When we last left Mike and Jeff, Mike was on his way in to check Mrs. Thomas' system to see if it had a TXV or fixed metering device. When Mike returns, he explains to Jeff what he found.
Mike: "It's a TXV. I checked and the the sensing bulb is securely fastened, it is on the horizontal pipe of the evaporator outlet, and the insulation is secure. Now what?"
Jeff: "Do you remember from your training, the proper way to check the system charge on a system with a TXV?"
Mike: "I'm not sure, but I think it's by checking the subcooling."
Jeff: "Okay, let's check it now."
Mike removes the temperature clamp from the suction line and reattaches it to the liquid line. Checking his notebook to make sure he is doing it right, he verifies that, SATURATION TEMPERATURE - LIQUID LINE TEMPERATURE = SUBCOOLING. He then records his findings, "High side pressure: 195 psig and liquid line temperature of 112F. 195 psig = 99F saturation temperature. 112F - 99F = 13 degrees of subcooling."
Mike: "13F of subcooling indicates the system is not low on refrigerant, I do know that much. But, I am not certain exactly what saturation temperature and subcooling is."
Jeff: "We'll talk about saturation temperature and subcooling a little later. First let's get Mrs. Thomas some cool air. Tell me what you have in your class notes about TXVs"
Mike: "TXVs have one opening force and two closing forces."
Jeff: "Good! Tell me about those forces."
Mike: "The opening force is from the sensing bulb. It is usually filled with the same type of refrigerant as the system. It is mounted at the outlet of the evaporator coil. When the evaporator outlet temperature increases, the refrigerant in the bulb expands pushing down on the top of the diaphragm in the TXV. This opens the TXV to allow more refrigerant into the system."
Jeff: "What about the closing forces?"
Mike: "The closing forces are the evaporator pressure and the superheat spring pressure. These oppose the sensing bulb pressure on the bottom of the diaphragm in the TXV and provide the closing force. So when the outlet of the evaporator coil gets cooler and the sensing bulb pressure decreases, the two closing forces begin to throttle back the refrigerant."
Jeff: "Excellent! You took great notes. So what do you think is happening?"
Mike: "The system charge looks good and the superheat is too high. High superheat means the coil is starved for refrigerant. I think for some reason, the sensing bulb may not be opening the TXV. So what do we do next?"
Jeff: "Let's remove the sensing bulb from the evaporator outlet and heat it up by submersing it in a glass of warm water" After the sensing bulb has been in the warm water for a few minutes, they check what is happening with the system.
Jeff: "Okay, Mike what's happening?"
Mike: "I just checked the system pressures and subcooling again and nothing has changed."
Jeff: "If the TXV was good how would submersing it in a cup of hot water do?"
Mike: "The sensing bulb would heat up, expand the refrigerant in the bulb, and open the the TXV. This would increase the suction pressure and decrease the superheat."
Jeff: "Exactly! So it looks like the sensing bulb has lost its charge. Let's go inform Mrs. Thomas what we've found and change out this TXV."
What did we learn from Mike and Jeff?
Mike: "It's a TXV. I checked and the the sensing bulb is securely fastened, it is on the horizontal pipe of the evaporator outlet, and the insulation is secure. Now what?"
Jeff: "Do you remember from your training, the proper way to check the system charge on a system with a TXV?"
Mike: "I'm not sure, but I think it's by checking the subcooling."
Jeff: "Okay, let's check it now."
Mike removes the temperature clamp from the suction line and reattaches it to the liquid line. Checking his notebook to make sure he is doing it right, he verifies that, SATURATION TEMPERATURE - LIQUID LINE TEMPERATURE = SUBCOOLING. He then records his findings, "High side pressure: 195 psig and liquid line temperature of 112F. 195 psig = 99F saturation temperature. 112F - 99F = 13 degrees of subcooling."
Mike: "13F of subcooling indicates the system is not low on refrigerant, I do know that much. But, I am not certain exactly what saturation temperature and subcooling is."
Jeff: "We'll talk about saturation temperature and subcooling a little later. First let's get Mrs. Thomas some cool air. Tell me what you have in your class notes about TXVs"
Mike: "TXVs have one opening force and two closing forces."
Jeff: "Good! Tell me about those forces."
Mike: "The opening force is from the sensing bulb. It is usually filled with the same type of refrigerant as the system. It is mounted at the outlet of the evaporator coil. When the evaporator outlet temperature increases, the refrigerant in the bulb expands pushing down on the top of the diaphragm in the TXV. This opens the TXV to allow more refrigerant into the system."
Jeff: "What about the closing forces?"
Mike: "The closing forces are the evaporator pressure and the superheat spring pressure. These oppose the sensing bulb pressure on the bottom of the diaphragm in the TXV and provide the closing force. So when the outlet of the evaporator coil gets cooler and the sensing bulb pressure decreases, the two closing forces begin to throttle back the refrigerant."
Jeff: "Excellent! You took great notes. So what do you think is happening?"
Mike: "The system charge looks good and the superheat is too high. High superheat means the coil is starved for refrigerant. I think for some reason, the sensing bulb may not be opening the TXV. So what do we do next?"
Jeff: "Let's remove the sensing bulb from the evaporator outlet and heat it up by submersing it in a glass of warm water" After the sensing bulb has been in the warm water for a few minutes, they check what is happening with the system.
Jeff: "Okay, Mike what's happening?"
Mike: "I just checked the system pressures and subcooling again and nothing has changed."
Jeff: "If the TXV was good how would submersing it in a cup of hot water do?"
Mike: "The sensing bulb would heat up, expand the refrigerant in the bulb, and open the the TXV. This would increase the suction pressure and decrease the superheat."
Jeff: "Exactly! So it looks like the sensing bulb has lost its charge. Let's go inform Mrs. Thomas what we've found and change out this TXV."
What did we learn from Mike and Jeff?
- SATURATION TEMPERATURE - LIQUID LINE TEMPERATURE = SUBCOOLING. NATE Air Conditioning/Heat Pump
- TXV operation. NATE Air Conditioning/Heat Pump
- TXV opening and closing forces. NATE Air Conditioning/Heat Pump
- Subcooling is used to check system charge in a TXV system. NATE Air Conditioning/Heat Pump
- Checking that a TXV is properly mounted. NATE Air Conditioning/Heat Pump