What is the difference between pound-mass and pound-force in HVAC equations?

  • Thread starter russ_watters
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In summary, the psychrometric chart on the website says "pounds", but the equation to calculate the typical hvac cheater equation for pressure vs velocity uses 14 lb/ft^3 as the mass or weight density of air.
  • #1
russ_watters
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I'm having a slow day (pun intended) today, and I need a little help. I'm getting pound-mass and pound-force mixed up in my head. On a psychrometric chart, it just says "pounds", but is 14 lb/ft^3 the mass or weight density of air? I've been using cheater equations for the past few years, but now I need to do derive the typical hvac cheater equation for pressure vs velocity:

VP=4005 V^1/2

VP = velocity pressure in inchess of water
V = Velocity of air in feet per minute

So where does the 4005 come from...?
 
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  • #2
Ok, so it would appear that I'm not a complete idiot. See, the problem my boss and I were having is that we were not able to reconcile equation 1 on PDF page 55 http://www.alnor.com/downloads/manuals/1980497B.pdf with our "cheater" equation. The problem -- the equation is wrong in our flow-hood manual. Still, I'd like to finish the derivation...
 
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  • #3
(P1-P2)/Rho = V^2/2g (P1 is the total pressure, against the flow, and P2 is the static pressure, perpendicular to the flow, of the measurement)

Density of air at STP is, approximately, 0.075lbm/cu.ft.
1lbf/sq.ft = 0.1922 inches wg
g = 115820 ft/min^2

So, V = [2g(P1-P2)/Rho]^1/2 = [2x115820(VP)/(0.075x0.1922]^1/2 = 4008.66(VP)^1/2

Better conversion factors yield the result close to the standard constant 4005.
 
  • #4
(eh.. with all hatred towards IP system) Change that g to gc (32.2 lbm.ft/lbf.sec^2)

By the way, the constant 4005 is derived not because better unit conversions are used but the decimal value is chopped off (courtesy : ASHRAE handbook of fundamentals 2005)

Only correction required to the eq. given in the above reference is to put the density under the square root.
 
  • #5
Thanks - I must've worked on that for an hour and couldn't get my units straight. I've only been in "the real world" for 4 years and already I'm starting to get rusty...
 
  • #6
I know the feeling! Possibly the best thing about the ISO/SI system is that it forces a clear distinction between mass and weight?
 
  • #7
russ_watters said:
I'm getting pound-mass and pound-force mixed up in my head. On a psychrometric chart, it just says "pounds", but is 14 lb/ft^3 the mass or weight density of air?
One of the primary reasons I prefer metric/SI! IP is so . . . :yuck:
 

1. What is the difference between pound-mass and pound-force?

Pound-mass and pound-force are two different units of measurement for force and mass, respectively. Pound-mass (lbm) is a unit of mass, representing the amount of matter in an object, while pound-force (lbf) is a unit of force, representing the amount of force required to accelerate an object with a mass of one pound at a rate of 32.174 feet per second squared.

2. Why do we use both pound-mass and pound-force?

The use of both pound-mass and pound-force dates back to the English engineering system of units. Pound-mass was originally used to measure mass, while pound-force was used to measure force. However, due to the confusion and inconsistency caused by using the same unit for two different quantities, the use of pound-mass has been largely replaced by the more precise unit of measurement, the kilogram (kg).

3. Can you convert between pound-mass and pound-force?

Yes, it is possible to convert between pound-mass and pound-force using the formula F = ma, where F represents the force in pound-force, m represents the mass in pound-mass, and a represents the acceleration in feet per second squared. However, it is important to note that this conversion is only accurate in Earth's gravitational field, and may not be accurate in other environments.

4. Is pound-mass the same as pound-mass?

No, pound-mass and pound-mass are not the same. Pound-mass (lbm) is a unit of mass, while pound-mass (lbm) is a unit of force. This confusion often arises due to the fact that both units use the same symbol, "lbm". However, they represent two different quantities and should not be used interchangeably.

5. Which unit of measurement is more commonly used, pound-mass or pound-force?

Pound-force (lbf) is more commonly used than pound-mass (lbm) in everyday life, as it is a unit of force and is often used in the measurement of weight. However, in scientific and engineering applications, the use of pound-force has been largely replaced by the more precise unit of measurement, the newton (N).

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