Shell-and-Tube Heat Exchangers. For process cooling, these typically consist of a carbon steel shell containing a package of copper tubes. In a typical cooling application, the refrigerant flows in the tubes, and the water or brine flows in the shell surrounding the tubes (figure 1). Copper plates (baffles) mounted on the tubes direct the liquid to be cooled in a turbulent crossflow pattern. The dual shell-and-tube exchanger shown in figure 1 has two plates located approximately 0.5" (1.2 cm) past the two refrigerant inlets. These plates partially obstruct the incoming liquid-gas mixture, thereby ensuring that refrigerant is distributed evenly among the tubes.
Brazed Heat Exchangers.Constructed as a number of corrugated plates between front and rear cover plates (figure 2), these exchangers typically are made of vacuum-brazed stainless steel with pure copper filler. During vacuum brazing, the thin foil of filler material forms brazing joints between the plates, creating channels in two separate circuits. Note that the pressure drop shown in the outer circuit in figure 2 is slightly lower than that in the inner circuit. This difference occurs because the outer circuit has an additional channel.
It is well known that a highly turbulent flow promotes efficient heat transfer. In brazed heat exchangers, the turbulence often is achieved by a herringbone plate pattern that creates convoluted channels. The convolutions ensure that the media flowing through the exchanger experience frequent changes in speed and direction, thereby providing high turbulence even at low flow rates and pressure drops. The high turbulence also keeps particles in the fluid in suspension, which helps prevent surface fouling.
Comparing Plate with TubeThe criterion for evaporator performance in the following comparisons is the temperature difference between the leaving water temperature (LWT) and the evaporation temperature (Tdew) as a function of the cooling capacity. This temperature difference provides a way of describing the force driving heat transfer; efficient heat exchangers have high cooling capacity at low temperature differences.
For the example, a shell-and-tube evaporator was selected and sized to operate at 386,000 BTU/hr (113 kW) under standard European chiller working conditions with R407C as the refrigerant. The inlet quality was chosen to be about 30 percent. The evaporation temperature and the degree of superheat were held constant at 40°F (4°C) and 9°F (-13°C), respectively. The water flow rate was regulated to achieve a water temperature difference of 9°F (5°C). The corresponding brazed exchanger was a dual-evaporator model with 98 plates. Laboratory testing was performed on both heat exchangers for both full- and partial-load conditions. The results are shown in figure 3.
Comparing Plate with CoaxialWhen comparing a coaxial heat exchanger and a brazed unit, the criterion used for evaporator performance was the same as that used for the performance comparison between the shell and tube and brazed unit. For this case, a coaxial evaporator was designed for 59,710 BTU/hr (17.5 kW) under standard European chiller working conditions with R407C as refrigerant. The inlet quality was chosen to be about 30 percent. The evaporation temperature and the degree of superheat were held constant at 40°F and 9°F, respectively. The water flow rate was regulated to achieve a water temperature difference of 9°F. A corresponding brazed model was selected.
The results of laboratory tests on the coaxial evaporator are plotted in figure 5 and compared with calculated values for the performance of the brazed exchanger. The plot shows that the evaporators had equivalent performance. Furthermore, their heat transfer characteristics also varied in the same way with the heat load. Note that the brazed unit was plotted only for heat loads between 60,790 BTU/hr (17.8 kW) and 102,400 BTU/hr (30 kW), because higher heat loads lead to experimentally unverified heat fluxes.
A similar conclusion can be drawn when comparing the heat exchangers as condensers. In a condenser role, a brazed unit is almost five times more weight efficient than the corresponding coaxial model. However, if a coaxial condenser were to be replaced by a brazed heat exchanger, pressure drop would probably be the limiting sizing factor. Although the brazed exchanger condenser would be oversized, the larger size does offer advantages. First, the condensation temperature would be reduced, resulting in improved system performance. Alternatively, the leaving water temperature could be higher, which reduces the water flow rate and, hence, the pressure drop.
The comparisons show that the heat exchangers performed equally well for the specified design cases. A compact brazed exchanger can provide the same performance with a smaller size package. Compared to a shell-and-tube heat exchanger used as an evaporator, an equivalent brazed exchanger is about 84 percent smaller. Compared to a coaxial heat exchanger, an equivalent brazed unit is about 10 times more weight efficient than the coaxial at equal evaporator performance. For a typical process cooling system, the benefit is a smaller yet highly efficient system.
Italian company Sipa manufactures polyethylene terephthalate (PET) bottles used
for many kinds of beverages. PET bottles can be produced using one- or two-step
methods, both of which the company uses depending on the specific packaging
requirements. Sipa’s manufacturing concept integrates an injection press with
the preform blowing process, which enables the company to produce a PET
container from plastic resin in a single machine.
Case Study of a Brazed Exchanger Solving a Cooling Problem for Sipa S.p.A.
Sipa’s process is divided into two stages. The first is injection pressing, in which the powder is melted to form a preform. In the second stage, the preform is blown. During the pressing stage, oil in hydraulic systems is used to open and close the presses. The hydraulic oil becomes hot and must be cooled. Strict hygiene is essential because the bottles are used for beverages. The environment in the bottle-producing units is subject to disinfection, and is therefore an aggressive environment for steel heat exchangers. In some cases, the bottle-making units are placed in a clean room, and the exchanger is washed with antiseptic liquid. The heat exchanger then is coated with paint to protect it from the aggressive environment.
In 2004, the company installed a compact brazed exchanger, primarily due to the inherent compactness of the brazed unit compared with the shell and tube. The new unit was supplied by Swep.
Since the retrofit, the hydraulic oil from the pressing stage of the PET bottle process is cooled in a Swep Model B50 brazed exchanger. The key technical details of the application are given in the accompanying table. The size of the brazed unit was based both on the size of the particular bottle-producing unit (which determines the volume of oil used) and on the properties of the available cooling water (flow rates, temperatures, etc.).
With the compact brazed exchanger, Sipa has been able to reduce the size of its heat exchanger while obtaining better cooling performance compared to the original shell-and-tube.