A thermostatic expansion valve (TEV) is a crucial component in refrigeration and air conditioning systems, designed to regulate the flow of refrigerant into the evaporator. The TEV ensures that the evaporator has the appropriate amount of refrigerant to achieve efficient heat exchange while preventing liquid refrigerant from returning to the compressor. The superheat setting is an essential parameter to ensure the TEV operates effectively.
Superheat refers to the additional temperature increase of a gas after it has fully transitioned from a liquid to a gas phase. In the context of a TEV, superheat is the temperature difference between the refrigerant gas at the evaporator outlet and its saturation temperature, which is the temperature at which the refrigerant transitions between liquid and gas phases at a given pressure. Superheat setting is the desired level of superheat that the TEV should maintain for optimum system performance.
The TEV senses the superheat in the system using a temperature-sensitive element, typically filled with a refrigerant or other temperature-sensitive substance. This element is attached to the suction line of the evaporator outlet. As the temperature of the refrigerant gas changes, the pressure within the sensing element also changes, causing the TEV to modulate the refrigerant flow rate accordingly.
Evaporator superheat is the difference between the temperature of the refrigerant at the exit of the evaporator and the evaporating temperature (saturation temperature) at the same location. The formula to calculate evaporator superheat is:
Superheat = T_refrigerant_exit – T_saturation
- Superheat is the evaporator superheat, typically measured in degrees Fahrenheit (°F) or degrees Celsius (°C).
- T_refrigerant_exit is the actual temperature of the refrigerant at the exit of the evaporator or the suction line, measured in °F or °C.
- T_saturation is the saturation temperature of the refrigerant at the given pressure in the evaporator, measured in °F or °C.
The proper superheat setting depends on several factors, including the type of refrigerant used, the desired evaporator temperature, and the application (e.g., air conditioning, refrigeration, or heat pump). Typically, the superheat setting ranges from 8 to 15 degrees Fahrenheit (4.4 to 8.3 degrees Celsius) for air conditioning systems, while it can be slightly higher for refrigeration systems. Proper superheat ensures that the evaporator operates efficiently, maximizes heat transfer, and prevents liquid refrigerant from entering the compressor, which could lead to compressor damage or reduced system performance.
Both high and low superheat can negatively impact the system’s performance and the compressor’s longevity. High superheat can result from inadequate refrigerant flow through the TEV, causing the refrigerant to absorb more heat than necessary in the evaporator. This condition may lead to reduced cooling capacity, increased energy consumption, and higher discharge temperatures, potentially shortening the compressor’s lifespan. On the other hand, low superheat may indicate excessive refrigerant flow through the TEV, which can result in liquid refrigerant entering the compressor. This phenomenon, known as “liquid floodback,” can cause damage to the compressor’s valves and bearings, eventually leading to compressor failure. Additionally, low superheat reduces the evaporator’s efficiency by not fully utilizing the available heat transfer surface. Therefore, it is essential to monitor and adjust the superheat settings in refrigeration and air conditioning systems to maintain optimal performance and prevent potential damage to the equipment.
Measurement and setting of superheat
To measure and set superheat in a refrigeration or air conditioning system, technicians need the following tools:
- Pressure gauge or manifold gauge set: A pressure gauge or manifold gauge set is used to measure the refrigerant pressure at the evaporator outlet or suction line. The manifold gauge set typically includes separate gauges for high and low pressures, along with hoses to connect to the system’s service ports.
The photo shows an example where the low pressure gauge is connected to a Schrader valve on the outlet of the evaporator, and the temperature probe is measuring the temperature adjacent to this point.
- Temperature probe or thermometer: A temperature probe or thermometer is required to measure the actual refrigerant temperature at the evaporator outlet or suction line. Various types of temperature probes are available, such as clamp-on probes, thermocouples, and infrared thermometers. Choose an accurate and reliable temperature measuring device that is suitable for the specific application.
- Pressure-temperature (P-T) chart or refrigerant slide rule: A pressure-temperature chart or refrigerant slide rule helps technicians convert the measured refrigerant pressure to the corresponding saturation temperature. These tools are specific to the refrigerant used in the system and can be found in the form of printed charts, mobile apps, or online resources.
- TEV adjustment tool or wrench: An adjustment tool or wrench is needed to change the TEV superheat setting. The specific tool required depends on the TEV’s design and manufacturer. Some TEVs use a hex key (Allen wrench), while others may require a small adjustable wrench or a specialized tool provided by the manufacturer.
- Protective gear: Safety glasses and gloves are essential when working with refrigeration and air conditioning systems to protect against potential injuries from high pressures, cold temperatures, or contact with refrigerants.
- Pen and paper or digital note-taking device: A pen and paper or digital note-taking device helps technicians record the measured values and calculations, facilitating accurate superheat adjustments.
Procedure to set superheat:
The system must be running and be fully charged to measure and adjust the superheat accurately. Follow these steps to measure and set the superheat:
- Turn on the system and allow it to stabilize: The refrigeration or air conditioning system must be running for an adequate period to ensure that it has reached a stable operating condition. This usually takes about 15-30 minutes.
- Attach the pressure gauge or manifold gauge set: Connect the pressure gauge or low-pressure side of the manifold gauge set to the service port on the suction line near the evaporator outlet. Ensure that the connections are tight and secure.
- Measure the suction pressure: Read the refrigerant pressure from the pressure gauge or manifold gauge set. Record this value.
- Convert the pressure to saturation temperature: Using a pressure-temperature (P-T) chart or refrigerant slide rule specific to the refrigerant used in the system, find the corresponding saturation temperature for the measured pressure. Record this value.
- Attach the temperature probe or thermometer: Place the temperature probe or thermometer on the suction line near the evaporator outlet or at the same location where the pressure measurement was taken. Ensure proper contact between the temperature probe and the suction line for an accurate reading.
- Measure the refrigerant temperature: Read the actual refrigerant temperature from the temperature probe or thermometer. Record this value.
- Calculate the superheat: Subtract the saturation temperature (T_saturation) from the actual refrigerant temperature (T_refrigerant_exit) to determine the superheat:Superheat = T_refrigerant_exit – T_saturation
- Compare the measured superheat with the desired superheat setting: If the measured superheat is within the desired range for the specific application, no adjustment is necessary. If the superheat is too high or too low, proceed to the next step.
- Adjust the TEV superheat setting: Locate the TEV adjustment stem, which is usually covered by a protective cap. Remove the cap, if necessary. Using the appropriate adjustment tool or wrench, turn the adjustment stem to change the superheat setting. Generally, turning the stem clockwise increases the superheat, while turning it counterclockwise decreases the superheat. Make small adjustments and allow the system to stabilize before rechecking the superheat.
- Recheck the superheat: After making adjustments and allowing the system to stabilize, repeat steps 2-7 to measure the new superheat. Continue adjusting the TEV until the desired superheat setting is achieved.
- Secure and finalize: Once the desired superheat setting is achieved, replace the protective cap on the TEV adjustment stem and disconnect the pressure gauge, temperature probe, and any other tools used during the procedure. Ensure all connections and fittings are tight and secure.
Superheat Log Table
A superheat log table is a helpful tool for technicians to record superheat measurements and adjustments over time. This allows for easy monitoring of a refrigeration or air conditioning system’s performance and helps identify trends or potential issues. A sample superheat log table may look like this:
Suction Pressure (PSI)
Saturation Temperature (°F)
Refrigerant Temperature (°F)
Explanation of table columns:
- Date: The date the superheat measurement was taken.
- Time: The time the superheat measurement was taken.
- Suction Pressure (PSI): The measured refrigerant pressure at the evaporator outlet or suction line, usually recorded in pounds per square inch (PSI).
- Saturation Temperature (°F): The saturation temperature of the refrigerant at the measured pressure, determined using a pressure-temperature (P-T) chart or refrigerant slide rule.
- Refrigerant Temperature (°F): The actual refrigerant temperature measured at the evaporator outlet or suction line.
- Superheat (°F): The calculated superheat value, obtained by subtracting the saturation temperature from the refrigerant temperature.
- Adjustment Made: The adjustment made to the TEV superheat setting, if any. This column records the change in superheat settings, such as “+1°” for an increase of 1°F or “-2°” for a decrease of 2°F.
- Technician: The name of the technician who performed the superheat measurement and adjustment.
In conclusion, understanding and maintaining the proper superheat setting in thermostatic expansion valves (TEV) is vital for the efficient operation and longevity of refrigeration and air conditioning systems. By following the recommended procedure for measuring and adjusting superheat, technicians can optimize system performance and prevent potential equipment damage. Regular monitoring and documentation of superheat values ensure a proactive approach to system maintenance and can identify trends or issues that may require further attention.
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