Five main areas of the typical vacuum heating or brazing system (Fig. 1) need to be reviewed to make a proper determination whether to upgrade or replace an existing system:
A. The vacuum vessel or chamber
B. The power control system
C. The vacuum-pumping components
D. Trending/data storage and reporting devices
E. The furnace “hot zone”
Let’s look at each area of the vacuum-heating system and see what information is required and how it can be obtained.
Integrity of the Vacuum Vessel or ChamberA considerable number of vacuum heating and brazing systems, which were supplied in the ‘70s and ‘80s, are still in operation. Some are even older. These systems have been producing quality product for many years. Many utilize a water-cooled vacuum vessel of either a rectangular or cylindrical design. Over the years, the internal structure of the vessel may have been compromised due to a buildup of rust or water-born contaminates leading to areas of the vessel that are running “hot.” There may also be areas of the vessel that have developed a reduction in wall thickness due to oxidation – the ultimate indication of which is a water leak in the chamber wall.
Determining if the Vessel Water Passages are Restricted
One method is to utilize an infrared laser thermometer. With the furnace on and water being circulated through the vessel, measure the temperature of the external vessel wall at various locations (Fig. 2). The areas near the water inlet to the vessel should be the coldest, while the areas near the water outlet from the vessel should be the hottest. If there are sections of the vessel that produce temperatures above the water outlet zone, this could be an indication of an area that is receiving little or no water cooling – perhaps due to a buildup of rust or contaminants.
(Note: Buildup inside the water passages of a system is dependent upon a number of factors: the materials used to construct the vessel, the type of water cooling/recirculating system being used, the quality of the water being used to cool the equipment, etc.)
There are two common ways to do this – utilize an electronic thickness gauge or similar device, or perform the “drill and fill” empirical method. Modern electronic thickness gauges can provide extremely accurate measurements of the remaining wall thickness of various metals (Fig. 3). If the metal of the vessel or chamber wall has suffered erosion over the years, the reduced wall thickness can be determined by either method. Some people prefer to double-check the electronic thickness-gauge readings by performing a random drill and fill to physically verify their electronic measurements.
Power Control SystemMany older vacuum heating and brazing units utilized a mechanical means of controlling the various “trim” areas within a furnace (Fig. 4A). It was oftentimes necessary for an operator to manually adjust (or “trim”) a number of rheostats based upon the meter readings or sensor indications within a particular area of the hot zone. This was a labor-intensive procedure and lent itself to the possibility of lengthy cycle times and/or human error. Today’s control systems are completely automatic. “Hands-off” operation is achieved by using electronic controllers that make use of computerized programs (or “recipes”) for the various components to be heated or brazed (Fig. 4B).
Vacuum-Pumping ComponentsDiffusion pumps, mechanical blowers and hold pumps – when properly maintained – generally provide many years of reliable service (Fig. 5). The determining factor as to whether a vacuum-pumping system should be repaired or replaced can usually be found in the maintenance records for the individual components. Most often, it is more cost-effective to rebuild the existing vacuum-pumping components. The exceptions to this rule would be the need to reduce the present pump-down time and the inability to obtain parts for an obsolete pump.
If the current pump-down time can be reduced significantly by the upsizing of the vacuum pumps, the additional production that can be attained may cost-justify this investment. This ROI calculation is relatively simple, but be sure to include the cost of the “lost production” that will result due to the time required to install the bigger pumps.
If you cannot obtain parts for an “old dog,” the decision to replace it becomes simple and one best made before it breaks down in the middle of a critical production run.
Trending/Data Storage and Reporting DevicesPerhaps the areas of AMS 2750D and Nadcap that have the most impact on existing vacuum heating and brazing systems are the trending and data storage and reporting requirements. If you were to summarize AMS 2750D in one word, it would be “documentation.” While existing paper chart recorders and manual record-keeping systems may still be used for these purposes, the availability of electronic trending/data-retention and recording devices will generally enable the vacuum-heating customer to “survive” a Nadcap audit with far fewer headaches (Fig. 6). Present-day, readily available computer hardware and custom software have been designed to take into account the data and format that an auditor will want to see and verify. In general, if the existing trending/data storage and reporting system is more than five to six years old (a lifetime in the world of electronics), it will be well worth the cost to upgrade and replace this portion of the system.
The Furnace Hot ZoneThe heating elements, metal shielding and insulation materials that comprise the furnace hot zone are generally designed for a specific application at the time the system is originally engineered. The power components are sized and selected to match the heating elements. Over a period of time, the elements may become dirty or damaged due to the heating process and/or operator considerations. It is sometimes necessary to repair or replace sections of the hot zone, and this is ordinarily done by the maintenance crew as required. Replacement of a complete hot zone may become cost-justified when the number of patched areas reaches a significant portion of the hot zone (Fig. 7). As a rule of thumb, when you find it necessary to make a third patch to the heating elements within an individual trim zone of the furnace, you should begin to plan for the replacement of the entire hot zone at the next convenient time.
SummaryIf your customers are beginning to specify Nadcap or AMS 2750D in their purchase orders and your vacuum heating or brazing system has some “age” to it, you will want to consider each of the five main areas of the system when making the determination whether to upgrade or replace your system. Consider not only the up-front costs but also the ongoing operational costs when making your ROI calculations. IH
For more information: Matt Orfe is product manager – vacuum brazing, PV/T Incorporated, An Inductotherm Group Company, 100 Indel Avenue, P.O. Box 156, Rancocas, NJ 08073; tel: 609-267-8000; fax: (609) 267-1366; e-mail: firstname.lastname@example.org; web: www.pvt-vf.com
Additional related information may be found by searching for these (and other) key words/terms via BNP Media SEARCH at www.industrialheating.com: Nadcap, AMS 2750D, vacuum heating, vacuum brazing, furnace hot zone, nondestructive testing, ultrasound, infrared thermometer