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  • Saturday 21 May 2016

    Pressure and its Altitude Dependence

    Good Day Viewers...!!

    Back with a post, Today i gonna give you a brief description on pressure, Everyone knows about pressure, but only an Engineer can know what an Excess Pressure can do, and what can lack of pressure can result in....!!!

    And also the beginner knows the types of pressure, but in an industry a pressure can do many unremarkable things, and also there is one hidden logic, which is unknown by many that pressure will vary from location to location, in Technical words, Pressure will vary based on Altitude. And i'll explain you how it will vary and what effect it can have on pumping and some industrial operations, even it can severly influence the feasibility studies of an industrial siting. 

    Here are Some Typical Interview Queries for an Engineer which will tend to creating some fun in those who donno this logic,

    Query 1: Same pumping setup is being provided at visakhapatnam and hyderabad to pump an identical height of 20 m, where the pumping will be completed first ? 

    Query 2: I've a solvent recovery process plant in Ahmedabad and Bangalore both of them will be like twins, and there wont be any difference in between them, not even a flange also, so where can i get more recovery yield ?

    When you hear these queries you will be astonished initially but there lies a serious logic behind those queries, as the pressure will vary as per altitude.

    So, Before going into the topic, we should be able to differentiate in between types of pressure, the below figure will give clear some of your confusion right now.

     There will be three types of pressure basically, which are primarily known,

    1. Atmospheric Pressure,
    2. Absolute Pressure,
    3. Guage Pressure,

    Also we can classify pressure based on direction,

    1. Absolute Pressure - Positive direction pressure,

    2. Suction Pressure   -  Negative Direction Pressure.

    Simple example of these pressure developing equipments were House hold Ceiling fans which will provide pressure while it is rotated in clockwise and Suction while rotated in anti-clockwise direction.

    Absolute Pressure: This is the positive pressure which is developed from 0 mm to positive infinite, will usually hold the units like Bar, Atm, mm, Torr, 

    Atmospheric Pressure: This will certainly hold some value 1 atm, which equals 760 torr, at 273.15°K. Atmospheric pressure will vary according to Temperature and altitude above Sea level.

    Guage Pressure: This is what we use to denote the readings in an industry, generally will be represented with Bar's and Kg/Cm2 

    Guage Pressure can be calculated as, Pg = Pab - Pat = Pab -1.

    Here, Pg- Guage Pressure, Pab -Absolute pressure, Pat - Atmospheric Pressure.

    So, now you are familiar with the types of pressures, and now i'll reveal the procedure to calculate the pressure dependence over altitude above sea level.

    For that we need to know the Altitude above sea level for those locations, which is denoted as T.

    The Percentage loss in pressure will be, dP = ( 1 - ( 2.25577 x ( 10^-5 ) x T ) ) ^ 5.25588,

    So the available pressure in that location would be, P =  760 x dP, in Torr.

    Ahmeadabad is located 53 m above sea level, so the dP will be 

    = ( 1-(2.25577 x (10^-5) x 53)) ^ 5.25588

    = (1-(0.0011955))^5.25588
    = (0.9988)^5.25588
    = 0.99373

    So, the pressure that is available would be, P = 760 x 0.99373 = 755.23 Torr.

    So, Now i think you got the logic how pressure varies based on the altitude, So whatever pressure you will be getting in that location will be your atmospheric pressure, and the negative value of that will be your available vacuum.

    And now i strongly believe you may have one more doubt that i'm working in a hyderabad industry and my vaccum gauge is showing me 750 mm Hg of Vacuum, how??

    And for that my answer will be some what funny, but you need to accept it, you may have seen Dinosaurs in Jurrasic Park movie, and whenever those animals appears in front of your eyes, immediately one word will hit your brain, and that is GRAPHICS, so for your Question your answer will be CALIBRATION.

    The persons who will be doing the calibrations will consider the maximum available pressure in that location as full pressure, i.e., 760 mm Hg, and then they will calibrate.

    So, if you understand this logic say Cheers.....!!  Or else ...!!

    Any queries feel free to ask...!! Always happie to help....!!!

    Whatever, Stay Tuned for more!! Share this on your timelines......!!!

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    About The Author

    Hi! I am Ajay Kumar Kalva, Currently serving as the CEO of this site, a tech geek by passion, and a chemical process engineer by profession, i'm interested in writing articles regarding technology, hacking and pharma technology.
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    1. Cheerz..!!! NEED Diff pressure dp formula clearly ..

      1. thats not differential pressure bro, that dP is just a notation to represent the % available pressure., and that pressure difference can be calculated from removing available pressure from 760 torr

    2. Bro Where can you derive the percentage loss in pressure equation

      1. percentage loss in transfer lines or where??

        Dear bro,

        Comment with ur real name next time.

        AJAY K

    3. Will vaccum vary at a point which is say around 20 m away from the source.

      1. Dear ,

        If the point that is 20 m away from source is in a closed system then there wont be any issue, of if the system is having any leaks or it is a semi closed system, then the vacuum will vary for sure.

        AJAY K

    4. The Percentage loss in pressure will be, dP = ( 1 - ( 2.25577 x ( 10^-5 ) x T ) ) ^ 5.25588,
      Dear अजय can you explain how you get that formulae.

      1. Dear Sanjay,

        Thanks for landing here and reading the post,
        I appreciate your enthusiasm, till now many fellas have gone through the post, but i've not received this question, and i'm quite happy to receive this,

        below is the base equation from where it was derived,

        P = Po x [2.72 ^ (( -m x g x h ) / ( k x T )) ].
        P - pressure at required altitude,
        Po - Atmospheric pressure,
        m - mass of the gas / molecule,
        g - gravity,
        h - height,
        k - boltzman constant: 1.380 ergs/K,
        T - temperature in kelvin.

        AJAY K

      2. dP = ( 1 - ( 2.25577 x ( 10^-5 ) x T ) ) ^ 5.25588 in this formula T seems typo error, instead of T, H must be there.

      3. Hii Kiran,

        Thats not a typo error, intentionally i've mentioned it as T, and in the post i've mentioned how i've denoted too...

        Best Regards,
        AJAY K


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