Equations 9.1 and 9.2 as well as charts such as the one in Fig. 9.2 predict pressure drops due to friction in horizontal pipes. When a pipe carrying liquid slopes upward in the direction of flow, an additional pressure drop results. When sloping downward, the liquid head contributes a pressure increase in the direction of flow.
The liquid line from the condenser or receiver to an expansion valve or level control valve is one location where an increase in elevation of the pipe can cause trouble.
A critical parameter in evaluating effects of rises in liquid lines is the drop in saturation temperature. Table 9.5 shows this parameter for several refrigerants and indicates that, of the four refrigerants shown, R-134a is the most sensitive and ammonia the least sensitive to rises in the liquid line. The physical arrangement of the plant usually dictates the need for a rise in the liquid line, and the only responses open to the designer or contractor if a flashing problem is anticipated is to install a heat exchanger to subcool the liquid or to elevate the pressure of the liquid with a pump.
Example 9.4. The liquid line, shown in Fig. 9.4, in an R-507 system rises 8 m (26.2 ft). At point 1 before the rise, the temperature is 30°C (86°F) and the pressure is 1525 kPa (221 psia). What is the temperature and the pressure at point 2?
Solution. Liquid R-507 at point 1 is subcooled as indicated by the fact that the pressure is higher than the saturation pressure at 30°C (86°F) which is 1465 kPa (212.5 psia). At 30°C (86°F) the density of liquid is 1022 kg/m3 (63.8 lb/ft3). In rising 8 m (26.2 ft) the pressure drop due to the loss in liquid head is:
The pressure at point 2 has dropped slightly below the saturation pressure at 30°C (86°F) which is 1465.4 kPa (212.5 psia), so some liquid will flash into vapor and the temperature will drop to the saturation temperature at this pressure. Vapor in the liquid entering an expansion valve may severely restrict the mass flow rate that the valve can pass.