ACHRNEWS

Btu Buddy 17: Examining A Cooling Tower Problem

August 21, 2004
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.

A manager of a group of theaters calls and tells the dispatcher that the air conditioning system is not working at a shopping mall that has six theaters. The manager explains that it was working last night, but is not working this morning.

Bob is sent to handle the job. He arrives and, since he has never been there before, the manager takes him to the equipment room. He finds a water-cooled chiller that looks to be supplying about 50 tons of air conditioning. He tells the manager that he wants to look around to see what the problem may be.

Bob looks over the chiller and discovers that the high-pressure control has the chiller off. He can tell because it has a red plunger that is sticking out of the control. He starts to push it in when Btu Buddy appears and says, "Why don't you look over the cooling tower before you reset it."

Bob looks at the cooling tower and all looks well. Water is going over the top and it is good clean-looking water. So he goes back into the equipment room and finds that the temperature of the water is cool. There are no working thermometers in the condenser water lines, but the pipe is cool.

Btu Buddy suggests that he take a temperature tester and place one lead at the top of the tower where the water comes out and one in the tower basin where he can check the temperature. The tower is just behind the equipment room on the ground. He also suggests that Bob put gauges on the compressor.

Bob gets everything set up and Btu Buddy says, "Remember the last water-cooled call and get ready to write down the performance information."

Condenser water outlet temperature: ___ degrees
Condenser water inlet temperature: ___ degrees
Temperature difference: ___ degrees

A. Condensing temperature (from head pressure): ___ degrees
B. Condenser water out temperature: ___ degrees
Approach temperature (A – B): ___ degrees

Bob pushes the reset button and the chiller starts up. It seems normal. The chilled water begins to lower.

Bob is looking at the gauges when the head pressure begins to slowly rise. He waits a few minutes and records the following:

Condenser water outlet temperature: 100 degrees
Condenser water inlet temperature: 95 degrees
Temperature difference: 5 degrees

A. Condensing temperature (from head pressure): 110 degrees
B. Condenser water out temperature: 100 degrees
Approach temperature (A – B): 10 degrees

About then, the chiller shuts down because of high head pressure.

Btu Buddy asks, "What do you know from looking at the readings that you took?"

Bob studies them a minute and says, "The leaving condenser water to the refrigerant condensing temperature looks (approach) like it should; the condenser is doing its job. The cooling tower water is too hot coming back from the tower basin. The fan on the cooling tower is running, so I would guess that the tower is not doing its job for some reason. I can't imagine why."

Btu Buddy says, "Let's go and take a look."

Bob inspects the tower and says, "I don't see anything wrong."

Figure 1. A typical induced-draft cooling tower. (All figures are from Refrigeration & Air Conditioning Technology, by William Whitman, William Johnson, and John Tomczyk, published by Delmar Publishers.)


Btu Buddy says, "Let's review how a tower works. The object is to allow water to evaporate at the correct rate in order to lower the temperature of the remaining water that returns to the condenser. The tower uses a pan in the top with calibrated holes in it to spread the water over the entire tower area (Figure 1). It has a fan to speed the rate of evaporation in a smaller space. The tower also has slats, called "fill material," that are supposed to spread the water out and give it more surface area for better evaporation (Figure 2). With that in mind, look the tower over very carefully and see what you can find."

Figure 2. This is an example of how the fill material spreads the water out for a better evaporation rate.
Bob looks the tower over for a few minutes and says, "The upper tower basin has a big hole in one end. It seems that too much water is flowing down that hole. This would cause it to flow past the fill material. Is that what I am looking for?"

Btu Buddy says, "Yes, that is the problem. This is a very old system and a rust hole has caused the water distribution to not flow over the tower fill like it should. This has been coming on for a long time and no one noticed it."

Bob asks, "Well, what can we do about it?"

Btu Buddy says, "Here are the options. We know that the tower must be replaced. It will take several weeks to get a replacement tower and the theater will lose a lot of money being shut down for that long. We could suggest to the owners that they convert this chiller to an air-cooled condenser that will be more reliable, but this would also take several weeks to get shipped to the job. Meanwhile, for the immediate future, we should patch this tower until a replacement can be located.

"How in the world can we patch a rusty mess like that tower basin?" asks Bob.

"Well, there are two options," says Btu Buddy. "You could get a welder over to make a temporary patch that would probably last the rest of the season, or you could make a plywood patch over a portion of the tower and drill some holes of the same size. That also would probably last the rest of the season."

Figure 3. This cooling tower has a large hole in the pan that allows too much water to flow in one spot. This will prevent the tower from reducing the water temperature and will cause high head pressure.


Bob asks, "Why don't we bring the manager out and show her what the problem is. She may want to talk to the owner about what to do."

The manager then comes out and looks at the tower hole, and asks Bob to let her call the owner.

She calls the owner on her cell phone and asks Bob to explain the options.

When Bob is through, she talks to the owner again and verifies what Bob has explained to him.

The owner tells the manager to have Bob make a plywood temporary fix and get a price on both the air-cooled and water-cooled options. He explains to her that he had had similar problems before and changed out to air-cooled and it had been a good experience.

Bob gets to work on the plywood patch and installs it by just fastening it to the tower basin with long sheet metal screws.

He then starts the chiller up again and, after it has run for about 30 minutes, he records the following:

Condenser water outlet temperature: 96 degrees
Condenser water inlet temperature: 86 degrees
Temperature difference: 10 degrees

A. Condensing temperature (from head pressure): 105 degrees
B. Condenser water out temperature: 96 degrees
Approach temperature (A – B): 9 degrees

Bob says, "Boy, that was a quick recovery for the theater. They really appreciated being able to have cooling so soon."

Btu Buddy says, "You have made another customer happy with your service. Your company is really beginning to appreciate your worth. I understand you got a raise. Did it help?'

"Yes," said Bob, "the company owner called me in and said that he had had several calls from customers that were satisfied with how my work was handled. They said that I had kept them informed as to progress and that they felt they were receiving quality service for their money. The owner was very pleased."

Bill Johnson has been active in the HVACR industry since the 1950s. He graduated in gas fuel technology and refrigeration from the Southern Technical Institute, a branch of Georgia Tech (now known as Southern Polytechnic Institute). He taught HVAC classes at Coosa Valley Vocational & Technical Institute for four years. He moved on to become service manager for Layne Trane, Charlotte, N.C. He taught for 15 years at Central Piedmont Community College, part of this time as program director. He had his own business for five years doing installation and service work. Now retired, he is the author of Practical Heating Technology and Practical Cooling Technology, and continues as a co-author of Refrigeration & Air Conditioning Technology, 5th Edition, all published by Delmar Publishers. For more information, he can be reached at 704-553-0087, 704-643-3928 (fax), or bmj@myexcel.com.

Publication date: 08/23/2004