There are several different types of Compact Brazed Heat Exchangers (BPHEs), depending on their material combinations, pressure ratings and functions. The standard material is stainless steel, vacuum-brazed with pure copper or nickel-based filler. The basic construction materials indicate the types of fluids that SWEP's BPHEs can be used with. Typical examples are synthetic or mineral oil, organic solvents, water (not seawater), glycol/water mixtures (e.g. water/ethylene glycol and water/propylene glycol) and refrigerants (e.g. HCFC). Please note that if ammonia is employed, BPHEs with nickel-based brazing material must be used.
The front plates of SWEP's BPHEs are marked with an arrow, either on an adhesive sticker or embossed in the cover plate. The purpose of this marker is to indicate the front of the BPHE and the location of the inner and outer circuits/channels. With the arrow pointing up, the left side (ports F1, F3) is the inner channel and the right side (ports F2, F4) is the outer channel. The outer circuit has a lower pressure drop because it contains one more channel.
The inner circuit consequently has a slightly higher pressure drop. Ports F1/F2/F3/F4 are situated on the front of the heat exchanger (see Figure 8.1). Ports P1/P2/P3/P4 are situated on the back. Note the order in which they appear.
In principle, the BPHE is constructed as a plate package (of corrugated channel plates) between the front and rear cover-plate packages. The cover plate packages consist of sealing plates, blind rings and cover plates (see Figure 8.2). 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 material. The design creates a heat exchanger consisting of two separate channels or circuits.
Sealing plates are used to seal off the space between the cover plate and the first and last channel plates. The number of cover plates varies, e.g. with the BPHE's type, size and pressure rating. Some BPHEs have a blind ring to seal off the space between the channel plate and the cover plate. In some BPHEs, the blind rings are integrated in the cover plate and first / last channel plates.
There are different types of BPHE product categories depending on their material combinations and design pressures (see Figure 8.3). They are defined as standard BPHEs, all-stainless BPHEs, Mo-steel BPHEs and highpressure BPHEs. The standard plate materials are stainless steel, S, of AISI 316 type, vacuum-brazed with a pure copper filler, C, or a nickel-based filler, N. 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 a higher content of molybdenum, M. BPHEs are available in standard pressure rating, S, or high pressure rating, H. The material and pressure denominations are shown below and in Table 8.1.
(X) is the plate material (S=stainless steel, M=Mo-steel)
(Y) is the braze material (C=copper, N=nickel alloy)
(Z) is the pressure rating (S=standard pressure, H=high pressure)
BPHE plates and channel types
Some BPHEs are available with different types of channel plates, i.e. with different herringbone patterns. The chevrons, i.e. the V-shaped pattern on the plates, can be obtuse (creating a high-theta plate) or acute (creating a low-theta plate) (see Figure 8.3). The thermal characteristics of the BPHE can be modified by mixing high- and low-theta plates. For example, it is possible to construct a BPHE with the same pressure drop on both sides despite different flow rates.
The fluids can pass through the heat exchanger in different ways. For parallel-flow BPHEs, there are two different flow configurations: co-current or counter-current (see Figure 8.4).
Design conditions and approvals
The standard pressure rating for SWEP BPHEs, i.e. maximum operating pressure, is 31 bar. SWEP's standard maximum operating temperature is 185°C for copper-brazed BPHEs, and 350°C for all-stainless BPHEs (nickel - based filler). However, because temperature and pressure are closely coupled, it may be possible to increase the pressure if the temperature is reduced. For details, please check the label (cf. Figure 8.6) and other technical documentation.
SWEP's BPHEs are approved by a number of independent bodies, e.g.:
- Canadian Standard Association (CSA)
- The High Pressure Gas Safety Institute of Japan (KHK)
- Underwriters Laboratories (UL) – USA
- Pressure Equipment Directive (PED) – Europe
SWEP also has design approvals, from e.g.: Lloyds Register (LR), Great Britain; Det Norske Veritas (DNV), Norway; American Bureau of Shipping (ABS), USA; Korean Register of Shipping (KR), Korea; Registro Italiano Navale (RINA), Italy.
Labeling system and operating conditions
All BPHEs carry an adhesive label (see Figure 8.6) with vital information about the unit, e.g. the type of heat exchanger, and SWEP's serial number. This indicates the basic BPHE model. The operating conditions state the maximum operating temperature and pressure as determined by the various approving organizations. The label also includes the serial number (see Figure 8.7). The engraved serial number provides information about where and when the BPHE was produced, etc.