You’ve made it: The pilot runs of various sizes and shapes are finally over. You now have a product line that requires a significant volume of similar parts on a continuous basis to keep up with the sales demand. Your process is going continuous!
Continuous ovens are used in many applications from low to medium temperatures, for curing and bake-out, to a range of higher temperature applications such as billet preheating. But, finding the right one for your continuous process is not as simple as poking around the Internet.
This article will focus on single-belt units, where a flat belt runs through a forced convection oven. The beauty of this arrangement is in its simplicity, but coming up with specifications from which a supplier can easily provide an accurate quote is not always simple. It requires some forethought and a lot of knowledge about the process and its requirements.
Any supplier will tell you that the more thorough and detailed the specifications, the better they feel about investing time and effort in preparing a quote. When given sketchy details, it is possible that they will create a quote based on their best estimate of your requirements, but another supplier, given those same sketchy details, will interpret them differently and quote something smaller and simpler or even larger and more complex. The job can be lost or awarded based on a quote that does not best satisfy the process because an accurate quote was not created, and an apples-to-apples comparison of quotes was not possible.
By providing accurate and detailed specifications, a prospective oven buyer will receive an accurate, detailed quote. So, before requesting a quote, it is important to consider five key aspects and provide thorough details about the following criteria.
1. Product Weight and Belt Loading
The weight and material of each part should be discussed. This information will be used — along with the production rate — to size the unit’s heating capacity.
In addition, be sure to describe the maximum weight that will be on the full length of the belt when the unit is fully loaded. There is a cross-over point where supporting the load on a slider bed is no longer practical and rollers need to be considered. However, rollers are more expensive, especially if the temperature in the chamber is high enough where the bearings need to be outside the hot zone, so they should only be designed into the system when needed.
2. Part Dimensions and Shape
Knowing the desired dimensions of the belt — the length and width of the belt as well as the clearance over the belt — permits the best layout of parts.
The shape of the parts determines the choice of airflow pattern.
3. Soak Time and Production Time Influence Unit Length
The two things you must have knowledge of when specifying a unit are:
• Soak time at temperature.
• Maximum production rate.
If you are going to err on production rate, err on the high side. Assuming your soak time at temperature is set, if you underestimate your production rate, the unit as delivered will not be long enough to accommodate a greater production rate than that for which it is designed. If you overestimate, however, you are on safer ground, and the unit will accommodate any rate up to the specified maximum.
4. Specify Only As Much Control As Needed
If your oven is to be designed for a continuous process, do not go crazy requesting PLCs and data communication. After all, you are asking for a quote on a unit where the zone temperatures are supposed to remain static. Instead, keep it simple with single setpoint controls for each zone, or a master in the center zone and slaves in the others. If you need a record of the temperatures in each zone to determine, for example, that they remained stable, consider either a simple chart recorder or a paperless recorder with data communications capability.
5. Consider Available Utilities
Most systems can be heated electrically or by gas. Even with gas heating, electricity still will be used for the conveyor drive and controls. If available, 480 V, three-phase and 60 Hz should be used.
If gas is available and it is desired to make use of it for heating the system, it may be direct or indirect. The choice usually is determined by whether the products of combustion impact the parts. Direct-gas-fired does not mean the flame will impinge on the parts. It means the flame will be in the heater box (normally on top) and that air from the chamber will pass through the flame. Indirect means that the burners will fire into a tube, and the air from the chamber will pass over the hot tube surfaces, much in the same way home gas heating systems work. In either case, you generally will find that having 5-psig plant gas pressure is needed.
Once you have gathered the information to get a number of apples-to-apples quotes to compare, you can select a supplier and get an order going. Enjoy your step up into continuous, and leave room in your facility for the next line.
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