I am hitting a road block here and

Question

I am hitting a road block here and

UtilityLouie (Mechanical)

(OP)

16 Aug 01 17:32

I am hitting a road block here and am hoping that this is the correct forum for this question.  If anyone thinks there is a better forum for this question, please let me know.

I am having a problem with a vacuum system on the press section of a paper machine.  To make sure my piping is not the problem, I have been trying to calculate a pressure drop on my vacuum piping.  I have looked at freecalc and efunda.  They both have scripts to calculate friction loss of duct, but neither addresses any change in the viscosity of air in the vacuum system.

I would guess that hot (180 degrees) saturated air has a different viscosity than standard air and all calculators use the viscosity of standard air.  I think I can make the final calculation myself, but I don't like using numbers that I don't think are correct.  Now if the difference in viscosities between saturated hot air and standard air results in less than 5% in the final answer, this should be stated.

I am breaking out the thermo book, fluids book and Crane 410 tonight, but all of the formulas result from there and do not address these concerns.

Am I worrying too much?

Looking for some confidence in the numbers.

TBP (Mechanical) · Answer 1 · 2001-08-17 13:41:31

I've got an older version (2.1) of the ABZ program (based on Crane 410), and if I punch in atmospheric pressure air (0 PSIG) at 70*F, it says the absolute viscosity is 0.018 centipoise, and the kin vis is 14.953. At 180*F, these change to 0.02 and 20.882 repectively.

Being an older version, it doesn't have any fancy graphics, but I don't care. I LOVE that program. One of the best $150 I ever spent.

dcasto (Chemical) · Answer 2 · 2001-08-18 16:37:09

Here the output from a simulator that saturates the air.  Notice that the air is 53% water at the temperature and pressure (14 psig and 180 F).

K-VALUES: ASOA     ENTHALPY: ASOA     DENSITY: STD    FLOW RATES /HR
  VAP VISC: NS81     VAP THC:  NS81

                      TOTAL   LIQUID    VAPOR    TOTAL    TOTAL
COMPONENT NAME       LBMOL    LBMOL    LBMOL     LB    MOL PCT   KVALUE
--------- ----     -------- -------- -------- -------- -------- --------
AIR                  26.3555       0.  26.3555  763.361   46.120
WATER                30.7898       0.  30.7898  554.689   53.880
                    -------- -------- -------- -------- --------
TOTAL                57.1453       0.  57.1453  1318.05  100.000
TOTAL   LB           1318.05       0.  1318.05

STREAM NUMBER    1
TEMPERATURE DEG F      180.00
PRESSURE    PSIA       14.000
ENTHALPY    BTU/HR     62859.
VAPOR FRACTION         1.0000

PROPERTIES              TOTAL       VAPOR
----------------      ----------  ----------
FLOWRATE   LBMOL/HR      57.1453     57.1453
MOLECULAR WT.            23.0649     23.0649
ENTHALPY BTU/LBMOLE    1099.9913   1099.9912
ENTROPY  BTU/LBMO/R       3.4325      3.4325
CP       BTU/LBMOL/R                  7.5872
CV       BTU/LBMOL/R                  5.5764
CP/CV                                 1.3606
DENSITY  LB/FT3                     0.047185
Z-FACTOR                            0.997059
FLOWRATE FT3/SEC T-P                  7.7594
         MMSCFD  STP                0.520380
VISCOSITY   CP                      0.012973
TH.COND  BTU/FT/HR/F                0.019687

ArtR (Mechanical) · Answer 3 · 2001-08-21 14:48:51

Don't forget that the delta-p should be less than 10% of the absolute pressure or the pipes will choke. Do you have welded or screwed joints? Threads leak (by definition)

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I am hitting a road block here and