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  1. #1
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    Central Plant Efficientcy

    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.

  2. #2
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    Quote Originally Posted by control$ View Post
    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.
    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.

  3. #3
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    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.

  4. #4
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    Quote Originally Posted by bob_scheel View Post
    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.
    I agree. Many factors and many different systems can lower the number. But it would seem to be a very easy question.

  5. #5
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    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.

  6. #6
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    Quote Originally Posted by Nuclrchiller View Post
    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.
    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.

  7. #7
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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.

  8. #8
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    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.

  9. #9
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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 @ 40F).

    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 50F supply at times, and it auto-resets based on the chilled water valve position of the major air handlers.

  10. #10
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    Quote Originally Posted by CraziFuzzy View Post
    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 @ 40F).

    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 50F supply at times, and it auto-resets based on the chilled water valve position of the major air handlers.
    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.

  11. #11
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    Quote Originally Posted by control$ View Post
    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.
    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.

  12. #12
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    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.

  13. #13
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    Quote Originally Posted by CraziFuzzy View Post
    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.
    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.

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