Energy conservation can significantly affect the bottom line of organizations. Soaring energy prices and expensive capital equipment are a dual challenge today.

Fig. 1. Depiction of heat-transfer process


This paper attempts to identify the “low-hanging fruit” of energy conservation that can radically save energy without huge capital outlays. The methodology and its scientific basis are discussed as simply as possible. Detailed knowledge of mathematics is not necessary. However, the mathematical basis is discussed for those who want to dig deeper.

The strategy described here is based on:
  • Simple methods that add up to energy savings of as much as 50%
  • Quick returns on investment – sometimes a week
  • No capital investment is required to get at the low-hanging fruit


Practical Example - Stress Relieving

The relief of residual stresses is a time-temperature related phenomenon parametrically correlated by the Larson-Miller equation.

Thermal effect = T(log t + 20)(10 -3) where T is temperature (Rankin) and t is hours

For example, holding a piece at 595°C (1100°F) for six hours provides the same relief of residual stress as heating at 650°C (1200°F) for one hour. The energy savings by going to the higher temperature are in the region of 75% or more. The return on investment is as long as it takes to use a calculator – a good example of low-hanging fruit. The catch is that it takes a certain amount of finesse to do the arithmetic, and there are metallurgical concerns that may call for good technical judgment/expertise.

Fundamentals

Energy used = Energy produced - Energy lost

The burning of natural gas is generally depicted stoichiometrically as follows:
CH4 + 2O2= CO2+ 2H2O + Heat

In industry, excess air is utilized to a great degree. Some furnace operators run their furnaces with as much as 300% excess air. This can result in huge energy losses as shown later.

Heat Transfer

Conduction, convection and radiation are the three principal modes for heat transfer.

Conduction
In conduction, the heat flows through the material, from a higher temperature to a lower temperature based upon the following law of conduction:

Q(heat flow per unit time) =k(thermal conductivity) xA(area perpendicular to the heat flow) xTemperature Gradient(difference in temperature/distance between the high temperature of the furnace and outside wall)

Convection
Heat transfer in convection is governed by the following equation:

Q(heat flow per unit time) =h(convective heat-transfer coefficient) xA(area of the surface) xTemperature Difference

The heat-transfer coefficient (h) will vary with factors such as turbulence, rate of air/fluid flow, etc. For air, “h” may vary from 10-100 Btu/hr-ft2-°F.

The equation can be used to figure out the heat lost in transporting a piece of metal for quenching. A piece of steel 1 ft2at 1700°F will lose heat by convection, as follows. Here “h” has been assumed to be 18 Btu/hr-ft2-°F and room temperature 70°F.

Q = 18 Btu/hr-ft2-°F x 1 ft2x (1700 - 70°F) = 29,340 BTU/hr

Radiation
Radiation is the dominant mode of heat transfer in furnaces. The heat transferred is

Q = esAtT4

where e= emissivity of the object radiating (furnace);s= Stefan-Boltzmann’s constant = 0.174 x 10-8 Btu/(hour ft2°R4) with °R=459.67+°F; A = area of the surface radiating; t = time of the radiation; T = temperature of the object.

For example, a normalizing furnace running at 1700°F with a gap between the door and furnace shell will radiate heat. Assuming it is a car-bottom furnace with a gap of 4 inches over a length of 6 feet, the area of the gap is 6 x 4/12 = 2 ft2. The emissivity can be assumed to be about 0.9. Using the law for radiation above we find out the heat lost is:

Q = 0.9 x 0.174 x 10-8Btu/(hour ft2°R4) (1700+459.67°R)4 x 2 ft2= 68,135 BTU/hr.

Hence, it is important to shut the furnace doors properly. If a gap exists, it should be covered with a ceramic-wool blanket. If the door is left open, however, the heat lost depends upon the area of the door. Assuming it is 6 x 6 = 36 ft2, the radiation equation yields Q = 1,226,433 BTU/hr.

Fig. 2. Furnace efficiency vs. excess air

Heat Losses

In furnaces, heat losses occur in the following ways:

1. Flues– More than 50% of the heat may be lost through the flues. This is mainly due to the use of excess air for combustion. In boilers, the flue-gas heat loss may be as much as 83%. Flue-gas heat loss is the single largest energy loss in a combustion process, and because the products of combustion are heated by the combustion itself, it is impossible to eliminate this flue-gas heat loss. Reducing the amount of excess air supplied to the burner, however, will lessen flue-gas heat loss. ....In heat-treating practice, it is assumed that excess air will contribute to furnace uniformity by causing turbulence in the furnace. At higher temperatures above 1400°F, however, more than 90% of the heating is done through radiation. Therefore, increasing turbulence does not help in achieving uniformity. It is best to go closer to stoichiometric amounts of needed air. This is best achieved by slowly fine-tuning the process. Start cutting back on excess air at temperatures above 1400°F, and use the dampers again, very gradually, to reduce the exit of gases taking into account safety of the operation first (Fig. 2). This method can reduce the heat losses by over 33%. It has to be achieved slowly, however, and one builds upon the shop-floor experience and constraints over time.

2. Conduction through the refractory walls– These are generally low, unless the refractory lining has been damaged. With proper maintenance, many of these losses can be reduced significantly.

3. Convection and radiation– These are generally visible, and the best shop-floor remedy is to patch the area where hot gases are escaping with a ceramic-fiber blanket. Pictures can also be taken with an infrared camera to detect heat losses.

Scheduling as a Tool for Energy Conservation

Another method, which does not rely upon costly new capital equipment but good shop-floor management, is scheduling. Smaller loads are very inefficient because the bulk of the heat is spent in heating the refractory lining and also exits through the flues. Bigger loads are more efficient because the losses are more or less the same, since the same amount of energy goes into heating the walls of the furnace.

Flue-gas losses are similar for large and small loads because the rate at which flue gases escape depends upon the energy of the flue gas. This is primarily dependant on temperature and factors such as flues and damper position, which are generally the same for bigger and smaller loads. Therefore, for larger loads the energy consumed per unit weight is less. Table 1 indicates the increase in efficiency by increasing the load size.

Loads can be combined for better economies, and furnaces should not be kept waiting for equipment and personnel. Proper scheduling can eliminate down-the-road bottlenecks and affect energy conservation. An example is an industrial facility that utilized the exhaust from its annealing furnaces to preheat boiler water. The calculated savings were about $320,000/year.

Energy Audit

A simple audit can be done, and this will require identifying how much natural gas/fuel is consumed every month. A breakdown of fuel consumed by each unit is then determined. These are the steps:
  • Collect energy-consumption data from each furnace and overall facility.
  • Collect other technical data needed for analysis. This may include loads heat treated/melted, exit gas temperatures, excess air use, broken insulation, condition of equipment, improper calibration of furnaces, fuel-oil leaks, steam leaks, bare hot surfaces needing insulation, burners out of adjustment, equipment idling when not needed, compressed-air leaks, gas leaks, product rejects, unnecessary handling of materials, frequent production interruption/shutdowns, unnecessary pressure-reducing stations, defective control instruments, defective steam traps, faulty installation of steam traps, plugged-up filters of blowers/compressors, dirty working environment and lack of lubrication – especially furnace wheel bearings. Fine-tuning can result in significant energy savings.
  • Identify improved operating/maintenance procedures.
  • Identify minor cost improvements.
  • Identify any steps that can be minimized/simplified to reduce energy use (inter-critical heat treatments may replace two heat treatments).
  • Identify any product rejects that can be minimized to reduce energy use.
  • Can waste heat be recovered for preheating water/material?
  • Are there any financial/tax incentives provided by the state or utilities?
The previously reviewed calculations can be done to evaluate current efficiencies. The goals can be set to achieve these targets. The goals, methodology and results should be shared with the shop personnel. As they get involved, the contributions add up. In one heat-treating facility where this approach was adopted, the unit stopped losing $40,000/month and began making a profit of $160,000/month. The following case studies further illustrate the dramatic impact of this approach.

Case Study - Forging Industry

Maintaining good temperature control in hot forging can greatly reduce rework and save a lot of energy. Flow stress for a material depends upon yield stress, strain rate, etc. Strain rate is a function of the ram rate. Yield stress is a function of temperature. For a 50°F drop in temperature, the yield stress of some aerospace alloys can double. Therefore, it is very important to maintain temperature control to reduce rework and save energy.

Case Study - Diffusion Bonding in the Aerospace Industry

In the aerospace industry, titanium alloys are used extensively because of their high stiffness to weight ratio. These titanium alloys are difficult to machine, however. Hence, it was decided to make large parts – some over four feet in length – by joining smaller parts using diffusion bonding. Diffusion bonding can be a simple process. Parts are heated and mated, and they bond together if proper temperature and pressure is used.

The problem was that in spite of tremendous capital expenditure, the process was having major problems – a reject rate of about 160%. The rejects were even getting rejected after rework. The energy costs were horrendous. Titanium diffusion bonding is done at high temperatures under pressure, and the rework required TIG welding and extensive machining.

A study revealed the problem was improper tooling setup. The parts never really mated in many areas when put under pressure, resulting in a higher reject rate. Once the tooling was properly aligned, the process worked and energy savings were tremendous. A multi-billion dollar program stayed on schedule as a result of this simple fix.

Conclusion

Energy conservation can be achieved using a well-integrated approach backed with good engineering skills and shop-floor management.IH

For more information:Shobhan Paul is a vice president for Starfire Technologies LLC., 31505 PCH, Malibu, CA 90265, tel: 310-457-6781; fax: 310-457-6782; e-mail: shobhan@verizon.net.

Additional related information may be found by searching for these (and other) key words/terms via BNP Media SEARCH at www.industrial heating.com: energy conservation, stress relieving, Larson-Miller, heat transfer, conduction, convection, radiation, emissivity

SIDEBAR: Case Study - Heat-Treating Industry

The energy lost in operations such as heat treating can be as much as 93%. Hence, if a heat-treating operation has an energy bill of $100,000/month, it may be wasting up to $93,000/month.

Here is a simple way of finding the approximate efficiency of a gas heat-treating furnace where the weight (W) of the steel or metal being heat treated (at 1700°F) in the furnace is 1,000 pounds. If weight is not known, make a rough estimate of the volume and multiply it by the density of the metal. References 1 and 2 are good sources for metals/materials properties for doing the calculations. The specific heat (H) of the metal = 0.116 BTU/lb°F. The energy needed to heat the metal is:

W x T x H = 1,000 lbs. x 1700°F x 0.116 BTU/lb°F = 197,200 BTU

Actual gas burnt = Final reading of the gas gauge – Initial reading of the gas gauge = 1,643,300 BTU. Therefore, the efficiency of this furnace operation is 197,200/1,643,300 ~ 12%, and the dollars wasted (per load) are calculated by multiplying the price of gas, $7/MMBTU (1 million BTU) or $0.000007/BTU, by the quantity of wasted gas.

Money wasted = (1,643,300 BTU – 197,200 BTU) x $0.000007/BTU = $10

When there are a number of car-bottom furnaces heating huge shafts (for instance), the losses could add up to maybe $200,000/month. Prices of gas will vary as will, accordingly, the losses.