ReplyPump power is largely determined by the length and rise of the run. VFDs can help with energy costs but every installation will be different in its requirements.Originally Posted by control$
Professional Member
What is a good KW/Ton for an entire central plant. This would include towers, primary and sec CHW pumps and CW pumps. I generally see around .45-.5 KW ton at the chillers. When I add in all the KW for all mechanical equipment and secondary flow, I average around 1 KW/Ton.
Professional Member
Pump power is largely determined by the length and rise of the run. VFDs can help with energy costs but every installation will be different in its requirements.
Professional Member
I run into chiller plants that all seem to have .45-.5 KW/ton at the chillers. Great, but meaningless if the energy is wasted all around the system. The only way I know to measure how efficient a plant is actual KW/Ton. That is taking the total plant KW and seconary tonnage. KW reading from chiller, pumps, towers, and all energy in the primary / secondary systems to get an real number.
Professional Member
I agree. Many factors and many different systems can lower the number. But it would seem to be a very easy question.
Professional Member*
Yeah, but easy question doesn't necessarily equal easy answer. But I expect somebody here to have calculated their plant efficiency. To make it even more complicated, are you planning to calculate how changes for efficiency in your plant affect the efficiency of the cooling load equipment, such as AHU's or process equipment? I've been curious about that on my site, but not enough to calculate it. I just recently got added to "the list" to recieve copies of utility bills here, but I've yet to see one. BTW, my site is actually two; two office buildings, all comfort cooling (AHU's). I'm hoping your question generates a good discussion. Thanks for asking it.
Professional Member
Most of these additional energy costs are associated with moving BTu's from a physical plant equipment room to the occupied space that it is needed in. The distance is the variable that eats up your efficiency. As a matter of comparison a .50kw/ton machine is equivalent to a 24 SEER package unit if you didn't have to deliver your BTUs to a remote space. Even at 1kw/ton that is 12 SEER. Not too bad when you consider the maintenance savings of not having 200 condensing units to repair/pm.
If equipment costs are no concern, then the most efficient plant I've seen is a geothermal with zoned variable refrigerant flow. It is very complicated and probably will be a major headache to trouble shoot problems when it gets older, but it can get that 24 SEER plus delivered to the space.
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I'm also thinking of things such as when chilled water temp is reset, and AHU supply air is reset, how (much) is efficiency affected by other things that happen in response to this. Things like VAV's opening further for more cooling (if/when needed), and VAV electric reheat operating less (less overcooling and shorter on-cycle time of reheat). These things seem tough to measure when there is only 1 electric meter to the site. Well, one to each of 2 buildings. They are completey separate.
Regular Guest
For the plant efficiency, the lower is better, as the equipment using less electrical kw to generate one cooling rt. In sg here, the rating any below 0.6kw/rt consider the best. this reading very dependent on the local climate. I had seen in tw, the plant system efficiency rated at 0.5kw/rt where the outdoor air around 24dC. to my experience the condensing temperature affect the efficiency a lot. if it is a vfd chiller, then regulate condenser pump flow during part load condition. Agree that increase the temperature setpoint definitely improve. Also over pumping, high condensing, poor equipment performance will also affect the efficiency.
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In most installations, the two factors that have the greatest impact on overall efficiency are the condenser water supply temp, and the chilled water supply temp. The closer you can make those, the less work the system has to do. However, there is sometimes a break-even point where it isn't worth cranking up the tower fans to max to drop another degree of condenser temp. A wet bulb based condenser supply temp does good for this.
I will say that depending on outdoor conditions, and building loads, we have seen as low as .5kW/ton whole system efficiency (that's including chilled water pumps, condenser pumps, chiller, and cooling tower fans). That is, however, with cool, dry air (wet bulb @ 40°F).
The other trade-off to be made is in raising the chilled water setpoint - this will end up causing more chilled water flow, so there may be a point where it isn't worth going higher, but I haven't seen that in this installation. We run chilled water up to 50°F supply at times, and it auto-resets based on the chilled water valve position of the major air handlers.
Professional Member
Total central plant system at .5kw/ton is excellent, thanks for the feedback. Been trying to get my average below 1.00 kw/ton with some success. Got it down to around .8 at times. (5) chillers and 1900 tons of capacity with a mix of new and old.
Professional Member*
You aren't really going to be able to get this low with older equipment. Only the newest VFD chillers, with oversized condensers, and special oil handling, can run this way. Most older chillers start having significant problems with the colder condenser water it takes to get this kind of efficiency. This is also only possible at part load, with the compressor wheel slowed down to 45Hz or so. I know our new Yorks are most efficient at about 40% load. We could likely get better efficiency by installing VFD's on the condenser pumps, which currently run way out of their efficiency curve if using just the small chiller.
Professional Member*
For total annual savings, in cold climates you can bump your numbers up by running a coupel cetrifugals as free cooling machines and using economizers where possible.
Also consider how your measuring it all. IF you have hot water reheat for dehumidification, unless it's you have hot water generation, I think you need to factor that in somehow. AN energy effciency fresh air and dehumidificaiton strategy can get overlooked.
Efficiency of the central plant itself is great, but NET energy use for all HVAC is another.
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So when operations has the cooling towers set in manual and the tower water loop is 35F is that just a missed opportunity?
Sometimes I think we should specify the equipment for the way we like to run it, not the way the industry (you know decades of engineers, field expereince and common sense tells you to run it) As cold as we run our tower water at times, I sometimes think we should be specifiying VFD and free cooling machines all the time. Then agian, the powers that be would prefer it be controlled off a simplisticly programmed and slow poke DCS system instead of tried and tested HVAC controls like Tracer Summit.
Professional Member*
We do all our controls on the BMS, and trust me, just about any BMS system is fast enough for chiller plant optimization. It is not a high-speed system, where you need thousands of computations a second. we run our reset schemes on 5 second intervals, and even that is excessive. The important part is that it is designed to be flexible, and let the system be optimized over time. I understand there is a need to get the system up and running simply, to complete the construction phase, but there should be included a post-opening/post-startup period of trending, analysis, reconfiguration, trending, rinse-repeat.So when operations has the cooling towers set in manual and the tower water loop is 35F is that just a missed opportunity?
Sometimes I think we should specify the equipment for the way we like to run it, not the way the industry (you know decades of engineers, field expereince and common sense tells you to run it) As cold as we run our tower water at times, I sometimes think we should be specifiying VFD and free cooling machines all the time. Then agian, the powers that be would prefer it be controlled off a simplisticly programmed and slow poke DCS system instead of tried and tested HVAC controls like Tracer Summit.
As for running a chiller in free-cooling, that is usually not worth the effort, and it is much simpler/cheaper to run a plate-frame exchanger. A better opportunity is to run the plate and frame as its own primary loop from the chilled water return, and then through the chillers. Great for those situations where you need a 44°CHWS, and have a 45°F tower water. It allows you to get as much of the free cooling direct from the condenser loop, and then top the cooling off with the chiller. Best of all, it's not hard to add to an existing plant, as long as you can find space for the heat exchanger and its own primary pump.
Professional Member*
Great idea on the he. however I. One area with most of our chillers, we run glycol with a setpoint of 0f. its p.s piping to a big storage tank without much stratification. So at low loads in winter I think the evap return is only 3f.
Id rather use a bms like tracer summit with preconfigured interfacers for trane chillers, lead lag schemes, etc. We have foxboro dcs. It configured to control most things with 10 to 100 second intervals. Mostly slow processes. It not the speed, but that pur controls enginers are nog used to this type of equipment. The current chiller stages on rla and tends to load multiple chillers evenly and does prioritize running the centrifugals over the rotaries. The are times where a single cvhe could hold the load but instread 2 rotaries are also partly loaded.
You cannot remotely view approaches, pressures or any useful info. They even wasted money on seperate high dollar rosemount transmitters for. Water temps.
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Regular Guest
One consideration to CHW temperature reset is that the lower the setpoint the more latent heat will be absorbed by the CHW loop and this will add tonnage to the chiller.
Professional Member*
This is only indirectly true. If the air handler is controlling to a discharge air temp, and is modulating the chilled water valve, then changing the chilled water temp will not affect the discharge air temp - which equates directly to the amount of moisture removed. If you have an air handler set to discharge 55°F air, and dewpoint of the incoming air is higher than that, then the amount of latent heat removed is going to be the same no matter what temp the chilled water is going in.
That's not to say that latent heat removal isn't to be considered, but that is an air handler optimization, not a chiller plant optimization. Ideally, you'd run the discharge temperature only as low as the building needs to meet desired indoor temperature and humidity.
Professional Member
I've seen kW/Ton Plant wide vary from 1.23kW/Ton to 0.83kW/ton. Sometimes district loops play a factor.
Improvements vary from 50% to 20% reduction - and that pertains to some serious plant optimization
strategies (eg, Hartman Loop).
Then there's just the 'politics' of the matter: what do you think saving ~300K/year means to a $4B biotech company?They just bought the optimization product to say they wave the 'green flag'.
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