3 replies to this topic

[rsk]

compressed air 5Nm3/min is flowing through a 10m steel pipe of 1" nominal dia. inlet pressure is 10kg/sc and discharging in atomosphere. In view of engineering change line rerouting is required. i wanted to know
1. what is the maximum length i can use for this service without affecting flow rate.
2. If i use length more than choked flow consideration, will this going to result in reduced flow or no flow at all.
please help on this.
regards
rsk

[Art Montemayor]

rsk:

First and foremost, never ever cite a pressure to another engineer - especially an old professional such as I - without identifying if you are talking about ABSOLUTE pressure or GAUGE pressure. This is so important - especially when you are dealing witn identifying the relative ratio of two pressures - that it should be expounded over and over again at the university level. You should never assume that others know what you are thinking or what you already know. If you don't specifically identify a value, you can't expect us to know.

If you have compressed air at 10 kg/cm^2 (absolute or gauge) and are expanding it freely to the atmosphere, you will generate Choked Flow (also known as sonic flow). You should know this and the reason why this is so. You should also know the value of that choked flow as well as the identity of the upstream pressure that will termininate the choked flow condition.

Your questions are answered as follows:

1. what is the maximum length i can use for this service without affecting flow rate.
This question makes little or no sense. Please rewrite it or compose it so that we can understand what it is that you mean or propose.

2. If i use length more than choked flow consideration, will this going to result in reduced flow or no flow at all.
Length does not play a role in choked flow. Difference in driving force (pressures) is what determines the Choked Flow. What do you mean by "no flow at all"? This makes little sense.

[rsk]

It is a receiver of 7m3 volume in discharge of compressor. receiver pressure is being controlled at 10kg/sc gauge.This receiver caters to air demand of various load through distribution. Receiver at bottom has a provision of moisture draining through 10cm, 1"sch40 steel pipe discharging in to atomosphere through automatic drain traps. Since in view of improper layout, some reorientation of line is to be done. I have a question, To what length i can extend this drain nozzle without affecting the draining capaity considered in design.

regards rsk

[Art Montemayor]

rsk:

You must first organize your thoughts and identify the basics of what you what to express. I have to assume you are a student since this is the Student Forum. Is this an academic or a real-life problem?

At first you said you were involved with "choked flow" and that you were venting compressed air to atmosphere.

Now, you return and state that the problem involves the draining of condensed water in an air receiver. There can be no "choked flow" with a non-compressible fluid under these conditions. Don't you know exactly what the problem at hand is? How is it that you have switched fluids "180 degrees"?

This is all very confusing and/or so simple that it escapes me. If it is an academic problem, then please so state. No one in his right mind worries about the length of a drain line from an air receiver. However, a Fluids Mechanics Professor might, and that might be the source of the problem.

Please do what I requested from the very beginning: furnish all the details and explain clearly what the problem is and what is it that you don't understand.