The overall performance of power industry projects depends upon maximizing the efficiency of the heat transfer processes that occur at any individual system of the plant.
Thus, heat recovery boilers base their operations on the energy transfer between the flue gas and the steam-water system; condensers focus their functionality on the conversion of steam into condensate by mean of cooling water; turbine extraction heaters are needed to increase condensate temperature; solar steam generators produce steam while exchanging energy with a thermal synthetic oil.
In all-of-the-above heat transfer process examples, the optimization and long-term reliability of the heat exchanger depends on the material selection, the properties of the fluid and the chemistry cycle control.
This article presents an outline of the main aspects for optimization, and an overview of how efficiency could be affected by the chemistry involved in the process operation.
Improvements of a heat transfer process can be addressed from three main perspectives:
- Material Selection: to enhance thermal conduction
- Fluid Properties: to improve thermal convection which can be attained by increasing fluid quality and enhancing hydrodynamic.
- Chemical Aspects: to preserve the state of cleanliness by maximizing chemistry conditioning during operation and executing periodic chemical cleaning
Conduction is the transfer of heat by the motion of molecules of a solid body. It relates the heat transfer fluxes to the gradient of temperature across the solid. This phenomenon depends on the composition, density and internal structure of materials; it varies with temperature, and the way molecules reorganize among each other. It is basically the ability of any material to let heat pass through, therefore, that directly intervenes in the energy transfer rate.
From a mechanical standpoint, the rate of conduction is directly linked to the mechanical design and it depends on the wall thickness, the cross-section area and the temperature gradient. For instance, thinner tubes will perform better in terms of temperature homogenization, but they could compromise the interconnection tube-baffle because of possible the lack of material in the welding. On the other hand, thicker tubes will increase its structural stability at the interface tube bundle plate, but it will lose reliability under thermal fluctuations during start-ups.
From other perspective, chemical incompatibilities can create slight variations in the electrochemical potential and produce corrosion cells.
Example 1: FAC Corrosion
This is a case of a heat exchanger failure due to wrong material specification and an inadequate chemical treatment program. The problem affected the formation of the passive protective layer, altered the thermal conduction through the tubes, and damaged the physical integrity of the equipment.
When a steel surface is in contact with water under specific chemical and thermodynamic conditions, an oxide protective layer of magnetite (Fe3O4) is developed. if you control the growth, the morphology and the solubility of such corrosion product, it is possible to minimize the risk of failure and extend the life-time of the plant.
The presence of singlet oxygen (O2) in the water chemistry is well known to be one of the most harmful corrosion catalysts. For this purpose, oxide scavenger is generally dosed as a protective agent which also contributes to build a film of magnetite. However, this common practice can be quite detrimental in the low temperature regions of the plant where the solubility equilibrium cannot be reached, and magnetite is formed in a temperature range of maximum dissolution. As a result, single-phase flow-accelerated corrosion will lead to failure.
The best solution to this problem comes from selecting materials which are compatible with the chemistry of the process water. An alternative solution is to keep O2 concentration high to change the kinetic of the Schikorr reaction to the formation of ferric oxide hydrate (FeOOH) which is stable at low temperature ranges.
Figure 1: Frontal bundle plate of a heat exchanger. General degradation by the effects of incorrect chemistry and incompatibility with material selected. Notice the heterogeneous morphology of the surface which affects the transfer of heat and creates areas of different temperature gradient.
Convection relates to the properties of the fluid. Density changes with temperature variations. These changes promote circulation currents in the form of heat convective fluxes in the fluid. The quality of the fluid can also alter its density; therefore, impurities can have negative impact in the transport of energy. In consequence, it is essential to have good control and monitoring of the chemistry cycle of the plant.
The type of flow can also have a significant repercussion in the way energy diffusion occurs. There are many correlations that helps engineers to analyse the dependency between the flow regime and the fraction of convective heat to conduction. As general rule, the more turbulent and developed is a flow, the better heat transfer rate because of the proportional dependency between Nusselt and Reynolds numbers.
For this purpose, it is common to find baffle plates, helical bars or additional devises to promote turbulences in the heat exchanger.
Computational fluid dynamics can provide valuable information about the fluid behaviour inside the heat exchanger. Based on that, the performance can be enhanced by minimizing pressure drop, increasing residence time of the fluid, overcoming bypass effects or even vortex effects that would potentially decrease the overall efficiency.
Deficiency in the hydraulic balance could potentially lead to accumulation of particles and deposition. It could also produce to any kind of thermal stress failure such us overheating.
The fluid properties will also provide relevant information to determine through what side each fluid will go. Thus, the shell side of heat exchangers allows lower pressure drops, suitable for laminar fluids, clean fluids, condensate vapours or fluids in the state of vaporization. Instead, tube side would be more appropriate for high pressure fluids, high fouling fluids, sea water, corrosive fluids or cooling water systems.
Example 2: High Velocity
A computational fluid dynamic simulation was carried out to study the effects of the flue gas velocity in the last stages of a HRSG preheater. The results allow us to visualize a map of temperatures in a preheater module, hence, to better understand how energy is distributed in that HRSG preheater. In addition, it provides valuable information to predict points of low efficiency and propose corrective actions for optimization.
Image 1 represents the cross section top view of a HRSG preheater module It also shows how temperatures vary at the back-end tubes. The increase of velocity in the flue gas would produce gradients of pressure between the two opposite sides of the tube. In consequence, this would eventually create a vortex effect and turbulences in the backside of the tubes. This is shown in image 2.
Turbulence would drop velocity down to zero near the surface, which would decrease the rate of energy transfer from the flue gas to the condensate. By the same token, the lack of flue gas circulation around the tubes would bring temperatures down below the dew point and end up creating droplets of condensation. Image 3 shows areas of high probability for this phenomenon to occur. In that event, a layer of condensation would develop all along the surface of the tubes, while drastically altering the heat convection and thermal conduction rate across the wall.
Why is that relevant?
Back-end corrosion occurs when the gas turbine’s exhaust temperature falls below the dew point of any combustion product. Subsequently, high corrosive liquid acid would form in the presence of moisture. When natural gas contains sulphur in its composition, the reaction products derived from combustion will have, in addition to carbon dioxide and water, an equivalent proportion of sulphur dioxide. This compound will oxidize to sulphur trioxide and create sulfuric gas as it combines with the humidity of the flue gas.
If the temperature of the flue gas falls below the dew point of the sulfuric gas, liquid acid will form on the surface of such regions.
The aggressiveness of this attack depends on the concentration of acid in the condensate, which depends on the equilibrium H2O – H2SO4.
This simulation allows us to predict points of high probability where condensation droplets might form in the preheater module, so satisfactory mitigation strategies can be implemented during the HRSG design.
The chemical composition of the fluid can be very significant as well since diffusion may interfere in the heat transfer exchange. There are also evidences concerning the influence of turbulences in the rate of deposition.
Under certain flow regimens, the Reynolds number can favour particle settlements and so, provide resistance to the convective heat fluxes.
Fouling, scales, dissolved salts or any type of deposits on the surface can decelerate the transfer of energy and decrease the general performance of the equipment. The state of cleanliness is a serious and very dynamic aspect in the performance optimization of a heat exchanger. Therefore, periodical chemical cleanings are mandatory to ensure the maximum efficiency for operation.
The effects of deposition can also create localize heat resistance and lead the equipment to overheating. Likewise, the effects of degradation due to the absence of preservation during the construction, along with poor chemistry control in operation can cause the comparable type of failures. 
Example 3: Deposition and Corrosion Cells
This is another example to present the effects of how chemistry conditioning interferes in the efficiency of a heat exchanger. Image 1 is a cross section shot that presents clear incompatibility of the material selected with the chemistry of the process water. Exfoliation and deposition were the defects encountered at first sight. A closer view of the same tube, Image 2, revealed how corrosion cells evolved in the form of pitting. The loss of thickness weakened the tubes and lead to thermal stress cracking in that area.
Image 3 shows the presence of deposition which would affect the thermal conduction and create temperature gradients across the surface.
Another problem found during the analysis of the samples was that the tube presents intergranular corrosion (image 3) which increased the effects of degradation and would inevitability shorten the life-time service of the equipment.
The efficiency of a heat exchanger depends on the thermal conduction and convective heat transfer. Both phenomena are significantly influenced by the cycle chemistry of the plant.
Conduction depends on the thermal conductivity of the material and the state of cleaning of the equipment as any deposition will function as thermal resistance. It also could lead to other derivative problems such as overheating or thermal stress cracking.
Convection depends on the quality and type of fluid. The quality will directly influence the cycle chemistry of the plant, modify the initial state of passivation and threatens the integrity of the equipment. It can produce failure in structural parts and localized areas of the heat exchanger tubes.
The fluid dynamic can also promote the formation of deposits under certain flow regimens, flow disruption and increase of pressure drop. This can produce substantial negative impacts on unit reliability and performance.
About the author: Andrés Rodríguez Pérez (M.S., Fluid Mechanics and Heat Transfer, Chemical Engineer, University of Huelva, Spain, and Instituto Superior Técnico, Lisbon, Portugal) is a senior commissioning specialist with international professional experience and has successfully participated in the design, engineering, and commissioning of rojects in the power industry in the U.S., Spain, Israel, Mexico, the United Arab Emirates, Poland and South Africa. His expertise includes the integral engineering and field execution of chemical treatments, steam blowing, chemical conditioning, corrosion engineering and power plant startup management. He can be reached at [email protected]