Temperature measurement in an ammonia compressor in industrial refrigeration is crucial to ensure system efficiency, operational safety and early detection of problems. Because ammonia can be hazardous to your health, reactive, corrosive, flammable, and in some cases explosive, selecting and installing your temperature measurement equipment demands strict adherence to established standards and best practices.
At Ashcroft, we’ve built our reputation on delivering accurate and reliable pressure and temperature instrumentation for mission-critical environments. Read this article to learn how temperature is monitored in a refrigeration system's compressor, and what to look for when selecting the best temperature measurement instruments to monitor ammonia systems safely and effectively.
You will also see links to additional resources about temperature sensors for your reference.
Why Temperature Monitoring Matters in Ammonia Refrigeration Systems
Industrial refrigeration systems using ammonia require close temperature monitoring at several key compressor points:
Compressor suction line: The ammonia temperature at the compressor inlet (suction) is important to check the superheating of the vapor before compression. Adequate superheating ensures that no liquid ammonia enters the compressor, which could cause severe damage (liquid slam).
Compressor discharge line: The ammonia temperature at the compressor outlet (discharge) is a critical indicator of compressor performance. High discharge temperatures can indicate problems such as a high compression ratio, excessive superheat in the suction, or failure of internal compressor components.
Compressor crankcase (oil): The temperature of the lubricating oil in the crankcase is monitored to ensure that the oil is within the ideal temperature range for lubrication and cooling of the compressor.
Temperature sensor reliability, accuracy and quick response are vital to detecting issues early and ensuring system efficiency. The most common temperature instruments that are used in refrigeration systems with ammonia are Industrial thermocouples, and RTDs, transmitters and threaded thermowells. Read on for a closer look at each:
1. Thermocouple Temperature Sensors for Ammonia Refrigeration Systems
Temperature instruments like the Ashcroft® S50 Industrial Thermocouples are robust and low-cost sensors, ideal for measurements over a wide temperature range. They work based on the principle that the junction of two different metals generates a small voltage proportional to the temperature.
The thermocouples Type T and Type E are excellent options for ammonia systems, due to their good tolerance limits and low temperature range (negative scale). Here's a high-level look at each type:
Type T Thermocouples (Copper–Constantan)
Temperature range:
- −200 °C to +350 °C (−328 °F to +662 °F)
- Excellent performance in sub-zero applications
Why Type T thermocouples are ideal for ammonia systems:
- Highly stable and accurate at low temperatures, ideal for suction line monitoring
- Good corrosion resistance in dry ammonia systems
- Consistent performance in cryogenic and refrigeration conditions
Type E Thermocouples (Chromel–Constantan)
Temperature range:
- −200 °C to +900 °C (−328 °F to +1652 °F)
- Highest EMF output per degree among base metal thermocouples
Why Type E Thermocouples are a good choice for ammonia systems:
- Superior sensitivity and signal strength in low-temp environments
- Useful for cold-side compressor monitoring
- Excellent low-end accuracy and non-magnetic properties
2. Resistance Temperature Detectors (RTDs) for Ammonia Refrigeration Systems
Resistance Temperature Detectors (RTDs) like the Ashcroft® S50 Industrial RTD offer high accuracy and stability and are often used in applications where measurement accuracy is essential. RTDs are resistors that change their electrical resistance as the temperature changes.
The most commonly used RTD for ammonia refrigerant applications is a Platinum 100 ohm. Pt100 has excellent corrosion resistance, long-term stability and measures a wide range of temperatures, from -200 to +850 °C.
These temperature sensors are designed with 2-wire, 3-wire and 4-wire connections to provide accurate readings. The 3-wire and 4-wire RTDs are the most commonly used options for this application.
3. Temperature Transmitters for Ammonia Refrigeration Compressors
In industrial refrigeration applications, especially those using ammonia, relying on the raw output from thermocouples or RTDs does have its challenges, including signal degradation, electrical noise or inconsistent performance over a distance. That’s why analog temperature transmitters remain a great choice for accurate and reliable temperature measurement in compressor systems.
These transmitters convert the millivolt or resistance signal from the sensor into a standardized electrical output, making it easier to integrate with modern control systems.
Ashcroft temperature transmitters offer:
- Analog outputs: 4–20 mA (2-wire), widely used across most refrigeration installations
- Compact, rugged construction: Designed for space-constrained compressor layouts and high-vibration environments
- Encapsulated electronics: For enhanced resistance to moisture, shock and thermal cycling
- Fast response time: Ensures real-time monitoring and control
For specialized applications, especially in systems with advanced process automation or supervisory control, Ashcroft also offers models that support digital communication protocols like HART and Fieldbus.
4. Threaded Thermowells for Added Temperature Sensor Protection
Thermowells provide a critical barrier between the sensor and the process media. However, due to the hazardous and reactive nature of ammonia, special care is required when specifying thermowells for use with temperature sensors in refrigeration systems. The thermowell must be able to withstand pressure, prevent leaks, and resist chemical corrosion over time.
In these environments, threaded gas connections paired with a robust one-piece bar stock construction, are essential for ensuring safe and reliable temperature measurement.
Ashcroft’s threaded thermowells offer:
- Material: Solid bar stock in 316 stainless steel for corrosion resistance
- Threaded process connections: NPT, BSPT, or G threads ensure compatibility with standard refrigeration systems
- Stem styles: Tapered, straight, or stepped stems balance response time with mechanical durability
- Sensor isolation: Thermowells allow sensor replacement and calibration without depressurizing the system, thus improving both safety and system availability
When used with Ashcroft sensors and transmitters, our thermowells complete a fully engineered, safety-compliant temperature measurement system built to withstand the challenges of ammonia refrigeration.
Important Considerations When Selecting a Sensor
Here are a few things to keep in mind when choosing a temperature sensor for refrigeration applications:
- Compatibility materials: Sensor and housing materials must be compatible with ammonia to prevent corrosion and leakage. Material like stainless steel in the construction of sensor sheath and thermowell are the first option to be selected.
- Temperature range: The sensor must be capable of operating over the expected temperature range of the measurement point.
- Accuracy and response time: Measurement accuracy and the speed with which the sensor responds to temperature changes are crucial to control and safety.
- Hazardous location certification: In some cases, sensors must be certified for hazardous areas (for example, intrinsically safe or explosion-proof), which are governed by approval agencies such as FM, UL, CSA, ATEX and IECEx etc.
Installation, Calibration & Best Practices for Ammonia Compressor Temperature Sensors
Typically, thermocouple or RTD sensors with a thermowell are used to protect the sensor while ensuring contact with the ammonia for accurate readings. In some cases, surface sensors without a thermowell can be used, though this approach is less common because of insulation and accuracy requirements. Regardless of the method, proper installation is essential to guarantee reliable temperature measurement.
Regular calibration is essential for all measuring instruments, including temperature sensors. This ensures that readings are accurate over time and prevents errors that could compromise the safety and efficiency of the system.
Temperature measurement in ammonia compressors is governed by safety standards and good practices for industrial refrigeration systems. International organizations like the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) and the International Institute of All-Natural Refrigeration (IIAR) have guidelines and important references.
Ready to learn more?
As you can see, temperature measurement in an ammonia compressor is a process that involves the selection of appropriate sensors, accurate installation and regular calibration to ensure safe operation. For more insights on temperature measurement, we have additional articles that may interest you:
- How Much to Temperature Sensors Cost?
- When to Use an RTD or Thermocouple Temperature Sensor
- What is Thermocouple Calibration?
Contact us anytime to talk to one of our product experts if you have additional questions. In the meantime, feed free to download our product guide to learn about other temperature solutions.