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

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

Efficiency through clean tubes

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.

Overview of Twisted Tape

Twisted tape is an advanced, on-line heat transfer enhancement technology comprising:

  • A twisted strip
  • Brackets and shafts
  • Connecting rings and gaskets

The above components are made of polymer or metallic materials that are chemically stable, strong, elastic, and corrosive and wear resistant. The twisted strip is slightly denser than water and counters the multidirectional force of cooling water very well, allowing it to float at the center of the tube.

How Twisted Tapes Work

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.

The effectiveness of Twisted Tape in Enhancing Heat Exchanger Performance

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

1. Wear and tear

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).

2. Pressure loss

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.      

3. Sensibility to debris

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).

4. Surface fouling

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.

5. Water quality

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.

The Way Forward

The twisted tape is relatively easy to install and maintain both in new systems and in retrofits. The fact that it can clean your condenser tubes while in operation means no standstill production loss. However, the drawbacks highlighted above could cause less-than-optimal production cycles. 

We endorse this application and recommend it particularly for large installations, such as power plant condensers, where brush cleaning systems cannot be applied (due to feed pipe diameters that are too large for reversal valve application).

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.

EXAMPLE COMPARISON OF OPERATING COST

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.

System

Brush

Tape

Useable

600 brushes

600 tape sets

Replacement

Every 20,000 running hours

Every 7,500 running hours

Replacement & inspection frequency

Every 48 months

Every 12-18 months

Value per unit (including installation cost)

US$ 2.00

US$ 5.00-10.00

Total useable units (10 yrs-50,000 hrs)

1,200 brushes

4,200 tapes

Cost of useable

US$ 2,400

US$ 21,000 – 42,000

(first set included in initial set-up)

 

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.

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