#HVAC: Five easy steps to estimate External Static Pressure drop in HVAC Ducts using Equal Friction Method.
Introduction
Estimating External Pressure drop in ducts is not a rocket science and it can be done easily in order to select the HVAC equipment's fan (Air Handling Unit, Fan, Heat Recovery Unit, Fan Coil Unit...etc). and accordingly, we can estimate as well the power consumption of such units.
What is External Static Pressure in HVAC Ducts?
External Static Pressure is basically and simply the pressure required by a fan (which is located inside any HVAC equipment) to deliver the required / design airflow rate and to allow air to reach from the HVAC equipment to the Index point so air can be discharged with the required airflow rate even from the furthest air outlet (Diffuser or air grille...etc).
To do that, the fan pressure has to overcome all loses in the direction of air flow (on the Suction and Discharge side of the equipment) including, but not limited to, Volume control dampers, Fire Dampers, pressure drop in air outlets, duct pressure drop, duct fittings losses, External air louvers (in case if the Outside Air Intake duct has the index run compared to the return air duct)... etc.
What are the steps and how to estimate / calculate the External Static Pressure of a fan?
The below calculation sheet is very simple and all engineers can make use of it in order to estimate / calculate the External static pressure (ESP) of any fan in any HVAC equipment.
It is worth mentioning here also that ESP has to be calculated for different scenarios to ensure that the calculated ESP is indicating the maximum ESP drop in the system which will be for the Index run noting that the index run is not mandatory to be the furthest duct run. The index run is the duct run with the maximum external static pressure drop.
Steps:
1. Write down the "DESIGN AIR FLOW RATE" required for the fan. In case of heating or air conditioning equipment, the design air flow rate is what you get from the cooling load program.
2. Go to the friction loss chart in any HVAC standard (CARRIER,..etc) or use a ductulator to get the "FRICTION FACTOR" based on the required air velocity and duct size according to the area of application (low pressure system, Medium pressure system,...etc).
3. Measure the duct length from the design drawings and check different scenarios as explained above since the index run is the duct run with the maximum external static pressure drop not the furthest duct run.
4. Use the below sheet (very simple excel sheet) to add the duct run, friction factor, pressure drop in all accessories and repeat the same for the return air duct (or Fresh air duct whichever is greater).
5. Add whatever required safety factor to secure the unit selection.
So, External Static Pressure drop value can be defined accordingly.
What about detailed calculation for External Static Pressure drop?
Detailed calculation is not far different from the above calculation except for the exact equivalent length of each and every duct fitting that will be used instead of the safety factors.
A lot of programs are available nowadays in the market to do this exercise. However, this easy way as summarized in the table is friendly to use, easy, error margin is always not exceeding 5% of the detailed calculated value.
Detailed calculation for the equivalent length of duct fittings can be found in HVAC standards such as SMACNA standard, CARRIER binder Part (2) Chapter (2),...etc.
