Experience tells us that small changes and investments can impact the bottom line significantly. This article will point out areas where small investments can produce fast payback both in energy savings and, in some cases, improved process control.
Combustion BlowerMany direct- and indirect-fired furnaces and ovens are equipped with multiple burners that have a common combustion blower. The blower is sized for producing the required combustion airflow and pressure to overcome the piping restrictions and provide enough CFM at the individual burners to sustain the combustion at high fire. The maximum airflow is used at the start of the production. At soak or stable output, however, the requirements for combustion CFM drops significantly. With typical turndown of 20:1 for the burners, the combustion air blower will be dead heading at low fire and soak.
By installing a pressure transducer on the combustion-air pipe and letting it transmit directly to a variable frequency drive (VFD) installed in the control cabinet, it is possible to control the combustion-air pressure in the optimum range for the burners. When the burners go to high fire, the VFD speeds up the blower. When they are at low fire, it slows the combustion blower down. A 50% energy savings has been documented on motor amps. In addition, the burner control becomes more linear since the combustion air is supplied at constant pressure at the control valves during all firing ranges. Added benefits are noise reduction and wear reduction on the blower due to the lower speed during most of the process.
Routine maintenance like cleaning of the combustion blower wheels can also increase efficiency significantly. Many blowers do not have easy access to the wheels for inspection and cleaning. Adding an inspection port to the blower housing can help in the cleaning and inspection task. Another area of periodic maintenance is the inside of the combustion-air piping. Dust and dirt accumulates on ridges and transitions in the system. We have found that control valves can be completely filled with dirt after just a few years of operation (Fig. 1). This prevents good control. The combustion-air piping should be designed/modified to allow easy inspections and cleaning of these components.
Indirect-Fired BurnersThe proper adjustment of the burners is critical to the efficiency of the system. A small layer of soot in the tube can reduce the heat transfer by up to 20%. Good preventive-maintenance practices like monthly/quarterly O2 checks plus yearly inspections of the burners are excellent ways of ensuring good process economics. It shall be noted that new proportionators and, in some cases, new burners will add to the energy savings. We have found that more linear and lower excess O2 levels are possible. Old systems are typically fired with 3% excess O2 at high fire and 13-16% at low fire. With new proportionators and/or new burners on the same system, we have been able to obtain good combustion with 2-3% O2 at high fire and 6-8% at low fire without soot in the exhaust. Improved linearity of the combustion translates to significant savings since any excess air wastes fuel.
The use of recuperation on the exhaust leg can recover 15-20% of the exhaust energy and re-introduce it into the burner. Please note that maintenance of these devices is an additional cost.
Oven and Furnace ExhaustDirect-fired equipment with exhaust blowers provides many good opportunities for energy savings. A typical system will have motorized or fixed dampers in the exhaust system with a fixed-speed exhauster. When the system goes to high fire, the furnace/oven-box pressure goes positive. When the system is in soak at a lower firing rate, it might be only slightly positive or even negative. To compensate for the negative pressure, manual dampers are often used to allow fresh air to enter the heater box. The fresh air introduces a heat load to the system, which is compensated by the burner output.
The solution used to remedy this is to install a pressure transducer on the furnace/oven box and connect it directly to the exhaust blower VFD. The pressure should be programmed to be about +0.05 to 0.1 inch WC to ensure good uniformity at the door seals and other openings. The exhaust blower will now speed up when the burner(s) goes to high fire and slow down when operating at reduced firing rates. The fresh-air intake can be reduced or eliminated, and energy savings is immediate. (Please note that minimum airflow is required if products with solvent are processed per NFPA 86.) It is recommended to install shaft sensors on the exhausters as part of the combustion safety equipment. Airflow switches will likely not work on the low fan speeds during soak. More than 50% power reduction to the fan motor is common plus savings in energy to heat the excess air influx.
High-Temperature Furnace ExhaustDirect-fired, high-temperature furnaces offer several ways of recovering energy. The simplest way is to ensure the furnace seals and insulation are in good shape. Secondly, the systems must be fired with as little excess air as the burners and process allow. If burners are off-line during the soak cycle, these burners should have absolute minimum air passing through to the heat zone. Most burners must have some air flowing through them for cooling to protect the equipment.
We have seen systems where burners are off-line, but the same amount of combustion air is injected to the zone as if they were on. Changing the combustion-air piping by adding shutoff/control valves eliminates this waste. In some installations it is possible to add heat exchangers to the exhaust and then preheat the combustion air for the furnace.
Preheating and drying aluminum before it goes to the smelters by using waste heat from the homogenizers is a very efficient way of utilizing the waste heat. The exhaust from the homogenizer can be ducted to the dryer oven and recirculated before it is exhausted to atmosphere. The preheat oven must have an independent heat source to compensate for the times the homogenizer is not operating.
Preheating make-up boiler water is another use. Adding a heat exchanger to the furnace exhaust and using high-temperature heat-transfer fluid in a closed-loop system can save significantly on the boiler’s operating cost.
Rebuilds and UpgradesThe decision to rebuild or upgrade an existing furnace/oven depends on many factors, such as the cost of replacing equipment with new or new product mixes for the plant. Two examples will be reviewed where existing machines were modified with excellent results both in product capability and energy savings. We have found that most insulated boxes are in good shape. The controls and the heating equipment pose the biggest challenges for the plants. The following is an example of an existing-equipment rebuild.
Bogh Industries LLC together with American Controls and Engineering Service completed a total heater and controls rebuild of a large cure oven. The project was initiated when it was discovered that the existing heat exchangers had reached the end of their duty cycle due to heat damage and numerous repairs over the years. Airflow measurements had also shown that up to 30% of the fans’ capacities were lost by the restriction in these heat exchangers.
Fresh air and exhaust were used for low-temperature operation due to the low turndown capability of the four existing burners. Significant energy losses were obvious.
- Oven chamber size: 40 feet long x 20 feet wide x 12 feet high
- Two heat zones with 10 million BTU heat input each.
- Heaters: Four Maxon Kinnemax burners firing at up to 5 million BTU each.
- Certified temperature range: 150-350 +/-10°F
The modifications consist of both burner and control changes.
1. The heat exchangers were replaced with eight new 10-inch-diameter “W” burner tubes attached to two easily removed burner plugs in the event of future repairs.
2. Eight 10-inch Eclipse Tuboflame burners rated at 2.5 million BTU each replaced the four 5 million BTU Kinedizer burners.
3. The pilot gas train was eliminated with the direct-ignition burners.
4. New high-resolution motors control each burner for precise heat input.
5. The two 50-HP combustion recirculation fans were eliminated from the design.
6. All fresh air for temperature control at low operating temperatures is eliminated due to better turndown with the eight burners compared to four burners. Burners are turned off when not needed.
7. Combustion air is controlled with blowers that have VFDs and run at a set pressure. This cuts the energy usage by 50%.
The airflow increased with an average of 200 FPM measured at the outlets of the supply duct. This resulted in better uniformity in the work zone. The maximum certified temperature was raised from 350-450°F.
Eliminating two 50-HP combustion recirculation fans, two 15-HP exhaust fans (now only used for cool down) and cutting 50% usage from the combustion blowers saves 155 HP worth of electricity from every hour of usage on the oven – an estimated savings of $1,400 per year.
Eliminating the use of fresh air for low-temperature operation is estimated to save $51,000-54,000 per year in natural gas cost. In addition, the improved heat transfer has shown that the oven has more heat available than needed for the process.
The updated and simpler maintenance-friendly gas train (pilots are eliminated) and components are saving the customer significant maintenance and downtime.
The cost of the repairs and modification was less than 15-20% compared to replacement of the equipment. In addition, the downtime was only five weeks compared to the estimated downtime of six months for the installation of new equipment. The oven has extended its usefulness another 20-25 years.
The decision to get new equipment should incorporate the latest energy-saving techniques as described above. The equipment must also be built to complete the process most efficiently. Using existing equipment might not be the best option for the overall process. Recently, we built a new oven using the techniques described above. This oven cut the customer’s heat-up time from 18-22 hours to 6.5 hours on 300,000 pounds of steel dies. This rate eliminated three older, existing furnaces used for the task in the plant.
The oven is a 46-foot-long x 13-foot, 7-inches-high x 19-foot-wide gantry oven for heating up to 300,000 pounds of steel dies per load up to 800°F. The oven was installed and commissioned in five weeks, and it utilizes furnace construction with two heating zones and reversing airflow.
The oven has reversing airflow to ensure uniform and faster heat up of the heavy load. Heat-up data shows parts coming to temperature in approximately 30% of the time it took in the old equipment. Energy consumption is reduced by using burners designed for air-heat applications and a VFD-controlled exhaust blower.
This design eliminated the need for a moving load car. The oven is very suitable for situations where large loads need to be heat treated. Loading the parts on the insulated pad by crane or forklift makes the operation simple. IH
For more information: Contact Bogh Industries directly at tel: 253-732-8476 or website: www.boghindustries.com or American Controls & Engineering Service at tel: 316-776-7500 or website: www.theacesinc.com.