Jeff and Mike are on their way to another service call. The service dispatch states that a local restaurant has a refrigerated prep table with a small reach-in refrigerator that is not cooling properly.
As they arrive a Mike jumps out of the truck and grabs his manifold gauges.
Jeff: "I don't think we'll be needing our gauges on this call."
Mike: "How are we going to check the system without out them?"
Jeff: "I'll tell you in a minute, just grab the tool bag and let's take a look."
As they enter the kitchen, they see the small refrigerated prep table already pulled out from the wall. After verifying there is power to the unit, Jeff instructs Mike to remove the back panel.
Mike: "What the heck is this coiled up piece of tubing and where is the TXV?"
Jeff: "These types of systems don't use a TXV and that coiled up tubing is the capillary tube. It is the metering device for this type of system."
Mike: "Why do they use a capillary tube instead of a TXV?"
Jeff: "Capillary tubes, or cap tubes, are very inexpensive and have no moving parts to fail. Unlike a TXV, cap tubes allow the pressure in the system to equalize quickly so the compressor does not need a start capacitor. Also, TXVs require extra refrigerant, and cap tube systems do not."
Mike: "So, if cap tubes are so cost effective, why aren't they used as the metering device for residential air conditioning systems?"
Jeff: "Cap tube systems work fine as long as they don't have a large heat load to cool down. For example, this system is stocked every morning with product from the large walk-in cooler that keeps everything at 35°F. This refrigerator is kept a 40°F, so it has no problem keeping the product cool... if there isn't a very big heat load."
Mike: "I understand how a TXV works. But how does a capillary tube meter refrigerant?
Jeff: "The friction of fluids flowing through a tube or pipe results in a pressure drop. A capillary tube is designed to change the high pressure liquid refrigerant into a low pressure spray of refrigerant. The amount of pressure drop is dependent on the length and inside diameter of the capillary tube."
Mike: "Okay, that makes sense. Let's get our gauges and see what's going on with this system."
Jeff: "Not so fast Mike. These capillary tubes are critically charged by the manufacturer. Their engineers determine the exact amount of refrigerant needed for the system to operate properly. If we hook up our gauges we will change the amount of refrigerant in the system and could make things worse. We'll hook up our gauges only as a last resort."
Mike: "So how do figure out what is wrong?"
Jeff: "First we'll check to see if this system has been overloaded with warm product. Next we'll check for bad door gaskets, or if the door has been left open or opened frequently. Remember, cap tube systems don't respond very quickly to large load changes. Finally we'll check for a dirty condenser coil, dirty or iced evaporator coil, and make sure fan motors are operational. Oh, we will also check to make sure the thermostat is working and set to the proper temperature. Why don't you go ahead and check all of this now?"
After checking everything, Mike reports back to Jeff.
Mike: "Everything is clean and in good order. What's next?"
Jeff: "It's almost time to hook-up our gauges. The most common problem with a capillary tube system is a restriction in the tubing. You see, the inside diameter of the tubing is so small, it can plug up easily.
Mike: "How does the capillary tube get plugged?"
Jeff: "Do you see this small filter drier? It is filled with desiccant. After years of use, the beads begin to flake and eventually plug the entrance to the cap tube. Usually the blockage is in the very first inch or so of the capillary tube. We'll be looking for the low side pressure to be in or near a vacuum and the high side pressure to slightly lower than normal."
Mike: "So if it is a plugged capillary tube, how do we fix it?"
Jeff: "We'll recover the refrigerant, cut out the filter drier, cut off an inch or two of the cap tube, install a new drier, evacuate and weigh in the proper amount of refrigerant. Then we'll start the unit without any product and give it time to come down to temperature."
Mike: "Seems pretty simple, what else can go wrong?"
Jeff: "We could also find a leak, an improperly charged system, or an inefficient compressor. We'll talk about these later. It's time to get to work."
What did we learn from Mike and Jeff today?
Condenser / condensing unit checks (NATE - Light Commercial Refrigeration) Evaporator unit checks (NATE - Light Commercial Refrigeration) Refrigerant line checks(NATE - Light Commercial Refrigeration) Low capacity(NATE - Light Commercial Refrigeration) Analyzing overall refrigerant circuit performance (NATE - Light Commercial Refrigeration) Locating problems based on refrigerant circuit temperatures and pressures (NATE - Light Commercial Refrigeration)
by Ron Walker
As they arrive a Mike jumps out of the truck and grabs his manifold gauges.
Jeff: "I don't think we'll be needing our gauges on this call."
Mike: "How are we going to check the system without out them?"
Jeff: "I'll tell you in a minute, just grab the tool bag and let's take a look."
As they enter the kitchen, they see the small refrigerated prep table already pulled out from the wall. After verifying there is power to the unit, Jeff instructs Mike to remove the back panel.
Mike: "What the heck is this coiled up piece of tubing and where is the TXV?"
Jeff: "These types of systems don't use a TXV and that coiled up tubing is the capillary tube. It is the metering device for this type of system."
Mike: "Why do they use a capillary tube instead of a TXV?"
Jeff: "Capillary tubes, or cap tubes, are very inexpensive and have no moving parts to fail. Unlike a TXV, cap tubes allow the pressure in the system to equalize quickly so the compressor does not need a start capacitor. Also, TXVs require extra refrigerant, and cap tube systems do not."
Mike: "So, if cap tubes are so cost effective, why aren't they used as the metering device for residential air conditioning systems?"
Jeff: "Cap tube systems work fine as long as they don't have a large heat load to cool down. For example, this system is stocked every morning with product from the large walk-in cooler that keeps everything at 35°F. This refrigerator is kept a 40°F, so it has no problem keeping the product cool... if there isn't a very big heat load."
Mike: "I understand how a TXV works. But how does a capillary tube meter refrigerant?
Jeff: "The friction of fluids flowing through a tube or pipe results in a pressure drop. A capillary tube is designed to change the high pressure liquid refrigerant into a low pressure spray of refrigerant. The amount of pressure drop is dependent on the length and inside diameter of the capillary tube."
Mike: "Okay, that makes sense. Let's get our gauges and see what's going on with this system."
Jeff: "Not so fast Mike. These capillary tubes are critically charged by the manufacturer. Their engineers determine the exact amount of refrigerant needed for the system to operate properly. If we hook up our gauges we will change the amount of refrigerant in the system and could make things worse. We'll hook up our gauges only as a last resort."
Mike: "So how do figure out what is wrong?"
Jeff: "First we'll check to see if this system has been overloaded with warm product. Next we'll check for bad door gaskets, or if the door has been left open or opened frequently. Remember, cap tube systems don't respond very quickly to large load changes. Finally we'll check for a dirty condenser coil, dirty or iced evaporator coil, and make sure fan motors are operational. Oh, we will also check to make sure the thermostat is working and set to the proper temperature. Why don't you go ahead and check all of this now?"
After checking everything, Mike reports back to Jeff.
Mike: "Everything is clean and in good order. What's next?"
Jeff: "It's almost time to hook-up our gauges. The most common problem with a capillary tube system is a restriction in the tubing. You see, the inside diameter of the tubing is so small, it can plug up easily.
Mike: "How does the capillary tube get plugged?"
Jeff: "Do you see this small filter drier? It is filled with desiccant. After years of use, the beads begin to flake and eventually plug the entrance to the cap tube. Usually the blockage is in the very first inch or so of the capillary tube. We'll be looking for the low side pressure to be in or near a vacuum and the high side pressure to slightly lower than normal."
Mike: "So if it is a plugged capillary tube, how do we fix it?"
Jeff: "We'll recover the refrigerant, cut out the filter drier, cut off an inch or two of the cap tube, install a new drier, evacuate and weigh in the proper amount of refrigerant. Then we'll start the unit without any product and give it time to come down to temperature."
Mike: "Seems pretty simple, what else can go wrong?"
Jeff: "We could also find a leak, an improperly charged system, or an inefficient compressor. We'll talk about these later. It's time to get to work."
What did we learn from Mike and Jeff today?
Condenser / condensing unit checks (NATE - Light Commercial Refrigeration) Evaporator unit checks (NATE - Light Commercial Refrigeration) Refrigerant line checks(NATE - Light Commercial Refrigeration) Low capacity(NATE - Light Commercial Refrigeration) Analyzing overall refrigerant circuit performance (NATE - Light Commercial Refrigeration) Locating problems based on refrigerant circuit temperatures and pressures (NATE - Light Commercial Refrigeration)
by Ron Walker