adiabatic compression

[PaulLag]

Hi there

hope everybody is fine.

please i am facing to following problem

1 - I have air @ 86F and 43 PSI abs

this air is compressed adiabatically @ ambient pressure

I need to know the new temperature.
I was thinking to utilize

TP^((1-y)/y) = const

where y = cp/cv = 1.4

But with my surprise when utilizing this formula I get an outlet temperature lower than 86F.
I would expect the opposite.

Please, can anybody help me ?

Many thanks

 

[ione]

To me, going from 43 psi(a) to atmospheric pressure is an expansion.......

 

[PaulLag]

my mistake

from

86F and atmospheric pressure to
43 PSI abs

sorry

 

[ione]

In the formula you should enter absolute temperature (Kelvin). I've got 284.738 F.

 

[PaulLag]

many thanks
may I understand my method is correct ?

One question: in case of humid air (air with 50% RH ?)

many thanks

 

[ione]

You could use the same approach for moist until you remain in a range of conditions which don't imply saturation/condensation phenomena. In any case the exponent present in the formula reported in your OP needs to be changed because the specific heats of moist air are different from those of dry air and are influenced by the water vapour mixing ratio.

 

[zdas04]

I've got some issues with your equation. This relationship is typically written as:

T[sub]disch[/sub]=T[sub]suct[/sub]*R[sub]c[/sub][sup](γ-1)/γ[/sup]

where R[sub]c[/sub]=P[sub]disch[/sub]/P[sub]suct[/sub]

I'm not sure where you got that T[sub]disch[/sub] was constant, but it is anything but constant. Also putting "P" into the equation is just confusing. If P[sub]suct[/sub] = P[sub]atm[/sub] =14.7 psia, then the I get 742R or 282°F, which is ione's answer ± some wobble in conversion factors and picking an atmospheric pressure.

There are MANY transient and unstable factors in this calculation and trying to adjust it for changes in humidity is going way outside of its value (i.e., is the discharge temperature going to be 280°F, 290°F, or 300°F?). When I'm using this relationship I never look at the "ones" column. Humidity variation changes it in the "tenths" or "hundredths" column--simply outside the accuracy of the calculation.

David Simpson, PE
MuleShoe Engineering

Law is the common force organized to act as an obstacle of injustice Frédéric Bastiat

 

[ione]

David,

I think that, even the formulation in the OP has not been as rigorous as yours, what was meant was the same thing

Tdisch = Tsuct*(Pdisch/Psuct)^[(γ-1)/ γ]

Tdisch/ Pdisch ^[(γ-1)/ γ] = Tsuct /Psuct)^[(γ-1)/ γ]

Tdisch* Pdisch ^[(1-γ)/ γ] = Tsuct *Psuct)[(1-γ)/ γ]

So what remains constant is not the temperature, but the product of the temperature T multiplied by the pressure P raised to the exponent [(1-γ)/ γ]

 

[zdas04]

ione,
Absolutely correct. Since that formulation is only useful in deriving the one that is (very) useful, I keep forgetting that step. Thanks for clarifying.

David Simpson, PE
MuleShoe Engineering

Law is the common force organized to act as an obstacle of injustice Frédéric Bastiat