This is the third in a series of steps to get the maximum energy efficiency from your ovens and furnaces. The first two dealt with optimizing scheduling, loading, operation and maintenance. Now it's time to look at equipment improvements.
There's an old joke about a Texas oilman so rich he bought a new car every time his windshield got dirty. We should all be so wealthy. However, over the years, I've met any number of people who have a similar attitude about their heat processing equipment. Capital funds aren't that easy to come by in most places, but if nothing gets spent on maintenance or incremental equipment improvements either, the oven or furnace becomes a poor performer, and people get so frustrated they want to junk it and start over.
Look at this as a two-tiered exercise. The second tier is outright replacement, but the first is retrofitting what you have to raise its level of performance. The advantages of this approach are the same you get for doing tuneups and maintenance first -- quicker results, less money spent and less disruption to your operations. Compare the cost-benefit ratios of upgrades and replacement before cutting any purchase orders. In determining what can be done, you can follow pretty much the same checklist you used to optimize your existing equipment.
Exhaust or Flue Gas Losses. These are a function of how much excess air your combustion system uses and how effectively you can transfer heat to the load.
Do you have a fixed air ratio control system, where only the fuel flow is controlled to regulate temperatures? This operating method has been a standard way of firing ovens, dryers and some furnaces for years. It gives you excellent temperature control sensitivity and firing rate flexibility, but it does chew up a lot of fuel. Many processes can be served just as well by burners with control systems that modulate both the air and fuel, using less excess air at reduced firing rates. The reduction in fuel consumption can be significant.
In my last column, I talked about the need to maintain a good pressure balance in ovens. The same holds true in high temperature furnaces. Here, stack draft can drag in cold room air past poorly fitting doors and deteriorating seals. This air has to be heated to furnace temperature, consuming energy, before being tossed out the stack without contributing anything to the operation. Doors and seal repairs should have been done in Step 2, but those components take constant abuse, and keeping them tight is a continuing battle. Look at a furnace pressure control system to help them out. This will automatically hold the furnace at a near-neutral internal pressure, keeping infiltrated air out.
Can you speed up your processing times by improving the heat transfer to the load? First, you have to be able to take advantage of this. If you're bound by a specification that says, for instance, you must hold the work at a certain temperature for a certain length of time, faster heat transfer might not be an advantage. Then again, most of those specs say the clock on the heating cycle doesn't start ticking until the load or furnace comes up to setpoint. If you can get a fresh load up to temperature sooner, your total cycle time can be shortened, productivity will improve, and your energy costs will decline.
How do you get better heat transfer? It depends on your equipment and process, but areas to look at include higher velocity fans or revised air baffling on ovens; high velocity burners on direct-fired furnaces; and redesigned material-handling equipment to give the work better exposure to the heat source. There are many others, as well.
Wall Losses and Heat Storage. The design of your insulation system affects both of these. If your present furnace lining is due for some major work, this might be the time to go beyond a straight-out replacement. As a rule, most modern insulation systems do a pretty good job of keeping wall transmission losses down to a reasonable level, but verify it anyway. You still have those skin temperatures you collected looking for hot spots, right? Compare them with the kinds of surface temperatures equipment manufacturers shoot for on their latest products. If yours are significantly higher, it might be time for more or better insulation.
If you're operating your equipment in a batch or semi-batch mode (heat up, cool down, heat up, cool down…), take a hard look at your heat storage. Do you really need those dense refractories everywhere in the furnace? If they have to carry heavy loads or are subject to mechanical abuse, maybe so, but elsewhere? The goal is to get the equipment up to operating temperature sooner and use less fuel doing it.
Radiation Losses. If you're making sure doors are open no longer than necessary and that radiation shields are in good repair, you've probably done all you can in this area. If you have to operate with a furnace door open all the time, however, consider adding a tunnel-like extension to the opening. If it doesn't interfere with the operation of material-handling equipment, it will trap some of that radiant energy trying to leave the furnace.
Cooling Media. Cooling water, liquids and air are usually looked on as sacred cows because they're protective in nature, and they're certainly not to be trifled with. But don't shy way from looking at new types of doors, seals and conveyors that have reduced cooling requirements.
Once you've tended to all these areas, you're positioned to make an intelligent decision whether to retrofit or replace.