Updated installed equipment cost formulation

[yoelcortes]

Previously, the installed equipment cost was calculated as follows:

C_BM = C_P * F_BM

Which only applies when the bare-module factor is for the given material, pressure, and design. Now we use the more complete formula:

C_BM = C_Pb (F_BM + F_D F_P * F_M - 1)

Where:

• C_BM is the bare module cost
• C_Pb is the baseline purchase cost (without accounting for design, pressure, and material),
• F_BM is the bare-module factor
• F_D is the design factor (i.e. type factor)
• F_P is the pressure factor
• F_M is the material factor

All previous code should still work, but if you're working with different design, pressure or material factors, you can use the new F_D, F_Pand F_M dictionaries to specify them. You'll also need to add values to the baseline_purchase_costs dictionary (not the purchase_costs dictionary), then BioSTEAM will take care of calculating both the actual purchase costs (after accounting for construction material and design) and the installed equipment costs.

Here is an example:

import biosteam as bst
class Tank(bst.Unit):
    _BM = {'Tank': 2.3}

    def _cost(self):
        self.F_M['Tank'] = 1.25
        self.baseline_purchase_costs['Tank'] = 1e6

bst.settings.set_thermo(['Water'])
T1 = Tank(ins=bst.Stream(Water=10))
T1.simulate()
print(T1.baseline_purchase_costs)
print(T1.purchase_costs) # Correctly accounts for material factor
print(T1.installed_costs) # Correctly accounts for bare-module and material factor

Output:

{'Tank': 1000000.0}
{'Tank': 1250000.0}
{'Tank': 2550000.0}

Note that adding values to the baseline_purchase_costs dictionary is preferable over the purchase_costs dictionary, regardless if you are using different material factors or not. You'll get a helpful warning if you add items to the purchase_costs dictionary instead of the baseline_purchase_costs. For more details, you can checkout the code:

https://github.com/BioSTEAMDevelopmentGroup/biosteam/blob/af1faa9548521cfa3732317bb5dedbc32d0513fc/biosteam/_unit.py#L310

@yalinli2, I also made BioSTEAM spit out a warning if the BM is not included for a purchase cost item (instead of raising an error).

Hope this helps!

[yalinli2]

Hi Yoel, thanks for the notice, some questions:

• I'm confused by what C_BM is. You said:

  Previously, the installed equipment cost was calculated as follows:

     C_BM = C_P * F_BM

  But you also said:

C_BM = C_Pb (F_BM + F_D F_P * F_M - 1)

Where:

• C_BM is the bare module cost

So is C_BM installed cost or the bare module cost? Are they the same?

• Also, in the _load_capital_costs function, can you add the reference to this equation?
• Additionally, is this equation only applies when F_D, F_P, and F_M are all >1? What if I want to use some cost from stainless steel at higher pressure to calculate cost for carbon steel at lower pressure? Will this result in negative value?

Thanks!

[yoelcortes]

Hi Yalin,

Here are the answers to your questions:

• Check out equation 16.12 of Warren's Product and Process Design Principles:

• The bare-module cost and the installed equipment cost are the same.

• F_D, F_P and F_M can all be any number (less or more than 1). What is important is for what material/design/and pressure was the BM applicable. Usually BMs are given for carbon steel at low to moderate pressures (which correspond to F_M, F_D, and and F_P values of 1). Negative values are impossible as BM is always greater than 1.

Let me know if you have more questions!

[yalinli2]

This all looks good, thanks for explaining and adding the references!

[yoelcortes]

Quick update; The factors are now public (so they are called F_D, F_P, and F_M now).

[yalinli2]

Cool! Is F_BM public?

[yoelcortes]

@yalinli2, not yet... but yeah, I've been thinking about making the _BM dictionary the default values for a public F_BM dictionary that gets created with the unit instance. I think this would be better than having bare-module factors as class attributes... which leaves little room for flexibility.

What do you think? Thumbs up and I'll go ahead and fix this up over the weekend.

[yalinli2]

I'm on board! 👍 Thanks Yoel!!!

[yoelcortes]

Done!

Additionally, to make it easier to understand for future new users, I renamed _BM to _F_BM_default. I also added equipment_lifetime as an instance attribute and renamed the _equipment_lifetime class attribute to _default_equipment_lifetime (again, to make it easier to understand without necessarily reading documentation).

I added some checks for Unit subclassing to make sure '_BM' and '_equipment_lifetime' are not used anymore. All tests passed on my computer.

Hopefully it's not too much trouble to make the renames in QSDsan (just import, let it error, and rename).

Thanks!

[yalinli2]

Sounds good, thanks Yoel!!!