The anti-fouling and corrosion effect of mitreh in heating and cooling systems

Investigating the effect of the chemical substance under the name of scale and corrosion inhibitor with the brand name Miter produced by Abrizan Research and Industrial Company in heating and cooling systems.

abstract

One of the main causes of sedimentation in construction facilities is the presence of calcium carbonate in water. Because the solubility of this compound in water is very low, and if the conditions are met, it precipitates quickly, one of the parts that provide these conditions. The surfaces in contact with water in the heat exchangers in domestic gas boilers are heat packages.

In addition to reducing the diameter of the water passage and thus increasing the pressure drop, sediment has high thermal resistance and reduces heat transfer. As a result, the efficiency of the heating system decreases and the pump has to work with more power, which all these factors will increase energy consumption and reduce the life of this equipment. Formation of deposits depends on parameters such as speed and temperature of the fluid, the type and amount of depositing material, the type and shape and subsurface of the converter.

One of the ways to prevent the formation of sediment is the use of chemical sediment inhibitors. In this plan, it is decided to investigate the effect of a type of sediment inhibitor under the brand name Miter produced by Abrizan Industrial Research Company in plate converters.

Sedimentation in heat exchangers

Introduction

One of the main problems caused by the formation of sediment and corrosion is the formation of sediment in heat exchanger equipment. Although many advances have been made in the knowledge of this phenomenon, there is still this problem and obtaining the relationship and effect of various characteristics such as fluid velocity, the concentration of mineral salts, temperature, etc. on the type and growth rate of sedimentation, especially in the converter Thermal properties are researched and investigated.

The sediment layer formed on the heat exchanger surfaces increases the thermal resistance and as a result decreases the heat transfer rate as well as the pressure drop in the path of fluid movement and in general causes a decrease in the efficiency of the equipment and costs more in design and maintenance.

Since 1960, extensive research has been done in connection with the understanding of the sedimentation process. Although at that time, an accurate estimate of the relationship between the sediment layer and its constituent factors and the knowledge of how this layer grows was not obtained, but after that, steps were taken in this regard, as well as the impact of sediment on the increase in thermal resistance, as well as the impact of sediment on costs. Service and maintenance and energy of heating and cooling systems were removed.

Usually, the sediment consists of calcium carbonate, calcium sulfate, magnesium and silica salts. At a temperature higher than 50, calcium bicarbonate turns into calcium carbonate by releasing carbon dioxide, which precipitates. At a temperature higher than 90, carbonate ions are hydrolyzed to hydroxyl ions, which combine with magnesium ions and produce magnesium hydroxide precipitation. Therefore, calcium carbonate and magnesium hydroxide are alkaline deposits, for example, magnesium hydroxide tends to form at high temperature and pH.

Calcium carbonate crystals exist in three forms: aragonite, calcite, and vaterite. All three types of these salts have inverse solubility and their solubility decreases with increasing temperature. Figure (1)

Figure (1) Solubility of calcium carbonate in water as a function of temperature

Sediment formation mechanism

Sediment formation can happen in five stages:

1. Beginning of sedimentation During this period that lasts for hours, the surface reaches the initial conditions for sediment formation.
2. Transport of solutes to the surface
3. Adhesion to the surface Not all substances transferred to the surface adhere to the surface. The properties of the particles and the nature of the surface play an important role in the adhesion of different particles to the surface.
4. removal from the surface sediment can be removed from the surface by different mechanisms, such as dissolution and wear, fluid velocity and surface roughness can also play an important role in the removal mechanism. Dissolution of solids may occur if the pH of the fluid changes. Abrasion can occur by the impact of particles or liquid on the deposit layer.
5. Sediment Aging Sediment grows with time until it reaches a stable value, but the mechanical strength of the sediment can be changed by changing the crystal structure or the chemical composition of the sediment. Aging of the sediment may strengthen or weaken the sediment.

Factors affecting sedimentation

Important factors affecting sedimentation include concentration, flow speed, surface temperature, etc. In this way, deposition with time asymptotically increases with increasing solution concentration and increasing temperature or increasing fluid velocity.

• The surface temperature: usually increases with the increase of the surface heat transfer temperature of the crystal deposit resistance by inverse solubility.
• Water quality
• The shear stress of sediment resistance decreases with the increase of shear stress. According to one theory, the shear stress applied in the vicinity of the sub-layer is enough to remove the upper layers of sediment. Another theory is that the sudden eruption of fluid to the wall causes the sediment to be removed. Increasing the speed means increasing the shear stress and removing the sediment.
• The fluid velocity increases with the decrease of sediment fluid velocity.
• Gender and subsurface

Sedimentation in plate heat exchangers

Nowadays, the use of plate heat exchangers has expanded greatly in the heating and air conditioning industry due to their compactness. The original idea of ​​making a plate heat exchanger was formed in 1923 by APV company, which was completed in 1950 by Alfalavan company. This type of converter consists of a series of corrugated plates together in such a way that in each layer, hot and cold fluid pass between the plates differently. figure (2)

The thickness of these plates is about 0.5 to 0.9 and they are wavy and wrinkled due to the increase of the heat transfer surface and the turbulence of the flow. These exchangers had a heat transfer coefficient of 3 to 8 times that of the existing shell and tube heat exchangers, which is the basis for increasing the efficiency of these heat exchangers. These converters can withstand pressure up to 20 bar and temperature up to 300°C.

Figure (2): A view of a type of plate heat exchanger

The formation of deposits on the heat exchange surfaces increases the resistance to heat flow from hot fluid to cold fluid and as a result reduces the efficiency of the heat exchanger, in order to transfer heat optimally, the temperature difference between the two hot and cold fluids must be very high and This is despite the fact that the temperature difference on both sides of the metal surface is very small. Due to the low thermal conductivity of these sediments, their thermal resistance increases, and with the decrease in the performance of the heat exchanger, depending on the amount of sediment, periodic cleaning becomes necessary.

Some of the factors that can reduce the formation of deposits in the converter are the turbulence of the flow, the presence of very smooth and rough plates, and the absence of areas in the path of the fluid passage that hinder the movement of the fluid. Also, if the converter plates are made of corrosion-resistant materials, the tendency to form deposits will decrease due to the lack of sedimentation of corrosion products that can contribute to the accumulation of deposits.

Relations related to plate heat exchangers

Several researches have been done regarding heat transfer in heat exchangers, in all of them, the point of view is regarding the estimation of the heat transfer coefficient between the walls of the individual plates of the plate heat exchanger and the hot and cold fluids that are moving between these plates.

In a plate-type heat exchanger, the total thermal resistance is equal to the sum of the following thermal resistances:

1. Thermal resistances of the displacement type of movement of hot and cold fluid, which is dependent on the boundary layer of these two fluids.
2. Thermal resistance of the wall of the exchanger plates.
3. Thermal resistance due to the formation of deposits on both sides of each exchanger plate (cold fluid and hot fluid flow side)

In this case, the overall heat transfer coefficient is obtained from the following equation:

If the thickness of the plates is small, due to the high rate of heat transfer coefficient of conduction of the metal plates, the thermal resistance of the metal plates can be ignored, so the overall heat transfer coefficient of the exchanger will be as follows:

In this case, if there is sediment on both sides of each plate (on the cold and hot fluid sides) and the thermal resistance of the sediment (Rf), the overall heat transfer coefficient will be as follows:

Rf: Thermal resistance of the deposit in

The rate of deposition and the increase in the thickness of the deposit layer on the heat exchanger surfaces can be shown as a function of time. Therefore, the increase in pressure drop and thermal resistance resulting from the increase in deposit thickness can also be shown as a function of time.

Pressure drop in plate heat exchangers

The pressure drop in plate heat exchangers is calculated based on the following equation:

f : Friction coefficient, which is obtained for turbulent flows from the equation

r : Fluid density

L: The length of the path the fluid travels.

De : Hydraulic diameter (De ≈2d , d: distance between two plates)

V:  Fluid velocity

Logarithmic mean temperature difference in plate heat exchangers (LMTD)

In thermal analyses of plate heat exchangers, the total heat transfer rate is related to the overall heat transfer coefficient, the heat transfer surface area of ​​the plates, and the average logarithmic temperature difference, which can be expressed by the following equation:

Q : Overall heat transfer coefficient

A : The heat transfer surface area of ​​the plates

∆Tmean : The logarithmic mean temperature difference, which is as follows:

Thi : Hot fluid side temperature at entry

Th0 : Temperature of the hot fluid side at the inlet

Tci : Cold fluid side temperature at entry

Tc0 : Cold fluid side temperature at outlet

In plate heat exchangers, the average temperature can be expressed based on the following equation:

Where F (correction factor) is approximately 1 for counter flow exchangers and less than 1 for other exchangers. When the flow rate ratio on the cold side to the hot fluid side is about 0.66 to 1.5, plate exchangers with equal numbers of hot and cold side paths can be used.

Project process and experimental work

In this project, it was decided to investigate the effect of chemical substances with the Miter brand produced by Abrizan company in preventing sedimentation or corrosion in heating and cooling systems. For this purpose, a test stands including a heating and cooling system was designed and implemented, which is schematically shown in Figure (3). As shown in the figure, the test stand includes a main plate heat exchanger through which cold and hot currents pass in opposite directions. Therefore, some of the thermal energy lost by the hot fluid is wasted in the medium and the rest is used to heat the cold fluid. Since it is one of the places with the highest potential for sediment formation in the piping system of the heat exchanger, in this work, this exchanger was considered to investigate the effect of chemicals. Therefore, it was considered to check the three parameters of temperature, pressure and flow rate in this converter. For this purpose, 4 thermometers were placed to check the temperature at the inlet and outlet of the hot and cold path of the converter. Also, 2 pressure gauges were placed in the hot and cold inlets of the converter and 2 transmitters were placed in the outlets to record the inlet and outlet pressure. In order to determine the flow rate in two routes, 2 flow meters were placed in the hot and cold route.

Figure (3): Schematic of the test stand

The fluid in the cold pipe path is cooled by an air-cooled chiller, and in the hot pipe path, through a secondary plate heat exchanger and heated by an electric heater that maintains a water source at a relatively constant temperature.

Equipment used in the project

• Heater tank with 6 kW electrical elements and water circulation pump
• Plate exchanger between the heater tank and the hot cycle circuit with a circulation pump to circulate water in the circuit
• 60-liter open expansion source to balance the excess pressure of the system and compensate for the lack of water in the hot circuit
• Air-cooled chiller
• 60-liter open expansion source to balance the excess pressure of the system and compensate for the lack of water in the cold circuit
• Main plate exchanger
• 4 PT100 thermometers with the Lutron brand to measure the temperature difference on both sides of the main plate exchanger in each of the hot and cold circuits
• Two pressure gauges made by WIKA with a capacity of 0-4 bar to measure the inlet pressure to the main plate exchanger in the cold and hot circuits (output is 4-20mA)
• 2 pressure transmitters made by YOKOGAWA that express the inlet and outlet pressure drop of the plate exchanger in each hot and cold circuit as a percentage.
• 2 vane flow meters up to a temperature range of 130°C made by ZENNER to measure cold and hot flow rates
• ADAM card to transfer temperature and pressure measurement equipment data to the computer
• Piping using combined PE-X/Al/PE-X pipes and fittings

(Figure 4) shows the test stand for testing anti-fouling materials and (Figure 5) shows the main plate heat exchanger section along with the measuring equipment.

(Figure 4): Test stand for testing Mitreh antifouling materials

(Figure 5): Main plate heat exchanger section with measuring equipment

The process of measuring parameters during the test

During the test period, the temperatures at the inlet to the plate heat exchanger and the outlet from it in the two hot and cold circuits, the pressures in both hot and cold circuits at the inlet to the plate heat exchanger, the pressure differences in both hot and cold circuits at the outlet from the heat exchanger were measured and recorded daily, and the flow rate in the two cold and hot circuits, TDS, and water hardness inside the circuit were measured and recorded weekly.

In order to keep the temperature of the hot water entering the plate heat exchanger approximately constant, the heater tank was set at 70°C. Also, in order to keep the temperature of the cold water entering the plate heat exchanger constant, an air-cooled chiller was used.

In the first stage, the hot circuit was filled with hard water (TDS: 408ppm, Hardness: 204ppm) and the cold circuit was filled with distilled water. After sufficient time had passed and equilibrium had been reached, the following were recorded:

Thi: Hot path inlet temperature to the exchanger

Th0: Hot path outlet temperature from the exchanger

Tci: Cold path inlet temperature to the exchanger

Tc0: Cold path outlet temperature from the exchanger

Phi: Hot path inlet pressure to the exchanger

DPhot : Difference in inlet and outlet pressure of the exchanger in the hot path

Pci: Cold path inlet pressure to the exchanger

DPcold : Difference in inlet and outlet pressure of the exchanger in the cold path

mh : Hot path flow rate

mc: Cold path flow rate

Hardness and TDS of water in the hot water line

Given the specific inlet and outlet temperatures and flow rates of both the cold and hot circuits, the amount of heat exchanged in the exchanger can be obtained:

Here, Ch and Cc are the heat capacities of water in the hot and cold circuits, respectively.

Some of Qh is spent heating the cold water in the plate heat exchanger and the rest is lost as energy loss to the environment. Therefore, the heat exchange rate is equal to Qc.

Once the heat exchange rate is known, the overall heat transfer coefficient of the exchanger can be calculated according to the following equation:

Uc : Overall heat transfer coefficient of the exchanger in the unscathed state

A : Overall heat transfer area of ​​the exchanger, which can be achieved considering the number of exchanger plates and the surface area of ​​each plate.

F : Correction factor and is a dimensionless number, (F is about 0.9 in the plate heat exchanger used)

∆TLMTD : Logarithmic mean temperature difference, which is determined by knowing the temperatures.

By obtaining Uc A in the case of no deposits and Uf A in the case of deposits in the exchanger from the above equation, the thermal resistance of the deposit can be measured over time according to the following equation:

Uf : Overall heat transfer coefficient of the exchanger in the state where the scale has formed

Uc : Overall heat transfer coefficient of the exchanger in the state without scale

Rf : Thermal resistance of the scale at any time of scale formation

On the other hand, by having the pressure at the inlet and outlet of the heat exchanger, the pressure drop of the exchanger due to sediment formation can be obtained over time.

Results of measured parameters

As mentioned, in this project, it was decided to investigate the effect of chemicals with the trade name Mitreh produced by Abrizan Company in preventing sediment or corrosion in heating and cooling systems. In the test stand implemented, heat exchange was carried out by a plate heat exchanger with two closed heating and cooling circuits.

In the first phase, distilled water was used in the cooling circuit and water with a hardness of 204ppm was used in the heating circuit, and over two months, temperature, pressure, flow rate and hardness parameters were measured. On most days, more than one measurement was taken.

Also, after 60 days, the main exchanger was opened and the sediment status was checked, as shown in (Figure 6). As can be seen in the images, significant sediment has formed in the heating line.

(Figure 6): Images of the main heat exchanger after 60 days (deposition is observed in the hot path)

In the second phase, the piping system and exchanger were cleaned with a cold Mitreh solution. Unlike descalers, this scale-removing solution dissolves all the deposits formed in the water without causing corrosion to the equipment.

Then, 25ppm (equivalent to 100ml per 50ml solution) of Mitreh MSA1110 solution was added to this water in the heating circuit. After adding Mitreh solution to the desired water, the ions forming the deposits became aggregated, in which case solid materials do not tend to adhere to surfaces and form deposits. The parameters of temperature, pressure, flow rate and hardness were measured again. Some of the measurement results are shown in (Figure 7). On most days, more than one measurement was taken.

Results for raw water in the heating circuit

Here, raw water is hard water with a hardness of 204ppm and without chemical additives.

As mentioned, scale formation reduces the heat transfer of the exchanger due to the thermal resistance of the scale. The thermal resistance of the scale increases with time, first due to the increase in the amount and thickness of the scale, and then stabilizes, given that no new hard materials are added to the circuit and therefore no new layer of scale is formed. This is shown in (Figure 7).

(Figure 7): Increase in thermal resistance of deposits over time in the heating circuit

(Figure 8) also shows∆TLMTD of the exchanger in the hot circuit. As is clear from the graph, as scale forms over time and thermal resistance increases, the difference in inlet and outlet temperatures of the exchanger decreases, meaning that less exchange occurs.

(Figure 8): Reduction of the difference in temperature between the inlet and outlet of the converter in the hot circuit over time in the heating circuit

In general, the passage of fluid through the exchanger is accompanied by a pressure drop due to the impact on the exchanger plates. As time passes and sediment forms and the water passage channels narrow, the water pressure drop increases. (Figure 9) shows an increase in the difference in pressure between the inlet and outlet of the exchanger.

(Figure 9): Increase in the difference in pressure between the inlet and outlet of the converter in the hot circuit over time in the heating circuit

Results for water containing mitreh in the heating circuit

Here, water with mitreh is hard water with a hardness of 204ppm and with mitreh chemical additives. As can be seen in (Figure 10), the thermal resistance of the scale is constant over time and has not increased. Also, its amount is low, which indicates that scale has not formed in the exchanger.

(Figure 10): Increase in thermal resistance of scale depending on time in the heating circuit And when Mitreh anti-scale agents have been added to the water.

Also (Figure 11),∆TLMTD shows the converter in the hot circuit. As expected, the temperature difference between the inlet and outlet of the converter is almost constant and has not decreased. This means that due to the lack of scale formation and the lack of increase in thermal resistance, the temperature difference between the inlet and outlet of the converter in the hot circuit is constant.

(Figure 11): Reduction in the difference in inlet and outlet temperature of the exchanger in the hot circuit depending on the time in the heating circuit And when Mitreh anti-scale agents have been added to the water.

(Figure 12) also shows the increase in the pressure difference between the inlet and outlet of the converter, which is almost constant.

(Figure 12): The increase in the difference in pressure between the inlet and outlet of the exchanger in the heating circuit depends on the time in the heating circuit And when Mitreh anti-scale agents have been added to the water.

References

1- Alahmad M. and Abdul Aleem F. , "Scale Formation and Fouling Problems Effect on The Performance of MSF and RO Desalination Plants in Saudi Arabia ", Desalination, 93, 287-310, (1993).

2- Knudsen, J. G. “Fouling in Heat Exchangers”, in “Hemisphere Handbook  of heat Exchangers Design”, G. F. Hewitt (ed.), Hemisphere, (1990).

3- Bird, M & Fryer, P (1991) An experimental study of the cleaning of surfaces fouled by whey proteins Trans IChemE, Vol 69, p i3-21