In operation, the two-phase liquid and vapor mixture is returned from the evaporator to a central accumulator vessel and separated; then, the dry vapor is directed to the compressor. Liquid refrigerant in the accumulator is drained by gravity to a dump trap and intermittently transferred to a controlled-pressure receiver (CPR). No liquid level is maintained in the suction accumulator, thus providing more liquid accumulation space when needed. From the controlled-pressure receiver, partially subcooled liquid refrigerant is fed to the evaporator at the required recirculating rate for return to the suction accumulator.
The advantage of a gas-driven recirculating system is it employs one central vessel (an accumulator) for all evaporators on a common suction system. Also, because no pumps are used, maintenance costs are reduced. This configuration provides:
- Liquid refrigerant slop-over protection for the compressor.
- Increased utilization of evaporator's internal heat transfer surface with an optimum liquid feed to evaporator to load demand.
Several variations on the gas-driven ammonia system exist.
Singe-Stage SystemA single-stage, or high-side, system typically is composed of a high-side liquid-level control, controlled-pressure receiver, suction accumulator and liquid return unit (figure 1). The high-side control maintains a liquid seal in the condenser drain line, allowing only the liquid to flow to the controlled-pressure receiver. In effect, the main receiver is converted into a controlled-pressure receiver because excess flash gas is vented by a back pressure regulator. From this vessel, partially subcooled liquid at controlled-pressure receiver pressure is fed to the evaporators through conventional hand expansion valves set to overfeed. The unevaporated liquid exits to the suction accumulator from the evaporator along with the vapor produced by the heat load. In a properly designed suction accumulator, the liquid is separated from the vapor, and the liquid return unit transfers the excess liquid back to the controlled-pressure receiver.
No compressor penalty is incurred due to dropping the liquid from the condenser down to an intermediate pressure receiver. All liquid must be expanded down to suction pressure eventually to do its work. The controlled-pressure receiver facilitates transfer of the overfed liquid back to its source by using higher pressure gas. Lower temperature liquid returning from the suction accumulator mixes with liquid that has flashed down to controlled-pressure receiver pressure, thus subcooling the mixture in the controlled-pressure receiver.
All liquid-return units consist of a dump trap, three-way valve, gravity-inlet check valve, piston-type outlet check valve and a float switch to initiate transfer of the overfed liquid back to its source. Net positive suction head (NPSH), an essential for liquid pumps, is not a concern in these systems. Work is required to move the liquid, and instead of mechanical pumps, some work is done each time the dump trap depressurizes after transfer and the volume of gas is recompressed.
Other SystemsLow-Stage System. In a low-stage system, liquid feed to the low temperature evaporators takes place from a specially designed intermediate pressure vessel (figure 2). This vessel also may act as an intercooler or suction accumulator with its own liquid return unit for recirculation on the high side of the system. The lower section of this vessel acts as a reservoir of subcooled liquid that is fed to the evaporators. The overfeed liquid is returned to this section. A low-side control supplies liquid to maintain the level in the reservoir. This liquid may come from a high-pressure receiver or a controlled-pressure receiver.
Typically, a downstream regulator is utilized in the gas supply line to the three-way valve to minimize the pressure required to move the excess liquid back to the intermediate pressure reservoir.
Totally Subcooled Liquid Feed SystemThe feed of exceptionally cold liquid specified for certain low temperature evaporators is accomplished with a totally subcooled liquid-feed system (figure 3). This low-stage system feeds the liquid makeup, generally from an intermediate pressure vessel, into the low temperature suction accumulator even though the level control is on the reservoir. The makeup liquid is flashed to the low temperature, and all the makeup and overfeed liquid is transferred to the reservoir by the liquid-return unit. The liquid-return unit employed in this system typically is one size larger than that used for a standard low-stage system.
A simple design translates to simpler operation. Recirculating systems simplify operation by eliminating the need for a pump and control panel wiring. Initial cost and maintenance requirements also are reduced. Yet, the system maintains a comparable rate of brake horsepower per tons of refrigeration while optimizing liquid feed to evaporators under varying loads.