Furnace Performance Depends on Dewpoint Measurement Accuracy

Furnace Performance Depends on
Dewpoint Measurement Accuracy


Written by James Tennermann fro Vaisala Inc. and reprinted with permission from Heat Treating Process/ASM International

Incorrect dewpoint levels can be an indicator that something, somewhere, has gone wrong in industrial applications such as metals heat treating. A dewpoint measurement instrument is invaluable for diagnosing potential process problems.

Water vapor is a critical component of many furnace atmospheres. Whether brazing, annealing, or sintering, there are few processes in which water vapor does not play a part. Target levels of water vapor (commonly expressed as dewpoint temperature) vary depending on the process, the metals, other furnace gases, binders, fluxes, etc. Dewpoint may vary widely between different furnace zones. Dewpoint temperatures from -70 to 10°C (-94 to 50°F) are common in heat treatment. Dewpoint can be actively controlled by gas generation or gas mixing, but it is also influenced by furnace design and sometimes even by the weather.

A closer look at a sintering furnace illustrates the complexity of atmosphere control with respect to dewpoint. Atypical sintering furnace has multiple zones that perform different functions and require atmospheres and dewpoint levels. In the front part of the furnace, a key function is to burn out the lubricant that was added to the powder mixture for facilitation of compaction. This usually requires an oxidizing atmosphere and relatively “wet” dewpoint levels of, for example, -10°C (14°F). The hot zone, where the sintered part becomes homogenous, requires a reducing atmosphere and a low dewpoint of perhaps -40°C (-40°F). The cooling zone is typically blanketed with pure nitrogen, which may have a dewpoint of -70°C (-94°F). To further complicate matters, gas mixture and dewpoint level will vary in the same furnace, depending on exactly what type of part is being made.

Maintaining Proper Dewpoint
How is dewpoint in a furnace maintained at the desired level? Three basic approaches are system design, direct dewpoint measurement at a specific location, and deriving dewpoint value from another measurement.

The first approach relies on the inherent design of the system. For example, a process might call for nitrogen blanketing. Nitrogen can be provided by an on-site nitrogen generator with a 1 ppmv water vapor specification (about -80°C, or -110°F, dewpoint at atmospheric pressure). It is assumed that the generator functions according to specification and no dewpoint measurement or control is required for the blanket gas.

The second approach is to measure dewpoint temperature at the point of interest. This requires an appropriate instrument that is installed, calibrated, and maintained correctly.

The third approach is to derive dewpoint from another gas measurement. This also relies on the correct installation, calibration, and maintenance of an appropriate instrument.

fig 1
Fig. 1 — Vaisala DryCap handheld dewpoint meter offers fast, accurate measurement of dewpoint in industrial applications such as metals thermal processing, monitoring compressed air, and polymer drying.

It is not difficult to imagine that much can go wrong with an atmosphere control system. This is particularly true when dewpoint is directly measured or derived and the resulting information is used for control. If the furnace dewpoint is out of specification, is it the measuring instrument, the process controller, a valve somewhere in the system, or something else?

Dewpoint Diagnostic System
Recognizing that dewpoint can be a critical indicator of furnace performance, Vaisala set out to create a simple dewpoint diagnostic system. The resulting device combines a proven dewpoint sensing technology, an intuitive user interface, and a sample system that can operate at positive pressures or extract a sample from a process at atmospheric pressure. The entire system is housed in a small industrial grade briefcase. ( Fig. 1)

The system is based on a portable dewpoint instrument that measures from -60 to 20°C (-76 to 68°F) dewpoint. The unique sensor technology used in this instrument actively checks and corrects itself. The user interface provides several functions that are easily navigated via language based menus. A graphical display (Fig. 2) shows the measurement over time, so it is easy for the user to determine when a stable measurement has been

fig 1
Fig. 2 — Graphical display on hand-held dewpoint meter helps the user to know when dewpoint stability is achieved.
achieved. Measurement data can be recorded for future reference if desired, and the instrument can be set up to log data over an extended period of time to identify problems that could occur during a complete process run.

The measuring instrument is integrated into a sample system that includes a filter, sample pump, flow meter, and needle valve (Fig. 3). The system can be configured to extract a sample from a furnace or to measure the dewpoint of a pressurized gas. The entire Vaisala Dew Point Meter is powered by a rechargeable battery system. Because this is a direct reading instrument, there is no operator error or sensitivity associated with its use. The measured dewpoint does not have to be corrected for gas type or for other variables.

The Vaisala Dew Point Meter is commonly used to check permanently installed dewpoint instruments. This is achieved by connecting the device to the gas outlet of the unit under test and flowing the sample gas past the test instrument. If there is no gas outlet, e.g., the unit under test is installed directly in a pipe, the test instrument can be installed in a spare port on the same pipe, or the unit under test can be removed and replaced with the test instrument. In any case, readings from both instruments are compared to determine the performance of the unit under test. Direct measurements of furnace atmospheres are also possible with the Vaisala Dew Point Meter. The user simply connects the sample system inlet to a sample port on the furnace and starts the sample pump. Aflow meter demonstrates that the dewpoint sensor is actually getting a sample from the furnace.

Portable instruments provide two major benefits to heat treaters. First, if a furnace or atmosphere control system includes any dewpoint instruments, it is essential to check their performance from time to time. The demanding nature of the heat treatment environment will often cause permanently installed sensing devices to drift from calibration. It is fast and convenient to conduct spot checks using a portable instrument. The portable instrument can easily be sent out for periodic calibration, and its calibration can in turn be transferred to the permanently installed instruments. In addition, a dewpoint instrument is invaluable for diagnosing process problems. Incorrect dewpoint levels can be an indicator that something, somewhere, has gone wrong. This may not identify or solve the underlying problem, but it is a first step in the right direction.