**4.4 Fouling removal**

562 Heat Exchangers – Basics Design Applications

2 750 kg/m³ and a diameter of 2 mm. In spite of this low seawater velocity, an overall heat transfer coefficient of 2 500 W/(m²·K) was achieved. From the equations above, it can be concluded that this desalination plant required only 0.125/1.8 × 173 = 12 m condenser tube length in series, which can be installed in only one vessel with an overall height of less than

Pumping power is influenced by the pressure drop across the heat exchanger and the pressure drop to support a stationary fluidised bed, which is determined by the following

For the MSF desalination plant with stationary fluidised bed condensers specified above, the pressure drop to support the bed weight amounts to 47 000 N/m². On top of this pressure drop we have to add a pressure drop caused by the flow distribution system of 4 000 N/m² for stabilisation of the flow through all tubes. Pressure drop due to wall friction has not to be taken into account because of the very low liquid velocities in the tubes of only 0.125 m/s. However, for this particular application, we have to add the lifting height for the liquid which requires an additional pressure drop of 120 000 N/m² resulting in a total

For the conventional MSF desalination plant we calculate a pressure drop of approx. 400 000 N/m² required by the wall friction in these very long condenser tubes with much higher liquid velocities, and when we take into account the losses in water boxes we end up

It should be emphasised that for this particular application the pressure drop influencing the heat transfer coefficient and required by the condenser bundle installed in the conventional MSF is a factor 400 000 / 51 000 = 7.9 (!!) higher than this pressure drop for the MSF equipped with stationary fluidised bed condensers. These differences in pressure drop directly influence the pumping power requirements for both installations. In general, when also considering 'circulating' fluidised bed heat exchangers operating at somewhat higher liquid velocities and using higher density solid particles, the differences in pumping power requirements will not be that much as presented above, although, for all applications, the differences in pumping power remain easily a factor 2 to 3 times lower for the fluidised bed

heat exchanger compared to the conventional shell and tube heat exchanger.

 

*P L t t sl t* 1 *g* (7)

 

ΔPt = pressure drop across the tube due to bed weight [N/m²]

ρs = density of the material of the solid particles [kg/m³]

εt = liquid volume fraction in tube or porosity [-]

pressure drop of 47 000 + 4 000 + 120 000 = 171 000 N/m².

with a total pressure drop of approx. 450 000 N/m².

15 m.

equation:

Where:

**4.3 Pumping power requirements** 

Lt = tube height [m]

g = earth gravity [m/s²]

ρl = density of the liquid [kg/m³]

Fouling of heat exchangers is experienced by a gradual and steady reduction in the value of the overall heat transfer coefficient. A closer look into this phenomenon shows that there are always two causes:


The first cause can be solved by the mild scouring action of the fluidised solid particles in the tubes. The second cause, at least of the same importance as the first cause but often neglected, can only be solved by the installation of a strainer upstream the self-cleaning fluidised bed heat exchanger. To minimise the cost for such a strainer and the ground area for the heat exchanger and its accessories, we have developed a proprietary self-cleaning strainer which forms an integral part with the inlet channel of the exchanger.

Now, let us pay attention to some of our fouling removal experiences in a fluidised bed heat exchanger and, therefore, we once more should pay attention to our MSF seawater evaporators:

It is known that natural seawater cannot be heated to temperatures above 40 to 50 °C because of the formation of calcium carbonate scale. Conventional MSF evaporators often operate at maximum seawater temperatures of 100 °C, but only after chemical treatment of the seawater feed which removes the bicarbonates from the seawater and prevents the forming of scale. Of course, this is a complication in the process and does cost money. The MSF evaporator equipped with the stationary fluidised bed condensers, using 2 mm glass beads, has convincingly demonstrated that it can operate at even much higher temperatures than 100 °C without scale forming on the tube walls. Although, the scale crystals are precipitating from the seawater on the tube walls these crystals are knocked off by the glass beads at an early stage, so that it never comes to the formation of an insulating scale layer and the tube walls remain clean and shiny. Here we have clearly demonstrated the fouling removal, self-cleaning or non-fouling behaviour of a fluidised bed heat exchanger operating under harsh conditions as the result of the scouring action of the fluidised particles. No doubt that this feature is of extreme importance for heat exchangers operating on severely fouling liquids.

Meanwhile, with many self-cleaning fluidised bed heat exchangers already installed in different industries, commercial operating experiences have shown that the self-cleaning fluidised bed heat exchanger, which can remain clean indefinitely, is a cost-effective alternative to the conventional heat exchanger which suffers from severe fouling in a couple of hours, days or weeks and even months. Any type of fouling deposit, whether hard or soft; biological or chemical; fibrous, protein, or other organic types; or a combination of the above can be handled by the self-cleaning fluidised bed heat exchanger. Moreover, later in this chapter it will be shown that the unique characteristics of this heat exchange technology allow for the introduction of major design changes of installations in traditional processes

Self-Cleaning Fluidised Bed Heat Exchangers

**Steam** Final heater

Temperature [°C]

Heat input section (final heater)

Fig. 9. Principle of conventional MSF and its temperature diagram.

Tf

Td

Tin

Tmax

period.

for Severely Fouling Liquids and Their Impact on Process Design 565

Fig. 8. Rounding-off effects of 2 mm stainless steel particles as a function of operating

Condenser

**Distillate Seawater**

**Blow down**

Seawater feed

Blow down and produced distillate

Heat recovery section (n stages accomodated in several vessels)

Flash chamber

**D**T = ( Tmax - Td ) / n

**D**Tlog = Tmax - Tin - **D**T / 2

Heat transfer surface [m²]

Heat recovery section Stage 1 Stage 2 Stage n

Stage 1 Stage n

Fixed interstage orifice

Time: 0 weeks Time: 2 weeks Time: 15 weeks

and, therefore, the advantages of this heat exchange technology does reach much further than solving heat exchanger fouling problems only.
