Overview of Electronic Descaling Technology
by Professor Young I. Cho, Dept. of Mechanical Engineering, Drexel University
e-mail: choyi@drexel.edu
Contents
Photo Albums of Applications of ED Technology
Scanning Electron Microscopy Photos of CaCO3 scales
Selected Publications on Electronic Descaling Technology
Related Patents and Ph.D. Theses
How is ED technology different from others?
Theory of Electronic Descaling Technology
Objectives: Fouling Laboratory at Drexel University has been conducting research to understand the basic fouling mechanism occurring in various heat exchangers and to control fouling. In order to control the fouling, the Fouling Laboratory has developed a new innovative technology called Electronic Descaling (ED) technology.
Background
When hard water is heated (or cooled) in heat transfer equipment, scaling occurs. When scales deposit on a heat exchanger surface, it is traditionally called "fouling". The type of scale differs from industry to industry, depending on the mineral content of the available water. One of the most common forms of scale is calcium carbonate, CaCO3, which is the subject of the present study.
Why does scale problem occur?
1. Hard water contains too much mineral ions such as calcium and magnesium.
2. Calcium carbonate is less soluble in hot water.
(Inverse solubility )
3. As water enters heat exchangers, the solubility of CaCO3 drops.
- uncontrolled precipitation
Why is scale a problem?
1. Scale layers act as thermal insulator, decreasing efficiency of HX.
Why? Small thermal conductivity of scales
(Conversion: 1 W/m K = 0.5781 Btu/hr ft oF)
* Oversized heat exchanger.
2. Narrowing of tube opening
- Flow rate through scaled tubes is significantly reduced.
- Controlled Precipitation
Ca+2 + 2HCO3- -----> CaCO3 + H2CO3
Sketch of cross section of a pipe
where induced electric field oscillates with time.
(a) for E field in the clockwise direction, positive ions move clockwise while negative ions move counterclockwise, resulting in collision and precipitation in bulk solution and (b) for E field in the counterclockwise direction, positive ions move counterclockwise while negative ions move clockwise.
Why is new scale prevented?
Solenoid-Induced-molecular-agitation precipitates dissolved mineral ions to insoluble mineral crystals. Hence, new scale is prevented.
Scanning Electron Microscope Photographs
taken with scale specimen from scaled tubes produced at a flow velocity of 0.78 m s-1 and a concentration of 10 mol m-3 (a) without electronic anti-fouling device and (b) with EAF device. Magnification = 1500.
Scale specimen for SEM was prepared with a utility knife by scraping small amounts of scales from the outlet region of each scaled tube. SEM photographs of scales produced without the EAF (electronic anti-fouling) device (a) revealed that CaCO3 scales were needle-shaped aragonite, whose dimensions were approximately 25 mm by 2 mm. Aragonite is a dangerous form of calcium carbonate scale, which is crystallized at a temperature above 308 K. It is sticky, dense, and difficult to remove. The long-needle shaped crystals confirm that the precipitation reaction occurred on the heat transfer surface without the EAF device.
In contrast, the scales produced with the EAF device (b) depicted a very different structure from the one produced without the EAF device. The scales produced with the EAF device were a cluster of small elliptic shape particles (e.g., 10 mm by 3 mm) with no particular orientation, suggesting that many fine particles were formed in bulk solution, attached to the heat transfer surface, and then grew in size through precipitation reaction.
The SEM photographs support the hypothesis of the EAF technology, which is to convert dissolved mineral ions into crystals in a bulk solution, thus reducing both the diffusion of dissolved ions to the heat transfer surface and subsequent precipitation reaction on the heat transfer surface. As a result, the production of aragonite type calcium carbonate is prevented. Calcium carbonate crystals formed with the EAF device appeared to be loosely connected. In other words, the scales formed with the EAF device may be removed at a small flow velocity (e.g., 1 m s-1), whereas the scales formed without the EAF device may not be removed even at a large flow velocity (e.g., 5 m s-1).
Effect of pressure change on scaling in pump and valve
1. The solubility of CaCO3 decreases as pressure decreases.
2. At the tip of vane in a pump, the local pressure significantly drops due to high rotating velocity (i.e., Bernoulli equation), resulting in uncontrolled precipitation.
How is ED etchnology different from others?
Photo Album of Applications of ED Technology
1. Shell and Tube Heat Exchanger
Before ED treatment
After ED treatment
2. Cooling Towers
Before ED treatment
After
ED treatment
3. Hot Water Heater, Steam in Tubeside
Scaled Tube Bundle
Tube Bundle treated by ED technology
4. Scale Prevention in Induction Heater Heating Element
Scaled Heating Element (zoomed view)
Chemical Cleaning of Scaled Heating Element
Scaled heating element is immersed in a strong acid solution.
ED Technology can be a clean and efficient solution to maintain heating element scale-free.
5. Chiller Tubes
Scaled Tubesheet
After ED treatment
Selected Publications on Electronic Descaling Technology
(updated 12/10/98)
