How to Specify the Right Heater for Your Application
Now that you have a general idea of the types of heaters that are available, how will you go about specifying the right one for your application?
To narrow your search from the beginning, it is important to always keep industry and application in mind. There are many heater designs that are suitable across multiple industries, but some are simply best suited for a particular industry. To be sure you are looking at the heater that is most relevant for your process, always keep your industry in mind. Similarly, what is the application? Will you need a heater that can run at high temperatures and pressures for a long time? This would be very different from a heater that will be used for short runs and lower temperatures and pressures.
Chris Horten, Heatec’s sales engineer for the Southeast United States, non-domestic Americas and Europe, [SES2] says temperature and flow rate are the two most important components to consider. It is key to know the type of medium being heated and the amount of heating power required. Some heaters have been specially designed to function in oils, viscous or corrosive solutions. However, not all heaters can be used with any material. Confirm that the heater will not be damaged by the process.
Another consideration is, of course, budget. A heater that costs more than a given price range is not an option, so this will also aid in narrowing your choices—especially if the budget is limited. Don’t, however, consider just cost up front.
Compare the estimated operating efficiency and long-term operating costs of the systems that seem to be within your price range to help you specify the option that is best for your plant in the long run.
Another item that might seem obvious to consider is where the system will be placed as well as its footprint. Sometimes the obvious can be overlooked. Horten says that he once had a customer who was trying to determine if a particular unit would fit in the space they had allotted on the plant floor. It was after 20 minutes of configuring and re-configuring that Horten asked the simple question, “Do you have space outside?” The customer admitted he had no idea that was even an option. Thinking outside of the box is something that both the purchaser and the seller should do.
Knowing your region’s emissions and efficiency requirements is important, too. Be sure to do your due diligence so that these factors can be a part of the selection process.
Of course, different manufacturers offer different designs. Consider key design elements to help distinguish between comparable heaters and determine which option is best for your specific application and facility.
An advantage of vertical heaters is their small footprint, so they can fit into small ground areas. Knowing that they can be installed outside is also significant.
Horten says that in thermal fluid heaters, for instance, heating coil design is a particularly important feature. “Flow should be maintained between 5 and 8 feet per second (fps)—any slower and you are oversizing the piping and not spending money properly on valving…it’s oversized. Any faster, and you are going to start having a lot of pressure drops and increasing your pump sizing.” In other words, to get better heat transfer in a small section of piping you are looking at a volumetric flow rate of 5-13 fps—that is what determines the pipe diameter and how many splits that are inside. Horten notes, “There is only so much that can be done with a coil: if there is too much volumetric flow rate, you’re going to want to use a heater like the Convectec™—one that has a large header with many splits that serpentines back and forth from one header to another. The more mass flow on the flue gas, the better,” he says.
Another application for this type of heater would be one that uses crude, a very viscous fluid. Horten says, “It's going to move very slowly. So that it has a low temperature rise and a mass flow going through you'll typically want to use a lot of splits—that way you’re not going to damage your process; you’re not going to coke up. High-circulating cooler flue gas is less aggressive than a flame.”
If your process is such that it is getting too cool, another option is a water bath. This is when knowing your industry and application is crucial.
Heatec Aquatec™ water bath heater is in operation at a natural gas compressor/booster station in South Dakota.
Water baths are suitable for the natural gas industry because typically it is high pressure. Horten explains, “You have a high-pressure vessel on the process side and you have a fire tube, which is firing natural flame through, and in between the two is filled with water glycol. The water glycol takes the heat of the fire tube, and the process bundle pulls the heat out of the water glycol.” Typically, this is anywhere between 50 and 130°F, which is too cool for a coil as it would start to condensate. Another application for water baths could be a power plant with its gas turbines. “They will want to preheat the gas for efficiency, so they’re looking for a temperature that’s ready for combustion,” he explains.
If your application is cooking oil, Horten says there are a couple ways to go about heating that. You can have some kind of intermediate fluid that’s going to circulate around your plant and heat the fryer indirectly. “So you’re going to send this hot oil through our heater on the carbon steel coil, and then it goes into your fryer’s stainless steel tubes to heat up the frying oil as the food picks the energy up,” he explains.
Another option does not require an intermediate fluid. “You can push the frying oil directly through our heater, but in order to do so, you need a stainless steel coil. Typically, these heaters are vertical so they can be free-drained, but it removes the need for the intermediate fluid.”
No matter what type of heater you get, they all require some kind of energy source—natural gas, electric or you can recover your own flue gas. “We can take the convection unit and make it into a waste recovery unit. So you can pull energy out of this gas that would normally just go to atmosphere and can use anywhere else in your process.” Of course a caveat for this option is less control over the heat source. “When we use an electrical element of natural gas flame, we can control how much energy is going in. With a flue gas stream, we recover heat but don’t have control of it,” he says. To alleviate that concern, bypass valves and large dampers can either send the process through a bundle or bypass it or somewhere in between. There are two safeties: one is a high-temperature safety sensor on the flue gas side and the other one is as a modulation thermocouple in the hot oil or heat transfer fluid outlet. “So as it starts to approach that temperature, it starts to throttle how much is going through the process bundle from the flue gas side. If a safety gets too hot, you want to protect your fins and the process bundles that may shut off that direction but in general modulates based on the outlet temperature of the process—similar to how we modulate a flame.” And to protect your fluid, there’s also a high-temperature shutdown on the process side as well.
Convection heaters like these are used for heating crude oil at a production facility in Angola.
Specifying the right heater can seem like a challenging task, but by identifying your industry, application, cost, footprint, emissions regulations and working with a knowledgeable supplier, you should be able to focus your search.
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