ASHRAE OR-05-10-1-2005 Brazed Plate Heat Exchangers and Their Applications《钎焊板式换热器及其应用》.pdf

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1、OR-05-1 0-1 Brazed Plate Heat Exchangers and Their Applications Eric Mencke Jenny Larsson Ylva Persson Andreas Olsson Adam Dahlquist Patrik Eriksson ABSTRACT This paper presents an overview of brazed plate heat exchangers and their applications and competing technolo- gies. First, an illustration of

2、the construction of the brazed heat exchanger and the basic refrigerant cycle is made. Further, the paper explains where the heat exchangers can be used in a refrigerant system. Diflerent refigerant applications with brazedplate heat exchangers arepresented, liquid chillers and residential heat pump

3、s, among others. In addition, the paper compares the performance of brazed plate heat exchangers with that of shell-and-tube and coaxial heat exchangers. A brazed plate heat exchanger that solves the same operating case as a shell-and-tube is considerably smaller and lighter. The brazed plate heat e

4、xchanger is also approved for higher working pressure. At equal performance, the comparison between a coaxial evaporator and the corresponding brazed plate heat exchanger, according to tests, shows that the brazed plate heat exchanger is about ten times more weight eficient than the coaxial evaporat

5、or. INTRODUCTION The compact brazed heat exchanger (CBE) is one of the most efficient ways to transfer heat from one medium to another. A CBE consists of corrugated plates combined to create complex channels through which a hot medium and a cold medium can be alternately distributed. The mediums com

6、e into close proximity inside the CBE, on either side of the corrugated plate without mixing, and energy is transferred from one to the other as they flow side by side. The modular product concept, with customizable parts, means the right product solution for every application can often be found. Th

7、e effective use of the material in a CBE leads to a compact and very competitive heat exchanger when compared to shell-and- tube and coaxial heat exchangers. CONSTRUCTION There are several different types of CBEs depending on their material combinations and design pressures. They are defined as stan

8、dard CBEs, all-stainless CBEs, Mo-steel CBEs, and high-pressure CBEs. The basic construction materials indicate the types of fluids that can be used with the CBEs. Typical examples are synthetic or mineral oil, organic solvents, water (not seawater), glycol/water mixtures, and refrigerants (e.g., HC

9、FCs, HFCs, HCs). The standard plate materials are stainless steel, vacuum-brazed with a pure copper filler or a nickel-based filler. Carbon steel can be used to some extent, e.g., for certain types of connections. For demanding applications, the plates can be made of SMO 254, a stainless steel with

10、a higher content of molybdenum. CBEs are available in standard pressure rating or high pressure rating. In principle, the CBE is constructed as a plate package of corrugated channel plates between the front and rear cover plates, the thickness ofwhich depends on pressure ratings. The cover plate pac

11、kages consist of sealing plates, blind rings, and cover plates, as shown in Figure 1. 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 mate

12、rial, which depends mainly on the working fluids. For instance, copper filler would be avoided for ammonia applications. The final design creates a heat exchanger consisting of channels with two separate circuits (Figure 1). The authors are with SWEP International AB, Landskrona, Sweden. 02005 ASHRA

13、E. 81 3 Figure 1 CBE plate package (top) and the direction of a counter-current flow through a CBE (hottom). The front plates of the CBEs are marked with an arrow. The purpose of this arrow is to indicate the front of the CBE and the location of the inner and outer circuitslchannels. With the arrow

14、pointing up, the left side (ports F 1, F3) is the inner channel and the right side (ports F2, F4) is the outer channel. The pressure drop over the outer circuit is lower because it contains one more channel. The inner circuit consequently has a slightly higher pressure drop. Ports FlIF2/F3/F4 are si

15、tuated on the front of the heat exchange and ports PllP2P31P4 are situated on the back. High turbulence is desirable for efficient heat transfer and is achieved by increasing the disturbances in a flow. A rough surface, which is achieved in the CBEs through a herringbone pattern, thus results in a m

16、ore turbulent flow than a plane surface. As the fluid passes through the channels in a CBE, it constantly changes its direction and velocity. This ensures turbulent flow even at very low flow rates and pressure drops. In addition, a turbulcnt flow will keep particles in the fluid in suspension, Le.,

17、 no particles are allowed to collect on the surface, which will avoid surface fouling. MAIN FUNCTION IN REFRIGERANT SYSTEMS Since the copper brazed plate heat exchanger became commercially available in the early 1980s, new applications have been found every year. The refrigeration industry was one b

18、usiness area that very soon realized and accepted the advantages of this hermetic, pressure resistant, compact, and highly efficient heat exchanger technology. Higher demands on efficiency and compactness for refrigeration systems have then improved the market penetration of CBEs as evaporators and

19、condensers for increasingly large capacities. The simplest refrigeration system consists of an evapora- tor, a compressor, a condenser, and an expansion valve, where CBEs typically are used as evaporators and condensers. Evaporators In all reftigerant applications, it is very important that every re

20、frigerant channel is surrounded by a secondary fluid channel on both sides. Normally, the refrigerant side is connected to the left-hand side and the secondary fluid circuit to the right-hand side on the front plate of the CBE. If the refrigerant and secondary fluid connections are transposed, the e

21、vaporation temperature will fall, with the risk of freezing and very low performance. CBEs specially optimized for evaporator duties are equipped with a special distribution device at the refrigerant inlet. The purpose of the distribution device is to distribute the refrigerant-normally made of a mi

22、xture of vapor and 81 4 ASHRAE Transactions: Symposia Figure 2 A wuter-cooled chiller; dimensions: 150 x 125 cm. liquid-evenly in the channels. The distribution device is composed of so-called v-rings, placed at the inlet of each chan- nel, forming a smooth consistent tube for the refrigerant mixtur

23、e. Each device has a carefully defined radial drilled hole, through which the refrigerant flow is forced. The posi- tion of each device at the entrance to each refrigerant channel ensures an even distribution and a stable and highly efficient heat transfer for large plate packs and high refrigerant

24、flows. Condensers When used as a condenser, the refrigerant is connected to the left connections and the secondary fluid circuit is connected to the right connections on the front plate of the CBE. As with evaporators, a countercurrent flow is the normal flow arrangement, resulting in the highest me

25、an temperature difference. A Co-current flow is also used in the case of revers- ible refrigerant systems, when the CBE evaporator is also used as a condenser. Condensers can be tilted with some perfor- mance loss. The condenser is normally less sensitive to tilting compared with evaporators, for wh

26、ich the performance losses are significant due to a stratification of the flow. REFRIGERANT APPLICATIONS WITH CBE Liquid Chillers-Water-cooled Liquid chillers are used in air-conditioning systems or in processes where there is a need for chilled water or brine. During the 1990s, brazed heat exchange

27、rs developed into a broadly accepted component in small and mid-size chillers. Today, they have become the preferred heat exchanger tech- nology for evaporating, condensing, subcooling, and de- superheating applications. By using CBEs instead of shell- and-tube heat exchangers, the system builders a

28、re able to Figure 3 Front und top view of un air-cooled chiller with scroll compressors and a dual circuit CBE evaporator: shrink the chiller dimensions, thereby reducing footprint, weight, and cost. Chillers are either water-cooled or air-cooled depending on the source of rejecting the heat from th

29、e condenser. Water- cooled chillers have the advantage of high efficiency and a very compact format since both the evaporator and the condenser are brazed heat exchangers. The popularity of modular chillers is evidence of the new possibilities when using brazed heat exchangers as evaporators and con

30、densers. Chiller modules with CBEs are more compact than traditional shell-and-tube designs, as shown in Figure 2, making them a popular choice for system retrofits or new construction due to the ability to fit them through standard doorways and eleva- tors. At site, an existing chiller can be repla

31、ced with a number of chiller modules equivalent to the total capacity needed. Liquid Chillers-Air-Cooled Unlike water-cooled chillers, air-cooled chillers do not depend on a cooling water circuit for rejecting the condenser heat. Instead, the surrounding ambient air is used for cooling the condenser

32、, The drawbacks of these chiller systems are their larger dimensions due to the condenser coil and their lower efficiency. A large part of air-cooled systems is dual refrigerant circuit systems where each circuit has one or several compres- sors in parallel. An example of such a system is shown in F

33、igure 3. Liquid Chillers-Dual Refrigerant Circuits With larger systems, two independent refrigerant circuits are commonly used to follow the changes in cooling demand better and to increase system reliability. If one circuit malhnc- tions, the one remaining in operation may still provide suffi- cien

34、t cooling until help arrives. A recent innovation in CBE technology offers two inde- pendent refrigerant circuits combined with a common second- ary fluid circuit, as shown in Figure 4. The patented plate technology ensures full countercurrent flow and full symme- try between the refrigerant circuit

35、s. A CBE with this config- ASHRAE Transactions: Symposia 815 Coolkg coollns ctqua 1 chqua 2 Figure4 Schematic sketch of a dual circuit CBE evaporator system. uration running with both circuits active operates no differently from a high-efficiency single-circuit evaporator with fll contact between re

36、frigerant and secondary fluid. Even if one refrigerant circuit is closed, i.e., half-load operation, all secondary fluid channels remain in contact with the active refrigerant channel. All the secondary fluids still transfer heat, and the leaving water temperature will therefore be the same as for f

37、ull-load operation provided the water flow is also halved. Typical applications for the dual-circuit CBEs are climate control systems and high-precision systems for food cooling cabinets in supermarkets. Indirect Refrigerant Systems There are two reasons for using an indirect refrigerant system. Fir

38、st, the amount of refrigerant can be kept at a mini- mum. Second, the risk of leakage of primary refrigerant is decreased. This means that refrigerants that are not wanted in systems in public buildings (e.g., ammonia) can still be used in the primary system, which can be kept in a safe sealed room.

39、 A good example of indirect refrigeration systems is in a supermarket, as illustrated in Figure 5. All the cooling effect required is produced in the machine room, far from the display cases, resulting in high flexibility and a lower risk of leakage ofprimary refrigerant. A heat recovery system, als

40、o situated in the machine room, provides heating for the supermarket when required. The use of dual-circuit heat exchangers is advanta- geous because the independence of the refrigerant circuits increases the safety of operation and the availability of cool- ing. Residential Heat Pumps-Liquid to Liq

41、uid A refrigerant system can be used to heat residential or commercial areas by utilizing the condensation energy. Heat required by evaporation comes from the surroundings through a loop of brine in the ground (Figure 6, top), from the bedrock, a lake, or the ambient air. Dedicated heat pumps are co

42、mmon in cold climates where the low requirement for air condition- ing during the summer makes reversible heat pumps less attractive. By optimizing the heat pump for heating only, the operating cost will be lower than for reversible heat pumps. l I Figure 5 Indirect reji-igeration system with a cent

43、ral cooling machine, separutedfrom the supermarket area by the harmless heat transferfluid. ._ - - . . . . . . - . Ib C41 Figure 6 Dedicated residential heut pump system with a secondary fluid loop buried in the ground (top) and using the exhaust ambient air (bottom). Dedicated heat pumps operate wi

44、th relatively low temper- ature differences in both evaporator and condenser to maxi- mize the coefficient of performance (COP), i.e., the heating capacity divided by the electrical consumption. The use of dedicated CBE evaporators with a distribution device is highly recommended to maximize the eff

45、iciency. 81 6 ASHRAE Transactions: Symposia n 4 Figure 7 One-stage absolption system. Residential Heat Pumps-Exhaust Air Heat Pumps In houses with ventilation systems in cold climates, the warm ambient exhaust air can be used to operate a heat pump, as shown in Figure 6 (bottom). The use of highly e

46、fficient CBEs as condensers makes it viable to utilize the warm air to evaporate the refigerant in an air coil before releasing it. This arrangement is a good way of minimizing the energy waste. Absorption Chillers Conventional compressor air-conditioning chillers are powered by electricity. Absorpt

47、ion chillersheaters, on the other hand, use high-temperature heat as their main energy source. Absorption chillers are used mostly for large installa- tions when electricity is limited and/or heat is abundant. The most efficient modern absorption cycle chillers use water as the refrigerant and a sol

48、ution of lithium bromide (LiBr) as the absorbent. A one-stage absorption chiller system is illustrated in Figure 7. CBEs are used to exchange heat between different flows in the system. Typical applications include heat exchange in the circulating stream between the hot LiBr stream from the generato

49、r and the low-temperature stream from the absorber. This heat exchanger is often referred to as “high-temperature.” Another CBE can be used to cool further the LiBr stream entering the absorber (“low-temperature”), and another to preheat further the stream entering the generator. ALTERNATIVE TECHNOLOGY- SHELL-AND-TUBE HEAT EXCHANGERS Construction The typical shell-and-tube heat exchanger consists of a carbon steel shell containing a package of copper pipes. A number of copper plates, so-called baffles, are mounted on the copper pipes in order to direct the heat transfer fluid in a more t