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Cold water coil ideal Delta T?

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Timco
Timco Member Posts: 3,040
edited September 2017 in THE MAIN WALL
Short story. 3) 5-ton air handlers and each has a double pass 1" coil for chilled water. 11th floor. 20psi. 41-43* water. Originally one AH now piped for three. I made the 1.25" feeds into a manifold that feeds all three with an isolation valve between the S&R take offs to establish supply and return. All 1" Wirsbo.


Delta T is between 10-15*. I'm thinking that's too great? It's underperforming and I want to get flow and DT right first. It has very unique challenges with heat gain to say the least.

The Schneider 3-way valves I used look pretty lame (restrictive). I'm thinking I can pipe one direct before adding a small pump.
Just a guy running some pipes.

Comments

  • hot_rod
    hot_rod Member Posts: 22,261
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    That's pretty cold water, maybe someone has cranked it way down to overcome a design or installation flaw. 45- 55° is a common design temperature, cool the space with the warmest possible temperature:)

    The coil or AH manufacturer should have graphs to show output at various flow and temperature conditions.

    Do a load calc to be sure the equipment is going to get the load.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • EBEBRATT-Ed
    EBEBRATT-Ed Member Posts: 15,626
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    Everything @hot rod said is correct. A 10-12 degree TD on the water is normal for chilled water.

    First.....do a load calculation to figure the btu needed to cool the space. Don't forget the dehumidification load affects the water and air TD.

    Second.........determine the air quantity needed to cool the space based on the above calculated load. Normal air temp would be about 74 deg return air and 55deg discharge off the coils.

    refer to the ahu & coil specs to see if they will deliver what you need.

    Then check pipe size and gpm to see if you have the available chilled water.

    You have to start from scratch with the calculations to get this right.

    Reverse engineering is difficult.

    Don't forget outside air for ventilation if needed. That can be a large load
  • hot_rod
    hot_rod Member Posts: 22,261
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    And working in your favor is the dry, desert climate of Utah :)

    Tighter ∆ will require more pumping power and cycle the chiller more often. Low supply temperature as you indicated also works the chiller harder and drops efficiency.

    The design should be to cover the load with the warmest supply 45- 55 is often used, and the lowest pumping flow and power consumption, 10- 15 is a common ∆. The coil spec will help determine that, since it is something you are probably not going to change out. Are the zone valve able to modulate?

    An accurate load calc is crucial if you are trying to upgrade or improve the cooling of the space, without that it is all a guesstimate. Could be you have a 6 ton load on those 5 ton chillers? Or inadequate coil or air handling to move the 5 ton. Sounds like a frustrating job, but the answers are always in the numbers.

    Cover all the windows, that will lower the loads :)
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Timco
    Timco Member Posts: 3,040
    edited September 2017
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    Thanks guys.

    I'm way short on return. -.45-.50"WC on returns. If I open the old shutters to outside air the numbers get better but I'm pulling 90* roof air for makeup. This is 5000sq' of 10th floor. Grey tile siding, all 1960's masonry, flat roof, mostly glass walls. They are getting tinting this fall but at 5pm it's 80* in one area and 78* in another.

    My air temps are 75 in and 64 out. Coil DT is 15* when I'm there and it's cycling. I'll need to swing by at 5pm, when it's at peak demand. As soon as the sun hits the building I'm toast and I'll bet it's far greater.

    The owner of this company is 82 and the guesstimater king, lol. Vell, 5 tons is more than ve can push through those vents anyvay so three 5-ton units vill do, hey...(German)
    Just a guy running some pipes.
  • hot_rod
    hot_rod Member Posts: 22,261
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    Used to be a 20° delta between outside and inside was considered adequate for AC. 90 outside, 70 inside.

    Recently I had an elderly lady contact me about buying my tiny home. She required a sleeping temperature of 65 for some unusual health condition!? 65 inside with over 100 outside in SW Missouri, yikes!

    Although I swear most restaurants around here feel like 65 in the summer.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
    Timco
  • Timco
    Timco Member Posts: 3,040
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    I own a home in Waynesville by St Robert. Nice area.
    Just a guy running some pipes.
  • EBEBRATT-Ed
    EBEBRATT-Ed Member Posts: 15,626
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    The colder the coil the better the humidity removal. May not be a concern in your area.

    Standard coils for comfort cooling are usually in the area of 75-80% sensible heat 20-25% latent heat. Determine by coil thickness
  • hot_rod
    hot_rod Member Posts: 22,261
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    Timco said:

    Thanks guys.

    I'm way short on return. -.45-.50"WC on returns. If I open the old shutters to outside air the numbers get better but I'm pulling 90* roof air for makeup. This is 5000sq' of 10th floor. Grey tile siding, all 1960's masonry, flat roof, mostly glass walls. They are getting tinting this fall but at 5pm it's 80* in one area and 78* in another.

    My air temps are 75 in and 64 out. Coil DT is 15* when I'm there and it's cycling. I'll need to swing by at 5pm, when it's at peak demand. As soon as the sun hits the building I'm toast and I'll bet it's far greater.

    The owner of this company is 82 and the guesstimater king, lol. Vell, 5 tons is more than ve can push through those vents anyvay so three 5-ton units vill do, hey...(German)

    Cut a hole in the roof and sell him a massive swamp cooler! I lived in Utah for 20 years without AC, just swamp coolers. Phoenix also was a swamp cooler market, back in the day.

    The added bonus was humidification to keep your skin moist and fingers from cracking :)

    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Timco
    Timco Member Posts: 3,040
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    Added a 26-99 to the supply to the attic. DT dropped from 17* to 9* during peak time. Based on the charts, I was at about 1.2 GPM. Chart says I'm at 11.7 with these numbers and that's the low side. I can speed up water to get to the upper end of the chart but now we are out of the 90's for the season finally.
    Just a guy running some pipes.