In some instances, it is necessary to do more than simply add heat to a product to achieve the desired result in a given heat-processing application. It is also beneficial to increase and control the relative humidity of the air in the furnace atmosphere. For example, the drying effect from convection heating can be reduced. Typical applications include: curing certain coatings on glass, drying sealants on aircraft parts, processing lead battery plates, paper processing, drying aluminum printing plates and curing special gasket material.
When thinking about humidity control, it is important to understand the two key variables: temperature and relative humidity. They are interrelated, with relative humidity being the dependent variable (see below). Thus, it is not enough to merely specify the relative humidity, one must also specify a corresponding temperature as well.
What is relative humidity?
In simplest terms, relative humidity is the ratio of water vapor (i.e., moisture) in the air at a specific temperature to the maximum amount of moisture the air can hold at that temperature. For example, hot air is capable of holding more moisture than cool air. Therefore, as the temperature of air increases, the relative humidity decreases.
If you want to maintain a constant relative humidity while heating the air, moisture must be added. And to increase the relative humidity of air already at an elevated temperature, you must add substantially more moisture.
How it Works
Moisture can be added to air in several ways. One of the simplest techniques is the introduction of a water mist into the air stream. Air and water can be mixed in a number of ways, including atomization by nozzles. Separate piping assemblies are used for air and water, each with its own valves, filters and pressure regulator. During system setup, both pressure and the flow of the air and water are adjusted to achieve proper nozzle misting. A sensor measures the relative humidity and opens and closes the water solenoid valve as required.
The nozzle technique is simple and inexpensive but is only suitable for lower temperatures and lower relative humidity. It can easily provide 20-30% relative humidity at 120°F, for example. In this case the heat present in the unit is often enough to evaporate the water, so increased heat input is not required to counteract the cooling effect of evaporation. A disadvantage is that not all of the mist generated by the nozzles immediately evaporates. Some moisture carried in droplet form collects on internal surfaces. To prevent water from dripping or spraying onto the product being processed, precautions must be built into the equipment design. A drip pan is typically required to collect any water present in the system.
Steam injection is another approach. A steam generator is required with this technique. The primary advantage of steam humidification is that higher relative humidity can be obtained (up to 100% at certain temperatures). Additionally, steam humidification is a constant dry-bulb process. This means adding humidity has little effect on temperature because the water is already evaporated before injection. Typically, the same types of sensors used in nozzle systems are suitable for steam. Other methods used by manufacturers can best be described as hybrid systems in which steam is generated inside the oven itself.
A discussion of the equipment required and various things to consider when adding moisture will be covered in part 2.
1. Grande, Michael, “Using Humidity Control in Heat Processing,” white paper