A study on the efficiency of ovens
Is one 12″ wide conveyor oven the same as other maker’s 12″ wide conveyor ovens? If so, shouldn’t you just purchase the cheapest oven among your choices? Each maker will try to show why their oven is the best one for your application in their quotation. Some of the arguments are quite accurate, but others are just marketing blather that doesn’t really deliver much value to the customer. So, how can you wade through all the information and make an informed choice? Pricing aside, this article focuses on the factors that affect the efficiency of ovens. You should be considering whether an oven was designed for efficiency, what the efficiency factor really is, and learn what you can do to boost or maintain your oven’s efficiency.
A simple calculation of energy usage is to multiply the total kW value of the oven by the number of hours it will take to complete a production run. The answer would be inaccurate for 2 main reasons: 1) The ovens will not demand electricity 100% of the time, and 2) you are assuming all ovens have similar efficiencies. In general, when springs enter an oven at a constant rate, the elements will only call for electricity between 30% and 85% of the time. It means the simple formula can be way off. Why such a wide range? That’s where an oven’ s efficiency becomes important to understanding the actual cost to heat treat parts.
There are several factors that affect the efficiency of an oven, but before we discuss them, please understand that the fundamental goal in using “the best oven” is to utilize the least amount of kilowatt-hours (kWh) to complete the project. Also, your efficiency will partly be determined by the amount of product you put through an oven relative to the total volume of the heat chamber (notice the term “volume” and not ” surface area”- it will become important for later discussion).
Heating Elements are far and away the biggest KWH-determining items in your oven, and they are responsible for probably 90% of the oven’ s overall electrical demand. Springmakers usually see conveyor ovens that use Stainless Steel Elements or Incoloy Elements. Elements work by radiating heat outward and the ” heating power” of a element is determined by its Watt Density, which is the total watts divided by the surface area of the element.
Take a look at this table that summarizes 3 characteristics of elements. Incoloy material contains a lot of nickel, which is paiily why Incoloy elements cost more than stainless. However, nickel provides low resistance to heat and electricity, so it radiates more heat through the element sheath instead of just heating up the sheath surface temperature. For each oven’s heat chamber shape, oven makers will design specific element diameters and watt ratings (as well as number of elements) and you should ask your oven supplier to explain their reason for choosing a particular wattage, element diameter and material for each oven. A quality oven maker will also use nickel jumpers, washers, nuts and terminals on the tips of the elements for the precise reason of letting electricity through and not heat up the materials so much; you want to keep the heat inside the chamber.
Since most elements are round and the heat radiates outward in all direct ions, an efficient oven will utilize fans to ” push” the heat downward onto the parts. Not all fans are equal, though. The size and pitch of the fan blades and the fan motor rotation speed will determine the amount of “push” realized. (Some ovens will indirectly drive the fan blades so they can additionally control the rotation speed to optimize that “push” on the heat. As an added bonus, indirect drives prevent heat from traveling up the fan shafts and into the motor, thus reducing down time to replace fan motors).
Since the fans will not push down all the heat, you want the rest of it to reflect back into the heat chamber. Any oven that utilizes a metal (hopefully stainless) heat shield will have a tremendously higher efficiency than exposed brick insulation. It also means if your heat shield is dirty or damaged, you can improve the oven efficiency if you clean and/or fix the heat shield. Although it may sound counter-intuitive, “too much” heat shield can actually lower your oven’s efficiency. By ” too much”, I am referring to the way the heat shield is installed. If all sides of the heat shield are attached to the oven frame, there will be too much metal-to-metal contact, and heat will just escape to the outside of the oven. Not only will you lose some efficiency, your oven shell can become quite hot and dangerous to touch. A cleverly designed oven can isolate the heat shield to minimize the metal-to metal contact.
The next and quite often overlooked components are the curtains on each end of the conveyor. Since 95% of conveyor ovens in operation today have fans, if you run an oven without any curtains, it is like running your home furnace with all the windows and doors open. Even a frayed, beaten up ceramic fiber curtain will help retain the hot air currents inside the heat chamber. Somewhat heavier stainless curtains do an even better job.
Now that you can look at an oven from the point of view of efficiency rather than purchase cost, here are some things you can do to boost efficiency on existing ovens.
- Use curtains – open ended ovens will spill out heat. Stainless curtains are imperative if you use exhaust hoods or else you will suck out all the heat along with oily fumes.
Many ovens have automatic turn-on functions and we often see ovens operating in spring plants that have no product going through them. Conveyor ovens get up to heat within 5 minutes (maybe 10 minutes for a large oven operating at a high temperature), so there is no need to turn on an oven until you start preparing the coiling equipment or loading the wire.
Learn how to use the automatic shutdown feature that allows oven chambers to cool while running the fan and belt-but not the heating elements. You can safely stop the belt if the chamber is below 300F. So, make sure the delay timer is not set to too long a period.
Insulation degrades over time. Replace discolored insulation that is discolored in an area larger than ½” from the point of an obvious heat source. Areas of special concern are the ends of the elements between the heated section and the element tip, the area surrounding the fan shafts there should be only enough room for the shaft to rotate—you shouldn’t be able to look down along the shaft, any pockets of air inside the oven shell should be filled with insulation.
Use the right-sized oven. The best sized oven will fill up the belt area with one layer of parts when the coiler runs at its most efficient speed. From there, adjust the belt speed to avoid tangling. Fairly rigid springs can handle 2 layers of parts on a belt (more if using an oven with elements above and below the belt). If you utilize less than 50% of the oven, you will waste electricity.
An oven’s electrical cost is the cost to maintain a steady heat inside the entire volume in the heat chamber and then produce extra heat as it transfers into the parts. This means don’t just look at the chamber height as a means to determine the max OD part that will fir. Consider that a 4″ chamber height is 25% larger than a 3″ chamber; you have to maintain a constant temperature over a much larger are before and heat-treatment occurs. Once an oven is up to temperature, the elements will only come on to replace the heat that gets sucked into the parts and also to replace the heat that is lost to the environment due to the above-mentioned efficiency factors. For this reason, you want to use the smallest volume chamber that still fits your parts and an oven with the right design to maximize the oven efficiency in every regard.
The bottom line is, it is entirely possible that a $5000 oven with a 30% efficiency rating, compared to a 90% efficient $8500 oven, could cost you more than $3500 annually in additional energy costs, thus negating any purchasing savings you thought you had in the first year. By the time this issue of SPRINGS is distributed (in time for the Spring World 2008 show, I believe), some long-term studies will be completed that precisely measures the affects of several efficiency. For example, these studies will reveal exactly how many kWh one can save by adding stainless curtains to an oven without any. There is also a study to measure the effect of using degraded insulation, using a solid-body oven vs. a clamshell oven, running insulated and internal belt returns vs. exterior belt returns, and others. These studies will be carried out by sophisticated instruments – some only coming onto the market in mid 2008—and results will be available upon request. A summary of the results will also be made into a future article for SPRINGS.