ACHRNEWS

Btu Buddy 54: A Cooling Tower Starved for Water

September 24, 2007

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.

Bob received a call from the dispatcher to go to an office building that has a 100-ton system that is water-cooled. The compressor is in an equipment room in the basement and the cooling tower is on the roof. The building manager said the building temperature was rising. The outdoor temperature was 95°F.

Bob arrived and talked to the manager to see what he could learn. The manager told Bob that the system had run all summer and there had been no problem, but the compressor was now shutting off because of high head pressure and he has been resetting the manual reset control, several times each day. Now the building temperature was rising so he called a service company. This was the first time Bob’s company has been called out to this job.

Figure 1. Manual reset high pressure control. The manual reset feature calls attention to the problem. (Figures are from Refrigeration & Air Conditioning Technology, 5th Edition, by William Whitman, William Johnson, and John Tomczyk, published by Thomson Delmar Learning.)

Bob and the manager went to the basement equipment room. The compressor was off again and the button on the high pressure control was out, signaling that the high pressure control had tripped (Figure 1). The manager reached to reset the control and Bob said, “Let me look around for a minute before we reset it.” The manager left the job to Bob.

Bob looked around the equipment room. The chilled water pump was running. The leaving chilled water was 75°F. No wonder the building is hot, he thought. The leaving chilled water should be about 45°.

Bob moved to the condenser water pump and it was running. The water coming from the tower was 85°, which was about right, so Bob reset the high pressure control and the compressor started running. The compressor started to load up towards full load and Bob thought things seemed good so he decided to just observe for a few minutes.

When the system had run for about 15 minutes, the compressor tone began to change. The compressor sounded like it was in a strain. The head pressure normally would run about 210 psig for an R-22 system, water-cooled, on a day such as this. The head pressure went up to 250 psig and was rising. The condensing temperature should be 105°, corresponding to 211 psig and would be determined by the entering water temperature to the condenser. The entering water temperature was 85°, so Bob was scratching his head and wondering what to do when Btu Buddy appeared and asked, “What’s up, Bob?”

Bob explained what he knew about what was going on and said, “All looks normal, except the head pressure is rising.”

Btu Buddy asked, “What is the leaving water temperature from the condenser?”

Figure 2. This diagram shows the relationship of the cooling tower to the compressor and condenser when things are operating normally.

Bob looked and said, “It is 105°. It should be 95°. The condenser is taking a lot of heat out of the refrigerant. It seems like there is too much heat for the cooling water.”

Btu Buddy then asked, “What should the temperature rise be across the condenser? What is the temperature rise across this condenser? What would happen if there was plenty of heat, but not enough water?”

Bob answered, “The temperature rise should be about 10° (Figure 2). The temperature rise across this condenser is 20°. If there is not enough water flow, there would be an increase in temperature rise. I think we don’t have enough water flow.”

Btu Buddy then said, “Let’s go to the roof and look around.”

They went to the roof and Bob said, “Boy, look at the steam rising off of the tower. That water is hot.”

Bob looked into the tower basin and said, “The water level is low. The tower must be evaporating more water than is being made up. The water flow from the float valve seems low (Figure 3). There is a water hose. I will use it to add water.”

Figure 3. The cooling tower and its makeup water system.

Bob brought the water hose over and started adding water. He started to lay the hose over in the tower basin to let it flow and Btu Buddy stopped him saying, “Tie the hose up high. Do not lay it in the basin.”

Bob asked, “Why, what is wrong with that?”

Btu Buddy said, “If you were to walk off and leave the hose in the basin and the building lost water pressure, the water from the basin would be sucked into the building water system. The water in the basin looks clean, but you can bet it is not. Laying the hose in the basin has caused accidents where people were hurt from drinking the polluted water. The water tower is actually a filter for whatever may be in the air.”

Bob said, “Boy, that is one of those things that I never would have thought of. Thanks for telling me. I wonder how many technicians know about that?”

Btu Buddy then said, “All technicians that work on water-cooled equipment should be aware of that safety step. Let’s go and find out what is holding the water back from the water tower basin.”

They followed the piping down to the basement where the water fill circuit started and found a kink in the copper pipe.

Bob shut off the water to that circuit and cut the kink out and put in a coupling and turned the water back on. They went back to the roof to observe the water flow to the tower. It was flowing well.

Bob said, “Good thing that the water hose and the tower feed water were not on the same circuit. The tower is nearly full now. Let’s go and see what is happening.”

Bob disconnected the water hose and they went back to the basement.

Bob said, “Wow, look at that compressor run. The head pressure is 210 psig and the leaving chilled water is 48° and on the way down. This job is fixed, thanks to you and another good experience.”

Bob went to the building manager and explained what had happened. The manager said, “Thanks for making a quick repair. The complaints have stopped coming in. I have informed the people that everything is working now.”

Bob then said to the manager, “This system really needs regular maintenance. Would you like a quote for changing your filters, oiling the motors, checking belts, and keeping the cooling tower up to standards. You have a good system here, but it should be maintained.”

The manager said, “You are right. I have been changing the filters. The system would be better served by an expert. I could then take care of the things that I am an expert in.”

Bob said, “I will turn the request into the service department and they will give you a quote. You will hear from them in a few days.”

As they were driving away, Btu Buddy said, “Great job on the service end, but also a great job on helping your company grow. Service contracts are good for the customer and the company. When service calls are down in mild weather, the company can keep technicians busy with service contracts. Revenue is coming in for the company and the technicians have steady work.”

Publication date: 09/24/2007