Having trouble following Trane example unit selection

[RandomUserName]

The following section is directly from the Trane SCWG Brochure (Unitary Vertical Self-Contained System). This is their example selection procedure:

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Design Conditions
Total gross capacity required = 368.7 MBh = 31 Tons
Total sensible capacity required = 259 MBh
Entering air temperature = 80/67°F
Entering water temperature = 85
gpm = 105

Selection Procedure
Airflow = 14200 cfm at 2.5-inch duct static pressure
Unit to include:
Constant Volume
Waterside economizer
Medium velocity throwaway filters

Unit Selection
Tentatively select a 35 ton unit. Refer to Table 13, p. 36 to obtain gross total and sensible unit
capacities, and gpm at the design conditions:

Total capacity = 370 MBh
Sensible capacity = 282 MBh
LWT = 95.4°F

Since the design cfm is greater than the nominal cfm, adjust the capacities and condenser water
delta T to reflect the higher cfm:

design cfm/nominal cfm 14,240/14,000 + 3% from nom. Cfm

Refer to Table 6, p. 29 to obtain the capacity correction factors for +3% of nominal cfm:

Cooling capacity multiplier = 1.005
Sensible capacity multiplier = 1.014

Multiply the capacities by the correction factors:

370 MBh x 1.005 = 371.85 MBh
282 MBh x 1.014 = 285.95 MBh

The SCWG 35 meets the total and sensible design requirements.

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I am not understanding the logic here I was hoping those with more experience than me could teach. See items in bold above. Their unit selection is for Qt = 368.7 MBh and Qs = 259 MBh. I would assume this means they have a latent load of Ql = 368.7 - 259 = 109.7 Mbh they need the coil to remove.

Their final unit selection has numbers of Qt = 371.85 MBh and Qs = 285.95 MBh. This would mean the coil can only condense Ql = 85.9 MBh out of the airstream.

Is this not a bad selection? I would think they would have humidity issues if the latent load was not met. I am under the impression that in this case, you would either reduce the fan speed if possible - making sure you are still meeting your sensible load and OA requirement - or "bump up" to the next size unit - in this case from 35 ton to 40 ton - to get your latent load.

Could you guys help a young engineer understand the logic here?

Thanks!

 

[chicopee]

I suspect that the reduction of the latent heat load from 109.7 Mbh to 85.9 Mbh may be due to a higher dew point on the fins when the CFM was increase by 3%. A higher dew point on the fins will mean a lower quantity of water vapor being cooled efficiently.

 

[chicopee]

So how much latent heat load do you have in your project?
Options: reduce fan speed to nominal rpm or explore the next size unit.

 

[RandomUserName]

chicopee,

This is not my project. The text above is directly from the Trane SCWG Brochure. It is an example. I like to read their selection procedure to make sure I understand their logic.

So then you would agree with my original post text of:

"Is this not a bad selection? I would think they would have humidity issues if the latent load was not met. I am under the impression that in this case, you would either reduce the fan speed if possible - making sure you are still meeting your sensible load and OA requirement - or "bump up" to the next size unit - in this case from 35 ton to 40 ton - to get your latent load."

?

 

[MintJulep]

I agree, the example evaluation does not consider Sensible Heat Ratio, or latent capacity.

 

[RandomUserName]

Thanks MintJulep for the affirmation of my reasoning.

 

[ssdd09]

Remember that your design requirements are :
Total gross capacity required = 368.7 MBh = 31 Tons
Total sensible capacity required = 259 MBh

The selected unit was :
370 MBh x 1.005 = 371.85 MBh
282 MBh x 1.014 = 285.95 MBh

which satisfies the requirements. The coil performance would "play along" the actual requirements (at given airflow), so your SHR will still be satisfied.

 

[317069]

Did you contact TRANE for this issue, can you post the SCWG you have.?

 

[317069]

This is from TRANE ( I see different numbers)
Selection Example
Design Conditions
Total gross capacity required = 420 MBh = 35.2 Tons
Total sensible capacity required = 315 MBh
Entering air temperature = 80/67°F
Entering water temperature = 85
gpm = 105
Airflow = 14200 cfm at 2.5-inch duct static pressure
Unit to include: Inlet guide vanes,Waterside economizer.
Medium velocity throwaway filters
Unit Selection
Tentatively select a 35 ton unit,model SCWG 35. Refer to Table PD-13 (page 33) to obtain gross total and
sensible unit capacities, and gpm at the design conditions:
Total capacity = 439.4 MBh
Sensible capacity = 329.7 MBh
LWT = 95°F
Since the design cfm is greater than the nominal cfm, adjust the capacities and
condenser water delta T to reflect the higher cfm: design cfm 14240 +3% from nom. cfm nominal 4000 cfm
Refer to Table PD-1 (page 25) to obtain the capacity correction factors for +3% of nominal cfm:
Cooling capacity multiplier = 1.005
Sensible capacity multiplier = 1.016
Multiply the capacities by the correction
factors:
439.4 MBh x 1.005 = 441.6 MBh
329.7 MBh x 1.016 = 335.0 MBh
The SCWG 35 meets the total and sensible design requirements.