RETA Breeze_MayJune_Final_2025

needed. A surge-type receiver would be smaller for a given system than the pass-through high-pressure receiver. The high-pressure receiver operates at system discharge pressure. As the dis charge pressure increases, the pressure within the high-pressure receiver increases, and the flow rate increases. This varying feed rate required early operators to adjust the liquid feed valves (expansion valves) that were in the engine room each time the pressure changed. This was labor intensive. It’s important to remember that at the time, this is how it worked. No one thought this odd or unusual. When the pressure was highest, the amount of liquid passing through the expansion valve was highest. As tempera ture always follows pressure, the tempera ture of the refrigerant was also high. The operator would adjust the valve, throt tling it further and increasing the pressure differential across the valve. As we know, as the refrigerant passes through the expansion device, flash gas is generated. The larger the pressure drop, the more flash gas is generated. Flash gas passes through the evaporator without doing any useful work. It is a necessary loss of efficiency. The flash gas generated is then drawn back to the compressor. So, when using a high-pressure receiver, the liquid pressure varies, leading to varying amounts of flash gas being generated, and during high pressure

conditions a decrease in evaporator efficiency. This also caused varying feed rates as the pressure differential across the expansion device grew as high-stage discharge pressures increased. Please note, in modern systems the high- pressure receiver may not feed liquid directly to an evaporator. In these cases, this inefficiency does not occur. CONTROLLED PRESSURE RECEIVER (CPR) As systems began to grow, the need to stabilize feed rates and minimize flash gas inefficiencies at the evaporator began to drive innovation. The concept of the controlled pressure receiver was born. The feed rate issue could be controlled by controlling the pressure in the CPR. By stabilizing (controlling) the pressure in the CPR, we could provide a feed rate to the expansion device that did not vary with high-stage discharge pressure. To control the pressure within the CPR two things were needed: 1. A way to reduce the pressure if it got too high; and 2. A way to increase the pressure if it got too low. To increase the pressure, an outlet regulator is used to provide high pressure vapor from the compressor discharge (bypass ing the condenser). This regulator assures

that a pressure above the minimum is maintained in the CPR. To decrease the pressure, an inlet pressure regulator is used to draw vapor out of the controlled pressure receiver when the pressure increases above acceptable levels (thus lowering vessel pressure). These two regulators work in conjunction with each other to maintain a set pressure range in the controlled pressure receiver. The CPR pressure can vary as much as 8–10 PSIG. But this is a much more stable pressure than was able to be maintained in a high-pressure receiver. An added benefit of a CPR is that the flash gas generated by lowering the pressure of the refrigerant liquid is drawn into the compressor without passing through the evaporator. This allows for more heat transfer at the evaporator. This was very useful in increasing capacity without changing the evaporators. Unfortunately, there is an additional factor. As designers lowered the pressure in the controlled pressure receiver, making the evaporators more efficient, the feed rate was lowering as well. There became a practical limit. The colder liquid became

offset by the low-liquid flow due to low-pressure differential. We were

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