(Audio Only) In this HVAC School podcast episode, Jim Bergmann covers static pressure, measuring delivered capacity, and ECM motor considerations. This episode is the first part of a two-part series.
In the HVAC industry, many techs confuse static pressure for airflow. There IS a relationship, but while you need static pressure to have airflow, it is NOT airflow and can fluctuate rather wildly depending on the duct conditions.
Static pressure is an indirect airflow measurement. We measure static pressure with a manometer; we measure total external static pressure (TESP) by placing the positive probe on the supply side and the negative probe on the return side.
Airflow is actually a measurement of velocity (such as with pitot tubes) that you then convert to a volume measurement (CFM). When it comes to airflow ratings and measurements, we also need to distinguish actual measurements from estimation methods; in the past, we usually just needed to use the static pressure reading or temperature rise to get a close enough estimation to pick the right tap on the blower. However, when you want to do capacity testing, you need to be able to take accurate measurements beyond static pressure, which is a challenge because we need extremely accurate sensors.
When we talk about a system that's moving air, we also have to think about the appliance, whether that's a furnace, heat pump, or straight-cool A/C system. The appliance produces the airflow, and the appliance manufacturer publishes airflow parameters, but there are appliance conditions that can negatively impact airflow over time. Appliance manufacturers and installers also tend to use estimation methods and assume that the appliance is delivering the proper airflow.
When we take static pressure readings, we may notice that readings change depending on the placement of the probes. That's because velocity pressure has an effect on static pressure, so we need to be mindful of the velocity pressure generated by the blower and general turbulence in the duct or air handler.
We must keep in mind that each measurement method has its uncertainty, whether we're measuring static pressure or velocity. The main challenge is to understand each method and what affects those measurements. We may have to normalize readings based on our understandings of those uncertainties. Making measurements is easy, but making them correctly is hard because we have to study so much about methodology and consider so much when we actually gather readings and interpret them.
As ECM technology has made its way into the blower motor marketplace, it has become easier to get accurate CFM outputs based on static pressure. Jim Bergmann thinks it is one of the best inventions in modern air-moving technology over the past 50 years. ECM assemblies work off RPM and torque, and they're programmed to speed up to move a constant mass flow of air across the coil; that is how they are different from PSC motors and might be advantageous in blower motor applications. However, these are not universal drop-in motors; the ECM must work with the entire assembly and design.
Overall, we need to understand that we're not cooling the CFMs of air; we're cooling the mass of air across the coil. It would be wise to remember that when we think about measurements and motor selection.
Jim and Bryan also discuss:
Static pressure ratings
Turbulence and variable pressures
Filters and their impacts on static pressure readings
The evolution of test instrumentation
Flow restrictions and their effects on amp draw
Mass flow rate vs. airflow
X13 ECMs and fail rates
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9 июл 2024
