An Energy Audit for Carburizing
However, speed and cost are not the only concerns in industrial processing. In a world where energy is in ever-increasing demand, the amount consumed to process a given part can be a critical factor.
Energy SourcesSources of available energy depend on location. To improve comparability, this study assumes, except where stated otherwise, that the ultimate energy source is natural gas. The energy required to extract and distribute it can therefore be disregarded. The efficiency of electricity generation has been taken as 50% (high for conventional gas-fired power station but low for a CHP station, and a reasonable mean for all forms of electricity generation [5, 6]).
Furnace EquipmentWhile it is tempting to compare two furnaces with the same loading capacity, they are not really comparable. The slightly longer cycle time of a low-pressure carburizing furnace (floor-to-floor) does not allow it to carburize as much product as a conventional furnace in a given time, even though the actual carburizing time is shorter. In this study, a typical gas-fired, sealed-quench furnace using an endothermically generated atmosphere (Fig. 1) is compared to an acetylene-based vacuum carburizing unit using a 15-bar nitrogen quench (Fig. 2), both having the same total throughput. The initial study assumes an optimal continuous 24/7 output. The effect of the sub-optimal loading typical of a unit integrated into a production line is discussed later.
Capital Cost and EnergyNo comparison of processing routes, in terms of either the total energy used or the total processing cost, can ignore the equipment the process uses. This is widely understood in terms of the processing cost where depreciation is averaged across all the units produced. The energy used to produce the equipment itself, however, is ignored by most studies because it is difficult to define accurately . This article assumes that the total energy cost of the equipment is equal to the energy cost of producing steel components (65 MJ/kg) and that the equivalent of the entire structure is replaced by spare parts over the equipment’s lifetime of 20 years.
FiringAs the vast majority of sealed-quench furnaces are gas fired and most vacuum furnaces are electrically heated, these have been taken as the fuels. The energy requirement for both furnaces is made up of several elements:
- The energy to heat the furnace from its loading condition – hot in the case of the sealed quench and room temperature in the case of the vacuum furnace – to the operating temperature
- The energy to heat up the load to its operating temperature
- The energy to maintain the temperature over the period of the treatment
In addition to the firing itself, other ancillaries require energy including control equipment, the fan in the sealed quench and the vacuum pumps. These need to be assessed on a per cycle basis as the power required – particularly for the vacuum pumps – varies during the cycle.
AtmospheresIn comparing the energy consumption for the atmospheres, only the consumables are considered because the equipment was included previously. The elements of the energy consumption of endothermically generated gas are well known – the natural gas for firing and for reaction gas and the electricity for the compressor, etc. It has been assumed that the generator runs 24/7 and that an average of 5% natural gas is added to the endo to control carbon potential.
The energy represented by the gas mixture for the LPC (acetylene, ethylene and hydrogen) is complex. The energy requirement of acetylene is high because of the large energy investment to make its precursor, calcium carbide. As most calcium carbide is made using hydroelectric power, the high efficiency of this type of plant – typically 90%  – has been used in the calculations. For the ethylene, its calorific value has been used. The energy required to make hydrogen was calculated based on electrolysis of water.
QuenchingThe sealed-quench furnace uses little energy for quenching. All that is needed is to recirculate the oil. The small amount of dragout has been incorporated into the maintenance energy requirement.
The vacuum furnace is more energy intensive. A high-power blower is needed to drive the nitrogen through the load. Additional energy is required to separate the nitrogen from air, liquefy it and transport it to the plant. In this latter calculation, plant efficiencies and transportation distances typical of the U.S. have been used.
WashingFinally, the parts may need to be tempered and, if necessary, washed. As both processing routes use the same tempering energy, however, this has been ignored in the comparison. As Fig. 3 shows, the parts from the LPC treatment are clean, so only the parts treated in the sealed-quench furnace and quenched in oil need to be washed.
ResultsA comparison of the total energy required by each processing route is shown in Fig. 4. It is immediately obvious that the LPC process uses less energy overall than the conventional sealed-quench processing route. The primary reason for this is that the majority of the energy used for the LPC process is in the form of electricity. Electricity must be generated from a primary energy source, in most cases, at fairly low efficiency. The conventional route uses natural gas firing, however, which is even less efficient. This is not to say that the LPC process is not more cost efficient as that depends largely on the relative cost of the fuel gas and electricity.
For more information: Paul Stratton is the project manager for research and development for Linde AG - Linde Gas Division, Carl-von-Linde-Strasse 25, 85716 Unterschleissheim, Germany; tel: +44 1484 328736; e-mail: email@example.com; web: www.boc-gases.com
Additional related information may be found by searching for these (and other) key words/terms via BNP Media SEARCH at www.industrialheating.com: low-pressure carburizing, endothermic atmosphere, high-pressure gas quench