FHC for water mist hydralic calcualtions

The Canute FHC hydraulic calculation software can be used to calculate water mist systems for low, intermediate and high-pressure systems as defined by NFPA 750.  Careful consideration needs to be given on which option you use in FHC for your calculation. This will depend on the system pressure, water velocity and if the system has any additives.

NFPA 750 allows the Hazen Williams formula to be used for water mist systems, including high-pressure systems (pressure exceeds 34.5 bar) if certain conditions are met, for intermediate and high-pressure systems, the water velocity must be below 7.6 m/s, and the smallest pipe diameter must not be less than 20 mm.  The infographic below summarises the differences between the three pressure classifications and when the Hazen Williams pressure loss formula can be used.

Water mist hydraulic calculations and NFPA 750

The Hazen Williams pressure loss formula is the standard method Canute FHC when calculating pressure losses in the pipework. Therefore, you do not need to make any changes, but you must be careful that you selected the appropriate minimum pipe size and your water velocity does not exceed 7.6 m/s.

Our experience has shown that you will benefit from lower system pressure by using the Darcy Weisbach pressure loss formula for all water mist calculations.  If you are designing a high-pressure system, then for practical purposes, you should select the Darcy Weisbach pressure loss formula by limiting yourself to pipe velocities of less than 6.7 m/s, and a minimum pipe size of 20 mm is probably impractical.

When making water mist calculations and your system meets the criteria which allow the Hazen Williams pressure loss formula to be used, you can specify a maximum water velocity in the calculation options as shown to the left.  This dialogue is displayed after you select calculate, you can now enter 7.6 m/s as your maximum velocity and if this is exceeded FHC will display a dialogue box at the end of the calculation and will display the pipes exceeding this velocity.

You can also find the pipes with the highest velocities by going to the menu -> reports -> pipe summary and you will find that any pipes exceeding the set velocity will be displayed in red.

Using Darcy Weisbach Pressure Loss Formula

If your system does not meet the requirements that allow you to use the Hazen William pressure loss formula, you will need to switch to the Darcy Weisbach pressure loss formula. To do this open the project data [Ctrl+P] and you will see a button to the right of fluid labelled ‘DWF’; press this button, and a dialogue box will appear asking you if you wish to switch the pressure loss formula. You can now select the Darcy Weisbach pressure loss formula and the water temperature (The temperature will change the default valves for density and viscosity).
The Darcy Weisbach pressure loss formula is considered the most accurate, and you may find that you get beneficial results when using it.  Although NFPA 750 allows you to use the Hayes Williams formula for certain systems under certain conditions, it does not preclude you from using Darcy Weisbach for all water mist hydraulic calculations.
Technical Data Sheet
You can download in full TDS 1 - FHC for water mist calcualtion as a PDF useing the download button.
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