Inquiries about boilers

Hello, may I ask you some questions about the boiler? Thanks for your help. Wish you a good day.

1. In BioSTEAM's cellulosic model, where does boiler ```blowdown``` go? Is boiler ```blowdown``` taken into account in the calculation of process water consumption? Boiler inlet ```makeup water```, that is, RO grade water needed, has been considered in the amount of RO water, but the boiler ```blowdown``` does not seem to be considered in the amount of process water?

2. What is the meaning of negative ```HXN``` in ```cooling water, chilled water, and low pressure steam``` in BioSTEAM's excel Utilities? Does it represent the utility usage saved by ```HXN```? If so, does the utility usage of other units in Utilities ```(e.g. -944330, -1850532，-295118，-17140)``` refer to the utility usage of other units prior to HXN execution? If so, after performing HXN, is the utility usage of the process the utility usage of the individual units plus the utility usage saved by HXN? That is to say, ```-944330 -1850532 -295118 -17140+1262407``` is the utility usage of the process?

3. With the code ```settings.get_heating_agent('low_pressure_steam'), settings.get_heating_agent('medium_pressure_steam'), settings.get_heating_agent('high_pressure_steam')```, the temperature and pressure of the three types of steam (```low pressure, medium pressure, and high pressure steam```) are inconsistent, but why is ```duty_over_mol = 39000``` used in the boiler the same (https://github.com/BioSTEAMDevelopmentGroup/biosteam/blob/master/biosteam/facilities/_boiler_turbogenerator.py#L306, https://github.com/BioSTEAMDevelopmentGroup/biosteam/blob/master/biosteam/facilities/_boiler_turbogenerator.py#L338)? Doesn't this mean that the duty of the three kinds of steam produced by the boiler are the same, but should actually be different? And the boiler code doesn't seem to distinguish whether the steam produced by the boiler is low-pressure steam, medium-pressure steam, or high-pressure steam (https://github.com/BioSTEAMDevelopmentGroup/biosteam/blob/master/biosteam/facilities/_boiler_turbogenerator.py)? Could I consult you how to check the duty of the three types of steam used in the process, that is, the amount of heat that 1mol of steam has or can transfer?

4. Why is the price of ```natural gas and ash stream``` in boiler in BioSTEAM 0?
```
from biorefineries import cellulosic
import biosteam as bst
bst.nbtutorial()
cs = cellulosic.Biorefinery('corn stover ethanol')
boiler = cs.cornstover_sys.units[-4]
natural_gas = 3-boiler
ash = boiler-2
print(natural_gas.price) # return 0
print(ash.price) # return 0
print(boiler.natural_gas_price) # not return 0
print(boiler.ash_disposal_price) # not return 0
```

5. In BioSTEAM, does the boiler burn purchased natural gas to produce steam for heating, rather than purchasing steam for heating? In this case, the ```heat transfer price and regeneration price``` of all kinds of steam (```low pressure, medium pressure, and high pressure steam```) should be set to 0, right? And the cost of heating for the process is the amount of natural gas I buy times the price of natural gas (i.e. ```boiler.natural_gas_price```), right?

6. In the model of ```lactic acid```, it appears that the price of ```natural_gas```, that is to say, boiler inlet stream, is used instead of ```boiler.natural_gas_price``` (https://github.com/BioSTEAMDevelopmentGroup/Bioindustrial-Park/blob/master/biorefineries/lactic/models.py#L285-L289)?