Low Temperature Calibration of Ammonia System
Temperature Devices in Coal‑Fired Power Plants
Background
Coal‑fired power plants continue to play an important role in base‑load electrical generation across the United States. To comply with environmental
regulations, many facilities rely on emissions control technologies such as selective catalytic reduction, or SCR, to reduce nitrogen oxide emissions in
flue gas.
In an SCR system, ammonia (NH₃) is injected into the flue gas stream where it reacts with nitrogen oxides to form nitrogen and water vapor. This
reaction is highly temperature dependent. If the injection temperature is too low or too high, reaction efficiency is reduced and excess ammonia can pass
through the system. This condition, commonly known as ammonia slip, can lead to downstream equipment damage, increased maintenance, and
compliance concerns.
To maintain proper system performance and safety, coal‑fired power plants depend on temperature sensors and temperature switches installed
throughout ammonia storage, transfer, and injection systems. These devices are often required to operate at temperatures as low as −20°F (−29°C) to
account for cold ambient conditions, ammonia expansion effects, and system safety interlocks. Accurate calibration of these devices is critical to
reliable plant operation.
The Calibration Challenge
Low temperature calibration at −20°F presents several technical challenges.
At sub‑zero temperatures, maintaining thermal stability and uniformity becomes increasingly difficult. Temperature gradients increase, stabilization
times lengthen, and many portable calibrators struggle to deliver consistent performance near the bottom of their operating range. When multiple
sensors or large sensor bodies are introduced, these effects are amplified.
Temperature switches add another level of complexity. Unlike standard RTDs or thermocouples, switches must be tested dynamically to determine
opening temperature, closing temperature, and hysteresis. Performing these tests manually at very low temperatures is time consuming and increases
the risk of operator error.
The physical environment further complicates calibration work. Ammonia systems are typically located in industrial areas where dust, residue, and
process contamination are present. Technicians often wear insulated gloves for chemical and cold protection, which can make delicate interfaces
difficult to operate. Calibration equipment must be robust, reliable, and easy to use under these conditions.
Supporting Both Liquid Bath and Dry‑Block Calibration
Traditional liquid baths have long been used for low temperature calibration due to their excellent thermal uniformity. In many applications, they remain an effective solution, particularly when calibrating odd‑shaped sensors or devices that do not fit easily into inserts. However, liquid baths can be
inconvenient in field environments due to handling requirements, contamination concerns, and cleanup time.
Dry‑block calibrators offer cleaner operation and easier transport, but many lack the low temperature performance, stability, or flexibility required for
demanding industrial applications.
The RTCt‑168B addresses this tradeoff by combining both technologies in a single portable instrument. It can operate as a traditional dry‑block
calibrator or transform into a liquid bath using a removable liquid container. This design allows technicians to choose the best method for the
application without changing instruments. Switching between dry and liquid operation is quick, clean, and safe, providing true flexibility for both
laboratory and field use.
The RTC‑168B Solution
The RTCt‑168B Reference Temperature Calibrator was selected by this coal‑fired power plant to meet its low temperature calibration requirements for
ammonia system sensors and switches.
With a temperature range down to −22°F (−30°C), the RTCt‑168B allows technicians to calibrate below the −20°F operating point. This capability
provides valuable margin when verifying switch operation and sensor performance, ensuring correct behavior under worst‑case conditions rather than
only at nominal setpoints.
The calibrator’s dual‑zone heating and cooling system creates highly uniform temperature zones within the block. The lower zone establishes the
primary calibration temperature, while the upper zone compensates for heat loss near the top of the well. This design improves stability and uniformity
at low temperatures, even when multiple devices are calibrated simultaneously.
Automated Switch Testing and Stability Control
A key requirement for this application was reliable temperature switch testing at low temperatures. The RTCt‑168B includes an automated switch test function that detects open and closed states and calculates switching temperature and hysteresis automatically. This removes the need for manual
observation and reduces variability between technicians.
The RTCt‑168B also supports automated temperature stepping and programmable hold times. Once a test sequence is defined, the calibrator executes
the routine automatically. Clear stability indicators inform the technician when conditions are suitable for recording measurements, which is especially
important at low temperatures where stabilization can be difficult to judge manually.
Designed for Industrial Field Use
The RTCt‑168B is well suited for harsh industrial environments. Its clear display and intuitive interface are designed for reliable operation. Robust
construction helps ensure dependable use in areas where residue or contamination may be present.
The reversed airflow system directs exhaust air away from the top of the calibrator, protecting both the sensor under test and the operator. Combined
with portable design and fast heating and cooling performance, the RTCt‑168B supports efficient in‑field calibration with minimal downtime.
Conclusion
Calibrating temperature sensors and switches at −20°F in ammonia systems is a demanding task that requires more than basic cooling
capability. It requires thermal uniformity, automation, flexibility, and equipment designed for real industrial environments.
By selecting the RTCt‑168B Reference Temperature Calibrator, this coal‑fired power plant gained a single solution capable of accurate
low temperature calibration, automated switch testing, and the flexibility to operate as either a liquid bath or dry‑block calibrator. The
result is improved confidence in emissions control systems, enhanced safety, and more efficient calibration practices in the field.
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