Stainless-steel brazed-plate evaporators commonly are used in the refrigeration industry because of their ability to provide high-efficiency heat transfer in a compact unit. Achieving and maintaining optimum system performance, however, requires the proper design and installation for the application.
The three most important steps required to apply a brazed-plate evaporator successfully are:
- Provide a complete engineering specification.
- Design the control system to prevent freezing and oil logging.
- Install the unit to allow for rated performance and proper servicing.
- Refrigerant type.
- Required refrigerant saturated suction temperature (SST).
- Required refrigerant suction superheat.
- Refrigerant liquid line temperature.
- Refrigerant feed type (direct-expansion, thermosiphon or pumped).
- Chilled fluid type.
- Chilled fluid temperatures.
An outdoor process chiller at a major U.S. food manufacturing facility routinely shut off because the low pressure cut-out limit had been reached. The situation was bad enough that a technician had to be stationed at the chiller 24 hours per day in certain weather conditions.
During normal operation, the chiller’s operating cycle started with a tank of food-grade process fluid at the outside ambient temperature that was chilled through a recirculation loop to a temperature of approximately -20°F (-29°C). At this point, the fluid could be pumped through the various food-processing stations in the factory. Unfortunately, the chiller could not provide enough cooling to satisfy the thermal load of all of the stations. The compressor clearly had enough capacity, and the evaporator’s saturated suction temperature was lower than expected for the load condition, yet the fluid setpoint temperature could not be maintained any lower than -10°F (-23°C). This limitation significantly affected production at the facility.
After nearly two years of less-than-adequate system performance, the end user told the packaged chiller system manufacturer to fix the system or replace it. The system manufacturer in turn contacted the manufacturer of the direct-expansion (DX) brazed-plate evaporator for assistance. The ensuing investigation indicated that the problems had started with the initial specification.
Unfortunately, an inexperienced engineer at the system manufacturer had specified the maximum load condition only for the evaporator and had specified it to be circuited with only one refrigerant circuit. This operating condition occurred during the initial temperature pull-down from ambient temperature to the process fluid setpoint. During this segment of operation, the system performed flawlessly.
Further testing showed that the operating load at the system setpoint, however, was approximately one-eighth of the design condition used to specify the evaporator. Field measurements on the evaporator indicated that the refrigerant-side pressure drop was higher than expected and that the saturated suction temperature was low. All of these symptoms suggested that oil logging was the problem.
Direct-expansion brazed-plate heat exchangers with more than 30 plates typically require fixed-orifice-type internal refrigerant distributors to be installed in the refrigerant liquid inlet. These distributors meter identical amounts of refrigerant gas and liquid to each flow channel between respective plates.
A fixed orifice has a specific range of operation. Outside the boundaries of this range, significant refrigerant maldistribution can occur. This maldistribution results in some channels being over-fed with liquid refrigerant so that refrigerant liquid is present in the suction line, which reduces the average superheat temperature. Other channels are fed only with gas, which results in single-phase gas cooling instead of two-phase heat transfer. Single-phase gas cooling is inefficient and does not adequately cool the process fluid. Orifice calculations indicated that the installed orifices were too large by at least a factor of two. The incorrect sizing explained why evaporator capacity was lower than expected (figure 1).
Because the engineer had specified only the maximum flow condition, the distributor orifices were sized only for the initial pull-down load and were significantly oversized for the operating setpoint condition. Another complication was that the refrigerant vapor velocity at the system setpoint was too low to carry the refrigeration oil out of the evaporator and into the suction line. Oil filled the bottom of the evaporator and partially clogged the distributor’s orifices. The restricted orifices explained the higher-than-expected refrigerant-side pressure drop.
In this case, the system manufacturer was fortunate that a two-step solution was possible. First, a field retrofit kit for the distributor orifices was developed by the evaporator manufacturer. Second, the evaporator, which was a dual-unit tandem manifolded to a single-circuit, was re-piped to dual circuits. The orifice modification reduced the refrigerant maldistribution and increased the thermal effectiveness of the heat transfer surfaces. The change to dual circuits allowed one circuit to be shut down during operation at the system setpoint, which doubled the refrigerant vapor velocity so that oil was properly carried to the suction line.
The system manufacturer averted having to perform an expensive system replacement; however, not every situation allows for such field modifications. Getting the entire specification right from the start can prevent headaches later.
Connection Types and SizesBrazed-plate heat exchangers can be obtained with a range of connection styles and sizes. Selecting the optimum connections for a system generally will result in the simplest piping, a reduced number of joints, and the lowest system cost.
Many brazed-plate units allow for additional connections to install temperature, pressure or pH probes directly in the internal fluid ports. These connections can further simplify the system-piping construction and offer better control-system response.
If notable vibration is expected in the piping, the connections might need to be welded onto the unit rather than brazed. Welded connections offer much larger radii at the connection joint with the heat exchanger body than those fabricated with the brazed fillers currently used in the industry. The result is a stronger joint with significantly reduced stress concentrations at the base of the connection.
To simplify system piping, it might be advantageous to have the brazed-plate unit manufactured with the primary (refrigerant) connections on the front face and the secondary (process) connections on the rear (back) face. Figure 2 demonstrates two popular connection arrangements. It is also possible to have three of the connections on one face and the fourth on the other. The main constraint is that the centerline locations of the connections cannot be changed. Most manufacturers can offer these connection location options with little to no impact on price and delivery, so engineers should ask for what they really need to fit their system properly.
Codes and StandardsMost brazed-plate heat exchangers manufactured for the North American market are available off-the shelf with UL, UL-C, and CRN (Canada) certifications. UL is acceptable throughout most of the United States for light industrial fluorocarbon-based systems. CRN is acceptable throughout Canada for most applications.
For large industrial applications and applications that involve poisonous or flammable fluids (for example, ammonia and propane, respectively), either ASME certification or National Board registration generally is required. For ASME certification, most brazed-plate units will carry a “UM” stamp instead of a “U” stamp up to 600 psig working pressure. The “M” in “UM” stands for “miniature,” which is defined by internal volume. Because brazed-plate units have small internal fluid volumes, they usually fit this designation. The advantage for the end-user is that the cost for a “UM” stamp is typically less than the cost for a “U” stamp. A point to be considered when purchasing a vessel that carries a “UM” stamp is that the procedures for the “UM” designation do not require the manufacturer to include the ASME U-3 form in the shipment with the unit. Instead, these forms are kept on file by the vessel manufacturer. Companies that must maintain a copy of the U-3 form for their safety records should request the form when they issue the purchase order.
For shipboard applications, Lloyd’s certification generally will suffice. Fortunately for end users and system manufacturers, most plate heat exchanger manufacturers can offer any of these code options with little to no impact on the product delivery.
Control SystemOne of the primary reasons for selecting brazed-plate heat exchangers over other heat exchanger types is because they traditionally offer the smallest fluid volumes. As a result, the amounts of expensive or dangerous fluids used with them are minimized, which lowers system cost and provides improved plant safety. A consequence of having significantly reduced fluid volumes is that the thermal responses of brazed-plate heat exchangers are much faster than those of other exchanger types. In many cases, this consequence is desirable. In some situations, however, the traditional system-control methods that are used with other exchanger types offer inadequate protection for brazed units. Such situations mostly occur when water is being chilled to below 38°F (3°C) with a brazed-plate evaporator.
For example, the installation of a freeze-stat in the water outlet line, which is traditionally used with tubular type heat exchangers, does not offer a fast enough thermal response to prevent a freeze inside a brazed-plate unit. By the time a near-freeze is sensed by the freeze-stat, the brazed unit usually is already frozen. Figure 3 illustrates the internal damage that is caused by a catastrophic freeze. In the figure, note the expanded and twisted channels, indicated by the box on the left side. A comparison to the right side of the heat exchanger section shows the significance of the damage. Also note the height difference (distance between the front and rear cover plates) between the left and right sides in the photo.
There are several proven methods for preventing freezeups in brazed plate evaporators:
- Install a flow switch on the water side so that when a low-flow condition is sensed, the refrigerant compressor is either off-loaded or shut off completely.
- Install a temperature sensor directly into the water port on the heat exchanger. Some manufacturers provide these sensors as standard; others offer them as options.
- Install an evaporator-pressure-regulating (EPR) valve on the suction line or a hot-gas bypass (HGBP) at the direct-expansion inlet connection to keep the saturated suction temperature above 32°F (0°C).
- Use an intermediate fluid loop when cooling water to below 33°F (0.6°C).
The second method is helpful because the sensor is near the point where freezing will occur, but it is not a fail-safe by itself. For this method to work well enough to be considered almost fail-safe, a laboratory-grade sensor that can be calibrated to and maintained at an accuracy of ±0.2°F (0.1°C) would be required. In practice, field-installed sensors are, at best, accurate to ±1.0°F (0.6°C), with ±1.5°F (0.8°C) more typical. Laboratory-grade sensors generally cannot maintain their precision under industrial conditions and thus cannot be relied on alone as a fail-safe.
The third method is important when the thermal load is expected to vary significantly. If the evaporator’s load is expected to fall below 25 percent of the full-load value, the refrigerant vapor velocity should be checked from the performance printout, which can be obtained from the manufacturer. If the velocity at the low-load point is lower than 100'/min (30 m/min), then the hot-gas bypass option is recommended to maintain the refrigeration oil flowing through to the suction line and prevent oil logging. If hot-gas bypass will be used, this information should be conveyed to the evaporator manufacturer to aid in sizing the correct distribution orifice.
The fourth method should be used when trying to control the water outlet temperature below 33°F. Most control schemes involve some type of time averaging to minimize large fluctuations. However, with the precision control required to keep the water from freezing, even small fluctuations can be significant.
InstallationBrazed-plate evaporators are designed to be installed in a vertical position with the liquid refrigerant feed on the bottom connection and the suction connection at the top (figure 4). The units can function when installed in other orientations; however, they will exhibit low thermal performance or liquid slugging into the suction line.
In addition to installing the brazed evaporator in a vertical orientation, it should be piped with the process fluid in counterflow (i.e., counter-current flow) with the refrigerant to achieve rated performance. When installation constraints do not permit piping the unit for counterflow, then the manufacturer should be contacted for a unit rating with parallel (i.e., co-current) flow. The main consideration with using the parallel flow arrangement is that the suction superheat is more difficult to achieve. In close temperature approach applications, it might be impossible to achieve a reasonable suction superheat without using counterflow. Many manufacturers design their systems to allow for suction superheats as low as 4°F (2.2°C) with brazed-plate evaporators; however, a typical design suction superheat value is 8 to 10°F (4.4 to 5.6°C).
In conclusion, brazed-plate evaporators offer efficient heat transfer in small packages. However, attaining the best performance and reliability throughout the life of these heat exchangers requires the proper specification, control and installation techniques. By following the guidelines offered, system designers can create systems that provide many years of trouble-free service.