PID Loops?


Thread Starter


Trying to control humidity and temperature and conserve a bit of energy at the same time.

Attached are two scans of graphs of loop sets.

The top line in each graph is temperature vs temp setpoint. The middle is cooling coil discharge temp setpoint and the bottom is humidity vs humidity setpoint.

I am getting oscillation in the humidity loops and the temp loops tend to drift a bit. Using a 6/1 P/I ratio on the steam valve and a 7.5/1 PI ratio on the humidity valve with zero derivitive on both.

Would guess that to most of you out there the steps to calibrate these are very obvious. I am still learning this and unfamilar with the next best step. Guessing that I need to slow down the Humid loops and speed up the temp loops but don't fully understand if I should be looking to the P or the I portions of each.


as an hvac inst tech i have to tune these types of loops often. keep in mind that the temp of the air leaving the temp section will immediatly affect the humidity sensor (the warmer the air temp, the more moisture it will hold). so you don't want the leaving loop to be more tightly controled than the entering loop. realize that you will not control the humidity any tighter than the temp because the temp loop directly affects the humidity loop. if you are using an energy management system with a loop tuning package --use it. otherwise, start at the temp loop and increase gain using the 1/4 wave process, the temp will settle alittle above or below setpoint, then began adding reset until you return to setpoint. next tune the humidity loop the same way but you may need to use less gain. these are just general procedures because there are many ways to accomplish humidity control and most of the installations i have had to deal with have had the sensors improperly located for accurate feedback. also, many humidity sensors are very slow, so don't expect tight control. you also need to check your humidifier itself. depending on your steam quality you might not be injecting properly. one other important thing, is the steam pressure at a proper level going to the control valves? your valves may be operating in a range below their stable control area, or they may be wide open at times. adjust the supply steam regulator so the control valve is at aprox 70% during maximum load. make sure the supply steam regulator is in good repair, it is a part of the control system and if it is wandering it will affect the downstream control loops. if you are using water atomizers for injectors, water treatment and quality are also problem areas. the above assumes all sensors are calibrated, in the correct position, control valves of proper size and not sticking, minimum dead time due to lash in control actuator attached to valve, and proper supply steam pressure to control valves.

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TDK Thanks for your input though. I hadn't looked in here in months. Will revisit my loops in the spring. I did solve the problem. Better loop tuning was a part of the solution. Primary concern was the ability to reduce overcooling of the air off the cooling coil and then do less reheating and humidifying. Needed to maintain reasonably tight control and the loops were all interdependent. Ended up adjusting the temp off the cooling coil based on the reheat and humidification valve positions. If one of these is near zero then precooling is optimally near minimum. If the temp or humidity starts to wander too much from setpoint due to changing weather conditions, I increase the action of the cooling valve operator proportionately. Works pretty well. Manufacturing space is kept in tolerance and so far it has responded well to quick weather changes (such as a summer storm blowing in and causing lower temps and higher humidity in a very short time). When weather is stable loops work slowly and the valves don't over shoot. Utility savings for the customer are in excess of 100K per month. Tom new email address: [email protected]