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Now, Bob’s company has promoted him to help train a new employee, right out of a school specializing in HVAC, just like Bob was. Bob is now Tim’s Btu Buddy. Tim is anxious to travel with Bob. Tim realizes that he is right out of school, with the theory and lab work that he accomplished in school, but still needs help. He knows that he worked with many of the components of the systems in the school, under ideal conditions with good light and air conditioning. Now it is into the field, sometimes under the house with poor lighting, or out on the rooftop in the sun, where the real action is. He is naturally and normally reluctant, but he has Bob to help guide him.
Bob and Tim were doing a routine service contract call on a 20-ton system in a mall. The contract called for them to put gauges on the system and check the pressures and temperatures. The condensing unit was on the roof and had to be accessed through a roof hatch. The air handler was in the stock room at the back of a large shoe store.
They had fastened the gauges to the compressor at the condensing unit and had a temperature lead fastened to the suction line at the evaporator coil. They were observing the pressures and temperatures and Tim said, “The suction pressure is not stable. Isn’t it supposed to be stable during a long running cycle?”
Bob said, “Yes it is and you are right. What is the suction line temperature doing?”
Tim said, “It is varying also. It is fluctuating between 45° and 55°F. This is not normal.”
Bob said, “It seems that the thermostatic expansion valve, or TXV, is hunting, trying to find equilibrium.”
“What can cause that?” asked Tim.
Bob explained, “Hunting is when the valve cannot settle down. It feeds too much liquid until the sensing bulb senses the superheat is too low, then it overcompensates and shuts the valve down and underfeeds liquid. Something isn’t right.”
Tim asked, “What can cause these symptoms?”
Bob said, “There are several things, and most all of them concern the sensing bulb. Remember we had a sensing bulb a while back where the bulb was mounted after the external equalizer line. The valve had a slight leak allowing a small amount of liquid refrigerant to reach the sensing bulb, starving the coil of liquid refrigerant. That system was not hunting. It was just starved of liquid causing the system to operate under capacity.
|Figure 1. This is an example of how a sensing element of a thermostatic expansion valve (TXV) should be mounted. (From Refrigeration & Air Conditioning Technology, seventh edition, by William Whitman, William Johnson, John Tomczyk, and Eugene Silberstein, published by Delmar Cengage Learning.) (©Delmar Cengage Learning.)|
“Hunting is usually related to the sensing bulb not responding correctly. The sensing bulb has to have an accurate relationship with the temperature of the suction line. It has to be able to sense the temperature of the suction line to the point of being the same temperature as the suction gas inside the line. It has to be in close contact with the right location on the line. There are several problems that can occur when fastening the bulb to the line:
1. The bulb can be loose.
2. The bulb can have something under it, such as a small amount of corrosion which will act as an insulator.
3. The bulb can be on the line crooked and not making good contact.
4. The bulb can be mounted on the bottom of the line. This is not good because oil travels down the line and will act as an insulator and the bulb will not read the temperature of the vapor in the line.
5. Sometimes the bulb is mounted with one end up on a fitting, again, not making close contact with the line.
6. The bulb can be mounted in the wrong location, such as on a vertical line or on top instead of on the side of a larger suction line (Figure 1).
“We should shut the system down for a few minutes while we examine the bulb location. If we leave the system running and handle the bulb, we will surely cause the valve to overfeed with hand temperature.”
Tim shut the system down and removed the TXV sensing bulb and said, “The bulb was tightened down tight, but one end was setting up a little on a drop of solder. I guess that it was not tight down on the line. I could see light under it.”
Bob asked, “Did you notice anything else?”
Tim said, “Before we shut the system down, there seemed to be a lot of airflow close to where the bulb was mounted.”
Bob said, “You are right. The bulb is mounted on the line just outside the air handler and the hole where the suction line comes through the cabinet has a flow of air blowing on the bulb insulation and that insulation was not very tight. I think that the air, which is return air temperature, is probably affecting the bulb temperature. We will need to seal that airflow off from the bulb when we insulate the bulb again.”
Tim said, “Now what are we going to do special to get this bulb in control?”
Bob said, “We are going to do a textbook mounting and insulation of this bulb:
1. Clean the bulb contact point with sandpaper.
2. Clean the suction line contact point with sandpaper. Remember that the contact point will be about 45 degrees off of the bottom.
3. Use this thermal conductive grease at the contact point. It is a grease that will improve the contact and prevent corrosion between the bulb and line where it is bare copper on the bulb and bare copper on the line.
4. Then we are going to strap the bulb on the line, making sure that it fits down flat.
5. Then we will insulate the bulb and line with closed cell insulation.
6. Then we are going to wrap the end next to the air handler with rubber insulating tape to keep air from getting under the insulation.
7. Then we are going to seal the hole in the cabinet with rubber insulating tape to try to contain the air inside the cabinet.
“With this procedure, the bulb will control the superheat and it will be a job well done for a long time.”
They completed the process and started the unit. The superheat was now holding steady at about 11°.
Tim then said, “We sure paid a lot of attention to details with that bulb, but it paid off.”
Bob said, “Many technicians would have just changed the TXV, which is a big job. They would probably mount the new sensing bulb correctly and it would have worked. In my time in the industry, I have seen very few TXVs that were defective. They seem to be really reliable and it is a big cost to change the valve when it is not necessary. Let’s button everything up and go home. This job is really working like it should.”
Tim said, “Yep, that was a good lesson.”
Publication date: 10/21/2013