Energy-Saving Potential in High-Vacuum Brazing Furnaces Using Diffusion Pumps
For many years, energy efficiency has been the number-one topic with manufacturers and users of vacuum furnaces for thermal treatment. The search for energy-saving possibilities concentrated mainly on the furnaces themselves and the optimization of the heat-treatment processes.
Furnace designs and operating conditions were established and optimized long ago. Measures for heat recovery and other concepts to reduce energy consumption were clearly gained. With the new digital age upon us, however, further investigation into energy-savings possibilities should again be explored.
The vacuum system, as part of the furnace, was often neglected in those efforts to reduce power consumption. Despite this, vacuum pump manufacturers worked diligently on the improvement of energy efficiency for their products. For example, modern screw vacuum pumps have been developed with a focus to reduce their energy consumption down to the level of long-standing oil-sealed rotary-vane or rotary-piston vacuum pumps. Improvements in enhanced robustness along with a marked reduction in maintenance costs were also attainable. In addition, next-generation roots pumps utilize modern built-in motor concepts to improve leak tightness, minimize power consumption and reduce parts wear.
In comparison to the absolute power requirement of a furnace of around 200-800 kW (depending on furnace size), the energy demand of the main fore-vacuum system only represents a small fraction of that consumption. The nominally installed power of a fore-vacuum system is typically in the range of 10-30 kW, while during most operation time, these pumps only require 30-50% of their nominal power. A realistic power-consumption reduction of between 1-4 kW by optimized pump design does not, therefore, offer significant savings. Nevertheless, even such small savings add up over the life span of the furnace.
Another vacuum pump technology mostly utilized in brazing furnaces that offers far-higher saving possibilities is the diffusion pump. Although these pumps have the highest power requirement of the entire vacuum pump system, optimizations to higher energy efficiency were completely neglected in the past. Very little development work was invested in this dated technology by pump manufacturers. A diffusion pump with 50,000 liters/sec. nominal suction speed has an installed heater power of 24 kW. Until now, this 24 kW was fully consumed during the entire operation of the pump. An energy-saving potential here would offer much larger cost reductions.
This article will describe measures that can help clearly reduce the power demand of a diffusion pump.
To understand the energy-saving potential, one must first understand how a diffusion pump operates. The main components of diffusion pumps are a cooled pump body with intake and exhaust ports, a system of nozzles and the pump boiler (Fig. 1).
The pump fluid contained inside a boiler is heated until it starts boiling. The uprising vapor stream is directed through a nozzle system into the pump body, where a vapor jet with ultrasonic speed is created that is streaming with a specific angle downward.
Reaching the cooled pump housing, the vaporized pump fluid condenses on the cold wall. The liquid fluid runs downward and returns into the boiler from where it will be evaporated again. Why does this principle pump at all?
Figure 2 shows a detailed view into the pumping mechanism of the diffusion pump. We can see the vapor jet on its way from the jet system toward the water-cooled wall.
The jet consists of “degassed” fluid vapor, which means it only contains extremely low partial pressure of the gases that should be pumped. The area above the vapor jet, therefore, contains a much higher partial pressure of the gas. Driven by partial-pressure difference, the gas from above diffuses into the jet stream to compensate.
The jet stream will push the gases toward the cooled wall and the next nozzle stage of the pump. Finally reaching the exhaust side of the pump, the gases will be pumped away by the backing pump. The vacuum inside the boiler will ensure that the reboiling fluid will contain the lowest partial-pressure of gases again.
The following measures have been identified as having the highest influence on the power consumption of a diffusion pump:
1. Pump design features
2. Electronic regulation of heater power
3. Regulation of cooling water
4. Housing insulation
5. Correct heater adjustment according to the selected driving fluid
6. Standby operation/intelligent process management
Pump Design Features
It is obvious that the total diffusion pump must already be designed to minimize consumption. To save energy and consumables, modern diffusion pumps are optimized with some design features:
• Heaters are positioned inside the boiler
* Reduction of heat-transfer losses, thereby lowering power demand
• Grooved pump body with built-in cooling-water coils
* Reduction of weight and therefore total energy for initial heating
* Optimization of contact surface/heat-transfer efficiency, thereby reducing cooling-water demand
• Water-cooled cold-cap baffle at inlet
* Reduction of fluid backstreaming into the vacuum chamber, thereby reducing fluid losses
• Water-cooled fore-vacuum baffle at outlet
* Reduction of fluid losses into the fore-vacuum line
Electronic Regulation of Heater Power
One general statement to first consider: More heating power does not automatically result in more suction speed!
As previously described, the main driving force for the pumping mechanism is the partial-pressure difference between the gas entering the pump and the vapor jet. As long as the vapor jet is stable, there is no difference in suction speed independent of more or less vapor streaming toward the cooled walls.
The heaters contained in today’s diffusion pumps are typically not regulated at all. The full heater power is required to ensure a short heating-up time and quick availability of the diffusion pump, but after reaching the optimal operation temperature, the full heater power only results in evaporation and degradation of fluid with no increase of the pumps’ suction speed. Depending on the chosen operation fluid (e.g., mineral oil or silicon oil), there are different temperatures that enable the fluid to deliver best performance.
Modern, digital controllers (Fig. 4) can be coupled with diffusion pumps and allow for the optimal temperature as the regulation point. A temperature sensor inside the boiler detects the actual temperature. As soon as the actual temperature reaches the optimal level, the heating power will be reduced. This regulation allows a reduction of power consumption in the range of approximately 15-35%, depending on the chosen fluid. A savings example of 30% would equate to approximately 7 kW if using a 50,000 liters/sec. pump, which is quite a high value.
To avoid overheating, a maximum fluid temperature is also monitored by the control unit. If this temperature is exceeded, the con-troller will transfer a warning signal toward the furnace control via its Ethernet or USB interface.
Regulation of Cooling Water
The operating principle of the diffusion pump requires a generous cooling of the housing surface because this works as a condenser for the fluid vapor. Approximately 80% of the heater power will be removed by the water cooling. By combining the diffusion pump with a thermostat valve, depending on water temperature, the cooling-water consumption could be reduced by up to 50-60% of the standard value.
For further optimization, the cooling-water lines for the housing must be separate from those used for the inlet and outlet baffles. The baffles must utilize the coldest-available cooling water for optimal operation, while the housing, which is also responsible for most of the cooling-water consumption, may be cooled with warmer water.
For a DIP20000, the cooling-water demand could, for example, be reduced from 720 liters/hour down to 380 liters/hour.
A significant heat loss is generated by the hot surfaces of the pump, especially in the boiler area. About 10% of the pump energy is lost over the surfaces. A suitable housing insulation will save an additional 2-4% of heating energy. In addition, the insulation will enhance the safety of the pump because operators can’t be burned by hot surfaces.
Correct Heater Adjustment According to the Selected Driving Fluid
Depending on the specific application, the user can choose between various fluids for their diffusion pump. These different fluids have unique vapor pressures and, therefore, different optimal operation temperatures to fulfill their full function.
Mineral oils are typically low boiling. They usually start boiling around 190˚C (374˚F). The vapor jet is not stable yet at this fluid temperature, but some suction speed is already measureable. To deliver full stable suction speed, the operation temperature is typically around 245-250˚C (473-482˚F). These temperatures will be exceeded by uncontrolled full-power heating, which will waste energy and even rapidly degrade the oil as temperatures begin to exceed 270˚C (518˚F). Choosing the right temperature setpoint, therefore, is as important for energy saving as it is for oil life.
For silicon oils, the temperature setpoint should be lower, typically around 235˚C (455˚F). The specific energy to evaporate the heavy silicon-oil molecules is clearly higher than for mineral oils. For this reason, power savings by heater regulation is clearly lower. The energy-saving potential is only up to 10%.
The selection of the right diffusion-pump fluid has a significant influence in total power consumption. Silicon oils are more expensive and require more power. The user should only select these if the application really requires such inert fluid.
Standby Operation/Intelligent Process Management
Often, diffusion pumps are idling between batches. Since a complete warm-up of a diffusion pump requires up to 45 minutes (depending on pump supplier), the pumps are typically not switched off and utilize 100% power even during downtime.
The usage of the innovative control unit opens new and different possibilities today. The aforementioned measures will not only ensure that the diffusion pump will work with minimized power demand at its operation point but also during downtime between batches.
During the idling time we simply reduce the temperature to a “holding temperature,” which keeps the pump in standby and allows a very quick reheating to full power. By reduction of the boiler temperature to approximately 170˚C (338˚F), the energy consumption during idling can be decreased by an additional 10-15%. The pump fluid stays degassed under vacuum, and the reheating of the pumps to full temperature can be done in 5-8 minutes (e.g., during the pump-down of the furnace with the fore-vacuum system).
Users of diffusion pumps have never challenged the power consumption of these products in the past. The pumps have been necessary, and the standard was that they didn’t offer any energy-saving possibilities except to switch them off if there is enough time in between the batches. This situation changes completely with the development of modern, digital control units. Diffusion pump users can easily reduce their costs in energy consumption, oil consumption and heater maintenance.
In vacuum brazing, after reaching operating pressure or during standby, the power consumption of a diffusion pump can be re-duced by more than 30%! Considering a large-capacity 50,000 liters/sec. pump, this equates to 8 kW less consumption or cost sav-ings of more than $8,500/year (based on 8,000 hours/year operation and electricity costs of $0.15/kWh), a value that cannot be ig-nored.
With such savings in mind, even a retrofit of existing pumps should quickly be considered. IH
For more information: Contact Mario Vitale, Sr. Manager – Regional Marketing and Sales Support, Oerlikon Leybold Vacuum USA, Inc., 5700 Mellon Road, Export, PA 15632; tel: 724-325-6565; fax: 724-325-3577; e-mail: firstname.lastname@example.org; web: www.oerlikon.com/leyboldvacuum