LACO received a request to design a cost-effective, space simulation vacuum system from a university that is testing items that will be used in the space program. Their requirements were for the system to be capable of achieving tested part temperatures ranging from 60˚C down to -40˚ C with a temperature variation of ±3˚ C.
Additionally, the client needed us to effect a ramp rate of 1-3˚ C per minute. Typically, a system with requirements of those temperatures would be built by using a high precision thermo regulator, known as a “chiller”. Thermo regulators are closed loop systems that use a variety of fluids and are highly effective at very precise temperature control, at a significant cost. The addition of a chiller to this system would have easily doubled its overall price tag, and priced it out of range of our client. To fulfill our customer’s needs, both technical and fiscal, we had to tap the expertise of our control engineers to provide a cost-effective solution.
Close up of platen
Inside View of Vacuum Chamber
Heating the platen to +60˚ C within the chamber could be effectively accomplished with a strip heater that comes with its supplied control unit. Our software engineer had only to call out the temperature required and the heater would heat to that point. Cooling was another matter entirely. The most effective and economical heat transfer medium available to universities and one that LACO is familiar with is LN2 or liquid nitrogen. Unfortunately, the boiling point of LN2 is -196˚ C so bringing the temperature of the platen to precisely -40˚ C would require judicious application of the liquid. In fact, using liquid nitrogen to cool to only -40˚ C is roughly analogous to attempting to roast a perfectly golden brown marshmallow with a flamethrower. It can be done, but only if you are very careful.
At first, we tried using a PID controller, but the results were not successful, temperatures fluctuated wildly and would not stabilize. In the end, LACO control engineers spent hours and hours devising their own way of setting the temperature with a valve and the advanced capabilities of LACOs new, HC-3000 controller. To add further complications, the valve that controlled the nitrogen could only be used once every 30 seconds; more frequent use would “seriously reduce the lifespan of the solenoid”. This precluded the minute, fiddly adjustments that are often necessary to keep a temperature from drifting from its set-point. In the end, it was best to undershoot the valved amount of nitrogen and then, in the next cycle, nudge the valve farther open to ease into the goal temperature. As the following chart demonstrates, they were very successful in reaching and sustaining target temperatures, without the use of traditional, more costly, methods.
If you can’t afford to throw money at a need, at LACO, we have found that throwing a little engineering know-how and brainpower at it can often help our customers find an economical solution to a previously cost-prohibitive problem.