Taking the Long Look
Sometimes, after a while studying the trees, it’s a good idea to step back and look at the forest. I’ve done a lot of columns on the specifics of saving energy in process heating. This might be a good time to look at how those specifics fit together.
I’ll begin with another look at our old friend, the Sankey diagram. This one’s been embellished with recommendations on how to reduce the different categories of energy losses. Some of these may be of great benefit to your operations, some none
at all, and most, somewhere in between. How do you know which ones to pursue?
Before you even start looking at equipment and process modifications, make the best of what you have. Optimize your equipment’s operation through tuneups, repairs and preventive maintenance. Sometimes, this is a hard sell once people have bought into the “newer is better” mindset. All they want to do is get on with tearing out that old clunker. Truth is, much of that old equipment may perform surprisingly well if given a little TLC.
There’s another reason to do this, even if you have your heart set on installing a new oven or furnace. Payback or ROI justification is a nearly universal hurdle to be cleared, and comparing the factory specs of the latest state-of-the-art device with misadjusted, out-of-tune equipment can produce some badly distorted results.
Once you’ve got the existing equipment running as well as it can, it’s time to establish your performance baseline. Collect data on production rates, energy consumption, operating schedules, exhaust gas temperatures, flows and analyses, wall, conveyor and radiation losses and heat storage requirements. Can’t estimate your energy consumption? Then you need a meter. If you have several pieces of equipment and lack the budget for a meter on every one, consider buying one meter and moving it from place to place.
With all your data in hand, run a heat balance to determine your efficiency level and the magnitude of your losses. If you’re not familiar with the fine points of heat balances, take advantage of the Department of Energy’s PHAST program (see links at end of article for details on PHAST). Identify the largest losses and evaluate improvements that will address them. This requires some judgment -- not every technology works in every situation, so here are some general guidelines about a few of the more popular technologies and where they fit.
Preheating Combustion Air. Preheating combustion air using heat exchangers, recuperators or regenerators makes the most sense on processes operating at 1,500oF (800oC) or higher. However, if your exhaust gas flow is high enough, you may find you can hit the payback threshold at temperatures of 1,000oF (550oC) and sometimes lower. The key thing to remember is that BTUs saved are what pay for the heat exchanger, and either high exhaust temperatures or large exhaust volumes can provide them.
High Accuracy Combustion Ratio Controllers. As a rule, these don’t offer justifiable energy savings unless the process is fairly large and operates at temperatures in excess of 2,000oF (1,100oC). Low temperature processes tend to operate at fairly high excess air levels, and small ratio fluctuations simply don’t have much of an impact on efficiency. However, where product can be adversely affected by excessive or uncontrolled levels of oxygen in the combustion gases, or where emissions of NOX must be closely controlled, precision ratio control may be beneficial, if not mandatory.
Low Density Fiber Insulation. Unless you’re dealing with a really old oven, chances are it will already have some sort of low density insulation. A good rule of thumb is that there should be 1" of insulation thickness for every 100oF (55oC) of oven temperature. If you have substantially less than that, you might be a candidate for an upgrade.
High Velocity Air Circulation. In convection ovens and dryers, air velocity is critically important. Unless it is sufficiently high at the point of contact with the product, heat transfer will suffer, leaving you with the choice of slowing the product flow or jacking up the temperature to get the productivity you need. In other words, you have a choice between poor productivity and poor efficiency. Getting the most out of your air-circulating system may call for higher fan pressures, less restrictive ductwork, redesigned air nozzles and slots, or a combination of these.
Obviously, there are two ways to apply energy-efficient technologies -- as retrofits to your existing equipment, or as part of a totally new installation. As long as your existing heat processing equipment meets your production requirements and is still in reasonably good shape, a retrofit may make more economic sense.