Recent power-company offerings of financial rebates for reducing power consumption have stimulated new interest in power usage reduction. Solar Manufacturing has devoted significant efforts in developing more energy-efficient systems and designs. These efforts have resulted in assisting customers with new furnace installations to receive rebates as high as $50,000.

This article will highlight advanced furnace design features, illustrating both yearly power savings and possible power-company rebates. Furnace improvements to be discussed and their respective power advantages include:

• Improved hot-zone insulations
• Variable-frequency drives (VFD) on vacuum pumps
• VFD on furnace gas-cooling motor
• Reducing diffusion-pump heater power (Solar ConserVac® System)

Improved Hot-Zone Insulations

Vacuum furnace hot-zone designs for the past 40 years have focused on good structural integrity, gas-cooling improvements, extending element life and performance. Little emphasis has been placed on heat-loss concerns and overall hot-zone power requirements.

Hot-zone insulation has evolved from the old “all-metal design” to the modern multiple graphite-felt layer design. Manufacturers and users seem satisfied with this approach.

However, the escalating cost of electrical power makes apparent some inefficiencies of modern hot zones that require additional power to maintain final cycle temperature. In the vacuum furnace hot zone, heat is conducted through the insulation from the hot side to the cold side, producing a temperature gradient across the insulation.

Figure 1 shows typical heat transfer through the insulation. The steel insulation support ring is eventually heated from the losses through the insulation. The ring temperature then radiates to the chamber wall, representing the true loss (point “A” to point “B”). Since the radiation equation used to calculate the loss includes the difference between temperature “A” to the fourth power minus temperature “B” to the fourth power (in degrees Kelvin), minimizing the ring temperature becomes the critical factor in establishing final heat loss.

Most conventional hot-zone designs use either graphite-felt layers with a foil hot face or a graphite board backed by layers of felt supported in a structural ring assembly. Testing of various insulations when holding at elevated temperatures yields varying support-ring temperatures (Fig. 2).

A new board material (Fig. 3) was compared with prior insulating materials. The board tested was 2-inch-thick graphite with a thin graphite-foil layer bonded to the hot face. The board was sealed to minimize moisture absorption during the process cycle. New board test results are also shown in Figure 2.

To demonstrate to the power company savings using the new board versus the old foil and felt, we will use a furnace with an internal useful volume measuring 36 inches wide x 36 inches high x 48 feet deep. Support-ring and front and rear supports have a radiating surface area of approximately 12 m².

We will assume an average furnace cycle like that shown in Figure 4, run 14 times per week or 728 cycles per year. The equation for calculating radiation losses is:

P = eσA(T⁴ – T_c⁴)

where P = net radiated power; e = emissivity of the radiating surface; σ = Stefan’s Constant (5.6703x10^-8 watt/m²K⁴); A = radiating surface area; T = radiating surface temperature (ring “A”); T_c = surrounding surface temperature (chamber inner wall “B”).

Using the furnace cycle in Figure 4, the annual savings shown in Figure 5 is seen when comparing most current designs using PAN graphite felt with foil hot face to the new board material.

Projected savings can be demonstrated to the power company for a possible rebate. Even if the initial cost for new insulation material adds $5,000 - 7,000, the investment is quickly recovered.

Variable-Frequency Drives (VFD) on Vacuum Pumps

A vacuum furnace uses large motors on the pumping system that are not required to be at full output during certain phases of a typical heat-treating cycle. Using a variable-speed or variable-frequency drive (VFD) could result in considerable power savings (Fig. 7).

A VFD is used for controlling the rotational speed of an AC electric motor by controlling the frequency of the electrical power supplied to the motor. Energy savings with these systems can be significant, often quickly paying for the cost of the VFD. In variable-torque applications such as vacuum pumps, torque required varies with the square of the speed, and horsepower required varies with the cube of the speed, resulting in a large horsepower reduction for even a small reduction in speed. A motor at 63% speed will consume only 25% as much power as at 100% speed.

Most furnaces include the blower pump ahead of the mechanical pump for pumping down from about 50 Torr to 50-60 microns, when it should be operating at full speed and capacity. Using the VFD, during other phases of the cycle it could operate at a much lower speed, saving considerable energy.

If completely off at the initiation of vacuum pump-down, the blower impedes mechanical pump throughput because it is between the vacuum chamber and the mechanical pump. The blower lobes restrict the pumping of the mechanical pump. A very slow rotation of the blower pump at higher pressure, controlling overload concerns, can reduce cycle time prior to initiating full speed of the blower. When the diffusion pump enters the cycle at 50-60 microns, a slow-rotation blower speed is helpful as the mechanical pump backs the diffusion pump.

A typical vacuum blower pump uses a 15-HP motor. This equates to approximately 11.2 kW at full speed. With blower speed at 50% or slower, we reduce power to approximately 2.5 kW, a significant long-term savings even after initial cost of the VFD. Running at reduced speeds also extends blower life and decreases maintenance requirements.

Projected energy savings using the VFD on the blower pump are summarized as follows. Referring to Figure 4 – about a 10-hour total cycle – full blower pump speed is only required for approximately one hour. Without the VFD, it would be operating at full speed for approximately nine hours of the cycle. Figure 7 demonstrates the energy savings and possible rebate based on 728 cycles per year at an average rate of $0.10/kWh.

A VFD for the blower motor along with labor and engineering would cost approximately $3,500, which would present a payback within the first year of operation.

Occasions when the mechanical pump could be reduced in speed during the cycle are minimal because it backs up the blower and diffusion pump after the initial pump-down. Therefore, we do not recommend a VFD for the mechanical pump.

VFD on Furnace Gas-Cooling Motor

A VFD on a gas-cooling-system motor is an excellent feature that provides energy savings and enhances cooling performance.

Most furnaces are designed for cooling at a certain gas pressure with a specific cooling gas, such as nitrogen or argon. This requires designing the recirculating fan and motor size for one of the gases and accepting lesser performance with the other gas. This is acceptable to some operations but not to the modern heat treater. New high-quench furnace designs incorporate a synchronous design concept with the following key components:

• A VFD to “over-speed” the fan motor
• A specially designed motor that can accept the over-speed requirement
• An appropriately sized fan to operate across the entire pressure range
• Full motor HP available under varying gas types and cooling gas pressures

A cooling motor VFD on a system designed for maximum performance with the heavier gas required can provide optimum performance for other gases. Reducing motor speed during portions of the cooling cycle will reduce total operating costs.

Most cycles require fast cooling on the initial phase – from final soak temperature to some lower temperature, when the blower motor speed can be reduced to 50-60%, saving considerable energy. Factors such as reduced heat of compression come into play at lower motor speeds, which actually speeds up cooling at lower temperatures.

Projected annual energy savings for a 50-HP gas-cooling-system motor when using a VFD are shown in Figure 8. Without the VFD, the motor would operate approximately two hours. With the VFD, we would have one hour at full speed and one hour at 50% speed. For Figure 8, we again used our average rate of $ 0.10/kWh for rebate possibility.

Reducing Diffusion-Pump Heater Power: Solar ConserVac^® System

A diffusion pump is only active during initial pump-down from crossover (50-60 microns) to low vacuum (10^-4 to 10^-6 Torr range) and for recovery of vacuum when product outgassing reaches a certain preset level. At other times, the diffusion pump can operate at less-than-normal power requirements.

The heaters for the diffusion pump draw power ranging from 8 kW for a 16-inch pump to 24 kW for a 35-inch pump. On this example furnace, we are using a 20-inch diffusion pump with heater ratings at 12 kWh.

By utilizing an SCR power controller in conjunction with an integrated control system, we can reduce power consumption by approximately 50%. This energy-saving feature is used after the furnace has achieved desired vacuum, when the furnace is quenching and when the furnace is being loaded or unloaded. This can yield significant power savings for possible rebate as well as extend the life of the diffusion-pump heaters.

To illustrate savings when incorporating this feature, let us take Figure 4 as a typical 10-hour cycle with the diffusion pump only operating at full power for about two hours. With the pump using 12 kWh of power when in full operation and 6 kWh for idling condition for 728 cycles per year, Figure 9 shows a chart for the pump using our average rate of $0.10/kWh.

The control-system initial cost of approximately $2,450 is quickly recovered and provides continued savings while extending heater life.

Summary

We can summarize our yearly projected power savings to be used for rebate application in Figure 10, which demonstrates the type of projected savings that can be presented to the power companies offering rebate programs. Adding some of these features can provide for a more-efficient system and continued yearly savings even without the rebates.

For more information:  Contact Patricia Niederhaus, executive technical administrator, Solar Manufacturing, 1983 Clearview Road, Souderton, PA 18964; tel: 215-721-1502 x1240; e-mail: patricia@solarmfg.com; web: www.solarmfg.com.