Airflow - Basic Fundamentals of Airflow
One inch w.c. is the amount of what?
Pressure needed to support a one-inch water column.
Static pressure is used as a measurement of what?
Resistance to airflow.
Dry bulb temperature drop over a coil tells only the what?
Sensible cooling performance. You need wet bulb for total cooling performance.
Too much airflow increases what?
Static pressure loss, places a greater load on the condensing unit, and reduces latent cooling capacity. Too little air reduces system capacity, and can cause compressor failure.
External static pressure, or ESP, is what?
The pressure difference between the return opening and the supply opening on the unit at a given airflow.
Unit airflow ratings are often given after what?
The pressure drop for internal components is deduced. Always check to see if performance data is given with or without coils, filters or heating elements.
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The pressure loss can be measured in inches of water column.
Total static pressure drop is what?
The sum of the individual pressure drops of all system components from return air grill to supply grill register.
The pressure drop of air flowing from point to point is actually what?
The total of the pressure drops of all resistances, like filters, coil, duct, supply grill, etc.
Latent cooling=what?
Total cooling-sensible cooling. Sensible ratio=sensible cooling divided by total cooling
How should you read static pressure?
When both readings are positive or when both are negative, you subtract the smaller number from the larger. When the reading is negative and the other is positive, add the numbers for the total pressure difference.
If, in a duct system, equipment air performance is marginal or the duct system is unusually long, don't use 0.1 per 100 feet of duct, use what?
0.08
Many filters are rated at what?
0.07 inches to 0.08 inches pressure drop at 300 feet per minute face velocity when cleaned. To calculate the correct filter size, divide CFM by 300 for the square footage of the filter area needed.
Propane burns at what temperature?
1750 degrees F. Silfoss burns at 1500 degrees F. When adding air to acetylene, you can take acetylene's burning temperature up to 2700 degrees F. But pure oxygen added to acetylene, and it burns at 5600 degrees F.
The capacity of a cooling system can be calculated with what type of an equation?
A "total heat" equation, requiring only the change in total heat across the cooling coil and the air flow rate in CFM. Total cooling capacity is found using the total heat chart and formula,
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A higher static pressure difference between two points causes more air to flow. Resistance to flow causes a drop in static pressure as the air moves from one point to another.
A filter, that is a little bit dirty, which is also too small, will have what type of pressure drop?
A much greater pressure drop. Velocity is greater through a smaller filter. PD2=V2 over V1, squared, times PD1
Pressure drop across a cooling coil is usually the most reliable way of measuring what?
Air quality with static pressure. The change in P= P High-P Low
The difference in static pressure, between two points, causes what?
Air to move, from high to low pressure. The amount of air which moves is determined by the amount of pressure difference between the two points and the friction, or resistance to flow, between those two points.
In the cooling mode, a temperature gain of 3 degrees F in a return duct and a 4 degree F gain in a supply duct reduces the net cooling capacity by how much?
As much as 30%. This means the loss of almost a ton of cooling capacity for a 3-ton system.
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BTUH=CFM times change in heat times 4.5. The change in heat= the total heat difference or change using wet bulb. The number of tons of cooling is determined by dividing the total cooling by 2,000
Formula for finding air distribution pressure drops?
PD2=CFM2 divided by CFM1, squared, and all that times PD1
Forcing air through a space causes a what?
Greater velocity. Twice the velocity means 4 times the pressure drop.
As airflow increases, temperature rise does what?
Increases
Static pressure drop, or loss, across a constant resistance, like a coil, filter or grill, does what as airflow increases?
Increases
High indoor airflow blanks the sensible cooling capacity, or what you feel and blanks the latent cooling capacity, dealing with humidity?
Increases the sensible cooling capacity, and reduces the latent cooling capacity.
How can you measure velocity pressure?
It cannot be measured directly. It is found as the difference between total and static pressures.
How does low air flow affect the condensate pan?
It causes excessive condensate fillup, while high airflow causes minimal water output.
The pressure drop through a clean filter, operating at 500 feet per minute can be calculated as what?
PD2= feet per minute 2, divided by feet per minute 1, both squared, all multiplied by PD1.