2019 Breeze Jan-Feb

The Shade Tree Mechanic, Volume XXXVL Hot Oil! And Cold Beer! SERVICE

So why doesn’t my compressor run hot? I get this question a lot. I always ask,“What did you do to it?” “Nothin’, I didn’t do nothin’. It was like my old Ford Maverick goin’up a hill, she just stopped.” Well, that can happen. But I’m always suspicious. A few weeks ago, I got a call from Bob. Same story.“I didn’t do nothin’.”So, I stepped out. As we were walking back to the engine room, Bob started tellin me that he hadn’t done“nothin” Bob told me,“I didn’t do nothin’. After we installed the new compressors, the old ones just started runnin’hot.” “What! You added three compressors and then the old machines ran hot!? OK, let’s get something straight. If you add 1500 HP in new screw compressors, you’ve done something!” As the refrigerant temperature went up, the oil cooler couldn’t get enough liquid to cool the oil enough to keep the compressor running. So, some background. Bob’s system had seven 500 HP rotary screw compressors. They were gravity fed thermosyphon oil cooled. Bob’s contractor added three 500 HP rotary screws, also thermosyphon oil cooled. The newmachines were installed between the thermosyphon vessel and the first compres- sor. The thermosyphon vessel and the thermosyphon supply mains were not upsized when the newmachines were installed. So, what happened? The oil coolers were getting all the refriger-

ant they needed to cool the oil from the compressors. When the newmachines were added, we needed 42%more cooling (7 to 10 compressors) but we have the same amount of liquid. Unless the thermosyphon vessel and thermosyphon supply (and return) mains were oversized originally, there isn’t enough liquid. Bob’s systemwas sized for the original seven compressors with no additional capacity. So, with a shortage of liquid for the cooling, Bob’s systemwas setup for overheating. So, I had to ask,“Why didn’t you upsize the liquid supply to the oil coolers?” Bob’s response was,“The contractor didn’t mention it.” Well Bob didn’t ask. Shame on the contractor, shame on Bob! The best project is not always done at the best price. When we called the contractor, his initial response was to blame the project scope.“They never asked for it.” His second excuse was even better. “If I had included it in my bid, I would not have gotten the job.” Well, he wouldn’t get another job fromme. But Bob isn’t me. When the problem first came up, ……… in June …………. in Alabama, Bob called the contractor. The contractor had a solution. All he needed was a check. Well, Bob cut a check and the contractor came back to“Fix it.” So, the fix was to install hand expansion valves, on the thermosyphon supply inlet of the oil coolers. Never, ever, ever do that! The concept is pretty simple. We have a limited amount of liquid, so the best hope for cooling is to try to spread the limited liquid supply over the longest area. Rarely does it work. It’s like rationing the macaroni and cheese so everyone gets a little and the fat kid doesn’t eat it all. It might work for a

birthday party, but if you ration food every day, everyone just starves slowly. Your thermosyphon oil cooler is the same way. Rationing liquid will limit the ability of the oil cooler to cool the oil. All the machines run a little hot. So back to Bob. With his new expansion valves, Bob balanced his liquid. Each oil cooler got a percentage of the liquid it needed, so each cooler was cooling as much as possible, and each machine was running warm. But everything was still running. As the outside air temperature increases, the system discharge temperature increased, so the refrigerant liquid temperature also increased. As the refrigerant temperature increases the temperature difference between the ammonia liquid and the oil decreases. Heat transfer slows down, and capacity of the oil cooler drops off. The ability of the thermosyphon oil cooler to cool is very dependent upon two things - the amount of liquid refrigerant touching the surface area and the difference in tempera- ture between the oil and the refrigerant. So, Bob’s problem? As the refrigerant temperature went up, the oil cooler couldn’t get enough liquid to cool the oil enough to keep the compressor running. As soon as the Alabama summer went into full swing, the temperature got high, the lack of cooling shutdown the compressors. Why did the issue show up in the older compressors?Well, they were further away from the liquid supply and the feed balance wasn’t perfect. So’s, I tell Bob,“Let’s go get us a beer.” After round two, we begin to draw pictures. After round three, we began to do math. Ultimately, we decided we can’t invent liquid, we only got what the pipe can give us. So, here’s what we did.

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