What is probably the most popular of the gas-pumping arrangements uses a controlled-pressure receiver (CPR), one version of which is shown schematically in Fig. 8.26. A key concept is that the receiver no longer operates at essentially the condensing pressure, but is maintained at a lower pressure, the controlled pressure, with a saturation temperature of approximately 15°C (59°F). Liquid from the condenser flows through a float valve which allows only liquid to pass and also drops the pressure from the condensing pressure to that of the CPR. The desired pressure in the CPR is maintained by the pressure-regulating valve which vents to the low-pressure receiver. Since essentially saturated liquid enters the float valve, some of the liquid flashes to vapor, and this vapor passes through the pressure regulator to the low-pressure receiver.
Liquid from the CPR flows to the evaporators in a quantity that overfeeds all the coils, and the liquid/vapor from these evaporators returns to the low pressure receiver. The vapor is drawn out by the compressor, and the liquid
drains to the pumping vessel. During the draining process the three-way valve connects the pumping vessel to the low-pressure receiver, allowing gas to vent from the pumping vessel. Check valve Ckv A prevents higher-pressure gas from the CPR from flowing into the pumping vessel while draining of liquid is taking place.
After the pumping vessel has accumulated some liquid, the three-way valve shifts its status to allow high-pressure discharge gas to enter the pumping vessel, forcing liquid through Ckv A into the CPR. Check valve Ckv B prevents access of the pressure of the pumping vessel into the low-pressure receiver. When the transitions between pumping and draining are controlled by a timer, less liquid accumulates in the pumping vessel during draining when the refrigeration capacity and refrigerant flow rate through the evaporators are low. If the transitions are controlled by the liquid level in the pumping vessel, the cycle time shortens at high rates of liquid transfer.
The CPR recirculation system shown in Figure 8.26 serves a single-stage plant. The CPR concept is adaptable to two-stage systems as well, and a typical flow diagram is shown in Figure 8.27. The main vessels are the CPR, the flashtank/intercooler, the low-pressure receiver, and two pumping vessels. The CPR feeds the intermediate-temperature evaporators and the intercooler supplies the low-temperature evaporators. The system incorporates three pressure regulators with Regulator A feeding high-pressure vapor to the CPR if its pressure drops, and Regulator C venting vapor to the intercooler if the CPR pressure rises too high. Regulator B provides high enough pressure to the high pressure pumping vessel to force liquid into the CPR. The liquid level is not controlled in the intercooler, so the option is not available to bubble discharge vapor from the low-stage compressor through liquid for desuperheating.