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.
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.)
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.
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.
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.
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.
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