Direct vs Indirect Fired Heaters for Spray Dryers
The question of direct or indirect fired spray dryers is one that is starting to be asked in the United States more and more often, especially in food production applications. Long a staple of European food production, the concerns of contaminants in food products is beginning to drive the conversion of spray drying facilities from Direct Fired to Indirect Fired heaters over the desire to remove the byproducts of the combustion process from the process air stream. GLM Hydro recently completed the commissioning of a Spray Dryer for a customer in Rochester, MN that was driven by the desire to eliminate Nitrates and Nitrites from their end products.
Nitrites and Nitrates are formed as a byproduct of the combustion process and begin to form when flame temperatures reach 2800 °F (~1500°C). As the flame temperature passes this threshold, any increase in temperature dramatically increases the creation of NOx gases from the combustion process. NOx also requires an excess of oxygen in the air and the formation is greatest at a fuel-to-air combustion ratio of 5-7% oxygen or 25-45% excess air. Many dryers operate on natural gas fired burners, and these burners have an average flame temperature of 3500°F (1950°F) – well in the range of optimal NOx generation. An indirect fired heater may be used to eliminate the burner as a source of nitrate and nitrite contamination by heating up the process air with a heat exchanger rather than directly with the combustion gases.
The decision to use an indirect air heater comes with some trade-offs that must be considered. First, it is significantly more expensive to install an indirect air heater vs a direct fired burner control. For the project in Rochester, the indirect air heater cost almost four times as much for the equipment in comparison to installing only a burner with the same heating capacity. The second trade-off is space constraints. The indirect fired heater is an air-to-air heat exchanger that requires a lot of surface area to transfer the heat from the combustion air to the process air. The heater for our project in Rochester has a diameter that is only 25% smaller than the drying chamber, is just under half as tall, and weighs almost 19 tons. Third, the indirect fired air heater has a thermal efficiency lower than that of a direct fired burner – where a direct fired burner is 100% efficient because it combusts directly into the process air, an indirect fired heater is typically in the range of 90% to 95%, depending on the heat recovery equipment installed to boost the thermal efficiency. However, a bonus of the indirect fired heater over the direct fired burner is that you do not have to consider the effect of water generation of the combustion process on the drying of your products.
When fuel combusts with oxygen it forms water and carbon dioxide. In a direct fired burner, this water generation reduces the ability of your drying air to evaporator water from the product droplets you are attempting to dry. For our Rochester project, we calculated that a direct fired burner would on average add about 340 pounds per hour of water into the drying air. At a desired evaporation rate of 4,500 pounds per hour, this would represent a 7.5% reduction in our ability to dry product at the same temperatures as an indirect fired heater, with as much almost 10% during the worst times of the year.
Another benefit of the indirect fired heating system is plant and system safety. By heating the air indirectly, you remove the possibility of product ever becoming exposed to the open flame of the burner. While unlikely, if the dryer air flows are not carefully controlled, it is possible for product to back draft through the drying system into the heater. If this were to happen, the burner could be ignited with dispersed powder in it, causing a fire or worse. This cannot happen with an indirect fired burner as these air flows are physically separated from each other by the walls of the heat exchanger.
There is a lot to consider when deciding on an indirect or direct fired burner. As regulations and customer preferences continue to evolve, it is likely more and more systems will be purchased with or retrofitted to accept indirect air heaters. It is a decision that must consider the financial and market realities of today as well as the potential of tomorrow.
Contact us for more information when you are considering the decision of installing an indirect or direct fired heater for your spray dryer or any of your evaporator and dryer questions.
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