Assuming this is for a rooftop unit or air handling unit. It means the supply air temperature or some people refer to as discharge air temperature (leaving air temperature) from the unit will reset as needed to maintain temperature. If this is on a DDC system you would have a process variable like space temperature or return air temperature.

So the heating & cooling will work as needed (depending on lockouts, limits, deadbands, etc.. in the program) to reset the SAT / DAT based on a variable like space temperature or return air temperature.

A simple reset would look like this;

Space Temp
75
70

Supply Air Temp
55
65

So if the space is 75 deg. we want a 55 deg. supply in a cooling mode, and on the other end if the space was 70 deg. we would want a 65 deg. supply. Many variables. Does system have reheat, is it constant volume or VAV etc.. The reset would usually be proportional in between those setpoint values.

Gotta get back to work. Hope that helps.

 

It is also possible to reset the DA based on OA. I.E. if it is 80 outside we may need cooler supply air than if it is 70 outside. So the discharge air would be adjusted based on what is "needed" to satisfy the space.

 

Everything can reset the DAT. I've seen OAT, RMT, fan speed, VAV damper positions, terminal loads .... you name it.

And ratio based reset like twisted pair described is not the only way. Periodical increasing/decreasing setpoint is another possibility.

 

zarembad and leaflying,

I agree with both of you. Based on the general question I thought I would answer it with some common or generic examples. I am sure there are probably even more ways.

He did not elaborate on what type of system this would apply to. So I figured my example would be a good start, but you both point out that there are many ways to accomplish supply air reset.

 

VAV Damper Pos resetting DATstpt? WOW! Never seen that one before. Havent got the slightest idea how that would be proper...

On a VAV unit:
I reset my Static Pressure Setpoint based on VAV Damper Position, and reset the DAT setpoint based on terminal load. I also have time limits on how fast the setpoints can change. Example: Static setpoint can not change more than x-"w.c. per x-time, etc...
Without going nuts like adding another variable in there for "riding the fan curve", that about as good as it gets IMHO.

 

On a VAV unit:
I reset my Static Pressure Setpoint based on VAV Damper Position, and reset the DAT setpoint based on terminal load. I also have time limits on how fast the setpoints can change. Example: Static setpoint can not change more than x-"w.c. per x-time, etc...
Without going nuts like adding another variable in there for "riding the fan curve", that about as good as it gets IMHO.

I've not been doing this as long as some but I think I'm seeing a potential for conflict there. Resetting static stpt based on damper position? Why wouldn't you let the damper change based on load & vary the VFD or guide vanes at the AHU to keep the static constant? The way I'm reading your statement you're changing 2 variables in the same equation and I'm curious how you keep making them add up.

Seriously, not being a wise guy.

Thanks.

 

Discharge air control, in some cases, will have an air handling unit supplying air at a neutral temperature that does not really heat or cool the space.

As the space starts to warm up or cool down, you can reset or change the supply air temperature to compensate.

Example a make up air unit in winter would normally supply room temperature air to the space, however if the space was starting to cool off the air temperature could be reset up to say 90F, to warm the space up.

 

This is kind of stealing the topic. But I am gonna do it anyways.

The other way of VAV system fan speed control I know in Asia is to calculate the total box air volume / design air volume. I havn't tried that myself but I can tell the change of the speed will be faster and not as smooth as other methods.

But the craziest idea stays in America. Thomas Hartman in his all VFD controlled system (from chiller to pump to fan), suggested that every little change of the cooling load will be evenly shared by all analog controlled device. I've not figured out how to exactly apply this to a real system.

Mr. Thomas Hartman has some famous patented control strategies. Has anyone here heard of or tried his stuff?

 

I saw Tom Hartman at an ASHRAE dinner one evening where he was the technical speaker. Very nice and very interesting. In a nutshell and in simplistic terms, what he was trying to get across was the fact that rather than varying only certain pieces of equipment (ie:just the fan for static pressure) he varies everything including the central plant (as many pieces as possible) - at least that was my understanding.

I believe it would be rather complex in terms of set up, but once it's done it seemed like it would work. I think one of the most difficult things would be to get a control guy to understand what he wants done and then doing it.

BTW...this whole thread has taken an interesting twist. I'm very curious about the Asian/ Japanese method.

It seems to make sense, because in the end all we care about is getting the CFM to the space as it is required.

 

Hartman's stuff is very interesting, but it tends to ignore cause and effect. By that I mean it seems to work well when you are turning the entire system down as requirements for cooling/heating/air movement etc. diminish. But when that demand reverses and you apply Tom's logic, it means you turn everything back up again to remedy an unbalance. This sequence will probably "fix the problem", but it does so my throwing a handful of pebbles at it instead of a well aimed single rock. It's more efficient to simply react to an event by altering the one restorative variable that's going to effect the biggest change for the smallest effort.

But - that aside - I've used Supply Air Reset extensively in VAV systems and consider it a mandatory component of good VAV system design. The setpoint itself floats between 1.0" and 2.5" as measured 2/3 the way down the main trunk on the floor furthest away from the AHU. The reset scheme is run every 10 minutes and adjusts the set point up or down by 0.05". This decision is a function of monitoring the damper positions of all the (occupied) VAV boxes associated to the AHU and discarding any that are open more than 98% and closed more than 50%. Boxes that fall beyond these parameters are usually involved in trying to heat/cool a space that's not being used to the original design load and need to be physically investigated to find out why they can't provide the heating/cooling required.

The theory behind this is simple - you want your main supply fan(s) to ride their curve. Fan energy is typically larger component of total system energy use than the energy required to cool a building and this reset scheme allows you to maintain your facility but to do so efficiently.

Nikko

 

Likewise Nikko.

We use that strategy for many VAV applications and have the same results as you.

 

Mr. Hartman's theory comes from a very simple question. What is the most efficient way to deliver certain amount of cooling/heating capacity? And he argued that when there is a over cooling/overheating, equally uploading all energy consuming equipments in the system will save the most energy.

I googled the term "The Equal Marginal Performance Principle", trying to understand the basics behind it. But it seems that no one has mentioned that before in the engineering field. And Hartman is the first one. I can only find some similar term in economics.

 

Tom's theory is quite interesting. Basically he says that instead of a bunch of independent control loops running a system, all systems should be treated as one big loop. In otherwords - think "system loop". If you don't need that loop to produce effort (which costs $$) to combat an unwanted situation, turn it down. And if you need to produce more restorative effort, turn it up.

But I'm not convinced that his method is any more efficient than a bunch of smaller independent tasks running together to control a piece of equipment. He uses chillers as the premise for his example, but let's look at a basic VAV AHU. In this unit you have several things under the direct control of the unit:

1) Discharge air temperature control (Heating/cooling)
2) Mixed Air temperature control (economizer)
3) Discharge pressure control
4) Building pressure control.
5) Minimum outdoor air (volume)

Dischage air temp control is simple in a VAV system. The system is designed to function to a static temperature setpoint and the unit controls to this value. I'll often tuck a small "if, if if, if, if" loop in there that says if everything else is completely backed off and the system's satisfied, start to reset the discharge air temp and sometimes I'll incorporate a small reset schedule based on OAT (if the system design is at all marginal), but generally, as a rule - I'll leave it alone.

Mixed air temp is also simple. You control it to a setpoint that's equal to the discharge air temp setpoint minus the delta T accumulated due to the supply fan heat (I start at 2 deg. F). If it's cool out but not cold enough to damage the unit, and if it's not too warm out, throttle the OA dampers to achieve setpoint.

For Min OA, never close the OA dampers beyond a point at which your minimum OA volume is being satisfied unless the unit's in danger of hurting itself. In which case, both of these previous loops quit, the OA damper closes and the unit recovers.

Supply air static pressure was explained in my last

Building air static pressure is controlled by the exhaust fan speed or damper position from a measurement point usually located i the main lobby but away form the elevators and front doors.

If you examine these various control strategies, none of them are really connected except maybe the Static pressure reset and Discharge air temp reset. But that's it. So there's no real advantage to putting all the various control-measurement point values into one big pot, mixing them up and magically deriving a bunch of control output points. It just doesn't work that way. In fact and as I said, It's easier and the "tunability" of the unit is far better if each of these control strategies is treated as an independent.

Now, chiller control may be different, but I'm still not convinced it's all that much different and that there's value in incorporating measured values into an equation where the result of the equation has no cause or effect on those measured values. Seems like a lot of extra work (and risk) for no reward.

Unless I'm missing something.

Nikko

 

Tom's theory is quite interesting. Basically he says that instead of a bunch of independent control loops running a system, all systems should be treated as one big loop. In otherwords - think "system loop". If you don't need that loop to produce effort (which costs $$) to combat an unwanted situation, turn it down. And if you need to produce more restorative effort, turn it up.

But I'm not convinced ......

Unless I'm missing something.

Nikko

I think Tom's theories are more suited to an Iterative control structure and not the industry-standard PID structure.

I try to limit my SA Temp reset as you get into humidity problems with increased IAQ concerns. I'd rather vary the fan static a bit on terminal loading and tend to minimize any SA reset unless it's low outside RH levels. (FALL/WINTER). Trane had an interesting set of articles on low temp delivery systems. Currently, I'm investing alot of time into minimizing air moving systems. I personally think that's a bad way to provide space comfort.

 

I pull in a stockmarket RSS feed from the internet and reset my discharge setpoints based on the value of Siemens Stock.

When the stock goes up, I reset the setpoint down to make everyone cool and happy. If the stock price goes down, I raise the setpoint so that people get hot and stuffy and then they call for service.

 

Perhaps - but the overlying concept is the same, iterative simply means a task will take small corrective steps instead riding a corrective ramp of response.

I'm still not sure how this global melting pot of data in and data out (the "System") translates into an actual "if,then,else" sequence of operations.

I'm all ears though...

Nikko

 

Nikko,

That bothers me too. I will revisit what I've read and try to put all puzzles together again. Good to have someone to discuss with along the road!