Some of the same benefits of direct flashing of liquid into vapor at the intermediate pressure can be achieved by liquid subcooling. A popular method of subcooling, shown in Figure 3.5, immerses a pipe coil in the liquid of the intermediate-pressure vessel. Warm liquid from the condenser enters the heat exchanger coil and transfers heat to the lower-temperature liquid. Liquid subcooling is a form of flash-gas removal, because some liquid in the vessel vaporizes and is drawn off at the intermediate pressure.
Compared to direct flashing of Figure 3.2, the liquid subcooler has the advantage of maintaining the liquid at a high pressure. Therefore, the subcooled liquid can travel long distances and endure some drops in pressure without flashing into vapor. The liquid leaving a direct flash tank is saturated and flashes into vapor when the pressure drops due to friction or a rise in elevation. Also, since the liquid is cool, it will absorb heat from the warm ambient, which may also cause flashing. The disadvantage of the subcooler in comparison to the direct flash tank is that liquid cannot be cooled all the way down to the saturation temperature of the liquid because the heat exchanger must operate with a temperature difference between the leaving subcooled liquid and the intermediate-temperature liquid.
The selection of the length of tube of the immersed subcooler is not usually the result of a detailed heat-transfer calculation. Instead, the fabricator often inserts as much tube as convenient, and the system lives with the result. In certain cases it may be profitable to design the coil more carefully to balance the installed cost against the saving. The overall-heat-transfer coefficient, which is the U-value in the equation:
Thde U-value is a function of the boiling heat-transfer coefficient at the outside of the tube and the convection coefficient of the flowing liquid refrigerant inside the tube. Using some typical values of boiling heat-transfer coefficients suggested by Ayub1 for R-22 and ammonia, approximate U-values can be calculated, as shown in Table 3.2.
A guideline sometimes used by one designer2 is to install a heat-transfer area of immersed coil of 2.5 m2 for every 100 kW (1 ft2 per ton) of refrigeration capacity at the evaporator. A heat exchanger of this size conforms with the data in the table to provide a reasonable temperature drop of liquid.
Another class of heat exchangers for subcooling liquid is shown in Figure 3.6, which depicts a shell-and-tube heat exchanger in which the boiling liquid is regulated by a thermostatic (superheat-controlled) expansion valve. The heat exchanger in Figure 3.7 is of the thermosyphon type, which is provided with liquid from the flash tank and this liquid partially vaporizes in the vertical tubes of the exchanger and circulates by natural convection. Higher heat-transfer coefficients are usually possible with the external heat exchangers in comparison to the immersed type of Figure 3.5, and they are much more practical for a retrofit installation.