How to optimize dryer and oven energy performance
July 17, 2012
by Douglas A. Zietlow, PE, senior process engineer, POWER Engineers
Dryers and ovens can use 10% or more of the total energy consumed by a processing facility. Often this energy use comes with great inefficiency. Removing 1 lb of water with a dryer requires at least 970 Btu, but the actual amount required depends on the altitude, product formulation, product temperature, and moisture content. Dryers often consume two to three times as much energy as required to remove a pound of water.
Depending on operating conditions, ovens can consume five times as much energy as required to heat and process a product. The dryer/oven heat source introduces additional energy inefficiencies. Based on primary fuel energy content, thermal energy efficiency is approximately 85 to 95% for gas burners, 70 to 80% for steam, and about 30% for delivered electricity. The unit energy cost of these heating sources reflects the energy efficiency of each.
Many sources discuss general industrial energy efficiency opportunities such as improved equipment insulation and fan motor efficiency. Although these can provide significant energy savings, even larger benefits often exist with less obvious opportunities. Design, operation, maintenance and sanitation can affect dryer/oven energy efficiency.
One important design consideration is the use of air recirculation, which reduces wasted energy because it reuses a portion of exhaust air from the dryer/oven. However, energy efficiency improvement with air recirculation is limited by the allowable humidity in the dryer/oven system. Alternatively or additionally, waste heat from exhaust air may be used to preheat dryer/oven makeup air, product into the dryer/oven, or water and or air for other uses.
Operating conditions also impact energy efficiency. For instance, operating a dryer or oven at the lowest exhaust air temperature and lowest air flow rate possible generally increases its energy efficiency. Reducing product moisture to a dryer will reduce the energy required for drying. Increasing dryer/oven humidity to the maximum extent possible reduces makeup and exhaust air rates, reducing energy usage.
For dryers and ovens using a belt for conveying product, product distribution is also important. Generally, the product should cover the full width of the belt as uniformly as possible. This will improve not only improve energy efficiency, but often product quality as well.
Air leakage in or out of the dryer/oven can be another source of energy loss. Air migration into the dryer/oven may require heating the supply air to the product to a higher temperature to compensate for the additional cold air migrating into the dryer/oven. Air leakage out heats the surrounding space rather than the product.
A well-maintained and properly tuned control system also helps ensure that air temperature, humidity, and recirculation rate are well controlled thus optimizing energy conservation.
Another opportunity for energy savings can be startup, idle and shutdown procedures. Needless to say, heating an empty dryer/oven is a waste of energy. Implementing procedures that minimize the amount of time a hot dryer/oven is empty and minimizing energy usage while it is empty will improve your overall energy efficiency.
Improving burner or boiler efficiency requires burning less fuel to get the same heat output. If steam is the dryer/oven’s heat source, then the design, maintenance and operation of the steam and condensate system will have a significant impact on overall energy efficiency.
Even your sanitation protocols can affect dryer/oven energy efficiency. Keeping fans, dampers, and heating elements clean will improve their performance by improving air flow control and thermal efficiency.
In addition to reducing energy costs and improving energy efficiency, these kinds of improvements can also achieve corresponding increases in dryer/oven production capacity. Increased capacity can often be worth more than your targeted energy cost savings.
Although most dryers and ovens have a number of opportunities for improving energy efficiency, which ones are worth implementing depends on the costs, benefits, and economic return of doing so. An engineering evaluation or audit can assist bakers and snack food manufacturers determine which dryer/oven energy saving opportunities are worthwhile. A structured program will establish an existing baseline on which energy savings opportunities, and corresponding costs can be benchmarked to provide you with the best cost and benefit opportunities for your facility.
This story is sponsored by POWER Engineers, which has one of the most comprehensive teams of engineers and specialists serving the baking and snack industry. As an extension of its clients' engineering teams, the company provides program management, integrated solutions and full facility design for the baking and snack industry. Learn more at www.powereng.com/food.