Airflow Basics – Part 1

Posted on:

May 15, 2019

Let’s look at the three basic components of airflow: volume, velocity and pressure.

Volume

Standard air is air that is 70ºF with 50% RH, at sea level. We measure air volume as a rate in cubic feet per minute (cfm).

1 ft3 of standard air weighs .0741 pounds

13.33 ft3 = 1 pound of air

13.33 cubic feet of air is about the amount of air in a typical residential refrigerator.

.244 is the Specific Heat of air. Specific Heat is the heat necessary to raise the temperature of 1 pound of a substance 1°F.

That makes air relatively efficient when compared to water. You probably already know that the specific heat of water is 1. If you don’t, then think of the definition of a British thermal unit: the amount of heat required to raise the temperature of one pound of water one degree Fahrenheit.

It only takes about a quarter of a Btu to raise the temperature of one pound of air one degree, but it takes a whole Btu to raise the temperature one pound of water (about a pint of water) one degree. That makes air about four times more efficient than water.

So, if we multiply the specific heat of air by the weight of a cubic foot of air by 60 minutes in an hour (remember, we’re going from Btuh to cfm), then, we have our air flow constant of 1.085 which is usually rounded up to 1.10 for convenience:

.244 x .0741 x 60 = 1.085 (often rounded to 1.10)

We’re going to use this airflow constant in the Sensible Heat Equation below:

cfm = Sensible Btuh

1.1 x ΔT x ACF

This is one way of converting cfm to Btuh. Remember, if customers call you and say that they don’t have enough heating or they don’t have enough cooling, then they are actually saying that they don’t have enough airflow (cfm). That’s how the heating and cooling Btu’s get to the rooms or areas to be heated or cooled, via the cfm.

Now let’s define all the components of the Sensible Heat Equation. You know that 1.1 is an airflow constant, and you know how it was derived. That brings us to ΔT. That triangle represents the Greek letter “d”, or delta. Anytime you see Δ in our industry it means “difference”. ΔP is a pressure differential across a pump or blower and ΔT means temperature difference. In the case of airflow, the temperature difference referred to is the difference between the temperature of the air leaving the furnace or evaporator coil (LAT) and the thermostat set point.

Lastly, ACF is the Altitude Correction Factor. It is taken from Table 10A in Manual J, 8th Edition. It is used to correct the 1.1 airflow constant (which was based at sea level) for altitudes greater than 1000 feet above sea level. The higher you go, the more cfm you will need for the same Btuh.

Velocity

We must deliver the volume (cfm) at a proper velocity. We measure this velocity in feet per minute (fpm). Typically, we would want to keep our trunk lines between 700 and 900 fpm and our branch run-out velocities, ideally, between 400 and 600 fpm, although there is some variability here. Proper register sizing can go a long way to improve low branch velocities.

Velocities less than recommended can produce poor (low) air flow and velocities greater than this will always produce noise.

Pressure

We measure pressure within a duct in inches of water column, a.k.a., IWC, IWG, “w.c. or even in. H2O. No wonder there is so much confusion regarding airflow. We have at least four different designations for pressure in a duct system!

There are two primary pressures in a duct system. The first is velocity pressure. The faster the air moves, the greater the velocity pressure. The other pressure within an operating duct system is static pressure. Static pressure is bursting pressure. An inflated balloon contains 100% static pressure. When you release that balloon, the static pressure converts to velocity pressure. Total pressure is velocity pressure plus static pressure.

1 psig = 27.72 IWC

Most of our residential duct systems are designed to operate in a range from 0.5 to 1.0 IWC.

The Duct Designer’s Dilemma

We know that a cubic foot of standard air weighs .0741 pounds. Therefore, a 3-ton system handles 90 pounds of air, a 4–ton system handles 120 pounds of air and a 5–ton system handles 150 pounds of air.

Think about it! That much air every 60 seconds (cfm) with a pressure that wouldn’t properly blow your nose (between .50 to 1.0 IWC). How is that even possible?

We do it all day long, one cubic foot at a time. That’s because we’re the HVAC industry and we know how to! More to come…

Author
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Jack Rise

Owner at Jack Rise HVAC Technical Trains

Jack Rise, CMS, is a Certificate Member Specialist of the Refrigeration Service Engineers Society.He has extensive industry experience having worked as a contractor for 18 years, as a distributor of HVAC products for 13 years and for a major HVAC manufacturer for 5 years.In 2004 Jack formed his own company, Jack Rise HVAC Technical Training, and works out of his office in Tampa, Florida.

Jack is an ACCA/EPIC certified instructor for all of the ACCA residential and commercial design manuals including the 8th Edition of Manual J (Residential Load Calculations), the 3rd Edition of Manual D (Residential Duct System Design), the 5th Edition of Manual N (Commercial Load Calculations) and Manual Q (Commercial Low Pressure, Low Velocity Duct System Design).Jack travels extensively around the country conducting seminars on residential and commercial load calculations, system design and on the use of HVAC computer based software.He was an RSES instructor for 10 years, currently runs training sessions for NATE and EPA/CFC Section 608 Certifications and ACCA/ANSI Standard 5 (HVAC Quality Installation Standard).

Jack has authored two books as companion manuals for both Manual J and Manual D.He is the author and presenter of the ACCA HVAC Essentials series and the NATE Essentials series of training CDs.He is a 35+ year member of RSES, a Life Member of the North Jersey Chapter of ACCA, an associate member of ACCA-Florida and FRACCA and a Life Member of the NATE Technical Committee.