Service & Maintenance / Extra Edition

Btu Buddy 50: Reciprocating Chiller With Fouled Condenser Tubes

May 21, 2007
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Bob is a service technician who is well trained and nationally certified. However, he sometimes suffers from the same confusion that all technicians occasionally do - the facts that he gathers may or may not point to the obvious cause of the problem or the best solution. But Bob has something that no one else has. He recalls his long-time HVACR mentor and imagines him accompanying him as “Btu Buddy,” someone who reminds him to take time to stop and think before rushing to judgment, helping keep him on the right track, even with facts that are confusing.

Figure 1. A high-pressure control that is manual reset. (Figures are from Refrigeration & Air Conditioning Technology, 5th Edition, by William Whitman, William Johnson, and John Tomczyk, published by Thomson Delmar Learning.)

The weather had been mild and then a hot spell moved in, the first one of the year. The dispatcher called Bob and gave him a service call at a motel. The motel had been operating its 100-ton chiller for about a month, but the weather had been mild. When the first 90°F day came, the chiller just shut down.

Bob arrived at the motel and the manager showed him to the equipment room where the chiller was located. It was not running. Bob began to look around and opened the control panel on the chiller and noticed that the manual reset high-pressure control button was out, indicating that high-pressure control had shut the chiller down (Figure 1). Bob was reaching to reset the high-pressure control when Btu Buddy appeared and said, “Why don’t you check around a little bit before you reset the control. It would also be a good idea to fasten your gauges to the compressor so you could monitor the pressures when it starts up.”

Bob then said, “Well, that is just like me, just push a button without thinking. You are right. I should think more before I act.”

Bob installed gauges on the compressor on the high and low side gauge ports. Then Btu Buddy said, “It is a good idea to check the water tower before you reset the control to see how dirty it is and to see if there is a proper water flow over the tower.”

Bob went to the tower and noticed that there was plenty of water flowing over the tower, but the water was dirty and there was a lot of green algae scum in the tower basin. He then told Btu Buddy, “There is no need to try to start this chiller until the tower is cleaned up.”

Figure 2. The cooling tower water is not taking the heat out of the condenser because the tubes are dirty. (Click on the image for an enlarged view.)

Bob called the office and asked for a helper to come out and help him clean the tower. The helper arrived with rubber boots, face masks, and chemicals for cleaning the tower. The strainer to the pump was also cleaned. A lot of caution was used cleaning the tower because the dirt in the tower can be unhealthy. It took two hours to get the tower and basin cleaned up. It was refilled with fresh water and they started the water circulating again. They were ready to start the chiller.

Bob then reset the high-pressure control and the chiller started. After running for about 15 minutes, the head pressure began to rise. The water from the cooling tower was returning to the condenser at 83° and the water from the condenser back to the tower was 88° (Figure 2). Bob then scratched his head. Btu Buddy asked, “What is the matter? What do you see?”

Bob said, “I thought this chiller should add 10° to the water and it is only adding 5°. The chiller is operating at full load; the head pressure is 229 psig (R-22) and climbing. I am not sure what I see.”

Btu Buddy said, “Feel the liquid line going to the expansion valve and tell me what you feel.”

Bob held the line in his hand and said, “It is really hot, with 83° entering water. You would think it would be cool.”

Figure 3. This technician is brushing the tubes in a condenser with a machine that rotates a nylon brush while flushing the tube with water. (Click on the image for an enlarged view.)

Btu Buddy then said, “It is obvious that the cooling tower water is not taking the heat out of the refrigerant. What could that mean?”

Bob then said, “I get it. There is plenty of capacity to remove the heat, but it is not being removed. The condenser tubes must be dirty, like the tower basin was.”

“Correct,” said Btu Buddy. “The only thing to do is shut it down and clean the tubes.”

Bob shut the chiller down and removed the head from the condenser. The tubes were really dirty. He again called the office and they sent a helper with a tube cleaning machine that turns a nylon brush and injects water at the same time (Figure 3). They cleaned the tubes and flushed out the condenser shell with fresh water and replaced the head on the condenser. It was ready to start up again.

Bob started the chiller while watching the gauges. The entering water to the cooling tower quickly rose to 85° and the leaving water went to 95°. The chiller was operating really well as the outside temperature was now 95° (Figure 4).

Figure 4. This cooling tower and condenser are working together to remove the heat from the system. Notice that the tower temperature difference is 10°F. (Click on the image for an enlarged view.)

“Well,” Bob said, “every job seems to have a surprise. You never get bored with all of the surprises.”

Btu Buddy then said, “You went about this with a systematic approach and you were able to find the problem. Now it is time to recommend to the owner that they get a water treatment program going as well as a service contract to keep the machine operating at peak efficiency. This machine was costing them extra money operating at high head pressures for such a long time. They were paying extra for poor efficiency.”

Bob gave the motel management a proposal and they decided that it was a good idea. Bob had sold a service contract that was worth something to his company, and a good value to the customer.

As they were riding away, Btu Buddy said, “Well, Bob, your service call was a good value to the customer and you added business to your company volume. It is hard to beat that for a good day’s work. Everyone is pleased with that. Every technician should try for this kind of day.”

Bob said, “Thanks for the advice along the way.”

Publication date: 05/21/2007

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