Practical Process Engineering

Also, this is another ebook they already published. Both the book and their website are strongly recommended.

2018 version of Best Tips of The Month was published today. This is a highly recommended ebook. It is a set of the published articles on petroskills website.

👆👆👆These videos are original videos downloaded from Aspen website. More videos will be uploaded later.👆👆👆

Setting up Screw Compressors in Aspen HYSYS

Model Plate Fin Exchangers in Aspen HYSYS Using Aspen Plate Fin Exchanger

Design an Air-cooled Overhead Condenser for an Atmospheric Crude Distillation Unit

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Here they did not use incompressibilty assumption and adiabatic formulae underpredict the amount of flowrate. ***Overprediction for using incompressible assumption instead of adiabatic formulae when the flow is adiabatic. ***Underprediction using adiabatic assumption instead of real formulae when the flow is being cooled down. Both above errors work in opposite direction in part cancelling each other. Therefore, if there is an adiabatic flowrate and incompressible assumption gives a significant error, cool the flow to increase the flowrate and decrease the error. Suppose you have two numbers A and B (A>B) and you would like to decrease the difference between them. There are two options for you, decrease number A or increase number B. Here we have the same situation: number A is the amount of flowrate based on incompressible assumption and B is the adiabatic flowrate. While we can't change the calculated flowrate comming from bernouli/darcy equations (number A), we increase adiabatic flowrate by cooling (number B) to decrease the difference (error) between them. On the other hand, if the stream is heated up, another overprediction shows up due to lower adiabatic flowrate and, thus, the difference (error) between incompressible assumption and actual adiabatic flow increased. So, the heat transfer can affect the error significantly. Conclusion: To use incompressibility assumption for a compressible flow: 1- Mach number MUST be less than 0.3 2- Stagnation pressure drop and its associated L/D ratio MUST show a reasonable error. 3- Heat transfer effects should be considered. large errors in adiabatic flows can be alleviated by cooling the stream down.

If stream temprature is decreased by cooling, we jump to lower line and temperature ratio increases. Thus, a negative error appears (UNDERPREDICTION) by using adiabatic formulae.

Above pictures show that incompressibility assumption results in poitive error (OVERPRIDICTION). It means if we uses bernouli and darcy-weisbach equations to calculate flowrate of an adiabatic stream, the result will be higher than actual amount. There is a common belief among practicing engineers that we can employ incompressible assumption safely up to 40% pressure drop. But above pictures prove that pressure drop should be associated with L/D to give us a right answer. Please note that the horizontal axis in above pictures is in terms of stagnation pressure drop and this is why I discussed stagnation conditions and its difference with static conditions earlier. Last but not least is the effect of heat transfer. Sofar, adiabatic flow has been discussed while almost all real processes transfer heat. ATF Arrow software developers proved that cooling a gas may result in a greatly increased flowrate. In contrast, heating a gas can cause the flowrate to decrease significantly. So, if an engineer is trying to design for a minimum flowrate, a gas stream that is cooling works in his or her favor by causing an UNDERPREDICTION of the flowrate when using adiabatic flow methods. See the below picture.

Both pictures are for adiabatic flow. The friction factor is the same for all conditions and absolute roughness is 0.00015 ft.

Higher L/D decrease the error. But, at low L/D, even low pressure drop makes a signifcant error.

Let's get back to our topic. Studies show that the pressure drop ratio should be limited enough for incompressibility assumption to be valid. I, personally, see this statement in many references. But another study was carried out by ATF Arrow software developers and they found L/D ratio or fL/D (f: friction factor) should be considered as well. Take a look at bellow pictures👇🏼👇🏼👇🏼👇🏼

Stagnation enthalpy. All stagnation conitions are represented by zero subscript.

If the Mach number is high enough for density changes in the flow to be significant, the temperature changes in the flow will also be important. All in all, Mach number can be used to define another flow regime; COMPRESSIBLE GAS FLOW. Engineers believe for M<0.3 the error comming from such assumption is low enough that gives us enough confidence to use bernouli/ darcy-weisbach for gases. This is not the end of our story. Other factors should be considered. Second parameter is pressure drop ratio : PRESSURE DROP/ INLET PRESSURE What pressure? STAGNATION PRESSURE (not static pressure). First of all, let's study stagnation conditions. ****STAGNATION CONDITIONS: There are two important reference conditions in incomprssible flow: stagnation and critical (M=1, SONIC FLOW). If the flow at any point in a fluid stream was isentropically (adiabatic+ reversible) brought to rest (velocity=0), we call this new condition stagnation. If the entire flow is essentially isentropic and if the velocity is essentially zero at some point in the flow, then the stagnation conditions will be those existing at the zero velocity point. Even if the flow itself is not isentropic, this concept is still usefull. Stagnation properties (Enthalpy, Pressure, Temperature, Density, etc.) can vary throughout flow field, but it is understood that all stagnation properties are constant along an isentropic flow. See the equations for stagnation conditions👇🏼👇🏼👇🏼