Service Hotline: More on Condensation, Cfm Calculations

Both Armaflex and Rubatex would recommend nearly 1 in. and a thorough vapor barrier job. If Loy will build up to 7/8 in. or 1 in. by spiral-wrapping Armaflex or Rubatex insulating tape with ample overlap, he will get the necessary “R” as well as improving the vapor barrier.

The fiberglass and canvas jacket actually increases the condensation by lowering the dewpoint of the Armaflex surface, although sometimes when rh is not too high, this padding will be a good blotter!

When rh gets up above 95%, he may still get some drip.

Cfm Calculations

Question:

From Gregory Daw Poughkeepsie, NY

The basic guideline for determining the proper cfm of a blower unit in a forced hot air furnace used for air conditioning is to use 400 cfm for each ton of cooling (12,000 Btuh). Most duct sizing calculations are based on the blower being located on the same level, or above the air conditioned space.

My question is, how is this calculation affected by the height, or vertical rise, of the duct system; by the blower being in the basement and cooling is required on the first floor, on the second floor, etc.?

What is the maximum height, or vertical rise, that cold air can be delivered from an evaporator coil and blower in the basement?

Must a single-speed blower in a hot air furnace always be upgraded to a two-speed blower when placing an evaporator coil in the plenum?

How much should the blower motor horsepower or cfm be increased to deliver the cold air from the basement to the first floor, second floor, etc.?

Answer:

From Gene Silberstein Consultant and Freelance Writer

The vertical run in a duct system is taken into account when the system is initially designed. When duct systems are designed, many individuals use a “design static” of 0.1 for the entire system, which is about average and will suffice in most situations.

The problem with this is twofold. When duct runs are too short, there is not enough resistance to airflow and the ducts and registers will whistle on these runs. On duct runs that are too long, there will be insufficient airflow to these registers. So it is important to take the “actual duct length” and the “equivalent duct length” into account when designing a duct system.

Two important charts are needed to successfully determine proper duct sizes. These charts are an “air friction chart” and a chart containing various duct fittings and their “equivalent footage.” Both of these charts are published by ASHRAE.

To give an example, we will look at a single duct run designed to supply 400 cfm of air. Using a 0.1 design static pressure, we will determine that the proper duct size for this run will be the equivalent of a 10-in. round duct. (Using the friction chart, we look at the crosshairs between 0.1 and 400 cfm and we find ourselves at the 1-in. round duct size.) To be more accurate in our calculations, we should use the “actual static,” which is calculated as follows:

Static = 10 ÷ by Total Effective Duct Length

Where:

Total Effective Duct Length = Actual Duct Length + Equivalent Duct Length.

The actual duct length is the total length of the run while the equivalent duct length utilizes the “equivalent footage” of all fittings in the run. (For this you need the ASHRAE chart.)

If the total effective duct length for this run was actually 30 ft, you would need to use a static of 0.33 (10/30). Looking up the crosshairs between 0.33 and 400 cfm, you would come to a duct size of 8-in. round. This duct size will provide more resistance to flow and still deliver the designed 400 cfm. If, on the other hand, the total effective duct length was 250 ft, the static could be only 0.04. Looking up the crosshairs, we find that we now need a 12-in. round duct to deliver the 400 cfm.

In response to your question regarding changing the speed of a blower, you need to determine the volume of air that the furnace is presently providing. This can be determined by measuring the average velocity of the air in the main supply duct and then multiplying it by the cross-sectional area of the duct in square feet. In other words:

Cfm = Air Velocity (ft/min) x Cross-Sectional Area (sq ft)

Another way to determine the air volume being delivered is to use the sensible heat formula. Briefly stated:

Q(sensible) = 1.8 x Cfm x TD

or

Cfm =Q(sensible)

1.08 x TD

Where:

1.08 = Constant

TD = Temperature difference across the furnace

Q(sensible) = Watts x 3.413 (Watts = voltage x current)

So, for an electric furnace drawing 90 A at 220 V, with 50° TD across the furnace, we get:

Cfm = 90 x 220 x 3.413 = 1,251 Cfm

1.08 x 50

If this blower was being used for a 3-ton air conditioning system, this blower would be sufficient (400 cfm/ton). If the blower was not providing enough airflow, a two-speed motor should be used.

To sum up your question, if the system’s ductwork is designed properly, the blower’s speed and motor size in horsepower should not have to be changed.

Got a technical question for the pros? Submit your Service Hotline questions via The News’ website, www.achrnews.com!

Publication date: 11/06/2000