, its own surface becomes vulnerable to fouling. This potentially reduces the rotating speed of the twisted strip and subsequently increases the water resistance. When the twisted tape loses its antifouling property, scaling sets in and takes a toll on the efficiency of the chiller system.

Twisted tape works well in good quality water. However, when the cooling fluid contains substances such as chlorine, potassium, and sodium salts, the rate of scaling and corrosion increases. The fluid temperature then rises and this deteriorates the operation of the chiller unit.

For smaller size projects where reversal valves can be applied (up to 600 mm pipe), an automatic tube cleaning system like Eqobrush may be preferable, due to low cost of ownership and low maintenance. This technology cleans the condenser tubes during normal operations by removing mineral deposits before they even settle and crystallize. This cleaning approach is effective in avoiding tube corrosion and leakage, two dangerous developments that can paralyze your operations.

A chiller (450 RT) runs with a 270 kW compressor motor with 600 tubes. The compressor running time is 5,000 hrs. per year over a period of 10 years. Total running time is 50,000 hrs.

This article is based on the findings published by Zhu Yu Jie and Liu Wei in 2013 on the “China Academic Journal”. A translated version of the article can be downloaded from our website. 

Graphics with thanks to Chongqing Huanji Low Carbon Saving Technology Development Co., Ltd.