Thanks for the great video. Why is humidity ratio unitless? Should it not have units of lb moisture per lb dry air? In the formula mdot_w = mdot_a*(deltaW) it is imperative that the lbs of air cancels.
Thanks for the thoughful question. Let me attempt to provide some clarity. It depends on how you view the units - If you view 'lb dry air' all together as a unit, which is entirely independent of 'lb vapor', then yes, it's imperative that you carry those units throughout the calculation as you have described. However, if you view the units for both of those as just the unit of mass, lb, then you have lb divided lb, which you can think of as unitless or 1. In that process of cancelling, information is lost, since there are an infinite number of ways you could get to that point. This is why we still normally present the humidity ratio with the explicit units lb vapor/lb da, so we don't get confused which variable is in the numerator or denominator. But when evaluating the formula, it can be considered unitless. In a unit system, you typically add things with like units. If I have 2 meters of aluminum and 2 meters of steel, I'm still normally allowed to add these together for a total length. However, if you take the viewpoint like you have done above, these would be considered different units, and this operation wouldn't be allowed.
MITCH, ASHRAE 2017 Fundamentals, 18:15, uses the number 1076, which they say represents latent heat at 75/50% minus the enthalpy of leaving condensate at 50F. Sounds reasonable, don’t you think?
Dear sir , in your converting mass flow rate of dry air m dot (lb/hr) = roh of air (lb/ft3) x Vol(ft3/hr) Now to convert Vol from (ft3/hr) to (ft3/min) We have Vol (ft3/min) = (1/60) x ft3/min So how come you are mutilplying by 60 instead of dividing by 1/60 Could y explain 🤔
@@MitchellPaulus but sir volume flow rate is all ready in ft3/hr unit and we need it to be in ft3/min So that we would have cfm as volume flow rate , that we can use it either in latent and sensible heat calculation , And as u stated if we x by 60 min/hr , Then we have vol dot = ft3/hr x 60min/hr Which lead to canceling hrs unit and we left with ft3/ 60 min which we can express as ft3/min x 1/ 60
It is not 4.5 (CFM) Δh, it is 4.5 (CFM) (Δω) (hv), where hv is the latent heat of vaporization, the amount of energy to take a specified amount of mass from a liquid to vapor state.
All good. I watched this video to know what steps I needed to complete for a job where I have no guidance. Applying thermo and fluids to real world applications can be tricky at first haha
Thank you!
Thanks for the great video. Why is humidity ratio unitless? Should it not have units of lb moisture per lb dry air? In the formula mdot_w = mdot_a*(deltaW) it is imperative that the lbs of air cancels.
Thanks for the thoughful question. Let me attempt to provide some clarity.
It depends on how you view the units - If you view 'lb dry air' all together as a unit, which is entirely independent of 'lb vapor', then yes, it's imperative that you carry those units throughout the calculation as you have described.
However, if you view the units for both of those as just the unit of mass, lb, then you have lb divided lb, which you can think of as unitless or 1. In that process of cancelling, information is lost, since there are an infinite number of ways you could get to that point. This is why we still normally present the humidity ratio with the explicit units lb vapor/lb da, so we don't get confused which variable is in the numerator or denominator. But when evaluating the formula, it can be considered unitless.
In a unit system, you typically add things with like units. If I have 2 meters of aluminum and 2 meters of steel, I'm still normally allowed to add these together for a total length. However, if you take the viewpoint like you have done above, these would be considered different units, and this operation wouldn't be allowed.
MITCH, ASHRAE 2017 Fundamentals, 18:15, uses the number 1076, which they say represents latent heat at 75/50% minus the enthalpy of leaving condensate at 50F. Sounds reasonable, don’t you think?
Certainly reasonable!
thank you so much, it's helpful
THANKS A LOT, JUST(DELTA w) are they from off coil and on coil ?
Yes, ω2 (leaving) would be off the coil and ω1 would on the coil (inlet).
Thank you
Dear sir , in your converting mass flow rate of dry air
m dot (lb/hr) = roh of air (lb/ft3) x Vol(ft3/hr)
Now to convert
Vol from (ft3/hr) to (ft3/min)
We have Vol (ft3/min) = (1/60) x ft3/min
So how come you are mutilplying by 60 instead of dividing by 1/60
Could y explain 🤔
We are multiplying by 60 minutes/hour, since we wanted hour as our unit of time in the denominator in the end.
@@MitchellPaulus but sir volume flow rate is all ready in ft3/hr unit and we need it to be in ft3/min So that we would have cfm as volume flow rate , that we can use it either in latent and sensible heat calculation ,
And as u stated if we x by 60 min/hr ,
Then we have vol dot = ft3/hr x 60min/hr
Which lead to canceling hrs unit and we left with ft3/ 60 min which we can express as ft3/min x 1/ 60
Sir,Would you please explain to me the term delta h from q = 4.5 CFM delta h, delta h is h1-h2, or h2-h1? Thank you,
It is not 4.5 (CFM) Δh, it is 4.5 (CFM) (Δω) (hv), where hv is the latent heat of vaporization, the amount of energy to take a specified amount of mass from a liquid to vapor state.
Shouldn’t it be mv2-mv1=mw? Since the sum of flow in equals the sum of flows out? Basically the first principle no?
Sébastien Yes - just a typo - this guy is just too good not to know that
Sébastien remember also the whole thing applies to adding moisture to the air stream also
Yes, my apologies, for the way I've drawn it I have the subscripts backwards. I'll try to add a note to the video to mention this.
All good. I watched this video to know what steps I needed to complete for a job where I have no guidance. Applying thermo and fluids to real world applications can be tricky at first haha