Chiller Efficiency and CWS Reset

[smokies]

I am hoping to get some help from you chiller guys. I'm looking for input on increasing the efficiency of a chiller by reseting the condenser water temperature. I can use wetbulb and drybulb to understand the conditions necessary to reset, but I would like some input from chiller folks to know when to do it, what are my limits, and what if the actual afffect. Cooling Tower manufacturers can provide input, but I care more about what it does to the chiller. Any thoughts? Do you know of any articles on the subject?

[pecmsg]

The US Department of Energy has several articles on energy efficiency and large chilled water plants. Start your search there.

If I remember correctly for every 1*F rise in chilled water supply and 1*F drop in condenser water in you pick up 1% overall. As far as the chiller itself you have to check with the manufacture.

[Tech Rob]

I'm far from an expert on the matter, so here's my unsolicited thoughts. Feel free to correct me if I'm wrong...

The chiller's manufacturer should be able to tell you the range as far as condenser water temps go. There is a point where condenser water temp and %FLA intersect that is where the chiller is most efficient. I would imagine that the closer you can stay to that point the better off you are. Any time you can lower the compression ratio or lift while staying within safe operating limits and not creating oil problems, you're saving money.

Sometimes it's better to run the 50hp tower fans harder to save on the 400hp compressor motor.

[Randy S.]

Is anybody resetting their tower fan setpoint based on wet bulb plus approach?

[Tech Rob]

Is anybody resetting their tower fan setpoint based on wet bulb plus approach?

I think I understand this concept, but would you mind explaining it to me?

[Randy S.]

In theory, they say, ya know, etc...

In some cases, especially with a variable speed centrifugal and trying to drive the condenser water as far down as you can, there are times when you can't reach the tower leaving set point because of the wet bulb and approach of the tower, and possibly are wasting fan energy trying to do so. The idea is to reset to the wet bulb temp plus the approach, or "how close to wet bulb temp you can get tower leaving temp".

It would take an accurate, absolutely reliable wet bulb reading to make it even try to work. I can't say that I've seen anything that reliable in the HVAC world, anyway, so it's really just a theory, as far as I know.

Before you go messing with condenser temps, know what your machinery can handle, and pay special attention to oil level as you mess with things.

The same thing applies to resetting the chilled leaving temp.

You don't just do either and walk away.

Good luck.

[klove]

I am hoping to get some help from you chiller guys. I'm looking for input on increasing the efficiency of a chiller by reseting the condenser water temperature. I can use wetbulb and drybulb to understand the conditions necessary to reset, but I would like some input from chiller folks to know when to do it, what are my limits, and what if the actual afffect. Cooling Tower manufacturers can provide input, but I care more about what it does to the chiller. Any thoughts? Do you know of any articles on the subject?

Simply put, you use less horsepower to move the same amount of refrigerant if your compression ratio drops. Same way they make residential units more efficient by using large condenser coils (a little oversimplified, but you get the meaning). If your evap pressure remains stable, it takes less energy to move XXX pounds of refrigerant into the condenser if it drops from 90* saturated p/t to 80* saturated p/t. Pure physics.

All machines like for their towers to unload naturally as the load drops off the machine, and some chillers can stand lower cooling water temps than others. Historically, some mfrs have had to keep the cooling water in the 75* +/- range minimum to make the machine work correctly. Some of the old guys would set tower temps at 85* because most chillers are rated at about 85* ecwt and it was better to deal with a screamin' chiller than cold water related issues. But even this 10* drop in tower water temp to 75* which won't cause you any problems relates to significant HP savings on the compressor. If your machine can accept lower tower water temps, drive them down as low as they will go within reason based on the mfrs recommendations for their machine, and take advantage of the energy savings. All this is said based on the fact that we're talking about centrifugals. Screws can do the same thing, but have narrower bands of forgiveness because of their design.

If you do the wet bulb reset thing with the tower, I'm also a big fan of dropping that setpoint from where the calcs say it needs to be (a little bit) because generally you can get a bit more out of the tower than the calculations say you can. Play with it and see what it will do. It will be close, but don't just set it there and leave it because somebody with a slide rule said to. Also look at Randy's previous post and pay attention - he brings up some good points on this subject.

[9675]

Your biggest concern is going to be oil return, I am a little partial but york is going to be the best for low water temp on condensor side of centrifugal Chiller, you can get around 10-12 degress above leaving chilled setpoint without problems.

[smokies]

Good feedback, thanks. What started this was we had a JCI factory sales guy come down and spend some time on their product. He spoke about how the effciency of the chiller can be increased by resetting the condenser water temp. I didn't get the nuts and bolts from him on how to accomplish this, limits, correct sequence, etc. I contacted our local guy for JCI and Trane to ask if they knew of the process, recommendations, or white papers. Both said they would look into it,but I've gotten no response. Basically they had no idea. This seems like a real oportunity and something that could set a company about if done correctly. In a time where every KW counts more than ever, it makes since to investigate. I don't think anyone else in our area is doing this. The chilled water side seems alot easier and more common.

I appreciate the help, thanks in advance.

[jayguy]

like many have said...it depends on your chiller and its design.

lower head pressures equal lower kw/ton "at any instantaneous moment". the real difference is the long term effect and whether YOUR chiller can handle it. the reference to oil in these posts refers to whether the oil that spills into the refrigerant side of the system (versus staying in the bearing areas) is able to be returned to the oil sump as fast as it is spilling out...if not, then you will trip (hopefully) on a lack of oil diagnostic.

the other side of the coin is that you spill more oil out (compared to higher condensing temperatures) but that it returns at the same rate. what does the oil do while it is out in the refrigerant side? it coats the tubes. an oil coated tube does not transfer heat very well. therefore, your chiller will then have to work harder to make the chilled water setpoint...the net gain of lower tower water temperatures may actually end up being a loss. it also may not be a loss at all. i know of chillers in my area that lose their oil drastically at 84F entering condensor water temperature but run fine at 85F (weird!). i also know of some that don't lose a drop at 65F. you will need to investigate your chiller.

by the way, you will gain more savings by investigating your load side more. a chiller that is not running hard to begin with uses less kw.