We continue our discussion on humidity control in furnaces and ovens from last time.
Most heat-treat furnaces and ovens are either gas-fired or electrically heated (Fig. 1). With gas-fired equipment, the products of combustion from the burner can disturb the moisture-to-air ratio when trying to perform humidity control. Therefore, gas-fired equipment must employ indirect heating in which the burner is fired through a heat exchanger, which is then used to heat the recirculating air indirectly.
Controlling the System
When enhancing relative humidity in any system, a means of sensing the humidity to provide closed-loop control of the steam or atomizing nozzles is necessary. Low-cost electronic relative-humidity sensors are available for most applications. For example, a thin-film polymer-type is suitable for temperatures up to 350°C (660°F). Another common sensor is the wet/dry bulb-type. This design has certain advantages, but it is limited to lower temperatures. More exotic sensors may be required by certain special applications for relative humidity measurement and control. Variable capacitive measurement devices can be used to measure relative humidity at temperatures up to 650°C (1220°F) or even 1000°C (1832°F). Finally, portable instruments that record the relative-humidity data and provide electronic interfacing to store the data are available.
Industrial ovens must incorporate special features for those applications in which moisture is being added to the process. Many relative-humidity-specific features are costly but necessary, and incorporating these at the time of purchase is prudent if moisture is to be added to the system.
If the system will be used frequently in high relative-humidity conditions, a stainless steel interior is recommended to avoid rusting and other deleterious effects. With mesh-belt units, a stainless steel or temperature-permitting plastic (acetal-resin) belt is also a good idea. To prevent condensation from infiltrating the insulation and eventually saturating it, it is important that the oven interior shell be sealed in most applications (as opposed to standard tongue-and-groove construction). Depending on the temperature and relative humidity, sealing the oven can be accomplished in a number of ways.
The choice of heating method (gas-fired or electrically heated) is the preference of the equipment buyer. But remember that to prevent the disruption of the humidity control in gas-fired systems the products of combustion and moisture generated by the burner must not be allowed to mix with the oven process air.
The main drawback of the steam-injection approach is greater complexity and capital cost. The steam generator must be purchased, installed, operated and maintained as a separate piece of equipment. Depending on the system design, water softeners may also be required. A drip pan is typically supplied, and the unit may even need to be located directly over a drain when high relative-humidity levels are required.
Heat treaters seldom think of the need for humidity control in their units. When the necessity arises, however, it is good to understand the basics.
1. Grande, Michael, “Using Humidity Control in Heat Processing,” white paper