6.8 BPHE evaporators

Since the copper brazed plate heat exchanger became commercially available in the early 1980's, new applications have been found every year. The refrigeration industry was one business area that very soon realized and accepted the advantages of this hermetic, pressure resistant, compact and highly efficient​ heat exchanger technology. SWEP has a very wide range of dedicated evaporators for single or dual refrigerant circuits.

All SWEP evaporator models have the refrigerant inlet at the lower left port, the F3 position (see Figure 6.26). The inlet can also be on the back, corresponding to the P3 position. The refrigerant outlet on all BPHE evaporators is the top left port, the F1/P1 position.

Each BPHE has a specific port size, i.e. the diameter of the holes cut in the plates. To connect the piping to the heat exchanger, a connection is brazed on during the manufacturing process. This connection can have the same nozzle diameter as the port hole, but may also be different to achieve an appropriate mixture of gas and liquid and thereby a better performing evaporator.

Higher demands on efficiency and compactness for air conditioning and refrigeration systems have improved the market penetration of BPHEs as evaporators and condensers for increasingly large capacities. Compactness, a small refrigerant hold-up volume and true counter-current flow make BPHEs the favored choice for many system builders.

With increasing system size, it is common to divide the total heat load between two independent refrigerant circuits. This increases the system safety and flexibility compared with using only one refrigerant circuit. Furthermore, most refrigerant systems are dimensioned to handle a nominal heat load higher than the normal cooling demand. The system consequently operates most of the time at part load with one or more compressors inactive. SWEP can offer two dual circuit technology BPHEs: the True Dual and the Back-to-Back dual. It is also possible to arrange two single circuit BPHEs in parallel to obtain a dual refrigerant circuit system.

Single circuit

The basic BPHE evaporator has one refrigerant and one cooling fluid side. The evaporating refrigerant flow enters at the lower left of the BPHE, through port F3 or P3 (see Figure 6.27). The distribution device is located in this port for the single circuit dedicated evaporators. The evaporating flow should normally be upwards because of the lower density of the refrigerant gas formed. Inverting the evaporator is therefore not recommended without contacting SWEP. For a counter-current flow arrangement, the secondary fluid enters at the top right connection, F2 or P2, and exits from F4 or P4, flowing downwards. The arrangement should be reversed for co-current flow.

The secondary side always has one more flow channel than the refrigerant side. The first and last channels in a BPHE therefore always contain the secondary fluid, surrounding the refrigerant channels (see Figure 6.28). This has two benefits: firstly, it ensures stable evaporation because all refrigerant channels receive the same heat transfer from the secondary side; secondly, it offers some insulation because it shields the BPHE cover from the cold refrigerant.

True Dual

The SWEP True Dual technology BPHEs (see Figure 6.29) are equipped with two independent refrigerant circuits combined with a common secondary fluid circuit. The patented plate technology assures fully counter-current flow and full symmetry between the refrigerant circuits. The True Dual models are available with or without the SWEP distribution device, and for evaporator or condenser duty. The channels are arranged in such a way that each refrigerant channel (R1 and R2) of the two circuits is surrounded by secondary fluid channels (W). Thus, all secondary fluid channels are always in contact with at least one active refrigerant channel.

Capacity control: A True Dual BPHE running with both circuits active operates no differently from a single circuit evaporator. Every water circuit is in contact with two operational refrigerant circuits (see Figure 6.30).

With one refrigerant circuit closed (half-load operation), all channels with secondary fluid will still be in contact with one active refrigerant channel. Thus, the temperature of the secondary fluid channels will be the same, and its leaving temperature will be stable and homogeneous, just as for full load. Because the secondary fluid channels surround the active refrigerant channels, the evaporating process remains stable. The True Dual channel arrangement therefore maintains high-efficiency evaporation even at half load, completely eliminating any additional freezing risk.

Combining the True Dual technology with tandem compressors (or variable speed compressors) is advantageous. In this arrangement, where each refrigerant circuit operates at half load (see Figure 6.31), 100% of the surface area is utilized for heat exchange. This capacity management method gives a higher thermal efficiency than operating one circuit at full load and closing down the other. As always, consideration should be given to ensuring oil return if the outlet vapor velocity becomes low.

The True Dual technology is based on alternating secondary fluid and refrigerant channels, so it is not possible to build an asymmetric True Dual with different numbers of refrigerant channels on each circuit; it must be a fully symmetric BPHE. To meet asymmetric capacity demands on the refrigerant circuits, the True Dual models must be designed with respect to the largest capacity, resulting in oversurfacing for the smallcapacity side.

System Design: The True Dual units have centered secondary fluid connections and refrigerant connections on either side (see Figure 6.32), which provides flexibility in the arrangement of the connections. The two refrigerant circuits can be located on either the front or the back, depend-ing on the customer's demands. The centrally positioned water connections can be positioned on the same or opposite sides, facilitating mounting inside the refrigeration system.

The True Dual refrigerant system is often characterized by extreme compactness, small footprint and uncomplicated piping.

The True Dual symmetric channel arrangement assures an even distribution of secondary fluid inside the unit. It saves the system builder the trouble of arranging the pipes for secondary fluid symmetrically, as is necessary for systems with two single circuit BPHEs in parallel. It is very common to mount the True Dual BPHEs directly on the side of the cabinet, with secondary fluid connections facing outwards and refrigerant connections inwards. This facilitates the assembly of the refrigerant circuits and further minimizes piping for the secondary fluid circuit. As a result, the True Dual systems (an example is shown in Figure 6.33) have a lower assembly time and thus a higher production throughput. This is very important, especially for original equipment manufacturers (OEMs) with large production volumes.

Back-to-Back Dual

A Back-to-Back dual circuit BPHE (see Figure 6.34) is based on two single circuit BPHEs mounted in parallel, with a common water circuit divided between the two plate packs. The refrigerant circuits work independently and affect only the water channels in their respective plate packs. SWEP Back-to-Back duals are available with or without a distribution system.

Capacity control: For full load with both refrigerant circuits active, a Back-to-Back dual operates in the same way as a True Dual or single circuit BPHE (see Figure 6.35). A Back-to-Back BPHE operates at nominal load at high efficiency, with all secondary fluid channels in contact with refrigerant channels.

If one refrigerant circuit is closed, the secondary fluid channels in the inactivated plate pack will not be in contact with any refrigerant. The entering and leaving secondary fluid temperatures for this plate pack will therefore be the same, and the final temperature will be an average of the two secondary fluid streams leaving the BPHE.

For a Back-to-Back evaporator with a fixed temperature for the leaving secondary fluid, the active side has to reach a much lower secondary fluid temperature to achieve an acceptable average leaving temperature for the secondary fluid. This will reduce the system performance due to the resulting drop in evaporation temperature, which is very likely to be below 0°C. This also dramatically increases the risk of freezing in those channels. For a Back-to-Back heat exchanger operating as a condenser, the decrease in system performance will still be visible but not as apparent as for the evaporator, because the high-pressure side has less influence on the system performance. There will of course be no additional risk of freezing on the condensing side.

Combining a Back-to-back dual with two sets of tandem compressors increases the performance and stability of operation at half load. All the heat surface area is utilized, and the leaving secondary fluid temperatures from both plate packs will be the same (see Figure 6.36). Precautions must still be taken to maintain the necessary channel and port velocities. However, trying to operate the Back-to-Back dual with one refrigerant circuit completely deactivated will again result in poor thermal performance and increased freezing risk, especially if the water flow is decreased to meet the part-load capacity. The reduced turbulence of the secondary fluid will combine with the decreased evaporation temperature, further increasing the freezing risk.

System Design: The Back-to-Back is less flexible than the True Dual in the arrangement of the water and refrigerant connections. Because the design is based on two parallel BPHEs, brazed back-to-back, the refrigerant connections are always located on opposite sides of the unit (see Figure 6.37). The water connections will therefore always be adjacent to a refrigerant circuit, and more pipe routing is necessary to connect the refrigerant circuits to the compressors and the water circuit to the outside of the cabinet.

Nevertheless, the central water circuit will distribute the water evenly inside the Back-to-Back BPHE, and no additional flow regulation is necessary to ensure an even flow over the two plate packs. Consequently, it is not possible to close one water circuit when operating at part load. The two refrigerant circuits of a Back-to-Back BPHE are built with separate plate packs, and thus it is possible to design the unit for asymmetric chiller capacities. A higher number of plates in one plate pack will give proportionately more refrigerant and water channels.

Single circuits in parallel

SWEP has a wide range of single circuit BPHEs for refrigerant applications. The B-models are the standard type, while the dedicated evaporators are equipped with the SWEP distribution device for evaporation duty. To take advantage of a dual refrigerant circuit system with single BPHEs, two units can be arranged in parallel with the secondary fluid circuit divided between the two BPHEs.

Capacity Control: At full load, two single circuit BPHEs in parallel function as a Back-to-Back dual (see Figure 6.38). This arrangement requires that the water flow be divided proportionately before entering the BPHEs. A high degree of symmetry in water piping is therefore necessary.

Deactivating one refrigerant circuit will leave the water in the inactive BPHE without any heat exchange, similar to the operation of a Back-to-Back dual. The entering and leaving secondary fluid temperatures will be the same in the inactive BPHE, and the final temperature will be the average of the two combined secondary fluid flows. The performance of the active refrigerant circuit will decline due to the consequent reduction of evaporation temperature. Such operating conditions might lead to very low evaporation temperature and thus a increased risk of freezing.

By installing a flow control valve it is possible to completely close the water flow through the inactive BPHE, thus maintaining high performance even at half load (see Figure 6.39). This arrangement requires a more complex pipe arrangement. In addition, the total water flow must be halved, because the pressure drop through the single active BPHE will otherwise be squared.

As with a Back-to-Back dual, it is possible to install two parallel BPHEs with a variable compressor system (see Figure 6.40). All the heat surface area is then utilized even at half load, and the leaving secondary fluid temperatures from both plate packs will be the same. However, precautions must be taken to maintain the necessary channel and port velocities.

System Design: Installing two BPHEs in parallel (see Figure 6.41) to obtain a dual refrigerant circuit requires the most complicated piping of the methods discussed in this chapter. Although the refrigerant circuits can be assumed to work independently, the water circuit must be divided proportionately between the two units. A very high degree of symmetry in the piping is therefore required. Additionally, in most cases a flow control valve has to be fitted to adjust the water flow. Due to the relatively complex piping work involved, which leads to longer assembly times in the factory, and the extra component costs, the popularity of this arrangement has declined in favor of the True Dual technology. The system configuration for single circuits in parallel is also less compact compared with both Back-to-Back and True Dual systems.

Every BPHE can be designed independently with refrigerant and water connections on the same or opposite sides. It is possible to have two differently sized BPHEs connected in parallel for asymmetric heat loads, but precautions must be taken to ensure that the water flow is proportionately divided between the units. Regulating valves are therefore often necessary to control the flow.

Single BPHE with two compressors

A special arrangement is to use a single circuit BPHE and to connect two or more compressors to the same refrigerant channel. This removes the safety aspect of having two separate refrigerant circuits. However, the advantages of such an installation are that the BPHE always uses 100% of the heat transfer area during operation and that the piping for the secondary fluid is less complex than for two BPHEs mounted in parallel.

Capacity Control: During full-load operation with both compressors (C1 and C2) running, the BPHE will operate as a normal single circuit unit, assuming good symmetry between the two refrigerant streams.

Deactivating one refrigerant circuit still leaves all refrigerant channels active (see Figure 6.42). The water temperature is homogeneous, and no extra freezing risk is induced. The relatively large oversurface benefits the system performance due to the increased evaporation temperature. However, it also induces a potential risk of oil retention if the unit is too large to allow the single wactive refrigerant flow to carry the oil.

System Design: A typical application for the arrangement with one single BPHE and two compressors is the use of tandem coupled scroll compressors, where the two refrigerant circuits are already combined before the evaporator (see Figure 6.43). Tandem compressors are currently available up to approximately 100 kW, but the capacity will grow with the increasing size of scroll compressors and the introduction of triplet/trio compressors.

The piping for such a system is fairly uncomplicated, but precautions must be taken with the oil return after the evaporator when operating at half load. Tandem compressor sets are factory assembled. They can operate in parallel, and have a piping design that avoids problems with oil distribution between the two units.

Because there is only one expansion valve for the two refrigerant circuits, it must be able to operate over the full capacity span of both circuits. This in turn imposes very high demands on the expansion device, which has to be able to cope with changing mass flows and evaporation temperatures. It is often necessary to use nonstandard thermal expansion valves or electronic expansion valves. The high cost of these special expansion devices will partly or completely offset the capital saving of using only one evaporator. Taking into consideration the lack of two independently operating refrigerant circuits, such as for a dual system, it is understandable that this arrangement is uncommon.

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