Oven building can be a challenging enterprise, especially where large industrial ovens are concerned. Small laboratory units or other small standard catalog ovens are relatively easy to help a customer select and specify -- unless the customer’s requirements are completely misinterpreted. For larger units that must be designed specifically for the application, customers with clear, comprehensive requirements are the easiest to understand, respond to and provide the right type of oven. To help you become one of those customers, here is what you should be thinking about when you talk to prospective oven vendors.
In real estate, the three primary considerations are location, location and location. For a large forced convection oven, the primary considerations -- as I see it -- are airflow, airflow and airflow. That sounds very basic, doesn’t it? But airflow determines a number of things, including how efficiently you will heat your load, how fast you will recover after a door-open condition if you will be taking parts in and out, and the temperature uniformity you can expect to achieve.
Adequate airflow is necessary to heat your product effectively. Before entertaining quotes, consider how you think it would be best to heat your product, and anything that would impede airflow through the oven. I have chatted with prospective customers who are entertaining quotes from other builders for horizontal-in-from-the-side, vertical-return-to-the-ceiling airflow patterns -- when it turns out their load would block vertical airflow. Or a customer could get a quote for an oven with horizontal side-to-side airflow, yet during the heating process they will stand large panels on end that will block the flow. Give some thought up front to what you plan to heat, and how that may affect airflow, and save yourself a lot of nightmares downstream.
Rate of Rise (Or, How Hot, How Fast?)
How fast do you want to get to temperature? It is one thing to say you want to get there in 45 minutes or so if you are heating a load that does not weigh much. Heating up a 40,000 lb weldment is another story. Be realistic. Depending on the weight and nature of your load, the air temperature may get there way before the core of your product.
Oven Temperature Uniformity
What temperature uniformity do you need? If the customer does not specify a certain level of temperature uniformity, oven builders assume it is not critical enough that they may need to modify the normal design. Yet if uniformity is important, be realistic about what is really required for the process -- please don’t tell the oven builder you need ±2°F temperature gradient in an oven you could move your family into. Aside from being difficult to achieve, it may make your oven unnecessarily more expensive than it has to be.
Many spec writers confuse uniformity with accuracy.
- Accuracy refers to the capability of a control instrument to achieve the temperature desired, and to have the temperature at the instrument’s sensor location be stable and repeatable to within a certain tolerance.
- Uniformity is largely a function of airflow. If the airflow is managed properly, the air temperature uniformity throughout the chamber will be tight, assuming the airflow is not blocked completely with the load.
Most builders will state the zone in the oven in which uniformity can be expected -- for example, to within what distance of the walls, ceiling and floor the uniformity will be achieved. This defines the “uniform zone.” Your builder should be up front with you in advising that. If the builder tests the uniformity before the oven leaves the shop, the builder will state that all uniformity statements and certifications apply to an empty oven at steady-state conditions.
One more thing about uniformity: Where you measure temperature influences how uniform your readings are. For example, if you specify ±10°F (±5.5°C) at 300°F (149°C), can you expect the readings taken during a survey to be 290 to 310°F (143 to 154°C) throughout the uniform zone? Only if you have physically located the sensor (the control thermocouple) at a point that represents the mean of all the temperature readings in the chamber.
Oven Temperature Control
Ahh…the biggest challenge for the spec writer. Pretty simple rules though: The simpler your operation, the simpler the controls should be. Take examples from one extreme to another. Suppose you are leaving the temperature in the unit at one point all day, and taking your parts in and out several times a shift. A single setpoint controller, teamed with a door switch that shuts down the heat and circulation when the door is opened for loading and unloading, will serve the purpose. At the other end of the spectrum is an application where you need to ramp the temperature up and down at a controlled rate, acquire data from vacuum transducers and part thermocouples, and make that data available to your computer system for archiving and print out. Often, this type of application requires a graphical human-machine interface (HMI) and a PLC. In between these two extremes are the programmable controllers, which will allow the heating cycle to run automatically, ramping up, holding, cooling down and shutting off.
Using a PLC to Manage Operating Parameters
The PLC itself is a control component that many spec writers either forget about, take for granted or have some foggy notion that lies in between.
In the early years, all actions -- turning on the heaters, turning them off in the event of an alarm condition, processing an alarm condition, or turning on an exhaust blower to cool the unit -- were handled with relays. A large system could have a daunting array of relays, usually marked in some manner that could tie them to the unit’s schematic, and they were an electrician’s nightmare. The poor fellow could spend hours with his meter trying to trace a problem and find the relay that was at fault.
Then along came the PLC, a black box that took the place of all those relays. The box could be programmed with what was referred to as ladder logic. On one side of the ladder -- which essentially was what a PLC program schematic looked like -- was a vertical line, which represented the inputs. The line on the opposite side represented the outputs. The rungs in between the two lines were the transmitters between the inputs and outputs. So, it could be set up so that the a number of inputs -- for example, power switch on and limits satisfied -- had to be on before the “rung” would tell the output to be active. This output could allow the heaters to be energized, the blowers to come on, and other oven functions.
You will not find a PLC in a small laboratory oven or even in many small industrial units where the operating logic is simple. The more complex the operating parameters are, the better the case can be made for a PLC.
Gas or Electric
One final consideration is whether the unit should be heated by gas or electricity. A sensible way to make the decision is to determine the expected power usage during the average operating day and use the local utility rates to compare costs.
Another way to make the decision is by the size of the unit. On a smaller unit, electricity may be the way to go to avoid what could be a more expensive gas system that will require more maintenance. At the other end of the scale are the very large units, where the cost of the gas system will pale in comparison to the very expensive components (relays, breakers, SCRs, wire sizes, etc.) that would be needed for a unit with a lot of electrical power.
Once you have given some thought to the system you need, you can contact some oven vendors confident that you will buy enough -- but not too much -- oven to handle your process effectively and even leave you some room for growth. PH