Marketing Bulletin. Heat Exchanger

Transcription

1 Marketing Bulletin Heat Exchanger

2 Fig. 1: Thermal resistance through the wall of a plate heat exchanger. Inlet media 1 Outlet media 1 Flow T Across turbulent region T Across surface film T Across product deposits T Across heat exchanger surface T Across product deposits T Across surface film T Across turbulent region Turbulent region Inlet media 2 Outlet media 2 Flow direction Flow Fig. 2: Typical plate heat exchanger regeneration system. Note the achieve ment of a 70ºC temperature rise in the incoming fluid with a direct energy input from the heating section of only 7ºC. Cold outgoing flow 4ºC Cooling media in Cooling media out Heating media out 11ºC 74ºC Heating media in 4ºC 67ºC 74ºC Cold incoming flow 4ºC T = 7ºC Cooling section T = 63ºC T = 7ºC Regeneration section Heating section Fig. 3: Schematics of laminar and turbulent flow regimes in a tube. Velocity maximum = 1.2 x average velocity Velocity maximum = 2.0 x average velocity 2

3 All living things depend on energy for their survival. The potential ener gy of large masses of water power turbines which generate electrical energy; combustion of petroleum fuels is used to heat and expand air which drives the internal combustion engine and the nucleus of an atom is split to re lease energy used to heat steam and generate electricity. Man continues to thrive as he finds new ways to harness nature s store of energy. A fundamental link in this conver sion process is the Heat Exchanger. In this Marketing Bulletin we shall address some funda mentals of heat transfer, the range of APV heat exchangers available and show some examples from the vast range of industries which have applied APV heat exchange technology. The fundamentals Elements of heat transfer Conduction. The rate of heat transfer from one fluid to another is a function of the temperature diffe rence and the thermal resistance in the heat transfer path. The rate of heat transfer through a substance (or combination of substances) is proportional to the temperature difference, the thermal conduc tivity of the heat transfer path and inversely proportional to its thickness. If the heat transfer wall thick ness is minimised, as for example in a plate heat exchanger, or the build-up of insulating deposits on the heat exchanger wall, as for example in a scraped surface heat exchanger, then we can expect a higher rate of heat transfer and so a smaller re quired heat transfer area for a given duty. Convection. The term convection heat transfer refers to the heat exchanged between a surface and a fluid moving over the surface. The rate of heat transferred is de pen dent on the following factors: the geometry of the surface; the nature of the fluid; the ve locity of the fluid close to the surface, and the temperature difference. This phenomenon of convection is often simplified by as suming that a barrier film of fluid is in immediate contact with the surface and that heat is transferred through this surface film by conduction. The rate of heat transfer from convection can be influenced by designing flow channels that induce higher turbulence in the fluid flows and thereby higher local ve locities close to the heat transfer surface, as for example in plate heat exchangers and corrugated tube heat exchangers (see Fig. 1 above). Regeneration In order to optimise the efficiency of a process in which two flows are simultaneously heated and cooled, heat exchange systems can be de signed to recover the heat from the heated fluid and use the thermal energy to preheat the incoming cold flow. A typical flow arrange ment is shown in Fig. 2. Turbulent vs laminar flow Flow through heat exchangers is generally considered to take place in one of the following two flow regimes or in a state of transition between the two. Laminar flow, where the fluid may be visualised as flowing in ordered layers. This streamlined flow in a tube leads to a low mean velocity of 50% of the maximum velocity. Turbulent flow, on the other hand, is best explain ed by reference to its name. In this type of flow, the mean velocity rises to 82% of the maximum. As the rate of heat transfer is related to fluid velocity, the rate of heat transfer from a fluid in a state of turbulent flow will generally be much higher than that obtained from the same fluid in a state of laminar flow (see Fig. 3). The factors which determine the type of flow achieved are: the diameter of the flow path system (D); the fluid velocity (V), the density of the fluid (r), and the viscosity of the fluid (μ). These factors come together to form the so-called Reynolds number. Re = DrV μ For tubular flow, the fluid is said to be in a turbulent state if the Reynolds number is greater than a figure of 2500, where as in a plate heat exchanger this figure can be as low as Heat exchange systems are design ed with narrow heat exchange channels so that the hydraulic diameter will be decreased and so give more efficient heat exchange. Examples are: smaller bore tubing in tubular heat exchangers or thin plate gaps in plate heat ex changers. 3

4 Fig. 5: A pre-war plate heat exchanger for pasteurisation of apple wort. Fig. 4: Flow configurations in tubular heat exchangers. T1 t1 T1 t1 t1 = (T1 - t1) t2 = (T2 - t2) T2 t2 T2 t2 Flow configurations In a heat exchanger, the product flow and the heating / cooling m e di u m can either flow in the same direction, socalled concurrent flow, or in opposite directions, known as counter-current flow. These two flow arrangements when applied to a Tubular Heat Exchanger can be seen in Fig. 4. temperature difference between the two fluids. Refer to Fig. 4 for the derivation of log mean tempe rature difference. Heat transfer The overall steady state heat trans fer between two fluids can be expressed as: Length Temperature difference Q = U A LMTD T1 t1 T1 t1 Counter current flow t1 - (LMTD) = t2 ( loge ( t1 / t2 ) ) Derivation of (LMTD), logarithmic mean temperature difference t1 = (T1 - t1) Length Concurrent flow t2 = (T2 - t2) T2 t2 T2 t2 The driving force for the exchange of heat between two fluids in a heat exchanger is their temperature difference. As can be seen from Fig. 4, this temperature difference may vary from one end of the heat exchanger to the other. With concurrent flow, both the hot and cold fluid flows start from the same end of the heat ex changer. For a given duty, this leads to a higher entry temperature difference between the two fluids. In counter-current flow, the exit temperature of one fluid will ap proach the entry temperature of the other. This flow configuration allows a more constant temperature difference between the two fluid flows throughout the length of the heat transfer path. With non-condensable fluids, this flow con figuration will provide more efficient heat exchange. As the temperature difference between product and medium varies over the heat exchange surface, we generally use the Log Mean Temperature Difference (LMTD) in order to arrive at an effective overall Where: Q = Rate of heat flow (W, Watts) U = Overall coefficient of heat transfer (W / m 2 / K). A = Heat transfer area (m 2 ) LMTD = Log mean temperature difference (K, degrees Kelvin) It is by the manipulation of these three variables that it is possible to match the right size and configuration of a particular heat exchanger to the desired application and fluid properties. By increasing the heat exchange area (A), the log mean temperature difference (LMTD) or the heat transfer coefficient (U), we can increase the rate of heat transfer in a designated process. 4

5 Fig. 6: Exploded view of a plate heat exchanger. THE PRODUCTS Plate heat exchangers Since the first commercially successful design was introduced by APV s founder Dr Richard Seligman in 1923, the plate heat exchanger has become one of the most versatile and cost-effective methods for the indirect heating and cooling of fluids. The basic concept is to run fluids on either side of a thin metal plate so that heat may be trans ferred between the two fluids. Construction The plate heat exchanger consists of a number of thin metal plates, supported between top (1) and bottom (2) bars and held together in a frame. Each plate is separated by a gasket. At one end of the Key: Welded plates Brazed plates Evaporator plates Gasketed plates machine is a fixed head (3)and at the other a move able follower (4), the whole plate pack being then compressed together by tie bars (5). Fluids can be admitted to the machine through ports (nozzles) (6), which can be located at the head, the follower or at intermediate grids (see Fig. 6). Plates APV heat exchanger plates come in over 34 different corrugation patterns and 60 different sizes, so that the heat exchanger can be matched to a particular duty. The largest frame can accommodate up to 1000 plates, giving a heat ex change area of more than 3500 m 2. Heat exchanger plates are typically mm thick. They are said to be thermally long (or hard) if they have corrugation patterns that run transverse or diagonal to the flow path. This will lead to a more turbulent flow path, im prov ed thermal efficiency, heat transfer and higher pressure drop. Plates are said to be thermally short (or soft) if they have corrugation patterns that run with the flow. This will lead to less efficient heat transfer but a lower fluid flow pressure drop over the plate. Plate pack configurations can be either long or short or a mixture of the two in the same plate pack. Examp les of APV plates can be seen in Fig. 7. Plate configuration Plates are arranged throughout the plate pack in passages and passes. The number of passages refer to the number of individual channels through which the fluids can flow, and these passages can then be arranged in a number of passes through the plate pack. More passages will generally lead to a lower pressure drop per pass and a larger heat transfer area. However, lower velocity of fluid over each plate can lead to a higher risk of fouling and so less efficient heat transfer. More passes will generally lead to a higher pressure drop over the plate pack, but a higher flow rate over individual plates. This in turn can lead to higher velocities in the individual passages and so a higher thermal efficiency. O12 PS007 T4 FFPE VO OA Fig. 7: A selection of APV plates. 5

6 Fig. 9: The new DuoSafety plate system. 6 Fig. 8: APV double seal of the plate heat exchanger port. Gaskets Gaskets are either glued on the plates or come as EasyClip gaskets where the gaskets are fixed on the plates with special sprues (lugs), the so called clip-on type. EasyClip gas kets allow for easier mainte nance and replacement of the gasket in comparison with the glue-on type, which must be bond ed to the plate with adhesive. The gasket provides separation between the two fluid flows and together with the port hole in the plates directs the flow through the plate pack. Gaskets are designed to 3A, hygienic standards, all with a special double seal around the plate port and a drain channel to atmosphere between the double seals. As can be seen in Fig. 8, this sealing system keeps the product and Fig. 11: APV ParaBrazed plate heat exchangers. Fig. 10: APV Paraweld laser welded plate pairs. heating / cooling medium from mixing should the port area of the gasket be damaged. Some advantages of the plate heat exchanger system are: All surfaces easily dismantled for cleaning Compact floor space for a given duty in comparison with tubular design High heat transfer coefficient. Easily expandable to suit new duties. Duo-safety plates Duo-Safety plates are specially designed to protect product from medium as a consequence of through-plate leaks from cracks or corrosion. They have a pair of special non-welded plates back to back, with the leak path between them open to atmosphere. These paired plates then have product and service medium flow on either side (see Fig. 9). Some advantages of the Duo-Safety plates are: Certainty that product and service medium are not mixed All surfaces easily dismantled for cleaning Different plate materials may be combined on product / medium surfaces No welded joints. Hybrid The Fully Welded Heat Exchanger the Hybrid combines the efficiency of the ParaFlow and a strong vessel construction. The Hybrid is available in all weldable and pressable materials. Compared to the traditional Shell & Tube Heat Exchanger the Hybrid is very compact and flexible in form and size. The Hybrid is suitable for applications like Con den sing, Evaporation, Liquid/Liquid, Gas/Gas and Liquid/ Gas. ParaWelded plate technology If a plate heat exchanger is to be used for duties involving an aggressive medium which would otherwise attack plate gaskets over time, or for evaporation or condensing du ties for ammonia or freon refrige rants, then the plates are manu factured in pairs. These pairs of plates are laserwelded together with a double bead of weld on the aggressive medium side in stead of being gasketed (see Fig. 10). Brazed plate heat exchangers Brazed plate heat exchangers have no gaskets, tie bars, top or bottom bars. The individual plates are permanently brazed together in a vac uum furnace. This gives a very compact and durable heat ex changer without gaskets. Typical applications have been in the area of district heating where the low cost and minimal maintenance have made this type of heat exchanger specially attractive (see Fig. 11).

7 ParaShell heat exchangers The ParaShell heat exchanger is con structed with the plates in a stack surrounded by an outer shell which encloses the plate pack. The product plates are welded together in pairs with the service medium then circulated through the outer case and between the plate pairs. The end cap can be fully-welded or manufactured so that it can be opened for inspection. The service medium is routed through indi vi dual passes via a baffle plate similar to the shell in the tubular type of heat exchanger. Compared with the normal plateand-frame heat exchanger, the ParaShell type has the advan tage of a higher working pressure limit (up to 40 bar) and working tempera tures up to 300ºC, and yet it has the compactness of the plate system. Typical applications have been water / refrigerants cooling and water / water heating and cooling (see Fig. 12). Fig. 12: The APV ParaShell consist of a fully welded plate pack, contained in a shell. The shell is available in a fully welded and an openable version, with the possibility for several parallel connec tions on shell side. Customized pass arrangements for e.g. asymetric flow is possible. Fig. 14: Cross-section of the SSHE barrrel showing blades and dasher orientation. Scraped surface heat exchangers The scraped surface heat exchanger (SSHE) is a specialised heat ex - chang er. Initially developed as a more effi cient freezer for making ice cream, it incorporates a scraping action to prevent a build-up of product on the heat transfer surface. It is now utilised primarily for high particulate, high viscosity heating / cooling duties or crys tal ising cooling duties such as cooling of peanut butter, ice cream or margarine. Product out Dasher Scraber Media in Construction Product in Cylinder Heat exhange cylinder Media Jacket Media out The scraped surface heat exchanger consists of a vertical or horizontal jacketed cylinder fitted with a rotating shaft (dasher) on which scraper blades are mounted. Product is pumped through the cylinder while Insulation Annular space (product area) Dasher Scraber blades heating or cooling medium is circulated in the annular space between the cylinder and the jacket. Bearings hold the dasher at either end with either a packing gland or a mechanical seal between the pro duct and atmosphere (see Fig. 14). Thinner wall Higher conductive wall Eg, chrome-plated nickel Higher heat transfer Standard wall Lower conductance Eg, 316 L S/S Lower heat transfer Larger dasher Decreased residence time Decreased product Increased product shear diameter particle size Higher dasher speed* Increased agitation Increased horsepower Increased heat transfer More blades** Increased wall scraping Increased horsepower Increased heat transfer Stagger blades Greater mixing Same horsepower Increased heat transfer *Overly high dasher speed will not always lead to greater heat transfer because a carry-around effect may occur. This means that the product does not get exposure time on the cylinder walls and tends to get carried around with the dasher. **An excessive number of scraper blades will not necessarily lead to greater heat exchange, as the product may be carried through the heat exchanger with little exposure to the cylinder wall. Table 1: General operational trends for scraped surface heat exchangers. 7

8 Operation The blades are fixed to pins so that as the dasher is rotated, centrifugal force will hold the blades on to the inside wall of the cylinder. The blades do not pump the product through the cylinder but constantly scrape the product off the wall, thus allowing new product to be exposed to the thermal treatment. For maintenance and inspection of the horizontal units, the dasher can a) c) be withdrawn through the rear door on a plastic cradle, in the case of the vertical units the cylinder can be withdrawn over the top of the dasher (see Figs. 14 and 15). Some advantages of scraped surface heat exchangers are: Able to heat / cool fluids with particulates up to 32 mm Able to heat / cool high viscosity fluids Easy to open for inspection. Fig. 15: The three SSHE models. a) Scraped surface heat ex changer, type VT. b) Scraped surface heat exchanger, type HD/HEXHD. c) HEX RT 672 with optional water cooled dasher. b) Tubular heat exchangers Tubular heat exchangers can be supplied with either straight tubes or corrugated tubes. They all follow a similar construction format with a single tube or number of smaller tubes enclosed within an outer shell. They are available in the four configurations shown in Figs and can be manufac tured as a fullywelded design, with removable end caps or with built-in thermal expansion protec tion of O-rings or pack ing between the shell and tube bundle. Some advantages of tubular heat exchangers are: Working Temperatures up to: 285ºC Design Pressures to: 60 Bar Flexibility in materials of con - struc tion from carbon steel to special alloys High particulate capacities Can handle high viscosity products Resistant to fouling Robust Easy maintenance. Coiled tube aseptic systems The coiled tube aseptic (CTA) heat exchanger is of compact design with an inner coiled tube contained within a baffled outer shell. It can withstand working pressures in excess of 300 bar with product velocities of up to 6 m / sec. It is manufactured in both horizontal and vertical configurations for use with low pressure steam or other heating / cooling media. Cylinder material Working properties Blade material Working properties Heat transfer General Hardness General corrosion corrosion resistance resistance Chrome-plated nickel High Low Stainless steel High Low 316 L S/S (standard wall) Low Medium Plastic Low High 316 L S/S (thin wall) High Low Inconel 625 Lowest Highest Duplex S/S Medium High Table 2: Standard cylinder and blade materials used in scraped surface heat exchangers. Note: Scraper blades must always be manufactured of less hard material than the mating cylinder so as to wear at a faster rate. 8

9 Concentrate heater For applications where it is a re quirement that products to be heated are maintained under high pressure, a concentrate heater may be utilised. In this heat exchanger, the product is pumped through a spiralled tube inside an outer vessel. Vacuum steam is admitted to the outer vessel, so that a relatively small temperature difference between product and heating media is realised (see Fig. 21 overleaf). Concentrate heaters are available with product pressure ratings up to 600 bar, and they are typically used prior to spray dryer atomisers enab ling heating, pasteurising and reduction of product viscosity to take place before the drying pro cess, resulting in increased capacity and improved powder qual ity. Plate evaporators During the plate evaporation process, liquid is pumped between thin plates with the heating medium on the mating surfaces. The product is then evaporated with the vapour generated forming a high velocity core. This expansion in volume causes a high-velocity, thin film on the heat exchange surface providing efficient heat transfer. Plate evaporators should be considered for applications that have: Capacities up to 27,000 kg / h water removal A need for future expansion Limited head space A particle size smaller than 600 Micron. Rising / falling and falling film evaporators In these evaporators, the evaporator plates are either arranged in pairs of one steam plate and one product plate (the falling film evaporator), or in sets of four plates (the rising / falling film evap orator). With the rising / falling film unit, the feed is partially evaporated ris ing up the first set of plates, with the product / vapour mix being duct ed for final evaporation in the second set of plates. In the falling film evaporator, the feed is first introduced into a chamber above the product plate where flashing occurs, then this liquid / vapour mix passes through a single product hole to a flash chamber which extends across the top of the adjacent steam section Fig. 16: Double tube. Application advantages: Unrestricted flow. Suited for products with high pulp or high fibre content. Products with a re quire ment for high particulate identity. Fig. 17: Triple tube. Application advantages: Low viscosity fluids. Newtonian fluids. Heat transfer from two sides. Fig. 18: Quadruple tube. Application advantages: Balanced pressure drop. Balanced heat transfer. Non-newtonian fluids. Fig. 19: Multi tube-in-tube. Application advantages: Compact design. Lower viscosity products. Maximum heat transfer. 9

10 Fig. 20: Model 1000 CTA complete with tube-in-tube pre-heater for production of juices / coffee / soy products. Fig. 21: Perspective drawing of APV Anhydro concentrate heater. (see Fig. 26 overleaf). This mix is then distributed through distribu tion holes in the separator. the bottom of the plate to the steam plate on to the top (distribution) With the falling film plate, it is section of the product plate. The product possible to achieve double concen / vapour mix distributes evenly across and tration in the one plate pack. In this then down the plate before travelling out of case only the left-hand side of the the vapour duct at plate is used for a pre-concentration of the feed, and then a final Orifice Chamber Flash chamber Product transfer hole concentration takes place down the right-hand side of the evaporator plate (see Fig. 23). Paravap The Paravap type of plate evapo rator is based on the same principle as the Rising Film Evaporator, except that the plate gap is reduced. This means with the same flowrate across the plates we will have high er velocities in the plate gap. The vapour then tends to carry the feed in the form of minute particles, which has the effect of lowering the apparent fluid viscosity within the plate than that in the final separated product (see Figs. 27 and 28). The Paravap evaporator is speci ally suited to products with high viscosity, as well as high-solids and nonnewtonian fluids. Distribution holes Fig. 22: Typical product and steam plate unit for falling film plate evaporator. 10

11 Paraflash The Paraflash type of plate evapo rator operates under suppressed boiling with the product circulated around the evaporator. Vaporisa tion does not occur within the heat exchanger plate, instead the prod uct flashes as it enters the vapour / liquid separator. Suppressed boiling combined with high liquid velocities helps to reduce scaling on the heat exchange surface (see Figs. 24 and 29). The Paraflash evaporator is speci ally suited to products with a tendency to crystalise, with a starch content or with suspended solids. Feed Product from separator Vapor and product to separator Steam Steam section Product sec- Section Water desalination unit One of the ways in which APV has combined the technologies of evaporation and condensing is in the creation of a water desalination unit. In this, sea water is pumped to a vacuum chamber which encloses two plate heat exchangers, one operating as an evaporator and the other as a condenser. The fresh water is evaporated off the saline solution in one heat exchanger and channelled to a condensing duty in the other. The unit comes fully en closed with titanium heat exchange plates and built-in flow diver sion if there should be high salinity in the fresh water. Condensate outlet Final product to separator Units can be supplied with a nomi nal fresh water capacity of m 3 per day (see Fig. 25 overleaf). Other evaporators APV also manufactures a full range of tubular evaporators. Pic tured overleaf are some ex am ples of the many diversified installations completed by APV. Fig. 23: Falling film plate evaporator flow arrangement. Fig. 24: Double effect SR95 Paraflash concentrates paint wash water at the rate of 3200 lb / h. Fig. 25: Water desalination unit showing enclosed evaporator and condensor. 11

12 OTHER HEAT EXCHANGE PRODUCTS APV also covers a wide range of heat exchange products for more specialised processes. These include: Ohmic heating Radio frequency heating Microwave heating Direct steam heating by infusion or injection. District heating units The APV compact district heating unit is a modular installation mounted on a frame consisting of plate heat exchangers together with pumps, automatic regulating equipment, thermometers, pres sure gauges and all necessary valves. The units are constructed so that a minimum of space is re quired and they can be quickly and simply installed. a) The range of the unit is kw from single family dwellings to distribution stations. CUSTOMER SERVICE FACILITIES APV service facilities are located in numerous countries all over the world. From complete system diagnosis to individual equipment overhaul. From these facilities the following product services can be provided: Heat exchangers and plate evaporators Gasket removal Chemical plate cleaning Dye penetrant testing On site service Plate inspection Re-gasketing Pressure testing Tubular heat exchangers Mechanical repairs Seal replacement Acid / alkaline cleaning Pressure testing Services in general Material diagnosis Maintenance contracts System expansion Shutdown maintenance Scraped surface heat exchangers Grinding of barrels Blade replacement Motor, gearbox maintenance Seal replacement b) c) Fig. 26: a) Two 30-tonnes APV evaporator calandrias. b) Falling film evaporator for the concentration of cleaning water from dye-stuff production. c) Six effect falling film evaporators with thermal vapour recompression 12

13 Fig. 27: Paravap utilised for the con centration of corn syrup. THE QUALITY APVs commitment to manufac turing quality products and pro vid ing high standards of service has earned us major quality certifications for the supply and manufacture of our heat exchange equipment throughout the world. Asme VIII (U) Stamp ISO 9001 ISO 9002 Lloyds quality assurance National accreditation of certification bodies. In addition heat exchangers can be designed to comply with any rele vant international pressure code such as: AD Merkblatter Stoomvezen BS 5500 Asme VIII. CONCLUSION In 1923, Dr Richard Seligman, the founder of APV, introduced the first commercially successful design of Plate Heat Exchanger. From this beginning of cast gun metal plates within a frame we have supplied heat exchange units around the world and diversified into the full range of commercially available heat exchangers. No other company in the world can offer such a com plete range of heat exchange equipment. From pilot duties to millions of litres a day we can supply a heat ex change component for your needs. APV has extensive test facilities in both Europe and North America to ensure that we are at the forefront of the industry. With over 80 years of application experience in a world-wide network, our depth of knowledge in your industry will ensure that we can fit the most energy-efficient and cost-effective solution to your process. 13

14 Fig. 28: Flow schematic af Paravap evaporator. Fig. 29: Flow schematic af Paraflasch evaporator. 14

15 PLATE HEAT EXCHANGER MATERIAL SELECTION GUIDE Plate heat exchanger gasket materials NBR std Paracil FPM Fluorelastomers Graphite General purpose material used for aqueous and fatty duties. Also vegetable and mineral oils. E.P.D.M High temperature general purpose material for chemical environments and steam applications. May not be used with fats, hydrocarbon solvents or where traces of mineral oil are present. EG compressor oil in refrigerants. Fluororubber material resistant to a wide range of organic solvents, chemicals. Resistant to sulphuric acid. Can be used with vegetable oils at high temperatures. Polychloroprene An elastomer which finds wide spread use in sealing applications involving ammonia (R717) and fluorinated refrigerant liquids such as Freon 13, 22 and R134a, but not 11, 21 or 112. Note: This table lists only some of the most common materials of construction available from APV. It is to be used for comparative purposes only, as specific applications will dictate material selection. The thermal and chemical properties of gaskets are exceptionally good and the gasket does not age. Grafoseal is resistant to the vast majority of plate heat exchanger media. Not recommended for strongly concentrated acids, such as sulphuric acid and nitric acid at high temperatures. Plate materials AISI 304 The cheapest grade of the Austenitic stainless steels. Whilst offer ing a general all-round corrosion re sistance to a range of organic and inorganic products, it exhibits a poor resistance to sulphuric acid and hydrochloric acids. It is particularly susceptible to crevice corrosion induced by chloride contain ing cooling waters and the marginal cost saving which can be achiev ed by its use, as an alternative to AISI 316 is rarely justified. AISI 316 A general purpose stainless steel suitable for use in a wide range of environments but with limited applications in sulphuric acid. The presence of molybdenum in the alloy confers a level of resistance to chloride induced crevice corrosion which will permit its successful application in most cooling waters. Alloy 904L The high nickel content imparts a good degree of resistance to stress corrosion cracking, a failure mode frequently associated with pitting and crevice attack in AISI 316. An alloy which offers an excellent compromise between price and corrosion resistance for a wide range of acidic and chloride environments. Alloy 254SMO / AL6XN Improved grades of AISI316 re sulting from the increased level of molybdenum which imparts in creased resistance to chloride in duced pitting/ crevice corrosion, and therefore use in brackish waters where the corrosion resist ance of AISI316 is inadequate. The performance in mineral acids is significantly better than AISI316. In general terms, the corrosion resist ance improves with increasing molyb denum content. Alloy C-276 An expensive alloy which is almost immune to attack by chloride ions at low ph values. It has an ex tremely good resistance to a wide range of sulphuric acid concentra tions and can be employed with certain strengths of hydrochloric acid. It is particularly resistant to wet process phosphoric acid. One of the few alloys suitable for use in hot, concentrated sulphuric acid. Titanium ASTM B265 Gr.1 Titanium is characterised by high price, low weight and much great er resistance to corrosion than stain less steel, especially in chlo ride containing liquids. It is in fact the material par excellence for chlo ride solutions. Titanium is not however immune to attack by some concen trated chloride solu tions at tem per a tures over 100ºC, and there fore where this condition prevails or is likely to arise through leakage, titanium-palladium is preferred. The resistance to corrosion of titanium is due to its passivity, just as for stainless steel. Note: This table lists only some of the most common materials of construction available from APV. It is to be used for comparative purposes only, as specific applications will dictate material selection. 15

16 PLATE EVAPORATORS VS TUBULAR EVAPORATORS SELECTION GUIDE Plate Evaporators Tubular Evaporators Falling Rising / Falling- Paravap Paraflash Tubular Water evaporation Low capacity < 2000 kg / h Medium < 10,000 kg / h. High > 20,000 kg / h. Particulates / suspended solids Less than 0.75 mm mm Greater than 1.5 mm diameter 1st effect evaporation temperature < 70 C > 120 C Fouling susceptibility Crystallising Viscous Inverse sol/ temperature scaling ph High > 12 Low < 2 Concentration ratio Medium/ high > 90% Residence time requirements Low Space requirements Restrictive head height Small overall dimension Maintenance Comparison: = Best selection = Suitable = Within working range = Not recommended Note: The table above is for general comparison of specific characteristics. It is not to be taken as definitive or absolute. For a specific duty more than one property may be relevant leading to a different selection. For optimum heat exchanger selection readers should refer to their local APV sales office. Addresses to be found at our website: 16

17 HEAT EXCHANGER SELECTION GUIDE Plate- Tubular- SSHE heat exchangers heat exchangers Scraped surface heat exchangers Plate in Para- Para- Hybrid Para- Double Quad- Multi CTA VT HD frame Shell Welded Brazed tube ruple tube ParaFlow tube Physical properties Viscosity < 10 Cp < 1000 Cp > 1000 Cp Shear sensitive particulates < 1mm diameter < 10 mm diameter > 10 mm diameter mm diameter Insensitive particulates < 1 mm diameter < 10 mm diameter > 10 mm diameter > mm diameter Fibres 2 mm 15 mm Fluid pressures < 1200 kpa (low) < 2200 kpa (medium) > 2200 kpa (high) Fluid temperatures < 150 C > 150 C Flexibility for expansion Maintenance Ease of inspection Compact/floor space Comparison: = Good selection = Suitable = Within working range = Not recommended Note: The table above is for general comparison of specific characteristics. It is not to be taken as definitive or absolute. For a specific duty more than one property may be relevant leading to a differant selection. For optimum heat exchanger selection readers should refer to their local APV sales office. Addresses to be found at our website: 17

18 APV transfers the energy to your world... Six APV SR23 heat exchangers containing 361 titanium plates per unit are used in a U.K. nuclear power station for cooling 4750 gpm of reactor water from 32.5 C to 22.2 C. These APV Paraflow heat exchangers installed in a USA power station have been pre-heating boiler make-up water for the last 16 years, utilising plant waste water. One unit with approx. 400 plates was amor tised within 865 hours of use. This 10 MW hot water distri bution unit comes factory tested. C/W heat exchanger pumps and automatic regulators. APV utility hot water unit comes as a com plete package in order to supply the whole house from a district hot water supply. This APV SR26 plate heat exchanger supplies constant temperature hot water to a 17 storey block of flats in Leichester, United Kingdom. It is fed with 95 C hot water on the primary side from the councils central boiler house. 18

19 ... to your office block The second water loop supplies chilled water to the air-conditioning units on the upper floors. Plate heat exchanger being compact and having low bearing weight is ideally suited to the application. Separate condensor water circuit for computer room cooling via plate heat exchanger tied to the cooling tower water circuit. Four Paraflow plate heat exhangers Two Paraflow plate heat exhangers 27th floor Secondary Condenser Evaporator Basement Clevelands Society Tower, USA. This building, the tallest in Ohio, rises 63 storeys above street level. It relies on nine large TR9 APV Para flows for HVAC duties. Eleven Paraflow plate heat ex changers in the basement allow the chiller to be by-passed for free cooling duty at low outdoor tempera tures. 19

20 ... out to sea Modern ships have huge cooling requirements with the main engines needing a capacity close to 25% of fuel consumption energy. APV heat exchangers are suited to the closed circuit cooling duty so often uti lised at sea with titanium plates allowing fresh water to be recirculated and cooled directly against sea water. APV plate heat exchangers are used on an offshore oil platform in the North Sea to cool over 495,000 kg/h of produced water and gas from 120 to 85 C with a cooling medium of 400,000 kg/h of seawater. APV water de-salination units can provide up to 60,000 l /day of fresh water using waste heat from engine cooling and sea water. The packaged units come complete with two titanium plate heat exchangers acting as evaporator and condenser. 20

21 ... in to industry This large plate heat exchanger installation uses over 70,000 sq.ft. of heat transfer area at a major United States soda ash plant. These units exchange heat from carbonation reactors to incoming raw brine from underground wells. These new plate & shell heat ex changers are installed in a United Kingdom plastics plant and are used to heat limpet coils around a reactor vessel which in turn heats polyurethane and derivatives up to 130 C. This M92 Paraflow plate heat exchanger is installed as an evaporator as part of refrigeration package used to cool over 270,000 kg/h of propylene glycol from 1 C to -2 C. The heat exchanger in cor porates 316 stainless steel plates with neoprene as a gasket material using a flooded ammonia system. A United Kingdom car manufacturer has installed APV s new double wall plate heat exchanger to provide security against cross contamination during the cooling of a 130,000 litre paint vat. The electrostatic paint is circulated at a rate of 75,000 kg/h through the PHE system to achieve a 3 C temperature drop before returning to the vat. 21

22 This special SSHE unit consists of seven RT672 cylinders together with an integral tubular holding cell. Three of the cylinders are used for heating, three for cooling, with the final cylinder utilised as a product stabilisation holding cell. Used for the thermal treatment of various sauces and dressings containing particulates. The gentle product treatment and short residence time of the unit provides for excellent product identity, texture and aroma. Juices and coffee creamers are produced in this coiled tube aseptic heat ex changer system complete with a tube-in-tube pre-heater. Installed in a major Australian dairy company producing 20,000 tonnes of cheddar cheese per annum, this large K71 heat exchanger pasteurises 55,000 l/h of cheese milk. The test center in Silkeborg, Denmark is one of many APV Centres around the world reinforcing APV s commitment to innovation and development of high added-value food products. The plate heat exchangers are utilised to UHT treat starter media prior to the manufacture of starter culture for cheese production. 22

23 ... and food processing This corrugated multi tube-in-tube heat ex changer is utilised as a hot-fill pasteu riser for the processing of juices, teas and isotonic drinks. This processing system shipped to Costa Rica aseptically processes banana puree using RT627 SSHE together with an SR95 Paravap evaporator and essence recovery system. This 1400 l/h aseptic system was delivered to Panama to process single strength juices and nectars. The system includes a 4-section stainless steel clad plate heat exchanger and parasol deaerator all controlled by an APV Accos 3 series plc. This aseptic processing plant was shipped to Cyprus to process a variety of products including crushed tomatoes, tomato juice, tomato paste and citrus juices. This versatile plant uses a straight multi tube-in-tube design. 23

24 We are only one click away We would like the opportunity to provide further information and explain the detailed benefits of APV products and services. For more information about our heat transfer technologies please contact your local APV sales representative. If you already have a duty for a heat exchanger and want to configure and design it yourself, please visit APV South America Industria e Comério Ltda. Rua João Daprat Sao Bernardo do Campo, SP Brazil APV China Ltd Shuanghe Road, Linhe industrial Zone, Shunyi, Beijing P.R.China. APV 1200 West Ash Street P.O. Box 1718 Goldsboro, North Carolina USA APV Deutschland GmbH Otto-Brünner-Strasse 7 D Artern Germany T: F: Heat.SouthAmerica@apv.com APV Platinvej Kolding, Denmark Phone: Fax: T: F: Heat.China@apv.com T: +1 (919) F: +1 (919) Heat.USA@apv.com T: F: Heat.Germany@apv.com For more information about our worldwide locations, approvals, certifi cations, and local representatives, please visit Copyright 2003, 2004, 2005, 2005, 2006, 2007, 2008 SPX Corporation GB The information contained in this document, including any specifi cations and other product details, are subject to change without notice. While we have taken care to ensure the information is accurate at the time of going to press, we assume no responsibility for errors or omissions nor for any damages resulting from the use of the information contained herein.