OA wetbulb temp. as it cooresponds to waterside economizer

[BlueRhinoPinkBully]

What is a good wetbulb temperature to set as a maximum temperature to allow a water-side economizer to be allowed to operate in a water-cooled chiller application with cooling towers. This application is in the mid-west (st. louis)

 

[RossABQ]

ASHRAE and the Air Force among others publish historical mean coincident wet bulb temps for any major city.

 

[NCDesign]

Do you have a separate system or are you just letting the water free fall on the cooling towers without the fans?

 

[BlueRhinoPinkBully]

it is all one system. I would still have to try to figure out the best control strategy for the tower fans, but for now I would say it would be best to have some sort of return water temperature reset with a change in outside air temperature

 

[atlas06]

Why are you controlling an economizer with OA? you should monitor your condenser water temp. You cycle your machines from load in the building, not from OA temp. OA temp is not a good representation of building load. Monitor you water temp is OK, but switching automatically from chiller to HX based on OA temp may not achieve heat removal. I do not see a need for a OA wet bulb to control economizer.

Hope some of the control guys will jump in and shed more light on this.

 

[NCDesign]

atlas06 is right on bluerhino. Just control to the optimum condenser water temperature for your setup. If you do it that way you will maximize economizer time.

 

[BlueRhinoPinkBully]

You still have to set a parameter at which the system switches out of economizer mode. So that parameter would still be the return condenser water temperature? I.E. if you wanted your return condenser water temperature to be 85, then if it got over something like 89 (adj.), you would have the economizer turned off? Thanks.

 

[NCDesign]

Your economizer is essentially your first stage of control for your condenser loop, when it can't keep up you start cycling through your tower fans as required.
I am still not clear on your exact economizer set-up. So I'm not really sure if your situation is setup such that economizer operation is independent of tower operation. That will have some effect on the control sequence but the overall theory is the same.

 

[MPcontrol1]

Well i hope i can answer a few of your questions here.As far as HX mode most control companies base there start off of outside wb. The sequence for the control is to find the more efficient way for energy savings. So if cooling demand is needed in the bldg the operation is to compare the outside wb and see if it is more optimal to run in HX or econnomizer rather than a chiller. Most programs are written with parameters and setpoints depending on each situation. Most towers are presumed to have a 10 Degree approach so if outsied wet bulb is 40 degrees you can make 50 degree water theoreticly. There are variables that can prevent that (clogged tower , fan not running , ect...) in which case I can monitor cdw temp and see if it is achieving that setpoint. We try and keep our cdw temp in hx mode to a maximum of 55 degree CDW so in our sequence we write that if (assuming 10 degree approach in the tower)we can disable hx mode on 45 degree wet bulb. there is a parameter for Dry bulb to. I would have to refrence that in the psych chart. It also goes the other way if the OA temp is below a certian point you would disable the fans in the tower such as 35 degree oa temp. (towers can make some serious ice) In a nutshell the controls for hx are based on OA wb for starting and stopping and we also can watch the CDWR temp to see if it is achieving that.

 

[BlueRhinoPinkBully]

that's the answer I was looking for, as I am indeed in energy conservation... I thought you could go into the low 40's, or even 45 if the site can indeed handle approximately 50-52 degree chilled water. The reason i say 50-52 is because I've been told that the approach you described can be as low as maybe 4-5 degrees, but I haven't confirmed it either way. In any event, thanks all for your time, it is all good insight.

 

[atlas06]

Thnaks MP for the clarifications on wet bulb control,
Bluerhino, the system described by MP is what's called "Chiller plant Optimization".
I think that you would use the scheme described by MP if your HX is sized for the laod of at least one chiller, AND the economizer water flow should be compared to CHW delta/CHW flow to determine the building load and verify if the heat exchanger water flow will be adequate before switching to economizer.

The other thing is: Is wet bulb control really reliable? never relied on it myself, I would like to hear news about it if you have any.

A variety of optimizations can be programmed in "what if's", say if you have a screw chiller at 0.77 KW/ton sized at 125 ton with two 250-ton centrifugals (at 0.56 KW/ton), your DDC will compare Amp draw of screw chiller at full load and centrifugal at part load and/or vice versa to see which combination will do you the most good.

But I admit, it appears that MP understood you question best.

 

[MPcontrol1]

As far as using wet bulb for a demand control i would agree with you. Really the wet bulb control is a signal that lets us know that conditions are ideal outside. Usually there are very few cooling demands when you are in a plate and frame mode.Therefore we can run the CHW water temp a little higher than normal from 42 to 52 degrees. In otherwards your not going to run your AC in your house when its 68 degrees outside. So it just give us a gauge on when it is applicable. If the conditions do not meet we will disregard plate and frame mode. Then you can go into staging chillers on demand. Thats where reading water temp becomes key to see if we need to step up the cooling process or slow it down depending on demand limit.And like you said there are many various ways we can write the program depending on what it calls for

Glad to have helped

 

[atlas06]

Thanks MP. Looking forward to seeing more posts from you.