Plate Heat Exchanger As Vaporizer and Condenser
Plate heat exchangers (PHEs) have been installed in larger heat pump systems and refrigeration systems since the 1980s. Tech-nicians who deal with these systems need to know some specific details of how they work.
Part one of this article (The News, Nov. 20) covered PHE design, on-site assembly and service, leak indication, freezing risk, and resistance from vibration, pressure, and seismic occurrences. This week the article continues its discussions of the plate heat exchanger as vaporizer and condenser.
VAPORIZERParticularly as a vaporizer, the PHE presents the characteristics of a high degree of turbulence and high shear forces in the complex channel geometry, which lead to:
SUBMERGED VAPORIZATIONThermosiphon (natural) circulation is obtained when the pressure drop within the exchanger balances the external liquid head; no pump is needed to feed the liquid to be vaporized to the exchanger.
The PHE is well suited to this process as the liquid head in the knockout drum is approximately on the same level as the upper PHE connection (outlet). (See Table 4.)
REFRIGERANT VOLUMEThe total system volume can be kept low and the pipework made in small diameter due to the low refrigerant volume and low circulation ratio (high exit vapor fraction) in the PHE. The knockout drum can also be of relatively small size.
Submerged vaporization with low liquid-phase velocity in the inlet manifold means that a large number of plates can be installed in the frame. The largest vaporizers have about 300 refrigerant channels (600 plates), with a capacity of about 6 MW on R-12 and 8 MW on ammonia.
OILIn ammonia systems, small quantities of oil can accumulate at the inlet. Oil draining at the lowest point can be used, but is not mandatory. The oil will be entrained in the ammonia and emerge with the partially vaporized fluid, thanks to the high shear forces and the turbulent flow even at low velocities.
The oil content in CFC vaporizers, however, affects heat transfer. At normal oil concentrations of about 1% to 2%, maximum heat transfer is obtained at an exit vapor fraction of about 0.7. Small oil concentrations can have a positive effect on heat transfer; surface tension effects can explain this.
DX VAPORIZATIONPlate heat exchangers have so far been used at capacities up to about 500 kW using direct expansion (DX). It is primarily the brazed PHEs used at these small capacities.
Pure counter-current operation provides reliable superheating. The pressure drop can be kept low in order to minimize the temperature drop for the refrigerant, and to keep the specific volume at the outlet as low as possible.
Distribution of the CFCH medium (usually as a two-phase mixture from the expansion valve) to the PHE channels is good provided the number of channels is not excessive. With direct expansion, this is more important, since channel distribution is a function of the relation between channel and inlet-outlet pressure drops.
An extra restriction at the inlet permits the use of more channels while maintaining good distribution. A fixed throttling at the inlet can, however, lead to difficulties with control; the vaporizing temperature cannot then rise maximally on turndown. The electronically regulated expansion valve cannot easily set the temperature at the vaporizer inlet if a fixed restriction is used.
Oil in the channels is entrained and removed from the vaporizer. At small fixed turndown, small quantities of CFC(H)/oil may remain in the inlet manifold, which has a small volume.
Capacity regulation down to about 20% turndown can generally be permitted. The advantage is that the entire heat transfer surface is available for heat transfer at turndown. The vaporization temperature can then lie above design point.
As already noted, low loads can be obtained at small nominal velocities, but the shear forces and turbulence mean that oil will still be swept up and out of the vaporizer.
Choices of vaporizer type and design calculations are made in close cooperation with the designer of the refrigeration plant.
FREEZE RISKThe PHE can be designed close to the freezing limit. It is worthwhile repeating the significance of this:
The PHE does not completely freeze, and the medium continues to circulate. This means:
The brazed PHE is a somewhat more rigid construction than the twin-plate PHE, having less internal flexibility. It is therefore somewhat more sensitive to damage due to repeated freezing than is the twin-plate, and is designed with a somewhat higher minimum surface.
Freeze indication is most readily and quickly obtained via a determination of the pressure drop on the liquid side. An increase in pressure drop occurs instantaneously with the onset of freezing. In systems with varying flow rate, however, this method is evidently less suitable. A low-pressure “pressostat” with a fair margin and/or regulation via minimum liquid temperature from the vaporizer is required.
Freezing tendency can be minimized through a calculated minimum wall temperature, which lies somewhat above the freezing point. Submerged vaporization permits higher vaporization temperature than does total vaporization-superheating.
Pure co- or counter-current operation affects the minimum wall temperature. The freezing point will be influenced by the composition of the water. The presence of solid particles increases freezing tendency compared with that for pure water; chemical impurities often reduce the freezing point.
INDIRECT SYSTEMSWith the PHE as brine cooler and condenser, the CFC/HCFH system volumes can be kept at an absolute minimum. The brazed units, particularly, permit installation entirely within the framework of the machine, meaning that refrigerant need not exist outside the chilling unit.
In order to reduce the pumping cost for the viscous brine, the flow in the PHE can be kept low while maintaining turbulent flow. Stable control is thus achieved without discontinuities, which would result from the instabilities, which are the inevitable result of transition to laminar flow regimes.
The possibility of small temperature differences enable a reduction in vaporization temperatures, compared to DX systems, to be reasonably made. A return to submerged vaporization becomes once more of interest.
Indirect systems can thus involve advantages not only from the low refrigerant volume, but also from the possibility of stable regulation and relatively high vaporization temperature. This can also be employed to use lower brine concentrations, which in turn has a positive effect on heat transfer rates and pumping cost.
Stromblad is with Alfa-Laval Thermal, 5400 International Trade Drive, Richmond, VA 23231.
Publication date: 11/27/2000