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  1. #1
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    Chilled glycol loop pressure differential

    I've been offered a chief engineer position at another data center. One of their priorities upon my acceptance is to get their energy use and efficiency improved. During the interviews, I sniffed around their facility and saw that they're running at 35psi differential on their chilled glycol loop. I run a 7psid on my loop at my present facility. I can't imagine how they're getting any kind of decent heat transfer from their CRAH units at the speed that glycol's screaming through their coils, but I'm not an engineer. Is there anyone out there who can offer any comparisons from their own facilities?

    Another interesting issue...our chilled loop's load comes in right about 900kW, which equates to about 240 tons (calculated from flow vs deltaT on my chiller). Their load is almost identical, but they're running almost flat out on their 500-ton chiller, and are preparing to start using TWO. Something's not adding up someplace.

  2. #2
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    The way I see it is with a higher flow rate the temperature rise of the glycol through the coil would be less. Because you maintain a higher temperature difference compared to the room temperature through the whole coil then you should be getting better heat transfer (higher temperature differential between the glycol leaving temp and the RA temp, and colder supply temp, unless you are reaching equilibrium with your glycol supply and discharge air.)

    A slower flow rate may feel like you are picking up more heat because of a higher temperature rise through the coil, but because your coil temp is closer to your return air temp it’s heat transfer would be less. Heat transfer is greater with a higher temperature difference. If you pick up the same amount of heat but spread it out over a larger GPM then the temp rise will be lower even though the BTUH was the same.

    Overall if your room heat load is the same (infiltration, equipment Kw, etc) and both setups are maintaining the same room temp then you are absorbing the same sensible load with both setups, but you may be preforming more latent work with the colder coil if it is operating below dew point. That may or may not be desirable to you depending on where you live. The higher loop pressure may allow for a higher flow rate through the coil but I would expect it will have a shorter run cycle with each call for cooling or dehumidification.


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  3. #3
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    Thread Starter
    Quote Originally Posted by thatguy View Post
    The way I see it is with a higher flow rate the temperature rise of the glycol through the coil would be less. Because you maintain a higher temperature difference compared to the room temperature through the whole coil then you should be getting better heat transfer (higher temperature differential between the glycol leaving temp and the RA temp, and colder supply temp, unless you are reaching equilibrium with your glycol supply and discharge air.)

    A slower flow rate may feel like you are picking up more heat because of a higher temperature rise through the coil, but because your coil temp is closer to your return air temp it’s heat transfer would be less. Heat transfer is greater with a higher temperature difference. If you pick up the same amount of heat but spread it out over a larger GPM then the temp rise will be lower even though the BTUH was the same.

    Overall if your room heat load is the same (infiltration, equipment Kw, etc) and both setups are maintaining the same room temp then you are absorbing the same sensible load with both setups, but you may be preforming more latent work with the colder coil if it is operating below dew point. That may or may not be desirable to you depending on where you live. The higher loop pressure may allow for a higher flow rate through the coil but I would expect it will have a shorter run cycle with each call for cooling or dehumidification.


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    Thanks for your reply, thatguy. That makes sense. You make a good point about the latent load, and I should have mentioned that they're running a 45F loop, while I'm running a 55F loop. So I'm not removing any latent heat at all in my room.

    That's one of the things I plan to look into, if I end up working there. They're removing humidity with their coils operating below dewpoint, but then they're adding it back in with humidifiers. I don't know if their loop temperature is a requirement to cool their equipment, or just a vestige from the "That's the way we always did it" days. I have no problems keeping a room full of servers cool with my 55F loop.

  4. #4
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    Keep in mind also that the higher differential didn’t equate to a higher flow rate if their valves aren’t opening to allow for the GPM to move. Valve off the discharge to a pump and you will still have a high side and low side but no flow.

    They might not have a need to add humidity back in if their room is not sealed correctly.

    What RH do they try to maintain in that space? Some people seem to be less concerned about it then others. With today’s equipment a steady RH is less important, but you still don’t want to approach less then 25% or more than 75% as a rough guideline.


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  5. #5
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    Get your hands on the building controls prints. The sequence of operation (SOO) on that should have the design differential pressure. Then go from there.

    When I first got my current job, one of my buildings had 3.5 psid on the chilled water loop, with the SOO showing 10 psid. This is all comfort cooling. Surprisingly, all AHUs were making supply air setpoint, and there were no complaints from the occupants. And we were experiencing 100+ degrees highs, when our location is designed for 95, lol.
    In honor of RichardL: "Ain't 'None' of us as smart as 'All' of us".

  6. #6
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    Without digging into the requirements it appears you could very well be right. I haven't balanced a server room in 5 or 6 years but the last ones I balanced had about half the cooling the first ones I balanced had. They told me tests indicated the equipment could handle a lot more heat than originally thought.
    Before lowering the loop differential pressure set point I would find out why it is set so high. There may be a reason (good or bad) it's so high. Find the reason and go from there.
    My best guess is they are living in the past.
    No man can be both ignorant and free.
    Thomas Jefferson

  7. #7
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    My money is on things being overridden. Likely suspect is the pump drive, if there is one. Or a bypass valve. Or more pumps running than required. This could be a long list of possibilities...
    In honor of RichardL: "Ain't 'None' of us as smart as 'All' of us".

  8. #8
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    Thread Starter
    I don't have a lot of the information at this stage, being that I'm on the outside. This is a larger colocated data center, with multiple raised floors in different rooms, of varying sizes. In one room they had too MUCH humidity (and they asked my advice in the interview...I saw that their CRAH fans were running at 100%. I suggested they back them down. I found out later, they did, and their humidity problem went away. Maybe that's why I got the offer...)

    As for my ability to look at their Sequence of Operation, there's another interesting thing. They've recently taken control of their DDC and are in the process of rolling out a complete rewrite of their plant manager DDC, which I'm expected to be a part of in the 2nd week of Nov. Looking over the basic descriptions between what they have and what they will roll out, I don't see anything that might be causing the issues we're discussing here. I DO see a couple things that could be done better, and we've discussed this. But my point is that their Sequence of Ops is....fluid...at the moment, so I'm not sure what value I can assign to it.

    Regardless, I agree with Wayne3298 in that I'm not going to be changing anything until I get more information. I just wanted to talk to you guys and see if my initial ideas about their high differential held any water.

  9. #9
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    The responses you have already received have pretty much nailed it . The only thing I can add is don't believe any readings you see on the DDC system . Take some baseline readings and check the sensors to see if they are even close before you do anything. Seen many guys look silly by trusting the computer . Good luck . P.S. I guess I can share a worthless story about my buddy . He was running the HVAC plant for a college with around 25 buildings . He was having trouble cooling a few of the buildings at the end of the loop . He told me he was running almost 80 psi differential . We looked at the loop pump and the motor was skin temperature and it was running at 42 hz . I said "Jimmy , you are not pumping hardly any water ." The actual differential pressure was 12 psi , we bumped it up to 20 and all was well . He had Trane replace the sensor .

  10. #10
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    Quote Originally Posted by bradluke0 View Post
    ... The only thing I can add is don't believe any readings you see on the DDC system . Take some baseline readings and check the sensors to see if they are even close before you do anything ...
    Good point. Also verify installed gauges and thermometers are correct.
    In honor of RichardL: "Ain't 'None' of us as smart as 'All' of us".

  11. #11
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    You’re gonna have to turn into Sherlock Holmes. Do they have primary secondary loops? Was the loop converted to variable pressure secondary from a constant pressure? 2 way valves? 3 way?

    Data center tech guys kill me with temp. “Requirements”.....they want it 62*, when it should be 72, or higher.

    Sounds like a fun challenge!! Good luck with it, let us know when you start digging into it!

  12. #12
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    Thanks for the encouragement. I'll be starting next Monday (28th). It's pretty scary...all my experience has been in a purpose-built, well-designed data center staffed by some extremely knowledgeable people from whom I could learn. The new situation is a very old building, a converted lighting distributor warehouse, reconfigured as a data center about ten years ago, then purchased by the company that just hired me four years ago. It's about as different an environment as one could imagine. Your statements about possible out-of-cal sensors is excellent advice...hell, I know some of the sensors at my CURRENT facility are out of whack.

    I expect I'll be coming here often to seek counsel, and I appreciate all your help.

  13. #13
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    Its best to apply for the free pro membership then once you have sufficient posts. We will be able to help you better.


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