Airstreams, Total Pressure and Standard Air¶
Summary¶
This chapter completes the air fundamentals: velocity pressure and its equation, total pressure, pressure rise, air power, the continuity equation, and the standard air condition that catalog ratings assume. It then classifies the airstreams fans must handle (hot, wet, corrosive, particulate-laden, and explosive) because those conditions drive many construction choices later in the book. After completing it, readers can describe an application's air conditions in the terms a fan specification uses.
Concepts Covered¶
This chapter covers the following 16 concepts from the learning graph:
| Concept | Concept Impact Score |
|---|---|
| Wet Airstream | 35 |
| Duct System | 235 |
| Blower | 6 |
| Pressure Rise | 3 |
| Feet Per Minute | 3 |
| Velocity Pressure | 85 |
| Standard Air | 66 |
| Corrosive Airstream | 46 |
| Particulate-Laden Airstream | 26 |
| Hot Airstream | 120 |
| Explosive Atmosphere | 77 |
| Air Power | 2 |
| Airflow Continuity Equation | 3 |
| Barometric Pressure | 34 |
| Total Pressure | 62 |
| Velocity Pressure Equation | 2 |
Prerequisites¶
This chapter builds on concepts from:
Two Questions on Every Fan Inquiry
Every fan inquiry eventually asks two questions: how hard does the fan have to push, and what exactly is it pushing? This chapter gives you the vocabulary to answer both in the terms a catalog and a specification use. By the end you will be able to turn a job-site description into air conditions an application engineer can select from. Let's follow the airflow.
Where This Chapter Picks Up¶
Chapter 1 described air with four measurements: airflow volume in cubic feet per minute (CFM), air velocity, static pressure in inches of water gauge (in. wg), and air density. Those are enough to say what a fan delivers, but not enough to read a performance table or write an order. A catalog rating also assumes a particular air condition, and a real job almost never matches it exactly. And the air itself may be hot, wet, corrosive, dusty or flammable, any one of which changes how the fan has to be built.
The chapter is in two halves. The first half finishes the pressure story: it adds velocity pressure to static pressure to get total pressure, shows how a fan's pressure rise and air power follow, and places the fan inside the duct system it serves. The second half covers barometric pressure and standard air, then the five airstream conditions that appear on almost every industrial inquiry.
Velocity, Area and Continuity¶
Air velocity in the fan industry is stated in feet per minute (fpm): the distance, in feet, that air travels along a duct or through an opening in one minute. Fpm pairs with CFM, which also uses minutes, so no unit conversion is needed between them. One fpm is 1/60 of a foot per second. Typical values range from a few hundred fpm across a filter bank, to 1,500–3,000 fpm in ventilation ductwork, to 4,000 fpm or more in ducts that carry dust.
Velocity, area and airflow are tied together by the airflow continuity equation. Air that enters one end of a duct with no branches or leaks must come out the other end, so the volume flowing past every cross-section is the same. Multiplying the cross-sectional area by the average velocity gives the airflow.
Airflow Continuity Equation¶
where:
- \( Q \) is the airflow volume, in CFM (ft³/min)
- \( A \) is the inside cross-sectional area of the duct or opening, in ft²
- \( V \) is the average air velocity across that area, in fpm
Strictly, continuity conserves mass. Treating it as conserving volume is accurate in ductwork because fan pressures change air density by only a few percent. The practical consequence is simple: when the same airflow moves from a large duct into a small one, the velocity rises in proportion to the area ratio. A 10,000 CFM airstream in a 24 in. × 24 in. duct (4.00 ft²) moves at 2,500 fpm; squeezed into a 20 in. × 20 in. duct (2.78 ft²), it moves at 3,600 fpm.
Velocity Pressure¶
Moving air carries kinetic energy, and that energy can be measured as a pressure. Velocity pressure (VP) is the pressure that would be produced if a moving airstream were brought to a stop. You can feel it by holding your hand flat against a strong wind: the push on your palm is velocity pressure being recovered. Velocity pressure is measured in in. wg, the same unit as static pressure, and it is always positive in the direction the air is moving.
Static pressure and velocity pressure answer different questions. Static pressure is the bursting or collapsing pressure that pushes outward or inward on the duct walls in every direction, and it is what overcomes the resistance of filters, coils, dampers and duct friction. Velocity pressure exists only because the air is moving, and it acts only in the direction of motion. A Pitot tube inserted into a duct, facing upstream, senses both; a side-wall tap senses static pressure alone. The difference between the two readings is the velocity pressure.
The Velocity Pressure Equation¶
The velocity pressure equation converts between velocity and velocity pressure. For standard air (defined later in this chapter as air with a density of 0.075 lb/ft³), it takes a form every application engineer memorizes:
Velocity Pressure Equation (Standard Air)¶
where:
- \( VP \) is the velocity pressure, in in. wg
- \( V \) is the average air velocity, in fpm
- 4005 is the constant that makes the units work for air at 0.075 lb/ft³
For air at any other density \( \rho \) (lb/ft³), the general form is \( VP = \rho \left( V / 1096 \right)^2 \). Substituting \( \rho = 0.075 \) recovers the 4005 version.
The equation is a square law. Doubling the velocity quadruples the velocity pressure, and that is why small, high-velocity ducts are expensive to run.
Worked example: velocity pressure in two ducts. Use the 10,000 CFM airstream from the continuity section.
- In the 24 in. × 24 in. duct, \( V = 10{,}000 / 4.00 = 2{,}500 \) fpm. Then \( VP = (2{,}500 / 4005)^2 = (0.624)^2 = 0.39 \) in. wg.
- In the 20 in. × 20 in. duct, \( V = 10{,}000 / 2.78 \approx 3{,}600 \) fpm (the exact area is 400 ÷ 144 = 2.778 ft², so a calculator on the rounded 2.78 shows 3,597; the book rounds to 3,600). Then \( VP = (3{,}600 / 4005)^2 = (0.899)^2 = 0.81 \) in. wg.
- Going the other way, a Pitot traverse that averages 0.25 in. wg of velocity pressure means \( V = 4005 \times \sqrt{0.25} = 4005 \times 0.5 = 2{,}003 \) fpm.
The duct area dropped by about 30 percent, but the velocity pressure roughly doubled. That energy has to come from the fan.
A Number Worth Memorizing
At standard air, 4,005 fpm is exactly 1.00 in. wg of velocity pressure, so 2,000 fpm is about 0.25 in. and 3,000 fpm is about 0.56 in. Keep those three points in your head and you can sanity-check any velocity pressure on a drawing or test report in seconds.
Total Pressure¶
Total pressure (TP) is the sum of static pressure and velocity pressure at a point in the airstream. It represents all of the energy the air carries at that point, measured as pressure.
Total Pressure¶
where:
- \( TP \) is the total pressure, in in. wg
- \( SP \) is the static pressure, in in. wg (positive above atmospheric pressure, negative below)
- \( VP \) is the velocity pressure, in in. wg (always positive)
The sign convention matters. On the inlet side of a fan, the fan is pulling air through the system, so static pressure is below atmospheric and is written as a negative number. Velocity pressure is still positive. The total pressure on the inlet side is therefore usually negative but smaller in magnitude than the static pressure.
Total pressure is the quantity that steadily falls along a duct in the direction of flow, because friction and fittings turn some of the air's energy into heat. Static pressure alone does not behave that simply: where a duct gets larger and the air slows down, some velocity pressure converts back into static pressure, and static pressure can actually rise downstream. Total pressure never rises unless a fan adds energy.
Static and Velocity Pressure Trade Places
Think of total pressure as the air's energy budget, with static and velocity pressure as two accounts it can move money between. A duct reducer moves energy from the static account to the velocity account; an expansion moves some of it back. Only the fan makes a deposit; every fitting and filter makes a withdrawal.
Pressure Rise and Air Power¶
The pressure rise of a fan is the increase in pressure the fan produces between its inlet and its outlet. The most complete measure is fan total pressure (FTP): the total pressure at the outlet minus the total pressure at the inlet. Catalogs more often list fan static pressure (FSP), which by industry convention is the fan total pressure minus the velocity pressure at the fan outlet. Chapter 10 shows how both appear on a performance curve; here the point is that both describe the same pressure rise, measured two ways.
Fan Pressure Rise¶
where:
- \( FTP \) is the fan total pressure, in in. wg
- \( FSP \) is the fan static pressure, in in. wg
- \( TP_{\text{outlet}} \) and \( TP_{\text{inlet}} \) are the total pressures measured at the fan outlet and inlet
- \( VP_{\text{outlet}} \) is the velocity pressure at the fan outlet
Air power is the rate at which the fan delivers energy to the air: the useful output of the fan, before any losses inside the fan, bearings or drive. In U.S. units it is calculated in horsepower from airflow and fan total pressure. The shaft power a fan draws, its brake horsepower, is always larger than its air power; the ratio of the two is the fan's total efficiency, which Chapter 10 covers.
Air Power¶
where:
- \( AHP \) is the air power, in horsepower
- \( Q \) is the airflow, in CFM
- \( FTP \) is the fan total pressure, in in. wg
- 6356 is the unit conversion between CFM × in. wg and horsepower
Worked example: an exhaust fan's pressure rise and air power. A fan exhausts 10,000 CFM. Its inlet duct is 24 in. × 24 in.; its outlet duct is 20 in. × 20 in. A field reading gives a static pressure of −1.20 in. wg at the inlet and +2.00 in. wg at the outlet. Using the velocity pressures already calculated (0.39 in. at the inlet and 0.81 in. at the outlet):
| Quantity | Inlet | Outlet |
|---|---|---|
| Static pressure, SP (in. wg) | −1.20 | +2.00 |
| Velocity pressure, VP (in. wg) | 0.39 | 0.81 |
| Total pressure, TP = SP + VP (in. wg) | −0.81 | +2.81 |
- Fan total pressure: \( FTP = 2.81 - (-0.81) = 3.62 \) in. wg.
- Fan static pressure: \( FSP = 3.62 - 0.81 = 2.81 \) in. wg.
- Air power: \( AHP = 10{,}000 \times 3.62 / 6356 = 5.70 \) hp.
- If the fan's total efficiency were 75 percent (an illustrative value), the shaft would need \( 5.70 / 0.75 = 7.59 \) hp.
Notice that subtracting a negative inlet pressure adds to the pressure rise. A fan that must pull hard on its inlet works just as hard as one that must push hard on its outlet.
Watch the Inlet Sign
A common trap is adding inlet and outlet static pressures as if both were positive, or dropping the inlet reading entirely because "the fan only blows." Write every reading with its sign before you subtract, and check that your inlet total pressure is negative on an exhaust or ducted-inlet fan; if it is not, recheck the gauge connection.
The Duct System¶
A fan never works alone. The duct system is everything the air passes through on its way to and from the fan: hoods, grilles, louvers, filters, coils, dampers, elbows, transitions, straight duct runs and the final discharge, together with the fan's own inlet and outlet connections. A system can be very short (a wall fan with a louver) or very long (a dust-collection network with dozens of hoods). The duct system is the reason a fan must produce pressure at all: every component takes away some total pressure, and the fan must put back exactly as much as the system uses at the airflow being moved.
Duct-system resistance depends on velocity, and velocity depends on airflow. Friction and fitting losses scale with velocity pressure, which scales with the square of velocity. The result is the most useful rule in system work: for a fixed duct system, the pressure required rises with the square of the airflow.
System Resistance Relation¶
where:
- \( P_1 \) is the pressure the system requires at airflow \( Q_1 \), in in. wg
- \( P_2 \) is the pressure the system requires at airflow \( Q_2 \), in in. wg
Plotting that relation for every airflow gives the system curve. Chapter 10 lays it over the fan curve to find the operating point, and Chapter 25 covers the "system effect" losses caused by poor inlet and outlet connections. Duct-system design itself is outside the scope of this book; what the book needs from it is the airflow and the pressure the system requires, stated at the actual air conditions.
Worked example: a requested airflow increase. The plant running the exhaust system from the previous example wants 20 percent more airflow, 12,000 CFM, through the same ductwork.
- Airflow ratio: \( 12{,}000 / 10{,}000 = 1.20 \).
- Pressure ratio: \( 1.20^2 = 1.44 \).
- New fan total pressure: \( 3.62 \times 1.44 = 5.21 \) in. wg.
- New air power: \( 12{,}000 \times 5.21 / 6356 = 9.84 \) hp, which is \( 1.20^3 = 1.73 \) times the original 5.70 hp.
A 20 percent airflow increase costs 44 percent more pressure and 73 percent more air power. Customers who ask for "just a bit more air" from an existing system are often surprised by that arithmetic, and it is why the duct system has to be described before a fan can be selected.
The MicroSim below lets you change the airflow in this same exhaust system and watch velocity, velocity pressure, static pressure and total pressure respond at both fan connections.
Diagram: Duct System Pressure Explorer¶
Run Duct System Pressure Explorer Fullscreen
Duct System Pressure Explorer
Type: microsim
sim-id: ch02-duct-system-pressure-explorer
Library: p5.js
Status: Specified
Bloom Level: Apply
Bloom Verb: calculate
Learning Objective: The learner will calculate duct velocity, velocity pressure, total pressure, fan total pressure and air power for a simple exhaust duct system, to within the tolerances stated in Feedback.
Prerequisites: airflow continuity equation, feet per minute, velocity pressure, velocity pressure equation, static pressure, total pressure, fan total pressure, fan static pressure, air power, duct system, system resistance relation (all defined in the sections above).
Evidence of Mastery: The learner answers five calculation challenges by typing a number before the sim reveals it. An answer is correct when it is within the tolerance listed for that challenge. Mastery is at least 4 of 5 correct on the first attempt. Moving the airflow control and reading the live values is exploration, not evidence.
Misconceptions: (1) Inlet static pressure is added as a positive number, so the fan pressure rise is underestimated. (2) Velocity pressure is proportional to velocity rather than to velocity squared. (3) A 20 percent airflow increase needs only 20 percent more pressure. (4) Static pressure always falls in the direction of flow.
Instructional Rationale: Apply-level calculation needs practice with a check, so each challenge locks the live readout of the target value until the learner commits an answer. Free exploration of the airflow afterwards lets the learner see the square law and the sign convention hold at every airflow, which confronts misconceptions 2 and 3 with numbers.
Content:
A labeled schematic of one exhaust system, in flow order: Hood → Inlet duct (24 in. × 24 in., area 4.00 ft²) → Station 1 (fan inlet) → Fan → Station 2 (fan outlet) → Outlet duct (20 in. × 20 in., area 2.78 ft²) → Discharge stack.
Values shown at each station: velocity V (fpm), velocity pressure VP (in. wg), static pressure SP (in. wg), total pressure TP (in. wg). Values shown for the fan: fan total pressure FTP (in. wg), fan static pressure FSP (in. wg), air power AHP (hp).
Clicking a schematic label reveals its definition:
- Hood: the entry point where air and contaminant are captured from the room.
- Inlet duct: the duct between the hood and the fan; the fan pulls on it, so its static pressure is negative.
- Station 1 (fan inlet): the measurement point just upstream of the fan.
- Fan: the only component that adds total pressure to the air.
- Station 2 (fan outlet): the measurement point just downstream of the fan.
- Outlet duct: the duct between the fan and the stack; the fan pushes into it, so its static pressure is positive.
- Discharge stack: where the air leaves the system at atmospheric pressure.
Challenges, in this order:
| # | Question shown | Correct answer | Tolerance | Feedback when wrong |
|---|---|---|---|---|
| 1 | At 10,000 CFM, what is the velocity in the inlet duct (fpm)? | 2,500 | ±25 | Use Q = A × V: 10,000 ÷ 4.00 ft² = 2,500 fpm. |
| 2 | At 10,000 CFM the outlet velocity is 3,600 fpm. What is the outlet velocity pressure (in. wg)? | 0.81 | ±0.02 | VP = (V ÷ 4005)²: (3,600 ÷ 4005)² = 0.899² = 0.81 in. wg. Velocity pressure goes with the square of velocity. |
| 3 | At 10,000 CFM, inlet SP is −1.20 and inlet VP is 0.39; outlet SP is +2.00 and outlet VP is 0.81. What is the fan total pressure (in. wg)? | 3.62 | ±0.03 | TP inlet = −1.20 + 0.39 = −0.81; TP outlet = 2.00 + 0.81 = 2.81; FTP = 2.81 − (−0.81) = 3.62 in. wg. Keep the inlet sign. |
| 4 | What is the air power at 10,000 CFM and 3.62 in. wg fan total pressure (hp)? | 5.70 | ±0.05 | AHP = Q × FTP ÷ 6356 = 10,000 × 3.62 ÷ 6356 = 5.70 hp. |
| 5 | The airflow is raised to 12,000 CFM in the same system. What fan total pressure is now required (in. wg)? | 5.21 | ±0.05 | Pressure goes with the square of airflow: (12,000 ÷ 10,000)² = 1.44, and 3.62 × 1.44 = 5.21 in. wg, not 3.62 × 1.20. |
Provenance: All values come from the chapter sections "Velocity Pressure", "Pressure Rise and Air Power" and "The Duct System". The system model is illustrative, chosen so that 10,000 CFM reproduces the chapter's worked example; the sim labels it "illustrative system".
Rules:
- Airflow Q: min 4,000, max 14,000, step 500, default 10,000, unit CFM.
- Air is standard air (0.075 lb/ft³) throughout.
- V1 = Q ÷ 4.00; V2 = Q ÷ 2.78 (fpm). VP = (V ÷ 4005)² (in. wg).
- Total pressure at each station follows the system resistance relation: TP1 = −0.81 × (Q ÷ 10,000)²; TP2 = +2.81 × (Q ÷ 10,000)² (in. wg).
- SP1 = TP1 − VP1; SP2 = TP2 − VP2.
- FTP = TP2 − TP1; FSP = FTP − VP2; AHP = Q × FTP ÷ 6356.
- Display velocities to the nearest 10 fpm, pressures to 0.01 in. wg and air power to 0.01 hp. Display computed values from unrounded intermediates.
- At the default airflow the sim shows V1 = 2,500, VP1 = 0.39, SP1 = −1.20, TP1 = −0.81, V2 = 3,600, VP2 = 0.81, SP2 = 2.00, TP2 = 2.81, FTP = 3.62, FSP = 2.81, AHP = 5.70 (matches the chapter table; small rounding differences of ±0.01 are acceptable).
Learner Activity:
- The learner reads challenge 1 and types an answer. The value the challenge asks for is hidden in the live readout until the answer is checked.
- The learner presses Check. The sim marks the answer, shows the correct value and its worked calculation, and moves to the next challenge.
- For challenge 5, the airflow control moves to 12,000 CFM before the question appears, and the FTP readout is hidden until the answer is checked.
- After challenge 5, free exploration unlocks: the learner changes the airflow and every station value and fan value updates at once. The learner should notice that SP1 is always negative, that TP1 is always smaller in magnitude than SP1, and that doubling the airflow from 5,000 to 10,000 CFM multiplies FTP by 4 and AHP by 8.
Feedback: Five challenges, fixed order, two attempts each. Correct: "Correct:
Starting State: Airflow at 10,000 CFM, the schematic with all labels, and challenge 1 displayed with the inlet velocity readout hidden. The prompt reads "How fast is the air moving in the inlet duct?"
Chapter Anchors: 10,000 CFM; 24 in. × 24 in. duct, 4.00 ft², 2,500 fpm, VP 0.39 in. wg; 20 in. × 20 in. duct, 2.78 ft², 3,600 fpm, VP 0.81 in. wg; inlet SP −1.20, outlet SP +2.00; TP −0.81 and +2.81; FTP 3.62 in. wg; FSP 2.81 in. wg; air power 5.70 hp; 12,000 CFM needs FTP 5.21 in. wg and 9.84 hp; airflow ratio 1.20, pressure ratio 1.44, power ratio 1.73.
Fans and Blowers¶
The word blower appears on nameplates, catalogs and company names, including Twin City Fan & Blower, so a new engineer needs to know what it signals. In everyday use, "blower" means a fan built to produce comparatively high pressure, usually a centrifugal design. Twin City Fan's current catalog describes its pressure blowers as "ideal for applications requiring high pressures at relatively low volumes of air," with performance that stays stable through the operating range and can be turned down to zero flow with a discharge damper. FanPedia notes that pressure blowers have narrower housings and impeller widths than standard centrifugal fans (FanPedia p. 4).
Engineering standards draw the line more precisely, using the pressure ratio: the absolute pressure at the outlet divided by the absolute pressure at the inlet. The commonly cited ASME definitions are:
- Fan: pressure ratio up to 1.11
- Blower: pressure ratio from 1.11 to 1.20
- Compressor: pressure ratio above 1.20
Absolute pressure is atmospheric pressure plus gauge pressure. At sea level the atmosphere is about 407 in. wg (the next section explains where that number comes from).
Worked example: is a 40 in. wg pressure blower a fan or a blower? Take a machine drawing air from the room at sea level and discharging at 40 in. wg.
- Inlet absolute pressure: 407 in. wg (atmospheric).
- Outlet absolute pressure: \( 407 + 40 = 447 \) in. wg.
- Pressure ratio: \( 447 / 407 = 1.098 \).
At 1.098, the machine is technically a fan under the ASME definition, even though its product line is called a pressure blower. The ratio reaches 1.11 only near 45 in. wg (\( 0.11 \times 407 \)), and 1.20 near 81 in. wg. The lesson for a specification is to state the airflow and pressure required, never just the word "blower"; the word tells you the customer expects high pressure, not which product family fits.
Barometric Pressure¶
Barometric pressure is the absolute pressure of the atmosphere at a location, the weight of the column of air above it. Weather reports and fan calculations state it in inches of mercury (in. Hg). At sea level under standard conditions it is 29.92 in. Hg. Since mercury is about 13.6 times denser than water, that equals roughly 29.92 × 13.6 = 407 in. wg: the atmosphere presses on everything with a force equal to a column of water nearly 34 feet tall.
Barometric pressure falls with altitude, because there is less air overhead. It also moves a little with weather, but for fan work the site elevation is what matters. Using the standard atmosphere, approximate values are:
| Elevation (ft) | Barometric pressure (in. Hg) | Ratio to sea level |
|---|---|---|
| 0 | 29.92 | 1.000 |
| 2,500 | 27.31 | 0.913 |
| 5,000 | 24.89 | 0.832 |
| 7,500 | 22.65 | 0.757 |
Air density is directly proportional to absolute pressure, as Chapter 1 explained, so the ratio column is also the ratio of air density at that elevation to air density at sea level, at the same temperature. That ratio is what corrects a fan's catalog performance for altitude.
Worked example: a fan for a plant at 5,000 feet. A catalog rating (illustrative values) says a fan at a certain speed moves 8,000 CFM against 2.00 in. wg static pressure and draws 4.00 bhp at sea level and 70°F. The fan will be installed at 5,000 ft, and the air will be at 70°F.
- Barometric ratio at 5,000 ft: \( 24.89 / 29.92 = 0.832 \).
- A fan is a constant-volume machine: at the same speed it moves the same 8,000 CFM, whatever the air density.
- Pressure scales with density: \( 2.00 \times 0.832 = 1.66 \) in. wg.
- Power scales with density: \( 4.00 \times 0.832 = 3.33 \) bhp.
If the duct system at the plant genuinely needs 2.00 in. wg at that airflow, this selection falls short. The fan must be selected from the catalog at \( 2.00 / 0.832 = 2.40 \) in. wg to deliver 2.00 in. at altitude.
Standard Air¶
Every fan catalog has to rate its fans at some air condition, and the industry agreed on one: standard air, air with a density of 0.075 lb/ft³. Dry air at 70°F and a barometric pressure of 29.92 in. Hg (sea level) has very nearly this density, so standard air is often described simply as "70°F at sea level." Unless a table says otherwise, the static pressures and brake horsepower in a fan catalog are standard-air values, and the 4005 constant in the velocity pressure equation assumes standard air too.
Real air rarely matches. The correction combines the altitude effect from the previous section with the temperature effect from Chapter 1 into one density ratio:
Density Ratio¶
where:
- \( \rho \) is the actual air density, in lb/ft³
- \( P_b \) is the actual barometric pressure, in in. Hg
- \( T \) is the actual air temperature, in °F
- 530 is 70°F expressed in absolute degrees Rankine (70 + 460)
Three rules follow from the fact that a fan moves a fixed volume at a fixed speed. At the same speed, airflow (CFM) does not change with density. Pressure changes in direct proportion to the density ratio. Brake horsepower also changes in direct proportion to the density ratio. The worked examples in the barometric pressure section above and the hot airstream section below both apply these rules; the table below collects them in one place.
| Quantity, same fan at same speed | Effect of lower density (hot air or high altitude) |
|---|---|
| Airflow, CFM | Unchanged |
| Pressure, in. wg | Multiplied by the density ratio (falls) |
| Brake horsepower | Multiplied by the density ratio (falls) |
| Mass of air moved, lb/min | Multiplied by the density ratio (falls) |
Two Directions, One Ratio
If density corrections feel slippery, that is normal; the hard part is remembering which way to apply the ratio, not the arithmetic. Use one rule: going from the catalog to the job, multiply by the density ratio; going from the job's required pressure back to the catalog, divide by it. Try it on the 5,000-foot example above until both directions feel automatic.
The Airstreams Fans Handle¶
Chapter 1 introduced the airstream, the air or gas mixture that actually passes through the fan, and the idea of a contaminated airstream. Specifications go further and name the specific condition, because each condition points toward specific construction, materials, arrangements and accessories. Five conditions cover nearly every industrial inquiry: hot, wet, corrosive, particulate-laden and explosive. An airstream can have more than one of them at once, and a good inquiry form asks about each separately. The construction details are covered in later chapters; this section teaches you to recognize each condition and to ask the right questions about it.
Hot Airstreams¶
A hot airstream is one whose temperature is high enough to change the fan's performance or to call for construction beyond the standard catalog build. There is no single threshold temperature, because the limit depends on the fan design. FanPedia's high-temperature pages show where special construction begins for different designs (FanPedia pp. 110–116):
| Fan design (FanPedia page) | Airstream temperatures covered by high-temperature packages |
|---|---|
| Centrifugal fans, Arrangements 1, 8, 9, 9F, 10 (p. 110) | 276°F to 1,000+°F; packages from 301–500°F up to 1,001°F and over |
| Centrifugal fans, Arrangement 4 (p. 111) | 181°F to 1,000+°F; packages 181–300°F and 301°F and over |
| Arrangement 4 plug fans (p. 114) | 181°F to 1,000+°F; 301°F and over requires engineering review |
| Axial fans (p. 116) | 106°F to 600°F, in packages starting at 106–150°F |
Other limits show up in the technical descriptions. A standard double-width, double-inlet fan in Arrangement 3 has both bearings in the airstream and is limited to clean air at 130°F (FanPedia p. 167). Arrangement 1 keeps the bearings on a pedestal outside the airstream, "which makes them ideal for high temperature or contaminated air applications" (FanPedia p. 162), while for an Arrangement 4 plug fan, with the impeller on the motor shaft, "high airstream temperatures may limit the use of this arrangement" (FanPedia p. 171). High-temperature construction adds features such as shaft coolers, heat shields, insulation, high-temperature grease and fixed-and-floating bearings; Chapter 21 covers them.
Hot air also changes performance, through density. Hot air is lighter, so a fan running at the same speed develops less pressure and draws less power in service than its standard-air catalog rating shows.
Worked example: selecting a fan for a 300°F oven exhaust at sea level. The duct system needs 3.00 in. wg static pressure at the operating temperature of 300°F.
- Density ratio: \( \dfrac{29.92}{29.92} \times \dfrac{530}{300 + 460} = \dfrac{530}{760} = 0.697 \). The actual density is \( 0.075 \times 0.697 = 0.0523 \) lb/ft³.
- Catalog (standard-air) pressure to select at: \( 3.00 / 0.697 = 4.30 \) in. wg, at the required CFM.
- Suppose the catalog shows 10.0 bhp at that point (an illustrative value). The operating power at 300°F is \( 10.0 \times 0.697 = 6.97 \) bhp.
- The catch: when the oven starts cold, the air is near 70°F and the fan draws the full 10.0 bhp until the air heats up. A motor sized only for the 6.97 bhp hot condition would be overloaded at every cold start. The order must state both the operating temperature and whether the fan must start and run on cold air, so the motor can be sized for cold start or protected by a damper or speed control.
The density ratio grows smaller as temperature rises and altitude increases. The chart below plots it, so you can read the correction for any combination before you calculate it.
Diagram: Density Ratio by Temperature and Altitude¶
Run Density Ratio by Temperature and Altitude Fullscreen
Density Ratio by Temperature and Altitude
Type: chart
sim-id: ch02-density-ratio-temperature-altitude
Library: Plotly
Status: Specified
Bloom Level: Apply
Bloom Verb: calculate
Learning Objective: The learner will calculate the air density ratio for a given airstream temperature and site elevation, to within ±0.01, and use it to convert a required pressure into a standard-air catalog pressure.
Prerequisites: standard air, barometric pressure, air density, density ratio, hot airstream, the rule that pressure and power scale with the density ratio (defined in the sections "Barometric Pressure", "Standard Air" and "Hot Airstreams" above).
Evidence of Mastery: The learner answers five challenges by typing a value before the chart marks the point. A density ratio answer is correct within ±0.01; a pressure answer is correct within ±0.05 in. wg. Mastery is at least 4 of 5 correct on the first attempt. Hovering curves and moving the temperature marker is exploration, not evidence.
Misconceptions: (1) Temperature in °F can be used directly in the ratio without adding 460. (2) Hot air raises the pressure a fan develops. (3) To find the catalog pressure, multiply the required pressure by the density ratio instead of dividing. (4) Altitude and temperature corrections cannot be combined.
Instructional Rationale: The chart makes the shape of the correction visible (steep at moderate temperatures, flattening at high ones; each altitude curve a fixed fraction of the sea-level curve), and the challenges then require the learner to compute values the chart can confirm, so the learner practices the calculation rather than reading answers off the plot.
Content:
- x-axis: airstream temperature, °F, from 0 to 1,000.
- y-axis: density ratio (actual density ÷ 0.075 lb/ft³), dimensionless.
- Four curves, one per elevation: 0 ft (29.92 in. Hg), 2,500 ft (27.31 in. Hg), 5,000 ft (24.89 in. Hg), 7,500 ft (22.65 in. Hg).
- A horizontal reference line at ratio 1.000 labeled "standard air".
- Hovering any point shows: elevation, barometric pressure, temperature, density ratio to three decimals, and density in lb/ft³ to four decimals.
- Reference points the chart can mark: 70°F at 0 ft = 1.000; 0°F at 0 ft = 1.152; 300°F at 0 ft = 0.697; 600°F at 0 ft = 0.500; 1,000°F at 0 ft = 0.363; 70°F at 5,000 ft = 0.832; 300°F at 5,000 ft = 0.580.
Challenges, in this order:
| # | Question shown | Correct answer | Feedback when wrong |
|---|---|---|---|
| 1 | Density ratio at 300°F, sea level? | 0.697 | 530 ÷ (300 + 460) = 530 ÷ 760 = 0.697. Add 460 to °F before dividing. |
| 2 | Density ratio at 70°F, 5,000 ft? | 0.832 | 24.89 ÷ 29.92 = 0.832; at 70°F the temperature factor is 1.000. |
| 3 | Density ratio at 300°F, 5,000 ft? | 0.580 | Multiply the two factors: 0.832 × 0.697 = 0.580. |
| 4 | Density ratio at 600°F, sea level? | 0.500 | 530 ÷ (600 + 460) = 530 ÷ 1,060 = 0.500. |
| 5 | A system needs 3.00 in. wg at 300°F, sea level. What standard-air pressure do you select from the catalog (in. wg)? | 4.30 | Divide by the density ratio when going from the job to the catalog: 3.00 ÷ 0.697 = 4.30 in. wg. Multiplying gives 2.09, which would under-select the fan. |
Provenance: The density ratio formula and the 0.075 lb/ft³ standard are from the chapter section "Standard Air". Barometric pressures are from the chapter's elevation table, computed from the standard atmosphere. Challenge 5 is the chapter's 300°F worked example.
Rules: density ratio = (Pb ÷ 29.92) × (530 ÷ (T + 460)), with T in °F and Pb in in. Hg. Density = 0.075 × ratio. Temperature marker: min 0, max 1,000, step 10, default 70, unit °F. Elevation selector: 0, 2,500, 5,000, 7,500 ft; default 0 ft. Ratios display to three decimals.
Learner Activity:
- The learner moves the temperature marker and switches the elevation; the marked point and its hover readout update. The learner should notice that the ratio at 600°F is exactly half the ratio at 70°F, and that each elevation curve is the sea-level curve multiplied by a constant.
- The learner opens the challenges. For each, the learner types an answer and presses Check; the chart then marks the correct point on the correct curve.
- After the five challenges the learner may continue exploring.
Feedback: Five challenges, fixed order, two attempts each. Correct: "Correct:
Starting State: The sea-level curve with the marker at 70°F and a ratio of 1.000, the other three curves visible, and the question "How much lighter is 300°F air than standard air?"
Chapter Anchors: 0.075 lb/ft³; 70°F; 29.92 in. Hg; 530; 460; 300°F at sea level = 0.697 and 0.0523 lb/ft³; 5,000 ft = 24.89 in. Hg and 0.832; 2,500 ft = 27.31 in. Hg (0.913); 7,500 ft = 22.65 in. Hg (0.757); required 3.00 in. wg at 300°F selects at 4.30 in. wg.
Wet Airstreams¶
A wet airstream carries enough water vapor, mist or steam that liquid water can condense and collect inside the fan. Typical sources are dishwashing and laundry exhaust, swimming-pool exhaust, cooling and washing processes, and any process that vents steam. Water vapor changes air density only slightly, so the main concern is not performance but what the water does to the fan: it pools in the housing, attacks steel and coatings, and can reach motors and bearings.
FanPedia lists several provisions for wet service:
- Drains and weep holes. Fan housings typically need an opening at the lowest point for draining moisture; most come standard with a weep hole, and a drain (a half-coupling pipe welded to the scroll, with an optional plug or valve) can be added. Inlet boxes may also get drains (FanPedia p. 147).
- High moisture construction, used "in applications where steam or condensation may collect in the fan housing." It applies to Arrangement 9 axial fans only and cannot be combined with high temperature construction (FanPedia p. 122).
- Motor enclosure. An open drip-proof (ODP) motor "should not be used in dirty and wet atmospheres"; a totally enclosed motor is the usual choice (FanPedia p. 73).
Worked example: reading a wet-service inquiry. A customer asks for an exhaust fan for a commercial laundry: warm, steam-laden air, ducted to the roof, with the fan mounted in the duct at its low point. Work through it the way an application engineer would:
- Will liquid collect in the fan? Yes: the fan sits at the duct's low point and the air is near saturation. Specify a housing drain at the lowest point, piped to a floor drain, rather than relying on a weep hole.
- Does the air pass over the motor? If the design puts the motor in the airstream, choose an arrangement that moves it out, or a totally enclosed motor; never ODP.
- Is the moisture carrying anything else? Laundry exhaust also carries lint, which makes the airstream particulate-laden as well (see below). Note both conditions on the order.
Corrosive Airstreams¶
A corrosive airstream contains gases, vapors or mists that chemically attack the fan's materials: acids, alkalis, chlorine, solvents and similar compounds, often made worse by moisture and heat. Mild steel, the standard material, can lose metal quickly in such service. The defense is material selection: coatings, stainless steel (SST) or fiberglass-reinforced plastic (FRP). Twin City Fan's current catalog states that its fiberglass fans "are specifically designed for the exhaust of moisture-laden, corrosive or chemically contaminated air" and that FRP "offers a more economical solution compared to stainless steel or other exotic alloys."
Mixing materials needs care. FanPedia lists buffer strips for use "when customer specifies that corrosion resistant (i.e. stainless steel) housing may not be welded to a mild steel pedestal" (FanPedia p. 85). Motors near corrosive air may call for a Severe Duty or chemical-duty motor (FanPedia p. 73).
Worked example: choosing a material path. An inquiry reads: "Exhaust from a chemical storage room; chlorine-bearing air; room temperature; humid." Before any material can be chosen, the application engineer needs four facts:
| Question | Why it matters |
|---|---|
| Which chemicals, and at what concentration? | Material compatibility is chemical-specific; "corrosive" alone cannot be quoted. |
| What is the airstream temperature, normal and maximum? | Every coating and FRP resin has a temperature limit that must be checked against the catalog. |
| Is the air wet or dry? | Moisture usually accelerates attack, and it adds the wet-airstream questions. |
| Where is the motor, and is it in the airstream? | It decides whether the motor needs protection or a severe-duty enclosure. |
For this inquiry, humid, chlorine-bearing air at room temperature points first toward the fiberglass fan line, with stainless steel as the alternative if the catalog's FRP limits or the customer's standard rule it out. The final choice is confirmed against the current catalog, not against a general rule.
Particulate-Laden Airstreams¶
A particulate-laden airstream carries solid material: dust, grit, chips, fibers, lint, paper trim or granular product. Particles cause two problems. They wear (erode) blades and housings, and they build up on surfaces, which unbalances the impeller and causes vibration. Fibrous and stringy material can also wrap around impeller parts. Particulate service therefore pushes toward rugged impellers, wear-resistant construction and bearings kept out of the airstream. FanPedia shows radial-bladed impellers designed for this work, including the open paddle impeller, the material handling impeller (wool) and the paper handling impeller (FanPedia p. 32), and taconite bearing seals "designed for dirty or abrasive environments" (FanPedia p. 100). Twin City Fan's current catalog offers pressure blowers "to meet clean or particulate-laden airstream applications."
Ducts that carry material must keep the air moving fast enough that particles stay suspended instead of settling and plugging the duct. That minimum is the conveying velocity, and for many dusts it is on the order of 3,500–4,500 fpm (a general industrial ventilation range; the right value depends on the material).
Worked example: airflow for a dust-collection branch. An 8 in. round duct must carry dust at 4,000 fpm (illustrative).
- Area: \( \pi (8/12)^2 / 4 = 0.349 \) ft².
- Airflow from continuity: \( Q = 0.349 \times 4{,}000 = 1{,}396 \) CFM.
- Velocity pressure: \( (4{,}000 / 4005)^2 = 1.00 \) in. wg.
That velocity pressure, about 1 in. wg in every branch, is lost at every hood entry and recovered nowhere. This is why dust-collection fans run at higher pressures than ventilation fans of the same airflow, and why the airstream description must say what the material is, not just "dusty."
Explosive Atmospheres¶
An explosive atmosphere is air that contains flammable gases, vapors, mists or combustible dusts in concentrations that a spark or hot surface could ignite. Solvent vapors, fuel vapors and fine grain, wood or metal dust are common examples. A fan in such service must not become the ignition source: a steel impeller rubbing a steel inlet can throw sparks. Twin City Fan addresses this with spark-resistant construction in three types, summarized here from FanPedia (p. 193):
- Type A, the highest degree of spark resistance: all fan components in the airstream are non-ferrous, assembled to reduce the possibility of contact between stationary and rotating parts.
- Type B: the impeller is non-ferrous, typically aluminum with an aluminum rub plate, and the airstream parts are assembled to reduce the possibility of contact.
- Type C, a minimal level: only reduces possible contact between rotating and stationary parts, typically with an aluminum inlet cone and an aluminum rub plate where the shaft passes through the housing.
Spark resistance concerns the fan's airstream parts. The motor is a separate decision: an explosion-proof (EXPL) motor is designed so that an internal arc or explosion cannot ignite a surrounding atmosphere of flammable dust or gas, and its rating is specified under the National Electrical Code by class, division and group (FanPedia p. 73). Chapter 20 treats the spark-resistant types in detail, including what each one does not protect against.
Worked example: separating the two decisions. An inquiry reads: "Exhaust for a solvent-recovery room; flammable vapors present; motor to be mounted in the same room."
- Is the airstream an explosive atmosphere? Yes, flammable vapors are present. The specification needs spark-resistant construction, and the type (A, B or C) must come from the customer's specification or engineer, because it sets how much of the fan must be non-ferrous.
- Is the motor in a hazardous location? Yes, it is in the same room. It needs an explosion-proof motor rated for the area's NEC class, division and group, which the owner or the authority having jurisdiction determines; the fan supplier does not assign it.
- If the motor could instead be placed outside the hazardous area, for example with a belt-driven arrangement and the motor beyond a wall, the motor decision changes, but the spark-resistance decision does not.
Spark Resistant Is Not Explosion Proof
Watch out for orders that say "explosion-proof fan." Spark-resistant construction lowers the chance that the fan's moving parts strike a spark; it does not make the fan, or the system, explosion-proof. Ask which spark-resistant type the specification calls for and, separately, what hazardous-location rating the motor needs.
Recognizing Conditions Together¶
Real airstreams often combine conditions: a laundry is wet and particulate-laden, a grain elevator is particulate-laden and explosive, a natatorium is wet and corrosive. The sorter below gives ten application descriptions; tag every condition each one has, or mark it clean.
Diagram: Airstream Condition Sorter¶
Run Airstream Condition Sorter Fullscreen
Airstream Condition Sorter
Type: microsim
sim-id: ch02-airstream-condition-sorter
Library: p5.js
Status: Specified
Bloom Level: Understand
Bloom Verb: classify
Learning Objective: The learner will classify ten application descriptions by every airstream condition they present (hot, wet, corrosive, particulate-laden, explosive atmosphere, or clean), matching the answer key on at least 8 of 10.
Prerequisites: airstream, hot airstream, wet airstream, corrosive airstream, particulate-laden airstream, explosive atmosphere (defined in the section "The Airstreams Fans Handle" above).
Evidence of Mastery: For each description the learner selects one or more condition tags and presses Submit. An item is correct when the selected set exactly matches the answer key. Mastery is at least 8 of 10 correct on the first attempt. Reading definitions by clicking a tag's info button is exploration, not evidence.
Misconceptions: (1) Each airstream has only one condition. (2) Warm, humid air is "hot." (3) Combustible dust is only a particulate problem. (4) Any chemical exhaust is an explosive atmosphere.
Instructional Rationale: Classification with multi-select tags makes the learner judge each condition separately against its definition, which is the Understand-level skill an inquiry form needs. Immediate per-item feedback with a reason corrects the overlap misconceptions while the description is still on screen.
Content:
Tag definitions, shown on each tag's info button:
- Hot: temperature high enough to change performance or call for construction beyond the standard catalog build.
- Wet: enough water vapor, mist or steam that liquid can condense and collect in the fan.
- Corrosive: gases, vapors or mists that chemically attack the fan's materials.
- Particulate-laden: carries solid material such as dust, grit, chips, fibers or lint.
- Explosive atmosphere: contains flammable gases, vapors or combustible dusts that a spark could ignite.
- Clean: none of the above; ordinary air near standard conditions.
Items, shown in shuffled order:
| # | Description shown to the learner | Correct tags | Why (shown as feedback) |
|---|---|---|---|
| 1 | Recirculating air inside a kiln dryer at 450°F. | Hot | 450°F is well inside FanPedia's high-temperature package ranges; nothing else is described. |
| 2 | Exhausting steam-laden air from a commercial dishwashing room. | Wet | Steam condenses and collects in the housing; it is warm, not hot. |
| 3 | Dry, room-temperature exhaust from a chemical storage room containing chlorine compounds. | Corrosive | Chlorine compounds attack fan materials; the air is dry and at room temperature. |
| 4 | Sand and grit from a foundry shakeout, at ambient temperature. | Particulate-laden | Sand is abrasive solid material; it is not combustible. |
| 5 | Exhaust from a solvent-recovery room where flammable vapors are present. | Explosive atmosphere | Flammable vapors could be ignited by a spark from the fan. |
| 6 | Filtered outdoor air supplied to an office floor. | Clean | Filtered outdoor air near 70°F has none of the five conditions. |
| 7 | Exhaust from an indoor swimming pool hall: warm, humid air containing chlorine compounds. | Wet, Corrosive | The humidity condenses and the chlorine compounds attack materials; the air is warm, not hot. |
| 8 | Grain dust drawn from a feed-mill bucket elevator. | Particulate-laden, Explosive atmosphere | Grain dust is solid material and is also a combustible dust. |
| 9 | Clean air leaving a heat-treating oven at 700°F. | Hot | 700°F calls for high-temperature construction; the air is stated to be clean. |
| 10 | Warm, moist exhaust from commercial laundry dryers carrying lint. | Wet, Particulate-laden | Moisture condenses in the duct and fan, and lint is fibrous solid material. |
Provenance: Definitions are from the chapter section "The Airstreams Fans Handle". Item temperatures and descriptions are illustrative applications written for this sim; item 1 and 9 temperature judgments rest on FanPedia pp. 110–116 as summarized in the chapter's high-temperature table.
Rules: Tags are multi-select. Selecting Clean clears all other tags, and selecting any other tag clears Clean. An empty selection cannot be submitted. Correct means exact set match; a partly matching set is incorrect and the feedback names which tags were missing or extra.
Learner Activity:
- The learner reads a description and may open any tag's info button to read its definition.
- The learner selects every tag that applies and presses Submit.
- The sim marks the item, shows the correct tags and the "Why" text, and presents the next item.
- After ten items the sim lists any missed items with their correct tags.
Feedback: Ten items, shuffled, one attempt each. Correct: "Correct:
Starting State: The first shuffled description with no tags selected and the prompt "Which conditions does this airstream have? Select all that apply."
Chapter Anchors: Ten descriptions; five conditions plus Clean; the chapter names laundry (wet and particulate-laden), grain elevator (particulate-laden and explosive) and natatorium (wet and corrosive) as combined examples; mastery 8 of 10.
Putting It Together on an Inquiry¶
Everything in this chapter ends up in a few lines of a fan inquiry or specification. A complete air-conditions description states:
- Airflow in CFM, and whether it is stated at actual or standard conditions.
- Pressure required, identified as static or total, at the stated airflow.
- Airstream temperature: normal operating, maximum, and the start-up condition.
- Site elevation, or barometric pressure.
- Each airstream condition present: hot, wet, corrosive, particulate-laden, explosive, with the material, chemical or hazard named.
With those five items, an application engineer can convert the duty to standard air, read the catalog, and begin choosing the construction the airstream demands.
You Can Describe the Air Now
You can now add velocity pressure to static pressure to find total pressure, work out a fan's pressure rise and air power, and convert a hot, high-altitude duty to the standard air a catalog uses. You can also name the airstream conditions that will steer construction choices in the chapters ahead.
Key Takeaways¶
- Velocity in fpm, area in ft² and airflow in CFM are linked by continuity, \( Q = A \times V \); for the same airflow, a smaller duct means higher velocity.
- Velocity pressure is the pressure of moving air brought to rest. At standard air, \( VP = (V/4005)^2 \): 4,005 fpm is 1.00 in. wg, and doubling velocity quadruples VP.
- Total pressure is \( SP + VP \). On a fan's inlet side, SP is negative; keep the sign when you subtract.
- A fan's pressure rise is fan total pressure, \( TP_{\text{outlet}} - TP_{\text{inlet}} \); fan static pressure is FTP minus the outlet velocity pressure. Air power is \( Q \times FTP / 6356 \).
- In a fixed duct system, pressure rises with the square of airflow and air power with its cube, so a 20 percent airflow increase needs 44 percent more pressure.
- "Blower" usually signals a high-pressure fan; by the ASME pressure-ratio definition, many pressure blowers are still fans. State airflow and pressure, not the word.
- Barometric pressure is 29.92 in. Hg (about 407 in. wg) at sea level and falls with altitude; air density falls with it.
- Catalogs rate fans at standard air, 0.075 lb/ft³. At the same speed, CFM does not change with density, but pressure and horsepower scale with the density ratio. Multiply going from catalog to job; divide going from job to catalog.
- Hot airstreams reduce operating pressure and power, raise cold-start power relative to the hot condition, and call for high-temperature construction whose starting temperature depends on the fan design.
- Wet, corrosive, particulate-laden and explosive airstreams each point to specific provisions: drains and enclosed motors, corrosion-resistant materials such as FRP or stainless steel, rugged impellers and adequate conveying velocity, and spark-resistant construction plus correctly rated motors. Airstreams often combine conditions, so ask about each one.