Heat Treating: Jet Impingement Furnace for Aluminum Heat Treating
November 15, 2006
Two important characteristics of heat-processing furnaces are heating rate and temperature uniformity. The furnace user and the furnace manufacturer strive to continuously improve these characteristics for obvious reasons. Improved heating rate reduces cycle time and improved temperature uniformity improves product quality and product-yield rate. Currently, there is also a desire to replace batch processes with in-line processes if heating rate and temperature uniformity can be improved dramatically. An in-line process would reduce product inventory and in many cases the heating and handling of product fixtures or baskets.
One such case would be short-cycle solution heat treatment of aluminum castings. Shorter cycles for heat treating aluminum castings have been accomplished in laboratory environments. The practical solution for industry is a production furnace that looks and operates like a conventional mass-flow convection furnace, but has the heating rate, heating-rate uniformity and temperature uniformity that makes shorter cycles possible. Temperature uniformity in mass-flow furnaces has already been addressed. Development in fan design and implementation of jet-impingement heat transfer has improved heating rate. But heating-rate uniformity, which is essential to the time-temperature requirements, has not been fully solved. The following briefly discusses the development program to solve the problem of heating-rate uniformity in a conventional mass flow - jet-heating furnace for heat treating aluminum components and gives a practical case study.
Jet-Impingement ConceptThe conceptual design of this new mass flow - jet-heating furnace is shown in Fig. 1. Generically, it looks like a conventional up-flow, convection-type, roller-hearth furnace. The recirculating fan is in the roof of the furnace, and the airflow exits the fan and is turned downward. Heat is introduced, the airflow is then turned upward through the rolls and is returned to the inlet of the fan. But in this furnace design what happens to the air between the fan outlet and the fan inlet is significantly different than what occurs in a conventional mass-flow furnace.
When the air exits the fan it enters the expansion chamber (E). Absent any turning vanes, the dynamic pressure of the air is immediately converted to static pressure when the air impinges upon the vertical wall of the chamber. Also, when the air strikes the vertical wall, there is equal probability that the air will be directed longitudinally (into and out of the paper) as well as downwardly. Uniform distribution of the airflow is initiated in this way. The airflow progresses downwardly and, again without any turning vanes, the air enters the distribution chamber where the longitudinal distribution of the air is completed before it enters the jet chamber (J) . The air then exits the jet chamber through a jet-impingement plate (P) that creates high-velocity jets for enhanced heat transfer to the work placed on the furnace rolls.
The design of the jet chamber is the final critical design of the furnace. Without proper design, the airflow would not be uniform horizontally across the jet plate. Since there are practical restrictions on furnace dimensions, the volume of the jet chamber cannot be made large enough to create uniform static pressure below the jet plate in order to create uniform airflow through it. Unless the bottom of the jet chamber is contoured or a distribution vane (V) is placed in the jet chamber, there would always be more airflow at the center than at the edges of the jet plate because of the forward velocity of the air in the jet chamber. Contouring would be furnace specific and non-adjustable, so the vane approach was chosen.
Furnace DescriptionA photograph of the development furnace is shown in Fig. 2. The hearth is a conventional roller with variable-speed driven rolls and water quench capability. Air is recirculated with two forward-curved multi-blade fans (one on each side) connected in parallel to a variable-speed drive. The total maximum airflow is in the range of 17,000acfm. The furnace is electrically heated with a total power input of 25KW, and the temperature is controlled by two proportional-SCR control loops (one for each side).
Airflow UniformityBefore beginning the trials, the pressure at the individual jet ports was measured in order to establish the uniformity of airflow over the entire area of the jet plate. The method of testing was to measure the total pressure at the center of a jet port. This arrangement, assuming the static pressure at the surface of the jet plate was uniform, would yield accuracy in the jet velocity of 0.5%.
As expected, the pressures in the longitudinal direction (front-to-back) were uniform. The absence of turning vanes and the two chambers (the expansion and the longitudinal distribution chambers) created this longitudinal uniformity. As anticipated, the pressures in the horizontal direction (sides-to-center) were not uniform, with the pressure at the center being the highest due to the dynamic energy in the air as it entered the jet chamber.
Pressure measurements were systematically and randomly made across the entire jet plate. The results of this investigation showed uniformity in pressure between individual jet holes was +/-4% with a corresponding variation in velocity of +/-2%. At maximum airflow conditions, pitot tube measurements showed the jet velocity was an average of 105ft/sec. The air temperature for this detailed testing was in the 150°F (65°C) range. Pressure measurements at higher air temperatures indicated that airflow uniformity improved as furnace temperature increased, and the jet velocity increased to an average of 125ft/sec (38.1m/sec).
Temperature Measurement and Data Acquisition SystemWhen designing new equipment to meet aerospace requirements, the product uniformity (per AMS specifications) is +/-5°F or less. Accurate "real time" temperature monitoring of the product is essential. The trailing thermocouple method with a custom-designed data-acquisition system was used for temperature monitoring of the product in the development furnace. Considerable care was given to the accuracy of the data-acquisition system, to the thermocouple accuracy and to the placement of the thermocouples in the product.
Temperature UniformityIn many heat-processing applications temperature uniformity of the product at the end of cycle is sufficient criteria for producing acceptable work. Examples of such processes would be reheating metal for rolling and forging and some applications of hardening steel. But there are many applications, particularly in aerospace aluminum heat processing, where time at temperature is the criteria. Even though the product temperature may be uniform at the end of cycle, the product may not be acceptable because different sections of the product will have had different time-at-temperature histories. Examples would be solution heat treating of aluminum alloy and carburizing steel.
Volume-to-area ratios of various product sections and load densities will affect time at temperature, but a more significant variable would be non-uniformity in the heat-transfer rate of various locations in the furnace. If the heat-transfer rate at different locations is not uniform, product temperature as a function of time, will not be uniform. Product temperature is a function of the heat-transfer rate, times the difference in equipment temperature and product temperature. At the end of cycle, all sections of the product will not have the same history, and all sections may not have acceptable properties. In many cases, extending the cycle time can minimize these differences in properties. But this is not a desirable solution and too much time at temperature may also yield undesirable properties. Therefore, the important characteristics of heat-processing equipment are temperature uniformity and heating-rate uniformity, not simply temperature uniformity. In all cases, uniformity of heat-transfer rate will improve product quality and reduce cycle time.
In furnaces where convection heat transfer and process-gas distribution contribute to product properties, the uniformity of airflow (process-gas flow) is a dominant variable with respect to acceptable product properties. The design and the cold-airflow testing mentioned were the starting points of the development program, but success could only be established through testing at temperature with usable production products. Figure 3 shows the product tested and the physical arrangement of the load.
Three differential aluminum castings were placed on a 1-inch-high open tray as shown. Each casting weighs 25lbs (11kg). Nine thermocouples, three on each casting, were placed as shown at a depth of 1 inch. The tray was then charged into the furnace and oscillated front-to-back during the test a distance of 9 inches (229mm). The steady state target temperature was 1000°F (538°C). The test duration was 35 minutes.
Figure 4 shows temperature vs. time for the complete test. Series 10 in the graph is furnace control temperature that shows a decreasing thermal head as the castings reached target temperature.
The data shows that the temperatures at the top of the castings, being the greatest distance from the jet plate, lagged slightly from those at the bottom and midpoint until all temperatures were above 995°F (535°C). At this point in the cycle, all temperatures were within +/-2½°F and none ever exceeded 1003°F (539°C). During the soak, the individual castings were within 1°F and side-to-side variation was within 2°F.
Multiple tests were run using various soak times followed by standard water quenching. Identical sections of each casting were then metallurgically tested. The soak time was the only variable in these tests, which varied from zero to 180 minutes. The casting material was a modified 319 alloy. An independent laboratory was commissioned for the evaluation and the results are shown in Table 1.
Heat Transfer RateThe jet plate in the furnace design serves two purposes. It is a means to distribute the airflow uniformly over the entire hearth area, and it produces high-velocity air jets that create an enhanced heat-transfer rate to the product. In order to compare the heat-transfer coefficient of this jet-heating furnace to that of a conventional mass-flow furnace, a series of heat-transfer-rate tests were conducted using aluminum plates. Thermal insulation was placed on five of the six surfaces of the plate with only the bottom surface exposed to the impinging jets of air. In this manner the heat-transfer area was well defined and all jets were perpendicular to the plate. In mass-flow systems, distance is not a variable with respect to the heat-transfer coefficient. But in jet systems, the distance from the jet plate to the product is a significant variable. Therefore, the heat-transfer-coefficient tests included target distance as a variable.
Figure 5 is a graph of plate temperature vs. elapsed time (heating curve) for a 1-inch-thick 6061 aluminum plate at a distance of 4¼ inches from the jet plate. The temperature of the plate was measured at four places - three corners and the center of the plate. The thermocouples were at a depth of ½ inch. Air temperature was measured at two places beside the plate. The seventh temperature shown on the graph is furnace-control temperature. Temperature data was recorded at 30-second time intervals.
The total heat-transfer coefficient was calculated at five temperatures of the heating curve at nominal temperatures of 400, 500, 600, 700 and 900°F. These calculations yielded a value of 39+/-4 Btu/Hr-Ft²-°F for the total heat-transfer coefficient. Using the same procedure, the heat-transfer coefficient was measured at target distances of 7½ and 14½ inches with values for the total heat-transfer coefficient of 29 and 14Btu/Hr-Ft²-°F respectively, compared to a value of 39Btu/Hr-Ft²-°F at a target distance of 4¼ inches. Mass-flow furnaces, due to maximum fan capacity with respect to furnace size, typically have a value of 10 Btu/Hr-Ft²-°F. The jet system provided significant improvement in heat-transfer rate.