The dispatcher notifies Bob that a motel owner has called and said that the 50-ton chiller that serves his motel is shutting off and the high-pressure control must be reset to get it to run again. Then it will only run for a short while and shut off again.
Bob arrives at the job with a little doubt in his mind because it is a strange job to him and he has never worked on a water-cooled unit. He has been thinking on the way to the job what he should do first. Since it is shutting down because of high pressure, he knows that he should check the part of the system that rejects heat, the cooling tower.
Bob goes to the equipment room where the chiller is located and finds the maintenance man standing there looking like he needs help. The maintenance man explains that he doesn't know anything about refrigeration equipment; he just pushed some buttons on the unit and when he pushed the high-pressure button it started up. He reached for the button again to restart the compressor and Bob says, "Let me look around for a minute before we restart it. There may be something obvious that is keeping it from operating."
Bob looks around the equipment room and identifies the condenser water pump; it is running and seems normal.
He then goes to the cooling tower for a look. Water is going over the cooling tower and it seems normal. A look into the cooling tower sump shows the water to be clean looking. All of this looks good, so Bob decides to install gauges on the chiller and start it up to see what happens.
Btu Buddy appears at this time and asks, "What do you expect the gauges to read when you start the compressor?"
Bob says, "I am not sure. I am going to start it up and see what they read."
Btu Buddy says, "You need to have some expectation or guideline before you start it up. Here are some numbers that you should commit to memory. The chiller is an R-22 machine. The suction pressure should operate at about 70 psig corresponding to 40 degrees evaporator when the chilled water leaving is 45 degrees and the entering water is 55 degrees. These are typical design conditions for a water chiller. The head pressure should be about 210 psig when the cooling tower is furnishing 85 degree water to the condenser. The entering chilled water is now running 80 degrees so the chiller really has a load on it. You can expect the suction pressure to be high and the head pressure to be lower than normal because the cooling tower is still operating. The cooling tower water is 5 degrees warmer than the entering chilled water.
"Here are some facts that you should know about water-cooled condensers.
1. The cooling tower should provide water that is about 7 degrees warmer than the outdoor wet bulb, so a 78 degree wet bulb should yield 85 degree water to the condenser. This is called the cooling tower â€˜approach temperature.' The cooling tower can reach an approach temperature of 7 degrees. There are towers with closer approach temperatures, but 7 degrees is typical. The dry-bulb temperature doesn't influence the cooling tower water temperature. It is controlled by the wet-bulb temperature which is related to the evaporation rate.
"There is a relationship between the leaving condenser water and the refrigerant condensing temperature of about 10 degrees. For example, if the refrigerant is condensing at 105 degrees, the leaving condenser water should be about 95 degrees. See Figure 1 for an illustration. This is important to know because when we start the compressor, we will record these conditions and see what they are. If they don't make sense, we can find out why by taking the following readings:
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 â€“ B): ___ degrees
"There is something that we should consider here. Are the two thermometers on the condenser water correct? They are permanent thermometers and have been in service for a long time. Since the chiller is not operating, both should read the same because the condenser is not adding any heat to the water. What are they reading?"
Bob looks and says, "They are reading the same, 84 degrees."
Btu Buddy says, "That is close enough. When you start the compressor, watch the head pressure and record it before it can shut down. It will probably run for a few minutes before it shuts off. Also watch both condenser water temperatures and record them."
Bob resets the high-pressure control. Everything seems fine for a few minutes, and the head pressure starts to rise. It rises to 240 psig and Btu Buddy says, "You better record your readings because it is going to shut off soon."
Bob takes down the readings and, just before it shuts off, Btu Buddy says, "Feel the liquid line leaving the condenser and I will ask you about it in a minute."
The chiller shuts off on high pressure.
Bob reviews the readings:
Condenser water outlet temperature: 90 degrees
Condenser water inlet temperature: 85 degrees
Temperature difference: 5 degrees
A. Condensing temperature (from head pressure): 117 degrees
B. Condenser water out temperature: 88 degrees
Approach temperature (A â€“ B): 29 degrees
Btu Buddy asks, "What do you see in these readings?"
Bob says, "I am not sure. I need help."
Btu Buddy says, "The condenser should have a 10 degree difference from inlet to outlet, and it only has 5 degrees. That tells you that it is not doing much work, actually only 50 percent of its workload if 10 degrees is normal. The approach temperature is 29 degrees when it should be about 10 degrees. That tells you that the condenser is not removing enough heat. Now for the question: What did the liquid line feel like when you touched it - cool, warm, or hot?"
Bob answers, "It was much warmer than hand temperature. Why does that matter?"
"Well," Btu Buddy explains, "if it was cool, the system could have an overcharge of refrigerant and a lot of subcooling due to the cool cooling tower water. If it was hot, the condenser is just not taking the heat out of the refrigerant. I would say that you have a case of dirty condenser tubes. About all you can do is shut the system off, drain the condenser, and go in for a look."
Bob shuts everything down and removes the condenser head, and is surprised to see a lot of slime in the condenser tubes.
"I don't understand. The water looked good," Bob exclaims.
Btu Buddy says, "The tower may have just been drained and cleaned, but the condenser is still dirty."
Bob uses brushes and cleans the condenser and rinses it out with fresh water. He then puts the cover back on the condenser.
The tower is filled and the water pump is turned on, and Bob is ready to start the system again and asks, "Should I look out for anything in particular?"
Btu Buddy responds, "Now is a good time to record the previous temperatures and then see what this chiller does with a clean condenser. That is good information to have on the jobsite and in your records for this job. The next time a different technician may be the one out here and those records would be appreciated."
Bob starts the chiller and lets it run for about 30 minutes and records the following:
Condenser water outlet temperature: 95 degrees
Condenser water inlet temperature: 85 degrees
Temperature difference: 10 degrees
A. Condensing temperature (from head pressure): 105 degrees
B. Condenser water out temperature: 95 degrees
Approach temperature (A â€“ B): 10 degrees
"That is perfect," Btu Buddy says.
Bob says, "The chilled water outlet temperature is 45 degrees, so the system is really working."
"These systems are very predictable if you take the time to understand them," Btu Buddy says. "You should take a couple copies of these figures and put one in the file and the other in the control panel of the chiller. This may save you some time in the future. Tell the maintenance man where they are and, someday, he may call you with a problem and you can ask him for the temperature readings and have some idea of what the problem is over the phone."
Bob shows the paper to the maintenance man who really appreciates what Bob has done.
Btu Buddy says as they are riding away, "You have made another customer happy. Just keep building happy customer files and you will always be in demand."
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 firstname.lastname@example.org.
Publication date: 07/26/2004