Energy Efficient Process Heating: Managing Air Flow

Much energy is lost through excess air flow in and out of process heating equipment. Energy saving opportunities from managing air flow include minimizing combustion air, preheating combustion air, minimizing ventilation air, and reconfiguring openings to reduce leakage. This paper identifies these opportunities and presents methods to quantify potential energy savings from implementing these energy-savings measures. Case study examples are used to demonstrate the methods and the potential energy savings. The method for calculating savings from minimizing combustion air accounts for improvement in efficiency from increased combustion temperature and decreased combustion gas mass flow rate. The method for calculating savings from preheating inlet combustion air consists of fundamental heat exchanger and combustion efficiency equations. This method accounts for the reduction of combustion air flow as fuel input declines, which is often neglected in many commonly-used methods. The method for calculating savings from reducing forced ventilation in ovens accounts for flow rate of ventilation air and air temperature when entering and exhausting the oven. The method for calculating savings from reconfiguring oven openings accounts for flow rate of air entering and exiting the oven due to buoyancy forces. INTRODUCTION Managing air flow is usually the most important aspect to consider when attempting to improve the energy efficiency of most process heating systems. The largest loss in fuelfired process heating is nearly always the loss through the exhaust stack, which is often greater than all other losses combined (Thekdi, 2005). For example, in a boiler, about 20% of input energy is lost in the exhaust gasses while only about 2% is lost through the boiler shell. For higher temperature applications, even more energy is lost in the exhaust gasses because they leave the system at higher temperatures. For example, boilers generating steam at 250 F to 350 F typically have efficiencies of about 80%. Furnaces that melt aluminum at 1,400 F have efficiencies of about 50%, and furnaces that melt glass at 2,500 F have efficiencies of about 30%. Although most or all air in a fuel-fired heating system leaves the system through the exhaust stack, air enters the system as combustion air, ventilation air, and infiltration air. Figure 1 shows a process heating system with the major categories of air flow.