**9. Cleaning of heat exchangers**

A decrease in the performance of a heat exchanger beyond acceptable level requires cleaning. In some applications, the cleaning can be done on line to maintain acceptable performance without interruption of operation. At other times, off-line cleaning must be used.

Garrett-Price et al. [27] presented some cleaning approaches for fouled heat exchangers. They specified on-line cleaning generally utilises a mechanical method designed for only tube side and requires no disassembly. In some applications flow reversal is required. Chemical feed can also be used as an on-line cleaning technique but may upset the rest of the liquid service loop.

On-line mechanical cleaning techniques are also in practice. On line tube side cleaning techniques are the sponge-ball and brush systems. The advantage of on-line cleaning is the continuity of service of the exchanger and the hope that no cleaning-mandated downtime will occur. The principal disadvantage is the added cost of a new heat exchanger installation or the large cost of retrofits. Furthermore there is no assurance that all tubes are being cleaned sufficiently.

Off-line chemical cleaning is a technique that is used very frequently to clean exchangers. Some refineries and chemical plants have their own cleaning facilities for dipping bundles or re-circulating cleaning solutions. In general, this type of cleaning is designed to dissolve the deposit by means of a chemical reaction with the cleaning fluid. The advantages of chemical cleaning approach include the cleaning of difficult-to-reach areas. Often in mechanical cleaning, there is incomplete cleaning due to regions that are difficult to reach with the cleaning tools. There is no mechanical damage to the bundle from chemical cleaning, although there is a possibility of corrosion damage due to a reaction of the tube material with the cleaning fluid. This potential problem may be overcome through proper flushing of the unit. Disadvantages of off-line chemical cleaning include corrosion damage potential, handling of hazardous chemicals, use of a complex procedure.

Off-line mechanical cleaning is a frequently used procedure. The approach is to abrade or scrap away the deposit by some mechanical means. The method includes high-pressure water, steam, lances and water guns. In off-line mechanical cleaning there are some advantages such as excellent cleaning of each tube is possible, good removal potential of very tenacious deposits. Disadvantages include the inability to clean U-tube bundles successfully, usual disassembly problem and the great labour needed.

Parkinson and Price [60] have reported significant reduction in fouling by the magnetic treatment as it helps in precipitating the salts. These salts stay suspended in the bulk liquid and are removed later. On the other hand Hasson and Bramson [61] informed that there is no effect of magnetic treatment at all on fouling. They observed that magnetic treatment neither decreased nor increased the rate of scaling. The nature of the deposits also remained unchanged. Bernadin and Chan [62] have also reported no influence of magnetic treatment on silica fouling. Muller-Steinhagen [37] has stated that magnetic mitigation devices in some cases actually increased fouling. Thus from the available information no conclusion can be

A decrease in the performance of a heat exchanger beyond acceptable level requires cleaning. In some applications, the cleaning can be done on line to maintain acceptable performance without interruption of operation. At other times, off-line cleaning must be

Garrett-Price et al. [27] presented some cleaning approaches for fouled heat exchangers. They specified on-line cleaning generally utilises a mechanical method designed for only tube side and requires no disassembly. In some applications flow reversal is required. Chemical feed can also be used as an on-line cleaning technique but may upset the rest of

On-line mechanical cleaning techniques are also in practice. On line tube side cleaning techniques are the sponge-ball and brush systems. The advantage of on-line cleaning is the continuity of service of the exchanger and the hope that no cleaning-mandated downtime will occur. The principal disadvantage is the added cost of a new heat exchanger installation or the large cost of retrofits. Furthermore there is no assurance that all tubes are being

Off-line chemical cleaning is a technique that is used very frequently to clean exchangers. Some refineries and chemical plants have their own cleaning facilities for dipping bundles or re-circulating cleaning solutions. In general, this type of cleaning is designed to dissolve the deposit by means of a chemical reaction with the cleaning fluid. The advantages of chemical cleaning approach include the cleaning of difficult-to-reach areas. Often in mechanical cleaning, there is incomplete cleaning due to regions that are difficult to reach with the cleaning tools. There is no mechanical damage to the bundle from chemical cleaning, although there is a possibility of corrosion damage due to a reaction of the tube material with the cleaning fluid. This potential problem may be overcome through proper flushing of the unit. Disadvantages of off-line chemical cleaning include corrosion damage

Off-line mechanical cleaning is a frequently used procedure. The approach is to abrade or scrap away the deposit by some mechanical means. The method includes high-pressure water, steam, lances and water guns. In off-line mechanical cleaning there are some advantages such as excellent cleaning of each tube is possible, good removal potential of very tenacious deposits. Disadvantages include the inability to clean U-tube bundles

potential, handling of hazardous chemicals, use of a complex procedure.

successfully, usual disassembly problem and the great labour needed.

made about the influence of the magnetic field on the scaling process.

**9. Cleaning of heat exchangers** 

used.

the liquid service loop.

cleaned sufficiently.

Frenier and Steven [63] describe general methods for cleaning heat exchanger equipment, including both mechanical and chemical procedures. They have given guidelines for selecting between chemical and mechanical cleaning, and among the various types of chemical cleaning processes. They stated that water-based fluids can transport and deposit a wide variety of minerals, and corrosion products form due to the reaction of the aqueous fluids with the metals of construction. Hydrocarbon and petrochemical fluids transport and deposit a variety of organic scales. Common inorganic scale forming compound includes various iron oxides, hardness deposits (carbonates and silicates). They stated that the entire cleaning situation must be considered when choosing between mechanical and chemical cleaning, as well as the specific technique within the general category. The general categories of mechanical cleaning are abrasive, abrasive hydraulic, hydraulic and thermal [64].

Frenier and Barber [63] stated that, for chemical cleaning of the heat exchanger tubes, it is very beneficial to obtain a sample of the deposit so that its composition can be determined. Based on the chemical analysis of the deposit, an optimal treatment plan can be developed and the best solvent selected. They have classified the deposits generically, as organic (process-side) or inorganic (water-side).

They stated that the process side deposits may range from light hydrocarbon to polymers and generally they are similar to the fluids from which they originate. They mentioned that the general categories of solvents for process side scales include aqueous detergent solutions, true organic solvents and emulsions. Aqueous detergent formulations always contain a surfactant-type component. In addition they can contain alkaline agents, such as sodium hydroxide, sodium silicate, or sodium phosphate. Builder molecules such as ethylenediaminetetraacetic acid (EDTA) suppress the effects of hard water, and coupling agents such as glycol ethers, improve the dissolution of some organic deposits.

Detergent formulations are effective only for removing the lighter deposits. Refinery fluids, aeromatics and terpenes are used to dissolve the organic deposits. N-methyl-2 pyrrolidinone also is a very effective polar solvent with low toxicity characteristics. They reiterated that the effectiveness of the application depends greatly on proper application conditions, such as flow rate and temperature. Combination of surfactants, organic solvents and water emulsions are good cleaning agents. Emulsions with an organic outer phase are particularly useful for cleaning large vessels. Oily rust deposits having both organic and inorganic compositions can be removed by acidic emulsions combining an acid and an organic solvent.

Water-side deposits usually contain minerals, such as iron oxides (corrosion products), hardness (Ca and Mg carbonates) and silica, in individual cases other minerals can also be found. The solvents for removing inorganic deposits usually contain mineral acids, organic acids or chelating agents.

Mineral acids used in chemical cleaning include hydrochloric acid (HCl), hydrofluoric acid (HF), sulphuric acid (H2SO4), phosphoric acid (H3PO4), nitric acid (HNO3) and sulfamic acid (H2NSO3H). Hydrochloric acid is the most common and most versatile mineral acid. It is used on virtually all types of industrial process equipment at strengths from 5 percent to 28 percent (5-10 percent is the most usual range). It can be inhibited at temperatures up to about 180 °F. HCl will dissolve carbonates, phosphates, most sulphates, ferrous sulphide,

Fouling and Fouling Mitigation on Heat Exchanger Surfaces 527

Mass flux kg/m2s

Increase of solids mass present in the fouling film kg/m2s

 Decrease of solids in the fouling film kg/m2s m Solids deposited in the fouling film per unit area kg/m2 P Pressure kPa *P* Perimeter m Pc Intercrystalline cohesive force N P Pressure drop kPa/m

Heat flow W

 Heat flux W/m2 R Ratio of the radius of inner and outer tubes of annulus - Rb Bonding resistance - Rg Universal gas constant J/mol.K Rf Fouling resistance m2K/kW

*Rf* Asymptotic value of the fouling resistance m2K/kW r Radius m *rH* Hydraulic radius m T Temperature oC Temperature at the surface of the fouling film oC T Temperature difference K or oC t Time s tind Induction time s U Overall heat transfer coefficient W/m2K u Velocity m/s *u* Local mean velocity m/s u\* Friction velocity, (w/) m/s u+ Dimensionless velocity, *u* /u\* ut Turbulent friction or shear velocity, *u f* / 2 m/s w Constant weight flow of fluid kg/s x Distance in x direction m *<sup>f</sup> x* Fouling film thickness m y Distance in y direction m

 Constant - Individual mass transfer coefficient m/s Time constant s Height of roughness m

*m* 

*md* 

*mr* 

*Q* 

*q* 

\*

**Greek** 

iron oxides and copper oxides. By using with appropriate additives, fluoride deposits, copper and silica can also be removed from surfaces with inhibited HCl. HCl is corrosive, so it has restricted use. HCl is not used to clean series 300 SS, free-machining alloys, magnesium, zinc, aluminium, cadmium, or galvanised steel because of the potential for generalised or localised attack. It is not desirable to contact the fouled metal with a strong mineral acid, because of the danger of damage to the equipment during or after cleaning. An alternative solvent family consists of aqueous solutions of chelating agents and organic acids with pH values of about 2 to 12.

Citric acid was one of the first organic acids used in industrial cleaning operations [65]. For removing iron oxide from steel surfaces, citric acid and a mixture of formic and citric acid could be used [66]. The mixture could hold more iron in solution than either of the acids alone could do. Ammonium citrate and sodium citrate solvents are currently used to clean a wide variety of heat transfer equipment, including boilers and various types of service water systems. The advantage of citric acid formulation is their low toxicity and ready biodegradability. EDTA is a versatile chemical that forms metal ion complexes with higher equilibrium constants than citric acid. As a result chemical cleaning solvents with pH values from 4.5 to about 9.2 have been formulated that can remove Fe and Cu, as well as Ca, Ni and Cr. The major advantage of the EDTA solvents is that they are much more aggressive than citric salts for removing very heavy iron oxide deposits especially if they contain copper. The disadvantage includes high cost per pound of metal removed and low biodegradability.

All of the chelating agents are also organic acids. Eberhard and Rosene [67] taught the use of solvents consisting of formic acid or citric acid for cleaning nondrainable tubes in super heaters. Reich [66] used a mixture of formic acid and citric acid to a proportion of 3:1, to remove iron oxide deposits. The advantage of these mixtures is that they avoid the precipitation of solids that formed in pure formic or citric acid solutions. Formulations of formic acid with hydroxyacetic acid and citric acid with hydroxyacetic acid can be used as a cleaning agent. Bipan [3] used acetic acid of concentration 3 percent to remove CaSO4.2H2O deposit on plate type SS heat exchangers. He said that with the increase in acid solution temperature the removal efficiency increases. Similar results were obtained by Kazi [68]. It reveals that a complete and systematic study of fouling on different metal surfaces and their mitigation by additives have been required to be done along with study of introducing a benign to environment technique for chemical cleaning of fouling deposits.

## **10. Nomenclature**


iron oxides and copper oxides. By using with appropriate additives, fluoride deposits, copper and silica can also be removed from surfaces with inhibited HCl. HCl is corrosive, so it has restricted use. HCl is not used to clean series 300 SS, free-machining alloys, magnesium, zinc, aluminium, cadmium, or galvanised steel because of the potential for generalised or localised attack. It is not desirable to contact the fouled metal with a strong mineral acid, because of the danger of damage to the equipment during or after cleaning. An alternative solvent family consists of aqueous solutions of chelating agents and organic

Citric acid was one of the first organic acids used in industrial cleaning operations [65]. For removing iron oxide from steel surfaces, citric acid and a mixture of formic and citric acid could be used [66]. The mixture could hold more iron in solution than either of the acids alone could do. Ammonium citrate and sodium citrate solvents are currently used to clean a wide variety of heat transfer equipment, including boilers and various types of service water systems. The advantage of citric acid formulation is their low toxicity and ready biodegradability. EDTA is a versatile chemical that forms metal ion complexes with higher equilibrium constants than citric acid. As a result chemical cleaning solvents with pH values from 4.5 to about 9.2 have been formulated that can remove Fe and Cu, as well as Ca, Ni and Cr. The major advantage of the EDTA solvents is that they are much more aggressive than citric salts for removing very heavy iron oxide deposits especially if they contain copper. The disadvantage includes high cost per pound of metal removed and low biodegradability. All of the chelating agents are also organic acids. Eberhard and Rosene [67] taught the use of solvents consisting of formic acid or citric acid for cleaning nondrainable tubes in super heaters. Reich [66] used a mixture of formic acid and citric acid to a proportion of 3:1, to remove iron oxide deposits. The advantage of these mixtures is that they avoid the precipitation of solids that formed in pure formic or citric acid solutions. Formulations of formic acid with hydroxyacetic acid and citric acid with hydroxyacetic acid can be used as a cleaning agent. Bipan [3] used acetic acid of concentration 3 percent to remove CaSO4.2H2O deposit on plate type SS heat exchangers. He said that with the increase in acid solution temperature the removal efficiency increases. Similar results were obtained by Kazi [68]. It reveals that a complete and systematic study of fouling on different metal surfaces and their mitigation by additives have been required to be done along with study of introducing a

benign to environment technique for chemical cleaning of fouling deposits.

A Heat transfer area m2 A0 Arrhenius constant m3/kg.s a1-a13 Proportionality constant c Concentration g/L or kg/m3 cp Specific heat capacity J/molK d Pipe diameter m E Activation energy J/mol H Head loss m H2O hc Heat transfer coefficient W/m2K KR Reaction rate constant (dimension depend on the order of n) m4/kg.s L Length m

acids with pH values of about 2 to 12.

**10. Nomenclature** 


Fouling and Fouling Mitigation on Heat Exchanger Surfaces 529

[5] Bohnet, M., Fouling of Heat Transfer Surfaces. Chemical Engineering Technology, 1987.

[6] Ritter, R. B., Crystallisation Fouling Studies. Journal of Heat Transfer, 1983. 105: p. 374-

[7] Reitzer, B. J., Rate of Scale Formation in Tubular Heat Exchangers. I & EC Process Design

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[9] Bott, T. R. and Walker, R. A., Fouling in Heat Transfer Equipment. The Chemical

[10] Middis, J., Heat Transfer and Pressure Drop For Flowing Wood Pulp Fibre

[11] Watkinson, A. P. and Martinez, O., Scaling of Heat Exchanger Tubes by Calcium

[12] Augustin, W., Verkrustung (Fouling) Von Warmeubertragungsflachen, in Institut fur

[13] Cooper, A., Suitor, J. W. and Usher, J. D., Cooling Water Fouling in Plate Heat

[14] Muller-Steinhagen, H. M., Reif, F., Epstein, N. and Watkinson, A. P., Influence of

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[17] Epstein, N., Heat Exchanger Theory and Practice, in: J. Taborek, G. Hewitt (eds.) heat exchangers in Heat Exchanger Theory and Practice, McGraw-Hill, 1983. [18] Epstein, N., Thinking about Heat transfer fouling: a 5 x 5 matrix. heat Transfer

[19] Epstein, N. (1981) Fouling in heat exchangers. In Low Reynolds Number Flow Heat

[20] Somerscales, E. F. C., and Knudsen, J. G. (eds.) (1981) Fouling of Heat Transfer

[21] Melo, L. F., Bott, T. R., and Bernardo, C. A. (eds.) (1988) Fouling Science and

[22] Standards of the Tubular Exchanger Manufacturers Association 7th ed. Tubular

[23] Chenoweth, J. M. (1988), General design of heat exchangers for fouling conditions. In

[24] Brusilovsky, M., Borden, J. and Hasson, D., Flux Decline due to Gypsum Precipitation

[25] Walker, G., Degradation of Performance, Industrial Heat Exchangers- A Basic Guide.

Fouling Science and Technology, L. F. Melo, T. R. Bott, and C. A. Bernardo (eds.),

Exchanger Manufacturers Association, New York, 1988.

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University of Auckland: Auckland, New Zealand.

Carbonate. Journal of Heat Transfer, 1975: p. 504-508.

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Technology. Kluwer, Dordrecht.

pp. 477-494. Kluwer, Dordrecht.

Engineer, 1971: p. 391-395.

378.

Germany.

Tube, Inc.


#### **Dimensionless Numbers**


### **11. References**


/d Roughness ratio - Fanning friction factor - Thermal conductivity W/mK Thermal conductivity of the deposits W/mK Dynamic viscosity kg/ms Density kg/m3 Density of the deposits kg/m3 Shear stress N/m2 w Wall shear stress N/m2 Shear stress exerted by the liquid flow on the fouling film N/m2 Kinematic viscosity m2/s Friction factor - Hydrodynamic boundary layer thickness m c Linear thermal expansion coefficient of the fouling film porosity 1/K t Thermal boundary layer thickness m Ratio of thermal to hydrodynamic boundary layers m

**Dimensionless Numbers** 

Nusselt Number *<sup>c</sup> h d Nu*

Prandtl Number Pr *<sup>p</sup> <sup>c</sup>*

Reynolds Number Re

New Zealand.

**11. References** 

[1] Bott, T. R. and Gudmundsson, J. S., Rippled Silica deposits in Heat Exchanger Tubes. 6th

[2] Crittenden, B. D. and Khater, E. M. H., Fouling From Vaporising Kerosine. Journal of

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[4] Muller-Steinhagen, H. M. and Middis, J., Particulate Fouling in Plate Heat Exchangers.

Chemical and Materials Engineering, 1994, The University of Auckland: Auckland,

 *u d* 

International Heat Transfer Conference. 1978.

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Heat Transfer, 1987. 109: p. 583-589.


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[48] Gainey, R. J., Thorp, C. A. and Cadwallader, Calcium Sulphate Seeding Prevents

[49] Rautenbach, R. and Habbe, R., Seeding Technique for Zero-Discharge Processes,

[50] Telkes, M., Nucleation of Supersaturated Inorganic Salt Solutions. Industrial and

[51] Pritchard, A. M., Cleaning of Fouled Surfaces: A Discussion, in Fouling Science and

[52] Keil, R. H., Enhancement of Heat Transfer by Flow Pulsation. Industrial Engineering Chemistry: Process Design and Development, 1971. 10(4): p. 473-478. [53] Ludlow, J. C., Kirwan, D. J. and Gainer, J. L., Heat Transfer with Pulsating Flow.

[54] Karamercan, O. E. and Gainer, J. L., The Effect of Pulsations on Heat Transfer. Industrial Engineering Chemistry Fundamentals, 1979. 18(1): p. 11-15. [55] Herndon, R. C., Hubble, P. E. and Gainer, J. L., Two Pulsators for Increasing Heat

[56] Edwards, M. F. and Wilkinson, W. L., Review of Potential Applications on Pulsating

[57] Gupta, S. K., Patel, R. D. and Ackerberg, R. C., Wall Heat /Mass Transfer in Pulsatile

[58] Thomas, L. C., Adaptation of the surface Renewal Approach to Momentum and Heat Transfer for Turbulent Pulsatile Flow. Journal of Heat Transfer, 1974: p. 348-353. [59] Kazi, S. N., Heat Transfer to Fibre Suspensions-Studies in Fibre Characterisation and

[60] Parkinson, G. and Price, W., Getting the Most out of Cooling Water. Chemical

[61] Hasson, D. and Bramson, D., Effectiveness of Magnetic Water Treatment in

[62] Bernardin, J. D. and Chan, S. H., Magnetics Effects on Similated Brine Properties

[64] Gutzeit, J., Cleaning of Process Equipment and Piping. 1997, MTI Publication,

[65] Loucks, C. M., Organic Acids for Cleaning Power- Plant Equipment. Annual Meeting

Chemistry: Process Design and Development, 1985. 24: p. 588-592.

Flow. Chemical Engineering Science, 1982. 37(12): p. 1727-1739.

University of Auckland: Auckland, New Zealand.

Chemical Engineering Progress, 1998: p. 37-44.

Materials Technology Institute, St. Louis.

Adaption to Electrodialysis. Desalination, 1991. 84: p. 153-161.

Chemical Engineering Communications, 1980. 7: p. 211-218.

Calcium sulphate Scaling. Industrial and Engineering Chemistry, 1963. 55(3): p. 39-

Technology, NATO ASI Series, Series E: Applied Science, Melo, L. F., Bott, T. R.

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Fouling Mitigation, PhD thesis, Chemical and Materials Engineering. 2001, The

Suppressing Calcium Carbonate Scale Deposition. Industrial Engineering

Pertaining to Magnetic Water Treatment. Fouling and Enhancement Interactions, 28th National Heat Transfer Conference. 1991. Minneapolis, Minnesota, USA. [63] Frenier, W. W. and Barber, J. S., Choose the Best Heat Exchanger Cleaning Method.

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43.

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of ASME. 1958. New York.


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[27] Garrett- Price, B. A., Smith, S. A., Watts, R. L., Knudsen, J. G., Marner, W. J. and Suitor,

[29] Rankin, B. H. and Adamson, W. L., Scale Formation as Related to Evaporator Surface

[30] Chandler, J. L., The Effect of Supersaturation and Flow Conditions on the Initiation of

[31] Marriott, J., Where and How to Use Plate Heat Exchangers. Chemical Engineering,

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[36] Freeman, W. B., Middis, J. and Muller-Steinhagen, H. M., Influence of Augmented

[37] Muller-Steinhagen, H. M., Fouling: The Ultimate Challenge for Heat Exchanger

[38] Muller-Steinhagen, H. M., Introduction to Heat Exchanger Fouling. Proceedings of

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Scale Formation. Transactions of Institution of Chemical Engineers, 1964. 42: p.


**20** 

*GEA PHE Systems* 

*Germany* 

**Fouling in Plate Heat Exchangers:** 

Due to their compact size, Plate Heat Exchangers (PHEs) are widely used in industrial processes. They have higher heat-transfer performance, lower temperature gradient, higher turbulence, and easier maintenance in comparison with shell and tube heat exchangers. For minimizing material consumption and space requirements compact models have been developed over the last years. By using thin plates forming a small gap, these compact models impress with larger heat transfer coefficients and, thus, smaller required heat

In Figure 1, plate heat exchangers are compared with shell and tube heat exchangers

Deposits create an insulating layer over the surface of the heat exchanger that decreases the heat transfer between fluids and increases the pressure drop. The pressure drop increases as a result of the narrowing of the flow area, which increases the gap velocity (Wang et al., 2009). Therefore, the thermal performance of the heat exchanger decreases with time, resulting in an undersized heat exchanger and causing the process efficiency to be reduced. Heat exchangers are often oversized by 70 to 80%, of which 30 to 50% is assigned to fouling. While the addition of excess surface to the heat exchanger may extend the operation time of the unit, it can cause fouling as a result of the over-performance caused by excess heat transfer area; because the process stream temperature change greater than desired, requiring that the flow rate of the utility stream be reduced (Müller-Steinhagen, 1999). The deposits

The advantages of compact heat exchangers over shell and tube ones at a glance:

 lower fouling due to high fluid turbulences (self-cleaning effect) significantly smaller required installation and maintenance space

**1. Introduction** 

transfer area.

lighter weight

 lower investment costs closer temperature approach

larger heat transfer coefficients

smaller heat transfer surfaces required

simplified cleanability especially for GPHE

regarding effectiveness, space, weight and cleaning time.

pure counter-flow operation for GPHE

**Some Practical Experience** 

Ali Bani Kananeh and Julian Peschel

