Many business leaders understand the basic sensibilities involved in reusing heated process air as a green solution for conserving energy during drying but when done correctly, it can also translate into tangible financial returns.
Here are three critical issues to consider before investing in the necessary recycling equipment, according to Larry Stoma, design engineer for the process equipment manufacturer The Witte Co., Washington, New Jersey. Witte developed this recycling concept during the 1970s to offset the jump in energy costs and has fine-tuned it in the years since to reliably improve efficiency, project savings, and support compliance with the maze of federal, state, and local rules and regulations.
1. Compare Process and Exhaust Air Temperatures
Materials such as salt and sand allow high process air temperatures upwards of 500 degrees F, which may generate exhaust air of 250 degrees F. Capturing and returning this heated air into the process makes sense since it is far less costly to raise 250-degree air to 500 degrees than it is to raise ambient air up to 500 degrees. Conversely, since delicate materials such as seeds, cereals, and some plastics require processing at low temperatures from 100 – 200 degrees F, the spread between the process air temperature and the exhaust air temperature may not be wide enough to warrant the investment in heat recovery equipment.
2. Product Drying Characteristics
Product characteristics such as particle size, shape, mass, porosity, and moisture content affect the rate and efficiency of energy transfer. Materials that dry easily such as carbon black, leave very little heat in the airstream to be recovered. Others that dry less efficiently, such as many vegetables, require longer drying times and/or higher drying air velocities that leave enough heat in the airstream to make heat recovery viable.
3. How Much Air to Recycle?
To the uninitiated, 100 percent may be desired. But drying releases water vapor into the exhaust air stream and recycling this water vapor back to the product would defeat the original purpose. Most commonly, 50 percent of the supplied air is recycled. To recover the most heat at the highest operating efficiency, the exhaust from the second drying zone needs to be recycled to the inlet of the first drying zone. This captured energy can then be used to preheat the air going to the first drying zone of the fluid bed. To do this properly, the captured air first needs to be cleaned using a bag collector.
4. Calculating Returns
Consider this sample process where a fluid bed dryer requires 10,000 Standard Cubic Feet per Minute (SCFM) of air heated to 500 F. This system would require $33.00 of natural gas per hour, assuming an ambient temperature of 65 degrees F and a natural gas cost of $7.00 per unit. By recycling 50 percent of the air at a recycled air temperature of 150 degrees F, the natural gas cost may be reduced to $29.80 per hour. This savings equates to a potential savings of $23,200.00 per year in gas alone. Given this type of drying system would likely require a baghouse dust collector at a cost of an additional $35,000.00 for the dust collector and ductwork required to recycle the air effectively, the recycling system would pay for itself in energy savings after two years of operation.
The Witte Company designs and manufactures drying, cooling, screening and related vibratory process equipment at its headquarters in Washington, New Jersey. Family-owned and operated since its founding in 1938, Witte has earned a stellar reputation for engineering expertise, craftsmanship and personal service. Witte equipment is guaranteed to work ...