External Static Pressure is the calculation of all the resistance in the supply & return duct system (Duct fittings, Dampers, Air outlets, Louvers, Sound Attenuator, etc.) that the fan has to overcome.
External Static Pressure and Air Flow are inversely proportional
The higher the ESP, the lower the Air Flow. The lower the ESP, the higher the Airflow. High ESP readings indicate that there is excessive resistance in the system. This may be caused by dirty filters, a dirty evaporator coil, closed dampers, restricted supply or return grills or undersized duct, sound attenuator and louvers. Similarly Low ESP readings indicate that there is less resistance in the system than calculated.
Let us analyze 3 cases for same fan curve.
Case : 1
ESP Measured at site = ESP Calculated.
Air flow will be same as per selection sheet.
Case : 2
ESP Measured at site < ESP Calculated.
Air flow will be greater than as mentioned in selection sheet as there is less resistance.
Case : 3
ESP Measured at site > ESP Calculated.
Air flow will be less than as mentioned in selection sheet as there is high resistance
HVAC Engineers shall do ESP Calculation as per ASHRAE Standard and consider 10% to 15% safety. So measured ESP will be less than ESP Calculated as mentioned in selection sheet (Case : 2). As a result, Airflow measured at site will be greater than as mentioned in selection sheet. Therefore it is recommended to install VFD driven control panel, so we can fine tune the system while testing and commissioning.
If ESP Calculations are not done as per ASHRAE Standard and by any means measured ESP at site is Greater than as ESP Calculated (Case : 3). As a result, Airflow measured at site will be lesser than as mentioned in selection sheet. In such case you need to contact supplier and check if required flow and ESP can be achieved by changing pulley or by replacing new motor with higher KW capacity.
What happens when static pressure is high?
When the static pressure is high, it means that airflow is restricted.
Hot pockets may be created due to reduced flow.
With reduced / restricted air flow the same AHU needs to run longer time to achieve the set point temperature.
Chances are there you may never achieve set point temperature, if air flow is reduced too much.
What happens when static pressure is Less?
When the static pressure is Less, it means restriction of airflow (via air outlets, dampers, duct fittings, etc.) is less than calculated.
Due to which fan shall deliver more air flow than duty point.
However with increased air flow, discharge at air outlets will also increase then designed airflow, which shall lead to increase in noise level and velocity.
Summarize
Therefore, External Static Pressure of AHU, Fan shall not be estimated, it is shall be calculated as per ASHRAE Standards, including safety of 10% to 15%. So we can avoid Case 3 (where measured ESP is more than duty point and air flow is reduced).
Better option is to be in category of Case 2 (where measured ESP is less than duty point and Air flow is increased), as it is practically not possible to estimate perfectly as mentioned in Case 1 ( where calculated ESP equals to measured ESP).
However with case 2, we need to install VFD to fine tune the system while Testing & commissioning.
At BTU Electromechanical Works LLC, Dubai, UAE, we provide technical support to MEP Contractor, HVAC Contractor, Fit Out Contractors, AHU Supplier & Fan Suppliers by providing ESP Calculations as per ASHRAE Standard. Also our team had successfully completed calculations of iconic projects such as The Dubai Frame, Expo 2020, Fly Dubai Head Quarters and many more. We can be reached at projects@btullc.com, www.btullc.com