2021 RETA Breeze Jan-Feb

QA & “ ” student experience was two years up to above 35 years in the industry. See what I mean about the opportunity? People with varying experiences and tenure getting together to prepare to take an important examination would not have had the

opportunity to learn from one-another if the association did not have a chapter that chose to offer the review session. The students were great about asking meaningful questions during the class. I am going to share a question that came after the last session. A few times during the course I described myself as an old guy who likes to talk about refrigeration. My student wrote: “Going to refer the subject after a quote (old guy reference) from your last session. So I remember answering a question about defrost etc…. The answer was 105 PSIG (hot gas feed into coil) to 75PSIG (back pressure regulator) So at the plant I operate in we keep hot gas infeed at 110 and 60PSI back pressure regulator. Would you be able to tell me if this is wrong? And if so why? Reason I ask is because of the amount of operators we have. They tend to clash when it comes to that, “their settings” One operator of 20 years wants a 120/95 and the other wants a 110/60 and me personally tend to stick around 90/70. But is the 105/75 optimal?”

Brian,

(thermal conductivity of aluminum compared to steel) and the usual thickness of the wall.

Your question gave me pause to think about why certain settings are decided on. Looking back at what we covered over the four weeks of January I can touch on a few topics that we can investigate: – we need to know what the evaporators are made of, especially those that had higher operating pressure values. If the units were steel tubes and hot-dip galvanized fins they would probably operate at a higher pressure because of the thermal conductivity and thickness of the walls of the heat transfer surface. Some older evaporators were made of schedule 40 pipe which is quite thick in heat transfer terms. - It was said that the greater the temperature differential, the faster heat energy would flow. The higher the pressure, the more dense (lbs per minute) would be delivered into the coil. - The time required for injection of hot gas would vary from an aluminum surface

- The defrost relief characteristics would need to be looked at. In the first several minutes of injection there is a significant amount of condensate formed. As the condensate is relieved back to the engine room in the low pressure suction or mid-pressure suction there is a bit of flash gas formed in that return line. As less and less gas is condensed and if the defrost system is a simple regulator (A4A) function set to maintain a pressure, regardless of the refrigerant passing through it being liquid or vapor, it can be that the hp gas injected basically drops a bit in pressure and escapes as an expanding high volume vapor into the lower pressure suction line. This sends a false loading to the engine room that the compressors have to deal with - The settings that the controls are set to typically have to work every time, regardless of humidity and ambient conditions. So, when a set of parameters

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