2. Practical advice

There are several different types of brazed plate heat exchangers, depending on their material combinations, pressure ratings and functions. The standard material is stainless steel, vacuum-brazed with pure copper or all-stainless paste. The basic construction materials indicate the types of fluids that SWEP brazed plate heat exchangers can be used with. Typical examples are synthetic or mineral oils, organic solvents, water (not seawater), glycol/water mixtures (e.g. water/ethylene glycol and water/propylene glycol) and refrigerants (e.g. HCFC). Please note that if natural refrigerants (e.g. ammonia) are employed, brazed plate heat exchangers with all-stainless-based or nickel alloy brazing material must be used.

The front plates of SWEP brazed plate heat exchangers are marked with an arrow, either on an adhesive sticker or embossed in the cover plate. The purpose of this marker is to indicate the front of the brazed plate heat exchanger and the location of the inner and outer circuits/channels. With the arrow pointing up, the left side (Ports F1, F3) is the inner circuit and the right side (Ports F2, F4) is the outer circuit. The outer circuit has a lower pressure drop because it contains one more channel. The inner circuit consequently has a slightly higher pressure drop. Ports F1/F2/F3/F4 are situated on the front of the heat exchanger (see Figure 2.1). Ports P1/P2/P3/P4 are situated on the back. Note the order in which they appear.

Figure 2.1

Construction

In principle, the brazed plate heat exchanger is constructed as a package of corrugated channel plates between front and rear cover plate packages. Cover plate packages consist of sealing plates, blind rings and cover plates (see Figure 2.2). The type of connection can be customized to meet specific market and application requirements. During the vacuum-brazing process, a brazed joint is formed at every contact point between the base and the filler material. The design creates a heat exchanger consisting of two separate circuits.

Figure 2.2

Sealing plates are used to seal off the space between the cover plate and the first and last channel plates. The number of cover plates varies, e.g. with the brazed plate heat exchangers type, size and pressure rating. Some brazed plate heat exchangers have a blind ring to seal off the space between the channel plate and the cover plate. In others, the blind rings are integrated in the cover plate and first/last channel plates.

Material Combinations

There are different brazed plate heat exchanger product categories depending on their material combinations and design pressures (see Figure 2.3).

Figure 2.3

They are defined as standard brazed plate heat exchangers, all-stainless brazed plate heat exchangers and high-pressure brazed plate heat exchangers. The standard plate materials are stainless steel, S, (AISI 316), or N (AISI 304), vacuum-brazed with a pure copper filler, C, stainless steel paste PS or a nickel-based filler, N. Carbon steel can be used to some extent, e.g. for certain types of connections. brazed plate heat exchangers are available in several pressure ratings, from L to U class, or from 4 bar to as high as 158 bar. Some materials and pressure denominations are shown below.

B35Hx40/P-(X)(Y)-(Z)

Where:
(X) is the plate material (e.g. S=stainless steel)
(Y) is the braze material (e.g. C=copper, PS=stainless steel paste, N=nickel alloy)
(Z) is the pressure rating (e.g. S=standard pressure, H=high pressure) 

Table 2.1. Examples of brazed plate heat exchangers with various materials and design pressures

Brazed plate heat exchcnger plates and channel types

Some brazed plate heat exchangers are available with different types of channel plates, i.e. with different herringbone patterns. The chevrons, i.e. the V-shaped pattern on the plates, can be obtuse (creating a high-theta plate) or acute (creating a low-theta plate) (see Figure 2.3). The thermal characteristics of the brazed plate heat exchanger can be modified by mixing high- and low-theta plates. For example, a brazed plate heat exchanger can be constructed with the same pressure drop on both sides despite different flow rates.

Flow Configurations

The fluids can pass through the heat exchanger in different ways. For parallel-flow brazed plate heat exchangers, there are two different flow configurations: co-current or counter-current (see Figure 2.4).Figure 2.4

There are several different versions of the channel plate packages. Some examples are shown in Figure 2.5.
Figure 2.5

 

Approvals

SWEP brazed plate heat exchangers are approved by a number of independent bodies, including:

• Canadian Standard Association (CSA)
• The High Pressure Gas Safety Institute of Japan (KHK)
• Underwriters Laboratories (UL) – USA
• Pressure Equipment Directive (PED) – Europe

SWEP also has design approvals from (e.g.) Lloyds Register, UK; Det Norske Veritas (DNV), Norway; American Bureau of Shipping (ABS), USA; Korean Register of Shipping (KR), Korea; Registro Italiano Navale (RINA), Italy.

Labeling System and Operating Conditions

All brazed plate heat exchangers carry an adhesive label (see Figure 2.6) with vital information about the unit, e.g. the type of heat exchanger, and SWEP’s serial number. This indicates the basic brazed plate heat exchanger model. The operating conditions state the maximum operating temperature and pressure as determined by the various approving organizations.Figure 2.6

The label also includes the serial number (see Figure 2.7). The engraved serial number provides information about where and when the brazed plate heat exchanger was produced, etc.Figure 2.7

Mounting

Never expose the unit to pulsations or excessive cyclic pressure or temperature changes. It is also important that no vibrations are transferred to the heat exchanger. If there is a risk of this, install vibration absorbers. For large connection diameters, the use of an expanding device in the pipeline is recommended. It is also suggested that a rubber mounting strip, for example, should be used as a buffer between the brazed plate heat exchanger and the mounting clamp.

In single-phase applications, e.g. water-to-water or water-to-oil, the mounting orientation has little or no effect on the performance of the heat exchanger. In two-phase applications, however, the orientation of the heat exchanger becomes very important. In two-phase applications, SWEP brazed plate heat exchangers should be mounted vertically, with the arrow on the front plate pointing upwards.

Several mounting suggestions for SWEP brazed plate heat exchangers are shown in Figure 2.8. Mounting stud bolts in various versions and locations are available as an option on brazed plate heat exchangers. For smaller brazed plate heat exchangers, it is also possible to mount the unit by simply suspending it from the pipes/connections.

Figure 2.8

Connections in General

All connections are brazed to the heat exchanger in the general vacuum brazing cycle. This process gives a very strong seal between the connection and the cover plate. However, take care not to join the counterpart with such force that the connection is damaged.

Depending on the application, there are many different versions and locations available for the connections, e.g. Compac flanges, SAE flanges, Rotalock, Victualic, threaded connections and welding connections (see Figure 2.10). It is important to have the correct international or local standard of connection, because they are not always compatible.

Figure 2.10

Some connections have an external heel (see Figure 2.12) to simplify the pressure and leakage testing of the brazed plate heat exchanger in production.

Figure 2.12

Some connections are fitted with a special plastic cap to protect their threads and sealing surface (see Figure 2.11) and to prevent dirt and dust from entering the brazed plate heat exchanger. This plastic cap should be removed with care, to prevent damage to the thread or any other part of the connection. Use a screwdriver, pliers or knife.

Figure 2.11

Threaded Connections

Threaded connections can be female or male (see Figure 2.12 above), in well-known standards such as ISO-G, NPT and ISO 7/1.

Soldering Connections

The soldering connections (sweat connections) (see Figure 2.13) are in principle designed for pipes with dimensions in mm or inches. The measurements correspond to the internal diameter of the connections. Some SWEP soldering connections are universal, i.e. fit both mm and inch pipes. These are denominated xxU, such as the 28U, which fits both 1 1/8" and 28.75 mm.Figure 2.13

All brazed plate heat exchangers are vacuum-brazed with either pure copper filler or nickel-based filler. Under normal soldering conditions (no vacuum), the temperature should not exceed 800 °C (1470 °F). The material’s structure can be altered if the temperature is too high, resulting in internal or external leakage at the connection. It is therefore recommended that all soldering uses silver solder containing at least 45% silver. This type of solder has a relatively low soldering temperature and high moistening and fluidity properties.

When soldering flux is used to remove oxides from the metal surface, this property makes the flux potentially very aggressive. Consequently, it is very important to use the correct amount of soldering flux, because too much may lead to severe corrosion. No flux should be allowed to enter the brazed plate heat exchanger.

It is important to degrease and polish the surfaces when soldering. Apply chloride flux with a brush. Insert the copper tube into the connection and braze with minimum 45% silver solder. Point the flame towards the piping and braze at max. 650 °C (1200 °F). Avoid internal oxidation, e.g. by protecting the inside of the refrigerant side with nitrogen gas (N₂).

Welding Connections

Welding is recommended only on specially designed welding connections (see Figure 2.14). All SWEP welding connections have a 30° chamfer on top of the connection. Do not weld pipes on other types of connections. The measurement in mm corresponds to the external diameter of the connection.

Figure 2.14

During the welding procedure, protect the unit from excessive heating by:

a) using a wet cloth around the connection.

b) making a chamfer on the joining tube and connection edges.

Use TIG or MIG/MAG welding. When using electric welding circuits, connect the ground terminal to the joining tube, not to the back of the plate package (see Figure 2.15). Internal oxidation can be reduced by using a small nitrogen flow.

Figure 2.15

Allowable Connection Loads for Pipe Assembly Conditions

The maximum allowable connection loads, as shown in Figure 2.15  and Table 2.2, are valid for low cycle fatigue. If high cycle fatigue is involved, a special analysis should be carried out.

Table 2.2. Allowable connection loads for different pipe assembly conditions.

Allowable Loads for Stud Bolt Assembly Conditions

Mounting stud bolts (see Figure 2.9) in different versions and locations are available for brazed plate heat exchangers as an option. These stud bolts are welded to the unit. The maximum allowable loads on the stud bolts during assembly are shown in Table 2.3.
Figure 2.9

Table 2.3. Allowable loads for different stud bolt assembly conditions.

Strainers

If any of the media contain particles larger than 1 mm, a strainer (see Figure 2.16) with a size of 16-20 mesh (number of openings per inch) should be installed before the exchanger. The particles could otherwise block the channels, causing low performance, increased pressure drop and risk of freezing. Some strainers are available as brazed plate heat exchanger accessories.Figure 2.16

Insulation

Brazed plate heat exchanger insulation (see Figure 2.17) is recommended for evaporators, condensers and district heating applications, etc. For refrigeration, use extruded insulation sheets, e.g. Armaflex or equivalent, which can also be supplied by SWEP.
Figure 2.17

Single-Phase Applications

Normally, the circuit with the higher temperature and/or pressure should be connected on the left side of the heat exchanger when the arrow is pointing upwards. For example, in a typical water-to-water application, the two fluids are connected in a counter-current flow, i.e. with the hot water inlet in connection F1 and its outlet in F3, and the cold water inlet in connection F4 and its outlet in F2 (see Figure 2.18). This is because the right-hand side of the heat exchanger contains one more channel than the left-hand side, and the hot medium is thus surrounded by the cold medium to prevent heat loss.
Figure 2.18

Two-Phase Applications

The heat exchanger should be oriented vertically with the steam inlet at the top and the condensate outlet at the bottom, as shown in Figure 2.19. In this orientation, condensate drainage is assisted by the force of gravity. Any other orientation, especially a horizontal one, makes it more difficult to drain off the condensate. Note the 'L' mark on the front plate of SWEP brazed plate heat exchangers. The unit should be installed with the point of the V uppermost and the steam inlet to the left of it. The heat exchanger should be connected for counter-current flow, i.e. with the steam and process media flowing in opposite directions to each other as shown in Figure 2.19. The advantage of this arrangement is that heat is transferred more quickly and efficiently than for co-current flow.
Figure 2.19

Corrosion is the term for a chemical or electrochemical reaction between a material, usually a metal, and its environment, which produces a deterioration of the material and its properties, e.g. rust. 

Types of Corrosion

A metal can be exposed to many different kinds of corrosion. The types of corrosion considered below are those most common when BPHEs are exposed to corrosive environments.

Pitting and Crevice Corrosion

In principle, pitting and crevice corrosion are the same phenomenon. Pitting may appear on exposed surfaces, for example attacking stainless steel if the passive layer is damaged. The passive layer is a protective surface film that is formed spontaneously when stainless steel AISI 316 is exposed to air. The attack may be sudden and can quickly cause leakage. Crevices may occur in welds that fail to penetrate, in flange joints and under deposits on the steel surface.

General Corrosion

General corrosion is a deterioration distributed more or less uniformly over a surface. This type of corrosion is more predictable than pitting. If a device has corroded 0.1 mm in one year, it will very probably corrode 0.2 mm in two years.

Corrosion in BPHEs

Provided the BPHE is exposed to the recommended environment, there should be no corrosion problems. Neither stainless steel AISI 316 nor copper corrodes easily. However, if BPHEs from the standard range are exposed to an unfavorable environment, corrosion can attack either the stainless steel AISI 316 or the copper brazing, as shown here.

left image is General copper corrosion / right image is Pitting corrosion in stainless steel 316

BPHEs do not resist high concentrations of chloride ions (Cl^-) in an oxidizing environment, because chlorides form a galvanic cell with oxygen and the metals of the BPHE. Stainless steel, in particular, is sensitive to this kind of attack, and the result may be pitting and/or crevice corrosion. It is also important to mention that higher temperatures make chlorides more aggressive towards stainless steel. When chlorides or other halogen ions (bromides, iodides) are present in high concentrations, SWEP recommends a Minex with channel plates in titanium.

When copper corrodes, it is degraded more often by general corrosion than by pitting. General corrosion will most probably attack copper exposed to ammonia (NH₃) or fluids with high sulfur contents. SWEP all-stainless BPHEs can have stainless steel paste or a nickel alloy as the brazing material (instead of copper), which is resistant to high sulfur and ammonia contents. Another threat to copper is the presence of dissolved salts in the fluid that affect the BPHE. Maintaining electrical conductivity within the recommended range will minimize this source of corrosion. General corrosion may attack both copper and stainless steel in strongly acidic solutions. However, copper is the more sensitive metal in an alkaline environment.

Water Quality

The corrosive effect of natural water can vary considerably with its chemical composition. Water quality is of great importance in avoiding corrosion in BPHEs.

City water is normally of good quality and is used as make-up water in cooling towers.

Well water is usually fairly cold and clean, which implies that it has a low biological content (see Practical advice/fouling. However, the concentration of scale-forming salts (calcium and magnesium sulfates and carbonates) can sometimes be very high. Pitting corrosion may be initiated under these salt deposits.

Cooling tower water (see Figure 2.31) is circulated in an open circuit between the BPHE unit and the cooling tower. The salt content can be ten times higher than in the make-up water, which is usually city water (very clean) or well water (fairly clean). In heavily polluted areas, the water may pick up dust and/or corrosive gases, such as sulfur and nitrogen compounds, during its circulation. The net effect could be a corrosive brew that requires regular treatment. As it is an open loop, treatment is fortunately quite easy.Figure 2.31

In river and lake surface water, the concentration of scale-forming salts is usually fairly low. However, there may be appreciable amounts of solids ranging from salts and soil particles to leaves and algae. Some type of pre-treatment is usually necessary, particularly to control biological activity.

Brackish water and seawater are not recommended in standard BPHEs (stainless steel 316/copper), all-stainless BPHEs (stainless steel 316/stainless steel paste) and nickel-brazed (stainless steel/Nickel) because of the corrosive action of the very high chloride concentrations. However, Minex models are available in titanium, which is compatible with seawater. Titanium forms a very stable, continuous and protective oxide film, giving it excellent resistance to corrosion. A damaged oxide film can generally heal itself immediately, provided traces of oxygen or water are present in the environment.

Avoiding Corrosion

In an environment containing halogen ions (e.g. chlorides or bromides) a Minex with titanium channel plates is recommended. Stable and turbulent water flow does not give corrosive substances the time needed to start the corrosion process. It is therefore important to maintain a stable water flow to avoid stagnant zones inside the BPHE. It is worth rinsing and drying the BPHE carefully before a long standby.

Bleed-offs and addition of make-up water to cooling towers must be carried out regularly, because the circulating water in an open cooling tower will be more or less contaminated with corrosive substances. When cooling tower water is used, a strainer should always be installed at the inlet of the BPHE. The recommended strainer stops particles larger than 1 mm, which correspond to a mesh size of 16-20 mesh (depending on the wire diameter). Smaller particles will pass through the heat exchanger due to the high turbulence.

In addition, salts, chlorides, pH and temperature, etc., must be kept within the recommended ranges to avoid BPHE corrosion.
For more specific information see Appendix A.

Fouling is a very undesirable phenomenon in the world of heat transfer and heat exchangers. In most heat exchangers, the fluid flowing is not completely free from dirt, oil, grease and chemical or organic deposits. In all cases, an unwanted coating can collect on the heat transfer surface, decreasing the heat transfer coefficient. The thermal efficiency of the heat exchanger will be reduced and the pressure drop characteristics may change.

This chapter discusses several types of fouling, the reasons for their occurrence and the preventive measures that can be taken avoid them. Fouling is a broad term, and can be divided more specifically into:

• Scaling
• Particulate fouling
• Biological growths
• Corrosion 

Scaling

Scaling is a type of fouling caused by inorganic salts in the water circuit of the heat exchanger. It increases the pressure drop and insulates the heat transfer surface, thus preventing efficient heat transfer. It occurs when there is a low fluid velocity (i.e. laminar flow) and uneven distribution of the liquid along the passages and the heat transfer surface.

The likelihood of scaling increases with increased temperature, concentration and pH. Studies have shown that high turbulence and a small hydraulic diameter, such as with SWEP BPHEs, have beneficial effects on this type of fouling.

Most scaling is due to either calcium carbonate (lime) or calcium sulfate (gypsum) precipitation. These salts have inverted solubility curves (see Figure 2.32), i.e. the solubility in water decreases with increasing temperature.
Figure 2.32

The salts are therefore deposited on the warm surface when the cold water makes contact with it. Pure calcium sulfate is very difficult to dissolve, which makes cleaning more complicated. In general, other types of scale are more easily removed.

The most important factors that influence scaling are:

• Temperature
• Turbulence
• Velocity
• Flow distribution
• Surface finish
• Composition and concentration of the salts in the water
• Water hardness
• pH

Scaling is more likely at a high pH, so a general approach to this problem is to keep the pH between 7 and 9. The risk of scaling generally increases with increasing water temperature. Experience shows that scale is seldom found where wall temperatures are below 65 °C (159 °F).

Removing the scale that has been formed restores the operating efficiencies of the equipment and the heat transfer surfaces. Other benefits from removing the scale are that it lowers the pressure drop, reduces the power consumption and extends the lifetime of the equipment.

Types of scale

Calcium carbonate (CaCO₃) can be formed when calcium or bicarbonate alkalinity (HCO₃^-, CO₃^2- and OH^- ions in the water) is present. An increase in heat and/or pH will cause precipitation of calcium carbonate according to (Ca(HCO₃)₂ -> CaCO₃ [s] + CO₂ [g] + H₂O).

Calcium sulfate (CaSO₄) is 50 times more soluble than calcium carbonate and will therefore precipitate only after calcium carbonate has been formed. This type of scale can exist in various forms, and its formation depends strongly on the temperature. An increase in temperature decreases the solubility of this salt and increases the risk of scaling (see Figure 2.37)

Figure 2.37

Water Scaling Tendency

In order to estimate the scaling tendency of natural water, several parameters must be analyzed and determined:

• pH
• Calcium content
• Alkalinity
• Ionic strength of the water

The first three parameters are relatively straightforward to determine. However, the ionic strength depends on the total amount of dissolved, dissociated compounds, i.e. salts and acids, as well as the relative concentrations of the various salts and acids.

The Langlier saturation index, I_s, is calculated from the amount of total dissolved solids (TDS), calcium concentration, total alkalinity, pH and solution temperature. It shows the tendency of a water solution to precipitate or dissolve calcium carbonate. In this method, the pH_s (the pH at the equilibrium state) is calculated from the total salt content (pS), the alkalinity (pAlk) and the calcium content (pCa). The pH_s is then compared with the actual pH for the water, giving the Langlier Index, I_s:

I_s = pH - pH_s

where

pH_s = pS + pAlk + pCa

The pH measurement is straightforward and is performed routinely. Because the pH may vary with the season and the climatic conditions, it should be measured on several different occasions. The calcium content, pCa, is normally expressed as the concentration of calcium either as calcium carbonate (CaCO₃)or as calcium ion (Ca²⁺). The bicarbonate alkalinity, pAlk, can be determined by titrating the water with an acid and a suitable indicator (e.g. methyl orange). The result is expressed in various ways, e.g. as the equivalent CaCO₃. The corresponding pAlk is obtained from the Langlier diagram (see Figure 2.37). The relative proportions of the various salts are fairly constant in naturally occurring water. Langlier uses the total salt content (mg/l), i.e. TDS (Total Dissolved Solids), as a measurement of the ionic strength. The corresponding amount of total solids, pS is obtained from the Langlier diagram (see Figure 2.37). All these measurements can be obtained from a general water analysis.

Note:

• For water analysis, mg/l is equal to ppm
  
  
• The relationship between calcium and calcium carbonate is:
   
  
  
• TDS = Salt content (mg/l), or possibly the conductivity x 0.63 μS/cm

If I_s is negative, the water has a tendency to be corrosive. This corrosivity is valid for carbon steel and, to a lesser extent, copper, but not for AISI 316 stainless steel. If I_s is positive, the water has a tendency to cause scaling. 

Example of the Use of the Langlier Diagram

 

Using the results from the water analysis above, the pCa, pAlk and pS can be found in the Langlier diagram as follows:

pCa
On the Langlier diagram (Figures 2.37 and 2.38), note the measured Ca concentration of 120 mg/l CaCO₃ (or 120 ppm). Read off the pCa value at the point where the calcium concentration meets the diagonal line for Ca/pCa. This gives pCa=2.92.

Figure 2.38

 

pAlk
On the Langlier diagram (Figures 2.37 and 2.38), note the measured alkalinity value of 100 mg/l CaCO₃ (or 100 ppm). Read off the pAlk value at the point where the alkalinity value meets the diagonal line for CaCO₃/pAlk. This gives pAlk=2.70.

pS

On the Langlier diagram (Figures 2.37 and 2.39), note the measured TDS concentration of 210 mg/l. Read off the pS value at the point where the TDS concentration meets the temperature line (in this case 49 °C). This gives pS=1.70.

Figure 2.39

Using the results extracted from the Langlier diagram, pH_s can be calculated, and then the saturation index I_s:

pH_s = pS + pAlc + pCa = 1.70 + 2.70 + 2.92 = 7.32

I_s = pH- pH_s = 8.0 -7.32 = (+)0.68

Because I_s>0, the water in this example has a tendency to cause scaling.

Determine Whether Scaling Has Occurred

To be able to clean the heat exchanger unit easily, it is important to detect the signs of scaling before the unit is completely clogged. This can be done by measuring the inlet and outlet temperatures of the heat exchanger, which indicate whether fouling has occurred. Fouling of the heat transfer surface decreases the heat transfer, resulting in a temperature difference smaller than specified. Another way to detect fouling is by measuring the pressure drop over the heat exchanger. Because fouling restricts the passages, and thus increases the velocity, the pressure drop will increase. When using this method, make sure that the water flow rate is as specified, because changes in the flow rate will of course also affect the temperature change and the pressure drop.

Prevention of Scaling

The formation of calcium carbonate can be controlled by adding acids or specific chemicals (phosphate compounds, e.g. AMP, or organic polymers such as polyacrylates) tailored to inhibit the precipitation of the compound. However, water treatment is not an easy task, and a water specialist should be consulted in order to determine the correct treatment. Improper use of acids can corrode the BPHE severely in a very short time. Calcium sulfate scaling can be controlled most effectively with chemicals such as polyacrylates or AMP.

Removing the scale that has been formed restores the operating efficiency of the equipment and the heat transfer surfaces. Other benefits from removing the scale are that it lowers the pressure drop, reduces the power consumption and extends the lifetime of the equipment.

Particulate Fouling

Particulate fouling is caused by suspended solids (foulants) such as mud, silt, sand or other particles in the heat transfer medium.

Important factors that affect particulate fouling are:

•   velocity
•   distribution of the flow
•   roughness of the heat transfer surface
•   the size of the particles

Velocity

The velocity is an important factor in the sense that it controls whether the flow is turbulent or laminar. Turbulent flow is desirable for several reasons. Turbulent flow will keep particles in the fluid in suspension, i.e. no particles are allowed to collect on the surface, which will avoid surface fouling. Another very important reason, of course, is that turbulent flow improves the heat transfer.

BPHEs have a high degree of turbulence, and the fluid has a scouring action that keeps the heat transfer surface clean. This is due to the unique design of BPHEs. As the fluid passes through the channels, it constantly changes direction and velocity. This ensures turbulent flow even at very low flow rates and pressure drops.

For shell and tube (S&T) heat exchangers, a much higher velocity is required to achieve turbulent flow. In a BPHE, the water flow will be turbulent at a Reynolds number of 150. S&T heat exchangers need Reynolds numbers above 4000 to achieve turbulent flow.

In an S&T, the water can flow either inside the tubes (common in condensers) or outside the tubes (common in evaporators). When the water passes through a tube, the maximum velocity is at the center of the tube. The turbulence at the walls is too low to keep particles in the fluid in suspension. These particles are allowed to precipitate and collect on the tube wall, which fouls the heat transfer surface. When the water flows outside the tubes, the flow rate is lower and low-flow areas are created, which increases the risk of fouling. This means that S&T heat exchangers are much more sensitive to fouling than are plate heat exchangers. In the design of S&T heat exchangers, the use of so-called fouling factors is recommended to account for the risk of fouling and the consequent decrease in performance. These are not used in BPHE design.

Distribution of the Flow

It is very important for the flow over the heat transfer surface to be well distributed to maintain uniform velocity. The flow distribution depends very much on the plate pattern. A special distribution pattern in the port areas of SWEP BPHEs ensures a well-distributed flow. In other heat exchangers (S&T, coaxial and other brazed heat exchanger brands), there can be areas of low velocity (resulting in laminar flow) due to uneven distribution of the fluid through the exchanger. These sections are sensitive to fouling. The fouling starts at the low velocity areas and spreads over the heat transfer surface.

Roughness of the Heat Transfer Surface

Rough surfaces are known to encourage fouling by collecting particulate matter. The material used in SWEP BPHE is AISI 316 or AISI 304 stainless steel, and the smooth surface of this material minimizes fouling. The round shape of the brazing points ensures that no pockets of stagnant water can be formed.

In applications where a cooling tower or other open system is used, the cooling water will be rich in oxygen. This can cause the corrosion of materials such as the carbon steel used in conventional heat exchangers. This corrosion is usually in the form of iron oxide scale on the carbon steel surface, but loose iron oxide can be deposited elsewhere as well. The stainless steel (AISI 316) used in SWEP BPHEs is not subject to the uniform corrosion that causes fouling problems. However, SWEP BPHEs are not completely immune to corrosion under certain conditions.

The Size of the Particles

Particulate fouling can influence the performance of the heat exchanger in two ways, depending on the particle size. First, if the particles are large (>1 mm) or have a fibrous structure, they may lodge in the inlet of the heat exchanger or clog the channels. The result is an increased pressure drop in the water circuit of the heat exchanger. Second, particles may adhere to the heat transfer surface and build up a layer of low thermal conductivity material. Initially, this leads to reduced heat transfer, and a higher pressure drop in severe cases of fouling.

Prevention of Particulate Fouling

Clean Cooling Water

The best way to avoid particulate fouling is to keep the cooling water clean and thereby prevent particles from entering the heat exchanger. However, in all cooling systems, and especially when using open cooling systems (with cooling towers), there will always be particles present in the cooling water. The correct maintenance of cooling towers will dramatically reduce the risk of fouling, including particulate fouling, scaling and corrosion. The evaporation of water in cooling towers is unavoidable, and they must be re-filled with make-up water. However, it is very important to bleed (discharge) water from the tower, otherwise impurities will accumulate and soon reach dangerous concentrations. This bleed water is called blowdown.

Strainer

A strainer is recommended before the inlet of the heat exchanger. A strainer will prevent large particles (>1 mm) from entering the heat exchanger. The recommended size of strainer for this purpose is 16-20 mesh or a mesh size of 0.5 to 1 mm. If a smaller mesh size is used, this will of course result in better filtration of the water, but the system will also need more frequent cleaning. It also creates an unwanted pressure drop.

Side-Stream Filtration

When the make-up water for the cooling tower contains significant suspended matter, it is advantageous to use side-stream filtration. A filtration unit (several types are available) is connected to the cooling tower basin. Water from the basin is then pumped through the filtration unit and back again. In general, passing a few percent of the re-circulating water through the side-stream filter will reduce the suspended solids by 80-90%.

Adequate Flow Rates

High flow rates will keep particles in suspension and prevent them from depositing on the heat transfer surface.

Chemical Water Treatment

Chemical treatment of water can also be an effective method of controlling suspended particulates. As in the prevention of scaling, polyacrylates disperse foulants (suspended solids) very efficiently. Concentrations of a few milligrams per liter are required in open re-circulating systems.

Biological Growths

Fouling through biological growths (also called biofouling) occurs when living matter grows on the heat transfer surface. In many cases, re-circulating cooling systems are ideal for promoting the growth of micro-organisms. Three types of living organisms are considered here: algae, fungi and bacteria.

Algae are easily detected by their green color. They need oxygen and sunlight to grow, and can therefore exist in cooling towers. In addition to reducing the thermal performance, algae can also have a severe impact on metal corrosion by providing conditions that increase the risk of corrosion.

Fungi are similar to algae but do not require sunlight. They require moisture and air, and exist on nutrients found in water or on the material they are attached to, for example bacteria, algae or wood.

Bacteria can live with or without oxygen. Water and other wet environments with organic content are suitable for the growth of bacteria. Heat exchangers can therefore provide an excellent environment for this type of micro-organism, which will reduce the heat transfer. Bacteria can also initiate pitting corrosion.

Prevention of Biological Growths

Biocides are the most practical and efficient method of controlling the growth of micro-organisms in cooling water systems. Biocides kill or inhibit the growth of the organisms. Although these chemicals inhibit biofouling, they will not remove material already adhering to surfaces. This emphasizes the importance of a clean system from the start.

Fouling Due to Corrosion

In some cases, fouling can be due to corrosion. The added layer of corrosion products on the heat transfer surface will reduce the heat transfer efficiency. The degree of corrosion depends very much on the water quality.

The main fouling risk is due to corrosion products from other parts of the system. These particles will be carried by the water and may adhere to the heat transfer surface or lodge inside the heat exchanger. This type of fouling should be considered as particulate fouling and prevented as for particulate fouling.

Fouling Resistance due to SWEP's BPHE Design

SWEP BPHEs provide good resistance against surface fouling for several reasons:

• The unique design of a SWEP BPHE allows the heat exchanger to operate at extremely low velocities while maintaining a turbulent flow. As the fluid passes through the channel, its direction constantly changes, which disturbs the boundary layer and ensures turbulent flow even at extremely low velocities. A SWEP BPHE is actually much less prone to fouling than other heat exchangers. This is because of its internal geometry (which ensures evenly distributed fluid), the higher turbulence and the hardness and smoothness of the stainless steel in the channel plates. With laminar flow, the velocity of the fluid close to the plate surface is very low, which means that the suspended particles are allowed to settle (cf. Figure 2.40).
  Figure 2.40
  
• The smooth surface of the channel plate material has a positive effect on minimizing fouling. Rough surfaces are known to encourage fouling, because they collect particulate matter by giving it a chance to adhere to the surface.

• The design of SWEP BPHEs ensures that no dead zones (where fouling compounds can settle) are created. In a dead zone, the liquid is stagnant and the suspended material has the chance to settle and accumulate.
• The particles are kept in suspension by the very high turbulence, even at low flow rates, caused by the corrugations in the plates. Turbulent flow and a small hydraulic diameter, such as with SWEP BPHEs, are important to prevent the suspended particles from settling. With laminar flow, the particles have a much higher tendency to settle.

Optimization of Factors that Affect Surfaces under Various Conditions

Some factors that affect the surfaces of a heat exchanger are discussed below:

• Use the highest possible water pressure drop. A high pressure drop implies higher shear stresses, and large shear stresses are always beneficial if there is any scale. The shear stresses work as descalers by constantly imposing forces on the adhered material that pull the particulate material away from the surface. The shear stresses also help keep the particles in suspension (see Figure 2.41).
  Figure 2.41

• For a heat exchanger with a temperature above 70 °C on the hot side and/or very hard water (and hence a danger of scaling), the pressure drop should be increased as much as possible on the cold water side and reduced on the hot side. This reduces the wall temperature on the cooling water side and increases the shear stresses, making it more difficult for the scaling compounds to adhere.
  
  
• Consider the use of co-current instead of counter-current flow. The warmest part of the hot side, the inlet, will then face the coldest part of the cold side. This normally decreases the maximum wall temperature on the cooling water side, which automatically limits the outlet water temperature.
  
  
• The normal practice is to let the cold water enter the lower port. This arrangement should be used whenever possible, because if the cold water enters through the upper port it could encourage debris to enter the channels.

 

Cleaning in Place

Provided the heat exchanger is not completely clogged, it is possible to clean the exchanger by circulating a cleaning liquid (cleaning in place, CIP). Heat exchangers should therefore be cleaned at regular intervals.

If the installation operates under difficult conditions, for example with hard water, installation of a heat exchanger with extra connections on the back for CIP piping is recommended to facilitate maintenance (see Figure 2.42). This makes it possible to connect and circulate the CIP solution through the system without having to disassemble the ordinary installation.Figure 2.42

The choice of cleaning solution depends on the problem, but a weak acid is a good start. This could be 5% phosphoric acid or, if the exchanger is cleaned frequently, 5% oxalic acid. The cleaning liquid should be pumped through the exchanger. For optimal cleaning, the flow rate of the cleaning solution should be at least 1.5 times the normal flow rate. Preferably, the flow should be in a back flush mode, which has a better chance of dissolving the scale because it attacks the deposits from the opposite direction to the usual flow.

After cleaning, the heat exchanger should be rinsed carefully with clean water. A solution of 1-2% sodium hydroxide (NaOH) or sodium bicarbonate (NaHCO₃) before the last rinse ensures that all acid is neutralized. One way to get an indication of the appropriate rinse time is to test the pH of the liquid at the outlet from the heat exchanger. A quick and easy method is to use litmus paper. The pH should be 6-9.

Circulation Systems

The circulation system could be a vertical peristaltic pump. In this type of pump, the liquid is forced forwards by an eccentrically rotating wheel connected to an engine (see Figure 2.43). Important features of a CIP pump are:

• The reservoir for the CIP solution should be manufactured in acid- and alkali-resistant material.
• Hoses should be made of PVC.
• It is an advantage if the pump is provided with a reverse flow device, which makes it possible to attack scale from both directions.
• It is an advantage if the pump is equipped with a heating device, which usually increases the cleaning effect.
• The required flow rate capacity depends on the size of the heat exchanger.

Figure 2.43

Eliminating the Scaling Problem: Principles 

There are several ways to eliminate the scaling problem. Usually, a commercial product containing additives to enhance the effect and/or prevent corrosion can be employed. Do not use any product containing ammonia if the filler material of the BPHE is copper. Take great care when using strong inorganic acids such as hydrochloric, nitric or sulfuric acids, because they are extremely hazardous. Under certain conditions, hydrochloric acid can corrode stainless steel in minutes, and nitric acid corrodes copper.

Chemical cleaning is the use of chemicals to dissolve or loosen deposits from process equipment and piping. Removal is uniform and generally at a lower overall cost. In principle, there are two steps in this process. The last step can sometimes be excluded.

Step 1: Chemical Cleaning Solutions

Mineral acids such as hydrochloric acid (HCl), sulfamic acid (NH₂SO₃H), nitric acid (HNO₃), phosphoric acid (H₃PO₄) and sulfuric acid (H₂SO₄) have a good ability to dissolve scale. However, they can also corrode the stainless steel or copper if used improperly. Organic acids are much weaker than mineral acids, in terms both of their dissolving ability and their ability to corrode the base material of the BPHE. This makes these acids more useful when attempting to remove scale from the BPHE because they are potentially less dangerous. They are often used in combination with other chemicals to bind the scale into complexes. Another advantage of organic acids is that they can be disposed of by incineration. Organic acids include formic acid (HCOOH), acetic acid (CH₃COOH) and citric acid (C₃H₄(OH)(COOH)₃).

Phosphoric acid is sometimes used at 2% concentration and 50 °C for 4-6 h to pickle and passivate steel piping. It is not as effective as HCl in removing iron oxide scale, but is preferred for cleaning stainless steels. Formic acid is generally used as a mixture with citric acid or HCl, because alone it is unable to remove iron oxide deposits. Formic acid can be used on stainless steel. It is relatively inexpensive and can be disposed of by incineration. Acetic acid is used to clean calcium carbonate scale, but it is ineffective in removing iron oxide deposits. Because it is weaker than formic acid, it may be preferred where extremely long contact times are necessary. Inhibitors are specific compounds that are added to cleaning chemicals to diminish their corrosive effect on metals. Finally, surfactants, or detergents, are added to chemical cleaning solutions to improve their wetting characteristics. They are also used to improve the performance of inhibitors, and act as detergents in alkaline and acidic solutions.

Step 2: Passivation

A passive surface is one where the corrosion rate is reduced due to the precipitation of corrosion products on the metal surface. These corrosion products usually consist of oxides that inhibit further corrosion in water or in air. The term passivation is applied to procedures that are used to remove surface iron contamination from stainless steel equipment. To passivate stainless steels, mild iron contamination may be removed using a mixture containing 1% each of citric and nitric acids. For more persistent contamination, strong nitric acid solutions must be used.

Water hammer occurs when the installation pipelines carry incompressible fluids such as water, ethylene glycol, etc., and the fluid flow suddenly changes its velocity. A common cause of water hammer is the rapid closing of a solenoid valve in the liquid line. Abruptly stopping the fluid flow produces a substantial pressure rise. High-intensity pressure waves will travel back and forth in the pipes between the point of closure and a point of relief, such as a larger diameter header, at extremely high speed. As it moves, the shock wave alternately expands and contracts the pipes.

Water hammer is the cause of many problems such as ruptured pipes and damage to valves, BPHEs and other equipment. In a BPHE, water hammer will cause a bulge in the front or back plate, resulting in internal/external leakage (see Figure 2.29).Figure 2.44

To avoid or eliminate these problems, the designer can install an air chamber or a water hammer arrester. Another way to control water hammer is to use valves with controlled closing times or controlled closing characteristics. The graphs in Figure 2.30 illustrate the difference between using standard quick-closing water valves and slow-closing time-controlled water valves.

Figure 2.45