Here is metioned that the total static pressure has to be used for the Fan Pressure Rise parameter in EnergyPlus.
Can someone explain why the total pressure (= total static pressure + dynamic pressure) should not be used? Thank you in advance!
A very old question, but I couldn't pass it up. I think @Anna Osborne Brannon 's explanation is not correct. It seems she is confusing "Dynamic pressure" (Velocity pressure) with "Dynamic loss". Dynamic loss which results from flow disturbances in the fitting correlates Dynamic pressure which is the kinetic energy per unit volume of a fluid, but they are differenct concepts.
Short answer: strictly speaking, total pressure (total static pressure + dynamic pressure) shoud be used, but energy modellers use total static pressure case by case. More details below.
In general, the following formula holds:
Fan power consumption [W] = Air flow rate [m3/s] * Fan total pressure [Pa] / Fan total efficiency
EnergyPlus uses the wording Pressure Rise instead of Fan total pressure, but they refer to the same thing.
Fan airflow and rated fan power are always in the fan equipment schedule, but other information available depends on projects.
For constant speed fans, the fan total pressure can be calculated backward by using the rated fan power in the fan equipment schedule as the excess pressure is adjusted with a volume damper i.e. Fan total pressure (=Pressure Rise)[Pa] = Fan Power [W] / Air flow rate [m3/s] * Fan total efficiecny
For variable speed fans,
Pressure Rise in the idf file. It is becasue the external static pressure calculated by MEP engineers is overestimated i.e. a safety factor of around 10% is taken into account or/and it is rounded up in 50Pa or 100Pa increments (e.g. Calculated external pressure + internal pressure = 930Pa → Entered total pressure in the equipment schedule = 1000Pa). On the other hand, dynamic pressure (= 1/2 * ρ * v^2) is normally less than 50Pa. Dynamic pressure is consequently included in the margin of the external static pressure. I think that's why many energy modellers ignore fan dynamic pressure and input only the total static pressure (= internal static pressure and external static pressure) into Pressure Rise field.Pressure Rise in the idf file, but this case is very rare. MEP engineers do not normally calculate fan dynamic pressure because their concern is whether fans can supply air to the farthest room, and because static pressure is the force that pushes air out by overcoming the resistance of ducts.Short answer, total static pressure in the HVAC community includes dynamic losses from fittings and is equal to total pressure. Losses from fittings (elbows, branches, tees) is sometimes converted to an equivalent length of "straight duct" and then applied a friction loss. This might be why there is the misnomer of total "static" pressure. Or the loss coefficient ("C") for a fitting is looked up in a table and multiplied by velocity pressure ("Vp") to determine the loss.
The total pressure includes external static pressure and internal static pressure. External static pressure includes losses from volume control dampers, fire dampers, air outlets, duct length pressure drop (i.e. friction losses), duct fittings (i.e. dynamic/velocity losses), louvers, etc.
Internal static pressure includes things like filters, cooling coils, heating cools, and heat exchangers.
So, you should use the "total" pressure in your numbers below, as it includes the dynamic/velocity pressure loss from fittings.
Thank you for your answer.
I would like to simulate a HVAC system with the following fan specifications: system effect: 76 Pa, external static pressure: 350 Pa, total static pressure: 831 Pa, dynamic pressure: 74 Pa, total pressure: 905 Pa.
So in this case I should also not use the total pressure?
I revised my above answer to be a bit more clear. You should use the "total" pressure in your numbers above, as it includes the dynamic/velocity pressure loss from fittings.
Asked: 2018-02-14 09:42:59 -0500
Seen: 2,214 times
Last updated: Mar 27 '23
