Some people would argue that Sequences of Operation are the most important thing to know when programming a controls system. They would say knowing what the engineer’s intent for the use of the system or piece of equipment and the capabilities of the unit are key to providing an efficient and capable controls system. They even think you need some sort of manufacturer’s documentation available for the equipment being controlled.
Those people have no spirit of adventure!
The truth is… THEY are absolutely correct and most of the time I happen to be one of THEM. It makes all too much sense that you have to have an idea of what the equipment was designed and made to do. Then it is up to you as the controls guy to figure out any better ways to accomplish the same effect and add in atleast one of these things:
- Added comfort
- Efficiency and Energy Savings
- Lower Maintenance Costs
- Cool Factor (yeah really)
We Got Comfortable With The COOL Factor
In our little Liebert Mini Mate retrofit we figured we hit on two of the above bullet points; Added Comfort and Cool Factor.
Comfort was accomplished to the delight of our 2 servers and phone system tower because they had been enduring 90°F room temps in the closet since the Liebert system had been off due to mechanical issues of a refrigerant leak.
Needless to say having the A/C running and doing what we wanted it to by programming it ourselves added some cooling comfort.
I mean c’mon… we’re a controls integrator and were running our unit off a stand alone thermostat. What!
The Cool Factor wasn’t about temperature though, it was that we could now play with the unit and get email alarms if we wanted to and not have to hear the stupid Liebert Thermostat blaring beeps every time the compressor short cycles because it’s getting too cold, too fast.
So now that we added a Delta Controls DAC-606, we now have it on our Building Automation System and can take a look at it from any computer… <insert movie trailer guy’s voice> … a n y w h e r e in the W O R L D!
It’s supposed to make cold air, DUH.
So I knew very little about any sequences to this unit and most of the time neither will you if you are walking up on an existing unit of a small project where no engineers are involved. This is why i say most of the time I want sequences, but… I’ve done this long enough to get the gist of mechanical systems and units like this one and what *I* want it to do. But what should you do if you don’t?
GOOGLE IT! So for this article, I googled ‘liebert mini mate 2 operation manual‘ and as you can see there are plenty of options right on the first page. You can do this with any kind of HVAC unit out there, though sometimes it helps to have the first few characters of the model of unit and not just a broad manufacturer and model name.
When looking for manuals always go for the PDF results as those are usually the official manuals offered up on websites and such.
Here is the Manual I chose which was 2nd on the list: Liebert Mini-Mate2™ Installation, Operation and Maintenance Manual – 1-1.5 Tons, 50 & 60Hz
If you look in Section 4 on page 37, you can read what the manufacturer has decided the various Sequence of Ops should be.
Ol’ Frank Sinatra would be proud. “…and I did it MY way.”
So being that we weren’t too concerned with energy savings for a server closet that housed our company’s data and the humidity levels weren’t much of a concern since the cold air pumping in to such a small area kept it within tolerance… I did it MY way.
It was getting late and we just wanted to get the sucker to run. In the last retrofit video you heard me mention I was only interested in the fan and compressor. With those two outputs we could get the unit cooling and running.
My little sequence was nothing more than utilizing the following inputs and outputs:
- Space Temperature (Input)
- Supply Air Temperature (Input)
- Supply Fan Status (Input)
- Supply Fan (Output)
- Compressor or Cooling Stage 1 (Output)
To accomplish this I/O, I used the following products:
This is how I Dooze it…
With those three parts we can control the simplified, let’s-hurry-up-and-finish-so-I-can-go-home-and-eat-something version. I utilized a database (Delta’s controller database file contains all the I/O, variables and programs to make a unit work) that I had already written for use in any kind of package unit, heat pump or fan coil that had a set number of digital or analog cooling/heating stages/valves.
I promise to show you in detail with a look over my shoulder of how that looks like in a future video.
PID Loop Control - Essentially, the sequence of operation is to utilize a proportional only PID Loop looking at space temp and space temp set point to change the Unit Mode to Heating, Cooling or Deadband. Deadband, meaning it is satisfied and the unit is waiting in the OFF position.
Once the PID Loop hits 100% I change the Unit Mode to Cooling. The Proportional Band is set really wide so that the unit will have longer cycle times, but the temperature will have a wide swing as well. looking back on the trend logs we saw the space temp would go from 68° to 78° which is exactly the 10° ΔT (differential temperature) I programmed.
Supply Fan Control – When the Unit Mode is in any mode other than Deadband the Fan is started.
Cooling Stage 1 Control – First off I always build in a safety to not turn on a compressor unless fan status is proven and that is where the current switch comes in. Once the fan status has been proven I allow the compressor to be turned on based on the PID Loop.
Since the programming is setup for the possibility that the unit could have heat I split the percentage numbers of the PID Loop so that at 100% turn on cooling and at 50% turn it off. If the unit had a heat stage then it would come on when the PID Loop hit 0% and back off at 50%. These percentages only work if they are in the proper Unit Mode (cooling/heating).
IMPORTANT!
When controlling DX compressors on an A/C unit, you MUST make sure your sequence/programming does not allow the compressor to start more then 6 times per hour.
This is a standard practice, just ask any mechanical or HVAC/R equipment guy and he will tell ya how frustrating it is that greenhorn Controls Guys don’t know how to control a compressor properly.
BAM! Now go show em what ya know.
If you don’t have any questions, you’re not thinkin’ hard enough…
I have just barely scratched the tip of the iceberg and have tried to keep everything very general and easy to understand. If you have questions or are confused about what I have laid out. Let me know.
Take a second to write your comments below and myself or any one of the experienced engineers and controls technicians lurking about will be sure to feed you with food for thought.
…you won’t do it…
