The past few weeks EPS President Ken Klein has been sharing tips for use when buying a large industrial oven, specifically a large convection oven. This week Ken wraps up with two last components for consideration: the PLC, and choosing between gas and electric.
PLC – The PLC (programmable logic controller) itself is a control component that many spec writers either forget, take for granted, or have only a foggy notion of. In early years all actions—turning heaters on or off (in an alarm condition), processing an alarm condition, activating an exhaust blower to cool the unit, or any number of other tasks—were accomplished using relays.
A large system could have a daunting array, usually marked in a way that tied them to the unit’s schematic, and they were an electrician’s nightmare. The poor guy would spend hours with his meter trying to trace a problem to find the individual faulty relay. Then along came the PLC, a black-box that took the place of all the 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 were the transmitters between the two.
You could set this up so that the number of inputs (power switch on, limits satisfied, etc.) had to be on before the “rung” would tell the output to be active. This output could allow the heaters to be energized or the blowers to come on. PLCs aren’t used in small lab ovens or even in many small industrial oven units where the operating logic is simple. The more complex the operating parameters the better the case can be for a PLC. Lights on the PLC can indicate the source of a malfunction.
Gas or Electric –Lastly, let’s talk about whether a unit should be heated by gas or electricity. One way to decide is to determine the expected power usage during an average operating day and use the local utility rates to compare costs. Another way to make the decision is by 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. On very large units the cost of the gas system will pale in comparison to the very expensive components (relays, breakers, SCR’s wire sizes, etc.) needed for a unit with a lot of electrical power. Another subject is direct vs. indirect gas but that is a subject for another article. Just keep in mind that you may want to avoid getting to know the president of the local power company on a first name basis.
That concludes our series of posts from EPS President Ken Klein, “Oven Selection and Specifications”. Contact us today for more information or a free quote.